Page 388 - Pipelines and Risers

Use of High Strength Sieel 357

The allowable hoop stress for the operating condition is 0.72 x SMYS for purposes of wall
thickness and material grade selection.

iii) Britannia Pipeline
The Britannia Field is a gas condensate reservoir in the Central North Sea approximately 200
km north-east of Aberdeen and 45 km north of Forties. Britannia Operator Ltd. POL) is a
joint venture established by Chevron and Conoco for the Operatorship of Britannia on behalf
of the Co-venturers.

Dry, dewpoint controlled gas will be exported in dense phase mode through a pipeline to an
extension of the Mobil SAGE Terminal at St Fergus. At the terminal, the gas will be
processed for delivery into the British Gas National Transmission System. Offshore
condensate will be delivered to the Forties Pipelines System through a condensate export
pipeline from the Britannia Platform to the Forties Unity Platform.

The Gas Export Pipeline is nominally 28-inch diameter, 186 km in length with a bore of 650.6
mm. The pipeline design pressure is 179.3 barg and the design life of the pipeline is 30 years.
The pipe grade is X70. The 14-inch Condensate Pipeline is 45 kilometers in length. The
Britannia pipelines were completed in 1997.

The section of pipeline between KP11 and KP126 was subject to reliability-based limit state
design techniques in order to justify a steel wall thinner than that permitted by BS8010.

Onshore lines are specified on the basis of transverse yield strength. The method of
manufacture of these steels (TMCP, UOE) means that the axial yield strength will be around 4
- 5 ksi (-30 Nmm-’) lower. Thus, X70 material specified for a land line may only be
equivalent to a subsea line specified to have X65 properties in the axial direction.

19.1.2 Usage of XSO Linepipe
i) General
High strength large diameter pipes are available from steelmakers e.g. Europipe for pipe
diameter 20 - 60 inches and wall thickness of 12 - 32 mm, see Graf and Hillenbrand (1997).

Grade X80 carbon steel linepipe is only now becoming accepted onshore and has not yet been
installed as subsea pipelines. Five onshore projects have been identified in which X80 pipe
has been used. Available details are summarized in Table 19.4.

The first two small projects (Engelman et al. (1986), Matouszu et al. (1987)) were conducted
on a trial basis by inserting X80 sections in X70 lines. They demonstrated production and
construction capabilities but the X80 sections are only required to operate under X70 design
conditions (i.e. operational stresses).

ii) Ruhrgas Pipeline