The objective of the paper is to provide further interpretation of the latest methods in hydrodynamic fatigue assessment for free spanning pipelines through analysis of three different pipelines on an uneven seabed. The theory behind free span fatigue assessment is reviewed and the procedure for designing against free span fatigue is outlined. Eigen-value analysis of an in-place pipeline is shown using the 3-D finite element method. The fatigue calculations are performed using a high-speed numerical tool incorporating wave loading and vortex induced vibrations. Various parameters (damping; environment directionality; water depth; and seabed gap) are shown to have an effect on the fatigue damage both for cross flow and in-line directions.
Pipeline inability to conform to the seabed irregularities cause free spans to develop. Pipelines spanning over an uneven seabed are prone to vibrate due to the hydrodynamic loads induced by the current and wave. Historically, simplified onset criteria were adopted to identify allowable span lengths based on simplified boundary conditions and general loads where the significant environmental loads induced severe fatigue damage. With the latest finite element models for in-situ behaviour of pipelines and the development of the recent guidelines for free spanning pipelines, it is possible to carry out fatigue assessments over the entire design life. Determining the pipeline behaviour under the environmental loads requires a long-term description of the wave and current distribution. The damage contribution is accumulated for all environmental conditions and all the pipeline temporary and operating conditions. The new design techniques enable the seabed intervention to be more optimised thereby reducing the overall project costs. There are three sources of pipeline free span fatigue as indicated in the DNV Guideline No. 14 1998.
In-line motion due to cyclic wave induced vibrations
In-line vortex induced vibrations
Cross-flow vortex induced vibrations
