B-DEOS Plans Schultz, Lampitt, Peirce & Boyle
B-DEOS Plans for establishment of long-term mobile interdisciplinary ocean observatory systems in the N and S Atlantic
Adam Schultz1, Richard Lampitt2, Christine Peirce3, Evan Boyle4
[email protected], [email protected], 3Christine.Peirce@- durham.ac.uk, [email protected]
1Cardiff Marine Institute, Cardiff University, Earth Science Department, Cardiff CF10 3YE, UK; 2George Deacon Division, Southampton Oceanography Centre, European Way Southampton SO14 3ZH, UK; 3Department of Geological Sciences, University of Durham, Durham, DH1 3LE , UK; 4Kvaerner Engineering & Construction Australia Pty Ltd.
Introduction baseline data, and to monitor natural and man-made changes to this system by: Advances in theoretical understanding of o Establishing a long-term, permanent the natural systems in the sea and in the and relocatable network of instru- Earth below have been closely associated mented seafloor platforms, moorings with new data sets made possible by tech- and profiler vehicles, provided with nological advances. Important and unanti- power from the ocean surface and in- cipated processes have been revealed as a ternal power supplies, and maintaining result of new capabilities in acquiring mar- a real- or near-real time bidirectional ine data. The plate tectonic revolution, the Internet link to shore. discovery of hydrothermal circulation, and many other examples can be attributed to o Examining the time varying properties the application of innovative new techno- of these different environments (solid logy to the study of the sea. earth, ocean, atmosphere, biosphere), exploring the links between them and A consortium of research groups and insti- the causes of the variability. tutions within the United Kingdom is plan- ning a regional system of multidisciplinary o Developing appropriate methods of ac- ocean observatories to study the linkages quiring data in real-time, assimilating between the physical, chemical and biolo- them into mathematical models of the gical processes regulating the earth-ocean- solid earth, oceans, and air-ocean in- atmosphere-biosphere system. Funds have terface, and promoting interpretation been released for engineering activities of these data for a truly synoptic un- leading to the design of the basic mooring derstanding of the linked earth-ocean- and telecommunications/power buoy sys- -atmosphere-biosphere system and its tem, and a UK NERC Thematic Pro- components. gramme is in the planning stage. Feasibility: Major advances in technology The B-DEOS (British - Dynamics of Earth have demonstrated the potential of geo- and Ocean Systems) observatory system is physical and oceanographic sensors moun- designed to allow studies on scales of or- ted on deep water seafloor platforms and der 10-3-102 km, as well as to supplement moorings for recording and transmitting on larger spatial scales the emerging glob- data to the surface. Major advances have al ocean and seafloor solid earth observat- also been made in the oceanographic dis- ory network. The facility will make it pos- ciplines over the past decade such that a sible to obtain requisite long-term synoptic wide variety of marine properties can now be measured in situ over prolonged time B-DEOS Plans Schultz, Lampitt, Peirce & Boyle periods (eg nutrients, phytoplankton con- The B-DEOS draft science plan currently centration, aqueous geochemistry, ocean calls for establishing in the first instance currents and temperature). Furthermore three observatory locations, employing at developments of subsurface vehicles to each a relocatable 200 km long 1D array carry such sensors are nearly ready for of oceanographic sensors bisecting a 50 large scale deployment. Satellite telecom- km on a side 2D array of multidisciplinary munications enable these data to be trans- sensor platforms. This scenario is based on mitted from remote areas (inaccessible to the staged installation of the first observat- shore cables) to land-based data centres in ory at a high latitude N Atlantic site. After real-time for dissemination via the Inter- an initial operating period of three years, net. These Internet links make it possible the bulk of the observatory array will be for shore-based scientists to modify exper- relocated to the next observatory site. The imental parameters and instrument opera- central node in the array is to be left in tions remotely in response to changing en- place, and modified for very long term op- vironmental or engineering conditions. eration as part of the global system of per- manent geophysical and meteorological A UK Natural Environment Funding stations. The process will be repeated a Council supported feasibility study by an second time, resulting in establishment of academic-industrial partnership (Cardiff three sites in the N and S Atlantic and University, Kvaerner Oil & Gas Australia Southern Oceans. From an engineering Pty Ltd, Southampton Oceanography perspective, the instrument array, buoys Centre) has culminated in a design spe- and moorings have been specified to satis- cification for a permanent ocean observat- fy the operational and scientific require- ory communications and power supply ments in each of the potential target areas. buoy system. The observatory buoy plat- form and moorings will be deployable Initial observatory site - Reykjanes from the largest class of research vessels Ridge: available to the UK fleet (e.g. RRS James The location proposed for the first deploy- Clark Ross), or international equivalents. ment of the UK observatory array is a 200 The scientific rationale for such an obser- km long (E-W) by 50 km wide (N-S) area vatory system is summarised below, and centred on the Reykjanes Ridge – the sec- the enabling technology of large ocean tion of the slow spreading Mid-Atlantic power/telemetry buoys envisaged for B- Ridge extending southwards from Iceland DEOS is outlined. to the Charlie-Gibbs transform system. The Reykjanes Ridge represents an area of Science planning: the North Atlantic at which multi-discip- linary data collected by the observatory ar- Our efforts are part of a larger internation- ray will be able to address fundamental al requirement for sustained multi-discip- questions relating to biogeochemical linary ocean-based observatories. The fluxes and cycling; to ocean circulation; present proposal falls under the umbrella and to the dynamics of the upper mantle of DEOS (Dynamics of Earth and Ocean and sea floor spreading. This also serves Systems), an initiative in advanced plan- as a component in the global observatory ning in the UK and the US (further in- array. formation about the DEOS observatory programme may be found at www.deo- s.org). Intersections between B-DEOS and other international ocean observatory ef- forts such as Euro-GOOS and CLIVAR are also taken into consideration. B-DEOS Plans Schultz, Lampitt, Peirce & Boyle
Fig 1. Location of 50 km 50 km dense observatory instrument array centred on Reykjanes Ridge, with 200 km long 1D instrument transect bisecting the array. This configuration permits study of local, regional and global scale processes. Each point in the array comprises three seafloor landers and a mid-water relay buoy forming a 10 km on a side triangle for high resolution site-specific studies.
The initial location of the observatory ar- magnetotelluric <
The surface buoy at the centre of each ob- servatory will be instrumented to measure the fluxes of heat, freshwater and mo- mentum between the atmosphere and ocean. The transect of time series observatories across the Reykjanes Ridge offers an op- portunity to study the boundary between two N. Atlantic biogeochemical provinces, the Atlantic Arctic province (ARCT) to the Northwest and the North Atlantic Drift province (NADR) to the southeast. We seek to characterise the two provinces which lie on either side of the Reykjanes Ridge in terms of their biology and chem- istry. This is to be done primarily by a comparison of the oceanographic charac- teristics at the stations at each end of the cross ridge transect. As seen in Fig 1, the intention is to deploy five time series moorings across the ridge, each of which supports a profiling vehicle carrying a suite of sensors to characterise Fig 2. Schematic of central the physics, and biology of the upper 1000 telemetry/meteorological mooring m of the water column. Measurements in- supporting acoustic communications to clude CTD, chlorophyll and zooplankton seafloor instrumentation platforms and to abundance and size distribution with the oceanographic mooring. incorporation of nutrient sensors later in the program. Profiles will be made once or Transect (Fig 1), is designed to measure twice per day with data being transmitted the structure and transport of deep bound- in real time acoustically to the surface ary currents on either side of the ridge. buoy, multiplexed with the other geophys- Physical oceanographic and upper ocean ical, oceanographic and meterological data biogeochemical data will be provided by a streams, and from there transmitted to the set of five vertical sub-arrays, distributed ground station by satellite. from WNW to ESE across the axis of the Reykjanes Ridge, and extending laterally Eventual permanent observatory in- for 200 km and vertically throughout the stallation entire water column. Each vertical sub-ar- Since the array is designed to be a re- ray will be fitted with standard physical locatable resource, much like a ship we oceanography sensors, together with a ver- envisage transporting it to new observat- tical profiling vehicle which will transit at ory locations over time. However one least once per day through the upper 1000 long-term objective is that the central sub- m of the water column. The vehicle will array on Reykjanes Ridge should be left measure conductivity, temperature and permanently installed on the ocean floor, depth (CTD), together with chlorophyll connected directly by power and data and zooplankton abundance and size dis- cables (rather than by acoustic telemetry, tribution. which is used during the three year wide area array operations period to relay data B-DEOS Plans Schultz, Lampitt, Peirce & Boyle between instrument platforms and the The Southern Ocean central satellite communications buoy) to The Southern Ocean plays a major role in a long-lived data telemetry surface buoy, regulating the Earth’s climate because it to form part of the DEOS global network serves as an important conduit for inter- of seismic and geomagnetic observatories. -ocean exchange between the Atlantic, In- At the centre of the array we plan there- dian and Pacific Oceans via the Antarctic fore to install a communications mooring, Circumpolar Current (ACC), which flows equipped with a large, tethered, surface eastward around the Antarctic continent. A buoy that will provide the satellite com- key to understanding the ACC is the munications link to shore. The buoy will Drake Passage - the natural “choke-point” also be fitted with a suite of meteorologic- between the southernmost tip of South al sensors, compatible with those installed America and the northernmost tip of the by the UK Meteorological Office on their Antarctic Peninsula. other autonomous ocean weather stations. Due east of the Drake Passage lies the Future observatory sites - MOMAR, Sandwich Plate, which extends just 200 Scotia Sea-Drake Passage km from the medium-fast spreading East Scotia Ridge, in the west, to the South The Mid-Atlantic Ridge southwest of the Sandwich Island Arc and trench in the Azores Archipelago contains four sites of east. The East Scotia Ridge itself exhibits high-temperature hydrothermal venting. a range in geological characteristics in- These are found along a 200 km section of cluding at least one axial magma chamber the ridge crest in varying geological set- beneath the ridge crest and a minimum of tings and at a range of different two discrete hydrothermal fields, located depths/pressures. The Lucky Strike vent at opposite ends of the. sites are distributed around a lava lake in the caldera of an axial volcano. The Lucky Within this area, the possibility exists to Strike segment, lying within EU waters, provide long-term continuous observations provides a natural laboratory (the Inter- of all the major components of the plate Ridge-designated “MOMAR area) for de- tectonic cycle in real-time with telemetry tailed multidisciplinary investigations in a either from moored arrays or through dir- range of seafloor environments, all within ect cable connections between the seafloor ready reach of the Azores. and land-based relay stations installed on the South Sandwich islands and/or South The MOMAR area is also of wide interest Georgia, an area of great strategic interest to researchers into whole water-column to the UK in which logitistical support is processes. This section of the MAR exhib- high. its a complex hydrography which is fur- ther affected by the ridge topography, the Mediterranean water tongue and the B-DEOS observatory buoy system Azores current and front. An engineering feasibility study has con- The water column overlying the ridge sidered competing designs for a stable, crest within the MOMAR area is noted for long-term buoy system to support sea-sur- an increased biomass, perhaps linked to face power generation, transmission of local upwelling around the islands as the power to the seafloor, and bidirectional ridge-crest shoals toward the Azores Ar- telemetry from the above-mentioned in- chipelago. The intrinsically high biod- strument array sites to/from the Internet. iversity observed within the mid-water fauna at these latitudes attracts the interest Disc buoys have the majority of their of a wide spectrum of scientists studying volume above or at the waterline resulting biogeochemical cycles. in very small heave natural periods (below B-DEOS Plans Schultz, Lampitt, Peirce & Boyle
the wave period) and consequently follow od of the buoy is in excess of any anticip- the sea surface resulting in relatively large ated wave periods. motions. Spar-type buoys have the ability to modify their natural period and hence Design service life ‘tune’ the response in union with the sea surface. The superior motion characterist- The buoy internal and external systems ics of spar buoys give advantages in terms and coatings have a design life of 20 of satellite communications and reliability, years, with 10 years between major refur- as well as mooring system load. bishment (involving recovery of the buoy and subsequent redeployment). The buoy The B-DEOS spar buoy (Fig 3) is charac- hull structure has been designed for a terised by two sections of different diamet- design life of 2 refurbishment periods (20 ers and a conical transition. Differing dia- years), assuming a scheduled maintenance meters allow for ‘tuning’ of the buoy to period of 2 years. This is based around a obtain the most favourable motion re- preventative maintenance program, includ- sponse. For SPAR type buoys the natural ing refuelling, replacement of consum- period is largely dependent on the surface- ables such as lubricants, and replacement piercing diameter. Further ‘tuning’ of the of relatively low reliability items. buoy response may be carried out by in- cluding a heave-suppression plate around Proposed Locations the keel. This allows for additional added mass which ensures the heave natural peri- To date, four buoy locations have been proposed for the B-DEOS program:
1 58 N 33 W Reykjanes Ridge 2 37.3 N 32.25 W MAR axis close to Lucky Strike, 250nm S. of Azores 3 59 S 63 W Centre of Drake Passage 4 59 S 27 W Midway between E Scotia rise and S.Sandwich Islands
Operations at water depths of 2500, 4000 and 6000 m have been considered during the design phase.
Operating Condition
Data communications requirements dictate the use of an INMARSAT/B-HSD array for the primary system, necessitating an actively stabilised antenna unit. This places a limitation on the buoy motions to meet the specified design communications uptime of 95% within each monthly peri- od. Wave heights occurring at Reykjanes Ridge are greater than for the other loca- Fig 3. B-DEOS buoy schematic showing tions for all return periods, and therefore two sections of different diameter for tuned wave height data from this location only response, and INMARSAT antenna dome. has been used for design. B-DEOS Plans Schultz, Lampitt, Peirce & Boyle
The wave periods chosen in the buoy Payload Requirements design specification is generally equival- ent to a Beaufort seastate of 6/7 (high/very The payload requirements are: high). Wave directionality is not a signi- ficant design parameter, as the mooring Electronics Package system is not taut, and the buoy will be Buoy Control CPU able to rotate easily inside its watch circle. Telemetry CPU / Communica- tions control hardware Heave and pitch are driven by a combina- Distribution panels tion of wave forcing and dynamics. The Battery Package - Batteries for wave forcing has two components: pres- 1000W continuous use sure driven and inertia driven. Generator Package - 1.5 kW gener- ator for recharging the batteries Fuel Bladders - 6000 litres of fuel The dynamic behaviour is characterised by in independent bladders a sharp resonant peak at the heave natural period. If the forcing cancellation and Mechanical/Buoy Systems heave natural period coincide, heave mo- tion is minimised over the operating range. In addition to the equipment included in This cancellation period can be manipu- the payload package, there will be various lated by changing the geometric properties equipment installed on the buoy hull struc- of the buoy; specifically the relative dia- ture to enable it to function. This shall in- meters of the cylinders and the cone clude the following. angles. Heave motion can also be reduced by increasing vertical damping, over the . Inmarsat radome antenna array (ap- frequency range where the dynamic com- prox. 1m diameter), other antennae as ponent is large. required . Piping systems (fuel, exhaust, ballast, Servicing etc.) . Bilge pump The intent is that the buoy will be access- . Hazard detection ible for servicing and retrieval at least 3 . Navigation Aids, including lights months during the year. Servicing of the . Compartment access ways and seals buoy will be carried out by either retriev- . Interface connectors ing the buoy onto the deck of a vessel . MetOcean equipment (either a research vessel or a supply boat) for repairs or maintenance or using stand- Power Generation and Supply ard connections for replenishment. The design of the payload package of the buoy One of the major requirements of the buoy is to allow simple removal and replace- is to generate power for both subsea in- ment of all major components in package strumentation and for buoy systems. Given form, with all connections between pack- the limitations of the buoy in terms of size, age systems and buoy hull systems to be weight, and reliability requirements, the made at a single location where possible. base case is diesel power generation. Servicing of seafloor packages will be However, it is important that alternate possible by ROV, including the new UK means of power generation are investig- national deep submergence facility cur- ated, as significant improvements in reli- rently being established. ability and fuel efficiency may be pos- sible. Possibilities may include fuel cells, B-DEOS Plans Schultz, Lampitt, Peirce & Boyle wind or wave power generation, or other by the stabilising system and the greater emerging technologies. the reliability.
Power requirements are initially defined Other backup systems are to be provided, as: 1.0 kW for the buoy system, with 0.5 including an emergency beacon, GPS pos- kW delivered to a subsea junction box for itional data collection, and lower band- provision of subsea instrumentation pack- width data transmission via LEO satellite ages. Power losses along the (omnidirectional antenna) in the case of a umbilical/mooring line can be significant failure of the INMARSAT system. (as much as 50%). The power supply methodology is DC current, 110V to max- imise supply efficiency. Based on these re- quirements, the power generation capacity required at the buoy is estimated to be 2.0 kW.
Satellite Data Transmission
The satellite telemetry requirements of the scientific payload are planned to lie within the capabilities of a 64 kbps communica- tions link (additional effective bandwidth may be obtained through the use of data compression techniques). The most cost- effective system to achieve this bandwidth is INMARSAT/B-HSD (high speed data) which can provide either 56 or 64kbps of bandwidth. There are a number of compet- ing satellite systems proposed for installa- tion over the next decade, which would also be capable of providing this band- width in a cost-effective manner. How- ever, recent experience has shown that at least some of these proposed systems will be abandoned before installation. B-DEOS have therefore implemented a conservative design, under the assumption that non-IN- MARSAT options will have less stringent antenna stability requirements, and there- fore all alternatives may be accommodated safely within the operating margins for IN- MARSAT/B-HSD.
Based on the use of INMARSAT/B-HSD a gyroscopically stabilised antenna is re- quired to maintain the pointing accuracy needed by the INMARSAT system. How- ever, the more that these motions can be reduced through the design of the buoy, the less complexity and power is required