A Trawl Net Model Tailor-Made for Simulation of Pipeline and Trawl Board Pull-Over Interaction

This paper proposes a new trawl net model tailor-made for simulation of trawl board and pipeline pull-over interaction. The trawl net is connected to the trawl board by a steel rope referred to as the sweepline. It is crucial that the correct tension level is simulated for the sweepline during the trawl board and pipeline interaction phase. In previous work, this was achieved by using a single node with quadratic drag properties for the trawl net. However, it was necessary to apply a time-consuming approach where the drag properties were tuned on a case-to-case basis and also stepwise changes of the drag properties throughout the analysis had to be applied in some cases. These drawbacks were avoided in the present work by developing a new trawl net model consisting of 4 nodes and 4 lines. The number of degrees of freedom (DOFs) is very small which makes the proposed model an attractive alternative compared to detailed net structure models consisting of numerous DOFs. Further, the model is applicable for use in standard commercial software since it employs basic structural elements and a quadratic Morison load model. This paper also presents a numerical study where the developed model is validated against model test results of trawl board and pipeline interaction. The trawl net model was found to predict sweepline tension levels in very good agreement with the model tests performed for velocities of 1.5 m/s and 2.06 m/s. The trawl net model was further applied to study the validity of a flume tank test method that applies a constant sweepline tension during trawl board and pipeline interaction. The study showed that non-conservative pipeline interaction loads resulted for the constant sweepline tension model. It is therefore recommended to include the complete trawl net in flume tank testing of trawl board and pipeline pull-over interaction.