Sea Technology

AUG 2017

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12 st / August 2017 www.sea-technology.com relief and a plateau formed by tectonic activity and mass wasting events origi- nating from the western valley wall. The vertical displacement is almost 1,000 m from the scarp edge to the valley floor. The seafloor consists of fine-grain sediments and basalt rock. A small area of gravel was also located during the operations. ROV Platform For the drilling operations, a stan- dard Forum Energy Technologies Perry XLX 150 ROV was used. The ROV weight is 4,800 kg and has main di- mensions of 3,526 by 1,893 by 2,070 mm (length, base, height). The vehicle can produce 900 kgf vertical bollard pull, which stabilizes the vehicle and drill rig during drilling. The hydraulic manipulators were removed during the core sampling. The ROCS was designed to be mounted and operated from a work- class ROV or similar subsea asset with comparable capabilities. The ROCS re- lies on the host ROV for hydraulic flow (50 L/min. at 200 bar), electrical power for the valve pack and telemetry. The ROCS concept faces several unique challenges associated with drilling from an ROV. To be portable and eas- ily fitted to a wide range of ROVs, the ROCS should not significantly degrade the capabilities of the ROV's maneuverability and buoyancy. This is accomplished by keeping the ROCS design compact, lightweight and rela- tively simple, with the goal of obtain- ing a single core sample 75 by 1,000 mm from a variety of seabed materials. The system drill capability can be ex- tended to 3,000 mm. Core drilling normally depends upon rotational torque and bit weight, usually supplied by a platform with large reactive weight firmly situated on the seabed. Without a large reactive mass to enable high downward force and stability on the seabed, the ROCS compensates through use of a thin wall drilling bit and barrel operating at high rpm with minimal down-force (weight of the ROCS plus the available down thrust provided by the ROV) and core lengths limited to 1 m (3 m optional). While low bit weight is utilized for drilling, significant drawback force is required to break the core sample from the substrate and extract it from the seabed. Drawback force of 4,500 kgf is provided by feed cylinders and a breakout clamp secured to the upper portion of the core barrel. Should the core barrel become stuck in the drill hole, emergency release is provided by detents in the rotary head that will release the core barrel at an upward force somewhat greater than the com- bined weight of the core barrel plus sample, thus leaving the core barrel in the hole but allowing the ROCS and ROV to be recovered. ROCS Design The ROCS design features a low torque (235 N-m max), low drilling force (100 kgf max) and high rpm to obtain samples of 75 mm in diameter by 1 m in length. Power for the cor- ing system comes from the hydraulic power system on the ROV. The ROCS consists of the frame assembly, rotary head assembly and cross head assem- bly. The frame assembly consists of ver- tical frame members that serve as the linear rails for the attachment of the ro- tary head assembly and the cross head assembly. The rotary head assembly consists of a two-speed hydraulic mo- tor mounted to a housing containing bearings, a shaft and an integral water swivel. This shaft contains an external spline that mates with the spline of the core barrel. This interface transmits the drilling torque and downward force. It also contains a ball detent system used to lock the rotary head to a groove inside the core barrel. To remove the tool, a force slightly greater than the weight of the tool plus the core sample must be applied. This function also serves to provide a low-force, simple emergency release system. It also con- tains a vertical positioning rail that pro- vides alignment of each assembly. The cross head assembly controls the drill- ing feed, core barrel pull back, vertical positioning and clamping operations for the drill tools. It comprises two ver- tical feed cylinders, a clamp mecha- nism and linear bearings to attach to the frame assembly. The hydraulic system consists of a 10-function valve pack powered by the ROV hydraulic system. Standard marine materials are used in construc- tion: 6061-T6 anodized aluminum, 316 stainless steel and UHMW PE plastic. The valve pack and drill head are pressure compensated; the only component requiring a 1-atm pressure housing is a linear sensor providing position of the cross head assembly. This housing is rated to 3,000 m; this

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