FPSO Fatigue Assessment: Feedback From In-Service Inspections
Bluewater developed a hull fatigue strategy for her FPSOs. This strategy supports the decision process with respect to fatigue damage during the hull selection, hull modification and lifetime extension, hull monitoring/ inspection and offshore hull repair. The essential element in this process is the correct identification of the fatigue sensitive structural details and determination of their remaining fatigue life. As this remaining fatigue life can not be measured directly, a calculation procedure is necessary to estimate this remaining life. In the presented paper, the hull fatigue strategy is discussed and the side shell structure of FPSO Uisge Gorm is used as an example. After seven years of service as an FPSO, the applied strategy for the Uisge Gorm is evaluated. Using feedback from tank inspections, the predicted remaining fatigue life is compared with the actual cracks found. This information is used to improve the assessment procedure for fatigue sensitive structural details. The consequences for the hull fatigue strategy are discussed. Attention is given to improved models that predict the loading on the structural detail. In addition, some attention is given to (future) in-service monitoring systems that can determine this loading while the FPSO is at a field. Using such methods, the remaining hull fatigue life predictions will be updated continuously during the field lifetime.