Structural integrity of pipelines using reeling installation method
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In the reeling method for installing offshore pipelines, several miles of line are wound onto a large diameter drum onshore mounted on a vessel. The vessel travels to the installation site where the line is unwound gradually installing it to the sea floor. This process involves repeated excursions into the plastic strain range of 1-3%. This study examines three structural integrity issues that arise from the process. A full-scale numerical modeling scheme incorporating nonlinear kinematic hardening plasticity is developed for the reeling/unreeling process. The first issue studied is the degradation of the cross section of pipelines and its effect on the collapse pressure. To capture the ovalization induced and assess its impact on the structural performance of the pipeline in deeper waters, the complete 3-D finite element model and a simplified 2-D model are presented to simulate reeling/unreeling of up to three cycles and subsequent collapsing under external pressure. Comparison of the results of such simulations with experiments highlights when fully a 3-D model is required and when the simpler 2-D model is adequate for evaluating the structural performance of a reeled pipe. The second issue investigated is the discontinuity in pipelines. In order to show how discontinuities in geometry and mechanical properties can lead to buckling and failure, the 3-D numerical model is applied to simulate the reeling/unreeling of pipelines. Discontinuities are shown to result in sharp local changes in accompanied by severe local straining and ovalization. These local effects can be reduced by increasing the applied tension at the expense of additional ovalization of the pipeline. The last part of the study examines the complication brought in by reeling pipes that exhibit Lüders bands. To simulate this process, the material is modeled by a stress–strain response with a negative slope over the extent of the Lüders deformation. During reeling with some back tension, Lüders banding produces inclined bands of localized deformation organized in clusters with distinctly different spatial distribution than that of pure bending. As a consequence, the ovalization develops axial undulations. The influences of problem variables are examined in a detailed parametric study.