Assuring the Integrity of Subsea Pipeline Butt Welds through Design, Construction and Operational Life.
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Subsea pipelines may be designed, using strain based approaches, to operate under thermal longitudinal tensile stress levels approaching and exceeding the pipe material yield strength. A particular concern is the integrity of the butt welds made using the mechanised methods required for economic pipe lay from a barge offshore. Weld flaws such as lack of fusion or penetration are unavoidable which presents a concern that they may propagate by fatigue and fracture, under operational stresses, to cause hydrocarbon release. The weld integrity is confirmed by fracture mechanics analysis, called engineering critical assessment (ECA), which predicts a maximum allowable flaw size for the automatic ultrasonic testing (AUT) conducted on the pipe lay vessel (barge). An important input to the ECA is the weld toughness (fracture initiation resistance) determined by fracture mechanics tests on welds at minimum design temperature. For pipelines conveying sour hydrocarbon additional fracture mechanics testing in simulated internal H2S environment was required which recorded significant reduction in toughness through the effects of corrosion. The toughness levels were in some cases insufficient for the ECA to confirm weld integrity using the published equations (BS7910, DNV-OS-F101). The problem was resolved by detailed 3D finite element analysis (FEA) of flaws assumed in the butt welds to improve accuracy of analysis and reduce excess conservatism. The management of ECA and associated design and construction activities (e.g. WPQT, AUT) to minimise the risk to project schedule is discussed. Published documents (e.g. DNV-OS-F101, EPRG) provide weld maximum allowable flaw sizes based on previous ECA and/or large scale tests as an alternative to ECA but they have not yet been widely adopted for subsea design/construction projects. Finally the paper reviews how the integrity of the butt welds in the operating pipelines is managed by external survey, monitoring of operating conditions and in-line inspection.
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