Page 164 - Pipeline Pigging Technology

Inspection of the Forties sea line

After the introduction of the inspection systems into full operational use
in British Gas, a decision was taken to offer the inspection service on a
commercial basis to oil and gas pipeline operators world-wide.
BP was one of the first companies to use the inspection system, with the
inspection of its 30-in crude oil pipeline between Kinneil and Dalmeny in
Scotland. Following this operation, and the subsequent inspection of the 213-
km, 36-in Forties landline between Cruden Bay and Kinneil, an agreement
was reached between BP and British Gas to produce a 32-in inspection system
to inspect the Forties submarine pipeline linking the Forties field with the
landline at Cruden Bay in Scotland.

PIPELINE DETAILS

The 169-km long Forties sea line was installed in 1973/4 to carry produc-
tion from BP's Forties field to the landfall at Cruden Bay in Scotland. This
pipeline is part of the 380-km of offshore and onshore pipeline which makes
up the Forties pipeline system (Fig.l).
When laid, it represented the biggest offshore pipeline diameter (32in)
that could be used at that time, being constructed of steel grade 5LX65 with
a wall thickness of 19mm. Design pressure of the pipeline was 2084 psig
(I42bar).
Since their discovery, the Forties field reserves have been increased four
times from an initial 1800 million barrels of oil to a current 2470 million
barrels. The field recently celebrated production of its two billionth barrel.
The pipeline also now carries production from the Buchan, South Brae,
North Brae, Montrose and Balmoral fields, as well as Hemtdal in the
Norwegian sector. BP's Miller field is scheduled to produce into the line early
in 1992.
Production feeding through the Forties system during the first three
months of this year peaked to 565,000 barrels during a 24-hr period in January,
1990, and has averaged some 500,000 barrels a day, of which nearly 275,000
barrels was Forties field production.
Routine conventional monitoring of the pipeline system by BP had already
identified the existence of some corrosion, and hence it was deemed
necessary for the British Gas inspection system to accurately locate and
quantify such corrosion in order to maintain the maximum operating through-
put of this strategic oil line.
This routine monitoring led to the replacement in 1986/7 of part of the
main sea line riser. The riser contained the internal metal-loss characteristic

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