Optimization of a Subsea Pipeline Route Profile with the Elimination of Free Spans

Abstract

The selected route for a subsea pipeline is usually uneven and requires extensive preparation. In the case of a relatively stiff pipeline, free spans that are too long may occur even on the prepared route (causing design issues like fatigue). To solve this problem, an optimization procedure has been developed to minimize route preparation costs and to eliminate free spans. The procedure is based on the Euler-Bernoulli beam and the quartic spline used for modeling the route profile in the vertical plane. The seabed is considered to be rigid and without friction. In the procedure, the contact load between the pipeline and the seabed is constrained. A minimum limit of contact load is set to ensure that the pipeline is in contact with the seabed for its whole length so that free spans do not occur. The effectiveness of the procedure is tested in a narrow sea channel with a relatively large diameter steel pipeline for oil transportation. Optimization results are compared with the static response obtained by the nonlinear finite element method (NFEM). The comparison shows that the use of a contact load constraint leads to a significant reduction or to the elimination of free spans.