Dynamic Simulation of Free-Spanning Pipeline Trawl Board Pull-Over

The main objective of this effort was to examine if finite element analyses can be used to predict pull-over loads from a trawl board. Here the trawl board was represented by a 6-DOF hydro-dynamic load model, where the mass and drag coefficients are expressed as functions of seabed gap, seabed inclination angle and heading angle. Both seabed proximity and forward-speed effects of the trawl board are hence included. The applied drag coefficients were established by model testing, while the hydro-dynamic mass was found by potential theory calculations. Compared to previous efforts, the hydrodynamic loading on the trawl board is represented in a far more consistent way in this paper. A simulation model which contained a polyvalent trawl board and a free-spanning pipeline was established. Several simulations were performed with span heights between 0 m and 2 m. In all simulations the pull-over force and pipeline response was sampled. The sampled results were thereafter validated by means of the analysis method and pull-over loading proposed in the DNV-RP-F111 code. Some differences could be observed in the response histories, but in summary the numerical model predicts a realistic pull-over. This indicates that the applied hydrodynamic load model captures the relevant effects during the pullover.