The 32" diameter sealine which runs for 110 miles from BP's Forties Field to the east coast of Scotland was completed last year. The flow of oil began on the 12th September 1975, making this the first of the pipelines serving the crude oil producing areas of the Northern North Sea to be commissioned. At the time of its conception and design, it represented a considerable step forward in offshore technology with regard to, the combination of diameter, depth and the hostile environment in which it had to be constructed. Larger lines are now under construction, some in deeper water in the North Sea, but this is perhaps an appropriate moment to review the technical achievement of Forties and to summarizes what has been learnt.
The main line was designed and built in 4 years. Work began on the design in Autumn 1971 and this was followed by a series of tests in 1972, culminating in the full scale Mediterranean trials using one of the barges which was subsequently employed in the North Sea. The construction of the line itself was spread over the 3 seasons 1973 to 1975.
BP had considerable prior experience of submarine pipelaying, based principally on work in the Middle East and, more relevant, on the construction of a 45 mile 16" gas line laid in 1966 in the Southern North Sea. In neither area however, did the water depth exceed 150'. Throughout the whole industry experience was limited in 1971 arid can be simply summarized as being in the range of a 20" diameter line in 350' to a 48" line in 125?. BP were faced with laying a pipe of 30" plus in diameter, over a length of 110 miles, the majority of which was in the depth range of 350' to 450'. Add to this expected weather downtime of 50% in Summer with seabed conditions ranging from sand waves to stiff clay, and it can be seen that a major task faced the company and its contractors.
On such an unpredictable job, a contractor would only accept responsibility for the cost of a buckle (and certainly never the consequential losses this might incur) by payment of an exceptionally large contingency sum. BP was therefore obliged to accept in the main reimbursible contracts with only limited incentive payments, and for this reason it was decided to exercise a stronger measure of control throughout all stages of the contract. This level of supervision was virtually unprecedented by the company and it is to our contractor's credit that their full co-operation was secured.