by David J. Miles
Subsea oil and gas exploration is increasingly moving into deeper water, where trenching of a pipeline for protection and to mitigate against upheaval buckling becomes increasingly impractical. In addition, the exploration of new reservoirs at higher temperatures and pressures than before leaves a submarine pipeline on the seabed more susceptible to lateral thermal buckling.
A novel small-scale compressible base model, with an expanded polystyrene base compressed beneath a silicone rubber strip, has been developed to represent the constrained thermal loading of a pipeline lying on the seabed. This physical model is used, in addition to a nonlinear finite-element analysis, for a case study of a real buckled pipeline. Dimensional analysis is used to provide a means of comparing the post-buckled behaviour of the model strip with that of the full-size pipeline. There is good agreement between the results of the post-buckled behaviour for the physical and finite-element models, and these results compare well with the survey data for the buckled real pipeline. General results from the physical model are also presented for strips with differing geometric and material properties, laid both straight and on a scaled lay-away curve.
A useful measure of the evolution of a buckle, the free end displacement is introduced. This is the axial displacement of the free end of a cut pipe, constrained to remain straight while undergoing thermal loading. This measure used in a study of the parameters which affect the far-post-buckling behaviour of a beam on a frictional foundation. The phenomenon of buckle lobe extinction, when a buckle lobe stops growing, is discovered for certain combinations of beam bending stiffness, axial friction coefficient and lateral friction coefficient. When the buckle length, buckle amplitude and free end displacement are formed into non-dimensional groups with these three parameters, curves for many parameter combinations are found to fall onto a single curve. The conditions for buckle lobe extinction, in terms of these dimensionless groups, may be determined directly from this universal curve.
Finally, the closely-related problem of the stability of a pipeline being built up a slope is investigated. A case study is made of a real pipeline, incorporating numerical and physical models and also a simplified analytical model. These models correlate well with each other, and enable the conditions for collapse of the real pipeline to be predicted.
[Cambridge University | CUED | Structures Group | Geotechnical Group]
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