Calibration of Partial Safety Factors for Two Adjacent Deep-water Free Spans

Fatigue damage of free spans has always been a primary concern of owners, especially deep-water pipelines which are more likely subjected to uncertainties compared with shallow water pipelines. In detail design, fatigue limits state (FLS) shall be checked by incorporating partial safety factors, in order to ensure the adequate safety margin. At present, the commonly-used design code for free spans, e.g. DNV-RP-F105, recommends safety factors for single and multiple free spans. However, the coupling of in-line and cross-flow loads and responses is not considered to calibrate partial safety factors in DNV-RP-F105. The objective of this paper is to propose a simplified analytical method used to calibrate safety factor(s) for FLS of two adjacent free spans. The structural model considering the coupling of in-line and cross-flow fluid loads is proposed in this paper to obtain the time-history of stresses. The reaction of span shoulder(s) is approximated by linear dampers and springs. All the uncertainties are assumed to be on load side and fatigue capacity. The partial safety factors are calibrated using the limit state function based upon S-N curve and compared with those recommended in DNV-RP-F105.