An efficient methodology has been developed for the dynamic analysis of offshore
floating structures. In this methodology, special attention was given to the second
order difference frequency forces and responses. According to this numerical scheme,
a MATLAB program named TRSPAR was developed to predict the dynamic
responses of truss spar platform in time domain. In this program, the truss spar
platform was modeled as a rigid body with three degrees of freedom. Hydrodynamics
of the structure, which include the linear and second order wave forces, mean drift
forces, added mass, radiation damping, wave drift damping and system stiffness were
included in the program. Current and wind forces were also considered showing their
effects on the slow drift responses. The wave forces, including inertia and drag forces,
were calculated using Morison equation assuming the wave field as undisturbed. An
efficient time domain integration scheme was adopted based on Newmark Beta
method.
Comprehensive experimental studies were conducted and the numerical
predictions were systematically compared with model test results. These comparisons
consisted of structure’s dynamic responses in different environmental conditions and
two structural situations. The first situation was the structure with intact mooring lines
and the other one was the structure under mooring line failure. The responses of the
platform with mooring line system damage were investigated with the emphasis on
finding the critical effects of line failure on the resonant responses.
The effects of the second order difference frequency wave forces on the truss spar
motion characteristics were examined numerically. Published numerical results were
used to verify the developed numerical model in predicting the truss spar dynamic
responses when subjected to combined wave, current and wind forces. The effects of strengthening mooring line system on the motion characteristics of the structure were
examined numerically. For the assessment of the fluid to mooring nonlinear
interactions, a deterministic approach based on lumped mass method with equations
of dynamic equilibrium and continuity was adopted. Finally, parametric studies on
deepwater mooring line analysis have been conducted for investigating the
contributions of the various design parameters on mooring line tension.
The experimental results verified the validity of the developed numerical scheme
for prediction of the wave frequency and low frequency motions of the truss spar
platform with its intact mooring and in the case of mooring line damage condition.
RMSD values for the numerical and the experimental results show that the simulated
wave frequency responses (WFR) trend was relatively agreed well with the
experiments compared to the low frequency responses (LFR). For the intact mooring
line condition, RMSD values for the WFR ranged from 109.9 to 182.4 while for LFR
were ranged from 499.6 to 550.2. The same has been noticed in the mooring line
damage condition in which RMSD values ranged from 107.4 to 323.6 and 209.1 to
1074 for WFR and LFR respectively. With regard to the peak responses, good
accuracy has been achieved between the predictions and the measurements. The
percentage errors for the peak responses in the intact mooring and the mooring line
damage conditions were ranged from 9.5% to 17.3%
