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Study of marine sediment drag forces on offshore pipelines
Abstract
Soft marine sediments move in response to gravity forces and bottom pressures produced by storm waves. These sediment movements produce forces on ocean structures, such as offshore platforms and marine pipelines, which are sufficient to fail the structures. Previous research had shown that the sediments exhibited viscoelastic characteristics in which the shear modulus could be expressed as a power law in time. This implies that the drag forces produced by sediments moving past marine structures will be a function of the relative velocity of movement between the sediment and the structure. Tests were performed to determine the drag forces produced by moving sediment on deep, shallow and partially buried model pipelines. The results were expressed in terms of generalized force coefficients, g[subscript i] which are independent of the sediment velocity, sediment shear strength and pipe dimensions. Two "drag" boxes were utilized to study various effects such as the sediment flow angle, total pressure, sediment shear strength and pipe surface roughness on the generalized force coefficients. Results showed that the drag forces normal to the pipe increased with depth of burial, as might be expected, until a burial depth of six to seven times the pipe diameter was reached, at which point the drag forces became constant. Axial drag forces were much smaller than the normal drag forces and reached a constant value when burial depths exceeded twice the pipe diameter. Other findings were that increases in total pressure produced increases in normal forces at burial depths greater than two pipe diameters, whereas axial forces were not affected. For burial depths less than one pipe diameter, increases of external pressure produced smaller axial and normal forces depending on the amount and duration of the pressure. At shallow burial depths, freely rotated pipes produced no surface roughness effect at failure points; however, clamped pipes exhibited somewhat higher normal forces for roughened surfaces. At large displacements, after pipe-sediment separation, roughness effects were minor. Results of the drag tests on pipes are compared with existing theoretical predictions for various cases of separation at the interface between the pipe and the sediment. The theoretical predictions for no separation compared closely with the experimental results for deeply buried pipes under the highest total pressures. Other theoretical cases of separation compared closely with actual results for shallow and partially buried pipe.
Description
Typescript (photocopy)Vita
Subject
Major civil engineering1987 Dissertation H874
Underwater pipelines
Marine sediments
Ocean engineering
Collections
Citation
Hsu, Tung-wen (1987). Study of marine sediment drag forces on offshore pipelines. Texas A&M University. Texas A&M University. Libraries. Available electronically from https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -26998.
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