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Evaluation of fatigue for service of ageing pressure vessels

TWI is working with an Industrial Member from the oil and gas sector to provide a cost-effective method of evaluating the fatigue life of in-service pressure vessels.

The company has many similar pressure vessels installed on platforms in the North Sea, which are reaching or have reached the end of their design lives and yet continue in service. The vessels were designed to BS 1515, which did not require fatigue assessment, so an evaluation of the remaining fatigue life is an important part of the life extension process.

Detailed fatigue life assessments for many vessels would be costly and time consuming, leading the operator to seek a low-cost methodology for justifying life extension of several vessels designed to older codes through fatigue analysis and a campaign of risk-based inspection. TWI is supporting this by developing a process that uses a benchmark vessel to justify life extension of other similar vessels remaining in-service.

The member company removed one of the vessels from service after processing was diverted to a different platform. TWI carried out detailed examinations and testing on this vessel to investigate the in-service degradation, manufacturing quality and material properties. Both non-destructive and destructive examinations were conducted at fatigue-critical locations and areas of high corrosion. The examinations revealed an overall good manufacturing quality, with low levels of in-service degradation and no evidence of fatigue cracking.

The results of this work will be used as evidence to support the life extension of the other vessels that remain in service. The final stage of the work will determine the commonality between the benchmark vessel and similar vessels remaining.

TWI will also develop criteria for selecting the vessels with the highest fatigue usage for more detailed fatigue analysis and inspection, based on the benchmark vessel findings. This will enable justification of life extension of a number of vessels, for a fraction of the cost required to perform a full life extension assessment on each vessel individually.

For more information on our work to support the oil and gas industry, please email

Benchmark vessel used for detailed examinations and testing
Benchmark vessel used for detailed examinations and testing

For more information please email: