Global Buckle Control with Dual Sleepers in HP/HT Pipelines
Authors: Bai Qiang , Qi Xinhai , Brunner Mark S. 
Published: July 2009

High pressure/high temperature (HP/HT) pipelines may significantly expand and contract longitudinally during operational heating and cooling cycles, resulting in global buckle formation at naturally occurring locations with high out-of-straightness (OOS). This global buckling behavior may be unacceptable if it is not properly managed.

To manage global buckling, planned buckle initiators such as distributed buoyancy sections or sleeper pipe upsets are often designed to ensure pipeline integrity. However, the commonly considered single-sleeper upsets are not always feasible due to pipeline operational limitations. The upset of a pipeline using two parallel sleepers separated by a short distance may initiate pipeline global buckling more easily under a lower compression load. The dual-sleeper will also provide lower stresses and stress ranges during thermal cycling when compared with the single-sleeper, especially for larger diameter pipelines.

Another application of dual-sleepers is at pipeline crossings. In this application, two parallel sleepers (with a diameter larger than the crossed pipeline) are used. One sleeper is placed on each side of the crossed pipeline. The HP/HT pipeline is designed to cross over the dual-sleepers and the crossed pipeline, which may cause the pipeline to buckle globally in the crossing area during operation. The buckled pipeline will not touch the crossed pipeline, as the net vertical upset of dual-sleepers is higher than the crossed pipeline.

This paper discusses the applications of dual sleepers as global buckle initiators, stress control devices, and vertical upsets for pipeline crossings, and presents results obtained through finite-element modeling. A case study is presented, in which buckle initiation reliability, stress mitigation in global lateral buckles, and fatigue due to thermal cycling and vortex induced vibration (VIV) are analyzed for the dual sleeper configuration. The dual-sleeper scenario may be the preferred option over a single sleeper, and this mitigation approach is worth considering on future projects.

Introduction

Deep water pipelines subjected to high pressure and high temperature may significantly expand and contract longitudinally during operational heating and cooling cycles. Increasing internal pressure and temperature will create a very high effective axial compressive force when the pipeline is restrained or semi-restrained by the pipe end devices and the soil. The high effective axial compressive forces can cause the pipeline to buckle globally, in which lateral buckling will occur if it is exposed on the seabed, or upheaval buckling may occur if the pipeline is buried or constrained in a trench. Uncontrolled global buckling can cause excessive plastic deformation of the pipeline (which could possibly lead to localized buckling collapse) and/or cyclic fatigue failure during operation due to multiple heat-up and cool-down cycles, if it is not properly managed.

Global buckle mitigation can be achieved by providing controlled lateral pipeline movements. Buckle mitigation devices are installed along the pipeline to provide buckle initiation sites, thereby forcing the pipeline to move laterally at these locations to relieve the axial load in the pipeline. Snake-lay pipeline sections (lateral imperfections), sleepers (vertical upsets), and distributed buoyancy sections (vertical imperfections with low submerged weights) are typical methods used to control pipeline behavior and ensure long-term integrity. This paper reviews some global buckle initiation/mitigation methods and discusses how dual sleepers may have advantages over single sleepers under certain circumstances.