A study on drained residual response of axially loaded flowlines on Gulf of Mexico clay

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Date

2020-09-03

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Han, Jiali

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Lateral buckling and axial walking are the main design issues for offshore pipelines and flowlines. Accurate pipe-soil interaction estimation can lead to optimizing design with significant cost reduction. The objective of this study is to characterize the interface interaction between pipelines and Gulf of Mexico (GOM) clay by carrying out physical model tests using 4.25-inch diameter pipes. T-bar tests were conducted after reconstitution of the clay test bed to capture the undrained shear strength, which allows the sensitivity of the clay to be interpreted. Pipe embedment tests and consolidation tests were performed to estimate the pore pressure dissipation rate of the GOM clay after the placement of the pipeline. After the test beds are mixed to the desired water content, model pipes were placed and allowed to settle for about three weeks. Then, four series of axial load tests were carried out to simulate pipe walking under various loading conditions. The soil-pipe interface axial resistance was measured, and factors including consolidation time, loading conditions, loading sequence, and effective overburden stress were investigated. Drained response of the soil-pipe interface in this study appeared to be mobilized by a “slow” axial motion rate at 1×10⁻⁵ in/s. Also, it was observed that the drained residual state could be reached by performing numerous “fast” sweeps. Once the drained residual state is reached, the axial shear resistance can stay constant with the motion rate increased by 100 times. Same as suggested by other literature, the axial resistance will increase if the test is paused for days. The axial resistance will then drop back to the drained residual axial resistance with continued shearing. For test beds with large water contents, consolidation of the pipe-soil interface may take longer, and the measured axial resistance will keep increasing with time. Also, possible stress concentration at the invert of the pipe might lead to a decreased axial resistance due to increased effective contact stress between the pipe and the clay. Eventually, axial load test results are compared with previously performed tilt table tests results. Both tests indicate that the drained residual interface friction coefficient decreases with increasing effective overburden stress.

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