**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]

This page is maintained by rcb@eng.cam.ac.uk (last update 18 November 1998)