Dynamic Lay Stresses for Pipelines

Abstract The modelling system Dynamic Seapipe is capable of simulating a wide range of physical pipelipe systems and can thereby serve as a tool for dynamic structural analysis of different pipelaying techniques and operations. The basic equations describing the dynamic equilibrium of a pipestring subjected to various external forces are presented, together with the procedure for solving the resulting ten independent partial differential equations. The dynamic analyses of three different pipelaying situations, two from the North Sea in shallow water and one from the Mediterranean in greater water depth, are referred to. Introduction The current intensification of offshore oil and natural gas exploitation brings with it a need for mor-e ambitious and expensive pipeline systems capable of operating in exacting marine environments. The engineering design of these pipeline systems then raises new problems of analysis of structures under exceedingly variable, fluid-interactive and dynamic conditions. The dynamic structural analysis that is developed (alongside hydraulic analysis) in order to solve these problems, is of such an order of complexity that it must be aided by the use of digital computers. This paper describes an approach to the problem of simulating the dynamic behaviour of a great variety of pipeline-stinger-lay-barge systems and pipe tow systems. The approach is that of constructing a modelling system, the Dynamic Seapipe System, that is capable of simulating anyone of a wide range of physical pipeline systems when presented with a description of the system in a design-oriented format. The system has now been developed, tested and applied to a number of situations. The applications include the simulation of different specific pipe-laying techniques and procedures, abandonment procedures, recovery operations, and emergency operations. They are not restricted to laybarge operations, but include J-method techniques, drill-ship pipelaying, and riser handling, pipelaying and lowering exercises with intricate buoyancy arrangements and different configurations of pipe tow as well as dynamic on-bottom situations. The models generated by the system are rapidly constructed, are reliable and accurate, and use relatively little machine time, so that they have exceptional cost-benefit characteristics. The dynamic seapipe provides a basic tool for controlling the laying of a pipe, for setting of tensioners, optimizing stinger angles and buoyancies and limiting the static as well as the dynamic momenta and thereby reducing the risks of buckling. It is also a tool for predicting and limiting the risk of fatigue stress failure in foul-weather holding situations and during pipeline towing. Formulation Of The Problem The elastic properties of a suspended pipestring are described by the rod theory from the classical theory of elasticity in a small strain/large displacement formulation, the axial and torsional deformations being neglected. The equilibrium of the pipestring is set up in the local orthogonal coordinate system LHV (see Figure 1). L is along the pipestring, V located in the vertical plane, and H is horizontal. The notation is given at the end of the paper.