Upheaval Buckling of Offshore Pipelines: Overview and Introduction

Abstract

ABSTRACT This paper provides a general introduction to and a brief overview of the state-of-the-art in the understanding of and the design against upheaval buckling of offshore pipelines. Although the emphasis lies on the results of a recently completed research programme, it also serves as an introduction to the special OTC90 session on upheaval buckling, where work carried out for subject research programme as well as related work is reported. The understanding of upheaval buckling and its Veining parameters has been substantially improved over the last few years, allowing safer and more economic design, installation and protection of high temperature pipelines. The overall design against upheaval buckling has to be strongly integrated with the requirements for and the availability of survey data to further improve reliability and minimise costs. INTRODUCTION When a pipeline after its installation is operated at higher than ambient temperatures and pressures, it will try to expand. If the line is not free to expand, but restrained by for example soil friction, the pipe will be subjected to an axial compressive load. When the line is trenched and/or covered the lateral soil restraint exceeds the vertical uplift restraint created by the pipe's submerged weight, its bending stiffness and, when present, the soil cover. In that case the pipe will tend to move in the vertical plane - or along the trench slope when the pipe is laying in a trench without cover - and (partly) release the expansion force until a stable equilibrium position has been reached. For large compressive (buckling) loads pipeline response might however be unacceptable in terms of vertical displacements (the pipe protruding through the cover or moving out of the trench), excessive yielding of the pipe material, or both. This phenomenon is called upheaval or thermal buckling (offshore), also known as overbend instability (onshore), and constitutes a failure mode that has to be taken into account for the design of trenched and/or covered pipelines subjected to high temperatures and pressures. Upheaval buckling is not an entirely new phenomenon for pipelines and has in the past been a concern for hot steam piping, high-pressure water injection lines and onshore gas lines traversing undulated terrain1. Pipelines that are neither trenched or buried are more liable to a different but related mode of buckling, in which the pipeline snakes laterally across the seabed. The movement and stresses that result from this mode usually have less severe effects than upward movement, except when combined with the effects of large accidental point loads such as for instance due to trawlgear pull-over. In the latter case the pull-over load initiates and intensifies localised buckling deformations as has been demonstrated elsewhere2. To protect smalldiameter pipelines (16 inch and below) against trawlgear impact and pull-over, these lines are usually trenched and/or covered for North Sea operations. Due to among others the relatively low bending stiffness, low cover uplift resistance and high axial load these small diameter lines are also more susceptible to upheaval buckling.