An analytical solution is presented for fatigue response due to crossflow vibration of free spanning submarine pipelines supported on semi-infinite elastic soil bed at the ends. Free vibration response parameters are obtained by solving the governing differential equations and satisfying the appropriate boundary conditions for both symmetric and anti-symmetric modes. These parameters are used for assessing the fatigue damage due to crossflow vortex induced vibration under wave and current action as per new draft guidelines of DNV. Results are presented illustrating the effect of axial force and soil stiffness on the fatigue life of a free span under a combined wave and current loading scenario. The maximum permissible spans obtained from the full fatigue analysis are compared with those obtained from onset of cross-flow vibration criteria.
Safety assessment of free spanning submarine pipelines has received distinctly increased attention among the pipeline engineers in the recent years. The effort has been on improving the methodology of assessment so as to accurately predict the risk level of a free span. The aim is to minimise span correction through seabed intervention, which is not only very costly but also dubious and difficult to execute, specially in deep water. A key element in free span assessment is vortex induced vibration (VIV) caused by current and or wave hi the suspended pipeline. Till recently, the industry practice in this regard has been to avoid cross-flow VIV by limiting the so called reduced velocity which is a traction of water particle velocity, pipe outer diameter and the natural frequency of the span. This VIV onset criteria not only yields conservative results particularly for long spans but also gives no clue to the fatigue risks if the actual reduced velocity exceeds the threshold value.