Detailed design of British Gas Blake Development including subsea manifold.

With a depth of experience in designing flowlines and subsea structures, APA was awarded the design contract for this prestigious development, located in the South Halibut Basin 60 miles north of St Fergus, by lead contractor Coflexip Stena Offshore Limited (CSOL).The centrepiece of the seabed structure, which has a capacity of 40,000 barrels of crude oil a day, is a 200 tonne manifold and control system which governs and connects eight subsea production wells. The manifold incorporates two subsea multiphase flow measuring devices and will be linked to Talisman's production storage and off-loading vessel Bleo Holm.

During the challenging 16 week schedule, APA prepared the full 'Approved for Construction' package for the Blake manifold which allowed the client to obtain complete and accurate tenders from fabrication contractors. An essential part of the design process was the use of 3D modelling which ensured that the structure's components didn't clash - which can and does occur if the structure is designed in 2D.

The Aberdeen-based APA project team optimised the structure layout ensuring it was as compact as possible whilst still meeting functional requirements. The structural design incorporated stringent protection requirements against potential damage from fishing gear and dropped objects.

Production pipelines connecting the manifold and Bleo Holm consist of a 10 inch inside a 16 inch 'pipe-in-pipe' insulated oil pipeline and a 12 inch pipeline coated with a thick layer of insulation. In addition there is a 12 inch, plastic lined pipeline for injecting water into the oil reservoir and a 6 inch gas lift pipeline for providing gas to oil producing wells to improve performance. APA prepared the detailed design of the pipelines for their hydrotest and operational design conditions taking account of the client's preferred method of installation, the reeling method.

All aspects of the design were covered including optimisation of the wall thickness and pipeline stability and calculation of the amount of rock-dump or soil backfill required to mitigate the potential for the pipelines to buckle during test or operation.

APA's design work confirmed that the proposed pipe-in-pipe system and insulation coatings ensured the production fluids reached the Bleo Holm at the necessary temperature to be compatible with the FPSO's production processes and to ensure the product remained in the optimum condition for flow in the pipelines.