Motion analysis and model study of a guyed tower structure in regular waves

Murray, John James (1981) Motion analysis and model study of a guyed tower structure in regular waves. Masters thesis, Memorial University of Newfoundland.

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    Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission.
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Abstract

The use of fixed platforms of either the gravity of jacket type in water depths exceeding three hundred and fifty meters would so escalate the size of these conventional platforms as to render recovery of oil uneconomical. Alternative platform concepts such as the guyed tower (Finn, 1976) take advantage of the effect of compliance to the wave action. However, such a concept introduces the main problem for deep-water platforms namely the dynamic interaction of waves and structure. Assuming the tower to be of uniform flexuaral rigidity and uniform weight per unit length, a modified Morison’s equation was used to determine the horizontal wave loads on the tower. The equation of motion for the horizontal displacement of the deck was set up and a Crank-Nicholson finite-difference algorithm was employed to solve the equation of motion of the tower. Water particle velocity and acceleration used in the wave loading computation were obtained using linear diffraction theory (MacCamy and Fuchs, 1954). In the development of the computer model the tower was represented as an equivalent be3am and the distributed wave load was resolved into concentrated nodal forces. Experimentally determined coefficients for damping, restoring and the mass of the tower were used for solving equation of motion. – In order to compare the predictions of the computer model with the performance of a physical model, a model of the guyed tower was constructed and tested in a wave tank. The tower was supported by eight guy wires each having a model weight per unit length of 5.21 N/m. Deck displacements of the tower were monitored by means of rotary transducers and the guy line tensions were monitored using ring transducers placed directly in the lines. The damping coefficient of the model was determined experimentally by displacing the model and using the logarithmic decrement obtained from a record of its free oscillation. The restoring coefficient was also determined experimentally by generating a plot of total restoring force versus deck offset of the model tower. Fairly good agreement between the computer model results and the physical model test results was found for the deck displacement.

Item Type: Thesis (Masters)
URI: http://research.library.mun.ca/id/eprint/5273
Item ID: 5273
Additional Information: Bibliography: leaves 18-19.
Department(s): Engineering and Applied Science, Faculty of
Date: 1981
Date Type: Submission
Library of Congress Subject Heading: Offshore structures; Drilling platforms

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