Research On Thermal Stress And Buckling Of HT Submarine Pipelines

This paper focus on strength and buckling analysis of typical CNOOC submarine pipelines under HT/HP conditions and developes analysis methods on thermal stress calculation and buckling evaluation of them.1) For thermal stress calculation of submarine pipelines, on the stage of base design, a new analytical model is developed for non-compliant PIP systems, in which the axial gradients of temperature and pressure on the pipeline are included, and the hoop stress and axial stress are expressed by the series of Modified Bessel Functions of the first kind and second kind. Coupling analysis with the friction forces of the seabed and expansion spool forces can be carried out by using these analytical formulae.For the strength check of detail design, ring-pipe substructure elements are developed, on which the subsea pipeline and expansion spools at the extremities can be simulated together in one cell-element model depending on current hardware and software resources, so repeated calculations of stiffness matrixes of same ring-pipe sections and errors induced by the simplification of the boundary condition of an interception model are avoided.2) For single pipe buckling, new FE analysis methods and preheated resolving schemes are studied, mainly on the judgements of critical buckling loads, postbuckling strength analysis, preheating temperature selections and buckle initiation technologies. At the same time, a grid perturbation method based on buckle modes is developed to model pipelay imperfection, so an entire snap-through process of pipeline buckling can be acquired.3) Compliant PIP systems have more risk of buckling as a whole, so the disposing distance of ring plates connecting the carrier pipe and the jacket pipe should be selected carefully to keep thermal stability and postbuckling safety. Non-compliant PIP systems generally would hardly buckle as a whole but the carrier pipe may buckle in the annular space, so the postbuckling process of the non-compliant PIP systems need to be fully evaluated and the reasonable preserving clearance in the annular space should be recommended. For the first time, the upheaval buckling of the crossing PIP systems section is studied, and the analysis results indicate that the postbuckling of such section may induce high stresses, even a mode transition, so reaction force and moment of the anchoring mattress should be checked.