Browsing by Subject "Offshore oil and gas"
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Item A study on drained residual response of axially loaded flowlines on Gulf of Mexico clay(2020-09-03) Han, Jiali; Gilbert, Robert B. (Robert Bruce), 1965-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.Item An investigation of factors affecting interface shear resistance between pipes and soils(2022-09-12) Hussien, Ahmed Mustafa; Gilbert, Robert B. (Robert Bruce), 1965-; El Mohtar, Chadi; Kumar, Krishna; Chen, JinboTo develop a better understanding of the behavior at a pipe-soil interface under axial loading, this study investigated the effects of pipe curvature, time, and cyclic loading through the use of model tests. These tests were performed as part of two studies: 1) an investigation of the pipe-soil response for axially loaded flowlines and 2) an investigation on the effects of cyclic loading on pipe piles driven in sand. To study the flowline-soil interactions, flat and curved model surfaces were installed horizontally in both highly plastic marine clay from the Gulf of Mexico and carbonaceous silty sand from the North West Shelf in Australia. The axial shear resistance between the soils and surfaces was measured using tilt table tests, cyclic medium-scale model pipe tests, and cyclic centrifuge-scale model pipe tests. By analyzing these tests, several conclusions were made: 1) no evidence of a wedging factor was observed, 2) a projected contact area can be used to interpret the stress distribution at the interface, 3) aging can increase the axial resistance but is lost with additional cyclic loading, and 4) the pipe-soil contact area may decrease with cyclic loading. To study the pipe-soil interactions of small-diameter piles in sand, piles were driven at Hornsby Bend in Austin, Texas and at Allen Harbor in North Kingston, Rhode Island. The capacity of the piles was predicted based on cone penetration test results and various methodologies. One group of piles was tested after about one week of aging and the rest were tested after aging for more than 70 days at Allen Harbor, or more than 140 days at Hornsby Bend. Three types of tests were performed: monotonic tensile load tests, low-frequency cyclic tests (to simulate normal operating conditions), and high-frequency cyclic tests (to simulate long-term operational effects). The main conclusions from this work are: 1) two of the five design methods provided estimates that were closest to the measured pile capacities, 2) the piles aged at Hornsby Bend and Allen Harbor experienced aging factors of about two and five, respectively, 3) no cyclic degradation or post-cyclic changes were observed at either site, 4) there were no signs of an interaction between aging and cyclic loading.