When used for field development the dual-activity rig becomes even more efficient. After landing the BOP stack the ship can be moved so that adjacent wells can be drilled by the auxiliary rig. The offset required to reach a new well is well within the riser angle limit. Up to nine additional top holes can be drilled and cased through the auxiliary rotary table while the main rig is drilling out under the 17-1/2 in. top hole in the first well. The BOP can then be moved to the next well without pulling to the surface, and the pressure section of the wells can be drilled batch style.
While drilling through the BOP the second rig can be used to rig up and run the production trees. The well is secured, the BOP released and the rig positioned to land the tree. The stack can be moved to another well or set atop a horizontal tree for completion. In any case the tree is safely run without having to trip the BOP. In the event the tree does not test, it is not necessary to double trip the stack to retrieve the tree.
Construction Vessel Capabilities
As a construction vessel the Discoverer Enterprise compares favorably to typical dedicated deepwater construction barges. The capacities of the derrick (used as a “J-lay” tower for pipeline installation), stationkeeping, and sea bed survey equipment on the Discoverer Enterprise surpass those found on the current fleet of deepwater construction vessels. Moreover, the dual derrick allows two pipelines to be laid simultaneously without a significant increase in spread costs. Table 1 sets out a comparison between the Discoverer Enterprise and a typical deepwater J-lay barge. Figure 2 illustrates the difference in size between the same two vessels.
Tower.
Hook Load. Large lifting capacities are required to J-lay pipe in deepwater. Flow assurance often demands heavy pipe-in-pipe insulation systems, and project economics may not allow export line wall thicknesses that can withstand the collapse pressure acting on an evacuated line. Wet pipe weights can be as high as 180 lb/ft and, coupled with the tension requirements and dynamic loading, can result in tower loads up to 1,500kips. According to Albaugh (Ref.1) typical capacities of existing J-lay barges run between 150 kips and 792 kips. This compares to the twin hookload capacity of 2.000 kips for the Discoverer Enterprise. There are two hoisting drawworks in the dual derrick. Each is fully outfitted and rated for 2 million lb hookload. The normal rig landing string is 6-5/8 in. 27.7 lb S drill pipe rated at 1 million lb. For larger loads the riser can be used. The 2 million lb hookload is simultaneous and can be achieved by both drawworks at the same time. This is useful for emergency recovery or for operation in 10.000 ft of water.
Motion Compensation. The drilling rig has two 1,000 kip motion compensators and can in fact provide a constant tension at levels higher than that of the ultimate lifting capacity of typical lay barge towers. The two compensators can be put to good use for delicate hingeover operations or for emergency laydown or recovery operations.
Dynamic Derrick. The Dual Derrick on the Discoverer Enterprise is rated at 2 million lb each hook (simultaneous) while suffering 40kt winds and 18 ft significant sea acting 30° off the bow. A hook load of 1,800kips on one rig simultaneous to 1,400 kips on the other rig can be maintained while in 60kt winds and a 25 ft significant sea acting 30° off the bow. The dynamic capacity of the derrick allows emergency operations in rough weather. Typical lay barge towers have reduced sea state and subsequent motion resistance capacity and are thus required to cease operations more often.
Dual Pipe lay. Both rigs are rated to 2 million lb and can be used to simultaneously lay two lines. The rotary tables are 40 ft apart and provide good separation between lines. As illustrated in Figure 3 the rig can change heading ±45° about the neutral point and still achieve adequate separation between pipelines. When 45° from square the pipelines are only 11 ft closer together which is insignificant in deep water. The ability to make a ± 45° heading change coupled with the very good motion characteristics permit the dual lay operation even in winter.
Stationkeeping Power. The Discoverer Enterprise is outfitted with six 7,000 hp thrusters and designed to stay on location during a 50 year Gulf of Mexico hurricane. The thrusters can keep station with a 60 kt wind directly on the beam and while only running at 80% capacity. The azimuthing thrusters can develop 1,200 kips thrust. This compares to up to 800 kips of thrust potential from the most powerful J-lay barges. The drillship power generation is 52,000 hp continuous and capable of 70,000 hp instantaneous. This increased stationkeeping allows the drillship to work year round and add to the enhancement of schedule flexibility.
Pipehandling Machines. The Discoverer Enterprise is equipped with two vertical pipehandling machines. Each unit is capable of working either rotary table or any setback and provides backup. The pipe handlers are designed to handle 138 ft stands of 3-1/2 in. - 20 in. pipe and can pick up a 125 ft stand of 9-1/2 in. drill collars (27,000 lb). The machines are designed to set and align threaded pipe for makeup and are computer controlled. The pipehandlers can be commanded to center the pipe in the rotary and align to vertical. The rig is outfitted with all manner of power slips and casing elevators to run the pipe.
Special Operations.
Subsea Power. There are four 2,200 hp high pressure mud pumps on the Discoverer Enterprise which can deliver 8,000 hydraulic hp @ 7,500 psi to seabed machinery. This enables the rig to power seabed winches, trenching machinery or other equipment that heretofore were limited by the hydraulic power available from the ROV, One such application is the burial of pipelines. No equipment of such capacity is normally provided on pipe lay barges.
