Background and Status of the FPSO Fatigue Capacity JIP

Abstract

Abstract A joint industry project on the Fatigue Capacity of FPSOs is being carried out with 19 participants from oil companies, designers, shipyards, classification soceieties and one university. This paper presents the background and the status of the project. The main objective of the project is to reduce the risk for fatigue cracks and fracture in FPSOs. Part of the work is carried out by a number of participants as work in kind. The project will provide small scale and full scale S-N data on typical fatigue sensitive details in FPSOs and tankers. It will also provide improved information on the important link between S-N data and finite element analyses for fatigue life assessment. Finally it will provide data to assess more reliable the risk for unstable fracture of FPSOs when fatigue cracks are present in the structure. Background In offshore oil production there is a trend towards Floating Production, Storage and Offloading Units (FPSOs), either on the basis of converted, used tankers or as newbuildings with a typical tanker structure. Experience from tankers built in the seventies and eighties have shown that especially the side longitudinals have been susceptible to fatigue1,2. Based on this experience the classification companies have introduced more strict fatigue requirements for tanker design3–7. Various FPSO units are now entering the North Sea and other harsh environments, such as the North Atlantic. FPSO units can be deployed in marginal fields, at remote locations and in deep water. A clear advantage with these units is storage of oil, possibility for relocation and rather early production. Fatigue cracks may lead to stop of the field production, which can hardly be tolerated. Early fatigue problems must therefore be avoided, as a repair during service life might be very costly when cost of lost production is considered. Therefore it has become more important to assess the fatigue life of an FPSO at a design stage in a more reliable manner than was considered necessary for sailing ships where fatigue cracks might be repaired as part of a normal maintenance program. Design requirements to fatigue of tankers have to a large extent been developed based on service experience. Floating production units have also been in service for several decades, but the concept of the permanently turret-moored tanker is relatively new. Most of the FPSOs in service have still to prove that they can meet the expectations regarding their fatigue life performance, as the majority of them have been in use for less than ten years. Nowadays sophisticated analysis methods are available for fatigue life predictions of details of FPSOs. However, the reliability of such analysis is significantly depending on the fatigue capacity of the considered details (S-N data) used as input. Therefore it is considered important to reduce the uncertainty associated with the S-N data that are being used for design of typical ship details in order to improve our confidence in the analysis procedure used to calculate the fatigue capacity of these details8–10.