FPSO Fatigue Assessment: Feedback From In-Service Inspections
Authors: Hoogeland Martijn G. , van der Nat Clemens G.J.M. , Kaminski Mirek L. 
Published: October 2008

Abstract
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.

Introduction
The hull design and construction of Floating Production Storage and Offloading (FPSO) units are often based on those of crude oil tankers. The design practices for a trading oil tanker are based on shipping experiences in which periodic drydocking for inspection and repair is requested by the classification societies. Due to the high cost of lost production, hookup and (re-) commissioning, temporary abandonment of a field is not economically feasible for a permanently moored FPSO. Consequently, an alternative hull inspection and repair program is required that takes into account the continuous production of the FPSO at the field. The FPSO will in principle stay at the field as long as the oil production is economic. The alternative inspection program must allow this continuous stay offshore and the typical structural loading due to the (severe) environment and loading/offloading regime.
The following sections focus on the Bluewater strategy to manage fatigue damage in FPSO hull structures. The objective of this strategy is to control fatigue damage such that continuous FPSO service is achieved without significant inspection and repair activities offshore. This strategy requires reliable fatigue damage predictions in which the assumptions are consistent with workmanship during conversion and operational procedures during oil production at field. The following section will show that different predictions are required for each phase of the FPSO life.

Life time extension strategy for FPSOs
The FPSO fatigue life strategy covers the whole lifetime of the FPSO. It starts when a hull is selected and ends when the vessel is taken out of service. The following steps are important during this lifetime with respect to hull fatigue:Hull selection;Hull modification and lifetime extension;Hull monitoring/ inspection;Offshore hull repair.