Pipeline Stability: State of the Art

Abstract

Ensuring subsea pipeline stability on the seabed is one of the fundamental aspects of pipeline design. A comprehensive on-bottom stability design will include a detailed assessment of the hydrodynamic loads acting on the pipeline, the pipe-soil interaction, the structural response and a careful consideration of the acceptance criteria. Pipeline stabilisation is a major cost driver in some locations around the world, where the designer is faced with extreme design challenges including severe metocean conditions, shallow waters, large diameter lines, and uncertain or difficult geotechnical conditions. These may all contribute to complex stabilisation solutions resulting in costly construction techniques. The current knowledge and engineering practice applied in pipeline stability design is mostly based on the work performed during the 80s by the Pipeline Stability Design Project (PIPESTAB) and on the research conducted by the American Gas Association (AGA) in another Joint Industry project (JIP). At the time, these studies were aimed at gaining an understanding of the physics governing pipeline stability, in particular hydrodynamic loads on pipelines and soil resistance. These two aspects were investigated independently from each other. Understanding pipeline stability has evolved over the last decade due to the application of this knowledge, findings from further research work, the introduction and requirements of new pipeline codes, and advances in the understanding of pipe-soil interaction. Recently gained understanding has raised the question whether alternatives to the present design approaches and acceptance criteria, as specified in the design codes, could be developed. The areas of debate include the approach used for addressing pipe soil interaction, the hydrodynamic coefficients to be applied, the design kinematics to be considered, the design methodologies, the acceptance criteria, and compliance with design codes limit states. This paper presents an overview of the current available knowledge for addressing pipeline stability. The aim is to briefly summarise the key aspects of the pipeline stability design process and to include some historical perspective. The paper discusses the advantage and shortfalls of the different approaches with a view to consolidate understanding, rather than to provide a ready-made solution to a complex design problem.