Upheaval buckling of buried pipelines
Upheaval buckling is a serious problem which can be encountered during the operation of buried, submarine, oil and gas pipelines. These pipelines are usually operated at high temperatures and pressures (well above the conditions under which the pipe was laid), and the resulting axial expansion can cause significant axial compressive loads in the pipe wall. Under certain circumstances buckling can occur, with potentially disastrous consequences. An important requirement for pipeline design is therefore to ensure that upheaval buckling does not occur. Most previous models of pipeline buckling have been based on the rigid base model; which is an old railway-track model that has been modified to suit the pipeline conditions. According to this model, a so-called minimum, buckling load can be calculated, and provided that the axial load is kept below this value, then buckling should not occur. In practice however, this model is unsatisfactory because buckling does occur at axial loads well below the so-called minimum value. The model is useful for analysing the postbuckling profile of the pipeline, but a better theory is required to explain the buckling process. A number of modified buckling models have been proposed in the literature. However, despite the volume of literature available on this subject, there appears to be very little experimental data published to verify the numerous theoretical models. The purpose of this project has been to perform a number of experiments on a small-scale model of a buried pipe, in order to determine various aspects of the upheaval buckling behaviour. These aspects included: the force-displacement interaction of the pipeline and the soil, and the behaviour of the pipeline due to various combinations of horizontal and axial loading. A simplified buckling model has been developed, and the results have been discussed in relation to this model. The model concentrates on the conditions under which buckling is initiated, and takes into account the force-displacement interaction between the pipe and the soil. The effect of cyclic loading was also investigated. Under repeated loading, the pipeline can displace by an incremental creep mechanism. A simple design model is suggested for the creep phenomenon.