Cyclic direct simple shear test on soft clay at low normal stress - As applicable to offshore pipeline axial walking problems

Offshore pipelines play a significant role in transporting energy resources such as crude oil and natural gas from offshore platforms to processing facilities. The on-bottom stability of offshore pipelines is influenced significantly by the geotechnical conditions at the seabed. Pipeline would undergo a number of thermal cycles during its operational life. At the end of each thermal cycle, some part of the expansion would recover, whereas the irrecoverable expansion would accumulate at the free ends and cause the pipe to move axially in one direction, known as Axial Walking. The test results from the Monash Advanced Pipe testing System (MAPS) imply that pipe axial walking would induce relative movements of the soil below the pipe. Since the pipe-soil interaction is extensively influenced by the soil response, the portion of the soil below the pipe which undergoes shearing, characterised as the Shear Zone, is significant for pipe axial walking assessment and thus needs to be investigated thoroughly. This paper presents an investigation into the behaviour of soil within the shear zone in pipe axial walking problems. Cyclic direct simple shear tests on soft clay at low normal stress are performed as applicable to axial walking problems. The soil response in the shear zone is characterised as undrained, partially drained, or drained based on cyclic shearing rates and the relationship between residual peak shear resistance and shearing rate is investigated. Finite element analyses are also conducted to capture the behaviour of soil within the shear zone, utilising advanced constitutive soil model. Based on the results from both experimental work and numerical analysis, a set of data is established which can be applied in the calibration of large-scale pipe axial walking modelling, and provide guidance on the design practice of offshore pipelines when considering on-bottom stability in axial direction.