Time-Domain Simulation of Floating, Dynamic Marine Structures using USFOS
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.
IdentifiersURN: urn:nbn:no:ntnu:diva-22233Local ID: ntnudaim:9599OAI: oai:DiVA.org:ntnu-22233DiVA: diva2:648688
Amdahl, Jørgen, Professor