Drained Residual Conditions for Axially Loaded Flowlines-Paper

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The study investigates drained residual axial resistance between flowlines and Gulf of Mexico marine clay using model tests with 108-mm polypropylene pipes. Tests varied pipe weight, clay strength, loading rate, and loading history; results were compared with tilt-table interface tests.<br /><br />Drained conditions were mobilized at an axial speed of approximately 0.0005 mm/s. After reaching the drained residual state, the loading rate could be increased by at least 100 times in the same direction without changing resistance. The same state could also be reached through repeated faster, nominally undrained loading cycles. Following pauses, resistance could temporarily rise, but returned toward the residual value after sufficient displacement.<br /><br />Axial-test results and tilt-table measurements both showed that drained residual resistance decreases substantially as effective normal stress increases. However, measured pipe resistance varied with time and shearing history. Continued consolidation at the pipe–soil interface appeared to affect results for more than 50–100 days after placement, even when further settlement was not visibly measurable. Changes in the pipe’s effective contact area—potentially caused by stress concentration beneath the pipe and gaps along its sides—may also alter resistance.<br /><br />The findings show that both loading rate and the evolution of pipe–soil contact must be considered when estimating flowline resistance, particularly during long-term movement.

Keywords

drained residual axial resistance

flowlines

Gulf of Mexico marine clay

model tests

polypropylene pipes

loading rate

effective normal stress

pipe-soil interface

consolidation

shearing history