Software for Submarine Pipeline Free-Spanning Assessment and Evaluation

Abstract

Free-spanning pipelines is a phenomenon occurring on uneven seabed and scouring phenomena around the exposed pipeline. To study how free-spanning pipelines are affected from these phenomena, it is necessary to study environmental hydrodynamic flow conditions surrounding the pipeline, such as steady flow due to current, oscillatory flow due to waves and combined flow due to current and waves. Combined wave and current loading include the long-term current velocity distribution, short-term and long-term description of wave-induced flow velocity amplitude and period of oscillating flow at the pipe level and return period values. The bending stresses and associated fatigue life are determined from the given span length and boundary conditions accounting for bending due to self-weight and environmental loading from combined direct wave action and vortex induced vibrations (VIV). The fatigue damage is calculated and integrated over all selected directions, corresponding long-term sea-states and current. Fatigue life is calculated for the in-line response model, in-line force model and the cross-flow response model. The design fatigue life for the in-line mode is a combination of the response model and the force model. Peak dynamic stresses are found from the extreme wave and current conditions and are calculated for cross-flow and in-line response. The premises for this paper are based on application development within pipeline free span evaluation in a software development project based on DNVGL recommended practice, DNVGL-RP-F105. It provides a brief introduction to a software application used to calculate parameters addressing how free-spanning pipelines are affected considering stresses, damage and fatigue life.