Straightener Settings for Under-Straight Residual Curvature of Reel Laid Pipeline

The pipeline inplace buckling design by residual curvature method for controlled thermal expansion during operation is a novel and cost effective method for initiating multiple expansion loops for a reel-laid pipeline. The controlled thermal expansion will be initiated and managed by planned buckle sites at regular intervals of say, 1km spacing with residual curvature corresponding to 0.20%–0.25% residual strain.The pipelines installed by the reel Lay process are required to be straightened by a reverse curvature process which results in pipeline with nominally zero curvature and axial strain. In order to achieve a residual strain of 0.20%–0.25%, two options are available either to over-straight or under-straight the pipelines. This paper discusses the methodology used laying of pipeline with 0.20%–0.25% residual strain at pre-determined intervals by an under-straightening process with a combination of a central 40m under straight section with a residual strain of 0.2%–0.25% and a 15m transition section on either side of the central section, at 1km intervals.The paper presents the methodology used to determine the settings for the top straightener module of reel lay vessel using finite element analyses, as the equipment settings for under or over straight pipes are not readily available from analytical methods. The optimum settings were obtained after substantial number of FEA simulations allowing for the statistical range in pipe material properties from “strong” to “weak” and these settings are used in reeling trials to make necessary adjustments. The residual strain, out-of-straightness and axial force distributions in pipe sections for straight, under-straight and transition sections are discussed.The paper discusses how these settings were used on-board the vessel and the modifications and adjustments required to select the final settings for the under-straight curvature for the field development of Statoil Skuld project as a case study. This paper briefly discusses the vessel equipment specific features and limitations that may need to be taken into consideration in finite element analyses to optimise the straightener settings for residual strain.© 2014 ASME