Abstract

Deep water field developments have introduced a new set of general conditions which can significantly affect the design of subsea pipelines compared to more conventional water depths experienced in the past. Deep water pipelines do not require a concrete weight coating due to the low wave and current loads, as a result these pipelines have a lower submerged weight. This combined with the lower resistance provided bythe soft soil found on the sea bed in deep water have resulted in pipelines being more susceptable to movements both in the lateral and axial directions as a result of pressure and temperature driven loads. Consequenthly the instability ofpipelines on the sea bed induced by pressure and temperature effects have taken a much larger importance. This paper will compare a range of solutions which can be used to mitigate these issues of lateral buckling and pipe walking, which can cumulated their effect and induce fatigue failures of the pipeline. The paper will be based upon the practical experienced gain in implementing these mitigation measures in deep water projedcts in West Africa. Based upon an understanding of the instability mechanisms and the practical aspects of installation, each mitigation method will be discussed and the impact on pipeline design, installation, and project cost and planning will be highlighted.

Introduction

The expansion of submarine pipelines under the influence of pressure and temperature is not a new subject area for the design of pipelines, risers and tie-in spools. However, previously, the issues were mainly concerned with the displacements of the pipe ends and the resulting stresses and strains in the pipe wall. In addition for trenched and buried pipelines the issues of up-heaval buckling have required specific attention in recent years. It is important to note that the pipe end displacements under these conditions can be considered to be more or less reversible. That is the expansion of the pipeline experienced during production start up is more or less equal to the retraction of the pipeline during a shut down situation. This paper deals with a relatively new phenomena influencing deep water flowlines layed directly on the seabed, where the displacements induced by pressure and temperature variations can result in progressive and cumulative movements of the pipeline. The factors influencing the progressive movements include :-

  • Constant and asymmetrical tension applied to the pipeline from a Steel Catenary Riser (SCR)

  • Relatively lightweight pipelines due to the absence of a concrete weight coating and use of insulation coatings which can have specific gravity less than 1.0.

  • ?Short in field flowlines, consequently the whole line is affected by the pressure and temperature displacements.

  • In general deep water soils are very soft with a highwater content and consequently provide relatively little resistance to pipeline movements.

The conditions described above for deep water flowlines can result in two different types of displacements.

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