Abstract
To assess FPSO condition, it is necessary to adjust the inspection process and the inspection planning in accordance with the safety target. Crack propagation may be a leading criterion for designing inspection planning and repair planning. In order to realistically anticipate the possible propagation of either cracks not detected during inspection or cracks initiated after this inspection, a joint research project between three French companies (Nexter Systems, CETIM, and Bureau Veritas) has been carried out to obtain improved approach. This approach involves various steps of fatigue crack behavior leading to failure. This paper presents a part of this research dedicated to the development of tools applicable to FPSO structural assessment. This method takes into account the analysis of simultaneous crack propagation. It involves interaction between these cracks. This technique avoids successive re-meshing all along the crack path. Multi-crack initiations are calculated using a multi-axial fatigue damage criterion based on a local approach. A Line Spring Method coupled with a fitted structural stiffness condensation method is used to calculate the stress intensity factors, with a view to determine the crack propagation rate. This method allows monitoring the redistribution of the stresses generated by crack opening. This approach has been then validated on a welded mock-up, which was designed to provide several crack locations in different zones of the structure. An extended instrumentation of this welded structure has allowed tests to check accurately crack initiations and to follow crack propagation. The results are in good agreement with calculation predictions. This approach is now used to assess fatigue crack propagation in a FPSO. An example is provided on a welded connection between bracket and longitudinal stiffener, on the transverse bulkhead where a propagation of four cracks has been followed. This analysis shows an important gain in the fatigue life span prediction versus current fracture mechanics analysis. This gain is mainly due to a better interaction analysis between local cracked area and global structural behavior.
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