Experience with interface shear box testing for axial pipe-soil interaction assessment on soft clay

Abstract

This paper presents experience and best practice for a new geotechnical laboratory test protocol – the low stress interface shear box (ISB) test – for determining soil properties relevant to pipeline-seabed friction. The paper is underpinned by a major database (>200 tests) that demonstrates the protocol and shows general variations in the key parameters that may be useful for early design purposes. By accurately quantifying shear resistance along the pipe-soil interface under low normal stresses imposed by subsea pipelines, ISB tests allow design ranges in axial friction to be narrowed and tailored to specific pipeline conditions. These improved geotechnical inputs to pipe-soil interaction can alleviate unnecessary axial expansion, walking or buckling mitigation, unlocking cost savings otherwise unavailable without accurate geotechnical parameter characterization. A large database is presented of recent industry experience with low normal stress interface shear testing using a modified direct shear box device. This device, while still considered novel, is emerging as the new industry standard for axial pipe-soil interaction testing, gaining wider adoption than the tilt table and torsional shear devices. The test database comprises several soft clays from various deep water hydrocarbon-producing geographical regions, and several types of pipeline coatings. The database populates a theoretical framework for axial pipe-soil interaction with new data, illustrating general trends for key parameters such as shearing duration, normal stress, interface roughness, and changing pipeline weight, each of which is shown to vary the axial resistance by a factor of two or more. The shear resistance can also change by a factor of two or more due to consolidation or swelling during and between individual cycles of movement, associated with pipeline operations. This database and the populated theoretical framework can guide pipeline and geotechnical engineering by providing a basis for initial estimates of axial friction, and an approach for improving these estimates via focused site-specific testing. The new approach has been applied through >200 ISB tests for >12 oil and gas projects conducted in the laboratories of Fugro and the University of Western Australia, to deliver better pipe-soil interaction data than is available via other means. This paper shares this experience to support new projects in two ways: Firstly, the large high quality database provides a significantly improved basis for estimates of geotechnical soil properties for subsea pipeline design where site-specific data is not yet available. Secondly, we provide guidance on the planning, execution and interpretation of low stress interface shear tests, to allow best practices to be adopted more widely across industry.

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ORCID for D.J. White: orcid.org/0000-0002-2968-582X

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