On the initiation and arrest of propagating buckles in offshore pipelines.

摘要

Buckling, collapse and propagation of collapse are major concerns for pipelines designed to operate in deep waters. This dissertation deals with two related problems affecting such structures. The first addresses the initiation of collapse from a section of pipe structurally weakened by local impact by a foreign body. The second is a parametric evaluation of the effectiveness of integral buckle arrestors used to limit the spreading of collapse once it is initiated.;It is well known that small geometric imperfections can have a critical influence on the collapse pressure of such structures. However, much less is known about the reduction of their collapse pressures as a result of local imperfections of larger amplitude (i.e., dents). Such dents can, for example, occur during the life of the structure due to accidental impact by a foreign object. This problem was studied through a combination of experiment and analysis. Denting reduced the collapse pressure significantly. As the dent size increased, the collapse pressure was found to approach the propagation pressure of the pipe. An ovality like parameter of the most deformed cross section of dented tubes was found to be a key measure of the severity of a dent from the point of view of its effect on collapse pressure. The collapse pressures of tubes indented with different diameter indentors correlated very well when plotted against this measure of denting.;The problem was concurrently analyzed numerically using a finite element model. The indention and collapse processes were successfully reproduced for all the problem parameters varied in the experiments. A method for generating a universal collapse resistance curve applicable to all dents is proposed.;In such pressure loaded structures, if local collapse is not averted, a propagating buckle can be initiated. Propagating buckles tend to propagate at pressures which are significantly lower than those required to initiate collapse. As a result, it is desirable to limit the extent of catastrophic collapse by using buckle arrestors. Buckle arrestors are devices placed periodically along the pipeline in order to locally reinforce the pipe circumferentially and resist further spreading of collapse. An arrestor commonly used in deep water applications is the integral buckle arrestor. It consists of a short section of pipe thicker than the mother pipeline welded periodically between two adjacent strings of pipe. Such arrestors must be designed in such a way as to ensure that, in the event a propagating buckle is initiated, collapse is limited to a section of pipe between two arrestors.;The arresting efficiency of such devices was studied parametrically. The dependence of the crossover pressure on the arrestor geometry and material properties was established. Two modes of buckle crossover were observed which depend on the thickness and length of the arrestor. The process of buckle propagation, its arrest by an arrestor, and the crossing of the arrestor at a higher pressure were simulated through a finite element model. The model was subsequently used to conduct an extensive parametric study of arrestor efficiency. (Abstract shortened by UMI.).