Traditional pipe-in-pipe systems installed by reel-lay are straightened onthe lay-vessel. The process results in the inner and outer pipes havingopposite residual moment and the inner pipe is under-straightened,which typically means hogging residual curvature in the sag bend. Thismay lead to significant twisting rotation of the inner pipe whilst laying.The residual curvature method for buckling control can be appliedto pipe-in-pipe systems. As for single pipes, it is better if the pipetwists during installation, so that the residual curvature rotates into thehorizontal plane. This increases as-laid horizontal out of straightnessand reduces free spans which is important for in-place performance.Therefore, using predictive analysis at the design stage, parametersshould be selected to promote rotation during installation.Analysis for rotation of pipe-in-pipes is presented, which is morecomplex than for single pipes. The following features are included:simplified calculation of residual curvature for inner and outer pipes;separate rotation for each pipe; distributed contact force between thetwo pipes and interpipe friction. Results are presented from sensitivitystudies on interpipe friction and the effect of route curves.
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