The upheaval buckling behaviour of heated and pressurised submarine pipelines is reviewed, with particular attention to the role of foundation imperfections.
Exact solutions for continuous foundations are discussed, and the results utilitised to produce empirical relationships for analysis and design purposes.
The Importance of seabed profile survey to the design process is established, illustrated by results obtained using a recently developed procedure.
Regions of highly localised deformation have recently been discovered in submarine pipelines operating at high temperature.
These disturbances of the regular pipeline profile, involving large plastics tram in the pipe wall, and fracture of coating materials, have been attributed to upheaval buckling.(1),(2),(3),(4)
These observations, together with the consequent rather expensive repair procedures, have directed the attention of designers to the need for further research into the mechanics of heated pipeline behaviour(5)
The concern of the offshore industry has been reflected by the establishment of a joint industry research project to produce design guidelines for upheaval buckling prevention(6),(7)
Meanwhile fundamental research effort has been directed to a detailed investigation of the upheaval process, in order to establish general rules of behaviour which can be applied in the preliminary design situation.
Upheaval buckling is sensitive to both the magnitude and shape of the foundation profile, which in nature is typically of airregular, random character.
However, any systematic theoretical study must of necessity take as examples idealised shapes of pre-determined analytical form
The work described here attempts to resolve this dilemma by considering variety of shapes, for each of which a rigorous analysis of upheaval behaviour through all stages has been carried out.
The envelope of results thus obtained a proposed as a basis for preliminary design, on the understanding that as investigations are extended to further shapes, the envelope may require some extension Large changes are, however, considered to be unlikely In conclusion, the problem of relating these results to the realities of the construction process are considered in a section concerned with seabed survey interpretation
A pipeline laid upon an uneven foundation, will bear down most heavily upon the high spots or crests of the foundation profile, less so in concave areas. This is due to a combination of negative buoyancy, and the natural tendency of the pipeline to return to its original straight form as constructed.
(the formula is available in full paper) This force will reduce in regions where axial slip is possible eg close to risers. The effect of this force is to reduce foundation reaction at crests (regions of negative curvature), and to press the pipe down into troughs
As a consequence there will be changes in the configuration of contact between pipe and foundation.
Generally, these changes will progress smoothly with increasing driving force However, at a certain load level, no adjacent equilibrium may be possible, and the pipe must jump to a different configuration, usually involving large deformations. This is what is known as an upheaval buckle, the effects of which are usually permanently damaging to submarine pipelines.