HPHT, Development of the Subsea Option

Abstract Shell UK Exploration and Production, operator in the UK sector of the North Sea for Shell and Esso. in the North Sea, have made a number of discoveries in and around the Central Graben area of the Central North Sea: these can be considered to be entirely or partially high pressure high temperature fields. Two of these fields, HeronIEgret and Shearwater, are currently under development, Heron and Egret are being developed by subsea technology. Several of these discoveries have field potential which will, because of their marginal nature or the extent of the reservoir, require the use of subsea technology for development. Introduction This paper deals with the field development issues, which have been overcome to make a subsea HPHT field a reality. In the early 1990's, when Shell Expro first considered developing HPHT reservoirs, subsea technology was still considered technically limited; particularly where difficult well conditions existed. Combinations of high pressures, temperatures, scales, salts and waxes were seen as precluding the use of subsea technology. This was due primarily to high operating costs, although technical risk was also a factor. It was a widely held belief at the time, that subsea equipment could not be made in a large enough bore size for wells to produce at economically attractive volumes. Shell Expro realised that an acceptance of these limits would result in a number of very attractive prospects remaining undeveloped for the forseeable future. Shell Expro therefore began addressing the well equipment related issues with their main suppliers, and have established that a viable subsea HPHT well is feasible with a 5" production bore. In May 1996 it was decided that funding should be committed by Shell Expro to both Cooper Cameron and ABB Vetco Gray to develop modular subsea HPHT Xmas Tree systems, based on the use of their conventional 18 ¾" marine wellhead systems. These systems were intended to be run from a semi-submersible rig. Study work was designed for the two vendors Cooper Cameron and ABB Vetco Gray to evolve their existing conventional subsea wellheadltree systems to a subsea HPHT design, capable of operating at 15,000 psi and 350°F. The Shell Expro effort has been supported by BP UK Exploration. These Vendors had existing supply and through life support agreements with Shell Expro for subsea equipment, and both agreed to support the development by joint funding on a 50150 basis with Shell Expro. Both Vendors offered proposals to develop their technology shortfalls in subsea HPHT Xmas Tree equipment. They highlighted areas where they felt confident that their own designs and equipment already existed in a proven design and would not need significant development to package in a subsea Xmas Tree system. They identified areas where testing and qualification would be required at the elevated temperatures and a program was designed to complete this. In the case of ABB Vetco Gray, they were encouraged to utilise experience and designs from the then recently awarded Texaco Erskine platform HPHT equipment; in particular valve design and testing.