Fatigue reassessment to modern standards for life extension of offshore pressure vessels

Life extension of ageing assets is becoming increasingly important for the offshore oil and gas industry. Many pressure vessels in service have reached or are about to reach the end of their design lives, but their continued operation is required until the economic field life is exhausted. Many vessels in-service were designed over 30 years ago, when fatigue assessment was not required by the design standards. Therefore, fatigue reassessment is a critical part of the life extension process. This paper presents reassessment of a benchmark vessel as a case study for life extension of other similar vessels. Life extension assessments are costly and time consuming, often hindered by a lack of information and a lack of access to the vessels. By determining the commonality between a vessel and the benchmark vessel, it may be possible with suitable on-going in-service inspection to justify life extension of the vessel without the need for a full fatigue life extension reassessment in every case. The case study considered in this paper is a condensate flash separator vessel constructed in the early 70s which was in operation for 25 years; and is similar to many pressure vessels still in service on offshore platforms. The fatigue lives of key features of the vessel have been calculated and compared using different modern pressure vessel design codes, supported by finite element analysis.