Pipe-soil structure interaction: best practices for spools, buckle initiators, end connections and sand waves

Pipe-soil interaction behavior forms a key input into the design of pipelines. The global response of the pipeline can be critically affected by the resistance provided by the seabed. Accurate modeling of pipe-soil interaction behavior leads to improved reliability and can lead to design optimizations that offer considerable cost benefits. Extensive recent research and advances in design approaches, particularly through the development of high pressure high temperature lines, has led to significant revisions and improvements to pipe-soil interaction modeling over the past decade. There are now well-established techniques for estimating the as-laid embedment and axial and lateral pipe-soil interaction responses - the non-linear 'springs' used in pipeline structural modelling - for pipelines laid on the seabed. However, these methods, as applied in practice, generally involve the key assumption that the pipe exerts a vertical force on the seabed, V equal to its submerged self-weight, W-the 'V=W assumption'. However, this assumption is not appropriate for pipe elements that are close to points of fixity, constraint or vertical upsets - e.g. in-line tees, buckle initiators and sand waves - or which form spools. This paper illustrates examples of all of these situations, showing pipe-soil interaction 'springs' generated with and without the 'V=W assumption', to show the contrasting behavior. Recommendations are provided for practical modeling of this soil-structure interaction behavior. In some design scenarios, the pipeline may be sufficiently robust that adoption of the 'V=W assumption' may not affect the integrity of the design. In other scenarios, it may be necessary to eliminate the 'V=W assumption' in order for a satisfactory design to be demonstrated. This may be achieved by performing integrated soil-structure interaction analyses, or through iterative refinement of the pipe-soil 'springs'. In all cases, an awareness of the 'V=W assumption' is required across the pipeline engineering and geotechnical teams, who should interact to determine the appropriate basis to demonstrate an acceptable design.