David A.S. Bruton

Design Challenges and Experience With Controlled Lateral Buckle Initiation Methods

Subsea pipelines are increasingly being required to operate at higher temperatures and pressures. The natural tendency of such a pipeline is to relieve the resulting high axial stress in the pipe-wall by buckling. Uncontrolled buckling can have serious consequences for the integrity of a pipeline. An elegant and cost-effective design solution to this problem is to work with rather than against the pipeline by controlling the formation of lateral buckles along the pipeline. Controlled lateral buckling is a relatively new design option which has matured as more projects have adopted the approach. As with all new design techniques, knowledge and understanding has improved and evolved with design application, installation and operational experience. Methods used to control the formation of lateral buckles include snake lay, vertical upsets, localised weight reduction and local seabed imperfections. Selecting the right buckle initiation method for a flowline can be a complex issue which is influenced by the flowline type, operating conditions, environmental conditions and pipe-soil interaction. Detailed lateral buckling design is normally concerned with achieving reliable buckle formation, minimising the peak strains in the flowline (local buckling) and controlling through life girth weld fatigue. However, as lateral buckling design has progressed, other design challenges have become apparent, which must be considered during design. This paper discusses commonly used buckle initiation methods and focuses on the key design challenges associated with lateral buckling, in the light of feedback from operational experience of recently installed flowlines. Many of the design challenges are common to all initiation methods, such as pipe-soil interaction or girth weld fatigue. However, there are a number of issues which can be specific to a particular buckle control method or pipeline project, these can include sour service operating conditions or complex flow assurance implications. The paper highlights key information required for lateral buckling design and outlines typical test programmes performed to support the design process. Crucially, many of the flowline design issues identified in this paper have been identified as a result of lessons learnt from operational experience. This affirms the importance of rigorous visual inspection and survey to monitor the performance of flowlines during the first months and years after start-up.Copyright

Pipe-Soil Interaction Behaviour during Lateral Buckling

This paper addresses lateral pipe/soil interaction behavior at the large displacements that occur with lateral buckling of a pipeline. Force-displacement-response models were developed by the Safebuck joint-industry project (JIP) to replace the use of simple friction- coefficient approximations. Such simplistic models are unrealistic for modeling large lateral displacements or the building of soil berms that occurs with cyclic lateral loading. The models are based on large- and small-scale tests carried out by the Safebuck JIP on deepwater soils from the Gulf of Mexico and west Africa, as well as on kaolin clay. To this database was added project-specific test data donated by JIP participants. Four stages of pipe/soil interaction are considered: Embedment of the pipe at installation. Breakout during buckle formation on the basis of different levels of initial pipe embedment. Large-amplitude lateral displacement as the buckle forms. Cyclic lateral displacement influenced by the building of soil berms. While breakout loads have been the subject of much research and published papers on pipeline stability, there is little guidance on modeling lateral resistance at the large displacements experienced in lateral buckling. There is also little guidance on modeling subsequent large-amplitude cyclic behavior, which occurs with each shutdown and restart of the pipeline. New equations were proposed where appropriate, and recommended models for each part of the characteristic response were developed. These models provide a valuable basis for lateralbuckling design guidance. They currently are being applied by JIP participants on a number of projects in which pipelines are being designed for lateral buckling.

The Influence of Pipeline Insulation on Installation Temperature, Effective Force and Pipeline Buckling

High-performance insulated pipelines are designed for long cool-down times in operation. During the installation of such pipelines, the heat within the pipe as it leaves the lay vessel is not easily lost to the surrounding seawater. The ambient temperatures on the lay vessel, combined with significant heat input during welding and field joint coating, will result in the pipeline leaving the lay vessel at a temperature well above ambient deck temperature. The insulation system ensures that a significant amount of this heat will remain within the pipeline as it descends to the seabed, resulting in a higher than ambient installation temperature. As the pipeline cools to ambient seabed temperature, it is restrained on the seabed by axial friction thus generating effective tension in the pipeline. The magnitude of the locked in tension will depend on various factors, including the overall heat transfer coefficient, the system heat capacity, the water depth, water column temperature and the lay rate. Any significant locked in tension will influence the buckling behaviour of the pipeline by inhibiting buckle formation and reducing feed-in to lateral buckles. This paper presents a method to assess the temperature loss through the water column during installation of an insulated pipeline and the location, relative to the touchdown point, at which the pipeline becomes fully constrained. The modified as-installed temperature will much improve the accuracy of predicted buckling response at hydro-test or in operation.Copyright