Temperature Dependent FPSO Ultimate Strength Reliability

Structural Reliability analysis is based on probability theory and a significant amount of literature exists on the subject and continues to be a subject of ongoing research. Most classification society criteria and design procedures are based on the first yield of hull structures together with buckling checks for structural components and not on the basis of the ultimate strength of the hull girder. These methods can be effective for intact vessels in normal seas and loading conditions but they fail to assess the safety of ageing or damaged structures, which might collapse under applied loads even smaller than design loads. When the structural safety of a ship’s hull is considered, the ultimate hull girder strength should be evaluated and analyzed using a stochastic approach. Existing literature on the subject often neglects the effect of temperature. This paper aims to determine the effects of temperature in ship structural reliability and to propose a procedure for analyzing structures by taking into account temperature effects. The ultimate strength of the hull girder was calculated using a component approach, where the behavior of the hull is evaluated based on the behavior of the single structural components. A sample analysis for Tanker/FPSO structures is provided where the reliability analysis was carried out using a First and Second Order Reliability Method (FORM and SORM) analysis. Annual reliability indices and probabilities of failure were calculated for hogging and sagging conditions showing the effects of temperature and improvements made when the scantlings were updated to accommodate for the effects of temperature.