LOW CYCLE FATIGUE IN RISERS AND PIPELINES

It is very-well known that fatigue is a primary driver and challenge for steel catenary risers (SCRs) and potentially for steel lazy wave risers (SLWRs) as well. This challenge can be overcome in several ways, some better than others. However, the first step is to predict the fatigue damage properly. Fatigue in risers designed utilizing the working stress design (WSD) method, where stresses are kept below yield, is typically predicted utilizing a stress versus number of cycles to failure (S-N) approach, referred to as high cycle fatigue (HCF). Riser designed utilizing the load and resistance factor design (LRFD), or limit state design (LSD), or strain-based design (SBD) methods are likely to experience yielding and inelastic strains, especially in extreme and survival conditions. Cycles of stresses exceeding yielding and producing inelastic strains occur also in reel-lay operations and are likely to occur in pipelines subjected to large deflections cycles such as over sleepers. Utilizing HCF approach to predict fatigue in this case is inappropriate and a strain versus number of cycles to failure (epsilon-2N) approach, referred to as low cycle fatigue (LCF), is typically utilized. A detailed approach to predict the LCF fatigue is presented herein along with examples.