Deep-sea mining: dewatering plumes, vortex-induced vibrations and economic modelling

by Carlos Muioz-Royo M.S. Naval Architecture and Ocean Engineering Universidad Politecnica de Madrid (2016)

Submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Master of Science in Ocean Engineering at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2018 2018 Massachusetts Institute of Technology. All rights reserved.

Signature redacted

Signature of Author ..... y...... Departme of Mechanical Engineering Signature redacted Mvay 11, 2018 Certified by ... . I I Thomas Peacock Professor of Mechanical Engineering Thesis Supervisor

Accepted by...... Signature redacted Rohan Abeyaratne MASSACHU-SETSINSTITUTE OF TECHNOLOGY Professor of Mechanincal Engineering Graduate Students JUN 252018 _ Chairman, Committee on LIBRARIES ARCHIVES

Deep-sea mining: dewatering plumes, vortex-induced vibrations and economic modeling

by

Carlos Mufioz-Royo

Submitted to the Department of Mechanical Engineering on May 11, 2018 in Partial Fulfillment of the Requirements for the Degree of Master of Science in Ocean Engineering

ABSTRACT

Deep-sea mining activities are expected to commence in the next decade; the International Seabed Authority (ISA) has already issued twenty nine exploration licenses for polymetallic nodules, polymetallic sulphides and polymetallic crusts. The ISA is seeking to approve the exploitation regulations for polymetallic nodules in the next two years, but there are significant research and knowledge gaps that still have to be explored. The discharge of dewatering plumes, the analysis of the effect of vortex-induced vibrations (VIV) on mining risers, and the development of updated and detailed economic models -which are among the most pressing ones - are addressed in this thesis. PLUMEX field studies were conducted in the Pacific Ocean to create and monitor six dewatering plumes. The data obtained from the experiments will be used to validate analytic and numeric plume models that will help to assess the environmental impacts. Additionally, a preliminary VIV analysis of a simple riser configuration was conducted to show the significant impact of VIV on the fatigue life. The results showed that the fatigue life could be reduced to less than one year. Lastly, an independent and thorough economic model is being developed at MIT to allow the simulation of different scenarios and forecast the economic result of a polymetallic nodule exploitation. The ISA will consider the results from the model to determine the royalties and fees that will be paid by future contractors in order to share their benefit with the humankind.

Thesis Supervisor: Thomas Peacock Title: Professor of Mechanical Engineering

3 4 Acknowledgements

I would like to start giving thanks to my advisor, Thomas Peacock, for sharing with me this exciting learning experience and guiding me towards new challenges. Also, I have to thank Matthew Alford, Jonathan Ladner, Spencer Kawamoto, Michael Goldin, Rohit Supekar, Andrew Rzeznik and many others who worked incredibly hard during PLUMEX field studies to make it happen.

Undoubtedly, all this work would not be a reality without the unconditional support of Margarita, my wife, who is always encouraging me to make the most of myself in all facets of life. Of course, I have to thank my parents, my sister and the rest of my family for their great support and help since the very beginning. Thanks to my old and new friends, who make life much more fun and enjoyable.

Finally, I want to thank Fundaci6n Bancaria La Caixa for their trust and economic support during the last two years through their Fellowship program.

5 6 Contents

Chapter 1 Introduction to Deep-Sea M ining...... 17

1.1. Deep-sea m ineral resources...... 17 1.1.1. Polym etallic nodules ...... 17 1.1.2. Polym etallic crusts ...... 21 1.1.3. M assive sulfides ...... 23

1.2. Polym etallic nodule m ining stakeholders ...... 25

1.3. Seabed m ining process...... 28 1.3.1. Exploration ...... 28 1.3.2. Resource assessm ent, evaluation and m ine planning ...... 30 1.3.3. Technology development...... 30 1.3.4. Nodule collection ...... 32 1.3.5. Vertical transport ...... 33 1.3.6. M ining vessel...... 34 1.3.7. Horizontal transport...... 35 1.3.8. M etallurgical processing ...... 35

1.4. Environm ental impact ...... 36

1.5. Regulations...... 38

Chapter 2 Polym etallic nodule m ining sediment plum es ...... 41

2.1. W aste water discharge plum es ...... 41

2.2. Bottom plum es ...... 43

2.3. Other sedim ent plum es in the oceans ...... 44 2.3.1. Vertical and horizontal extent ...... 46 2.3.2. Flow velocity...... 50 2.3.3. Flow rate ...... 53 2.3.4. Settling velocity ...... 55 2.3.5. Density...... 56

Chapter 3 PLUM EX: Waste W ater Discharge Plum e Experim ent ...... 59

3.1. Plum e m odel...... 59

3.2. Sco pe of the experim ents ...... 61

3.3. Plum e creation...... 65

3.4. Plum e m onitoring, sampling and tracking ...... 70 3.4.1 Near field m onitoring ...... 71 3.4.2 Interm ediate field m onitoring ...... 72 3.4.3 Far field tracking and m onitoring...... 73 3.4.4. Biological studies...... 74

7 Chapter 4 Polym etallic nodule m ining econom ics ...... 75

4.1. Introduction...... 75

4.2. Previous econom ic m odels...... 76

4.3. Main assumptions, estimates and forecasts on polymetallic nodule mining economic models...... 78 4.3.1 General assum ptions...... 78 4.3.2 M ain estim ates...... 79 4.3.3 Revenue forecasting...... 80

4.4. The new M I T m odel ...... 81 4.4.1 Analysis of a proposed econom ic scenario for polym etallic nodule m ining ...... 82

4.5. Future work...... 84

Chapter 5 Vortex-induced vibrations. Preliminary analysis for polymetallic nodule mining risers...... 85

5.1. Introduction...... 85

5.2. T he riser...... 86 5.2.1 Type a nd m aterial ...... 86 5.2.2 M a in dimensions ...... 87

5.3. VIV analysis...... 88 5.3.1 M ethodology...... 88 5.3.2 Background flow ...... 89

5.4. Prelim in ary results ...... 90

5.5. Future work...... 91

Chapter 6 Conclusions ...... 93

6.1. Future perspectives of polym etallic nodule m ining ...... 93

6.2. Technical challenges...... 94

6.3. Environm ental challenges...... 95

6.4. Econom ic challenges...... 96

Bibliography...... 99

8 List of Figures

Figure 1.1. Manganese nodule with a rough top surface related to the hydrogenetic precipitation and a smooth bottom due to the diagenetic precipitation process (Von Stackelberg, 2000)...... 18 Figure 1.2. The areas in red are abyssal plains with soft seabed sediment, sedimentation rates lower than 10 mim per thousand years, and low bottom current velocities where polymetallic nodules are likely to grow. The circled areas correspond to the main deposits that are being considered as potentially mineable. Adapted from (Petersen et al., 2016)...... 19 Figure 1.3. The location of the main areas where old seamounts with cobalt-rich crusts are likely to be found is colored in red. Up to a 54% of the estimated resources are located within the EEZs (colored in blue) of different countries (Petersen et al., 2016). The Prime Crust Zone (PCZ) gathers most of the polymetallic crusts of the Pacific...... 22 Figure 1.4. Massive sulfide deposits where geochemical analysis have been carried out (Petersen et al., 2016). Small triangles indicate deposits of less than 1 million ton. Red triangles are deposits considered of economic interest because of the content of copper (more than a 5%), zinc (more than a 15%) or gold (>5 ppm). Blue areas indicate the extension of the EEZs...... 24 Figure 1.5. CCZ licensed exploration areas for polymetallic nodules (ISA, 2017). The exploration licenses of some of the contractors that are included in the map, like Interoceanmetal or LFREMER, have already expired. Because of that they are not mentioned on Table 1.8...... 27 Figure 1.6 (a) Hydraulic seabed mining system tested by Deep Sea Ventures in 1970 (Flipse, 1969). (b) Two-vessel Continuous Line Bucket System (CLBS) proposed and tested in the Pacific ocean in 1972 (N ishi, 2012)...... 32 Figure 2.1. Sketch of a sample polymetallic nodule exploitation (not scaled). The waste water is discharged at a certain depth, creating a sediment plume that is advected by the background curren ts...... 4 1 Figure 2.2. Sketch of a sample polymetallic nodule exploitation (not scaled). The sediment plume created by the horizontal exhaust jet, and the sediment resuspension generated by the nodule collection vehicle tracks form an overall sediment plume that is expected to evolve as a gravity c urren t...... 4 3 Figure 3.1. Sample result from the model, showing how the vertical extent of the plume changes with the variation of the plume release depth, keeping all the other variables constant. In this example, the plume is 5 times longer if it is released at 1300 meters of depth, than if it is released at 100 m eters...... 60 Figure 3.2. Current velocity forecast at 100 meters of depth obtained using MSEAS. The forecasted area corresponds to the Gulf of Santa Catalina, in the Pacific Ocean, where PLUMEX field studies were conducted. A subsurface anticyclone with the higher current velocities is easily observed in the center of the area...... 61 Figure 3.3. Location of PLUMEX field studies. A contingency location was chosen in the lee of San Clemente Island to operate there in case of bad weather conditions. Both locations had a depth of more than 1000 meters, to ensure that the plume was not affected by the sea bottom...... 61 Figure 3.4. R/V Sally Ride. Length: 73 m. Beam: 15 m. Draft: 4.5 m. Displacement: 3090 t. Cruise sp ee d : 12 k ts...... 62

9 Figure 3.5. (a) Buoyancy frequency calculated from a CTD profile measured during PLUMEX field studies. In (b), there is a zoom-in view of the same buoyancy frequency profile, where the MLD - marked in red - can be easily identified at about 30 meters of depth, where the buoyancy frequency is maxim um ...... 64 Figure 3.6. Sketch of the plume creation concept from R/V Sally Ride (not scaled). The tanks and pumps used to create the plume are located on the main deck of the vessel. A suction hose is deployed from the stern of the vessel to pump water from the ocean surface to create the waste water plume, which is discharged through discharge lay-flat hose of 20 cm of diameter. The discharge hose is attached to a crane cable with a clump weight at the very end in order to facilitate the deployment and recovery of the hose, and also to minimize the deflection of the hose due to the background currents...... 65 Figure 3.7. PLUMEX plume creation system diagram. Firstly, the auxiliary pump is used to fill the tanks with sea water. Then, the salt, limestone or CCZ sediment is mixed in tank 2 (T2 in the diagram) with the help of the auxiliary pump, which is recirculating the content and transferring it to tank 1. The content of both tanks is recirculated together to make sure they have the same concentration. Finally, the main pump starts pumping water from the ocean surface and discharging it through the discharge hose. At the same time, the highly concentrated mixture from the tanks is injected into the main pump inlet, using the help of the auxiliary pump. Two flow rate meters (blue circles in the diagram) were installed to measure both the over plume flow rate, and the injection flow rate ...... 67 Figure 3.8. Preparation of the CCZ sediment for the plume creation. (a) Initial condition of the sediment. (b) Sediment liquefying process. (c) Pumping the liquefied sediment using two submersible pump; the black hoses used to transfer the mixture can be seen in the top of the picture...... 6 8 Figure 3.9. a) Deck layout including the salt pallets in red, the clay totes in brown, and the knuckle- boom crane range (20 feet) in green. Pictures b) and c) show the real deck layout onboard R/V Sally Ride. The mixing tank is the black tank next to the main blue tank. The CCZ sediment was stored in the blue and green totes that are shown in the bottom of c). In that same picture, the white bulk bags next to the blue tank contain the salt for some of the experiments. In d), the orange suction hoses are ready to be deployed from the stern...... 69 Figure 3.10. Virtual increase of the center of gravity due to the effect of the free surface of a tank. 70 Figure 3.11. a) PADS being deployed from R/V Sally Ride during PLUMEX fields studies. b) Raw Intensity plot obtained during one of the plumes release. The aligned red-to-green areas correspond to the reflection of the acoustic waves on the discharge hose steel joints, at 60 meters of depth the plume is being released, and the effect of the background current is clearly identified at 90 meters of depth. However, this is a plot obtained from the raw data, that has to be adequately filtered and processed to determine the velocity field...... 72 Figure 3.12. Fast CTD being deployed from R/V Sally Ride during PLUMEX field studies. The CTD and the fluorometer mounted on the vehicle were able to obtain intermediate field measurements of the plume characteristics...... 73 Figure 4.1. a) Capital and b) Operating expenses distribution according to the ten cost sectors defined by the MIT model (Nyhart and Triantafyllou, 1983)...... 76

10 Figure 4.2. Capital expenses distribution of a polymetallic nodule exploitation according to the economic model proposed by Van Nijen, Van Passel and Squires (2018). The processing plant includes the metallurgical processing of nickel, cobalt, copper and manganese. In this case, the horizontal transport is only part of the operating costs, as the authors have decided to charter the transport vessels from an external company instead of buying them...... 77 Figure 5.1. Vortex shedding around a bluff body creating a von Karman street (Bearman, 1984)... 85 Figure 5.2. Background current velocity profiles for some of the proposed scenarios. Original current data provided by Global Sea M ineral Resources...... 90

11 12 List of Tables

Table 1.1. Characteristics of the main nodule deposits (Jauhari and Pattan, 2000; Von Stackelberg, 2000; ISA, 2010; Hein et al., 2013, 2015). The data include only the characteristics of the commercially mineable areas. For instance, in the case of the Cook Islands deposits, the total surface where nodules can be found is larger than 1 million km2 , but only about a 10% of the total area has the optimum characteristics to be mined (Hein et al., 2015)...... 19 Table 1.2. Comparison between the mean content of metals in land ores (US Geological Survey, 20131; Van Gosen et al., 20132; Li and Yang, 20141) and the mean composition of the polymetallic nodules in the four most important areas (Hein and Koschinsky, 2013). It is important to note that the composition of the nodules may vary significantly between different locations in the sam e area...... 20 Table 1.3. The comparison between land (U.S. Geological Survey, 2017) and polymetallic nodule reserves (Hein et al., 2013) emphasizes the relevant role that the latter may have in the future supply of metals. The CCZ reserves of manganese, nickel and cobalt exceed the actual land reserves...... 2 1 Table 1.4. Comparison between the mean content of metals in land ores (Mudd, 20101; US Geological Survey, 20132; Van Gosen et al., 2013'; Li and Yang, 20144) and the mean composition of the polymetallic crusts in the four most important areas (Hein and Koschinsky, 2013)...... 22 Table 1.5. Comparison between the land, PCZ, and CCZ metal estimated reserves (Hein and Koschinsky, 2013; M., L., Zientek, J., D., Causey, H., L., Parks, R., J., 2014; U.S. Geological Survey, 2017). PCZ mineral reserves stand out because of the cobalt, titanium, and nickel reserves. It is also important to note that the reserves of REE and platinum are approximately 10% of the actual land reserves that are controlled mainly by China and South Africa, resp ectively ...... 23 Table 1.6. Comparison between the most common land deposits grades (US Geological Survey, 2013; Emsbo et al., 2016; U.S. Geological Survey, 2017) and different massive sulfide deposits (Petersen et al., 20 16)...... 25 Table 1.7. Inferred reserves of copper, gold, silver and zinc at Solwara I and Solwara 1 North deposits in Papua New Guinea EEZ (Lipton, 2012)...... 25 Table 1.8. Current exploration licenses issued by the ISA (ISA, 2017). A total of 16 exploration licenses for the CCZ have been issued, and 6 of them have already expired. Only one exploration license was issued in 2001 to the Indian Government for the exploration of the Indian Ocean...... 26 Table 2.1. Vertical extent field data of hydrothermal plumes. The vertical extent ranges from 90 to 700 meters, with a mean value of 340 meters...... 47 Table 2.2. Vertical extent of volcanic plumes. In most cases, volcanic plumes reach the ocean surface, therefore, their vertical extent is limited. Because of the violence and unpredictability of these events, there is considerably fewer data than in the case of hydrothermal plumes...... 47 Table 2.3 Horizontal extent of hydrothermal plumes. The horizontal extent ranges from 2 to 70 km. According to these figures, it is reasonable to expect deep-sea mining dewatering plumes with an extent within this order of magnitude...... 48

13 Table 2.4. Vertical extent of resuspension phenomena. Values range from 4 meters above the seafloor, up to 70 meters in the case of some trawling-induced resuspension phenomenon. The activity or natural phenomenon which generated the sediment resuspension is indicated in the second colu m n ...... 4 9 Table 2.5. Horizontal extent of sediment resuspension phenomena. The horizontal extent ranges from 135 meters to 22 km ...... 49 Table 2.6. Vertical extent of gravity currents. Values range from 7.5 meters, up to 175 meters for the m ost m assive events...... 49 Table 2.7. Horizontal extent of gravity currents. Because of the characteristics and difficulty to predict this phenomena, only two field data have been found, ranging between 26 to 70 km...... 50 Table 2.8. Flow velocity at the exit orifice of hydrothermal vents. Values range from 0.5 to 3.4 m/s...... 5 1 Table 2.9. Volcanic plumes flow velocity estimates. Values range from 0.5 to 4.0 m/s...... 52 Table 2.10. Velocities of the head of the observed gravity currents. Values range from 0.2 rn/s up to 20 m/s for the most violent phenomena...... 52 Table 2.11. The bottom flow velocities indicated in this table generated sediment resuspensions in the noted locations. Values range from 0.15 to 1.2 n/s...... 53 Table 2.12. Field data of Hydrothermal vents flow rates. Values range from 0.5 up to 170 I/s. Dewatering discharge flowrates for seabed mining activities are expected to be around 500 1/s (O ebius et al., 200 1)...... 54 Table 2.13. Settling velocities of particles obtained from samples of hydrothermal plumes. The mean particle size is included in the table, as it is the governing parameter. The mean particle size in the CCZ is in the order of 10 to 100 pm (Oebius et al., 2001)...... 55 Table 2.14. Settling velocities of particles from resuspension phenomena. Values range from 8.8E-3 to 5 m m /s...... 55 Table 2.15. Initial density of hydrothermal plumes. The density has been obtained applying the plume models with field data from hydrothermal vents. The main characteristic of hydrothermal plumes is that, due to their high temperature, they are buoyant...... 56 Table 2.16. Density of gravity currents. Values range from 1029 to 1173 kg/m3. All the gravity currents have a greater density than the background sea water. According to Oebius et al. (2001), the bottom plume created by the collection vehicle may have a density of 1120 kg/m3 ...... 5 7 Table 3.1. Main parameters of the six plume experiments conducted during PLUMEX field studies. The range of parameters was selected to better validate the models and understand the behavior of the plumes. The diameter of the discharge hose used to create the plumes was 20.3 cm. Rhodamine dye was added to track the plume...... 62 Table 3.2. Comparison of the main waste water discharge parameters. Data from PLUMEX field studies is compared with OMI and OMA seabed mining pilot tests, and estimates for commercial seabed mining operations from Oebius et al. (2001) and Morgan, Odunton and Jones (1999). Sediment mass flow rate and concentration values from PLUMEX field studies correspond to the limestone and CCZ sediment plumes. In the case of commercial operations estimates, it includes not only the sediment, but also nodule fines...... 63

14 Table 3.3. Weight, footprint, load density and vertical position of the center of gravity (VCG) of the main equipment and materials for PLUMEX field studies. The maximum payload of R/V Sally Ride is 100 tons, with a maximum deck load density of the main deck is 7.5 t/m2 , and a m axim um payload V CG of 1 m ...... 69 Table 4.1. Estimated capital and operating costs for an annual production of 3 million tons of dry nodules ...... 83 Table 4.2. Metal grade of the nodules and recovery yield rates of the metallurgical processing ...... 83 Table 4.3. Royalty alternatives considered for the economic analysis ...... 83 Table 4.4. Results: IRR, average annual royalties, royalties NPV ...... 84 Table 5.1. Main dimensions and characteristics of the riser for the VIV analysis. Some of the values were obtained from Hong (1997), Chung, Whitney and Loden (1981), and Oebius et al. (2001)...... 8 8 Table 5.2. Characteristics of the dominant mode of vibration for each scenario...... 90 Table 5.3. Estimated fatigue life for the six proposed scenarios...... 91

15 16 Chapter 1 Introduction to Deep-Sea Mining

In December 1872, the Challenger Expedition sailed from Portsmouth, UK, to survey and explore the oceans all around the world onboard HMS Challenger. Among all the discoveries made by the expedition, a large number of manganese nodules was recovered from the seabed and catalogued (Murray and Renard, 1891). However, it was not until the 1960s that manganese nodules were seriously considered as a likely source of supply of metals like nickel, cobalt, or copper due to the significant increase in their demand (Antrim, 2005). J. L. Mero's book, The MineralResources of the Sea (1965), was the starting point of the extensive deep-sea mining research carried out during the 1970s and 1980s. But, in the early 1980s, a significant price drop of nickel slowed down the investments on research and development projects. It has not been until the last decade that the interests in deep-sea mining have risen again. The significant increase in the demand of nickel, cobalt, or copper and the positive perspectives for the future have focused attention on seabed mining again (Morgan, 2011). New technologies, green energy industry and electric vehicles are primarily responsible for this expected increase in the demand (Hein et al., 2013). Nevertheless, the complexity of the deep-sea environment results in the need for additional research projects, including laboratory and field experiments, to better understand the likely consequences of mining the ocean. This additional knowledge will allow development of both the international legal framework and technologies to minimize the environmental impact of seabed mining activities. Accordingly, this thesis aims at providing a better understanding of one of the main potential environmental impacts of deep-sea mining activities: the dewatering plumes that will be created from surface mining vessels.

1.1. Deep-sea mineral resources

Mineral resources from shallow waters, like diamonds, sand or gold, are being already mined, and many others are expected to start soon (Hannington, Petersen and Kratschell, 2017). However, only three types of deep-seabed mineral resources have been considered of likely commercial interest because of the relatively high content of some metals. None of them has yet been extracted for commercial purposes. These potential resources are polymetallic nodules, polymetallic crusts, and massive sulfides. In this section, the main characteristics of the three types of deposits will be described. However, the following sections will focus on polymetallic nodules, the type of resource that is more likely to play a significant role in the future of the mining industry.

1.1.1. Polymetallic nodules

Polymetallic nodules, also known as manganese nodules or ferromanganese nodules, are the result of a long precipitation process of metal oxides present in the sea water and seabed sediments. They are composed mainIy by manganese oxide and iron oxyhydroxide, but they also contain other metals of commercial interest such as nickel, cobalt and copper (Hein and Koschinsky, 2013). Additionally, traces of rare earth elements (REE) and lithium are commonly found in the composition of the nodules. The size of the nodules ranges from I to 20 cm, but small and medium size nodules between 1 to 5 cm are the most commonly found (Morgan, 2000). They are characterized by a high mean

17 porosity, up to 60%, a mean dry bulk density of 1350 kg/m3, and a wet bulk density of 1950 kg/M 3 (Jauhari and Pattan, 2000).

The precipitation process starts on the seabed around a hard nucleus; usually a shark's tooth or a whale's earbone (Glasby, Li and Sun, 2014). Manganese oxides and iron oxyhydroxides precipitate first. Manganese oxides have a negative charge on their surface, and iron oxyhydroxides a positive one (Hein et al., 2013). These electric charges attract other metallic ions present in the ocean water.

There are two main different precipitation processes that result in the formation of the nodules: the diagenesis and the hydrogenesis (Dymond et al., 1984). The diagenetic process consists of the precipitation of the metals dissolved in the pore waters within the seabed sediment. The metals in the sediment have their origin in the fecal matter from the sea surface. Because of that, the presence of a soft seabed sediment with a semi-liquid sediment layer is one of the basic characteristics of the polymetallic nodule deposits. On the other hand, in the hydrogenetic process the metals precipitate directly from the ocean water to the nodule surface. In general terms, the formation of the nodule is the result of both processes. But, in some cases, one process may have a greater importance than the other. For instance, according to Hein et al. (2015), the manganese nodules found in the Economic Exclusive Zone (EEZ) of the Cook Islands form mainly by hydrogenetic precipitation. On the contrary, according to the results showed by Von Stackelberg (2000), the diagenetic accretion predominates in the Peru Basin (PB).

The precipitation process has a direct impact on the growth rate of the nodules. Diagenetic nodules grow much faster, reaching growth rates up to several hundred mm per million years (mm/My). Hydrogenetic nodules have a growth rate between 1 to 10 mm/My (Petersen et al., 2016). Additionally, precipitation process and other factors seem to have a certain effect on the surface roughness of the nodules. The Hydrogenetic precipitation process generates a rougher surface than the diagenetic process (Figure 1.1).

Figure 1.1. Manganese nodule with a rough top surface related to the hydrogenetic precipitationand a smooth bottom due to the diagenetic precipitationprocess (Von Stackelberg, 2000).

The nodules are located in abyssal plains with soft sediments that enable the diagenetic precipitation, at depths between 4000 and 6500 m. These areas are characterized by their stable physical conditions, as bottom current velocities are extremely low and the sedimentation process is very slow, with sedimentation rates lower than 10 mm per thousand years (Hein and Koschinsky. 2013). Abyssal

18 plains with these characteristics can be found all around the world, as was discovered during the Challenger expedition.

However, a minimum abundance and metal grade are required to consider that a specific location is potentially mineable. Because of that, most research projects have been focused on four main areas; three of them are located in the Pacific Ocean and another one in the Indian Ocean (Figure 1.2). The Clarion Clipperton Fracture Zone (CCZ), located in the Pacific Ocean between Hawaii and Mexico, is the location of the largest deposit of manganese nodules (Hein et al., 2013). The Peru Basin (PB) and the Penrhyn Basin (PEN) are the two other deposits of interest in the Pacific (Petersen et al., 2016). On the other hand, a deposit located in the Central Indian Ocean Basin (CIOB) has been the focus of some research projects (Jauhari and Pattan, 2000). Table 1.1 shows the main characteristics of the four major deposits of manganese nodules around the world.

Table 1.1. Characteristicsof the main nodule deposits (Jauhari and Pattan, 2000; Von Stackelberg, 2000; ISA, 2010; Hein et al., 2013, 2015). The data include only the characteristicsof the commercially mineable areas. For instance, in the case of the Cook Islands deposits, the total surface where nodules can be found is larger than I million km2, but only about a 10% of the total area has the optimum characteristics to be mined (Hein et al., 2015).

Characteristics CCZ Peru Basin Cook Islands CIOB Nodules (million ton) 21,100 NA 2630 1335 Mineable surface (thousand km 2) 3830 1018 124 300 Average abundance (kg/M 2 ) 15 8.5 25 1.10 Depth (m) 4200 4100 4800 4800

Figure 1.2. The areas in red are abyssal plains with soft seabed sediment, sedimentation rates lower than 10 mm per thousand years, and low bottom current velocities where polymetallic nodules are likely to grow. The circled areas correspond to the main deposits that are being considered as potentially mineable. Adapted from (Petersen et al., 2016).

The CCZ covers an extension of 5.2 million km2 . In this area, 3.83 million km2 (Hemn et al., 2013) are considered of commercial interest, with nodule abundance reaching values up to 75 kg/rn 2 (wet weight). Most common found densities are around 15 kg/rn 2 (Petersen et al., 2016). The highest abundance has been found at 4200 m of depth and between a latitude of 120 to 160 N according to the geological model carried out by the International Seabed Authority (ISA, 2010). The model estimates

19 that approximately 21,100 million metric tons of dry nodules are located in the CCZ. This numbers make the CCZ the largest mining area all over the world (Antrim, 2015). Compared to other areas, nodules in the CCZ have a higher grade of nickel, copper, and cobalt (Table 1.2).

Studies held in the PB, that has almost half the size of CCZ, have detected an average abundance of 8.5 kg/M 2 with maximum values up to 66.8 kg/M 2. The higher densities have been found below 4100 m depth (Von Stackelberg, 2000). One of the characteristics of this area is a higher presence of buried nodules, which has to be considered for the design of the nodule collection system. The diagenetic accretion predominates in the formation of PB nodules, and it is directly responsible for their high manganese content (Table 1.2).

In contrast to CCZ and PB, which are located in international waters, a significant part of the Penrhyn and Samoa Basins is located in the EEZ of the Cook Islands. The size of the deposits is considerably smaller than in CCZ, covering around 1.1 million km 2 with abundances up to 32 kg/m 2 . Overall, the estimations consider a total dry weight of 8860 million tons, characterized by its high cobalt content (Kingan, 1998) and a lower content of manganese (Table 1.2). Around 2630 million tons of these nodules are located in areas with a higher nodule density than 25 kg/M 2.

In the Indian Ocean, the Central Indian Ocean Basin is the only deposit that has the minimum abundance to consider it as a mineable area according to the criteria established by the United Nations Ocean Economics and Technology Branch (UNOET, 1987). These basic criteria are that the abundance has to be higher than 5 kg/M2, the grade of nickel, copper, and cobalt together has to be higher than 2%, and the slope of the seabed has to be lower than 30. More than 4 million km 2 were surveyed by different research expeditions during the 1980s, but only two areas with a total surface of 300,000 km 2 met the criteria. These areas have an abundance between 4.5 to 5.1 kg/M 2 and a total of 1335 million tons of nodules. Overall, the nodules contain 21.84 million ton of nickel, copper and cobalt (Jauhari and Pattan, 2000).

Table 1.2. Comparison between the mean content of metals in land ores (US Geological Survey, 20131; Van Gosen et al., 201 32 ; Li and Yang, 20143 ) and the mean composition of the polymetallic nodules in the four most important areas (Hein and Koschinsky, 2013). It is importantto note that the composition of the nodules may vary significantly between different locations in the same area.

Element Land CCZ Peru Basin Cook Islands CIOB Iron (wt%) 30 - 63' 6.16 6.12 16.2 7.10 Manganese 20 - 531 28.4 34.2 16.9 24.4 Nickel 0.2-2.61 1.3 1.3 0.38 1.10 Copper 0.2-21 1.1 0.6 0.23 1.04 Cobalt 0.05 -0.41 0.21 0.05 0.41 0.11 Titanium 1 - 122 0.28 0.16 1.28 0.43 Molybdenum (ppm) 100-2,5001 590 574 295 570 Lithium 680-1,6001 131 311 51 97 REE+Y 500-60,0003 813 403 1,665 1,039

20 The best way to evaluate the importance of the resources contained in the estimated reserves of polymetallic nodules is to make an updated comparison with the estimated in-land reserves as shown in Hein et al. (2013). The results (Table 1.3) demonstrate that manganese nodules have the potential to ensure the supply of key metals like cobalt, nickel, or copper needed to develop many technologies such as renewable energies or batteries for electric vehicles. For instance, the reserves of nickel and cobalt in the CCZ are 3.5 and 6.3 times larger, respectively, than the actual land reserves. Manganese is also another important element to consider but, due to its lower value and demand, it is less relevant than nickel, cobalt and copper.

Table 1.3. The comparison between land (U.S. Geological Survey, 2017) and polymetallic nodule reserves (Hein et al., 2013) emphasizes the relevant role that the latter may have in thefuture supply of metals. The CCZ reserves of manganese, nickel and cobalt exceed the actual land reserves.

Element Land (Mt) CCZ (Mt) Cook Islands (Mt) CIOB (Mt) Iron 82,000 1299.8 1435.3 94.8 Manganese 690 5992.4 1497.3 325.7 Nickel 78 274.3 33.7 14.7 Copper 720 232.1 20.4 13.9 Cobalt 7 44.3 36.3 1.5 Titanium 830 59.1 113.4 5.7 Molybdenum 15 12.4 2.6 0.8 Lithium 14 2.8 0.5 0.1 REE + Y 120 17.2 14.8 1.4

1.1.2. Polymetallic crusts

Polymetallic crusts, also known as ferromanganese crusts or cobalt-rich crusts, have an origin and characteristics similar to that of polymetallic nodules. The crusts are the result of a hydrogenetic precipitation process. In this case, the precipitation takes place from the ocean water to the rocky substrate of old seamounts, ridges, plateaus, and abyssal hills. Unlike abyssal plains where manganese nodules grow, these locations are characterized by the presence of currents that avoid the settling of the sediments during millions of years (Hein et al., 2000). In general terms, ferromanganese crusts occur at lower depths than manganese nodules, between 400 and 4000 meters. However, the most attractive crusts from a commercial perspective are found at depths from 800 to 2500 meters.

As it is expected from a hydrogenetic precipitation process, the growth rate of the crusts is very slow, ranging from 1 to 5 mm per million years (Hein and Koschinsky, 2013). The thicker crusts reach values up to 250 mm. Due to the slow growth rate and according to Klemm et al. (2005), crusts that are more than 70 million years old have been catalogued. Crusts are considerably older than the oldest manganese nodules which are around 15 million years (Hein and Koschinsky, 2013).

Regarding the physical properties, the crusts' density is 1300 kg/M 3, and the porosity mean value is 60% (Hein et al., 2000). A direct consequence of these properties is a high mean specific surface area of 300 m2/g.

21 The Pacific Ocean has the highest number of seamounts, where crusts are likely to be found. However, they can be found in some areas of the Atlantic and Indian oceans (Figure 1.3). Polar regions are also likely to locate polymetallic crust deposits, but there is little information about the seabed geology of those areas (Hein et al., 2013).

Figure 1.3. The location of the main areas where old seamounts with cobalt-rich crusts are likely to be found is colored in red. Up to a 54% of the estimated resources are located within the EEZs (colored in blue) of different countries (Petersenet al., 2016). The Prime Crust Zone (PCZ) gathers most of the polymetallic crusts of the Pacific.

From a commercial perspective, the best area to extract polymetallic crusts is a region of 6.5 million km2 in the western Pacific called the "Prime Crust Zone" (PCZ) (Petersen et al., 2016), where the oldest seamounts are located. Therefore, the crusts are thicker, and they also have a higher content of metals.

Table 1.4. Comparison between the mean content of metals in land ores (Mudd, 20101; US Geological Survey, 20132; Van Gosen et al., 2013'; Li and Yang, 20141) and the mean composition of the polymetallic crusts in the four most important areas (Heinand Koschinsky, 2013).

Element Land PCZ South Pacific Atlantic Indian Iron (wt%) 30-632 16.9 18.1 20.9 22.3 Manganese 20- 532 22.8 21.7 14.5 17.0 Nickel 0.2 - 2.62 0.42 0.46 0.26 0.26 Copper 0.2-22 0.10 0.11 0.09 0.11 Cobalt 0.05 - 0.42 0.67 0.62 0.36 0.33 Titanium 1-l123 1.16 1.12 0.92 0.88 REE+Y 0.05 - 6.04 0.24 0.16 0.24 0.25 Platinum (ppm) 0.1 -41 0.5 0.5 0.6 0.2

Polymetallic crusts stand out because of their content in cobalt, higher than land ores and polymetallic nodules. But they also have a significant content of nickel, titanium, and platinum. In the case of the platinum, most land ores have a content around 1 ppm. Usually, platinum grade is considered along with other metals like palladium, rhodium, ruthenium, iridium and osmium (Mudd, 2010). This group of metals is known as Platinum Group Metals (PGM). In this case the information provided in Table 1.4 corresponds only to platinum itself.

22 There are few data available about crusts. Because of that lack, there is only an estimation of the crusts reserves in the PCZ, where about 7533 million tons of crusts are expected. Nevertheless, crusts reserves have a significant content of metals like cobalt, nickel, titanium, REEs, and platinum. In the case of cobalt, nickel, and titanium, these are metals needed for green and digital technologies, for which demand is expected to grow in the next decades. On the other hand, REEs and platinum are two important metals from a strategic perspective, mainly because China controls nearly 100% of the REE production, and South Africa has a 96% of the world reserves of platinum (U.S. Geological Survey, 2017).

Table 1.5. Comparison between the land, PCZ, and CCZ metal estimatedreserves (Hein and Koschinsky, 2013; M, L., Zientek, J, D., Causey, H., L., Parks, R., J, 2014; U.S. GeologicalSurvey, 2017). PCZ mineral reserves stand out because of the cobalt, titanium, and nickel reserves. It is also important to note that the reserves of REE andplatinum are approximately 10% of the actual land reserves that are controlled mainly by China and South Africa, respectively.

Element Land (Mt) PCZ (Mt) CCZ (Mt) Iron 82,000 1273 1300 Manganese 690 1717 5992 Nickel 78 31.6 274.3 Copper 720 7.5 232.1 Cobalt 7 50.5 44.3 Titanium 830 87.4 59.1 REE+Y 120 18.1 17.2 Platinum 0.042 0.004 NA

Although crusts could supply a significant amount of key metals during the next decades, they have some important disadvantages as compared to manganese nodules. From a technical perspective, the extraction of the crusts is considerably more complex and requires more sophisticated drilling tools (Chung, 2005). Additionally, the slope of the seamounts and the surrounding bathymetry make operating there with large machinery more difficult. Finally, the main disadvantage of cobalt-rich crust mining is the considerable environmental impact due to the removal of the crusts that constitute the actual seabed in that particular area (Levin et al., 2016).

1.1.3. Massive sulfides

Massive sulfides are the consequence of the hydrothermal activity on the seabed. The process that generates this type of deposit starts with the filtration of cold ocean water through the seabed. Due to the volcanic activity in the area, the temperature of the water increases with depth reaching values up to 4000 C. The hot water reacts with the surrounding rock and some metals and sulfur are dissolved. Because of the high temperature, the density of the fluid decreases and it rises up to the seabed until it is discharged through a precipitation chimney generating a black or white smoker. Then, the hot fluid mixes with the cold ocean water and most of the dissolved and swept metals precipitate in the form of sulfides or sulfates nearby (Krasnov, Poroshina and Cherkashev, 1995). Because of that, this type of deposit is known as massive sulfide.

23 The nature of this active process results in a much higher growth rate than in the case of manganese nodules and cobalt-rich crusts. According to Hannington, De Ronde and Petersen (2005), the growth rate may reach values up to 1000 tons per year. However, the characteristics of the deposits in terms of size and composition is highly variable and depends on many parameters. Additionally, due to the 3D nature of these deposits, it is required to drill them in multiple points to estimate the resources. The size of a massive sulfide deposit has a wide range from few tons up to more than 20 million ton (Petersen et al., 2016).

The location of the massive sulfides is directly related to those areas with high underwater volcanic activity (Figure 1.4). Because of that, 65% of the known resources are located in mid-ocean ridges, 22% in back-arc basins and 12% around submarine volcanic arcs (Hannington et al., 2004). Figure 1.4 shows that a significant part of the deposits, especially in the western pacific) are located in the EEZs of different countries. Most deposits are found at depths between 1000 to 3000 meters.

W j 9,

Figure 1.4. Massive sulfide deposits where geochemical analysis have been carriedout (Petersen et al., 2016). Small triangles indicate deposits of less than ] million ton. Red triangles are deposits considered of economic interest because of the content of copper (more than a 5%), zinc (more than a 15%) or gold (>5 ppm). Blue areas indicate the extension of the EEZs.

The composition of the massive sulfides deposits presents a high variability depending on the location and characteristics of each thermal vent. However, the elements that have drawn the attention from a commercial perspective are mainly copper, zinc, gold and silver (Table 1.6).

24 Table 1.6. Comparison between the most common land deposits grades (US Geological Survey, 2013; Emsbo et al., 2016; U.S. Geological Survey, 2017) and different massive su6fide deposits (Petersen et al., 2016).

Type of deposit Cu (wt%) Zn (wt%) Pb (wt%) Fe (wt%) Au (ppm) Ag (ppm) Land ores 0.2-2 5-15 5-15 30-63 1 - 10 10- 800 Sediment-free MOR 4.5 8.3 0.2 27.0 1.3 94 Ultramafic-hosted MOR 13.4 7.2 0.1 24.8 6.9 69 Sediment-hosted MOR 0.8 2.7 0.4 18.6 0.4 64 Intraoceanic back-arc 2.7 17.0 0.7 15.5 4.9 202 Transitional back-arc 6.8 17.5 1.5 8.8 13.2 326 Intracontinental rifted arc 2.8 14.6 9.7 5.5 4.1 1260 Volcanic arcs 4.5 9.5 2.0 9.2 10.2 197

The first massive sulfide deposit that will be mined is located in the EEZ of Papua New Guinea. There, the company Nautilus Minerals expects to start mining Solwara 1 deposit in 2018. This first deposit contains 2.6 million ton of massive sulfides at 1500 meters depth (Table 1.7).

Table 1.7. Inferred reserves of copper, gold, silver and zinc at Solwara I and Solwara I North deposits in PapuaNew Guinea EEZ (Lipton, 2012).

Element Reserves (t) Copper 228,800 Gold 17.9 Silver 93.6 Zinc 23,400

Massive sulfide extraction has similar characteristics than cobalt-rich crusts, with the main difference that the former requires to drill the seabed dozens of meters. This aggressive extraction has a significant impact on the local environment. However, the main advantage of massive sulfides is that the deposits are very local, so the area affected by the extraction is much smaller than in the case of crusts and nodules.

1.2. Polymetallic nodule mining stakeholders

It is important to identify the main stakeholders of polymetallic nodule mining to fully understand the complexity and uncertainties of this potential source of metals. They could be classified into three main groups: regulatory bodies, prospective mining companies, research institutions. Additionally, there are other secondary stakeholders like NGOs, the metallurgical industry, insurance companies, investors, the government of the countries where the processing plants will be located, etc.

There are two main types of regulatory bodies depending on the location of the polymetallic nodules. If the resources are located in international waters, beyond the EEZ of any country, the International Seabed Authority (ISA) is the institution responsible for the regulation and licensing of the exploration and extraction activities. However, some resources are located in the EEZs, like the polymetallic nodules near the Cook Islands (Kingan, 1998) or French Polynesia (Le Meur et al.,

25 2016). In this case, the country itself will establish the regulations and issue the exploration and extraction licenses.

It was shown in section 0 that most of the polymetallic nodule deposits of economic interest are located in international waters. In order to manage and control these resources, the United Nations (UN) established the ISA under the United Nation Convention on the Law of the Sea (UN, 1982) and the 1994 Agreement Relating to the Implementation of Part XI of the UNCLOS (UN, 1994; Lodge, 2006). The ISA has to control and organize the seabed mining activities that take place beyond the EEZs. Because of that, the ISA is responsible for the development of deep-sea mining regulations, and the issuance of exploration and extraction licenses (Clark, Cook Clark and Pintz, 2013). According to the UNCLOS (1982), seabed resources located beyond the EEZs are declared as common heritage of the humankind (Bourrel, Thiele and Currie, 2016). Therefore, the ISA has to guarantee the equity between developed and developing countries, achieving a sustainable development of the resources (Jaeckel, Gjerde and Ardron, 2017). In that sense, part of the benefits obtained from deep-sea mining in international waters have to be shared with developing countries by imposing an international royalty to mining companies (Antrim, 2005; Lodge, 2006).

In order to obtain an exploration or extraction license, mining companies have to be sponsored by a country that has signed the UNCLOS. Then, sponsoring countries, that have delegates at the ISA, are important stakeholders from the perspective of both the regulation and the mineral extraction. For instance, the United States (US) has not signed the UNCLOS. Then, US companies cannot be sponsored by the US to get an exploration or extraction license through the ISA. Because of that, Lockheed Martin, a US company, has created a subsidiary company called UK Seabed Resources that is owned by its branch in the United Kingdom (UK), Lockheed Martin UK (Antrim, 2015). Then, Lockheed Martin has been able to obtain, indirectly, an exploration license sponsored by UK. Table 1.8 presents the current polymetallic nodule exploration licenses issued by the ISA, and Figure 1.5 shows the location of the licensed areas in the CCZ.

Table 1.8. Currentexploration licenses issued by the ISA (ISA, 2017). A total of 16 exploration licensesfor the CCZ have been issued, and 6 of them have already expired. Only one exploration license was issued in 2001 to the Indian Governmentfor the exploration of the Indian Ocean.

Company/Institution Sponsoring Country Location Period China Minerals Corporation China CCZ 2017 - 2032 Cook Islands Investment Corporation Cook Islands CCZ 2016 - 2031 UK Seabed Resources United Kingdom CCZ (II) 2016 - 2031 Ocean Mineral Singapore Singapore CCZ 2015 - 2030 UK Seabed Resources United Kingdom CCZ (I) 2013 - 2028 Global Sea Mineral Resources Belgium CCZ 2013 - 2028 Marawa Research and Exploration Kiribati CCZ 2015 - 2030 Tonga Offshore Mining Limited Tonga CCZ 2012 - 2027 Nauru Ocean Resources Nauru CCZ 2011-2026 Federal Institute for Geosciences and Natural Resources of Germany Germany CCZ 2006 - 2021

26 Cl:rn-Cppwton Zone Explortlon Areas for Polymep Ic Nodules s E A wof I nators thatu APe od in$ eercanmtal r E a e

m~f IWuqab hedui m w~b duMWWhi* Plum) Ghdmuabm..P)G.E Becus imU stM ar nbft ent iOned" n .

- lugw(OMM."O 64am Cf. C" Usp. Pd. momfed.W")M 01ft ssM bew.mMMfi IW.u)______

Figure 1. 5. CCZ licensed exploration areasfor polymetallic nodules (ISA, 2017). The exploration licenses of some of the contractors that are included in the map, like Interoceanmetalor IFREMER, have already expired. Because of that they are not mentioned on Table 1. 8.

Some of the companies included in Table 1.8 are potential mining companies that are gathering the oceanographic and geologic data they need to evaluate the technical and economic feasibility of the mineral extraction. The lack of data and the uncertainties are the main challenges for these companies. Probably, the most important source of uncertainty is the lack of mineral extraction regulations, which will contain essential elements like the applicable royalty system, and some technical and environmental limitations (Antrim, 2015). All these aspects generate serious uncertainties as they have a direct impact on the economic and business models of the prospective mining companies.

Research institutions have been playing a main role since the 1970s. The lack of data and understanding of the deep-ocean environment makes difficult to forecast the potential impacts of seabed mining activities. After 40 years of research, there are still a lot of questions that have not been answered. Therefore, research institutions should continue working to provide a better understanding of the environment and the likely impacts generated by the mining activities. This understanding is essential to back the development of the regulations and the mining technology. Accordingly, this thesis aims for answering one of these questions regarding the environmental impact of the dewatering plumes discharged from mining vessels.

27 1.3. Seabed mining process

Deep-sea mining does not only consist on the extraction of seabed minerals. It is a complex process which starts with the exploration of the area that is expected to be mined, and ends with the sale of the processed metals to the markets. The main components of this process are the exploration, the resource assessment - including the evaluation and mine planning - the technology development, the extraction of the nodules, which involves the collection and vertical transport to the surface operation vessel, the horizontal transport to the shore, and finally, the metallurgical processing to separate the metals of interest (Chung, 2004; ECORYS, 2014a; Boomsma and Warnaars, 2015).

The first three components of the process - exploration, resource assessment, mine planning, and technology development - take place before the exploitation of the resources. In fact, the ISA has considered that a period of 15 years is adequate for the exploration stage. Because of that, the length of an exploration license is 15 years (ISA, 2013). After that period, it is expected that the contractors will be able to apply for a 20 or 25-years exploitation license. Therefore, they should have developed the technical resources to extract the nodules from the seafloor.

On the other hand, the exploitation of the resources can be divided into three main sub-processes: the extraction of the polymetallic nodules from the seabed, the transport to the shore, and the metallurgical processing of the nodules to separate the metals of interest. The extraction of the nodules consists of their collection from the seafloor, the vertical transport to the surface, and the storing - and likely crushing - onboard the mining vessel. Then, the nodules have to be transferred from the vessel to a ship, which will transport them to the shore, where the processing plant is located (Chung, 2005).

1.3.1. Exploration

Although deep-sea mining research began 40 years ago, the main polymetallic nodule stakeholders are still focused on the exploration of the seabed, the collection of oceanographic data, and the development of the technology to collect, lift up to the surface, transport to the shore and process the nodules. The apparent slowness of the research and exploration stage is directly related to the large extent and depth of the areas of interest. For instance, the average depth of the CCZ is 5096 meters and the overall surface is larger than Europe (ISA, 2010). Additionally, the complexity of this stage is significantly increased by the lack of previous oceanographic data. In general terms, the exploration is focused on four main areas: seabed mapping, characterization of the nodules, biodiversity and physical oceanography.

Seabed mapping is very important to identify flat areas that are ideal for nodule collection. Despite the CCZ is considered an abyssal plain, there are seamounts, valleys and ridges, that have to be avoided (ISA, 2010). The most common equipment to obtain the high-resolution bathymetry and mapping are multibeam and side-scan sonars. However, if these systems are mounted on the ship, it is not possible to achieve the required resolution because of the great depth. The use of deep-towed instruments solves this problem, decreasing the distance between the sonar and the seabed (Kuhn,

28 RUhlemann and Wiedicke-hombach, 2011; Rtihlemann et al., 2011). The exploration license, allows the contractors to explore specific areas of the ocean with a total surface up to 150,000 km2. Therefore, it may take between 30 and 50 days to map the whole exploration area (Madureira et al., 2016).

There are two main characteristics of the nodules that are essential for the economic feasibility of their commercial exploitation. The first one is the abundance. According to the UNOET (1987) criterion, the minimum nodule abundance is 5 kg/M 2. However, some authors (Ssreide, Lund and Markussen, 2001; Glasby, 2006) and the ISA prefer to establish 10 kg/m2 as a more adequate value to ensure the economic feasibility of the project. The only direct method to determine the nodule abundance consists on taking samples from the seabed using free-fall devices, grab samplers or box corers. The main disadvantage of these methodologies is that they provide a very local information, and it is well known that the nodule abundance changes irregularly within tens of meters. Because of that, different methodologies are being developed to estimate the nodule abundance using images (Kuhn, Ruhlemann and Wiedicke-hombach, 2011) or backscattering techniques (Ruhlemann et al., 2011).

The second essential characteristic is the composition of the nodules. In this case, the UNOET (1987) establishes that the nodules have to contain at least a 1.8% of nickel and copper. Glasby (2006) suggests a minimum of a 2.5% of nickel, copper, and cobalt. The large amount of manganese is another aspect to consider. However, due to its lower value and demand, it has to be analyzed carefully because of the effect that it may have in the market. In this case, the only way to determine the composition of the nodules is by taking samples from the seabed (Hein and Koschinsky, 2013).

After 4 decades of deep-sea mining research, the lack of knowledge about the fauna that will be potentially affected by commercial deep-sea mining is a great concern for marine scientists (Amon et al., 2016; Vanreusel et al., 2016; Kersten, Smith and Vetter, 2017; Leitner et al., 2017). Because of that, special attention has been focused on the study of the biodiversity and biogeography during the last few years. In most cases, data about megafauna, macrofauna, meiofauna, and microorganisms is obtained from core samples (Sharma et al., 1997; Amon et al., 2016; Glover et al., 2016; Goineau, A. and Gooday, 2017). Also, the fauna attached to the nodules themselves is usually analyzed (Veillette et al., 2007). Some studies made also use of baited traps and/or video cameras (Amon et al., 2016; Glover et al., 2016; Leitner et al., 2017). Plankton pump samples have been also collected to characterize the meroplankton -plankton organisms, which live in the benthic zone - in the CCZ (Kersten, Smith and Vetter, 2017). Because of the lack of data about deep-water fauna, most of the mentioned studies report the identification of new species.

Finally, physical oceanography parameters such as temperature, salinity and density profiles, or currents have also a great importance for many aspects of deep-sea mining. Currents play a main role in the nodule formation process, and also to determine the area impacted by the spreading of the sediments in suspension due to mining activities. Ocean stratification has also a significant impact on the spreading of dewatering plumes, which are discharged from the surface mining vessel. These parameters usually present daily and seasonal variations. Because of that, moorings for long-term data acquisition are commonly used (Klein, 1993; Yamada and Yamazaki, 1998).

29 Another important objective of the exploration stage is the development and testing of monitoring systems (Madureira et al., 2016). The assessment of exploration activities impact is required by the ISA, and it is expected that the environmental monitoring will be mandatory during the nodules exploitation.

1.3.2. Resource assessment, evaluation and mine planning

After the analysis of the collected information during the exploration phase, it is expected that the contractors will focus their attention in smaller regions of their licensed area. According to the exploration regulations approved by the ISA (ISA, 2013), the contractors have an initial licensed area for exploration of up to 150,000 km2. But, after the first three years of the exploration period, the contractor has to reduce the area by a 20%, an additional 10% by the end of the fifth year, and an additional 20% after 8 years. Overall, the licensed area has to be reduced a 50%. According to most authors (Nyhart and Triantafyllou, 1983; UNOET, 1987; Soreide, Lund and Markussen, 2001; ISA, 2008; Sharma, 2010), a production of 3 million tons of dry nodules per year is the reference value for the economic feasibility of the exploitation. Considering that the expected duration of the exploitation license would be 25 years, and assuming an average nodule abundance of 7.5 kg/m2, a total surface of, approximately, 10,000 km2 will be mined. In other words, around a 7% of the initially licensed area would be mined. Because of that, the resource assessment and the mine planning is essential to identify the best minable areas in terms of nodule abundance, and bathymetry. For instance, according to (Knodt et al., 2016) only a 18% of the German exploration licensed area meets the minimum requirements to be commercially mineable.

The data gathered during the exploration phase is the starting point to identify the most suitable areas. Numerical models are developed to estimate the local abundance, metal grade, and the associated uncertainty (ECORYS, 2014b). Similar techniques than for the exploration stage - including samples, video images and backscatter - will be extensively used in the identified areas. These detailed data, and the high-resolution mapping of the seabed, will define the final mine planning. The high- resolution mapping is important to avoid seabed areas with greater slopes or presence of obstacles.

Medium, short, and long-term mine planning has to be defined in terms of production, logistics, capital and operational expenditures, environmental impact monitoring, mitigation actions, and maintenance, among others (ECORYS, 2014b).

1.3.3. Technology development

The proposal, development, and test of technologies for the extraction of polymetallic nodules started in the late 70s (Knodt et al., 2016). However, the development slowed down during the 80s due to the significant decrease of commodity prices (Van Dover, 2011). Because of the inherent complexity of seabed mining, most of the proposed solutions include technologies that have been already proven in other ocean industries in order to reduce the technical risks. The similarities between deep-sea mining and the offshore oil and gas industry resulted in the transfer of many concepts and

30 technologies. Oil and gas industry has extensively tested many technologies and equipment that are easily adapted for ocean mining; such as the vertical hydraulic lift system, rigid and flexible risers, riser handling system, material transfers at sea, or dynamic positioning systems (Williams, McBride and Kinnaman, 1977). Another source of tested technologies is the dredging industry, in which rocks and sediments are lifted from the seabed using pumps designed for such purpose (Knodt et al., 2016).

Certainly, deep-sea mining has a number of singularities that require a significant research and development effort. Therefore, the transferred technologies from other fields are not directly applicable. They have to be adapted to work with manganese nodules, at very deep waters, and from a moving vessel. Other systems, like the nodule collector, require the development of a completely novel design.

Since the 70s a number of systems and sub-systems have been developed, and even tested. Most of the initial concepts were based on three main elements: a nodule collection system, a lifting system, and a surface platform or vessel (Kaufman, 1971).

The very first seabed mining tests were carried out in 1970 by Deep Sea Ventures on the Blake Plateau in the Atlantic Ocean, off the coast of Florida, at a depth of 1,000 m (Flipse, 1969; Kaufman, 1971). The system was composed by a nodule collection vehicle, a riser to lift the nodules, and a surface vessel (Figure 1.6a). Two years later, a totally different concept was tested: the continuous line bucket system. (Masuda and Cruickshank, 1997). This solution consists of the use of buckets to collect and transport the nodules to the surface (Figure 1.6b). An endless line with multiple dredge buckets is operated from one or two surface vessels. At least, other three scaled mining tests - based on hydraulic and/or air lift systems for the vertical transport of the nodules - were carried out in the 80s:

- Between 1977 to 1978, the R/VDeepsea Miner II was equipped and tested for a scaled version of the Ocean Mining Associates (OMA) nodule mining system (Kaufman et al., 1985).

- In 1978, the Ocean Management Incorporated (OMI) consortium tested a mining system onboard the converted drillship SEDCO 445 (Bath, 1989). Two types of vertical transport systems were successfully tested: one using centrifugal pumps, and the other using an air-lift system.

- Also, in 1978, The Advanced Deep Sea Mining System was developed and tested by the Ocean Management Inc. (OMCO) in the Pacific Ocean, at 5,000 meters depth, onboard the Glomar Explorer (Welling, 1981).

31 a b Shop I ~U CAR~ ~ I

Retuming line Ascending line 3hip 2 Touchdown point (TDP)

Sea floor

.4 Dredging line De~ scending line

Figure 1.6 (a) Hydraulic seabed mining system tested by Deep Sea Ventures in 1970 (Flipse, 1969). (b) Two- vessel Continuous Line Bucket System (CLBS) proposed and tested in the Pacific ocean in 1972 (Nishi, 2012).

Even though some systems have been already tested, there are still significant challenges that have to be addressed building a commercial-scale system (Knodt et al., 2016).

A similar scenario can be described for the inland metallurgical processing plants. Different pyrometallurgical, hydrometallurgical and hybrid solutions have been proposed since the 70s. Furthermore, some pilot plants have been built and tested to determine the feasibility, and the recovery rates for the different metals (Abramovski, Vladislava P Stefanova, et al., 2017).

1.3.4. Nodule collection

Most of the nodule collection systems that have been proposed since the very early technical developments of deep-sea mining can be divided into two main groups. The first group, which is the most popular and likely to become a commercial solution, is based on the use of an underwater collection vehicle to pick up the nodules from the seabed (Figure 1.6a). The second one is the CLBS (Figure 1.6b), which makes use of an endless line with buckets to both pick and lift up the nodules from the seafloor.

A CLBS was tested from a whaling ship in 1970 (Masuda, Cruickshank and Mero, 1971). During some of the tests, the line was entangled, and other mechanical problems arouse. A new test was planned for 1975, using two ships instead of one. But the lack of funds did not enable the trials of this new configuration.

In the case of the nodule collection vehicles, they can be divided into two main groups: active and passive vehicles. Active collectors have their own propulsion, which in most cases consists of the use of caterpillar tracks, or Archimedes screws. Meanwhile, passive collectors are towed by a surface vessel. According to the ISA (2001) report on technologies for polymetallic nodule mining, active collectors, and caterpillar tracks in particular, showed a better overall behavior. Active vehicles have a lower risk of clogging, and present a much better maneuverability and collection efficiency. On the other hand, passive vehicles have the advantage of a higher reliability and survivability.

32 Both, passive and active collectors have been successfully tested. Passive collectors were used by OMA (Kaufman et al., 1985) and OMI (Bath, 1989) in 1978, or by Japan's deep-sea mining system in 1997 (Yamada and Yamazaki, 1998). On the other hand, active collectors were tested also in 1978 by OMCO (Welling, 1981), using Archimedes screws for the propulsion of the vehicle. More recently, in 2010 and 2012, the Korean tracked vehicles MineRo and MineRo II were tested in shallow and deep waters (Hong et al., 2010; Min et al., 2013; Knodt et al., 2016). Also Indian researchers have been testing tracked vehicles for seabed mining purposes (Janarthanan et al., 2015).

Another key aspect for the nodule collection is the technology used to collect the nodules. Several solutions have been proposed and tested in the last 50 years using hydraulic, mechanical, hybrid, and magnetic principles to pick up the nodules form the seafloor.

Hydraulic systems, which make use of Coanda effect achieve a collecting efficiency of up to 87% (Yamada and Yamazaki, 1998; Handschuh et al., 2001). Because of that, this technology is considered as the most likely for a commercial exploitation. However, a significant amount of seabed sediment is disturbed, and it is lifted up to the surface with the nodules. This is an undesirable aspect because the sediment has to be discharged again to the ocean.

A purely mechanical collector was built by OMI for their seabed mining tests. However, they lost the vehicle during the recovery. According to Yamazaki and Kajitani (1999), mechanical and hybrid collection systems have the advantage of the sediment separation from the nodules during the pick- up process. On the other hand, mechanical collectors are constituted by a number of moving parts, reducing its reliability and long term endurance (ISA, 2001).

Other solutions, such as hybrid collectors that combine both hydraulic and mechanical systems, or even magnetic systems have been also proposed (Handschuh et al., 2001). Hybrid collectors present some of the advantages and disadvantages of both methods: they have more moving parts than hydraulic systems, but they also disturb a significant amount of sediments because of the use of water jets. Magnetic systems require a significantly higher power consumption, which is a great disadvantage in comparison to other methods.

Most proposed technologies consider the crushing of the nodules in the collection vehicle to facilitate the vertical transport process (Min et al., 2013).

1.3.5. Vertical transport

The nodules have to be transported from the seabed to the surface, where the mining vessel or platform is located. In the case of the CLBS, the nodules are lifted up to the surface inside the buckets that pick them up from the seafloor. But, if a collection vehicle is used the nodules will be lifted up using a riser: a vertical pipe that connects the collection vehicle with the surface vessel. This system is based on oil and gas industry technologies (Williams, McBride and Kinnaman. 1977). At least,

33 three different technologies have been considered for the hydraulic lift of the nodules: air-lift system, centrifugal pumps, and dense slurry pumps.

The air-lift system consists of the injection of pressurized air into the riser. The volume of the air bubbles increases as they rise due to the decrease of the hydrostatic pressure. Then, the mixture of air, water, and nodules is accelerated, and the nodules are lifted up to the surface. This technology has been tested during OMI and OMA pilot tests (Kaufman et al., 1985; Bath, 1989). The main advantage in comparison to the pumping solution is the fact that all the air compression station is located on the surface vessel. Therefore, any maintenance operation is easier, faster, and cheaper than in the case of a submerged pump. However, it requires a riser with a larger diameter (Bernard et al., 1987), and the clogging and clustering are more frequent in comparison to the centrifugal pumping system, as it was shown during the OMI tests (Welling, 1981). Furthermore, the technical risk of this approach is considerably higher, since it has not been used in any other industry before.

The use of pumps for the vertical transport of the nodules is based on the extensively proven dredging technologies. This solution is more efficient and reliable, and less problems related to clogging and clustering were observed during OMI trials (Welling, 1981). The main disadvantage is that any maintenance operation is much more complex, because the pumps are submerged within the riser. The main difference between centrifugal and dense slurry pumps, is that the latter requires additional nodule crushing, but the riser diameter can be smaller (Bernard et al., 1987).

An important element of the hydraulic systems is the buffer module, a small storage tank located at the bottom-end of the riser. It is essential to control the likely differences between collection and lifting rates, and also to avoid the clogging of the riser (Hong et al., 2010; Knodt et al., 2016).

1.3.6. Mining vessel

The mining vessel is a key component of the mining system. The mining vessel is the platform from where the riser and the collection vehicle, or the CLBS, are launched. It also includes the power generation plant for all the mining systems, and the propulsion of the vessel itself (Handschuh et al., 2001).

Other systems that are located on the vessel are the nodule separation facilities, bunkering holds, the waste water discharge system (Oebius et al., 2001), or the nodules transfer system to discharge them on the ore carriers to transport them to the shore. Also, the vessel accommodates the crew, technicians and staff to conduct the mining operations.

Some conceptual designs have been presented but, none of them has been built. All of the already mentioned nodule mining pilot tests were carried out onboard of adapted drilling ships, research or even whaling vessels.

34 1.3.7. Horizontal transport

Once the nodules are on the surface vessel or platform, they have to be transferred to a bulk carrier, and then transported to the shore for the metallurgical processing (Flipse, 1969; Santo et al., 2013). The surface vessel will have a very limited storage capacity, as it happens in most oil and gas platforms. Therefore, at least one bulk carrier has to be "in attendance" during the mining operations to receive the nodules that are being lifted up to the surface (Kaufman, 1971).

One more time, most approaches to transfer the nodules from the mining vessel to the bulk carrier are based on a concept widely used at the oil and gas industry: the use of a hose attached to a buoy, where the bulk carrier is moored. Then, a mix of crushed nodules and water is pumped from the mining vessel to the holds of the bulk carrier (Flipse, 1969; Glasby, 2006). The bulk carrier will count with a system to dewater the nodules in an effective way.

Depending on the distance from the mining area to the unloading port, located near to the processing plant, 3 or 4 bulk carriers are required, at least, to avoid interruptions of the nodule recovery operations (Soreide, Lund and Markussen, 2001). Likely locations for the processing plant are India, in the case of the Central Indian Ocean Basin nodules, Mexico or British Columbia for the CCZ, and New Zealand for the nodules in the Cooks Island area.

1.3.8. Metallurgical processing

Lastly, the metallurgical processing of the nodules is one of the key aspects to ensure the economic feasibility of the entire exploitation. According to most published economic models, the land-based processing plant represents two thirds of the total investment and the operating expenses of a complete polymetallic nodule exploitation (Sharma, 2011). Therefore, the selection of an adequate processing technology is essential for the success of the exploitation.

Most authors talk about the extraction of three - nickel, cobalt and copper - or four metals - the three already mentioned plus the manganese - from the nodules (Vranka and Kotlinski, 2005; ISA, 2008; Sharma, 2011; Haiki, Okazaki and Ishiyama, 2015; Abramovski, Vladislava P. Stefanova, et al., 2017). Other elements, such as titanium or REE, are not economically feasible to extract at this point.

Processing technologies can be divided into two main types: hydrometallurgical, or hybrid pyro- hydrometallurgical processes (Abramovski, Vladislava P. Stefanova, et al., 2017). Pyrometallurgical processes require the smelting of the nodules, hence, this process is energy intensive, and the operating costs of hybrid methods are higher. On the other hand, hydrometallurgical processes consist of the leaching of the nodules using acids to separate the metals of interest.

Haiki, Okazaki and Ishiyama (2015) reviewed the existing methods that had been proposed for seabed mining:

35 - Roasting under reducing process and ammoniacal leaching method. - Cuprion ammoniacal leaching method. - Chloridizing roasting and water leaching method. - High temperature and high pressure acid leaching method. - Smelting and sulfuric acid leaching method. - Carbon reduction and ammoniacal leaching method. - Sulfurous acid ammoniacal leaching methos. - Smelting and chlorine leaching method.

Among them, the smelting and chlorine leaching method was ranked by the authors as the best one, based on different criteria such as the recovery rate, the cost, or the toxicity of the reagents. Conversely, the chloridizing roasting method was discarded because of the use of hydrogen chloride, which is very corrosive for the equipment. In other words, the maintenance cost is significantly higher in comparison to other methods. Also, the high temperature and pressure method requires a higher investment, as it makes use of an autoclave.

The ISA (2008) recommends the use of the cuprion process developed by the Kennecott Consortium. There are four main advantages identified by the ISA report. The first one is the fact that no heating nor pressurizing is required. The second advantage is directly related to the first one: the energy consumption is low in comparison to other methods. Also, the reactive agents that are consumed during the process are cheap or recyclable. And the last one is that few corrosive and toxic reagents are used.

Despite the fact that most of the proposed technologies have been already tested at an experimental scale (ISA, 2008), some authors claim that larger scale tests should be done before starting a commercial exploitation (Abramovski, Vladislava P. Stefanova, et al., 2017).

1.4. Environmental impact

Since the very beginning of seabed mining research, environmental impact has been one of the main worries of several authors and research institutions. Even though, there are still a lot of uncertainties regarding the likely impacts mining operations will generate, mainly because of the lack of knowledge about the deep ocean environment.

Oebius et al. (2001) identified some of the most important impacts that the main components of a mining system may generate. And others like Thiel (2001), directly list the impacts and evaluate their relevance. But, most environmental impact studies for seabed mining activities are focused on the following aspects:

- Waste water discharge - Nodule extraction - Nodule collector sediment plume - Sediment structure modification

36 Hydraulic vertical transport systems designed to lift the nodules up to the surface, will not only transport nodules, but also sediments, water and some organisms present in the seabed environment. At the surface mining vessel, the nodules will be separate and transport to the land-based processing plant. However, the sediments, water, and present organisms are planned to be returned to the ocean (Morgan, Odunton and Jones, 1999; Oebius et al., 2001). This waste water discharge has a significant amount of potential impacts to the environment, whose severity will be highly dependent on the discharge depth, and the concentration of sediments and organisms. Chung et al. (2002) listed the potential water-column impacts generated by the plumes. Most of them are directly related to the different species present in the water column, from plankton to marine mammals. From a spatial perspective, some numerical simulations have shown that the standard deviation of the particles' position average is located between 741 to 887 km from the discharge point, under different scenarios (Rolinski, Segschneider and Sundermann, 2001).

Probably, the benthic effects of the nodule collection are the most significant impacts of seabed mining (Morgan, Odunton and Jones, 1999; Thiel, 2001), even though, the extent of the impact is still unknown. The first direct impact is the extraction of the nodules, which constitute the hard substrate where many species live (Morgan, Odunton and Jones, 1999).

The nodule collection vehicle, is the other main responsible of the benthic impacts. Firstly, the vehicle induces a bottom sediment plume, particularly important in the case of the hydraulic extraction, where a water jet is responsible of the nodule picking up process. The main impacts of this type of plume are the resuspension of toxic heavy-metals, and the burial of the benthic fauna because of the plume sediment settling (Chung et al., 2002).

Another relevant impact generated by the tracks of the collection vehicle is the modification of the sediment structure. The semi-liquid layer, where a significant amount of fauna live, will be dispersed (Thiel, 2001). Also, the first layers of the soft sediment will be compacted (Madureira et al., 2016), which has a direct impact on the benthic fauna.

An environmental issue that is not frequently mentioned is the management of processing plant tailings. Even in the best scenario, in which the manganese is extracted from the nodules, more than 2 million tons of tailings will be generated annually (Sharma, 2011).

Some experiments have been conducted since the 80s to better understand and evaluate the potential impacts of seabed mining on the environment. Most of them are focused on the benthic impacts due to the action of the nodule collector (Sharma, 2011). Examples of that are the "Disturbance and Recolonization" (DISCOL) experiment (Thiel et al., 2001), the "Japan Deep-Sea Impact Experiment" (JET) (Fukushima, 1995), or the "Indian Deepsea Environment Experiment" (INDEX) (Sharma et al., 1997, 2000). Most of these experiments consisted on the use of a benthic disturber to simulate the effects of a nodule collector. The problem of these experiments is that they did not consider that the most important component of the bottom plume induced by the collector is due to the water jets used to pick up the nodules. Additionally, it is more likely that this sediment plume will be in the form of a horizontal jet, than a vertical one (Handschuh et al., 2001; Hong et al., 2010).

37 From the perspective of the benthic environmental impacts, it is important to note that a very small percentage of the seabed will actually be mined. For instance, considering that the contractors have a licensed area of up to 150,000 km2, an annual production of 3 million tons of dry nodules during 20 years, and a very conservative average nodule abundance of 7.5 kg/m2 , only a 5.3% of the licensed area will be directly impacted. This constitutes about a 0.15% of the total surface of the CCZ.

However, only one experiment has been focused on the monitoring of waste water discharge plumes (Yamazaki and Kajitani, 1999; Yamazaki, 2011). It was the "Deep Ocean Mining Environmental Study" (DOMES), which took place during the OMI experiment. This study consisted on the monitoring of the surface and benthic plumes, including an analysis of the biological effects (Burns et al., 1980). Unfortunately, during the experiment the waste water was directly discharged from the surface vessel to the ocean surface, which is a considerably different condition than the expected for commercial deep-sea mining. According to several authors, the discharge will happen well below the euphotic zone and the pycnocline (Oebius et al., 2001; Santo et al., 2013; Schriever and Thiel, 2013a).

Consequently, there exists the urgent need to conduct two specific experiments, under more realistic conditions, to support the development of the environmental regulations for polymetallic nodules exploitations. The first one would consist on the creation and monitoring of a bottom plume from a horizontal jet, to study the effect of the water and sediments discharge of a hydraulic nodule collector. The second one is the release and monitoring of a mid-water negatively buoyant plume, discharged below the pycnocline, to analyze the dynamics and propagation of the plume. These two experiments are part of the Plume Experiment (PLUMEX), conducted by the author of this Thesis with a team of MIT, Scripps Institution of Oceanography (SIO), ISA, and Global Sea Mineral Resources (GSR) scientists and engineers. The design, execution, and results analysis of PLUMEX experiment are the main core of this Master's Thesis.

1.5. Regulations

The regulations of land-based mines depend on the country they are located. However, in the case of seabed mining, this only would happen in the mining areas that are located in the EEZ of a country. A good example of that are The Cook Islands, which has significant reserves of polymetallic nodules in their EEZ (Kingan, 1998; Soreide, Lund and Markussen, 2001; Hein et al., 2015). Most of the polymetallic nodules reserves of economic interest are located in international waters. Therefore, their exploitation has to be regulated by United Nations, through the ISA.

For that reason, the ISA approved a set of regulations on the prospecting and exploration activities of areas of interest like the CCZ, or the CIOB (ISA, 2013). These regulations established aspects such as the maximum area that can be licensed to a contractor for exploration purposes, environmental requirements, fees, and other contractual details.

The ISA is developing a draft of the exploitation regulations (Clark, Cook Clark and Pintz, 2013; ISA, 2016), which are expected to be finalized in the next two years (Miller et al., 2018).

38 Nevertheless, it is a complex challenge that requires a deep knowledge about the potential economic and environmental risks. Essential economic aspects such as exploitation fees, or royalties have to be defined taking into account the expected profitability of the business, as well as the risk of the investment. From the environmental perspective, there are still relevant questions, like the minimum depth for the discharge of waste waters, that have to be answered as soon as possible.

39 40 Chapter 2 Polymetallic nodule mining sediment plumes

Two of the identified environmental impacts in the first chapter of this thesis, which are also two of the less studied, are the waste water discharge and bottom plumes created by seabed mining operations. There have been very few modeling and field studies regarding the impacts of the bottom plumes that will be created by nodule collectors (Thiel & Schriever, 1990; Jankowski & Zielke, 2001), and even less has been done on the nature of the dewatering plumes that will be discharged from surface operation vessels (Burns, 1980; Rolinski et al., 2001).

2.1. Waste water discharge plumes

According to most deep-sea mining concepts, a significant amount of sediment will be lifted up to the surface with the nodules and cold water from the seafloor (Morgan, Odunton and Jones, 1999; Handschuh et al., 2001; Oebius et al., 2001; Miller et al., 2018). All these sediments from abyssal depths will have to be returned to the ocean (Figure 2.1), mainly because it would not be feasible to transport them to the shore with the nodules. Therefore, after separating the nodules from the sediments, a mixture of sediments, nodule fines, and water from both the seabed and the ocean surface, has to be pumped and discharged at a certain depth. The depth of discharge of the sediment plume is still a matter of discussion, and one of the key parameters that are expected to be determined in some way by the ISA regulations for polymetallic nodule exploitation (ISA, 2016). Most authors expect the release to happen somewhere along the water column, below the photic zone and the permanent oxygen minimum zone (Oebius et al., 2001; Thiel, 2001; Schriever and Thiel, 2013b). Therefore, the minimum release depth is expected to be between 500 to 1500 meters. Even though, some authors propose the release of the plumes only a few meters above the seafloor to minimize the impact on the water column (Santo et al., 2013).

WMIt waef

Figure 2.1. Sketch of a sample polymetallic nodule exploitation (not scaled). The waste water is discharged at a certain depth, creating a sediment plume that is advected by the background currents.

41 The discharged water and sediment mass flow rates will depend on the technology used to collect the nodules (Handschuh et al., 2001) and, of course, on the annual production of the exploitation. In the case of Nautilus Minerals project in Papua New Guinea for massive sulfides mining, the waste water plume will have a total flow rate of 0.3 m3/s (Nautilus Minerals, 2008). In the case of polymetallic nodule mining, Morgan, Odunton and Jones (1999) estimated a similar flow rate: 0.298 m3/s. But, Oebius et al. (2001) expect a considerably higher discharge rate of up to 0.56 m3/s.

The release of the plumes may have a significant number of impacts, from the decrease of oxygen saturation because of the presence of different bacteria on the suspended sediment, to impacts on the marine mammals, or on fish mortality because of the presence of heavy metals (Chung et al., 2002). Also, the discharge of the plume may have a significant influence on the vertical migrations of nekton (Thiel, 2001). However, the lack of knowledge and data about waste water plumes (Thiel, 2001; Miller et al., 2018) makes very difficult to determine and quantify the relevance of these potential impacts.

As it has been already mentioned in the first chapter of this thesis, some nodule mining pilot experiments were conducted in the Pacific Ocean, and all of them included a waste water discharge system. Unfortunately, the waste water was discharged directly to the ocean surface, and very few data about the consequent plumes is available. Furthermore, the fact that the plume was directly released to the ocean surface, in the mixed layer and exposed to surface currents, has a direct influence on the dynamics and spreading of the plumes. During both the OMI and OMA mining tests, the average flow rated of discharge were between 0.095 m3/s, to 0.160 m3/s (Lavelle et al., 1982), about a fifth of the estimated values for commercial seabed mining. The solid discharge rate for the air lift systems during the tests was 0.55 kg/s for the OMA tests, and 0.68 kg/s for the OMI ones. In the case of the vertical transport using pumps during OMI tests, the solid discharge rate was considerably higher: 2.03 kg/s. Overall, the concentration in volume of solids was between 0.25% to 0.55%. Also, it was noted that the discharge water temperature was 3 to 8 'C higher than the water temperature at the seabed.

Despite the fact that several research projects on seabed mining environmental impact have been conducted since the early 70s, only during OMI mining tests the discharge plume was monitored (Yamazaki and Kajitani, 1999; Yamazaki, 2011). Also Thiel (2001) highlighted the lack of research and discussion about the adequate discharge depth, and the effects of the plumes. Therefore, considering the fact that the ISA has already published a draft of the exploitation regulations (ISA, 2016), and that it is expected that the final version will be ready by 2019, there is an urgent need to conduct field experiments to monitor waste water plumes. These experimental plumes should be released below the pycnocline to adequately mimic the conditions of commercial seabed mining. In order to answer this urgent need, PLUMEX field studies were conducted in the Pacific ocean onboard Research Vessel (RV) Sally Ride. The details of the planning and execution of the experiment, and the preliminary results constitute the third chapter of this thesis.

42 2.2. Bottom plumes

On the other hand, the nodule collection vehicle will generate a significant disturbance of the seabed sediment (Thiel et al., 2001; Madureira et al., 2016). The most significant source of potential environmental impact - apart from the recovery of the nodules - will be the horizontal plume created by the hydraulic collection system (Oebius et al., 2001; Yamazaki, 2011). A jet of several cubic meters per second of water and sediments will be released from the back of the nodule collector (Figure 2.2).

As it was explained in the first chapter, the most promising and likely technology to pick up the nodules is the hydraulic one, mounted on a self-propelled collection vehicle. This technology presents a number of advantages compared to other proposed technologies but the use of water jets to pick up the nodules results in a high disturbance of the seabed. Inside the vehicle, the nodules will be separated from most of the sediment using a classification grid. As a result, a horizontal jet of water and sediment will be discharged from the back of the nodule collection vehicle, 3 to 5 meters above the seafloor (Oebius et al., 2001; Yamazaki, 2011).

Additionally, the vehicle itself will resuspend a certain amount of sediment while driving through the seabed (Figure 2.2). But, the effects of this resuspension are expected to be considerably smaller compared to the horizontal plume created by the hydraulic system.

Figure 2.2. Sketch of a sample polymetallic nodule exploitation (not scaled). The sediment plume created by the horizontal exhaust et, and the sediment resuspension generatedby the nodule collection vehicle tracksform an overall sediment plume that is expected to evolve as a gravity current.

The details and main parameters of the nodule collection vehicles are still unknown. However, Oebius et al. (2001) estimated the resuspension of a mass flow rate of 85 kg/s of sediment, and the creation of a wake field with a flow rate of 54 m3/s, and a sediment mass concentration of 1.6 kg/m3 .

43 Several authors have explored the potential impacts that these plumes will have in the abyssal ecosystem. One of the main concerns is related to the fact that actual sedimentation rates in the abyssal plains is in the order of magnitude of only few millimeters per thousand years (Hein et al., 2015). Therefore, the settling process of the plume created by the collection vehicle may result in the burial and entombment of seabed organisms (Fukushima, 1995; Chung et al., 2002). Additionally, according to Jankowski and Zielke (2001) models and Thiel, Schriever and Foell (2005) analysis, these bottom plumes may reach distances of up to 2 km from the original source location.

During the benthic environmental impact studies conducted in the 80s - such as DISCOL, BIE, INDEX, or JET - relevant data about bottom plumes, blanketing, and effects on the abyssal fauna was collected. However, none of these experiments were based on the technologies that, nowadays, are expected to harvest polymetallic nodules. For instance, the DISCOL experiment (Thiel et al., 2001) consisted on towing a plough in a specific area of the Pacific ocean. Therefore, the only resuspended sediment was due to the plough itself, which is expected to be a minor component of the total disturbance created by hydraulic collectors. On the other hand, the BIE (Jankowski and Zielke, 1997), INDEX (Sharma et al., 1997, 2000), and JET (Fukushima, 1995) experiments, made use of benthic disturbers to create a plume that was vertically discharged, instead of horizontally as it is expected by the latest designs of collection vehicles (Handschuh et al., 2001; Hong et al., 2010).

Consequently, there is also a significant lack of experimental data to back the development of horizontal bottom plume models to assess the area affected by commercial seabed mining activities. This is another key aspect that will have to be considered by the ISA on the environmental regulations for the industry.

2.3. Other sediment plumes in the oceans

The uncertainty of most of the design and operating parameters of seabed mining - like the mass flow rates of the water and sediments that will be discharged from both the collection vehicle and the waste water discharge system - makes it more difficult to quantify the extent of the expected environmental impacts. Apart from that, it is not possible to validate analytical and numerical models of both midwater and bottom plumes because of the lack of seabed mining impact field studies and experimental data.

However, sediment plumes of different nature occur in the oceans all around the world, and some of them have been thoroughly studied. Good examples of that are the plumes discharged from hydrothermal vents, sediment resuspension due to bottom currents or anthropogenic activities, gravity currents, or even underwater volcanic eruptions. The available field data about these types of phenomena constitutes a good starting point to validate the principles and assumptions of analytical and numerical models of seabed mining activities. Then, an adequate analysis of the data can also be helpful to quantify, or at least estimate, the potential impacts of such mining operations. In this regard, the following phenomena are considered of interest:

44 - Hydrothermal vent plumes. The nature of hydrothermal plumes was explained in the first chapter of this thesis. The cold water from the seabed is filtered through the porous seabed. Then, the volcanic activity in the area heats up the water up to 400*C, decreasing its density and dissolving metals and sulfur present on the surrounding rocks. Finally, due to the lower density of the solution, the fluid rises up to the seabed, and it exists in the form of a buoyant plume known as white or black smoker (Baker and Massoth, 1986; Krasnov, Poroshina and Cherkashev, 1995).

- Volcanic plume. The eruption of submerged volcanoes is a violent and massive phenomenon very difficult to directly monitor. Because of that, very few subsea volcanic plumes have been monitored (Walker et al., 2008). Volcanic plumes are constituted by particles of very different sizes, mainly due to the fact that they are usually clustered (Wiesner, Wang and Zheng, 1995).

- Gravity current. A gravity current consists of the flow of a denser fluid stream within another fluid of smaller density (Benjamin, 1968). In this case, the interest is focused on a specific type of gravity current, known as turbidity current, which commonly transports sediments and mud, but also, in the most massive and destructive cases, it drifts big stones and even tree branches (Khripounoff et al., 2003).

- Sediment resuspension. This is usually referred to the resuspension of seabed sediment because of a natural (Rosa, 1985; Jones et al., 1998; Yuan et al., 2008) - such as strong tidal currents, wave orbital flows - or anthropogenic phenomenon (Friedrichs and Battisto, 2001; Durrieu De Madron et al., 2005) like, for example dredging activities, or bottom trawling.

Data from buoyant hydrothermal and volcanic plumes can be useful to validate waste-water discharge plumes models, and estimate the significance of their impacts. In all three cases, they are vertical jets of sea water and particles that are discharged in a stratified environment. Despite the fact that seabed mining plumes are expected to be negatively buoyant, both the dynamics of the plumes, and the advection by the ocean currents, make them of great interest for this purpose.

On the other hand, the nodule collector will generate a sediment resuspension while driving on the seabed, and it will discharge a horizontal exhaust jet of sea water and sediment from the hydraulic nodule-picking system. Therefore, existing field data from other types of sediment resuspension can be helpful to validate models and assess the consequences of the sediment resuspension created by the vehicle. Additionally, it is expected that both the sediment resuspension and the horizontal jet will evolve as a gravity current. Consequently, the available data of this phenomenon can be used to validate far-field models.

There are several parameters of interest to compare these phenomena with polymetallic nodule mining plumes. However, it is important to note that some parameters cannot be measured, or they just have no sense, for some of the selected phenomena. For instance, a parameter of interest like the volume flow rate is completely meaningless in the case of a sediment resuspension, because a

45 resuspension by itself does not implies the displacement of a measurable volume of fluid. Also, there are other parameters that are impossible or very difficult to measure, therefore, very few field data is available. A good example of that are the volcanic plumes, which are very difficult to monitor in-situ because of the violence of such a massive phenomenon. Overall, the following parameters have been considered of interest: vertical and horizontal extent, flow velocity, volume flow rate, settling velocity, and density.

Another piece of information that is considered useful are the different technologies used in the field studies of these phenomena. They constitute a helpful starting point for the design of environmental monitoring strategies.

The aim of this section is to present a review of the data from some field studies of hydrothermal plumes, volcanic plumes, sediment resuspensions, and gravity currents. These data can be easily used to conduct analytical and numerical models validations, and as a basis for the environmental impact assessment of seabed mining activities.

2.3.1. Vertical and horizontal extent

The vertical and horizontal extent of the plume is a key aspect for both bottom and dewatering plumes. These two aspects will determine the total volume of water, and seabed surface directly affected by the plumes. Furthermore, field data about the extent of plumes with comparable characteristics to the seabed mining plumes is a good resource for model validation.

In this regard, hydrothermal vents (Table 2.1) and volcanic plumes (Table 2.2) have some characteristics that make them interesting for the study of seabed mining waste water dewatering plumes. Therefore, some of the available field data results useful for the validation of different models, such as plume near, intermediate and far field dynamics models, and also mesoscale sediment transport models.

46 Table 2.1. Vertical extent field data of hydrothermalplumes. The vertical extent rangesfirom 90 to 700 meters, with a mean value of 340 meters.

Vertical Location Methods Source Extent (in) 90 Fiji Basin CTD and water samples (Nojiri et al., 1989) 150 Juan de Fuca Ridge (Endeavor Segment) CTD and transmissometer (Thomson et al., 1992) (Little, Stolzenbach and 150 East Pacific rise CTD, transmissometer, flowmeter and thermistors (Von Herzen, 1987) CTD, echosounder, Light scatter sensor (LSS) and 150 Wallis and Futuna Region (Konn et al., 2016) water samples 200 Juan de Fuca Ridge (Endeavor Segment) CTD (Lupton et al., 1985) 200 Mid-Atlantic Ridge CTD, nephelometer and water samples (Severmann et al., 2004) 205 Juan De Fuca Ridge (Cleft Segment) CTD and transmissometer (Baker, 1994) 290 Mid-Atlantic Ridge Optical backscatter sensor (OBS) (Marbler et al., 2010) 312 Juan De Fuca Ridge (Cleft Segment) CTD and transmissometer (Baker, 1994) 320 Mid-Atlantic Ridge CTD (Rona and Speer, 1989) 340 Juan De Fuca Ridge Water samples, CTD and LSS (Baker and Massoth, 1986) 350 East Scotia Ridge CTD and LSS (Hawkes et al., 2013) 350 Wallis and Futuna Region CTD, echosounder, LSS and water samples (Konn et al., 2016) 350 Wallis and Futuna Region CTD, echosounder, LSS and water samples (Konn et al., 2016) 400 East Scotia Ridge CTD and LSS (Hawkes et al., 2013) 430 Juan De Fuca Ridge (Cleft Segment) Water samples, CTD and LSS (Massoth et al., 1994) 500 Wallis and Futuna Region Cm, echosounder, LSS and water samples (Konn et al., 2016) 600 N. Gorda Ridge CTD, LSS, colotimetric and water samples (Massoth et al., 1998) (Horibe, Kim and Craig, 700 East Pacific rise Water samples 1986) 700 Juan De Fuca Ridge (CoAxial Segment) CTD, transmissometer and colorimetric detection (Massoth et al., 1995) 700 N. Gorda Ridge CTD, LSS, colotimetric and water samples (Massoth et al., 1998)

Table 2.2. Vertical extent of volcanic plumes. In most cases, volcanic plumes reach the ocean surface, therefore, their vertical extent is limited. Because of the violence and unpredictabilityof these events, there is considerably fewer data than in the case of hydrothermalplumes.

Vertical Extent (in) Location Methods Source 25 Myojinsho Photographs and bathymetric data (Fiske et al., 1998) 50 Kavachi Volcano CD, LSS, and water samples (Baker et al., 2002) 50 El Hierro CD, transmissometer, water samples and acoustic data (Fraile-Nuez et al., 2012) 300 Kavachi Volcano CTD, LSS and water samples (Baker et al., 2002) 700 NW Rota-I volcano CTD, turbidity and acoustic data (Embley et al., 2006)

The horizontal extent of hydrothermal plumes can be used as a reference of how far a sediment plume can be advected by the background current. This data is useful to validate sediment transport models, and also to assess the extent of the area affected by the plume.

47 Table 2.3 Horizontal extent of hydrothermalplumes. The horizontal extent rangesfrom 2 to 70 km. According to these figures, it is reasonableto expect deep-sea mining dewateringplumes with an extent within this order of magnitude.

Horizontal Extent (km) Location Methods Source 2.0 Northeast Lau Basin CTD, OBS and oxidation-reduction sensors (Baker et al., 2011) 2.5 Mid Atlantic Ridge CTD, transmissometer, MAPR and OBS (Marbler et al., 2010) 5.0 Endeavour Ridge CTD and transmissometer (Thomson et al., 1992) 5.0 Juan De Fuca Ridge CTD, transmissometer and chemical analyzer (Massoth et al., 1995) 5.0 Juan De Fuca Ridge CTD, transmissometer and chemical analyzer (Massoth et al., 1995) 5.0 Gorda ridge CTD, LSS, colotimetry and water samples (Massoth et al., 1998) 6.5 Juan De Fuca Ridge CTD and transmissometer (Baker et al., 1989) 7.5 Reykjanes Ridge CTD, transmissometer, nephelometer and water (German et al., 1994) samples 9.0 Juan De Fuca Ridge CTD and transmissometer (Baker et al., 1989) 10.0 Juan De Fuca Ridge Water samples, CTD and transmissometer (Massoth et al., 1994) 10.0 Juan De Fuca Ridge Water samples, CTD, Acoustic sensor (Baker and Massoth, 1986) 12.0 Gorda ridge CTD, LSS, colotimetry and water samples (Massoth et al., 1998) 13.0 Juan De Fuca Ridge CTD and transmissometer (Baker et al., 1989) 16.0 Juan De Fuca Ridge CTD and transmissometer (Baker et al., 1989) 20.0 Juan De Fuca Ridge CTD and transmissometer (Baker, Massoth and Feely, 1987) 25.0 Lucky strike CTD and water samples (Jean-Baptiste et al., 1998) 50.0 Manus Basin Water samples (Gamo et al., 1993) 50.0 Mid Atlantic Ridge CTD, nephelometer and water samples (Severmann et al., 2004) 70.0 Carlsberg Ridge CTD, MAPR, OBS and water samples (Murton et al., 2006)

On the other hand, data from sediment resuspension (Table 2.4 and Table 2.5) and gravity currents phenomena (Table 2.6 and Table 2.7) are a good starting point for seabed mining bottom plumes and sediment resuspension analysis.

48 Table 2.4. Vertical extent of resuspensionphenomena. Values rangefrom 4 meters above the seafloor, up to 70 meters in the case of some trawling-induced resuspensionphenomenon. The activity or naturalphenomenon which generatedthe sediment resuspension is indicated in the second column.

Vert ical Location Extent (in) (Source) Methods Source 4 James estuary (Friedrichs and Battisto, Water samples, OBS, and ADCP (Dredging) 2001) 8 Changjian estuary Acoustic suspended sediment monitor, water samples (Shi et al., 1997) (River estuary) 20 North sea CTD, water samples, PAR and transmissometer (Jones et al., 1998) (Winter storm) 25 North sea CTD, water samples, PAR and transmissometer (Jones et al., 1998) (Winter storm) 40 La Fonera canyon Turbidity meters and ADCP (Martin et al., 2014) (Trawling) 70 La Fonera canyon Turbidity meters and ADCP (Martin et al., 2014) (Trawling)

Table 2.5. Horizontal extent of sediment resuspension phenomena. The horizontal extent ranges from 135 meters to 22 km.

Horizontal Location Extent (km) (Activity) Methods Source CTD, transmissometer, fluorometer, water samples, LSS (Durrieu De Madron et al., 0.1 Gulf of Lion (Trawling) and ADCP 2005)

2.1 James estuary Water samples, optical backscatter, and ADCP (Friedrichs and Battisto, 2001) (Dredging) Estonia 20.0 Muuga, Satellite images (Kutser et al., 2007) (Dredging) Estonia 22.0 Sillamae, Satellite images (Kutser et al., 2007) (Dredging)

Table 2.6. Vertical extent of gravity currents. Values rangefrom 7.5 meters, up to 175 meters for the most massive events.

Vertical Extent (in) Location Methods Source 7.5 A British Columbia Fjord Sediment traps (Prior et al., 1987) 12.0 Bute inlet (British Columbia) Sediment traps, current meters and deflection vanes (Zeng et al., 1991) 30.0 Zaire submarine valley Transmissometer (Khripounoff et al., 2003) 32.0 Bute inlet (British Columbia) Sediment traps, current meters and deflection vanes (Zeng et al., 1991) 55.0 Eel Canyon, California margir Current meters, optical backscatter sensor, CTD and ADCP (Puig et al., 2004) (Xu, Noble and 75.0 Monterey submarine canyon Transmissometer Rosenfeld, 2004) (Xu, Noble and 170.0 Monterey submarine canyon Transinissometer Rosenfeld, 2004)

49 Table 2.7. Horizontal extent of gravity currents. Because of the characteristicsand difficulty to predict this phenomena, only two field data have beenfound, ranging between 26 to 70 km..

Horizontal Extent (km) Location Methods Source 25.9 British Columbia Fjord Sediment traps (Prior et al., 1987)

70.0 Bute inle Sediment traps, current meters and deflection vanes (Zeng et al., 1991) (British Columbia)

2.3.2. Flow velocity

The flow velocity is another parameter that has a direct effect on the plume dynamics. Because of the nature of the different plume phenomena that are being analyzed in this chapter, the flow velocity has to be adequately defined for each one:

- Hydrothermal and volcanic plumes: the parameter makes reference to the velocity of the flow at the very beginning of the plume. In most cases, the velocity is measured few centimeters from the plume exit orifice. Therefore, this parameter can be directly compared to the flow velocity of the \Waste water discharge plumes at the exit of the nozzle of the discharge pipe.

- Sediment resuspension: in this case, the flow velocity indicates the velocity of the bottom current that generated the resuspension of the seabed sediments. This value can be compared to the flow velocities within the wake of the nodule collection vehicle, and also to the velocities induced by the horizontal exhaust jet of the hydraulic system to pick up the nodules.

- Gravity current: the flow velocity is defined as the spreading velocity of the gravity current head. It is expected that the plume created by the nodule collection vehicle will evolve as a gravity current. Then, this values may be considered in this regard. However, it is important to note that, in most cases, natural gravity currents occur in steep areas of the seabed, which cannot be compared to the relatively flat abyssal plains where the nodules are commonly located.

50 Table 2.8. Flow velocity at the exit orifice of hydrothermal vents. Values rangefrom 0.5 to 3.4 m/s.

Flow velocity (m/s) Location Methods Source 0.50 Galdpagos Rift Vane-type flowmeter (Corliss et al., 1979) 0.68 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 0.76 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 0.77 Juan De Fuca Ridge (Cleft Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 0.78 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 0.85 Juan De Fuca Ridge (Cleft Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 0.88 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Motti and Von Herzen, 1994) 0.90 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 0.91 East Pacific Rise Turbine flowmeter (Converse, Holland and Edmond, 1984) 0.93 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 0.99 East Pacific Rise Turbine flowmeter (Converse, Holland and Edmond, 1984) 1.00 Trans-Atlantic Geotransverse area Visual observations (Rona and Speer, 1989) 1.00 Juan De Fuca Ridge (Cleft Segment) Visual observations (Baker, 1994) 1.02 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Motti and Von Herzen, 1994) 1.06 Juan De Fuca Ridge (Cleft Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.07 Juan De Fuca Ridge (Cleft Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.11 Juan De Fuca Ridge (Cleft Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.13 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.16 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.19 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.24 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.27 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.38 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.44 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.44 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.47 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.59 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 1.81 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994) 2.22 East Pacific Rise Turbine flowmeter (Converse, Holland and Edmond, 1984) 2.50 East Pacific rise Visual observations (Macdonald et al., 1980) 3.00 East Pacific rise Visual observations (Macdonald et al., 1980) 3.36 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter (Ginster, Mottl and Von Herzen, 1994)

51 Table 2.9. Volcanic plumes flow velocity estimates. Values range from 0.5 to 4.0 m/s.

Flow velocity (m/s) Location Methods Source 0.5 Myojinsho volcano, Japan Estimates based on surface boils size Fiske et al. 1998 0.5 NW Rota-i Video Deardoff et al. 2011 3.0 NW Rota-i Video Deardoff et al. 2011 4.0 NW Rota-i Video Deardoff et al. 2011

Table 2.10. Velocities of the head of the observed gravity currents. Values rangefrom 0.2 m/s up to 20 m/sfor the most violent phenomena.

Flow velocity (m/s) Location Methods Source 0.2 Spencer Gulf Current meter (Nunes and Lennon, 1987) 0.3 Bute inlet (British Columbia) Current meter (Zeng et al., 1991) 0.397 Monterey submarine canyon ADCP (Xu, 2010) 0.566 Monterey submarine canyon ADCP (Xu, 2010) 0.596 El canyon, California margin Electromagnetic current meter (Puig et al., 2004) 0.75 Monterey submarine canyon ADCP (Xu, Noble and Rosenfeld, 2004) 0.75 Bute inlet (British Columbia) Current meter (Zeng et al., 1991) 0.908 Monterey submarine canyon ADCP (Xu, 2010) 1.02 Bute inlet (British Columbia) Current meter (Zeng et al., 1991) 1.08 Monterey submarine canyon ADCP (Xu, 2010) 1.131 Monterey submarine canyon ADCP (Xu, 2010) 1.21 Zaire submarine valley ADCP (Khripounoff et al., 2003) 1.55 Monterey submarine canyon ADCP (Xu, Noble and Rosenfeld, 2004) 1.6 Monterey submarine canyon ADCP (Xu, Noble and Rosenfeld, 2004) 1.843 Hueneme canyon ADCP (Xu, 2010) 1.9 Monterey submarine canyon ADCP (Xu, Noble and Rosenfeld, 2004) 2.011 Hueneme canyon ADCP (Xu, 2010) 3.35 A British Columbia Fjord Doppler current meter (Prior et al., 1987) 3.35 Bute inlet (British Columbia) Current meter (Zeng et al., 1991) 3.7 Kaoping Canyon and Manila Trench Cable break locations and times (Hsu et al., 2009) 5.7 Kaoping Canyon and Manila Trench Cable break locations and times (Hsu et al., 2009) 17.9 Balearic Abbysal plain Cable break locations and times (Heezen and Ewing, 1955) 20 Kaoping Canyon and Manila Trench Cable break locations and times (Hsu et al., 2009)

52 Table 2.11. The bottom flow velocities indicated in this table generated sediment resusnensions in the noted locations. Values rangefrom 0.15 to 1.2 m/s.

Flow velocity (m/s) Location Methods Source 0.15 South of Cape Cod, MA Current meter (Dickey et al., 1998) 0.25 Lake Erie ADCP (Valipour et al., 2017) 0.3 Hillsborough Bay, Florida Electromagnetic current meters (Schoellhamer, 1996) 0.5 Jiaozhou Bay, Yellow Sea ADCP and ADV (Yuan et al., 2008) 0.6 Hillsborough Bay, Florida Electromagnetic current meters (Schoellhamer, 1996) 1.2 Changjian estuary Current meter (Shi et al., 1997)

2.3.3. Flow rate

The flow rate is a parameter that only can be consider for hydrothermal and volcanic plumes. Although, due to the fact that volcanic plumes are massive and violent, it is practically impossible to determine their flow rate. Consequently, only data for hydrothermal plumes is presented in this section. Again, these numbers may be helpful to make a comparison with the expected flow rates of dewatering discharge plumes, which are expected to be around 0.5 m3/s (Oebius et al., 2001). As it is shown in Table 2.12, values for hydrothermal plumes are an order of magnitude below. But, in any case, hydrothermal plumes data can still be used for model validation purposes.

53 Table 2.12. Field data of Hydrothermal vents flow rates. Values range from 0.5 up to 170 1/s. Dewatering dischargeflowrates for seabed mining activities are expected to be around500 1/s (Oebius et al., 2001).

Flow rate (/s) Location Methods Source 0.5 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 0.6 East Pacific rise Turbine flowmeter and video (Converse, Holland and Edmond, 1984) 0.7 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 0.7 Juan De Fuca Ridge (Cleft Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 0.8 Juan De Fuca Ridge (Cleft Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 0.9 East Pacific rise Turbine flowmeter and video (Converse, Holland and Edmond, 1984) 1.1 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 1.2 East Pacific rise Turbine flowmeter and video (Converse, Holland and Edmond, 1984) 1.3 Juan De Fuca Ridge (Cleft Segment) Video (Baker, 1994) 1.3 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 1.4 East Pacific rise Turbine flowmeter and video (Converse, Holland and Edmond, 1984) 1.7 East Pacific rise Turbine flowmeter and video (Converse, Holland and Edmond, 1984) 1.8 Juan De Fuca Ridge (Cleft Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 1.9 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 2.0 East Pacific rise Turbine flowmeter and video (Converse, Holland and Edmond, 1984) 2.1 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 2.2 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 3.4 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 3.5 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 4.1 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 4.5 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 5.1 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 5.9 Juan De Fuca Ridge (Cleft Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 6.0 Galipagos Rift Vane-type flowmeter (Corliss et al., 1979) 8.3 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 10.9 East Pacific rise Turbine flowmeter and video (Converse, Holland and Edmond, 1984) 12.1 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 19.5 Juan De Fuca Ridge (Endeavour Segment) Test collar (Tivey et al., 1990) 19.6 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 93.2 Juan De Fuca Ridge (Endeavour Segment) Turbine flowmeter and video (Ginster, Mottl and Von Herzen, 1994) 110.0 East Pacific rise Video (Macdonald et al., 1980) 170.0 East Pacific rise Video (Macdonald et al., 1980)

54 2.3.4. Settling velocity

The settling velocity of the sediment particles present in seabed mining plumes determines the time required for the plume settling and, therefore, the distance that the sediment particles will be advected by the background currents. This aspect is essential to assess the area impacted by both dewatering and bottom plumes. The settling velocity mostly depends on the particle size, but there are other aspects, such as the flocculation, that have a significant effect on this parameter. Therefore, comparisons with natural plume phenomena have to be carefully conducted.

Table 2.13. Settling velocities of particles obtainedfrom samples of hydrothermal plumes. The mean particle size is includedin the table, as it is the governing parameter. The mean particle size in the CCZ is in the order of 10 to 100 pm (Oebius et al., 2001).

Settling velocity Particle size (mi/s) Location Methods (Pm) Source 1.00E-4 Juan De Fuca Ridge Water samples <1 (Baker et al., 1989) 2.3 1E-3 Mid-Atlantic Ridge Particulate material samples 1 (German and Sparks, 1993) 9.26E-2 Mid-Atlantic Ridge Particulate material samples 5 (German and Sparks, 1993) 0.9 Juan De Fuca Ridge Sediment traps samples 2.1 (Dymond and Roth, 1988) 2.0 Juan De Fuca Ridge Water samples 150 (Baker et al., 1989) 25.0 Juan De Fuca Ridge Sediment samples 500 (Barreyre, Soule and Sohn, 2011) 29.0 Juan De Fuca Ridge Sediment traps samples 140 (Dymond and Roth, 1988) 39.0 Juan De Fuca Ridge Sediment traps samples 180 (Dymond and Roth, 1988) 55.0 Juan De Fuca Ridge Sediment traps samples 340 (Dymond and Roth, 1988) 120.0 Juan De Fuca Ridge Sediment samples 1000 (Barreyre, Soule and Sohn, 2011)

Table 2.14. Settling velocities of particlesfrom resuspensionphenomena. Values rangefrom 8.8E-3 to 5 mm/s.

Settling velocity (mm/s) Location Methods Source 8.8E-3 Lake Ontario Sediment traps (Rosa, 1985) 4.OE-2 Gulf of Lion Transmissometer (Durrieu De Madron et al., 2005) 0.2 North sea Water samples (Jones et al., 1998) 0.27 Jiaozhou Bay ADCP (Yuan et al., 2008) 0.41 Jiaozhou Bay LISST (Yuan et al., 2008) 0.57 Jiaozhou Bay ADV (Yuan et al., 2008) 1 North sea Water samples (Jones et al., 1998) 2 James estuary Water samples, OBS (Friedrichs and Battisto, 2001) 5 North sea Water samples (Jones et al., 1998)

55 2.3.5. Density

Finally, the density is another important parameter for the plume dynamics. In the case of dewatering plumes, the initial density of the plume will depend on the mass flow rate of discharged sediment, on the flow rate and density of the deep-sea, and on the flow rate and density of sea surface water added during the processing onboard the vessel.

Table 2.15. Initial density of hydrothermal plumes. The density has been obtained applying the plume models with field datafrom hydrothermal vents. The main characteristicof hydrothermalplumes is that, due to their high temperature, they are buoyant.

Density (kg/m3) Location Methods Source 653 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 701 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 772 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 784 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 787 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 800 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 821 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 840 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 859 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 870 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 870 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 871 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 883 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 883 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 890 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 914 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 949 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 991 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 993 Mid-Atlantic Ridge Temperature, velocity and equation of state (Thurnherr and Richards, 2001) 996 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994) 1009 Juan De Fuca Ridge Turbine flowmeter, thermocouple and plume model (Ginster, Mottl and Von Herzen, 1994)

56 3 Table 2. 16 Density of gravity currents. Values range from 1029 tn 173 kg/m . All the gravity currents have a greater density than the background sea water. According to Oebius et al. (2001), the bottom plume created by the collection vehicle may have a density of 1120 kg/M3.

Density (kg/m3 ) Location Methods Source 1029 Spencer Gulf (South Australia) CTD Nunes and Lennon 1987 1035 Grand Banks of Newfoundland Piston cores Piper et al 1988 1038.5 Bute inlet (British Columbia) Sediment traps Zeng et al., 1991 1124 Grand Banks of Newfoundland Piston cores Piper et al 1988 1173 Grand Banks of Newfoundland Piston cores Piper et al 1988

57 58 Chapter 3 PLUMEX: Waste Water Discharge Plume Experiment

Previous chapters were focused on the thorough analysis and review of the state of the art, and the most relevant research projects of polymetallic nodule mining. This analysis ended up with the identification of some relevant research gaps, such as the lack of field data to validate sediment plumes models, which are essential to adequately assess the consequent environmental impact. Because of that, the PLUMEX project - a field study led by the Environmental Dynamics Laboratory

(ENDLab) at MIT - was conducted in the Pacific Ocean from February 2 4th to March 7 th 2018, onboard Research Vessel (R/V) Sally Ride. PLUMEX field studies were focused on the creation and study of the near, intermediate and far-field of the sediment plumes created by polymetallic nodule mining waste water discharges. The results from the experiments will be used to determine which type of analytic and numeric models are more accurate, and should be used to assess the extent of this type of sediment plumes. One of the plumes discharged during the PLUMEX field studies was created using seabed sediment from the CCZ. The sediment was extracted by Global Sea Mineral Resources NV (GSR) - an ISA contractor with an exploration license in the CCZ - from their exploration area.

3.1. Plume model

As part of PLUMEX studies, an analytic and a numeric model were developed by MIT ENDLab in order to predict near-field characteristics of sediment plumes. The analytic model is based on the traditional integral plume equations (Morton et al., 1956). Equations 3.1, 3.2 and 3.3 establish the relation between volume (Q), momentum () and buoyancy (F) fluxes for the plume:

= 2aM1 (3.1)

dM -QF (3.2) dz M dF QN2 (3.3) dz where a is the entrainment coefficient of the plume, and N is the buoyancy frequency of the ambient stratification. This plume analytic model assumes that, in the near-field, the plume behaves as a single-phase fluid. Therefore, the initial density of the plume fluid is calculated considering the mass flow rates and densities of the discharged sea water and sediment particles. In contrast, a multi-phase model would consider both the fluid and the solid particles as independent elements. This is something that can be done using numeric models, which require significantly more computational effort.

Additionally, the effect of the background currents on the plume dynamics was also implemented in the analytic model. Under certain conditions, the current can even halve the vertical extent of the plume.

59 The model was programmed including a Graphic User Interface (GUI), so the user can easily modify the main parameters to analyze their effect on the plume characteristics (Figure 3.1). Some of the most important parameters that can be modified are the environmental conditions (background stratification and currents), flow rate, discharge nozzle diameter, release depth, initial density of the discharge sea water, and mass flow rate and density of sediments and fines.

Peramner sweep

E 2O~ X-A~e i~~" X Axt

5W0 1000 1510 Wessae Depth im)

Figure 3.1. Sample result from the model, showing how the vertical extent of the plume changes with the variationof the plume release depth, keeping all the other variables constant. In this example, the plume is 5 times longer if it is released at 1300 meters of depth, than if it is released at 100 meters.

The outputs from the model are the vertical extent and the velocity, density and width distributions along the plume. Some preliminary results from the model showed that the vertical extent of the plume was non-monotonic in the initial flow rate and release depth, having some local minimums. These results are very interesting, as they may help to control the vertical extent of the plume, what may have a significant impact on the advection and far field evolution of the plume.

Apart from the near-field analytic model developed by ENDLab, another research group at MIT has been developing the Multidisciplinary Simulation, Estimation, and Assimilation System (MSEAS). It is a numeric model used to study and quantify tidal-to-mesoscale processes over regional domains with complex geometries and interactions. This model can be applied to simulate and forecast a sediment plume transport process (Coulin et al., 2017). The model was used during PLUMEX field studies to forecast the currents and the ocean stratification in the area where the experiment was conducted (Figure 3.2). These forecasts facilitated the monitoring and tracking of the sediment plumes.

60 I K30 3 1 1 I'l 50 1 17'W

27 3330 25

-- 23

22 \~-~ ~20 33N W~'

16 16 15

32'30 P.4~C. h13 11

8 32N 6

4

AMn= 3,90409-01 Max- 3.90401+01 2.13 Day Forecast : 3:00:00 5 Mar 2018 Figure 3.2. Current velocity forecast at 100 meters of depth obtained using MSEAS. The forecasted area corresponds to the Gulf of Santa Catalina, in the Pacific Ocean, where PL UMEXfield studies were conducted. A subsurface anticyclone with the higher current velocities is easily observed in the center of the area.

3.2. Scope of the experiments

In 2018, between the 2 6th of February to the 6 th of March, a total of six waste water discharge plume experiments were conducted in Santa Catalina gulf (Figure 3.3), in the Pacific Ocean, from aboard R/V Sally Ride (Figure 3.4). The discharged plumes were monitored in the near, intermediate and far fields using different technologies that will be detailed in this chapter.

Figure 3.3. Localion ?/ PLUIMEXfield studies. A contingency location was chosen in the lee of San Clemente Island to operate there in case qf bad weather conditions. Both locations had a depth qf more than 1000 meters, to ensure that the plume was not qffected by the sea bottom.

61 Figure 3.4. RIV Sally Ride. Length: 73 m. Beam: 15 m. Draft: 4.5 m. Displacement: 3090 t. Cruise speed: 12 kts.

The experiments were designed to mimic sediment plumes expected to be discharged during seabed mining activities, and also to validate the analytic and numeric models. Because of that, three different types of plumes were created. The first type consisted of the discharge of a hypersaline plume, which had a higher density than the background sea water due to the salt addition. The objective of this type of plume is to validate the single-phase models, which consider the plume as a fluid with a certain initial density. Therefore, as the salt is completely dissolved in the water, the single-phase assumption is fully accomplished by these plumes. The second type of plume was created by mixing sea water and limestone with a mean grain size below 40 microns, within the range of the CCZ seabed sediment mean grain size (Oebius et al., 2001). This second type of plume was chosen as a good alternative to replicate a CCZ sediment plume, and also as a good practice before creating a CCZ sediment plume. Finally, the third type of plume is the CCZ seabed sediment plume. Because of the limited amount of CCZ sediment, only one plume of this type was discharged.

The objective of PLUMEX field studies was to mimic the expected commercial deep-sea mining dewatering plumes, considering always the constraints of space, payload and lifting capabilities onboard R/V Sally Ride. After a careful study of all the plume parameters and physical constraints, a total of six experiments were designed (Table 3.1).

Table 3.1. Main parametersof the six plume experiments conducted during PLUMEXfield studies. The range of parameters was selected to better validate the models and understand the behavior of the plumes. The diameter of the discharge hose used to create the plumes was 20.3 cm. Rhodamine dye was added to track the plume.

Discharge Flow rate Salt Limestone CCZ Temperature* Rhodamine Duration Plume Type depth (m) (L/s) (kg) (lbs) (lbs) (0C) (ml) (min) I Salt 41.5 95 2720 0 0 N/A 0 90 2 Salt 41.3 123 5440 0 0 N/A 700 90 3 Salt 59.5 101 5440 0 0 14.80 3000 44 4 Limestone 59.0 101 0 3310 0 14.90 4600 48 5 CCZ 59.0 48 0 0 1540 14.82 4700 45 6 Salt 58.9 48 1360 0 0 14.58 4000 45

62 The best way to analyze each of the selected parameters for the experiments is to compare them with some of the expected values for commercial seabed mining operations, and also with the OMA and OMI discharge plumes during their polymetallic nodule pilot tests.

Table 3.2. Comparison of the main waste water dischargeparameters. Datafrom PL UMEX field studies is compared with OMI and OMA seabed mining pilot tests, and estimates for commercial seabed mining operationsfrom Oebius et al. (2001) and Morgan, Odunton and Jones (1999). Sediment mass flow rate and concentration values from PL UMEXfield studies correspond to the limestone and CCZ sediment plumes. In the case of commercial operations estimates, it includes not only the sediment, but also nodulefines.

Parameter PLUMEX OMI OMA Commercial operation Discharge depth (m) 40-57 Surface Surface 500-1500 Flow rate (1/s) 48-123 100-160 95 298-560 Sediment mass flow rate (g/s) 570-1150 680-2030 550 4600 Sediment concentration (g/l) 11.4-11.9 6.8-12.7 5.8 8.2-15.4 Temperature 14.6-14.8 7-10 4.4-5.2 N/A

The discharge depth is, probably, one of the most important parameters for seabed mining plumes, mainly, due to the impact on the design, complexity and cost of the dewatering system. As it was mentioned in the second chapter, it is expected that the ISA will include a regulation on this aspect. Most authors have suggested that the discharge of the plume should occur below the photic zone and the permanent oxygen minimum zone (Oebius et al., 2001; Thiel, 2001; Schriever and Thiel, 2013b), at depths between 500 to 1500 meters. However, from the perspective of the plume dynamics and model validation, the key aspect is to release the plume below the pycnocline, where the density gradient is almost constant, and much lower than in the mixed layer. In the CCZ, the mixed layer depth is usually between 50 to 200 meters (Ozturgut et al., 1978; Burns et al., 1980; Lavelle et al., 1982). But, during PLUMEX field studies in the gulf of Santa Catalina, the mixed layer depth (MLD) was 32 meters, as it is shown in Figure 3.5. Consequently, all the plumes created for PLUMEX were released below the pycnocline. The obtained field data is, therefore, adequate to validate plume analytic and numeric models, and to assess the main characteristics of commercial polymetallic nodule mining plumes. Unfortunately, data from OMI and OMA pilot tests cannot be reliably used for this purpose because the plume was directly discharged from the operation vessel to the ocean surface.

63 0 10

20 MLD

40

50 300 70

4r 4400

10 iso

am 120 130

10 7 4 0 150

170

190

0 0.2 0.4 0.6 0.3 1 1.2 0 0.2 0.4 0.6 0.6 I 1.2 2 2 N (s-2) 103 N (5,2) -10- (a) (b) Figure 3.5. (a) Buoyancyfrequency calculatedfrom a CTD profile measured duringPLUMEXfield studies. In (b), there is a zoom-in view of the same buoyancy frequency profile, where the M LD - marked in red - can be easily identified at about 30 meters of depth, where the buoyancyfrequency is maximum.

Regarding the flow rates, PLUMEX values were within the range of OMI and OMA plumes. Some of the PLUMEX plumes had a lower flow rate in order to increase the initial density or particle concentration. The flow rate, has a direct impact on the initial momentum of the plume. But, according to the single-phase plume models, the momentum dissipates considerably quick and, consequently, it does not have a major impact on the plume extent. Compared to the expected flow rates for commercial seabed mining, PLUMEX values were between 2.5 to 11.5 times smaller.

Another important parameter, from the perspective of the plume dynamics, is the sediment concentration. In the case of PLUMEX, this parameter is only directly applicable to the limestone and the CCZ sediment plumes. The sediment concentration from our field studies is within the range of what is expected for commercial seabed mining. During OMI tests, two different systems were used for the vertical transport of the nodules: a hydraulic system using centrifugal pumps, and an air- lift system. In the case of the hydraulic system, which is the most likely to be used for commercial operations, the concentration was 12.7 g/l, very similar to the values used for PLUMEX. On the other hand, the air-lift system, which was also used on OMA tests, resulted in a much lower sediment concentration.

Finally, the temperature of the discharge is another key aspect to consider. In a commercial seabed mining operation, the water discharged from the operation vessel is a mix of water from the deep ocean, where the nodules are located, and water from the ocean surface that is used to help in the separation and cleaning process of the nodules. Therefore, a significant temperature increase will take place. From the perspective of the plume dynamics, the relevant aspect is the temperature difference between the plume and the environment. Then, assuming that the temperature values obtained from OMI and OMA tests can be applicable to commercial operations, the initial plume temperature can

64 be easily 2 to 4*C warmer than the ocean water at 1000 meters of depth. During PLUMEX field studies, a Seabird SBE 56 thermistor was mounted on the discharge hose nozzle to measure the background temperature before the discharge, and the initial temperature of the plume during the discharge. The measurements showed that the background temperature at the release depth was 11.4 *C, and the plume temperature was between 14.6 to 14.8*C. This means that the initial temperature difference between the plume and the environment was 3.3*C, within the range of the expected values for commercial deep-sea mining.

Of course, salinity would be another parameter of interest to compare with. At PLUMEX field studies, water samples of the discharge were taken from a bleeder located right before the discharge hose, and salinity analysis, among others, were conducted. Unfortunately, there is no plume salinity field data from OMA or OMI, or any estimate for commercial deep-sea mining operations.

3.3. Plume creation

There are different ways to create a water discharge plume. The main challenge of PLUMEX studies was to create a plume that is only three to five times smaller than a commercial seabed mining. The plume was discharge from a conventional research vessel, which has significant space and payload constraints. Also, it is important to note that, in the case of mining operations, the sediment lifted up from the seabed is already mixed with water. However, in the case of PLUMEX field studies, the salt, limestone or CCZ sediment had to be mixed with sea water onboard the vessel, and then, discharged at a certain depth through a discharge hose (Figure 3.6).

Suction hose

Discharg hose Crane cable

Clump Weight

/Plume

Figure 3.6. Sketch of the plume creation concept from R/V Sally Ride (not scaled). The tanks and pumps used to create the plume are located on the main deck of the vessel. A suction hose is deployed from the stern of the vessel to pump waterfrom the ocean surface to create the waste water plume, which is discharged through discharge lay-flat hose of 20 cm of diameter. The discharge hose is attached to a crane cable with a clump weight at the very end in order to facilitate the deployment and recovery of the hose, and also to minimize the deflection of the hose due to the background currents.

65 Then, the first step was to decide which was the best option to mix the salt, limestone or CCZ sediment to create the waste water for the plume. In order to compare and evaluate the alternatives, it is important to define the main functional requirements of the system. In this case, it was considered that reliability and safety were the two most important ones; the solution had to be simple, safe and effective. Other desirable functional requirements were accuracy - as we needed to make sure we were able to create a plume with specific characteristics - and cost-effectiveness, because the financial resources for the project were limited. Overall, three possible alternatives were considered to create the plume.

- Static mixer: there are several industrial processes that require to mix a granulated or powdered material with a fluid. This is particularly common in the food industry, where they make use of static mixers designed for such purpose. Compared to other alternatives, this one had the advantage of having a very low weight and footprint, very important aspects onboard a vessel. However, these machines are designed to work on land, with no motions and accelerations. Additionally, the high moisture of the marine environment will also affect the granulated particles of salt and limestone making it difficult, even almost impossible, to feed the mixer. Therefore, this alternative was discarded because of the high uncertainty.

- Water tanks: a highly reliable option was to create the waste water mix using tanks located on the main deck of the vessel. Then, the content of the tanks is pumped through the discharge hose to create the plume. Unfortunately, due to the payload and deck space limitations, the largest tanks that were possible to use, were not large enough to store the water needed to create a plume with a high flow rate to accomplish the objectives of the studies.

- Mix injection from storage tank: this solution, which was finally used for the plume creation, is a mid-way point between the two previous alternatives. The concept consists of the injection of a highly-concentrated mixture - which is stored in a tank located on the main deck of the vessel - into a main flow of water that is being pumped from the ocean by a centrifugal pump.

The main elements of the plume creation system are a main centrifugal pump, an auxiliary centrifugal pump, two submersible pumps, a main tank and a mixing tank, as it is shown in Figure 3.7.

66 Tank 1

Main Pump - fAux. Pump

8" Discharge hose 12" Suction hose 8" Suction hose

Figure 3.7. PLUMEXplume creation system diagram. Firstly, the auxiliarypump is used to fill the tanks with sea water. Then, the salt, limestone or CCZ sediment is mixed in tank 2 (T2 in the diagram) with the help of the auxiliary pump, which is recirculatingthe content and transferringit to tank 1. The content of both tanks is recirculatedtogether to make sure they have the same concentration. Finally, the main pump starts pumping water from the ocean surface and dischargingit through the discharge hose. At the same time, the highly concentratedmixture from the tanks is injected into the main pump inlet, using the help of the auxiliary pump. Two flow rate meters (blue circles in the diagram) were installedto measure both the over plume flow rate, and the injectionflow rate.

The basic plume creation process starts filling both tanks with water from the ocean surface using the auxiliary pump. Then, the auxiliary pump recirculates the content of the tanks. The submersible pumps, located inside the tanks, are turned on to facilitate the mixing process and avoid the particle settling. The cylindrical shape of the mixing tank is very helpful during the mixing process, as it allows to circulate all the content of the tank.

At this point, everything is ready to start dumping the salt, limestone or CCZ sediment into the mixing tank. The salt and the limestone were stored in 900 kg bulk bags with a bottom spout. These bags were craned on top of the mixing tank to dump their content. On the other hand, the CCZ sediment was in the form of a compact clay (Figure 3.8a) and, consequently, it was not possible to directly dump it into the mixing tank. The procedure in this case started fluidizing the CCZ sediment in 8 totes using artificial sea water to avoid the biological contamination during the preparation time. An electric paint mixer and a custom-made plunger were used to mix the sediment and mimic the effects of the pumps during commercial operations (Figure 3.8b). The objective was to make sure that the sediment was adequately dissagregated, and in a similar condition than in real mining operations. Two submersible pumps were used to transfer the liquefied CCZ sediment to the mixing tank (Figure 3.8c).

67 (a) (b) (c) Figure 3.8. Preparationof the CCZ sediment for the plume creation. (a) Initial condition of the sediment. (b) Sediment liquefying process. (c) Pumping the liquefied sediment using two submersiblepump; the black hoses used to transfer the mixture can be seen in the top of the picture.

Finally, the highly-concentrated solution mixed in the tanks is injected into a main ocean water flow that is being pumped by the main pump. The solution is injected right before the inlet, so the pump itself and the high turbulence of the flow ensure that the injected solution is mixed with the ocean water flow, obtaining the desired particle concentration.

An additional challenge was to ensure that it was feasible to install the proposed system onboard R/V Sally Ride. In this regard, the main constraints were the available space on the main deck, the overall weight, the weight distribution along the deck, the ship stability, and craning capabilities. Figure 3.9a shows the deck layout designed for PLUMEX. Additionally, in Figure 3.9 b) and c), two pictures of the actual layout of all the elements on the main deck of R/V Sally Ride are included. In Table 3.3, the weight, footprint, deck load density and vertical position of the center of gravity of the main equipment and materials are specified. In all cases, the values were below the vessel operational limits.

68 20 ft

T 2

I T2EankJL

Main Pum

(a) (b)

(c) (d) Figure 3.9. a) Deck layout including the saltpallets in red, the clay totes in brown, and the knuckle-boom crane range (20feet) in green. Pictures b) and c) show the real deck layout onboardR/V Sally Ride. The mixing tank is the black tank next to the main blue tank. The CCZ sediment was stored in the blue and green totes that are shown in the bottom of c). In that same picture, the white bulk bags next to the blue tank contain the salt for some of the experiments. In d), the orange suction hoses are ready to be deployedfrom the stern.

Table 3.3. Weight, footprint, load density and vertical position of the center of gravity (VCG) of the main equipment and materialsfor PLUMEXfield studies. The maximum payload of R/V Sally Ride is 100 tons, with a maximum deck load density of the main deck is 7.5 t/m2, and a maximum payload VCG of ] m.

Weight Footprint Load Density VCG 2 Item (t) (m2) (t/m ) (m above deck) Main tank (full) 38.5 18.2 2.1 1.1 Aux. tank (full) 9.2 4.9 1.9 0.9 Salt 18.1 20.4 1.2 0.8 Limestone 9.9 10.2 1.2 0.8 CCZ Sediment 2.2 9.6 1.2 0.5 Main pump 4.2 10.1 0.42 0.9 Auxiliary pump 3.2 5.4 0.59 1.0 Total 85.3 78.8 - 0.95

69 Another aspect that had to be analyzed was the effect of the tanks' free surface on the stability. The presence of a free surface when the tanks are not completely full is translated into a loss of stability that, in some cases, may have a significant impact on the ship's safety. The presence of the free surface is translated in what is called a virtual decrease of the metacentric height. Then, the effect is equivalent to an increase of the height of the center of gravity (VCG) from G to Go (Figure 3.10).

G"Z.

G'$- -i Z G Zk

Figure 3.10. Virtual increaseof the center of gravity due to the effect of thefree surface of a tank.

This effect only depends on the moment of inertia of the free surface (i), the density of the fluid inside the tank (pt), and the displacement of the ship (A).

5.66-2.4 3 12 .-1024.5 GGO= 1-Pt = 12 =0.0265m (3.4) - 3024-1

The calculation considering the characteristics of the main tank used for PLUMEX shows that the virtual increase on the ship's VCG generated by the free surface is 2.65 cm (1"). This value is, according to the ship's characteristics, considered a very small increase and, therefore, the stability and safety of the vessel will not be compromised because of the tank's free surface.

Overall, the plume creation system worked very well during the experiment. Only minor mechanical problems with the centrifugal pumps delayed the first plume experiments. Undoubtedly, the most challenging aspect was the deployment and recovery of the discharge hose; the initial system to attach the hose to the crane cable slipped during the first recovery of the hose. The system was improved using ratchet straps and crosby clamps to attach the joints of the discharge hose sections to the crane cable.

3.4. Plume monitoring, sampling and tracking

The development of a good and reliable plume monitoring strategy was one of the main objectives during the design of the field studies. After several iterations, it was decided to focus on four different aspects:

- Near field monitoring. It is focused on monitoring the dynamics of the plume while it is being released from the vessel. Therefore, the objective is to obtain data about the extension, shape,

70 and velocity field of the plume. The validation of the near-field analytic models rely on the data measured during this first phase.

- Intermediate field monitoring. As soon as the plume is being released, it is advected by the background current. The intermediate field monitoring is designed to determine aspects such as the vertical extent of the advected plume at a distance of tens of meters from the discharge.

- Far field tracking and monitoring. After the plume discharge, the head of the plume has already been advected a few hundred meters. The objective of this phase is to track, monitor and sample the plume during up to 6 hours to determine its size, and better understand its behavior.

- Biological studies. The impact of the plume on the micro fauna and flora present on the ocean is another of the key uncertainties of seabed mining environmental impact.

In order to facilitate the intermediate and far field monitoring, a significant amount of Rhodamine WT dye was mixed with the highly-concentrated solutions in the tanks. Concentrations as low as 13 ppb were detectable with the Sea Bird Eco Triplet fluorometer, which also is able to measure turbidity, a parameter of interest to track the CCZ sediment plume.

3.4.1 Near field monitoring

The near field monitoring of the plume dynamics during the discharge is essential to validate the models, and also to determine the neutrally buoyancy depth. When the plume is released from the discharge hose, the momentum and the negative buoyancy result in the sinking of the plume. But, due to the entrainment of ocean water and the loss of momentum, the plume vertical velocity decreases. At the very bottom of the plume, where the vertical velocity is zero, the background density is slightly higher than the plume's one. This results in a rebound of the plume, that starts going up until it reaches an ocean layer with the same density.

The near field monitoring was conducted using two main instruments, a Phased Array Doppler Sonar (PADS) (Figure 3.11 a) developed by Scripps Institution of Oceanography (SIO) (Smith, 2002; Pinkel, 2016), and a Teledyne Workhorse Sentinel ADCP.

The PADS were deployed from the starboard side of the vessel, at a longitudinal distance of 4 meters from the discharge hose, and at a depth of 7 meters. The PADS consist of 128 independent acoustic receivers working at 200 kHz and covering a fan of 700 (Figure 3.1 lb). The processing of the backscatter, including the phase difference, allows PADS to obtain the velocity field of the covered area by the fan. The range may change depending on the configuration, but in the case of PLUMEX studies, it was configured to reach up to 300 meters. Because of the small width of the covered fan, a tilting system was design to enable the control of the orientation of PADS. Therefore, adjusting the orientation, it was possible to obtain a cross section of the plume velocity field.

71 a) b)

WML_

401

Figure 3.11. a) PADS being deployedjrom RIV Sally Ride during PLUMEXfields studies. b) Raw Intensity plot obtained during one of the plumes release. The aligned red-to-green areas correspondto the reflection of the acoustic waves on the discharge hose steel joints, at 60 meters of depth the plume is being released, and the effect of the background current is clearly identified at 90 meters of depth. However, this is a plot obtained from the raw data, that has to be adequatelyfiltered and processed to determine the velocityfield.

The ADCP was mounted on the clump weight, looking down, to measure the velocity field along the vertical extent of the plume. Unfortunately, the ADCP acoustic waves interfered with the PADS. As the PADS gives more information about the plume dynamics, it was decided that the ADCP was only going to be used during the third plume experiment.

Additionally, a thermistor was mounted on the nozzle to calculate the temperature at the release depth before the plume discharge, and the initial temperature of the plume during the discharge. This is an important input parameter for the model validation.

3.4.2 Intermediate field monitoring

Another innovative instrument developed by the Multiscale Ocean Dynamics Laboratory at SIO was used to conduct intermediate field measurements during the discharge of the plume. In this case, the instrument used was a prototype of a fast CTD (Figure 3.12), which consists of a towed vehicle deployed 15 meters from the stern of the ship using a custom-made crane. The vehicle had a CTD sensor and a fluorometer mounted on it. The ship heading was modified to ensure the detection of the plume, considering the currents heading at the plume neutrally buoyancy depth.

72 .'JIJ

Figure 3.12. Fast CTD being deployed from R/V Sally Ride during PLUMEXfield studies. The CTD and the fluorometer mounted on the vehicle were able to obtain intermediatefield measurements of the plume characteristics.

3.4.3 Far field tracking and monitoring

This monitoring phase has the objective of tracking the plume to analyze the advection process, and determine how it evolves with time. An important aspect is to determine the evolution of the plume concentration. It is assumed that the big particles, and also likely clusters of particles, will sink quickly, and their horizontal transport will be smaller. But the finest particles can spend long periods of time in suspension, while being drifted by the currents up to hundreds of kilometers.

All the required sensors to determine the presence and characteristics of the plume during the tracking mission were mounted on the CTD rosette rig to send real-time data to the vessel. The rosette was tow-yo'ed at horizontal velocity of 0.5 knots, and vertical velocity of 1 m/s. The following sensors were mounted on the rosette:

- CTD sensor. It was used to measure the background stratification in terms of conductivity, temperature and pressure. - Oxygen saturation sensor. In the long term, one of the expected effects of a plume discharge is a decrease on the oxygen saturation levels (Chung et al., 2002). - Photosynthetically Active Radiation (PAR) sensor. One of the likely effects of the plume due to the presence of the sediment, is a decrease on the light radiation. - Turbidity sensor. Similarly, turbidity due to the presence of sediment particles is a sediment plume characteristic. Turbidity levels can be related to the sediment concentration, helping to understand the evolution of the plume. - Transmissometer. This is another tool to measure the presence and concentration of sediment particles. - Fluorometer. The fluorometer makes possible to detect very low concentrations of Rhodamine dye. This type of dye was added to the highly-concentrated waste water stored in the tanks.

73 - Niskin bottles. They were used to obtain water samples at different depths. Some of the samples were used to determine the sediment concentration and characteristics of the plume in the far field, and others were taken for biological analysis.

As soon as the plume discharge was finished, the PADS and the suction hoses had to be recovered before the ship can start the tracking mission. The PADS had to be recovered because they were deployed using the CTD rosette crane, and the suction hoses because they were deployed from the stern, near the propellers, and there was a certain risk of entanglement. Initially, it was also planned to recover the discharge hose before starting the tracking mission; a process that took at least 90 minutes. However, after the first plume tracking tests, it was noted that it was essential to conduct the first cast few minutes after the end of the plume release, otherwise, it was very challenging to find the plume. Because of that, it was required to leave the discharge hose in the water. Therefore, the tow-yo to track the plume was conducted with the discharge hose in the water, and the ship bearing at a velocity of 0.25 m/s. The dynamic positioning system made possible to have the ship bearing in the desired direction, while the heading of the ship was modified to keep the discharge hose far from the propellers.

The sampling strategy consisted of finding the head of the plume by bearing in the same direction than the currents at the neutrally buoyancy depth. As soon as the fluorometer, turbidity and transmissometer signals show that we have passed the head of the plume, the ship started conducting transects perpendicular to the current direction. This made possible to determine the width and length of the plume.

3.4.4. Biological studies.

As part of PLUMEX studies, two different sources of data were used to determine the effect of the plumes on the nekton vertical migrations. The impact of the plume on the micro fauna and flora present on the ocean is another key uncertainty. As part of PLUMEX studies, two different sources of data were used to determine the effect of the plumes on the nekton vertical migrations.

Firstly, a Simrad EK80 fisheries acoustic sonar was gathering data before, during, and after the discharge of the plume. The sonar was working at 5 different frequencies: 18, 38, 70, 120 and 200 kHz. Each frequency is adequate for a different range of depths. Data from the sonar was recorded 24 hours per day, to make possible a comparison between the usual conditions in the experiment area, and under plume release effects.

Secondly, water samples were taken during the plume tracking phase using the Niskin bottles mounted on the Rosette rig. The samples were taken at different depths below, within and above the plume. Then, they were filtered using precombusted filters, and frozen at -80'C for later biological analysis to determine whether there is a significant trace of the plume effects. Also, water samples were taken in the area of the experiment before the first plume experiment.

74 Chapter 4 Polymetallic nodule mining economics

In the late 70s, polymetallic nodule mining was a promising and exciting new industry aimed to become a very important source of cobalt, nickel and copper. However, in the early 80s, metal prices were lower than expected due to a slow growth of minerals demand. Also, the improvements on land- based mining technologies resulted in a higher and cheaper supply (Knodt et aL., 2016). Nowadays, the decrease of land-based ore grades and the emergence of new and green technologies - which are increasing the demand of certain metals - have restored the excitement and interest in seabed mining. The lessons learned during the last forty years have shown the need of conducting deeper analysis and understanding of the economics, risks and uncertainties related to such a complex activity. Therefore, this chapter is focused on the elements required to build a thorough economic model, and the identification of the main sources of uncertainty.

4.1. Introduction

Economic models are a very common tool to analyze and forecast the economic feasibility of any business or economic activity. Because of that, they are one of the most important elements of the decision-making process of any investment. Models provide a number of metrics that pretend to inform the investors about the expected results of the investment in economic terms. In most cases, these metrics have a probability distribution associated to the number of uncertainties in place.

In the case of polymetallic nodule mining, economic models are an essential tool not only for potential investors, but for the ISA and other regulatory bodies. Investors need to develop their own economic models to analyze the risks and likely profitability of the investment under different scenarios. On the other hand, the ISA has to determine the royalties, environmental bonds or any other type of payment regime that they will include on the regulations. These payments will be either aimed to share the benefits within the common heritage regime, or to implement some environmental measures. In order to determine these royalties and fees, the ISA needs to understand the economics of the activity, and ensure that, even including these payments, polymetallic nodule mining is still attractive for the investors. In this regard, it is essential to consider the technological risks associated to the use of new technologies at a commercial scale, compared to land-based mining activities.

The elaboration of a complete economic model for seabed mining requires, at least, three main elements. The first one is a cost model to determine both the capital and operation expenditures. The cost model will depend on the technical solution adopted by the contractors. The novelty of this new industry and the use of new technologies, adds a significant uncertainty to the model due to the associated technological risks. The second element of the economic model is the revenue model. The revenue depends, basically, on two main aspects: the amount of metals recovered from the nodules and the metal prices. Finally, the third element is the computation of the expected cash flows for every year. These cash flows have to include all the project stages, from the prefeasibility analysis to the decommission. Therefore, they are not only focused on the operation phase, when the revenue is generated.

75 The economic model itself can be difficult to analyze, and the values obtained from the cash flows can be easily misleading. Undoubtedly, the best way to analyze the models is through the use of some useful metrics that can be calculated from the expected cash flows. Good examples of useful metrics commonly used are the Internal Rate of Return (IRR), the Net Present Value (NPV), or the Payback Period.

4.2. Previous economic models

Several economic models have been elaborated since the early 80s. Probably, the most relevant and detailed one was developed by Nyhart and Triantafyllou (1983) at MIT. The model, known as the MIT model, was designed as a frame to be used and modified by potential investors or interested parties with their own assumptions and estimates. It consists of two main sections. The first one discusses all the assumptions and estimates related to the costs, revenues and taxes. The second section, makes use of the results from the first one to conduct the financial analysis itself, including a cash-flow model integrated with different taxation and regulatory assumptions, and a simple revenue model.

As most economic models, the MIT model introduced a number of assumptions and estimates which have a significant associated uncertainty. The way the authors dealt with all the uncertainties was by calculating three different scenarios - optimistic, neutral and pessimistic - and conducting a sensitivity analysis. The sensitivity analysis concluded that metal prices and operating costs were the two aspects that had a greater influence on the IRR of the mining activity.

The MIT model divided the capital and operating costs into ten different sectors. As it is shown in Figure 4.1, the capital costs associated to the processing and mining infrastructure constitute two thirds of the total investment. On the other hand, almost half of the operating costs correspond only to the nodule processing plant. a) b) 1'0% 2% 0% 2

2% a Processing a Mining * Transport

G&A a Onshore transportation a Exploration. R&D v Ore discharge terminal

a Waste disposal a Marine support a Continuing preparations

Figure 4.1. a) Capital and b) Operating expenses distributionaccording to the ten cost sectors defined by the MIT model (Nyhart and Triantafyllou, 1983).

There are other examples of seabed mining economic analysis that have been published after the MIT

76 model. For instance, the United States Bureau of Mines elaborated an economic model in 1985 (Hillman and Gosling, 1985), including also a sensitivity analysis. The results showed a very low IRR; from 3.5% to 6.0%, below the values of up to a 9% obtained by Nyhart and Triantafyllou (1983). In fact, the authors highlighted that, with the forecasted IRR, venture capital investors would not be attracted, as it actually happened in the 80s and 90s.

In 2001, Soreide, Lund and Markussen (2001) published a study on the feasibility of polymetallic nodule exploitation in the Cook Islands EEZ. The economic analysis concluded that the investment was not feasible at all because of the fact that the obtained IRR was only 2.7%. However, according to their sensitivity analysis, the introduction of new processing technologies based on leaching - which started to seem feasible by that time - and an increase of the price of cobalt could change the scenario, obtaining an IRR of up to 30% according to their model.

Recently, in February 2018, Van Nijen, Van Passel and Squires (2018), published the latest economic assessment of a polymetallic nodule exploitation. The technical improvements and significant increase on the demand and price of cobalt mentioned by Soreide, Lund and Markussen (2001), are becoming a reality. Consequently, the interest in commercial polymetallic nodule mining is increasing. The economic model presented by Van Nijen, Van Passel and Squires is a good reflection of the state of the art of both technology and regulatory framework. Their model emphasizes aspects that were not even mentioned in past analysis, such as the royalty and environmental payments, or the selection of the processing plant location based on current energy prices. All this decisions have a significant influence on the economic result, and should be effectively integrated in the analysis. It is also interesting to see that, according to this model, the capital cost of the processing plant is a 60% of the total capital costs (Figure 4.2), compared to the 40% suggested by Nyhart and Triantafyllou (1983).

4Processing - Mining a Feasibility

Figure4.2. Capitalexpenses distributionofapolymetallic nodule exploitation accordingto the economic model proposed by Van Nyjen, Van Passel and Squires (2018). The processingplant includes the metallurgical processing of nickel, cobalt, copper and manganese. In this case, the horizontal transportis only part of the operating costs, as the authors have decided to charterthe transportvessels from an external company instead of buying them.

The basis of the economic assessment proposed by Van Nijen, Van Passel and Squires (2018) is directly related to the management of the uncertainty; the model is based on the Monte Carlo Risks Analysis (MCRA) methodology. This approach assigns a probability distribution of some kind to the

77 estimated variables, like the capital and operating expenses. Then, thousands of simulations using random values based on the defined probability distributions are run to determine the most likely result. In this case, the metric of reference is the IRR. According to the authors, the IRR has to be above what they call a "hurdle rate" of 18%, in order to attract potential investors considering the risks of the investment. In all the proposed scenarios, the obtained IRR was between a 15 to a 20%. In this case, the metals price forecast -one of the most important parameters according to most sensitivity analysis - is based on the historical one, five and ten-years moving averages.

4.3. Main assumptions, estimates and forecasts on polymetallic nodule mining economic models

All the economic models are built upon a number of assumptions and estimates to forecast the future economic behavior of the activity of interest. In the case of well-established economic activities, the uncertainty of most input data is very small, or even non-existent. Unfortunately, in the case of seabed mining activities, no commercial activity has taken place yet. Therefore, the risks and uncertainties associated to these new unproven technologies are considerably high. Furthermore, the fact that the revenue is directly dependent on the price of the metals, which are very difficult to predict, increases even more the overall uncertainty and complexity. However, in section 4 of this chapter some techniques to control and better understand these uncertainties will be addressed, as they are part of the new economic model developed at MIT.

4.3.1 General assumptions

The starting point to build the economic model is to first define a set of general assumptions that will frame the mining activity. In this regard, one of the main aspects is to decide whether the mining exploitation will be focus on the extraction of three or four metals. In the case of a three-metal system, only cobalt, copper and nickel will be considered. On the other hand, the four-metal system includes also manganese, which requires an additional metallurgy processing plant to extract it from the tailings after the extraction of cobalt, copper and nickel. Nowadays, most approaches consider a four- metal scenario (Van Nijen, Van Passel and Squires, 2018). However, some economic models analyzed both possibilities (Hillman and Gosling, 1985) with uncertain outcomes. The high content of manganese increases the complexity of price forecasting due to the effects that such addition of offer to the manganese market may have; an annual production of 3 million tons of dry nodules is translated into an increase of a 10% of the global manganese supply (Van Nijen, Van Passel and Squires, 2018). Consequently, the uncertainty of a four-metal system will always be higher.

The annual production is another relevant assumption that has a direct impact on the mining equipment and, consequently, on the capital and operating expenditures. Most authors consider annual productions of whether 1.5 or 3 million tons of dry nodules (Nyhart and Triantafyllou, 1983; Hillman and Gosling, 1985; UNOET, 1987; Van Nijen, Van Passel and Squires, 2018). In this regard, there is a certain consensus among ISA contractors and other experts that a minimum production of 3 million tons is required in terms of economic feasibility of any polymetallic nodule mining project (ISA, 2008).

78 Additional general assumptions are related to technical and logistical details. Aspects such as the metallurgy processing method, or the location of the processing plant, have a significant impact on both capital and operating costs. According to Van Nijen, Van Passel and Squires (2018), the location of the processing plant may have an impact greater than a 3% on the IRR. Energy price is the main responsible of such impact, but other elements like the horizontal transport costs are also affected by this decision.

4.3.2 Main estimates

Once the main assumptions are in place, it is time to estimate capital and operating costs of the main elements of the mining, transport, and processing systems. In most cases, the technologies are similar to those from other industries. Therefore, it is always an option to consider the capital and operating costs of these technologies from well-established industries, scale them correctly, and use them as a reference for the lower cost threshold.

In the first chapter, it was shown that an important number of technology transfers from the offshore oil and gas industry are part of most seabed mining concepts. The vertical transport system and the operation vessel are two good examples of this technology transfer. However, it is important to note that seabed mining operations occur at much deeper areas. This means that, for instance, the riser to lift the nodules up to the surface will have to withstand much higher loads because of its higher own weight, and its larger area exposed to the ocean currents. Furthermore, depth is responsible of a significant increase of the capital cost of any kind of equipment that has to be deployed at the seabed. In any case, the use of new technologies - such as the nodule collection vehicle or the singularities of very deep operations - and the fact that there are no previous commercial seabed mining exploitations as a reference, introduce a significant uncertainty in both the capital and operating costs. In this regard, to deal with this uncertainty, both the MIT and Van Nijen's economic models consider that the capital and operating costs may deviate up to a 25%, up and down, respect to their estimates. In the case of the MIT model, the authors built three scenarios: a base one, a downside and an upside. Van Nijen, Van Passel and Squires (2018) model - based on MCRA - introduces this uncertainty as a triangular probability distribution of the capital and operating costs that will be considered during the random simulations.

Other technical aspects, such as the nodule abundance in the mined areas, the grade of the nodules, or the recovery yield during the metallurgy processing would also have to be estimated. The first two values and their uncertainties can be determined by an adequate exploration of the mining areas. While the third one depends on the chosen metallurgy processing technology. From an economic perspective, there is special interest in the Cuprion process, which has a lower energy consumption compared to other proposed techniques. However, no previous commercial-scale plants have been built. Therefore, recovery yield rates, power consumption values and capital costs are based on a pilot test conducted by Kennecott Corporation in the 70s. The scale-up of these technologies is not always a straightforward process, and relevant modifications may be required.

79 The horizontal transport system is, probably, the less uncertain process. The maritime transport is a very well established activity, and there is a lot of data available about capital and operating expenses. From a technical point of view, the main challenge is related to the nodule dewatering system that will have to be included on board. According to most approaches, the nodules will be fluidized to be transferred from the operation vessel to the transport vessel (Flipse, 1969; Glasby, 2006), but all the water used for the transfer will have to be discharged minimizing the loss of nodule fines.

4.3.3 Revenue forecasting

It has been already highlighted that, according to several sensitivity analysis, any change on the metal prices - which determine the revenues of the activity - have the greatest impact on the overall economic result of a mining exploitation (Clark, Cook Clark and Pintz, 2013). Consequently, it is essential to develop reliable forecasts that account the high uncertainty of metals markets.

There are several ways to forecast the metal prices, but all of them are based on the analysis of historical prices. However, in most cases these forecasts are far from being reliable. There are a number of external factors that may have a significant impact on the markets. For instance, only in the last two years, the price of cobalt has increased a 438%. This increase is related to the high demand of cobalt for batteries (Zhang et al., 2017) - which are used by electric cars and renewable energy plants - and superalloys used for many high-technology applications (Hein and Koschinsky, 2013). Also, in the case of cobalt, it is crucial the fact that a 50% of the global supply comes from a single country: the Democratic Republic of Congo (Zhang et al., 2017). Because of that, any political or social instability in the region, may have a significant impact on the global supply of cobalt.

A very straightforward way to forecast cobalt future prices consists of only a linear regression of historical data. This alternative, and also previous studies on the cobalt market are the basis of the MIT model metals price determination. The problem is that it only captures the long-term trend of the market, but misses the relevance of any short-term variation. In the case of cobalt, this short-term changes have been significant in the past.

Van Nijen, Van Passel and Squires (2018) based their price forecast on the calculation of historical moving averages. In their model, they considered different scenarios using one, five and ten-years moving averages. Then, they used a uniform probability distribution from the minimum to the maximum obtained value as an input for the MCRA.

There are more sophisticated options like the short-term/long-term model proposed by Schwartz and Smith (2000), also known as the two factor model. It consists of the calculation of two parameters based on the historical prices of the commodity. One of the parameters captures the long-term trend of the market, and the other one the short-term variations.

Unfortunately, even the most sophisticated available options for price forecasting, can reliably accomplish their mission. The significant influence of external factors like technology improvements, or political and social instabilities, cannot be predicted by any model.

80 4.4. The new MIT model

The MIT model was a very good frame and reference for most economic analysis during the 80s. However, in the last 30 years the scenario has changed: the ISA is developing a set of regulations that include the payment of certain royalties and fees, and new technical solutions and improved modeling tools have been developed. As requested by the ISA, the Materials System Lab at MIT has developed a new economic model to serve as a basis for the development of the regulations. The model was presented at the 2 4 th annual session of the ISA in Jamaica, in March 2018.

The new MIT model is built as a MCRA, to better deal with the uncertainties. The results presented to the ISA were obtained after the calculation of one million of random alternatives for each of the proposed scenarios. One of the objectives of the model is to analyze the impact of ISA royalties on the economic result. The ISA has to make sure that, even with the royalties, the mining exploitation is still attractive for the investors.

The ISA is considering up to three alternatives to establish the payment regime of the royalties. The first option consists of basing the royalties on the mass of dry nodules extracted from the oceans. This alternative is the less risky for the ISA, as it does not depend on the metal prices. However, in case there is a significant increase on the prices, the ISA would not benefit from it. The second alternative is known as ad valorem. In this case, royalties are based on the value of the extracted nodules, instead of the mass. Consequently, there is a total exposure to the market prices, and the revenue obtained from the royalties may drop or grow in accordance to that. The last alternative is to base the royalties on the profits obtained by the mining companies. This alternative may seem reasonable, but the international nature of seabed mining makes difficult to control how each company calculates its own profit. Furthermore, it is well known that there are different accounting techniques to decrease the profits of any company by strategically allocating the expenses. Additionally, in the case of companies that also have other sources of incomes and expenses, it is not trivial to ensure they are only reporting the profits from the mining activities, and they are not considering non-related expenses in their calculation. As it has been shown, all the alternatives have their advantages and disadvantages. Because of that, it may be reasonable that a blend of two or even the three of them is finally used. The mass based royalty would ensure a minimum constant and certain revenue for the ISA, and the ad valorem and profit-based alternatives would ensure that the ISA benefits from the upsides of the market prices.

The capital and operating costs are estimated based on the analysis of previous economic models, and also on the values estimated by the different ISA contractors based on their knowledge. Furthermore, all the values were associated with a certain probability distribution to account the likely deviations from the estimates. In this regard, the costs of the metallurgical processing of the nodules account up to a 60% of the total capital and operating costs. The high cost of the processing plant opens the possibility of, instead of having a single company that owns and conducts both the extraction and processing of the nodules, having a polymetallic nodule market where there exist companies that only extract the nodules and companies that buy these nodules to process them. This is a good way to

81 decrease the risks associated with such an investment, especially if there is the possibility of using the plant to process other land or ocean-based ores.

As it has been already explained, the price forecast of nickel, copper and cobalt is very complicated because of the influence of external factors on both supply and demand. Additionally, as Van Nijen, Van Passel and Squires (2018) propose, some contractors are considering the option of selling the manganese at up to four different markets: high, medium and low-carbon ferromanganese, and electrolytic manganese. The reason why they are considering this, is because the introduction of all the manganese - over a 15% of the global demand - produced by a single polymetallic nodule exploitation, would significantly impact the market prices. The new MIT model price forecasting captures both short and long-term variations of the prices, but always based on historical data. In the next section, an alternative to better understand the markets and minimize the uncertainty will be addressed.

4.4.1 Analysis of a proposed economic scenario for polymetallic nodule mining

Using the new MIT model as a frame, it is possible to build different scenarios by introducing the required input values, such as the annual production, the capital and operating costs, or the metal content of the nodules among others. In this section, an scenario based on the data available on the literature is presented including the overall economic results forecasted by the model.

The first step is to define the main general assumptions of mining operations. In this case, a mine life of 25 years and an annual production of 3 million tons of dry nodules will be considered. The mining system will be composed by a nodule collection vehicle, a hydraulic vertical transport system, an operation vessel, three transport ships, a Cuprion - to recover nickel, copper and cobalt - metallurgy plant, and a manganese processing plant. From this point, the capital and operating costs (Table 4.1) are estimated based on the values found in the literature (Nyhart and Triantafyllou, 1983; Hillman and Gosling, 1985; Sharma, 2011; Van Nijen, Van Passel and Squires, 2018). The estimated input values for operating and capital costs include a triangular probability distribution that covers plus and minus a 25% of the defined central value. Similar probability distributions are used with other input parameters such as the annual production, which can be impacted by bad weather conditions or technical issues with any equipment, the mean grade of the nodules, or the recovery yield rates of metals during the metallurgical processing (Table 4.2). With all these inputs and associated probability distributions, a MCRA is conducted to forecast the economic results.

82 Table 4.1. Estimated capital and operating costs for an annual production of 3 million tons of dry nodules

Capital costs (million USD) Operating costs (USD/ton/year) Prefeasibility 35 Feasibility 325 - Collection system 95 9 Lifting system 285 22 Operation vessel 550 66 Transport vessels 150 23 Processing plant 2400 220 Total 3840 1020

Table 4.2. Metal grade of the nodules and recovery yield rates of the metallurgicalprocessing

Metal Nodules grade (%) Recovery yield (%) Manganese 29 95 Nickel 1.3 95 Cobalt 0.21 85 Copper 1.14 90

Regarding the royalty system, two different alternatives were compared: ad valorem system, and a blend of ad valorem and profit-based systems. The ad valorem alternative is chosen with a 2.5% royalty for the first 5 years, and a 5% for the rest of the operation life. In the case of the blended alternative, a 1% of the ad valorem revenue and a 1 % of the profit is applied during the first 5 years, and both values are multiplied by two for the rest of the mine life. Additionally, 1% of the revenue is paid to an environmental liability fund, with a maximum total payment of $500 million.

Table 4.3. Royalty alternatives consideredfor the economic analysis

Ad valorem system Blended system Ad valorem royalty years 1-5 (%) 2.5 1 Ad valorem royalty years 6-25 (%) 5 2 Profit-based royalty years 1-5 (%) - 2 Profit-based royalty years 6-25 (%) - 4

Overall, a total of 100,000 simulations were run considering the probability distributions defined for every input value in order to determine the most likely result of the defined seabed mining exploitation. From the perspective of the mining company and the investors, the most important outcomes from the model are the IRR and NPV. On the other hand, for the ISA, the focus is on the average annual royalty and the NPV of the royalty payments during the entire life of the mine. According to the results (Table 4.4), the proposed blended scenario results in a higher royalty payment and, consequently, a lower profitability for the investors. In both cases, the IRR is near the 18% threshold defined by Van Nijen, Van Passel and Squires (2018) to make sure the investment is attractive for potential investors.

83 Table 4.4. Results: IRR, average annual royalties, royalties NPV

Ad valorem system Blended system IRR (%) 18.2 17.7 NPV (million USD) 264 220 Average annual royalty (million USD) 108.3 138.2 Royalty NPV (million USD) 1439 1962

4.5. Future work

There are still some relevant analysis that would help to increase the reliability of economic models, as well as the understanding of the overall economics of seabed mining.

The first point to consider is the need of a deeper research on the technologies and their costs. Most models are still based on the estimates done in the early 80s, and should be updated considering the advances of technologies from the offshore oil and gas industry.

Additionally, it has been realized that most models follow the same assumptions and business model. However, it would be interesting to explore other alternatives like the possibility of separating the nodule extraction and the nodule processing. Then, a deeper analysis on the processing alternatives, and the possibility of using the processing plant to process other ores is needed. These options could increase the interest of potential investors.

It has been highlighted that metal prices are the variable with the highest impact on the economic result of seabed mining. A better understanding of the nickel, cobalt, copper and manganese market would be essential to build a more reliable supply and demand model that would help to assess the associated risks. The introduction of polymetallic nodules may have a significant impact on the global supply of metals, especially in the case of the manganese. Therefore, a more detailed model of the actual markets would help to estimate the likely impact of the nodules on them.

From the perspective of the ISA, two additional topics would be of interest. Firstly, the study of hybrid royalty schemes would be helpful to define the most suitable payment regime. Secondly, it is a responsibility of the ISA to understand and assess the potential environmental and social costs of seabed mining activities. A deeper research on this topic would be needed to guarantee that the regulatory framework is developed considering the outcomes of an adequate cost/benefit analysis.

84 Chapter 5 Vortex-induced vibrations. Preliminary analysis for polymetallic nodule mining risers.

5.1. Introduction

Vortex-induced vibrations (VIV) is probably one of the most studied flow instability-induced excitations (Eduard and Donald, 2005). The fact that this phenomenon affects most bluff cylindrical structures, such as offshore oil and gas platforms, has attracted significant research attention in the last decades.

VIV are directly related to the formation of a staggered array of vortices at a certain frequency in the wake of a bluff body. The flow around a bluff body easily separates even at very low Reynolds numbers (Re) (Triantafyllou et al., 2016). Any kind of small instability of this separated flow is the origin of this array of vortices, also known as a von Karman street. The frequency (f) of the vortex shedding - which depends on the diameter of the bluff body (d) and the flow velocity (U) - is given by the Strouhal number (St), which is St =fd/U. The Strouhal number may vary with the cross-section geometry of the bluff body, but it has been widely studied for most commonly used geometries (i.e: Bearman, 1969; Achenbach, E.; Heinecke, 1981; Okajima, 1982).

Figure 5.1. Vortex shedding around a bluff body creating a von Karman street (Bearman, 1984).

The alternate shedding of vortices results in an alternate pressure drop during their formation (Eduard and Donald, 2005). This dynamic variation of the pressure - which occurs with the frequency determined by the Strouhal number - generates an alternate force perpendicular to the incoming flow direction. As a result of this applied dynamic force, the bluff body is excited in the form of vortex- induced vibrations. Apart from this, the vortex shedding has also an impact on the drag coefficient of the bluff body, which can be significantly increased (Triantafyllou et al., 2016).

Vortex-induced vibrations are thoroughly studied for the design of offshore oil and gas risers. This type of cyclic forces may significantly reduce the fatigue life of the risers (Triantafyllou et al., 1999), especially if the vortex shedding frequency is close to the natural modes of vibration of the riser (Zheng et al., 2011). If this resonance phenomenon happens, the amplitude of the vibration increases

85 significantly and there is a significant risk of collapse. In the case of the offshore oil and gas industry, the collapse of the riser would generate an ecological disaster because of the consequent oil spill.

This phenomenon - and its consequences - also has to be carefully considered by the polymetallic nodule mining industry: the most likely vertical transport system for polymetallic nodule mining is a riser; a pipe that will connect the nodule collection vehicle at the seabed with the operation vessel at the surface. The riser - typically a cylinder with circular cross-section - is a bluff body immersed in a background flow constituted by ocean currents. Therefore, it is expected that vortex shedding will occur along the riser with the risk of having three main undesired consequences. The first two are directly related to the VIV: the decrease of the fatigue life and the collapse of the riser in case of resonance. In this regard, it is important to include VIV in the fatigue analysis of the riser, and ensure that the natural frequencies of the riser - in particular the first modes of vibrations - are not close to the vortex shedding frequency. Lastly, as it has been already mentioned, VIV are also related to an increase of the riser drag coefficient, which will have to be considered to calculate the propulsion power required by the operation vessel during mining operations.

It is important to note the complexity of the VIV analysis of a deep-sea mining riser. The riser will be excited by the background currents, which are far from being constant in magnitude and direction along the depth of the riser. Consequently, the riser will be excited in different directions and magnitudes. This complex distribution of exciting forces will result in a specific dynamic response of the riser. But to obtain the dynamic response, it is essential to determine the natural frequencies and modes of vibration of the riser. All these calculations have to consider not only the riser itself, but other aspects such as the added mass or the damping ratio. These parameters, which are usually obtained using experimental data, may also have a dependency on the vibration frequency.

5.2. The riser

The first step before conducting the VIV analysis is to define the characteristics of the riser. In this regard, there is a significant variability of the main parameters and characteristics of polymetallic nodule risers on the literature, and in most cases the parameters are based on the OMI and OMA pilot mining tests. A good example of that is the diameter of the riser, which can vary from 244 to 500 mm (Chung, Whitney and Loden, 1981; Hong, 1997; Oebius et al., 2001). In this section, the main parameters and characteristics of the riser are going to be discussed in order to choose a realistic combination for the analysis.

5.2.1 Type and material

The offshore oil and gas industry has been developing new riser technologies during the last decades. Therefore, there are a wide variety of solutions depending on the specific characteristics of the operation. Overall, the oil and gas risers can be classified into four main categories: free-standing, top-tensioned, catenary and flexible risers. Although, some authors include flexible risers as a sub- type of catenary risers (Miller, 2017). In the case of seabed mining, the bottom-end of the riser will

86 be "freely" hanging; only constrained by a flexible connection with the nodule collection vehicle. Because of that, none of these types of risers can be directly applied.

Very few studies have focused on the analysis of riser concepts for seabed mining. In most cases, the riser is mentioned as part of the mining system, but very few details apart from the diameter are provided. In general, the riser is described as a steel pipe hanging from the operation vessel (Bernard et al., 1987; Oebius et al., 2001), far from the sophisticated concepts of flexible risers with buoyancy modules that are commonly used by the oil and gas industry in deep waters. Handschuh et al. (2001) proposed the use of a flexible riser, but very few details were provided. Additionally, the authors determine an inner diameter of 80 mm, which is considerably lower than the recommended values to avoid clogging (Bernard et al., 1987).

For this preliminary analysis, a rigid steel riser hanging vertically from the operation vessel will be considered, agreeing with most proposed concepts. The boundary conditions of the riser are defined as pinned in the upper end and free in the lower end.

5.2.2 Main dimensions

The length of the riser is determined by the depth of the operation area. However, due to the fact that the risers are built in-situ using sections of a standard length, the overall length can be easily adapted. The lower end of the riser will be hanging at least a few tens of meters from the seabed. Considering the typical depths of the abyssal plains where the nodules are located, the total length of the riser for the analysis is 5000 meters.

The inner and outer diameters of the riser have also to be defined for the analysis. As it has been mentioned, the literature shows a significant range of optimal diameters and several studies are based on the values used for the OMCO pilot tests. The first aspect to consider is the need to minimize the risks of clogging. To do so, Bernard et al. (1987) suggest the use of inner diameters at least three times the expected diameter of the larger nodules found in the seabed. In this regard, it is true that some mining concepts include a first grinding stage on the collection vehicle, or a size selector that will expel the larger nodules to avoid clogging. As a starting point for the analysis, the values given by Hong (1997), Chung, Whitney and Loden (1981), and Oebius et al. (2001), were considered to define the diameter and thickness of the riser (Table 5.1). Also, two centrifugal pumps and the buffer located at the lower end of the riser are included as part of the riser system for the analysis.

87 Table 5.1. Main dimensions and characteristicsof the riserfor the VIV analysis. Some of the values were obtainedfrom Hong (1997), Chung, Whitney and Loden (1981), and Oebius et al. (2001).

Length (m) 600

Section 1 Outer diameter (mm) 450 Thickness (mm) 16.1 Mass per unit length (kg/m) 192.5 Length (m) 10 Pump 1 Mass per unit length (kg/m) 4760 Outer diameter (mm) 1500 Length (in) 1400 450 Section 2 Outer diameter (mm) Thickness (mm) 12.7 Mass per unit length (kg/m) 152.6 Length (m) 10 Pump 2 Mass per unit length (kg/m) 4760 Outer diameter (mm) 1500 Length (m) 3000 450 Section 3 Outer diameter (mm) Thickness (mm) 11.1 Mass per unit length (kg/m) 134.2 Length (in) 9 Buffer Mass per unit length (kg/m) 8300 Outer diameter (mm) 3000

5.3. VIV analysis

The VIV analysis will be conducted using the software VIVA developed at MIT by Professor Triantafyllou and his research group (Triantafyllou et al., 1999; Zheng et al., 2011). The software, as most VIV analysis tools, is based on strip theory and the use of a database with experimental data of hydrodynamic force coefficients. From this point, the structural response of the riser is calculated in order to determine the fatigue life of the riser, which is the ultimate objective of the software.

5.3.1 Methodology

The code includes a numerical methodology that accounts for the effect of the currents on the calculation of the hydroelastic natural frequencies of the riser. As the motion induced by VIV is small compared to the wavelength of the vibration itself, a linear model is used for the structure, accounting the mechanical properties of the riser. The solution of the riser response is obtained in the frequency domain, as a result of a non-linear eigenvalue problem. Second order finite differences are applied to the discretized riser to obtain non-linear equations. The equations are solved following three steps.

88 Firstly, the software calculates the natural frequencies of the riser with no background current. Then, the problem is solved iteratively using frequencies near the obtained natural frequencies in the previous step. Finally, after the iterative process, the hydroelastic frequencies are determined. Regarding the calculation of the excitation, strip theory is used on the calculation of the lift generated by the current; experimental hydrodynamic lift coefficients are used for such purpose. VIVA software introduces the possibility of having a response with a variable phase, which is a consequence of the current variability along the length of the riser.

5.3.2 Background flow

As noted by Triantafyllou et al. (1999), currents are the main cause of riser problems directly associated to VIV. Because of that, this preliminary analysis will only consider the effects of the flow around the riser, neglecting the effect of the operation vessel motion due to the waves. However, in the case of seabed mining operation, it is essential to also consider the motion of the ship during mining operations to follow the collection vehicle. Therefore, the overall flow will be the addition of the ship velocity - around 0.5 m/s - and the background currents.

The software considers the flow velocity perpendicular to the riser, and also its direction to capture the effect of shear in the results. The problem is that, this increases the complexity of modeling the background flow. The flow velocity due to the ship displacement is constant in magnitude and direction along the riser, but not the background currents. Consequently, the combination of both flows for the analysis is not trivial, as the effect of shear has to be considered.

In order to conduct a complete preliminary analysis, the following scenarios will be considered and compared:

- Scenario 1. Background current profile: in this first scenario the operation vessel is stationary, and a real current profile from the CCZ will be used to obtain the response of the riser. - Scenario 2. Constant flow of 0.6 m/s: analyzes the behavior of the riser under a constant flow, in magnitude and direction, along the riser. This is the result of adding the 0.5 m/s flow due to the ship motion and a background current of 0.1 m/s in the same direction. Of course, this is not a realistic scenario but, compared with the other scenarios, it will help to determine the relevance of shear. - Scenarios 3a, 3b, 3c and 3d. Background current profile and ship velocity: this is a combination of the current profile from the CCZ, and the flow velocity due to the vessel displacement. This scenario will include the analysis of four different vessel headings - north, east, south and west, respectively - under the same current profile.

89 ...... SOW441

244 ......

~2442 a S42 t I443 -Ol- V*L*"

Figure 5.2. Background current velocity profiles for some of the proposed scenarios. Originalcurrent data provided by Global Sea Mineral Resources.

5.4. Preliminary results

VIVA software provides very detailed results of the response of the riser, including a fatigue life- estimate. In this section, the most representative results for each scenario are presented. The first set of results compares the dominant mode of vibration, frequencies and amplitudes of the six scenarios (Table 5.2).

Table 5.2. Characteristicsof the dominant mode of vibrationfor each scenario.

Scenario Dominant mode Coupled frequency (Hz) Max amplitude (m) RMS amplitude (m) 1 2 0.0147 0.155 0.109 2 19 0.2142 0.232 0.164 3a 17 0.1925 0.203 0.144 3b 17 0.1818 0.414 0.293 3c 19 0.1692 0.459 0.325 3d 16 0.1777 0.313 0.221

Three main outcomes are directly inferred from the results shown in Table 5.2. As it was expected, the first scenario, which has the lowest flow velocities, also has the lowest values of vibration amplitude due to VIV with a maximum amplitude of 0.155 meters. Therefore, this indicates that the flow velocity component due to the displacement of the ship has a major impact on the severity of VIV. Additionally, The maximum amplitude of the second scenario is 33% smaller than the average maximum amplitude of scenarios 3a-d. This is a good indicator of the effect of shear - which clearly increases the severity of VIV - due to the variability on the current direction along the riser. Lastly, the comparison of the last four scenarios leads to the conclusion that the ship heading may have a significant influence on the results as they vary from 0.203 meters to 0.459 meters of maximum amplitude, even though it constitutes the dominant component of the overall flow velocity.

90 The results from the fatigue analysis, summarized in Table 5.3, clearly show that the flow velocity due to the operation vessel displacement has a major impact on the fatigue life of the riser. In any case, the main outcome from the obtained results is that VIV are a major concern for seabed mining risers.

Table 5.3. Estimatedfatigue life for the six proposed scenarios.

Scenario Fatigue life (years) 1 6495 2 0.40 3a 0.29 3b 0.46 3c 0.36 3d 0.26

It is also remarkable that, even though the vibration amplitude of the second scenario was a 33% lower than the average amplitude for scenarios 3a-d, the fatigue life of the second scenario is only a 16% higher than the average for scenarios 3a-d. This is a direct consequence of the fact that the calculation of the fatigue life accounts all the excited modes of vibration, not only the dominant one. In Table 5.2, only the results for the dominant mode are shown. But, it turns out that other vibration modes are also considerably excited under the conditions of the second scenario, where the current was uniformly distributed.

This preliminary analysis is only the very first step of the design process of seabed mining risers, but it has the intention of highlighting the relevance of VIV for risers operating in deep waters. From this starting point, it would be recommended to explore other riser configurations - like flexible risers - and materials. Additionally, the use of VIV suppressors, such as strakes or fairings, should also be considered in order to ensure an adequate and safe fatigue life of the riser.

5.5. Future work

A future analysis should include the consideration of different riser configurations, including flexible risers made of different materials, and including with buoyancy modules to decrease the mean stress level of the riser material. Also, to improve the fidelity of the study, it would be possible to model the effect of the flexible connection between the nodule collection vehicle and the buffer. This can be modeled adding a spring and a dashpot at the lower end of the riser.

The oil and gas industry has developed very effective technologies to eliminate VIV on risers. The two most common solutions are the addition of strakes around the riser and the use of fairings (Bearman, 1984; Vandiver, 1998; Triantafyllou et al., 1999; Vandiver et al., 2009). However, they increase the cost of the riser, and the complexity of its deployment and recovery. Because of that,

91 they are only installed at certain segments of the riser, where the vortex shedding has a greater impact on the overall VIV.

Another aspect to improve is the availability of full-depth currents historic data from potential seabed mining areas like the CCZ. For this preliminary analysis, data from a specific research cruise in the CCZ has been used. But it would be more adequate to be able to reproduce the most likely conditions, and also the most severe ones.

One of the observed consequences of VIV is an increase on the drag coefficient of the riser. In the case of seabed mining, this would result in an increase of the propulsion power required by the mining vessel during the operations. Therefore, this is an aspect that should be addressed and included in the analysis of the power requirements of the vessel to design and optimize the power plant.

The fatigue life has been determined based on the use of a specific fatigue curve, which includes the use of experimental parameters. A deeper analysis on the most suitable fatigue model and parameters values would be essential. This is especially relevant in the case of using other materials less studied than steel.

Lastly, according to the studies conducted by Hong (1997), the axial vibration natural period of a seabed mining riser is around six seconds, which is within the range of waves period. Therefore, the ship response due to the waves, and in particular the vertical absolute motion, may excite the riser in a frequency close to the natural one, with a significant risk of resonance. Even though this is not directly related to VIV, the analysis of the effects of high and low frequency wave-induced motions of the vessel is a relevant study that should be included to guarantee the safety of seabed mining risers.

92 Chapter 6 Conclusions

6.1. Future perspectives of polymetallic nodule mining

Polymetallic nodule mining is, apparently, closer to become a reality than ever. The increasing interest of the main stakeholders is now tangible: the ISA is seeking to approve exploitation regulations in the next two years, up to seventeen ISA contractors have been licensed to explore some areas of the CCZ, the European Commission is funding "Blue Mining" project to support industrial and academic research on the topic, and the offshore industry is working on key technical developments to adapt current offshore technologies to seabed mining activities. However, a number of technical, environmental, and economic challenges still have to be overcome in the next few years to ensure the success of this new metal supply industry.

It is important to note that the ultimate objective of polymetallic nodule mining is to become an alternative source of metals required for advanced and green technologies, essential for the world's sustainable development (Hein et al., 2013). In this regard, nickel and cobalt play a main role; nickel- based superalloys are used in many high-technology applications, and all the batteries needed to substitute conventional vehicles for electric ones, or to back-up the use of renewable energies, need a significant amount of cobalt. Nowadays, the demand of cobalt cannot be satisfied only by recycling, and the grades of land-based ores are continuously decreasing. This decrease of the grades results in a higher environmental impact to obtain the same amount of metals: larger land areas are devastated and more pollution is generated. Furthermore, several NGOs are concerned about mining conditions - mainly because of the lack of safety and the use of child labor - in one of the main global suppliers of cobalt: Democratic Republic of Congo. Because of all this, seabed mining is an attractive alternative. However, it is essential to develop a deeper knowledge in certain areas to minimize the impacts on the delicate deep-sea environment.

In a few years from now, some of the fifteen-year exploration licenses issued by the ISA will have expired, the ISA will have already approved the exploitation regulations, and the first exploitation licenses will be issued. From that point, it will take around three years to build all the elements required to start the production. But, before all this happens, some urgent research gaps and technical challenges - most of them identified in the first chapter of this thesis - will have to be solved. In this regard, this thesis has addressed three of the most pressing challenges on three different areas: environment, technology and economics.

Polymetallic nodule mining is not only a good potential source of supply of very demanded raw materials, but a good chance to improve the knowledge about the deep ocean and its ecosystems and dynamics. All the research attention attracted by this topic is being already transformed into a better understanding of the ocean thanks to the collected field data and the models built to study the impacts of seabed mining.

93 6.2. Technical challenges

Polymetallic nodule mining implies a wide range of technical challenges: from the design and construction of a reliable mining system to the scaling of metallurgical processing technologies. However, there are certain aspects that will require a special attention in the short time because not much research effort has been invested into them in the last decades.

In this regard, the design of the riser is one of the aspects that will require a deeper analysis. It is important to note that, nowadays, the deepest offshore operations using risers are occurring at 3,000 meters of depth, which is about half of the required depth for seabed mining. Among the technical challenges that will have to be solved, VIV stands out because of its complexity and consequences. Because of that, the fifth chapter of this thesis was dedicated to conduct a preliminary VIV analysis of a simple seabed mining riser configuration. The results showed that VIV may reduce the fatigue life of the riser to less than one year. The main responsible of the VIV severity is, undoubtedly, the operational speed of the mining operations vessel. The vessel will have to follow the nodule collection vehicle, which is expected to drive at speeds of about 0.5 m/s. Therefore, the entire mining system - including the riser and the operation vessel - will have to displace at that same velocity, which is significantly higher than the usual background currents in the CCZ. Future studies should include the analysis of more sophisticated alternatives, such as flexible risers with buoyancy modules and maybe made of other materials than steel. Additionally, it would be of interest to analyze the possibility of including VIV suppressors as the offshore oil and gas industry does to minimize the impact of VIV on the riser fatigue life.

Some prototypes of nodule collection vehicles have been already tested. Undoubtedly, reliability and robustness are two of the most desired characteristics for such a vehicle, as the entire mining operation relies on it. Therefore, it is important to minimize the risk of failure during the design phase by reducing the complexity and number of moving parts and electronic systems. From the environmental perspective, it would be very interesting to investigate different alternatives to separate the sediment from the nodules before their vertical transport. This would decrease the amount of sediment that will have to be returned back to the ocean. At this point, separation systems have not been deeply studied, and most concepts are based on the use of a mesh to separate part of the sediment. Consequently, the feasibility of other passive technical solutions such as the use of hydrocyclones should be analyzed.

Regarding the metallurgical processing of the nodules, many alternatives have been proposed in the last decades. ISA contractors have payed special attention to the Cuprion process because of its lower energy consumption. However, as noted by Haiki, Okazaki and Ishiyama (2015), the use of carbon monoxide and the low rate of cobalt recovery may suggest that other alternatives should also be considered. One of the main challenges of the metallurgical processing is the scaling up from the already tested pilot plants, to commercial-scale plants. This is, undoubtedly, a field with a significant margin of improvement and optimization. It is important to remark that metallurgical processing accounts more than half of the total capital and operating costs of a seabed mining exploitation. Therefore, any small improvement may have a significant impact on the overall economic result.

94 6.3. Environmental challenges

It is true that seabed mining operations will have a significant impact on the local environment, as it occurs with land-based mines. Polymetallic nodule mining is based on picking up polymetallic nodules from the seabed, where they constitute the only hard substrate that make possible the life of different kinds of species. This, of course, is an unavoidable impact, but there are other potential impacts that may affect larger areas and can be significantly minimized. However, to achieve this objective, it is essential to make an additional research effort before the environmental regulations are set by the ISA.

In the first chapter of this thesis, the main research gaps from the environmental perspective are identified. Furthermore, the second and third chapters are focused on one of the most pressing environmental challenges: sediment plumes created by seabed mining operations. The lack of seabed mining plumes field data is the reason why the second chapter is focused on the review of past field studies related to sediment plume phenomena of different nature. These phenomena present some characteristics that are similar to those of mining plumes and, therefore, they can be used as a starting point to build and validate plume models. The third chapter goes a step beyond this to fill the lack of field data: the planning and execution of PLUMEX field experiments in the Pacific Ocean, where six plume experiments were conducted. The preliminary results of the experiments will be presented during the ISA meeting in July of 2018. During PLUMEX experiments the plumes were created and then monitored in the near, intermediate, and far-field with the objective of not only validating the models, but obtaining a better understand of the behavior of the plume and its interaction with the background environment. Some biological studies were also conducted during the experiments to determine the effects of the plume on the vertical migrations of nekton, and also on some other biomarkers. All this data will be helpful to assess the potential impact of the plumes and build reliable models that will help to optimize the designs in order to minimize the impacts. PLUMEX experiment was also a good opportunity to test different monitoring technologies. This is a very important aspect, as it is expected that the ISA will require the contractors to implement an adequate environmental monitoring program. This is, undoubtedly, a field that will require further work to develop the adequate technologies to monitor vast and deep mining areas. Of course, marine unmanned vehicles will play a significant role in order to minimize costs and extend the monitoring capabilities.

Likewise, bottom plumes created by the nodule collection vehicle are also of great interest, as they may have a significant impact on the benthic environment. The development of models and field studies in the next few years will be essential to minimize their effects and extent.

The management of the environment in areas of interest such as the CCZ, has to be thoroughly analyzed. In this regard, the ISA has already taken some measures that should be backed by adequate scientific analysis, such as the introduction of certain protected areas where it will not be allowed to conduct mining operations. Additionally, in order to justify the exploitation of polymetallic nodules in the deep ocean, it would also be of significant interest to compare the impacts with actual land- based mines to determine whether it is adequate or not to start the operations.

95 6.4. Economic challenges

The complexity, novelty and scale of polymetallic nodule mining activities are the main reasons why there are also significant economic-related challenges to overcome. The overall mining process, from the nodules lying on the seabed to the processed metals ready to be used by the industry, has a significant number of components that are already complex and challenging by themselves. From the economic perspective, seabed mining has a double objective: it has to be economically feasible and attractive for the investors, and at the same time it has to benefit and compensate the humankind.

From the investors point of view, seabed mining has many associated risks due to its technical novelty and complexity. Because of that, it is essential to understand not only the process as a whole, but the singularities of every step in order to conduct a thorough analysis. In this regard, economic modeling is a helpful decision-making tool as it was shown in the fourth chapter of this thesis. The main objective of the new MIT economic model is to provide a frame that captures the most significant technical singularities and alternatives. Then, it is possible to compare the economic result of different technologies, approaches and scenarios. The results from the studies will, hopefully, justify or at least help to understand the risks and uncertainties. Probably, one of the most relevant risks is related to the metallurgical processing plant. In the fourth chapter it was noted that most of the available studies indicate that the processing plant will account more than half of the total capital and operating expenses. Therefore, it is essential to analyze and compare the wide variety of available alternatives - and how they are scaled up from the pilot or laboratory tests to the commercial scale operation - to determine which one is more adequate considering not only the costs, but also the risks. For instance, a possible way to decrease the risk of the investment on the processing plant is to select a processing alternative that could be used or easily adapted to process ores from other land-based or ocean-based mines. It was also noted in the economic model chapter that it would be essential to study the impact of seabed mining on the metals markets and, in particular, in the manganese markets. In order to do so, the first step would be to analyze the structure, costs, and characteristics of the actual land-based supply markets.

From the perspective of the common heritage of the humankind, the ISA is the institution that has to determine the economic benefit - in the form of royalties - that the contractors will have to share with the humankind. This is a very complex task, because a lot of factors have to be accounted. On the one hand, the ISA has to consider the potential environmental harm that will be generated by mining operations, and the associated cost to the humankind. Also, it is important to highlight that the nodules are not owned by any particular country or group of countries. Consequently, the ISA has to determine how to benefit all those countries that do not take part on the mining activities. In this regard, it is very likely that most, if not all, the benefits from the royalties will be invested in developing countries. On the other hand, the ISA has to ensure that, apart from befitting the humankind, seabed mining operations are attractive for the investors. Otherwise, seabed mining in international waters will never take place. Overall, the ISA needs to find a delicate balance to satisfy both the humankind interests and the economic feasibility of the investment.

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