Technology Editor, Subsea & Seismic
PERTH, Australia --A new fiber rope deployment system (FRDS) contributed to the successful subsea installations made at Independence Hub in about 9,000 ft of water in the Gulf of Mexico. In a paper for the Deep Offshore Technology International Asia-Pacific Conference & Exhibition in Perth, Australia, Murray Dick of Subsea 7 described how the system worked.
The newly developed fiber rope and FRDS was used for all the deepwater lifts including installation of one wellhead as well as umbilicals, manifolds, subsea distribution units, stab and hinge over structures, flowline jumpers, and electrical and hydraulic flying leads, said Dick. The system was necessary because the depths exceeded that of existing crane and winch wires on the installation vessel.
In describing the installation events, Dick pointed out that the methods used to install the equipment generally were similar to those used to install equipment in shallower water. In fact, Dick said, the project shows that the industry has not yet reached the practical depth limit for subsea oilfield equipment installation.
The main parts of the FRDS are:
• CTCU: Six sheaves with individual drives de-tension the rope
• Storage winch to store the rope at low tension and to assure a constant back tension for the CTCU.
• Inboard damping device: to smooth the tension between the CTCU and the storage winch, to assure constant back tension for the CTCU, and to give constant spooling tension
• Outboard damping device used for constant tension control.
• Accumulator system: to reduce power peaks during active heave compensation
• HPU: hydraulic power pack
• Computer for dynamic control of individual machines and interactions between machines, monitoring and alarm functions and rope management system.
• Load Capacity -- 50 Te
• AHC Speed capacity -- 2 m/s
• Deployment speed @ 50 Te -- -- 1 m/s
• Recovery speed @ 46 Te -- 0.5 m/s
Bend optimized braid rope of about 56 mm (2.2 in), 12 by 12 strand MBL 218T which weighed 2.5 kg/m dry and 0.5 kg/m wet was used.