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This paper addresses the influence of pipe-soil interaction on the design of pipelines susceptible to lateral buckling and pipeline-walking. The pipe-soil response is the largest uncertainty in the design of such systems, and has a significant influence on the structural limit states. Generic guidance has been developed to guide the design process, but project-specific physical model testing is often necessary, and new advancements in the understanding of pipe-soil interaction are rapidly being made. Force-displacement response models were developed during Phase I of the Safebuck joint-industry project (JIP) based on large- and small-scale tests carried out by the Safebuck JIP and project-specific test data donated by JIP participants. These models are currently being applied by JIP participants on a number of projects, to quantify the susceptibility to lateral buckling and pipeline walking, and design safe and effective means to control these phenomena. However, of all the design parameters, the soil response causes the greatest uncertainty in design because of the extreme sensitivity of design solutions to the axial and lateral resistance imposed by the soil. Improving the understanding of pipe-soil response provides the greatest scope for refining the design of such systems. The purpose of this paper is to outline the significant influence that pipe-soil interaction has on the pipeline design process and highlight the ways in which the uncertainty in pipe-soil resistance severely complicates pipeline design. The paper then reviews and updates previous force-displacement-response models (published at OTC 2006) incorporating more recent experience from large and small-scale tests. Significant new data is included to illustrate the behaviour of ‘heavy’ pipes, which display a lateral response that differs significantly from most previously-published data, mostly related to ‘light’ pipes. The response of soil berms during cyclic lateral loading is also highlighted, demonstrating the cumulative rise in lateral pipe restraint. 1 Introduction Subsea pipelines are increasingly being required to operate at higher temperatures and pressures. The natural tendency of a hot pipeline is to relieve the resulting high axial stress in the pipe wall by buckling. Such uncontrolled buckling can have serious consequences for the integrity of a pipeline. The need to control lateral-buckling has led to a radical advance in pipeline engineering with a greater need for robust lateral-buckling design solutions. The Safebuck JIP was initiated to address this challenge and aims to raise confidence in the lateral-buckling-design approach and to improve understanding of the related phenomenon of pipeline walking. The pipe-soil force-displacement response is the largest uncertainty in the design of such systems. With lateral buckling it is necessary to understand the soil behaviour at large displacements, and through many cycles of loading, well beyond the point of failure. Such behaviour is outside the bounds of conventional geotechnics or extensive earlier research on pipeline stability. Most previous research into pipe-soil interaction has been related to stability under hydrodynamic loading, with the aim being to ensure the pipe remains in place. A lateral buckling design requires the pipe to break out from the as-laid position and move across the seabed, typically by several diameters.

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This paper outlines recent research into axial pipe-soil interaction from the geotechnical elements of the SAFEBUCK Joint Industry Project. The operational axial pipe-soil friction strongly influences the initiation and cyclic development of lateral buckles, and also controls the magnitude of pipeline end expansions as well as rates of axial walking. Results from model tests performed at the University of Cambridge are presented in this paper, and provide new insights into the axial pipe-soil response on fine-grained clayey soils. A simple test arrangement was used to pull an 8 m long plastic pipe axially over a bed of soft natural clay collected from a deepwater location offshore West Africa. Many axial sweeps were performed, spanning a wide range of velocities (0.001 mm/s - 5 mm/s) and a wide range of intervening pause periods (up to several days). Both of these variables had a strong influence on the axial pipe-soil resistance - or ‘friction’. The peak values of equivalent friction factor were as high as 1.5 and the residual values were generally in the range 0.2 - 0.5, but fell to below 0.1 in some cases. Higher peak values are associated with longer waiting periods between axial sweeps. The lowest residual values are associated with the fastest rates of shearing. This wide range of axial resistance was observed in a single test using the same pipe resting on the same soil, which is disconcerting from a design perspective. To identify the origin of this variability, an interpretation based on the generation and dissipation of excess pore pressure is explored. This provides a reasonable explanation for the results, but some unexpected aspects of the behavior remain. The results show the important influence of pore pressure effects, consolidation, and the level of drainage during sliding. They also highlight the complexity of axial pipe-soil interaction. For these experimental results, conventional design calculations do not provide adequate predictions of the observed behavior except for during very slow drained movements. The undrained behavior is not captured by conventional design calculations, which provides a cautionary warning to designers. In particular, in the slow-draining natural clay used in this experiment, very low equivalent axial friction factors - as low as F/W' is ~ 0.1 - can be sustained for a long period of movement. The SMARTPIPE® is a recently-developed tool for performing pipe-soil interaction tests in situ offshore, using an instrumented model pipe mounted on a seabed frame. Selected results from a SMARTPIPE® cyclic axial pipe test performed at a deep water location are also presented and discussed. The results support the proposed interpretation based on the generation and dissipation of excess pore pressure. Some differences exist between the in situ and model test data but they can be explained by the smaller magnitude of axial velocity tested, the higher coefficient of consolidation of the in-situ soil and the absence of pause periods between sweeps. Minimal data from experiments on axial pipe-soil interaction is in the public domain, so the results provided here represent a significant contribution to the available knowledge. This research is continuing within the SAFEBUCK JIP, via additional model testing using a new facility that is described in this paper. The aim is to establish new and more robust design guidance for pipe-soil interaction, to support the reliable and efficient design of seabed pipelines.

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.