ABSTRACT

The paper presents an analytical method, investigation results, and real applications of the phenomena of subsea pipeline sinusoidal lateral deviation or "snaking" due to thermal expansion of the pipeline. Thermal expansion of a pipeline arises from transmission of a product at an elevated temperature relative to the ambient sea water around the pipe. A new design approach described as pipeline self-stabiIization or self-limiting deviation phenomenon avoids expansion loops or offsets along the pipeline path and at riser locations. An energy variational method is applied to determine the lateral deviation and natural wave lengths of unburied subsea pipelines. The lateral deviation and natural wave lengths are used to determine the pipeline sinusoidal shape, resultant pipeline stresses, and a more realistic pipeline expansion length. A computer program was developed to calculate pipeline resultant shapes, stresses, and other results. The new analysis method has been applied in practice to realize a safe, subsea pipeline construction with a significant cost reduction and installation efficiency.

INTRODUCTION

A subsea pipeline is often operated with an elevated temperature product which is substantially warmer than the ambient temperature corresponding to pipeline installation conditions. Additionally, the pumping process may be associated with a high internal pressure. Both factors result in pipeline expansion defined by temperature difference and thermal expansion coefficient, pressure difference and Poisson's ratio, and the pipeline restraint conditions on the seabed. Pipeline restraint conditions are typically either of two different types of subsea pipeline arrangements: buried and unburied pipelines. A pipeline, buried or unburied, has both a moving portion and a fully restrained portion. A buried subsea pipeline has a larger contact area with bottom soil and, consequently, most parts of the pipelines are anchored by the combined effect of soil pressure and friction. In other words, a buried subsea pipeline even under high temperature and internal pressure has a minimum mobility and consequently less possibility of expansion.

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