Offshore pipelines have been installed for a period of' some 10 to 20 years using a variety of methods. This paper presents a new analysis method for the optimization of certain operation parameters in laying continuous pipe under tension from a barge. This method of optimization is now being used by one of the major offshore contractors. The solution is given by matching geometrical parameters through graphical overlays of network charts obtained by a computerized finite element iterative solution the solution is based on the requirement that the tension level be optimized with respect to either the minimum stinger length or the maximum average radius of curvature of the stinger.
The problem that this paper deals with is that of laying oil and gas-pipeline offshore (Fig. 1). The problem consists of, how one might place the pipeline on the sea floor without kinking or buckling the pipe. This is done by ensuring that the stress is always below some allowable stress; for example, 80 percent of yield stress. The stress is proportional to the bending moment. Typically, the stress induced by the bending moments is an order of magnitude to two orders of magnitude larger than the stress induced by either the tension level at which the pipe is being pulled or the hydrostatic pressure of the sea water.
In order to relieve the bending moment in the sag bend (and thereby reduce the stress) the pipe is pulled by a tensioning device, typically capable of producing a horizontal tension level, H, ranging from 0 to 120,000 lb.
As the pipe comes off the barge, it rests in a flexible buoyant support called the stinger. The stinger of length, S1, is partitioned off into a series of tanks and each tank can be filled with either water or air. The stinger, therefore, has some average radius of curvature, p. other methods of varying stinger curvature exist such as varying support roller heights with respect to the stinger.
The other operational parameters are the entrance angle, a, of the pipe coming into water off the barge; the rigidity of the pipeline, EI; the submerged weight per unit length, q, of the pipe j' and the depth of the water, WD. The contractor has control over the tension, H; the stinger radius of curvature, p; and until the stinger has, been built, the length, S1, of the stinger.
It should be pointed out that, prior to this system, the analysis was accomplished'by fixing all seven of the variables, solving for the moment as a function of arc length using a complex computer program, and checking each moment. If one of the moments happened to be greater than the allowable, then one or two or three of the parameters were adjusted, based on "engineering intuition".
