Stress Analysis of Offshore Pipelines

ABSTRACT The intent of this presentation is to describe an iterative method for determining the stresses and the three dimensional nonlinear configuration of subsea pipeline suspended between the ocean floor and a lay barge with or without stinger. If a stinger is used, it may be considered as composed by elements of desired length, rigidly or semi-rigidly connected or hinged to each other. The final shape of the pipeline can be 'improved and the stresses at the critical area can be minimized by applying a sufficient amount of tension from the barge to the pipeline. The stiffness matrix method is used to determine the final deformed configuration and the stresses at every assumed nodal point. The external loads such as the pipe's own weight, coating weight, buoyant forces, drag forces are considered as static forces. 'The effect of dynamic loads and plastic deformations are not taken into account, but as it will be mentioned later they can be introduced to the computation without any difficulty. A general and flexible computer program has been prepared to perform the analysis. The information related to the characteristics of the pipeline and the stinger, boundary conditions, external loads, slope of the pipeline on the lay barge, the depth and the slope of the sea floor and the velocities of the sea current and the liquid passing' through the pipe are supplied to the computer as data. All the required information is printed out on the output sheet. INTRODUCTION The suspended configuration of pipelines constitutes a geometrically nonlinear problem due to the large displacements occurring under external loadings. Many solution techniques have been proposed. Depending upon the water depth the suspended configuration is assumed to be a natural catenary (1) or stiffened catenary (2), (3). The application of tension is suggested in order to increase the suspended length, of pipeline and shorten the stinger (4). A solution based on the initial value analysis is given, which takes into account the effect of pipeline stiffness, any boundary condition, current profiles, weight of coatings, auxiliary support buoys, crosscurrents and lateral barge drift (5). The suspended portion can be made compatible with any stinger by specifying either moment or slope at the lift off point. As the profile is defined by adding beam segments, the calculation and round of errors accumulate until at some distance from the origin accuracy is lost. Therefore there is a maximum suspended length that the initial value procedure can generate. The influence of lay barge motions on a deep water pipeline laid under tension is investigated (6) and maximum barge motions compatible with the integrity of the pipeline are determined. The conventional stinger is a long stiff structure which prevents excessive bending of the pipe along its suspended portion.