Underwater Drilling Rig for Offshore Geotechnical

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Underwater Drilling Rig for Offshore Geotechnical Explorations for Oil & Gas Structures By G. SPAGNOLI and T. FREUDENTHAL*

Abstract High quality sampling of the sea floor is the basis for geotechnical investigations within the offshore industry. In order to bridge the gap between the use of expensive drill ships and simple sampling tools like a gravity corer, the MeBo was developed as a new sea floor drill rig. The manuscript presents and describes some case histories of the MeBo. This drill rig is capable of sampling soft sediments and hard rocks down to 80 m at the sea floor. It can be operated in water depths up to 2000 m. The MeBo can be deployed from standard research vessels. Based on the MeBo, the second generation of the drill rig and its possible offshore applications for oil & gas are also briefly described.

Introduction Drilling for natural resources offshore (e. g. gas, oil or marine minerals), or for soil sampling, in some instances hundreds of miles away from the nearest landmass, poses a number of different challenges over drilling onshore. The actual drilling mechanism used to delve into the sea floor is much the same as can be found on an onshore rig. However, with drilling at sea, the sea floor can sometimes be thousands of meters below sea level. Dedicated drill ships are used in science and in the industry in order to get long cores both from soft sediments and hard rocks. The drill string has to be built up through the entire water column before sampling can be started. Precise navigation and heave compensation are required in order to minimize disturbances during the coring process and to control drill-bit pressure. Especially in the upper tens to one hundred meters, core quality is usually poor when drilling with a drill ship into the deep sea floor. Besides, these special vessels are expensive and their operation is not time effective when only short drilling is required. The average day rate for drill ships working up to 1500 m water depth is about 240,000 US $ [1], whereas the MeBo day rate with a complete crew is estimated to cost eight times * Giovanni Spagnoli, Dept. of Maritime Technologies, BAUER Maschinen GmbH, Schrobenhausen, Germany (E-mail: [email protected]); Tim Freudenthal, Dept. of Marine Geology, Marum Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany 0179-3187/13/IV © 2013 URBAN-VERLAG Hamburg/Wien GmbH

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less than the drillship. Ultra deepwater floating vessels, including both drill ships and semi-submersibles, cost generally around 400 million US $, whereas the second generation of MeBo has a cost of about 5 million Euro. Subsea drilling equipment began to appear in commercial use in the 1970’s. The earliest examples of these units were diver operated, marinized terrestrial single-shot rotary diamond core drilling rigs used for shallow rock coring for survey purposes. They also found application installing the small diameter piles, which are used to restrain and stabilize oil and gas pipelines in shallow water zones with rocky sea beds, such as those found in the Persian Gulf and NW Australia coastal oilfields [2]. More recent developments focus on multi barrel sea bed drill rigs for deeper penetration (Table 1). The MeBo Rig The MARUM Center for Marine Environmental Sciences at the University of Bremen (MARUM) has developed and operated the sea floor drill rig MeBo (“MeeresbodenBohrgerät”, German for “Sea Floor Drill Rig”). This drill rig (Fig. 1) is capable of sampling soft sediments and hard rocks down to 80 m at the sea floor. It can be operated in water depths up to 2000 m [4]. The MeBo can be deployed from multipurpose research vessels like RV METEOR and RV SONNE. The development of the rig was funded by the German Ministry of Education and Research and by the Bremen State Government. The MeBo is a portable drill rig which is remotely operated from the vessel. A steel armoured umbilical is used to lower the MeBo to the sea bed. Copper wires and fibre optics in the umbilical are used for energy supply and for communication. MeBo uses the Schilling Robotic’s modular Digital Telemetry System for subsea robotic devices. Prakla Bohrtechnik (sister company of BAUER Maschinen GmbH) was responsible for the development of the drill technology and the hydraulic design of the MeBo. The mast with the feeding system forms the central part of the drill rig (Fig. 2). The drill head provides the required torque and rotary speed for rock drilling; it is mounted on a guide carriage that moves up and down the mast with a maximum push force of 4 t. A

Fig. 1

The sea floor drill rig MeBo launched from the research vessel RV SONNE during the research expedition SO221 in May 2012

water pump provides sea water for flushing the drill string, for cooling of the drill bit, and for removing the drill cuttings. Drill rods and core barrels for a drilling depth of more than 80 m below sea floor and coring of more than 70 m are stored on two magazines on the drill rig. The required drilling tools are selected by a loading arm. Two chucks assist in building up the drill string at the sea floor. The system utilizes commercial wire-line core barrels with diamond or tungsten carbide bits for hard rock rotary core drilling. Special adaptions have been made for optimizing core quality in hemipelagic muds typical for the continental slopes and the deep sea. Core diameters are 55–67 mm. The deployment depth of MeBo is currently limited to a maximum of 2000 m below sea level by the length and strength of the umbilical. Four legs are extended before landing to increase the stability of the rig (Fig. 2). Case histories of MeBo From 2008 to 2012 MeBo was employed in nine expeditions on five different vessels. 66 deployments took place between 10 and 2050 m water depth with a maximum drillOG 185

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Fig. 3

Fig. 2

Basic components of sea floor drill rig MeBo [4]

Table 1

Highly indurated carbonate layer drilled during research expedition GUINECO-MeBo with the sea floor drill rig MeBo (Sultan, N., Bohrmann, G. and the GUINECO-MeBo shipboard scientific party, 2012). The embedding of the crust within a soft formation, which is a major drilling challenge, is shown on the right. Being able to drill from a stable platform on the sea bed with optimal control on weight on the bit is a prerequisite for optimal core quality within complex geology.

Heavy duty subsea drill rigs currently on the market with reference to some of the key design features (modified after [2, 3])

Rig Name

Operator

Type

Water depth

Penetration

BGS RD1

BGS

Single barrel rock drill + vibrocorer

2000 m

5m

BGS RD2

BGS

Multi-barrel

3000 m

15 m

BRIDGE Oriented corer

BGS

Single-barrel rock drill, oriented core

5500 m

1m

MeBo

Marum, Bremen

Multi-barrel, wire-line

2000 m

80 m

BMS

Metals Mining Agency, Japan


6000 m

30 m

A-BMS

Japanese Oil, Gas and Metals National Corporation


4000 m

100 m

ACS

National Institute of Ocean Technology, India


4000 m

100 m

PROD 1

Benthic Geotech, Australia

Multi-barrel, In-situ geotechnical testing

2000 m

125 m

PROD 2 & 3

Benthic Geotech, Australia

Multi-barrel, In-situ geotechnical testing

3000 m

125 m

Rovdrill

Perry Slingsby

Multi-barrel, ROV-operated

3000 m

19 m

MBARI ROV Rockdrill

WHOI

4xsingle barrel, ROV-operated

4000 m

1m

DWACS

National Institute of Ocean Technology, India


3000 m

100 m

Rovdrill Mk 2

Canyon Offshore

Multi-barrel, ROV-operated

2500 m

120 m

Rovdrill 3

GEMS

Multi-barrel, ROV-operated (vertical), CPT

3000 m

90 m (expandable to 200m)

Gregg Seafloor Drill

Gregg

Multi-barrel, wireline CPT

3000 m

150 m

MeBo 200

Marum, Bremen

Multi-barrel, wire-line (optional)

4000 m

200 m

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ing depth of 80.85 m. 1649 m were drilled whereas 1445 m were cored in sedimentary hard rocks, gas hydrates, gravel, sand, till and hemipelagic mud. 1135 m of core were recovered, thus yielding an average recovery rate of 79%. Recovery rates were especially good for cohesive sediments and hard rocks, while sand and gravels are difficult to sample using the wire-line core drilling technique. Spectrum Gamma Ray borehole logging has been used since 2010 and the in-situ temperature probe since 2012. Moreover MeBo was used for sensor installations within the drill string for long-term monitoring of pore water pressure at four sites off Japan in 2012. During the Guineco-MeBo expedition off the coast of Nigeria, MeBo was deployed on board the oceanographic research vessel RV POURQUOI PAS? from November to December 2011. The two main objectives of this campaign were to determine the distribution of gas hydrates and to study the possible link between gas hydrate dynamics and the observed sedimentary deformations within a pockmark field. During this campaign, several innovative tools were used, including the MeBo drill rig and geotechnical monitoring tools developed by Ifremer (Penfeld pentrometer and piezometers). The campaign was a success as evidenced by the amount of data acquired: 375 m of drilling during 12 deployments, 336 m of coring with average recovery rate of 83% within a complex geology, including gas hydrates and alternating soft and hard formations due to authigenic carbonate precipitates within hemipelagic muds (Fig. 3). In May 2012 the MeBo was used for paleoceanographic studies in the South China Sea on board the German research vessel FS SONNE. The aim was the recovery of long continuous cores for the study of the monsoon history in that area over several glacial/interglacial cycles. In total 373 m were drilled during five MeBo deployments OIL GAS European Magazine 4/2013

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Fig. 4

Sketch of the MeBo 200

at two sites. The maximum drilling depth was 80.85 m below sea floor. In total 341 m were cored with an average core recovery of 94% within fairly homogenous hemipelagic mud. MeBo 200 and its Future Applications The MeBo 200 is the next evolution of the MeBo, developed and manufactured under the supervision of MARUM, with enhanced parameter application, such as deployment and drilling depth. The increasing search for marine natural resources and increasing research needs are pushing the demand for using drilling rigs in higher water depths (up to 4000 m) and for increased drilling capability. Therefore MARUM and BAUER Maschinen GmbH are collaborating in developing an upgraded system. The maga-

Fig. 5

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zines will have an increased storage capability for drill rods and core barrels (96 slots instead of 68). The stroke length will be 3.5 m. The magazines can be modified for the use of a wide range of barrel diameter according to the project requirements. The weight of the MeBo 200 is about 10 t in air and 8 t in water. The torque which will be provided is 1700 Nm for 180 rpm and 450 Nm for 450 rpm. The feeding system provides a push force of up to 50 kN and a pulling force of 100 kN. The installed electro-hydraulic power is 2 x 75 HP, 3 kV, 50/60 Hz. The flush water pump will reach 60 bar, and flow of 160 l/min. Four legs are extended before landing to increase the stability of the rig and leveling due to possible unevenness of the sea bed. BAUER Maschinen GmbH is developing the drilling technology, the mechanics, the hydraulics and the Launch and Recovery System (LARS), whereas MARUM is developing the overall system design, the telemetry, the energy supply, the compensation and the control system. The MeBo 200 is currently under construction and it will be tested in summer of 2014 in the North Sea. The wireline core drilling method is likely to be used and the method is most commonly used for mineral exploration drilling, where undisturbed core samples are essential for understanding the structure of the deposits. The mast with the feeding system and the power swivel forms the central part of the drill rig. It is mounted on a guide carriage that moves up and down the mast with a maximum push force of 5 t. A water pump provides sea water for flushing the drill string, for cooling of the drill bit, and for drill cuttings removal. The system utilizes rotary core barrels with diamond or tungsten carbide bits. In the framework of the project Sugar 2 (Submarine Gas Hydrate Deposits: Exploration, Exploitation and Transport), financed by the German Ministry of Education and Research and by the German Ministry of the

Economy, whose aim is to develop a safe, environmentally compatible process for combining submarine methane exploitation with carbon dioxide storage, the use of the MeBo 200 might come into play. These resources will become reserves only if the gas contained therein can be produced economically. To drill wells in a gas hydrate formation, a conductor casing is needed to allow close loop circulation of the mud, if different from seawater. However, drilling through gas hydrate becomes increasingly difficult due to the variety of the morphology of gas hydrate sediments that one can encounter. Hence, it is likely that no universal drilling formula can be developed, but understanding gas hydrate behavior while drilling will help reduce the inherent risks and improve the process [5]. MeBo 200 might be used as drilling rig in order to start pilot production boring (as shown in Fig. 5). However, at the current state further development is need in order to apply the drilling rig to such operations. A platform can be installed on the submarine seabed. This is connected to a conductor pipe. Axial conduit and production pipes are sunk and connected to the conductor pipe. The forces are transferred to the platform. In the case that free gas bubbles are encountered, the platform might be connected to the soil with helical piles. The stability force depends on the mechanical properties of the soil and this is yet unknown. Alternatively a blowout preventer might be installed between the guide base and the MeBo. The conductor pipe might be driven by means of hammers, the MeBo will be deployed, and axial conduits are released. MeBo is recovered and BOP might be released. MeBo is again deployed, production casing will be installed by means of the drill rig and the MeBo is recovered to the vessel. MeBo 200 can be also deployed in deepwater where very soft sediments can be found. The seabed at deepwater sites typically comprises soft fine-grained sediments

Preliminary sketch for the installation platform for the MeBo 200


DRILLING [6, 7], with low undrained shear strength values ranging from 3 to 40 kPa [8, 9]. Estimates of Brazilian pre-salt reserves indicate a potential for 70 to 100 billion boe, but the exploration process for this wealth of resources is still in its early stages. Offshore Brazil at a water depth of 970 m the first 15 m gave values of cu (undrained shear strength) about 4–40 kPa [8]. At the beginning of deepwater, Petrobras decided to consider suction piles as a potential technology to be applied as a foundation solution in such soft sediments. The main problem observed in this kind of foundation was the high weight caused by dead mass, necessitating installation by a special vessel with a heave compensator, which was not available in Brazil and the charter cost was very expensive. Due to these problems Petrobras decided to invest in another technology, like Vertical Load Anchors (VLAs) and a new concept foundation called Torpedo pile [10]. In order to properly investigate such soft sediments, MeBo 200 can be used. MeBo has already experience in soft sediments. During the Cruise MSM15/3 in 2010 MeBo cored a 51.9 m-thick sediment from the undisturbed slope apron upslope the Northern Twin Slide scar [11]. The deposits showed low over-consolidation ratio (OCR = 0.24–0.4) and low internal friction angle (20–22°) around 28–45 m. The sediment appeared to be under-consolidated. Values of the effective cohesion (c’), ranged from 0 to 30 kPa for granular to clay-rich sediments [11]. Conclusions The first generation of the MeBo system showed a strong improvement of the sampling possibilities for the marine geosciences. The new generation of the MeBo is


designed for ultra-deep water and deeper drilling with a coring possibility. MeBo provides a new and cost effective alternative to the services of drill ships. It will be the only system available for offshore technologies that can reach drilling depths of up to 200 m from standard research vessels. MeBo also has the major advantage that the drilling operations are performed from a stable platform independent of any ship movements caused by waves, wind, or currents. The development of the MeBo was funded by the German Federal Ministry of Education and Research and by the State Government of Bremen.

References [1] Y. Bai, Q. Bai: Subsea Engineering Handbook, Gulf Professional Publishing, 2012, p. 919. [2] J. Edmunds, J. Machin, M. Cowie: Development of the ROVDrill Mk. 2 seabed push sampling, rotary coring and in situ testing system. Proc. of Offshore Technology Conference 2012, OTC 23395. [3] C. J. MacLeod, T. Freudenthal, D. McInroy, D. J. Smith: Seabed rock drilling: previous scientific results and future applications. Geophysical Research Abstracts, 11, EGU2009-10358, EGU General Assembly 2009. [4] T. Freudenthal, G. Wefer: Scientific drilling with the sea floor drill rig MeBo. Scientific Drilling 5, 63–66, 2007. [5] C. Teodoriu, G. Falcone, A. Afolabi: Investigation of drilling problems in gas hydrate formations. Proc. of the ASME 27th International Conference on Offshore Mechanics and Arctic Engineering, June 15–20, 2008, Estoril, Portugal, OMAE2008. [6] M. Randolph: Offshore Geotechnics – The challenges of deepwater soft sediments. GeoCongress 2012, March 25–29, 2012, Oakland, United States, 241–271. [7] S. Micic, J. Q. Shang, K. Y. Lo: Improvement of the load-carrying capacity of offshore skirted foundations by electrokinetics. Canadian Geotechnical Journal 40, 949–963, 2003. [8] T. Lunne, T. I. Tjelta, T. By: Deepwater geotech-

nical challenges offshore Norway. Proc. of the International Conference on Deepwater Technology; Advancements and the Challenges, Mumbai, India, 1997. [9] J. L. Colliat, P. Boisard, P. Sparrevik: Design and installation of suction anchor piles at a soft clay site in the Gulf of Guinea. Proc. of Offshore Technology Conference 1996, OTC 8150. [10] C. Amaral: Personal communication 2013. [11] F. Ai, J. Kuhlmann, K. Huhn, M. Strasser, A. Kopf: Submarine slope stability assessment of the central Mediterranean continental margin: the Gela Basin. Advances in Natural and Technological Hazard Research. Springer (in Press).

Dr. Giovanni Spagnoli is the product manager for maritime technologies for BAUER Maschinen GmbH. Spagnoli holds a B.Sc. and a M.Sc. in Geotechnologies from the University of Milan-Bicocca(Italy) and a PhD from the Faculty of Georesources and Materials Engineering from RWTH Aachen University (Germany). He is member of the TC209 for offshore geotechnics and of the Helical Pile and Tiebacks Committee of DFI. He is adjunct lecturer at the University College Dublin. He has worked for Trevi, University of Bremen and Fugro before joining BAUER Maschinen GmbH. Dr. Tim Freudenthal (freuden @marum.de) is the project manager for the development of the sea floor drill rig MeBo 200 for University of Bremen/ MARUM (Germany). He holds a degree in Geology and a PhD with the topic “Reconstruction of productivity gradients in the Canary Islands region off Morocco by means of sinking particles and sediments” both from University of Bremen.

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