Time-variant reliability assessment of FPSO hull girder with long cracks

An efficient time-variant reliability formulation for the safety assessment of an aging Floating Production Storage and Offloading (FPSO) vessels with the presence of through-thickness cracks (i.e. long cracks), is presented in this paper. Often in ship structures, cracks are detected by means of close visual inspection when they have already propagated through the thickness. The propagation of long cracks in stiffened panels is therefore considered, as they may be present in critical details of the deck and/or bottom plating of the vessel. The mean stress effect due to the continuously varying still-water loading as well as residual stresses is explicitly accounted for in the crack growth calculation. An analytical model is established for determining the equivalent long-term stress range. Although it has been found that stiffened panels are tolerant to fatigue cracking, the probability of brittle fracture of an aging hull structure becomes more relevant with the presence of long cracks. The continuously varying still-water load effects due to the operational nature of FPSOs introduce additional uncertainties in the estimation of fatigue damage as well as in the likelihood of fracture failure mode. These uncertainties are accordingly considered in the analyses.