This paper address 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 so generic guidance has been developed to guide the design process.
Force-displacement-response models were developed during Phase 1 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 applies 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 responses cause the greatest uncertainty in design because of the extreme sensitivity of design solution 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 to highlight the way in which the inherent uncertainty in pipe-soil resistance severely complicates pipeline design. The paper than describes the research, development and model refinement that is ongoing to reduce the uncertainties.
Subsea pipeline 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 pipeline. Consequently, the industry has generally sought to restrain pipelines by trenching, burying and rock dumping, or to relieve the stress with inline expansion spools.
A far more elegant and cost effective solution is to work with rather than against the pipeline by controlling the formation of lateral buckles along the route. Controlled lateral buckling is an efficient solution for the relief of axial compression. Indeed, as operating temperatures and pressures increase further, lateral buckling may be the only economic solution. This challenges has led to a radical advance in pipeline engineering with a greater need for robust lateral-buckling design solutions. The SAFEBUCK JIP1 was initiated to address this challenge and aims to raise confidence in the lateral-buckling design approach and to improve under-standing 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 displacement 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 required the pipe to break out from the as-laid position and move across the seabed, typically by several diameters.