Time-variant reliability assessment of FPSO hull girders

Floating production, storage, and offloading (FPSO) systems have been used for the development of offshore oil and gas fields. This paper presents a methodology for the time-variant reliability assessment of FPSO hull girders subjected to degradations due to corrosion and fatigue. The corrosion defect is modeled as an exponential time function with a random corrosion rate. When corrosion occurs, the plate thickness is assumed to be uniformly reduced. The crack propagation is predicted based on a Paris–Erdogan equation. A modified Smith method and an ultimate strength equation are proposed to predict the progressive collapse and ultimate strength of the hull girder with an effective width of plating and beam-column formula. The modified Smith method and equation account for the interaction of axial compression, lateral pressure, transverse stress and shear stress. The Ferry–Borges method is applied to combine the stochastic processes of still water and wave-induced bending moments. The environmental severity factors are introduced in order to calculate the wave-induced bending moments of specific-site conditions. Failure is defined as the first outcrossing from the safe domain to failure domain for t>0. An efficient response surface approach is used to approximate the outcrossing rate with the independent excursions of high levels. The time-variant reliability and the parametric analysis are quantified. The COV of ultimate strength of hull girders is suggested for simple FORM. Finally, an example calculation is conducted to evaluate the effect of environmental severity factors and the ship's principal parameters on the required hull girder strength of FPSO.