Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"

CHALLENGES OF DREDGING IN THE ARCTIC AND OTHER DEEP OCEAN LOCATIONS

R. E. Randall1 and C. K. Jin2

ABSTRACT Dredging in the Arctic Ocean is challenging due to ice cover, permafrost, iceberg scour, whaling season, ice gouging, and remote location. In the deep ocean 1000 m (3280 ft), dredging is a technique for recovering minerals from the deep ocean waters where water depth is a major challenge for the pumping system. Another application for dredging in the deep ocean and beneath ice covered waters is for recovering petroleum reserves. This paper reviews the recent literature of current systems used for deep ocean mining and the need for developing oil and gas development beneath ice covered water. Conceptual ideas are discussed for overcoming the dredging challenges that include the use of remotely operated vehicles, trenchers, hopper dredges and self-propelled cutter suction dredges.

Keywords: Dredging, trenching, arctic, ocean mining, deep water.

INTRODUCTION The demand for essential industrial resources, such as oil and gas, has been increasing, which accelerates the decrease in such resources on land and in coastal regions. Accordingly, expanding the exploration to harsh environmental regions, especially the Arctic and deep oceans, turns out to be a promising solution since they are far less developed. For example, it is reported that the area north of the Arctic Circle comprises 13% and 30% of the undiscovered oil and gas in the world, respectively (Gautier et al. 2009).

As one of the most important applications in a severe ocean environment, dredging has been utilized to excavate material and minerals, or even improve the environment (Bray et al. 1996). Various innovative concepts for dredging have been developed, including the usage of remotely operated vehicles, trenchers, trailer suction hopper dredges (TSHDs), and self-propelled cutter suction dredges (CSDs).

The major difficulties of developing reserves in Arctic Oceans are to protect pipelines from ice and sensitive climate, which can be accomplished with trenching techniques. Considering that repair costs for downtime occupy up to 75 % of the capital expenditure (CAPEX) related to pipeline facilities (Vershinin et al., 2008), it is essential for the pipeline to be protected from possible hazards. In particular, the damage of subsea pipelines from ice gouging, strudel scour, and upheaval buckling should be well managed (Abdalla 2008; Jukes et al. 2011). The primary way is to bury pipeline below seafloor through trenching, where the water depth and trench depth are the main factors. Currently, conventional excavation, hydraulic dredging, ploughing, jetting, and mechanical trenching have been used as the typical trenching methods in Arctic areas (Paulin et al. 2014a). In addition, unique concepts have also been suggested, such as the arctic subsea bucket ladder trencher (Vaartjes et al. 2012).

Dredging in the deep ocean is technically and environmentally challenging. Dredging in deep water calls for the improvement in current equipment, which leads to an increase in capital expenditures (CAPEX). Besides, the maintenance of dredging equipment working under such conditions can result in increased operating expenditures (OPEX) and environmental disruption (Stuifbergen 2012; Vershinin et al. 2008). Furthermore, environmental aspects could not be ignored as well. For example, building islands and developing oceans or lands from dredging may destroy natural habitats and prohibit restoring the environment or recreating habitat. Therefore, both eco- friendly equipment and safe operations are required (Bray, 2008).

Actually, dredging is an essential technology in deep water for oil and gas production as well as for ocean mining. Firstly, dredging has been used in the offshore oil and gas industry to prepare for the development of petroleum

1 Professor and Director, Center for Dredging Studies/Haynes Coastal Engineering Laboratory, Texas A&M University, College Station, Texas 77843-3136, USA, T: 979-845-4568, Email: [email protected] 2 Ph.D Candidate, Texas A&M University, College Station, Texas 77843-3136, USA, T: 979-204-3454, Email: [email protected]

345 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" reservoirs and the installation of subsea components such as subsea manifolds and separators. Since rugged seafloor topography can have a negative influence on the installation of these components, dredging is often performed to make the seafloor more flat (Stuifbergen, 2012). Secondly, dredging in the deep sea for mining is also used given that such materials near shore have a tendency to be exhausted. For example, sea sand acquired from the deep ocean turns out to be a promising and feasible substitute for sand on land or coastal areas (van Duursen and Winkelman 2011a; van Duursen and Winkelman 2011b). The objective of this paper is to review current challenges of dredging and dredging technologies that can be applied to the Arctic and deep ocean locations. Innovative techniques that include the usage of trenchers, trailer suction hopper dredges, self-propelled cutter suction dredges, and remotely operated vehicles are discussed.

TRENCHING IN ARCTIC REGIONS

Challenges for Arctic Subsea Pipeline and Trenching

Arctic subsea pipelines are exposed to a harsh environment mainly because of the presence of ice. Possible environmental loads that pipelines may experience are the result of ice gouging, strudel scour, thaw settlement, and upheaval buckling (Paulin et al. 2014b). The formation of each environmental load and the corresponding influences are explained. Moreover, even though trenching can be applied to eliminate the loads, it also shows potential difficulties related to a regional location, operation, maintenance, expense, and environmental issues, which is also discussed.

Ice gouging is formed by either icebergs or sea ice ridges. The iceberg comes from the breaking off of a glacier, and the ice ridge is formed by wind and current forces that act on ice movements to pile up sea ice (Barrette, 2011). When the depth of an ice keel is greater than the water depth, gouges can be developed as a result of lower parts of an ice keel contacting the seafloor. Ice gouging can exert substantial loads of 10 to 100MN on the seafloor (Kenny et al. 2007; Palmer and Tjiawi 2009), which can result in the pipeline damage. Since changing the design of pipeline to guard against the loads is impractical in high loading conditions, trenching and pipeline burial is often used to protect subsea pipelines. It is reported that the trench depth of roughly 6m is sufficient to protect pipelines (Vaartjes et al. 2012).

Strudel scour is generated by bottomfast ice. The bottomfast ice sheet is normally formed in near-shore arctic areas during the freezing season. When river flows meet the bottomfast ice sheet during melting season, special flows passing through holes in the bottomfast ice sheet are created. If the speed of the seawater flow is high enough, it leads to scour of seabed sediments with high hydrodynamic loads acting on subsea pipelines (Reimnitz 1974; Paulin et al. 2014a). The strudel scour tends to take place in offshore area close to river deltas in water depths of 2 - 9m (Leidersdorf et al. 1996).

Thaw settlement is another significant challenge for arctic subsea pipelines. Since permafrost is not uniformly distributed in space and time, there is uneven temperature distribution. When production of hydrocarbons is conducted in permafrost, the temperature of surrounding area of the pipeline increases. These local rises in temperature cause permafrost thawing, creating local thaw bulbs (Abdalla et al 2008). In this condition, pipelines experience significant overstress and bending strain (Lanan and Ennis, 2001). As a result, trenching is required to remove the permafrost and replace the area with stable materials to guard against a thaw (Paulin et al. 2014a).

Upheaval buckling of a pipeline takes place when the temperature and pressure of an operating pipeline are higher than that in the installation period. Since pipeline movement is constrained because of surrounding soil, the axial compressible load is inevitable. This condition leads to the longitudinal expansion of the pipeline and making the pipeline move upwards. This phenomenon referred to as upheaval buckling. In this case, the pipeline may be exposed to seawater; thus, insufficient trenching can lead to additional damage by the ice gouging. Even though it is not a unique phenomenon in arctic regions, the temperature difference in arctic areas merits more emphasis than that in other ocean conditions (Abdalla et al. 2008; Paulin et al. 2014b).

Pipeline trenching is one of the optimum methods to prevent the ice gouging loads on pipelines in Arctic area. However, it also has several difficulties because of the severe Arctic environmental conditions. The areas are remote from land and generally covered by ice even during summer. In addition, highly developed low light cameras may

346 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" be needed to conduct trenching due to darkness even during daytime. Climate conditions also lead to challenges with respect to operation and maintenance of trenchers, and the utilization of high performance trenchers. Difficulties due to the transport of dredging equipment to the Arcttic area leads to increased CAPEX and OPEX as well as trenching lead time. Since environmental issues cannot be ignored, trenching should be conducted with a minimum emission of working fluid, COX, and NOX to environment while also preserving the local ecosystem (Jukes et al. 2011).

Current Trenching Methods in Arctic Locations

Since each trenching method has its own unique characteristics, selecting an appropriate method for trenching is essential. Various kinds of trenching methods are now discussed and are focused on not only principles of trenching but also on primary and secondary considerations. The primary considerations are water depth and trench depth; while the secondary considerations are seabed geology, seabed slope, backfill method, and environmental sensitivity (Paulin et al. 2014a).

Conventional Excavation Conventional excavation uses trenching by means of a hydraulic baackhoe and clamshell bucket dredges as shown in Figure 1. Normally, the trenching machines are operated on a barge, which maintains its location by using spuds. When trenching is undergoing shore crossing regions, a berm is buillt in the near-sshore region that is an alternative to a barge. However, it should be noted that using a long-reach or extended backhooe only works with a combined 15- meter water and trench depth, and often takes a longer working period (Paulin et al. 2013; Paulin et al. 2014a).

Figuure 1. Hydraulic backhoe ‘OPTIMUS’ (Wasa Dredginng Ltd. 2015).

Ploughing Figure 2 shows one of the multi-pass ploughs. Ploughs have been used for the past 30 years as primary equipment for trenching. It is normally towed with a large tug, a derrick barge, or pulley systeem from onshore. This method can be applied to both post-lay and pre-lay trenching. Because of its relative high trenching speed, it is normally used for long pipelines. The design and size of a plough are determined by the required force to pull the plough, which is normally calculated by trench depth and the type of soil (Paulin et al. 2013; Paulin et al. 2014a). Single-pass ploughs could reach a trench depth of 2 to 3 m, and multi-passes ploughs can achieve a maximum trench depth of 8 to 10m, with the trench width of 8 m (Jukes et al. 2011). Because there is no limitation of water depth, current ploughs can be applied in a water depth up to 1,000 to 3,000 m (Paulin et al. 2014a). Normally, ploughs have a relatively high speed of 400 to 500 m/hr, with a maximum speed of 800 to 1,100 m/hr, with consideration for the trench depth and the type of soil (Deng et al. 2010; Jukes et al. 2011).

347 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"

Figure 2. Multi-pass plough ‘AMP 500’ (Deep Ocean Group 2015).

Jetting Jetting is a trenching method that uses either jet sleds or jetting ROVs as shown in Figure 3. Jet sleds and jetting ROVs are equipped with a system for injecting high-pressure water to break the soil and track the designated route to remove the sediment on the seafloor. Equator or ail-lift pumps are utilized to transfer the excavated sediment (Paulin et al. 2014a). Since the maximum trench depth for single-pass jetting is 2.5 to 3 m, it is feasible to achieve the trench depth of 6m using multi-passes jetting that has a trench width of 2 to 3m. Jetting can be utilized in deep water up to 3,000 m, and the jetting rate for the trench depth of 2 m is approximately 300 to 400 m/hr depending on soil types. Careful operations are required for multi-passes jetting to prevent damage to the pipeline (Jukes et al. 2011).

Figure 3. ROV trenching unit ‘T1200’ (Helix Energy Solutions Group 2015).

Mechanical Trenching Mechanical trenching is divided into two main types, which are barge-mounted chain cutter and crawler style trencher. Figure 4 shows the chain cutter trencher. Normally, mechanical trenchers are often controlled remotely. The barge-mounted mechanical trenchers have been efficiently operated in the relatively shallow water depth of 100 m or less while the crawler type trenchers have been used in the water depth of 1,500 m. The trench depth for single- pass mechanical trenching is 3 to 4 m, and the maximum trench depth is roughly 7 m by means of multi-passes the trenching with the maximum trench width of 2 m. Mechanical trenching can be used not only to hard soil but also to soft cohesive and non-cohesive soils. However, using mechanical cutters in sand is not a proper application. Careful operations of mechanical trenchers are necessary to protect pipelines. (Jukes et al. 2011; Paulin et al. 2014a).

348 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"

Figure 4. Chain cutter trencher ‘T3200’ (Deep Ocean Group 2015).

Dredging Equipment Historically, two hydraulic dredges, cutter suction dredges (CSDs) and trailing suction hopper dredges (TSHDs), have been frequently uses to conduct arctic trenching, which is shown in Figure 5. The trench depth of these dredging vessels is more than 5 m, and the maximum trench width is more than 10 m (Jukes et al. 2011).

The CSD breaks hard materials through a rotating cutting tool located in front of the suction inlet. The broken materials are transported from the seafloor through a pipeline to barges that are offloaded to open water placement areas located a hundred meters away from dredged areas to prevent the accumulation of dredged material (Tang et al. 2009; Paulin et al. 2013). The CSDs are used in shallow water with the maximum depth of approximately 30-35 m (Jukes et al. 2011; Paulin et al. 2014a).

The TSHD erodes the sediments through the use of a drag head and the pumps on the drag arm and dredge and transfers the dredged material to the hopper or through a pipeline to the placement area. The dredged materials are stored in hopper or discharged into the sea through a pipeline connection on the bow (Jukes et al. 2011). The TSHDs can be applied up to the water depth of 155 m. However, more poowerful vessel is necessary to operate trenching with long drag arm. A wide trench depth is normally achieved since the suction pipe has flexibility (Paulin et al. 2014a).

Figure 5. Dredgers. Left: Self-propelled cutter suction dreddger ‘D’ARTAGNAN’ (Royal IHC 2015a), Right: Trailer hopper suction dredger ‘1000 M3’ (Hydromec Maritime Solutions 2015).

Recently, a new dredge has been introduced, named as the arctic subsea bucket ladder trencher (ASBLT). Significant improvements in the bucket ladder trencher have been acchieved, which can be applied in Arctic locations as shown in Figure 6 (a). The proposed concepts are the “compact ladder” and “triangular ladder” trenchers. The

349 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" compact ladder trencher has a smaller size and less weight than normal bucket ladder trenchers. Meanwhiile, the triangular ladder trencher guarantees a better bucket emptying ability by leaving more space and time, which increases the possibility for buckets to be emptied. The scraperr enhances the emptying operation. The bucket emptying process is conducted in subsea by using a screw conveyoor passing through an actuating wheel. Since only the submerged weight of soil is considered, a significant increase in trenching speed can be obtained. For pipeline handling, a twin ladder method is suggested to cut a trench as shown in Figure 6 (b). This method conducts trenching without damaging pipeline by tilting the trencher. ASBLT are designed to perform trenching at the trenching depth of 6m and trench speed of up to 70 m/hr (Vaartjes et al. 2012).

(a) Left: compact ladder trencher (b) Twin ladder method Right: triangular ladder trencher Figure 6. Principles of arctic subsea bucket ladder trench (Vaartjes et al. 2012).

DREDGING IN DEEP WATER LOCATIONS

Technical Challenges for Dredging in Deep Water

Even though, dredging in deep water for ocean mining and production of oil and gas reserves is promising, performing dredging in deep water is very challenging. The major difficulties in deep sea dredging are now discussed, such as cost for equipment, site exploration, excavation, transportation, etc. (Grima et al. 2011).

Dredging in deep water is conducted with a huge investment. Highly technical machines and suitable materials, which are normally expensive, are required to endure the harsh conditions. Risers to transport the dredged materials to a surface support vessel (SSV) and umbilical to supply electricity to pump and other machines are manufactured with an extremely long length. Anti-fatigue and anti-corrosion mateerials for risers and other pipelines must be used to prevent fatigue and corrosion. Moreover, ocean going dredges are normally large, and the dredging equipment such as pumps, cutters, buckets and pipelines, is only practically utilized during dredging. These factors can lead to an increase in both CAPEX and OPEX (van Duursen and Winkelman 2011a; van Duursen and Winkelman 2011b).

Conducting site exploration and excavation are difficult. Highly developed systems for seafloor bathymetric surveys are required. ROVs and autonomous underwater vehicles may be necessaryy for high-efficient operation and positioning of seafloor mining tool (SMT) (Stuifbergen 2012). Seafloor mining tools should be manufactured to not only endure environmental loads but also be provided enough poweer to perform well in deep water.

Transportation is one of the main costs in conducting dredging in deep and ice covered water. Risers, which are normally long, are designed to endure hydrodynamic loads contributing to displacements and bending moments of pipes/risers. Effects of arrangements of pumps on displacements and bending moments of pipes must also be considered (Grima et al. 2011). Pumps require higher power to deliiver dredged materials to surface support vessels (SSVs); thus, series and parallel connections of several pumps are feasible solutiions (Vercuijsse and Lotman 2010, Vercruijsse et al. 2011). In addition, since places in which dredginng is conducted are normally far away from land,

350 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" the development in transportation methods can also be important to save time and cost. Inappropriate transportation methods can make SSVs being “dead cargo” (van Duursen and Winkelman 2011a).

Environmental aspects should not be ignored. It is clear that dredging activities will change the environment, and it has been reported that inappropriate dredging activities have negative environmental influences on aquatic ecosystem (Bonvicini Pagliai et al. 1985; Brown et al. 1990; Stronkhorst et al. 2003). For example, during excavation and transportation, different soil layers can be mixed, and dilution can take place between dredged materials and seawater. Moreover, CO2 gas emission and a noise generated by dredges and other machineries can also have a negative effect on environment (R.N. Bray 2008). As a result, it is a prerequisite to conduct comprehensive investigations of environmental impacts with respect to both long-term and short-term effects (CEDA 2009).

Technologies for Deep Ocean Dredging

Appropriate selections of dredging equipment in deep sea are directly connected to the efficiency of dredging. Various dredging concepts have been developed in deep water that include hydraulic transport systems (HTSs), remotely operated vehicles (ROVs), seafloor mining tools (SMTs), and surface support vessels (SSVs).

Hydraulic Transport Systems (HTSs) There are two technologies developed for the transportation system from the seafloor to SSVs, which are mechanical lifting systems and hydraulic transport systems (HTSs). Normally, HTSs are more efficient because HTSs normally show steadier and higher production rates over mechanical lifting system.

The most traditional transportation system is an airlift (AL), as shown in Figure 7. Compressed air is provided at the injection point located at the bottom or a certain depth to create a mixture of compressed air and fluid. This mixture has lower density compared to the surrounding fluid, which makes fluid travel upward. The compressed air is generated using an air compressor located in the surface support vessel (SSV) and is delivered to the injection point through a pressurized pipe. With fewer mechanical parts below the sea surface, this system is simple and trustworthy that has less maintenance. However, the efficiency of the airlift system will decrease, as the length of riser increases, since isothermal expansion of air decreases the water content in the upper portion of the pipe (Verichev et al. 2012).

There are two alternative methods to deal with the problem of AL, which are the injection of solid buoyant particles (SBPs) and light hydrocarbons (LHs) as shown in Figure 7. These use same principle as the AL; however, higher efficiency than the AL is obtained. Solid buoyant particles (SBPs) that have lower density than water are provided to an injection point. In this case, no isothermal expansion occurs; thus, there is no water content variation with depth, resulting in higher discharge velocity than AL at the outlet of riser. However, a separator is necessary to remove SBPs from the dredged materials. A suitable pumping facility is also needed. The major considerations in selecting the type of SBPs are high strength, low cost, high buoyancy, and low compressibility. The injection of light hydrocarbons (LHs) that are lower density than water is another alternative. However, unlike the injection of SBPs, more equipment is required to deliver, pump, separate, and store the LHs. In particular, a complex separator is needed to extract the LHs from water and environmental issues cannot be ignored in this method (Verichev et al. 2012).

However, the previously described systems are not suitable for deep water. The use of centrifugal pumps and positive displacement pumps may be a promising solution. The centrifugal pump has a high flow rate at a relatively lower pressure, and the positive displacement pump working at a higher pressure has a relatively low flow rate. Four types of pump configuration are suggested to reach a proper flow rate and pressure in deep water as shown in Figure 8 (Vercruijsse and Lotman 2010 and Vercruijsse et al. 2011). In configuration 1, higher flow rate and pressure are obtained by placing positive displacement pumps in parallel or centrifugal pumps in series near the bottom of a riser as shown in Figure 8 (a) and (b). In configuration 2, the uniform distribution of centrifugal pumps along the riser prevents high pressure concentration at one location as shown in Figure 8 (c). Therefore, with such a flexible design, it is feasible to reduce the thickness of the riser. In configuration 3, centrifugal pumps are installed in series at a location close to the surface support vessel, which leads to easier maintenance and installation of the pumps as shown in Figure 8 (d). In configuration 4, the elimination of the pumps on the seafloor mining tool (SMT) results in

351 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" less power requirement, smaller size, and lower weight of the SMMT as shown in Figure 8 (e). The appropriate selection of these configurations based on project types and locations may greatly reduce CAPEX and OPEX.

Figure 7. The principle of lift system, including air, solid buoyant particles, light hydrocarbons (Verichev et al. 2012).

(a) Configuration 1, positive (b) Configuration 1, centrifugal (c) Configuration 2, centrifugal displacement pumps in parallel pumps in series pumps distributed over the HTS

(d) Configuration 3, centrifugal (d) Configuration 4, centrifugal pumps installed in near to the pumps without pumps on thhe SMT surface Figure 8. Pump configurations (Vercruijsse and Lotman 2010; Vercruijsse et al. 2011).

352 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"

It is common to use the rigid steel pipes as risers, but flexible pipes can also be useful in terms of low critical velocity and low weight. The flexible hose with a standard turning gland and 200 mm internal diameter is constructed to replace the steel pipe as shown in Figure 9 (van Duursen and Winkeelman 2011a). The critical velocity of the hose on that reel is 2-3 m/s, which is lower than that of the rigid steel pipe. In addition, the hose on a reel has a reduced possibility to clog because the dredged materials do not settle even if the sands do not move for 5 hours.

Figure 9. A flexible hose on a reel (van Duursen and Winkelman 2011a).

Seafloor Mining Tools (SMTs) The surface mining tool (SMT) plays an important role in deep sea dredging, which aims at thee excavation of target materials and transportation of these materials to the HTS. The selection of tthe SMTs is based upon the soil conditions. There are basically two types of seafloor mining tools, which are a pllain suction nozzle and mechanical excavation tools (Vercruijsse and Lotman 2010 and Vercruijsse et al. 2011). The soil conditions actually play a key factor in determining the type of excavation tools in deep water. A plain suction nozzle might be sufficient in case of the soil with little cohesion. The mechanical excavation tools are required for the sand or rock, as shown in Figure 10. If mechanical excavation tools are utilized for dredging, these tools have to withstand the reaction force during mechanical excavation, and the power needed has to be well supplied from the surface support vessel (SSV).

Using a submersible dredging unit is another promising solution (van Duurseen and Winkelman, 2011a). This dredging unit is connected to a hose and moves up and down using winches on tthe SSV. It is similar to a Trailing Suction Hopper Dredge; however, it could efficiently work at a depth of over 100m. The whole dredging system is made up of a submersible dredging unit, a flexible hose, a reel, gantries with winches, power supply unit, umbilical unit, and wave compensator as shown in Figure 11.

Figure 10. Seafloor miningn tool (Vercruijsse and Lotman 2010; Vercruijsse et al. 2011).

353 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"

Figure 11. Submersible dredging system (van Duursen and Winkelman 2011a).

Visualization and Exploration of Seafloor Topography For oil and gas fields, it is necessary to conduct bathymetry investigation before drilling and installing equipment. In addition, highly accurate positioning and operation of an excavator is required for efficient development of the oil and gas reservoirs. Stuifbergen (2012) uses a combiined method to check seafloor topography and position of the excavator by the using different kinds of sensors, acoustic positioning systems, and monitoring software. First, the positioning sensors are installed on the excavator to monitor height, roll and pitch motions of the excavator. These sensors prevent the excavator from shifting or tilting. When one of the sensors reaches a certain dangerous threshold value, an alarm system is activated. An inspection remotely operated vehicle (ROV) is also used for further checking the excavating activities. Second, the SeaBat forward looking system and mulltibeam echosounders are used to monitor and enhance the accuracy of the operation in deep water. LLast, the PDS2000 software is used to track and display the motion of the excavator and seafloor topography in 2D and 3D views. This software also collects information from ROV activities. This combined method greatly enhances the working efficiency in deep wateer, and Figure 12 shows an example of the combination of positioning sensors and PDS2000 software.

Figure 12. Example of PDS2000 software combined with the positioning sensors (Stuifbergen 2012).

354 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"

Surface Support Vessels (SSVs) Deep sea dredging is normally conducted in areas far away from land, and therefore, the rolle of surface support vessels (SSVs) is important to increase dredging efficiency. The trrailing suction hopper dredge (TSHD), which is shown in Figure 5, are normally utilized not only at a depth of more than 100 m but also in aarctic locations. The combination of the SMT, HTS, and SSV is the most valuable concept in deep wwater as shown in Figure 13. The SMT breaks a rock into small pieces and delivers the pieces to the HTS. The HTS transports the dredged material to the SSV by means of an AL system or hydraulic pumps. The SSV stores the material and transports the material to open water placement regions. Highly efficient and cost effective dredging is feassible with a proper selection of the SMT, HTS, and SSV in deep sea (Verichev et al. 2012).

Duursen and Winkelman (2011b) introduce a new type of dredge, Ro-Ro Deep Dredge (RDD), which is shown in Figure 14. RDD is equipped with submersible dredging system as shown in Figure 11 and a flexible hose as shown in Figure 9. Based on soil conditions and the length of hose, RDD caan conduct dredging at a depth over 100m.

The dredges used in deep water are expensive. After storage facilities are filled with dredged materials, dredging equipment changes into dead cargo during sailing the dredge for placement. Barges can be a good substitute (van Duursen and Winkelman 2011a). Barges fill with dredged material can sail or be towed to placement areas and place the dredged materials in open water. Meanwhile, the dredge continuously performs its dredging without the unnecessary waste of time required to sail to the placement location. Duursen and Winkelman (2011b) suggest an optimal layout by means of connecting three ships as shown in Figure 14. Sincee barges are normally not equipped with dynamic positioning (DP) system and due to its cost, the dredge and another ship can take a role of adjusting their locations without a DP system.

Figure 13. Dredging system in deep wateer (Royal IHC 2015b).

Figure 14. Ro-Ro deep dredge (Left) and its example of arranngement (Right) (Duursen and Winkelman 2011a and b).

355 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"

New Concept for Deep Ocean Dredgiing

In deep sea dredging, the hydraulic transport system (HTS) is important but expensive. For example, a riser must be long enough and manufactured to endure high hydrodynamic loadss. A complex umbilical system is used to provide electricity to all pumps. A high level of preventive maintenance annd repair is required to avoid failure of the HTSs. In this section, new concepts that do not use the HTSs are discussed.

Two new concepts that use containers can be applied as a substitute for the HTSs. One concept needs a crane and containers equipped with a pump on top of each container as shown in Figure 15. Containers are sunk to seafloor by filling with ballast water and connected to the seafloor mining tool (SMT). The SMT can conduct dredging and transfer the dredged material to a container while de-ballasting is accomplished using the pump. If the container is fully filled with the material, the crane is operated to transport the ccontainer to SSVs. When the container moves to the surface support vessel (SSV), another container is connected to the SMT and dredging is a continuous operation. In this case, an additional umbilical is required to control the pump, and an additional ROV might be necessary to connect the SMT and containers. The length of wire on the crane must be long enough to sink a container on the seafloor.

Another concept is using the buoyancy of container as shown in Figure 16. Once the container is sunk on seafloor using ballast water and connected to the SMT, the dredged material is transportedd to a container during de-ballasting of water. When the buoyancy is slightly higher than the weight of both a container and the material, a container naturally moves to the SSV. An initial lifting force from ROV can increase the speed of transportation to the SSVs. In this case, containers replace the role of HTSs. However, accuurate calculation of buoyancy and weight of a container and the material are necessary. For example, if the dimension of a steel container is 4 m long, 2 m wide, and 2 m deep, and the thickness of the container is 0.01 m, tthe buoyancy of fully submerged container is approximately 160,000 N with a seawater density of 1,020 kg/m3. In addition, the weight of the container is roughly 30,400 N with a steel density of 7,750 kg/m3. In this case, the dredged material is transported to the container until the height of the material in the container is 0.81 m considering that the density of wet sand with gravel is 2,020 kg/m3. In addition, calculation of the hydrodynamic force acting on a container and the investigation of seafloor topography must be conducted to locate the container in the dredging region. Several sensors are also required to measure the height of the material and to confirm the location of the containers.

Figure 15. New concept using containers and a crane. Fiigure 16. New concept using buoyancy of containers.

356 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015"

SUMMARY AND CONCLUSION

Trenching in arctic locations is essential to prevent possible environmental loads, which are ice gouging, strudel scour, thaw settlement, and upheaval buckling acting on subsea pipelines. Selecting an appropriate trenching method is based on the primary considerations, which are water depth and trench depth, and the secondary considerations, which are seabed geology, seabed slope, backfill method, and environmental sensitivity. Trenching methods are conventional excavation, ploughing, jetting, mechanical trenching, and hydraulic dredging equipment.

Dredging in deep water locations are challenging due to the great depths. Currently, the combination of the seafloor mining tools (SMTs), the hydraulic transport systems (HTSs), and the surface support vessels (SSVs) is considered as an optimum method for deep water mining or dredging. The SMT breaks and gathers the dredged material and transfers to the HTSs. The HTSs transport the material to the SSVs. Diverse methods can be used to transport the materials. The use of airlift (AL), solid buoyant particles (SBPs) and light hydrocarbons (LHs) are possible solutions. Both centrifugal pumps and positive displacement pump can provide constant transportation rate, and various configurations of pumps are possible. The SSVs are essential to store and place the material. During sailing to place the material, the SSVs are considered dead cargo. Thus, using additional barges substitute for the role of placement.

New concepts that use containers and cranes can be practical. The HTSs can be replaced by the use of containers. The transportation of the dredged materials can be done by filling containers with the dredged material and by moving it to SSVs. The transportation of containers can be conducted by means of a crane or the buoyancy of a container.

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CITATION Randall, R.E. and Jin, C.K. “Challenges of dredging in the Arctic and other deep ocean locations,” Proceedings of the Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015", Houston, Texas, USA, June 22-25, 2015.

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