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Time-Domain Simulation of Floating, Dynamic Marine Structures using USFOS
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Marine Technology.
2013 (English)MasteroppgaveStudent thesis
Abstract [en]

This master thesis deals with calculation of the convolution integrals that constitute the memory functions in the linear time-domain wave response equation. Basic theory needed to calculate the forces acting on floating structures is presented. The main motivation for the thesis is to investigate the feasibility of including frequency dependent added mass and damping in USFOS. USFOS is a computer program originally developed to simulate the response to extreme environmental and accidental loads for fixed offshore structures. The program has been modified to be able to handle also floating offshore structures. Floating structures are however subjected to diffraction and radiation terms often simulated as added mass and damping terms. So far, these terms are assumed to be constant in USFOS. This is not correct, as the added mass and damping terms are frequency dependent in reality. The frequency dependence can be accounted for by adding memory functions expressed as convolution integrals. The convolution terms are integrated from 0 to t, i.e. from the beginning of time and up to this moment. This is neither possible, due to computer restrictions, nor necessary, due to damping effects. This thesis concentrates on finding the amount of ?memory? that needs to be included in the convolution integral. A single degree of freedom system illustrating a platform oscillating in heave is analysed in MATLAB, both for regular waves and irregular seas. The convolution integral is calculated with different time intervals included in the memory function, and the deviations from the full integral are analysed. Finally, a brief description is given of how the experiences obtained with the single degree of freedom system can be utilized in the implementation of frequency dependent added mass and damping terms in USFOS.

Place, publisher, year, edition, pages
Institutt for marin teknikk , 2013. , 109 p.
URN: urn:nbn:no:ntnu:diva-22233Local ID: ntnudaim:9599OAI: diva2:648688
Available from: 2013-09-16 Created: 2013-09-16 Last updated: 2013-09-16Bibliographically approved

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