ABSTRACT

A model to predict the lateral buckling behaviour of hot pipelines exposed on the seabed is described. Lateral buckling design predictions are compared against the observed movement of an operational pipeline. The implications for the design and construction of high temperature pipelines are discussed.

INTRODUCTION

A subsea pipeline which operates at temperatures and pressures above seabed ambient conditions will tend to expand. If this expansion is restrained, for example by axial fiiction between the pipeline and the seabed, then an axial force will be developed in the pipeline. This force may be large enough to induce Euler (bar) buckling of the pipeline.

Many subsea pipelines need to be trenched or buried to provide physical protection against seabed objects such as trawl gear. Large axial forces will cause a buried pipeline to buckle upwards through the overburden and will cause a trenched pipeline to buckle up the side wall of the trench. In both cases the pipeline may become exposed and at risk from impact. This upheaval buckling problem has been studied in detail and design techniques have been developed to ensure against it[1].

Some subsea pipelines are laid directly on the seabed. Large axial forces will generally cause an exposed pipeline to buckle sideways, or laterally, on the seabed. Lateral buckling of an exposed pipeline does not affect the risk of impact of the pipeline and has less severe consequences than upheaval buckling. Lateral buckling has therefore received rather less attention from pipeline designers. However lateral buckling may still be a problem if the bending deflections induced in the pipeline exceed design criteria and subsequently threaten the integrity of the pipeline.

A number of buckles have been observed in operational pipelines in the North Sea. The better known cases are due to upheaval buckling but there are a number of other cases where lateral buckles have been observed. These buckles provide valuable data which can be fed back into the design process, and which allow the validity and accuracy of pipeline design techniques to be assessed.

This paper addresses lateral buckling and compares the predictions of a design model against the observed lateral movement of the Total Alwyn North Extension flowlines. In particular, the paper addresses the formation and development of a number of buckles within a pipeline, and discusses the effect of pipeline out-of-straightness on buckle formation and development. The implications for pipeline design and installation are discussed.

LATERAL BUCKLING
Theoretical Buckling Analysis

A hot pipeline exposed on the seabed could buckle upwards, sideways or at an angle to the seabed. The actual direction of movement will depend on the resistance against movement and on any out-of-straightness of the pipeline. Typical lateral friction coefficients are less than one, and so the uniformly distributed soil resistance against lateral movement is less than the resistance against upwards movement provided by the submerged weight of the pipeline. Assuming that the pipeline has no severe component of out-of-straightness in the vertical plane, then a pipeline laid directly onto the seabed without cover will buckle sideways on the seabed rather than upwards.

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