Abstract

Subsea 7 is currently planning for the installation of PiP flowlines in the Norwegian Sea. A case study has been performed for a 8 × 12in PiP to be installed in a water depth between 320 m to 420 m. Fishing activities are frequent in this area. Therefore, the integrity of the pipeline in case of trawl pull-over must be checked. It is found that pipelines with residual curvatures could behave very differently from pipelines without residual curvatures when they are pulled over by trawl gear. However, the effect of residual curvature on pull-over resistance capacity of rigid pipelines has not been mentioned in DNVGL-RP-F111 [1]. Therefore, an optimised methodology involving FE analyses and Monte Carlo simulation has been used in this project to check the integrity of the pipe-in-pipe flowline for the trawl pull-over load case.

This paper focuses on the FE analyses of the pipe-in-pipe flowline pulled over by trawl gear. The related Monte Carlo simulation has been discussed elsewhere [2]. To understand in detail the behaviour of the pipeline with trawl pull-over loading, the pipeline was modelled using a combination of beam, shell and brick (solid) elements. The advantage of the model was demonstrated by comparing output from the model with corresponding output using beam elements. The effects of some result-sensitive parameters were studied, which include centralizer location, pressure, trawl contact area and wall thickness. Special attention was paid to these parameters because their effects are not able to be captured with the normal beam element. Finally, the impact of residual curvatures on the trawl pull-over behaviour was studied. It was found that the pipeline pull-over resistance capacity is sensitive to residual curvature direction and contact location, but not sensitive to RC spacing and RC shape. Based on the advantage of this analysis methodology, it is believed to be a good option for pipeline trawl pull-over analysis, especially with complex pipeline configuration.

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