Case Histories Of Selected Subsea Developments
Authors: Brookes D.A. 
Published: October 2008

The paper describes BP's subsea production projects on the Buchan, Magnus, Don, SWOPS and High Island fields and the operational experience and lessons learnt on these developments. It concludes with a summary of how BP will be meeting the future subsea challenges from past experience.

BP'S ACHIEVEMENTS

The BP involvement in subsea completion's started more than 12 years ago with the development of the Buchan oil field, and now there are some six separate producing developments world-wide (see figure 1). Four of these have what can be called "permanent" facilities, with the other two subsea completion's being accessed through BP's novel Swops vessel, the "Seillean".

Buchan

The Buchan oil field lies some 154 kilometres ENE of Aberdeen and 46 kilometres NW of BP's Forties field. Economic appraisal of the field with a complex and relatively small reservoir required a low-cost, fast track approach to the development.

The development now has the following subsea components (see figures 2, 3 and 4):A floating oil production platform converted from a semi-submersible drilling rig installed in a water depth of 118 metres over a subsea template and manifold arrangement,There are five production wells drilled through a steel template, with four satellite wells, the furthest being some 2.5 kilometres from the platform, linked to the manifold by 4-inch rigid steel flowlines and associated hydraulic control umbilicals.A production/export riser system consisting of a single 12-inch export riser surrounded by nine 4-inch production risers and one 4-inch service riser. These nine production risers are clamped in tandem with 2-inch gas lift risers.Two wells have 2-inch flexible gas lift flowlines connected to them. This was the first time BP used flexible lines in the North Sea. Oil production began in May 1981, and various management improvements throughout the fields life have been undertaken. The Buchan floating production system has proved technically and commercially successful as a means of crude oil recovery from a marginal field such as the Buchan reservoir.

Buchan was BP's first subsea production system and although there have been problems it has been a technical and commercial success which is still in production. The fundamentals of the design are based on similar systems with 1970's drilling technology, In particular the rigid riser system and controls would seem dated today but overall they have worked well albeit with a high maintenance cost and steep learning curve in the first few years. Particular issues have included:-DHSV failures, earlier flapper type valves were changed out for dual piston type.Tubing hanger orientation problems were overcome with improved tools.The manifold and tree valves were an early source of problems with numerous leaks, this led to an intensive and successful programme for improved materials, assembly procedures and QA.The direct hydraulics system has worked safely but is very cumbersome in the moonpool area. There have been substantial corrosion problems due to incorrect materials, poor QA and contaminated hydraulic fluid. This led to hydrate formation in the control lines.