Simulation of Trawl Loads on Subsea Pipelines

Abstract

The main objectives in this thesis was to investigate the effect of oblique trawl board crossings, increased trawl board added mass due to seabed proximity and the effect of a more rectangular trawl board geometry. In addition a new hydrodynamic load model which handles the seabed proximity and forward speed in a more consistent way was examined.

All simulations in this thesis are performed by means of the computer software SIMLA. A brief description of methods applied in SIMLA and nonlinear finite element analysis is therefore included. The thesis contains also a chapter which describes trawling concepts and trawl boards used in Norwegian waters.

Design loads from trawl gears on subsea pipelines are nowadays based on recommendations from the DNV-RPF111 code. Simulation models with a 4500 kg polyvalent trawl board were established to verify the DNV recommendations for free spans of height 0 m and 1 m.

The simulations demonstrated that increasing trawl board added mass due to seabed proximity did not have any influence on neither pull-over loading nor pipeline response.

The effect of a rectangular trawl board geometry was most pronounced for a span height of 0 m because the duration increased by 0.5 s and the horizontal pull-over force was kept constant throughout the pull-over. A slighty larger pull-over loading compared to the polyvalent board was observed for a span height of 1 m.

Oblique trawl board crossings were examined for 6 different hit angles. The major finding was that a perpendicular crossing did not predict the largest pull-over load. On a general basis the simulations for a span height of 1 m underpredicted maximum pull-over force, duration and pipeline displacement compared to the DNV-RP-F111 recommendations. The 0 m span height simulations indicate that DNV predicts a different shape of the load time history and is slightly nonconservative in terms of maximum pull-over load.

The new hydrodynamic load model which includes the effect of forward speed and seabed proximity was used to simulate a perpendicular trawl board crossing. Here the span height of 0 m indicated that the DNV-RP-F111 code is nonconservative in terms of the pull-over load. The simulation for a span height of 1 m was however in very good agreement with the DNV-RP-F111 code in terms of duration and horizontal pull-over load. Therefore it is recommended that future simulations are based on the new hydrodynamic load model.