RIASSUNTO
Abstract
BP has combined existing technology with appropriate SIMOPS (Simultaneous Operations) planning to safely achieve significant cost reductions in deepwater field development by running several subsea wellhead components offline of drilling activities. This was achieved by utilizing conventional subsea wellhead tools that have been adapted for use in open water with the support of a subsea winch located in the moonpool on a mobile offshore drilling unit (MODU) and an ROV. The traditional method of deploying and installing the equipment using drill pipe requires that this work be done in the critical path of operations, using the rotary table. This takes more time because running drill pipe takes significantly longer than deploying wire rope off a winch.
Offline operations included deployment of wellhead corrosion caps, gasket seal prep protectors, and two different stack-up height measurement tools, installation of lockdown hangers, guidelineless re-entry assemblies (GRA), brushing the wellhead bore, and injecting corrosion inhibitor. The ROV system capability has been key in removing these operations from rig critical path by facilitating the operation, installation and retrieval of the tools.
BP's Holstein Drilling Team challenged itself to generate best in class performances by installing the lockdown hangers and GRAs, using offline subsea wellhead tools and ROV technology. This offline operational approach was successfully used on BP's Gulf of Mexico Horn Mountain project. The Holstein Team refined the concept and redesigned the tools from lessons learned, allowing the scope of the work to be increased to 16 wells. Removal of the operations from the critical path resulted in an estimated savings of 31 days of rig time for the project. These repetitive operations on multiple wells, in a field development scenario, resulted in significant cost reduction of $9.75 million, while meeting all safety expectations.
Introduction
Holstein, a BP operated field in the Gulf of Mexico, will be the worlds largest SPAR platform when it starts producing in mid 2004. The Holstein SPAR located in 4344 ft. of water, will be configured as a truss SPAR with a 149 ft. diameter hull. A truss SPAR is composed of a solid cylindrical hull at the top that protrudes through the surface, a solid structure at the bottom that is filled with fixed ballast and separated from the hull by a truss structure with heave plates. All SPARs to date have incorporated air cans that individually provide buoyancy to top tension the surface piercing risers and support the production trees. On Holstein the hull tensions the risers using tensioners somewhat like those used on TLP's. The result is that the risers stabilize the hull of the SPAR, but due to this coupling to the hull, the risers are subjected to higher loads than air can systems.
The well systems for Holstein include a unique configuration of the vertical top tensioned production risers, including a dual casing riser and production riser tensioner. The risers on Holstein are larger than previous production risers in order to permit usage of the risers for both production and drilling operations.
To increase startup efficiency of the SPAR, the subsea wellheads were temporarily abandoned in a full ready condition during the MODU phase of the development. The riser design required ancillary equipment installation to reach its full ready condition, which included an inner riser lockdown hanger with a lockdown profile, a GRA to capture the outer riser and a corrosion cap. These subsea wellhead components, although critical to the SPAR operations, were not crucial for continued MODU drilling operations and could be removed from the critical path to improve operational efficiencies. It was planned to remove these wellhead operations off the critical path by using modified conventional subsea technology.