Engineering critical assessment of pipeline welds
Connect, no.77, June 1996
Total Oil Marine (Total) has installed a co-axial insulated pipe system for the Dunbar Ellon Subsea project with help from TWI engineers in ensuring pipeline integrity. The system consists of a 16 inch pipe of X60 strength, surrounded by a 20 inch external pipe of X52 strength. The interspace is filled with fibre glass and provides very good thermal insulation, so that the nominal gas / oil mixture temperature loss is minimised over the 22km pipeline length. The pipeline is laid in water with a temperature of about 5 degrees C. The insulation avoids the formation of detrimental crystals.
Sections of the system of 24m long are pre-fabricated onshore and completed on a laybarge. Several onshore welds are difficult to inspect, or the X-ray and UT records hard to interpret. During the design and the pre-fabrication stage of the project, CTOD testing was carried out and TWI was approached to carry out an engineering critical assessment of several welds.
Calculations were carried out to assess the welds within the co-axial insulated pipe system. Conservatively assumed surface flaw dimensions were assessed and found to be safe. In addition, tolerable surface flaw dimensions were calculated. Results indicate that fully circumferential surface flaws of at least 4.4mm depth can be tolerated in each of the welds assessed, giving the client confidence that the integrity of the pipeline is ensured throughout its design life.
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