Pipelines are designed and operated under specific regulatory requirements. When faced with evaluation of the effect of loadings that are not identified or examined in the regulations, designers and operators often turn to additional guidance offered by industry standards, such as ASME B31.4 and B31.8, as well as ISO and DNV. Generally, the mechanical behavior assumed in industry standards, such as ASME B31.4 and B31.8, as well as ISO and DNV, are based on a comparison of the pipeline stress state to a limiting stress. However, there are several potential loadings, such as significant ground deformation, in which a pipeline’s longitudinal strain is well beyond that associated with typical working stress limits. Strain Based Design (SBD) criteria was developed to deal with loadings that involve imposed displacements on the pipeline, such as earthquake faulting, liquefaction, frost heave or thaw settlement, and ice gouging. This approach provides pipeline operators with a meaningful tool to adequately address buried pipeline design for displacement-controlled loadings.
In 2012, PRCI funded a program aimed at further developing and validating strain-based design and assessment of pipelines. The first initiative was a state-of-the-art report which provided an overview and introduction to the use of strain-based design and assessment (SBDA) approaches. It provided a summary of various approaches to ensure mechanical integrity for new pipelines (design focus) and existing pipelines (operational assessment focus) subjected to significant ground deformation. The goal of this initial project was to advance the understanding of SBDA within the PRCI community and to identify future work that should be conducted in order to facilitate the appropriate use of SBDA (for both new and vintage pipelines) to meet industry’s goal of ensuring pipeline integrity.
The research studies thus far demonstrated that appropriately designed pipelines subjected to significant ground deformation can accommodate longitudinal bending loads that induce tension and/or compression loads well beyond yield without impacting pressure containment. The fundamental concepts and structure of the strain-based design approach are well-established, but not widely known across industry or consistently addressed by regulatory and standards bodies. The report also examined the various aspects of the development of SBD principles to be employed in design of new pipelines, as well as the techniques that can be used on a site-specific basis on existing pipelines subjected to unexpected significant ground deformation.
Research continues in 2013 with the aim to conduct a review of current industry practices for pipeline integrity management and then to develop the additional requirements that would need to be in-place for pipelines that may be subjected to either slow-acting (e.g., permafrost, slope creep) or rapidly-acting (e.g., fault crossing, liquefaction, ice keels) displacement-controlled loads that can result in pipeline longitudinal strains beyond 0.5%. The objective of this study is to develop an integrity management approach that would meet industry needs and could be the basis for codes and standards development by relevant pipeline code committees. Follow on work will develop specific wording and to provide proposed modifications to the code writing committees, e.g. ASME B31.8 Committee, ISO TC67SC2 Committee to allow the SBDA integrity management supplements to be included in industry referenced documents.