Abstract

This paper addresses the phenomenon of pipeline walking, which can cause cumulative axial displacement of a whole pipeline, leading to potential failures at tie-ins or risers. This phenomenon can massively complicate the design of deep water flowlines and has significantly impacted field layouts on number of recent projects.

Pipeline walking occurs over a number of start-up and shutdown cycles, under the following conditions:

  • Tension at the end of the flowline, associated with a steel catenary riser;

  • Global seabed slope along the pipeline length;

  • Thermal transients along the pipeline during start-up and shutdown.

The SAFEBUCK JIP has developed new analytical equations, from first principles, that predict the rate of walking for all three load conditions. These equations have been successfully validated against FE (finite element) models, and bring welcome simplicity to conceptual design assessments.

Introduction

The SAFEBUCK JIP was undertaken with the intention of developing a guideline for the design of high temperature pipelines prone to lateral buckling. Part of the JIP included an investigation into the little understood pipeline walking phenomenon, which has occurred in a number of pipelines and lead to at least one failure to date.

The aim of this task within the JIP was to define the key factors that influence pipeline walking and provide guidancefor assessing the severity of the walking problem.

This paper summarizes the work done on pipeline walking and presents simple analytic expressions which can be used to assess pipeline walking at a conceptual design stage.

Pipeline Walking Mechanisms

When a pipeline is laid on the seabed and heated, it will tend to expand. The expansion is resisted by the friction generated by the seabed. When the pipeline is cooled, it contracts but the effects of seabed friction mean that the pipeline ends cannot contract to the original position. On subsequent heatup and shutdown cycles, the pipeline ends cycle between the fully heated position and the cool-down position; this behavior is addressed in the traditional approach to pipeline expansion design.

However, in some cases thermal cycling can be accompanied by global axial movement of the pipeline; this global translation of the whole pipeline is termed pipeline walking. Over a number of start-up and shutdown cycles walking can lead to significant global displacement of the pipeline. Walking itself is not a limit state, but without careful consideration can lead to:

  • Overstressing of spoolpieces/jumpers;

  • Loss of tension in a SCR (steel catenary riser);

  • Increased loading within a lateral buckle;

  • Need for restraint using anchors;

  • Route curve pullout of restrained systems.

Walking is a phenomenon that can occur in short, high temperature pipelines. The term ‘short’relates to pipelines that do not reach full constraint in the middle, but instead expand about a virtual anchor point located at the middle of the pipeline. Walking involves a global axial movement which occurs on cyclic load and does not reduce with the number of cycles.

Keywords:
pipeline length, mechanism, mobilization displacement, displacement, equation, gradient, force profile, analytic model, constraint, friction force
Subjects:
Pipelines, Flowlines and Risers, Offshore pipelines
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2006. Offshore Technology Conference
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