The decision by British Petroleum to develop offshore fields west of the Shetlands in water depths exceeding 500 meters within three and half (3- 1/2) years of discovery posed a unique submarine pipeline installation challenge. This paper summaries the salient features of a fast track program [o install a driverless sub sea pipeline system using rigid reeled pipe technology in an offshore frontier area.
There has been exploration for oil and gas in the Faeroes/Shetland Trough area since 1972. In 1992 British Petroleum (BP) Exploration and their co-venture, Shell U.K. Exploration and Production realized the correct mix of recoverable reserves and state-of-the-art driverless sub sea and floating production technology existed to make it commercially and technically viable to be the first Operator to undertake oil production West of the Shetlands
The location is frequented by unique and largely unpredictable seabed current regimes resulting from the interaction of the warm Gulf Stream, cold Arctic Stream, and various other intermittent flows. The nearly uniform current velocity through the water column is representative of a "slab current", With surface speeds of up to 4 kts range down to 1.5 kts at the seabed the current profile poses a technical challenge. With the location exposed to the expanse of the Atlantic the site is influenced by a broad range of environmental factors including tropical disturbances occurring as far away as the Caribbean.
Given operating limits of 2-3 meter significant wave height a loosely defined but narrow offshore construction season typically ranges between May and August. The prevailing sea directions during the work season are from the West and Northwest.
The Foinaven Phase I Development consisted of twelve flow lines joining two drilling centers to the FPSO and two flow lines to outlying water injection sites along with associated control and chemical injection umbilical.
Once laid the flow lines are connected using flexible pipe jumpers and the Driverless Maintained Cluster (DMaC) connection system.
In keeping with the UK CR[NE initiative and the Alliancephilosophy a minimum set of functional specifications were developed by MMCL to effect the design of the sub sea system. A key precept of the functional specification was that existing proven technology should be used where ever possible.
Principal characteristics of the lines are shown in Figure 1,
Salient pipeline design considerations:
No requirement for pipeline burial,
25 year design life,
For the water injection lines MDPE liner were used in lieu of exotic metals or reliance on chemical injection to control corrosion.
A "Reelable" Line Pipe specification based on APL-5 LX60, with the actual tensile strength of at least 15ksi greater than the minimum yield strength and the yield strength not greater than 78 ksi (538 Mpa). The metallurgy to give carbon equivalent of not more than 0.35
In order to ensure sufficient ductility a minimum elongation of 30°/0 was specified, Mill tests also required that samples be subjected to a strain ageing tests.
