Effect of Geometric and Material Discontinuities on the Reeling of Pipelines

Abstract

Reeling remains one of the most efficient methods for installing pipelines offshore. The process results in plastic bending, straightening, and reverse bending to strain levels that can be as large as 2–3%. Thus, despite many years of practice, occasional failures during the reeling and unreeling process continue to take place resulting in costly disruptions and repairs. A common cause of such failures is local buckling that can precipitate fracture. This paper presents the results of a study of how discontinuities in geometry and mechanical properties can lead to buckling and failure. Large-scale nonlinear finite element models are used to simulate the reeling/unreeling of pipelines. The pipeline is modeled using shell elements and contact with the hub of the reel is treated appropriately. The elasto-plastic behavior of the steel is modeled using nonlinear kinematic hardening. Typically, a section of pipeline is taken through a winding and unwinding cycle on a reel of a given radius at a constant value of tension. Discontinuities in wall thickness and yield stress such as those that can occur at girth welds are shown to result in sharp local changes in curvature that extend over 3–4 pipe diameters accompanied by severe local straining and ovalization. The combination of these can lead to local buckling. Increase in the applied tension can reduce these at the expense of additional ovalization of the pipeline.