Offshore pipelines may require a Direct Electric Heating (DEH) cable for auxiliary heating purposes which will be strapped as close to the pipe wall as possible at pre-determined intervals for efficient thermal performance particularly during start-up and shut-down conditions. The straps are provided with tension to maintain the orientation of the DEH cable intact. The positioning of DEH cable causes differential buoyancy and gravity loads and the associated positioning vectors giving a tendency to rotate the pipeline in the catenary.
If the pipeline inplace buckling design by residual curvature method for controlled thermal expansion during operation is adopted for the pipeline with a DEH cable, the additional imperfection in the pipeline will contribute to the pipeline’s tendency of rotation together with the imbalance in buoyancy and weight. This paper discusses the analytical approach used based on the Stiffened Catenary Calculations Method and Simplified Energy Balance Approach Method to determine the roll, the pipeline may experience to reach stable equilibrium in combination with the residual imperfection. The total work required to bend and twist the pipe from surface to seabed is estimated for twist angles at touch down between 0 and 180 degrees and in water depth of 360m.
The paper further discusses the numerical finite element analyses performed to estimate the amount of twist in the pipeline as the pipe is paid from the vessel. The analytical and numerical approaches support the view that the pipeline will have restoring moment to lay the pipeline with DEH cable in the sideways for pipe laid on seabed. Comparisons are made with the post lay test data from other projects to confirm the findings from the assessment are in agreement. The unlikely scenario of pipeline being laid with residual curvature section in a position other than in the horizontal plane and its effect on the lateral buckling behaviour on application of steady state temperature and pressure was also investigated and the results are presented in the paper.
