Minimization of maximum moment in offshore pipeline during installation

Abstract

During the laying of an offshore pipeline from a lay barge, the suspended length between the stinger end of the lay barge and sea bed is subjected to significant bending moments. The maximum moment in the pipeline depends on horizontal tension and the lift off angle at the stinger end. The problem of minimization of maximum moment in offshore pipeline is formulated by finding suitable values of horizontal tension and suspended length (and hence lift off angle) such that the maximum bending moment in the pipeline is as low as possible, without violating constraints on the lift off angle and geometric constraints. This is a nonlinear analysis and nonlinear optimization problem. For analysis Datta and Basu's formulation is used and the resulting nonlinear equations are solved by finite difference technique iteratively. The improved move limit method of sequential linear programming is used for optimization. In the present study only static loads are considered and optimum moments are found for one of the pipelines to be used in Bombay High. Progress of optimization, convergence to global optimum and suitability of the optimization technique used are discussed for a particular depth. Then optimum designs are obtained for various depths. It is found that by suitably adjusting the lift off angle the maximum moments can be considerably reduced. The optimum lift-off angle reduces with the depth. 1986 Computational Mechanics Publications.