Pipelines subject to high pressure and temperature loading are prone to thermal buckling instabilities in the presence of seabed or pipeline imperfections. Recent investigations have revealed a higher degree of design uncertainty with softer cohesive soil types and short wave length imperfections. This paper presents some of the findings from a programme of vertical buckling tests using small-scale pipes which encounter different vertical imperfections on a settling base. The tests primarily observed the nature of the developing instability when the pipe is in the small deformation state.
A web of complex non-linear interactions are associated with the displacement and stability of an axially compressed pipeline within a trench. For conceptual and detail design purposes it is usual to generate look-up charts of the overburden requirement for a certain temperature band against different heights and lengths of seabed imperfection. These features can then be identified offshore following an out-ofstraightness survey. For simplicity when carrying out such a study it is usual to assume the presence of a single mound of perfect symmetrical geometry (but variable wavelength L 0 and maximum amplitude A 0) along an otherwise flat rigid seabed. The most basic form of ground imperfection is the isolated prop which occurs when the imperfection wave-lengths are shorter than the suspended lengths associated with raising and supporting the pipeline by a single point; this can represent for example a substantial rock or the crossing of another pipe. For buried pipelines it is more usual to assume that contact is maintained between the pipeline and a smooth mound as a consequence of the infilling of voids by trench backfill. Generally it is assumed that the shortest wavelength which can maintain a high degree of contact to the point of instability is the most conservative case for design purposes.