{(J/I.{( J ( () ISSN 01410814. Journal of rhe Sociely for Underlvater Technology. Vol. 25. No. 2. pp. 77-82.2002 ec n ogy

A Novel Approach for the Study of -ca North Drill Cuttings Accumulations: u oe'e The Combined Use of an ROVand Benthic u ~ Lander for In situ Measurements •••

E. BREUER, O. C. PEPPE and G. B. SHIMMIELD

Scottish AssociationJor Marine Science. Dunslaflnage Marine Laboralory. Oban. Scotland, PA37 lQA. UK

Abstract laboratory. While this has proved a reliable methodology for benthic biological studies [4-8] Two benthic lander systems were used as part the very unstable structural integrity (Black et al., of a project investi gati ng the geochemical char- this volume) and location of the cuttings material, acteristics of a North Sea drill cuttings pile. the very steep gradients of chemical reactivity [2] Complications associated with deploying and the assorted debris (scaffolding, pipes, wire, sampling gear in close proximity to the etc.) scattered throughout the pile make sampling platform required an innovative technique to for geotechnical, geochemical and microbial obtain the necessary measurements. To process studies difficult [2] and lead to sampling achieve this goal, two deep-sea autonomous induced artefacts in analytical results [2]. Benthic free fall vehicles (benthic landers) were lander instrumentation enables measurements to adapted for deployment and transport by be made in situ thus reducing sampling artefacts remotely operated vehicle (ROV). The landers and providing invaluable data that were pre- were fitted with a microelectrode profiling viously unattainable [9]. Benthic landers were system for obtaining high-resolution oxygen, introduced into the oceanographic community sulphide and pH profiles, and a rig to deploy in the 1970s to allow for the measurement of gel probes into the sediment to examine dis- physical, chemical and biological parameters in solved trace metal profiles within the cuttings the . Progressive research and develop- pile. The use of an ROV for deployment of ment over the years has also allowed for in situ the landers enabled the simultaneous collec- measurements via landers to occur in a diverse ti on of data within a defined spatial context range ofharsh environments such as sulphide oxi- while providing real-time visual feedback of dising bacterial mats, hydrothermal areas, and the landers operating on the . Eight anthropogenically perturbed environments [2,9- deployments were made over 5 days, totalling 12].Over this period benthic landers have proved over 100 hours of sampling time. The to be robust and effective in acquiring data. successful deployment and retrieval of the The potential for employing benthic landers in instrumentation, the quality of data obtained studies associated with sea bottom sampling and the limited disruptions to the ongoing ship under and around oil and gas production operations demonstrate the value of the lander platforms has yet to be evaluated. This account system as an efficient and versatile tool for details the delivery of the first in situ geochemical in situ biogeochemical investigations in areas da ta from drill cuttings piles in the North Sea. associated with sampling and measuring diffi- culties such as North Sea drill cuttings piles. 2. Lander description

1. Introduction Dunstaffnage Marine Laboratory (DML) cur- rently operates two identical geochemical benthic The eventual decommissioning of North Sea oil lander systems purchased from KC Denmark and and gas production platforms is causing increased Unisense AIS. In their autonomous mode the concern regarding the behaviour and ultimate landers comprise an upper buoyancy frame fate of associated drill cutting accumulations bolted to a lower instrumentation frame, (Figure [l, 2]. Large quantities of drill cuttings have I) but for the study described here the buoyancy been accumulating under and around offshore frame was not used. The instrumentation frame, oil and gas installations in the North Sea since an aluminium tripod ].5 m high, can be equipped the 1960s [3]. The present method employed to with a number of different modules. For this study these cuttings piles is to make observations study on the drill cuttings pile of Beryl Alpha a on retrieved sediment sampies and carry out micro-electrode profiler and gel deployment experiments either on-board ship or in the system were deployed. The system was controlled

77 E. Breuer et al. A Nove/ ApproachJor the Study oJ North Sea Drill CUllings Accumu/ations

by a mini-computer contained in a cylindrical 2.1 Micro-electrode profiler pressure housing along with the analog amplifier The system used for the measurements of oxygen and sampling circuitry. Power was supplied by a and sulphide microprofiles was the Profi/ur [11] 12V 76Ah Deep-Sea Power and Light Sea- (Figure 2). The micro-electrode profiler used a Battery. motor with a very high ratio reduction gearbox to drive electrodes into the sediment in 1011m increments. Micro-electrodes with a sensing tip diameter of < 1011mand 90% response time of I s were mounted on the Profi/ur for in situ mea- suring of oxygen, sulphide and pR concentrations in the interstitial water of the seabed. The micro- electrodes were mounted on the boUom of the vertical pressure cylinder containing the control computer and were specially pressure compen- sated within an oil-filled perspex housing. The electrodes were around 100mm long, enabling the probes to penetrate the sediment by up to 80mm (Figure 3). Once the lander had settled

Figure 3 Micro-electrodes with tips 01less than 100 l!m attached Figure 1 The DML-KC Lander being deployed in autono- to the Profi/ur. This photograph was taken after retrieval demon- mous mode. The principle system components are labelIed. strating the durability of the electrodes when in use. As seen in the Photograph courtesy of Kevin Black. photograph only one micro-electrode broke during deployment. I \ Worm gear and motor · ~ · Profiling rack •· · • ~ ..• ·• Computer housing · · · ~ · Adjustable legs · · 106m r-· .•.· • •· •· • • Micro-electrodes ~ · 0 0 •· ~ · • •·

1- 1.4m Figure 2 A schematic showing the major components of the Profilursystem.

78 Underwater Technology, Vol. 25 No. 1,2001 on the seabed the electrodes were moved con- of any residual sediment partieles by a jet of tinuously down from their retracted position to from hoses mounted on the cover plates. within a few millimetres above the sediment surface. From here the electrodes were driven 3. Sampling Methodology into the sediment in steps of 25l!m with a pause of a few seconds at each step to allow the Deployment of the benthic landers was per- signal response to settle before sampling. On formed on the Beryl 'A' cutting pile (Figure 5) reaching the pre-programmed maximum depth which is located 346 km north east of Aberdeen, the electrodes were withdrawn from the sediment. Seotland in a water depth of 119m. A preliminary Three 750 ml water bottles attached to the lander inspection of the surface conditions of the drill collected ambient bottom water sampies for cuttings pile by remotely operated vehide calibration of the oxygen electrodes. (ROV) revealed a diverse sedimentological mixture of poody sorted weakly cohesive oil- rich silt, drill cutting chips, shells, stones and 2.2 Gel deployment system sand with bacterial mats scattered over the top The gel deployment system (GDS; Figure 4) of the pile in varying degrees of eoverage. Four consisted of a moving rack holding up to four Profi/ur deployments and four GDS deployments trace-metal gel probes [13, 14]. These perspex were made onto the drill cuttings piles during a probes had an active gel surface of 145mm x 5-day cruise. The benthic landers were used in a 10mm and were driven into the sediment by a non-autonomous mode, without buoyancy and computer-controlled motor. Gel probes were ballast, and were deployed and reeovered using mounted on the GDS module for the deter- a Pioneer 11 work-dass ROV and ship's crane mination of high-resolution trace metal profiles (Figure 6). Once on station, the ROV was used using the diffusion gradient in thin film (DGT) to reconnoitre the cuttings pile to ascertain the technique [13]. As the probes were lowered, the best possible locations to deposit the benthic perspex plates covering the gel surface for pro tec- lander. This reconnaissance induded observation tion during deployment and recovery were raised of the loeation of objects protruding from the pile up exposing the gel to the surrounding seawater (pipes, meta I framework, etc.), slope angle, and sediment. Once the probes had been inserted proximity to the apex of the pile and location of into the sediment to the required depth the motor the cuttings discharge pipe. Once a suitable site was switched off and the gels left to reach was found, the ship's crane placed the lander on equilibrium. After 24 hours in the sediment, the the seabed in the approximate location. The ROV probes were retrieved, and as the perspex covers then disconneeted the lander from the ship's fell back in place the gel surfaces were washed free crane, picked it up and moved it to the specified

Warm gear and motor

•• Adjustable legs

Moving rack 1.6m Pump

Computer housing • 0 o· •· •·

Cleaning hose

Perspex cover

Gel probe

I·------1.4m ·1

Figure 4 Schematic representation of the gel deployment system (GDS).

79 E. Breuer et al. A Novel Approachfor the Study' oI North Sca prifl Cuttmgs AccumulatlOns

Figure 7 Visual inspection 01 the benthic lander after placement on the seabed by the ROV.

% saturation % saturation o 25 50 75 100 o 25 50 75 100 -I ·1 8a 8b o . E Figure 5 Locatlon of the Beryl Alpha drill cu\llngs pile. E :;; 2 Ö. Q) 3 3 Q

4 4

5 5 65m 165m (a) 6 (b) 6 % saturation 0 25 50 75 100 -1 8c 0 ...... ~ S S '-' ..<:: 2 Ö. Q) Cl 3 Figure 6 The lander instrumentation Irame lilted with the microelectrode- proliling module, being deployed adjacent to the Beryl 'A' plalform. Photograph courtesy 01 Murray Roberts. 4

5 posItIon (Figure 7). Onee in plaee, the landers 300m (c) 6 were allowed to stabilise for I hour prior to com- Figure 8 Dissolved oxygen profiles measured using micro- meneement of measurement operations. electrode within the Beryl Alpha drill cuttings pile. The dashed line represents the sediment-water interface. 4. Results

The down sediment profiles of dissolved oxygen form euttings pile. The use of in situ O2 and and sulphide eoneentrations measured in the sulphide miero-electrodes has allowed us to porewater of the cuttings pile are presented in measure the distribution of these eonstituents at Figures 8 and 9. Figure 8 presents the down sedi- a very high spatial and temporal resolution [12] ment profiles of dissolved oxygen measured in the within the euttings pile. By inserting them, interstitial water from three sites at 65, 165 and under in situ conditions, into the cuttings with 300 m distanee from the apex of the Beryl plat- sub-millimetre resolution, the eoneentration

80 Underwater Teclmology, Vol. 25 No. 1,2001

SulphidepM (e) SulphidepM (e) into or out of the piles and the depth zonation of o 150 300 450 0 150 300 450 the key biogeochemical reactions are crucial for ~ ~ 9" understanding the environmental impact of Water 9b Water extant cuttings during subsequent disturbance o -...... 0 -. caused by removal operations or subsequent Cultings Cuttings fishing activities. These cuttings accumulations 5 5 are a uniquely anthropogenic environment with large concentrations of labile organic matter, oil 8' 10 10 and sulphide which are subject to severe dia- .g oS... genetic reactions within millimetres to centimetres ~ 15 15 of the sediment-water interface [2]. The presence of filamentous, colourless sulphur-utilising bac- teria Beggiatoa spp. on the pile is indicative of a 20 20 sulphide-rich area [10, 11). Beggiatoa mat form- ation occurs where diffusing sulphide from below 25 25 the sediment-water interface and oxygen from above it meet. The filaments oxidise sulphide, 30 30 0 25 50 75 100 with oxygen competing efficiently with the spon-

Oissolved Oxygen % saturation (0) taneous chemical oxidation, thereby establishing (a) (b) steep, opposed gradients of oxygen and sulphide. This steep gradient makes sampie acquisition and Figure 9 Dissolved oxygen (0) and sulphide (e) profiles within the drill cuttings pile. The dashed line represents the measurement of reduction-oxidation (REDOX) sediment-water interface. parameters without disturbance-induced artefacts extremely difficult. The challenge, therefore, was to conduct suitable measurement and sampling techniques that allow in situ measurement of key gradients can be identified without significant constituents at sufficient resolution within the cut- disturbance of the sediment structure. tings pile. Figure 8a, b, c provides evidence for the hetero- The oxygen profiles shown provide qualitative geneity in the dissolved oxygen profile in that two information on the rapid rate of oxygen con- or three electrodes mounted on the same electrode sumption by microbial activity over approxi- holder have been inserted into sediment simul- mately 30cm2 of the sediment surface. The taneously. rapid consumption of oxygen within the sediment Figure 9 presents measured in situ dissolved may be interpreted as microbially mediated sulphide concentrations from the Beryl cuttings diagenesis of labile organic matter [2, 15]. The pile and a comparison between sulphide and location in the depth profile at which dissolved dissolved oxygen profiles within the cuttings oxygen reaches zero and sulphide starts to make pile. Figure 9a shows the production of sulphide an appearance in the porewater (Figure 9b), is starting to occur at 10mm depth within the pile. probably controlled by the level of nitrate The concentration of sulphide shows a steady reduction and dissolution of higher manganese increase up to approximateIy 15mm depth into oxyhydroxides [2, 15, 16]. The likely outcome of the cuttings where the end of the micro-electrode increased porewater sulphide is the precipitation insertion takes place. Figure 9b compares the of metal mono- and disulphides within the pile, vertical dislocation in the depth at which which may have different degrees of stability with dissolved oxygen reaches zero and dissolved respect to reoxygenation [17, 18, 19]. This latter sulphide starts to make an appearance in the point is important when considering the potential porewater. of the pile to oxygenation on disturbance, The mechanical and electronic operation of the perhaps associated with the decommissioning. GDS system worked relatively weil with the gel probes being inserted partially into the sediment. Problems encountered with the gear ratio of the 6. Conclusions motor prevented complete insertion into the sediment. This, along with problems encountered The use of the lander systems within the frame- in the chemical analysis of the gel probes, work of the project provided an innovative and prevented us from obtaining any metal results reliable method for obtaining detailed in situ using this method. geochemical data from a North Sea drill cuttings pile for the first time. The data obtained show evidence for high rates of biogeochemical dia- 5. Discussion genetic reactions c10se to the apex of the pile, and give c1ear indication of the sharp interfacial The distribution of dissolved solutes in the pore- gradients in oxidant and reactants within the water of the cuttings piles, the diffusion gradient porewater of the cuttings pile [2). These data

81 E. Breuer et al. A Nove/ ApproachJor the Study oJ North Sea Drill Cuttings Accumu/ations

obtained through the use of lander technology community. Marine Ecology Progress Series., 91, have not only contributed toward understanding 277-287. the behaviour and fate of chemical constituents 8. Daan, R. and Mulder, M., 1996, On the short-term within these piles but also proved the effectiveness and long-term impact of drilling activities in the of landers in obtaining necessary measurements Dutch sector of the North Sea. 1CES Journal of in the difficult locations around offshore oil and Marine Science, 53, 1036-1044. 9. Tenberg, A., De Bovee, F.D., Hall, P., Bereison, gas production platforms. W., Chadwick, D., Ciceri, G., Crassous, P., Devol, A., Emerson, S., Gage, J., Glud, R., Graziottini, F., Gundersen, J., Hammond, D., Acknowledgements Helder, W., Hinga, K., Holby, 0., Jahnke, R., Khripounoff, A., Lieberman, S. and Nuppenau, The authors are pleased to acknowledge the S., 1995, Benthic chamber and profiling support and collaboration of the Captain and landers in - A review of design, crew of the Kommander Subsea; Jens technical solutions and functioning. Progress in Gundersen and all the staff from UNI SENSE; Oceanography, 35, 253-294. Roger Walls and Anne Walls from Mobil Ud; 10. Gundersen, J.K., J0fgensen, B.B., Larsen, E., and Denise Cummings and Murray Roberts. This Jannasch, H.W., 1990, Mats of giant sulphur project was made possible through [unding [rom bacteria on deep-sea sediments due to f1uctuating the UK Offshore Operators Association hydrothermal f1ow.Nature, 360, 454-455. (UKOOA) and the NERC [unded thematic 11. Moller, M.M., Nielsen, L.P. and Jorgensen, B.B., MIME programme. 1985, Oxygen responses and mat formation by Beggiatoa spp. Applied and Environmental Microbiology, 50, 373-382. References 12. Gundersen, J.K. and Jorgensen, B.B., 1991, Fine- scale in situ measurements of oxygen distributions I. Rice, AL, 2001, Man-made objects on the sea- in marine sediments. Kieler Meeresforsch, 8, f100r 2000. Underwater Technology, 24, 169-174. 376-380. 2. Shimmield, G.B., Breuer, E., Cummings, D.G., 13. Davison, W., Zhang, H. and Grime, G.W., 1994, Shimmield, T. and Peppe, O.C., 2000, Performance characteristics of gel probes used for Contaminant leaching from drill cuttings piles of measuring the chemistry of pore waters. the northern and central North Sea: Field results Environmental Science and Technology, 28, 1623- from the Beryl "A" cuttings pile. UKOOA Drill 1632. Cuttings Initiative - Research and Development 14. Breuer, E., Howe, JA., Shimmield, G.B., Programme, Report 2.2.2. 3. De Groot, SJ., 1996, Quantitative assessment of Cummings D. and Carroll, 1., 2000, Contaminant the development of the offshore oil and gas indus- leaching from drill cuttings piles of the northern try in the North Sea. ICES Journal 0/ Marine and central North Sea: A review. UKOOA Drill Science, 53, 1045-1050. Cuuings Initiative - Research and Development 4. Hartley, J.P. and Ferbrache, J., 1983, Biological Programme, Report 2.2.3. monitoring of the Forties Oilfield (North Sea). 15. Canfield, D.E., Thamdrup, B. and Hansen, J'w., In: Proceedings, 1983 Oil Spill Conference 1993, The anaerobic degradation of organic matter (Prevention, Behavior, Control, Cleanup). San in Danish coastal sediments: iron reduction, man- Antonio, Texas. ganese reduction and sulphate reduction. Geochim. 5. Davies, 1.M., Addy, J.M., Blackman, RA, Cosmochim. Acta. 57, 3867-3883. Blanchard, J.R., Ferbrache, J.E., Moore, D.C., 16. Pedersen, T., 1984, Interstitial water metabolite Somerville, H.J., Whitehead, A. and Wilkinson, chemistry in a marine tailings deposit, Rupert T., 1984, Environmental effects of the use of oil- Inlet, British Columbia. Canadian Journal of based drilling muds in the North Sea. Marine Earth Science, 21, 1-9. Pollution Bulletin, 15, 363-370. 17. Pedersen, T.F., 1985, Early diagenesis of copper 6. Gray, J.S., Clarke, K.R., Warwick, R.M. and and molybdenum in mine tailings and natural Hobbs, G., 1990, Detection of initial effects of sediments in Rupert and Holberg inlets, British pollution on marine benthos: an example from Columbia. Canadian Journal of Earth Science, 22, the Ekofisk and Eldfisk oil fields, North Sea. 1474-1484. Marine Ecological Progress Series, 66, 285-299. 18. Saulnier, I. and Mucci, A., 2000, Trace metal 7. Kroncke, 1., Duineveld, G.C.A., Raak, S., Rachor, remobilization following the resuspension of E. and Daan, R., 1992, Effects of a former estuarine sediments: Saguenay Fjord, Canada. discharge of drill cuttings on the macrofauna Applied Geochemistry, 15, 191-210.

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