Upheaval Buckling of Offshore Pipelines: Overview and Introduction

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.

Reliability Analysis of Subsea Pipeline against Upheaval Buckling

“…These combinations can be visualized as the domain in the n-dimensional space of X variables (where n is the number of variables in the set X) on one side of the function g≤0. Thus the failure probability is expressed by: (6) Where f(X) is the multivariate density function for the random vector X. The limit state function g(X), is so defined so that:…”

“…When the lateral restraint of a trenched pipeline exceeds the vertical restraint force against vertical displacements created by the pipeline submerged weight, the pipeline bending stiffness, and the covered soil resistance, the pipeline tends to move upward, and considerable uplift movement may occur. This phenomenon is called upheaval buckling [6,7,8], which is a failure mode that has to be taken into account in the design of buried and trenched pipelines. The pipeline moves upward due to upheaval buckling, leading to possibly unacceptable local plastic deformations or collapse or vulnerability to fishing gear and other third-party activities.…”

Model test based soil spring model and application in pipeline thermal buckling analysis

“…When the value exceeds the constraint of foundation soil on the pipeline, sudden deformation will occur to release internal stress, which is similar to the sudden deformation of strut due to stability problems. The upheaval buckling may make the pipeline destroyed suddenly, as a result, the effective prediction method and protection measure against this phenomenon are important in the design of buried submarine pipelines (Nielsen et al, 1990;Guijt, 1990;Liu et al, 2005).…”

Laboratory tests and thermal buckling analysis for pipes buried in Bohai soft clay

“…Upheaval buckling may suddenly destroy the pipeline leading to substantial financial loss including the cost associated with repair and that associated with the loss-of-serviceability. Consequently, effective prediction of upheaval buckling and viable protection measures against this phenomenon are important aspects of design of a buried submarine pipeline network [11,19,20].…”