Prediction of Pipeline Rotation during S-lay Installation

摘要

During S-lay installation of offshore pipelines, the pipe may becomerotated. This is of major concern when installing in-line structures withstrict rotational tolerances on the as laid configuration. The main drivingmechanisms for rotation are: Strain energy minimisation if plasticcurvature is present, caused for example while passing over the stinger;And gravitational energy minimisation in the presence of significanteccentric weights positioned unstably, for example structures welded inabove the pipe centreline. Additional torque leading to rotation maycome from friction on the seabed when installing in curves, or dragforces from transverse sea current.The pipeline is restrained against rotation at the laying barge and at theseabed, and the effect is transferred into the sag bend via the torsionalstiffness of the pipeline. However the fixedness is not total: friction atthe seabed is limited by effective weight and coefficient of friction, andit has been demonstrated that the pipeline can rotate during pull throughthe tensioners. Therefore predicting the rotation is a challenging problem.An analysis tool is presented here which accounts for the abovefactors, and results are compared to a documented incident of pipelinerotation. The capability to predict rotational phenomena can contributeto preventing undesired rotation in future pipeline projects by guidingpipeline design, installation parameters or vessel choice.