This paper deals with determination of trawl gear pull-over loads on subsea pipelines by use of finite element analysis. A new contact element that handles interaction between a general 3-dimensional rigid body and a pipeline modeled by beam elements is proposed. The contact element is based on a frictionless penalty formulation, which in contrast to previous work by the authors, employs a continuous normal vector description at the contact interface.

A comparison study with existing model test results was carried out to verify the performance of the contact element. The model test setup was reconstructed by a full-scale finite element model containing a trawl board, a flat frictionless seabed surface and a short pipe section. In this study several span heights were examined for both fixed and flexible support conditions. Consistent behavior with the model test was demonstrated for span heights of 3 m and 6 m with fixed support conditions.

The proposed contact element performed satisfactorily in terms of computing time, and due to the continuous surface normal vector a good numerical robustness was also provided. The contact element is believed to be especially useful for pipeline pull-over scenarios outside the scope of present design codes. This can include analysis of free-spanning pipelines with high flexibility, simultaneous interference of two trawl boards and establishment of design pull-over loads caused by novel trawl gears. To expand the application area it is recommended in future work to verify, and possibly improve the contact element, for handling of subsea template pull-overs and oblique trawl gear crossings of pipelines.

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