Reliability analysis of FPSO components and calibration of safety factors using response surfaces

In this paper a procedure is presented for the reliability analysis and calibration of safety factors for ultimate limit states of the hull and mooring system of an FPSO system. The formulation of the limit state equations is given first. The method of response surface (RS) is used to relate extreme environmental variables to bending moment on the hull and to tensions on the mooring lines. Second order polynomials are used for the RS, and its coefficients are estimated by regression using data from structural analyses. For the purpose of assessing the ultimate capacity of mooring lines, these are modeled as series system composed of many elements. The moorings capacity is then estimated as a function of the number of elements and their individual tension resistances using extreme value theory. For the calibration of safety factors, a simplified method, which decouples reliability analysis, is used. A hull model and three mooring system models at different water depths were used as an application example for calibration of safety factors considering metocean conditions of the Mexican region of the Gulf of Mexico.