Standard

API TR 2PSI

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Abstract

General The scope of work is divided into four tasks: a) Collect and catalog project-specific interface shear test data from various laboratories to establish an updated database of fine-grained interface shear test data and supplementary soil parameters; b) Review of the updated test database to establish a framework for data set definition and refinement; c) Recalibrate and refine existing design models to the updated data sets, and d) Develop guidance for best practice. Motivation The interface frictional response of subsea pipelines subject to cycles of expansion and contraction has gained considerable interest over the past two decades as developments are increasingly using subsea tie-back pipelines to existing infrastructure. Energy developers are now routinely testing shallow seabed soils in a variety of custom-built modified direct interface shear box (ISB) devices and tilt table devices designed to apply low normal stresses to fine-grained or coarse-grained soils sheared against a structural surface representing the pipeline coating. The range of interface shear strength measured for a given soil type and interface surface combination is influenced by several factors, for which there is no standardized investigation approach. As a result, ranges in interface shear strength derived from site-specific testing can vary by a factor of 3 (or more) and may be only marginally narrower or even wider than published trends from large databases of both fine-grained soils (Westgate et al. 2018) and coarse-grained soils (Westgate et al. 2021). The scatter in data are due to sensitivity in measured shear resistance to specimen preparation, testing procedure and equipment, variability of surface seabed soils, and surface roughness effects. These factors affect selection of design interface shear strength values. As a result, shear test program outcomes often present operators and their pipeline contractors with wide ranges of axial PSI inputs to pipeline design, leading to expensive solutions to mitigate pipeline expansion and associated lateral buckling. The lack of guidance means that the application of axial PSI in design will continue to vary based on operator experience. This may increase future risk of unplanned axial pipeline behavior including rogue buckle formation, excessive buckle feed-in, end expansion and through-life walking, resulting in potential buckle pullout and compromising the reliability of a project’s controlled lateral buckling strategy. Current approaches to mitigate these issues using strategies such as rock dumping, concrete mattresses, and hold-back anchoring are expensive, but can be used to avoid the significant environmental and financial consequences of excessive axial pipeline movements and their impact on structural pipeline system integrity. Objectives This research focuses on addressing gaps in current practice and improving the recommended practice for PSI under axial loading. The outcome is a set of design and assessment guidelines related to safe operation of new and existing pipelines as well as input to inspection, maintenance, and evaluation for life-extension/continued-service to support. The 2025 updates to ISO 19901-4 include discussion on testing and analysis methods for derivation of axial friction parameters for subsea pipelines in fine-grained soil, inspired by work performed within the SAFEBUCK Joint Industry Project (Atkins 2015). Much of the SAFEBUCK output is now included in the DNV Recommended Practice F114 (DNV, 2021); however, it is referred to as an ‘alternative method’. An axial PSI testing database used to calibrate SAFEBUCK PSI models has been published (Westgate et al. 2018). The objective is to update current guidance to include guidance on axial PSI testing and analysis, including the statistical database presented in Westgate et al. (2018) and recently re-examined in Westgate (2022). Reducing uncertainty in the axial PSI inputs through improved code-based guidance can accelerate deployment of resilient hydrocarbon development. The proposed project aims to use the wealth of project-specific data collected over the past several years to expand and refine the established database of Westgate et al. (2018) related to predictive models for low stress interface shear resistance.

Document information

  • Standard from API_AC
  • Published:
  • Version: 0
  • Document type: IS
  • Additional information
    • FIRST EDITION