Methodology for Time Domain Fatigue Life Assessment of Risers and Umbilicals

Abstract

Risers and umbilicals are exposed to dynamic loading from waves and floater motions. These structures are known to have a pronounced non-linear response characteristic. Non-linear time-domain finite element analyses is in general required to give an adequate description of the non-linearities involved. Analyses of a large number of short-term environmental conditions considering stochastic wave loading are required to give a representative description of the long-term fatigue loading on the structure. Short term fatigue damage is established by means of rain-flow cycle (RFC) counting in each stationary short-term condition. It is has been experienced that significant statistical uncertainties can be present in the short-term fatigue damage estimates. This is because the accumulated fatigue damage in a stationary condition normally has significant contribution from the largest stress cycles in the realisation. Selection of proper simulation length is hence essential to obtain reliable fatigue life estimates. Applicable codes and standards for risers and umbilicals provide Design Fatigue Factors (DFF) to secure adequate safety against failure due to wave induced fatigue. The total uncertainty in the calculated fatigue damage comes from various sources and the DFFs in e.g. DNV-OS-F201 “Dynamic Risers” and API-RP-2RD corresponds to a certain uncertainty level in the fatigue damage estimate. A recommended target value for the statistical uncertainty of the fatigue damage estimates is given with basis in these design codes. The objective of this paper is to give a description of a methodology recommended for time domain fatigue assessment. Special focus will be on the importance of adequate simulation time for predicting the short-term fatigue damage and selection of the short-term seastates in the scatter diagram. Statistical uncertainty is one source that the analyst actually can influence by selecting proper analysis methodology. A statistical uncertainty meassure can be used to evaluate the robustness in the estimated fatigue life. Assessment of statistical uncertainty in fatigue damage estimate is demonstrated by case studies. The fatigue assessment methodology discussed in this paper, will be described in an update of DNV-RP-F204 “Riser Fatigue” 2010.