UBC Theses and Dissertations
Numerical simulation of wave grouping effects on moored structures Pemmireddy, Venkata Rami Reddy
The present thesis investigates wave grouping effects on the low-frequency drift force and the low-frequency surge response of a moored structure. The smoothed instantaneous wave energy history, SIWEH, which may be characterized by a number of parameters, is considered as an acceptable approach to describe wave groupiness. A numerical simulation of wave records is carried out for a specified wave spectral density and SIWEH incorporating various wave grouping parameters. The low-frequency drift force and the surge response of a large moored circular cylinder, modeled as a single degree of freedom system, is estimated on the basis of Pinkster's approximate method, which involves the use of computed drift force coefficients. Results show that the wave grouping parameters have a significant effect on the low-frequency drift force and surge response. It is also observed that the low-frequency drift force can be related to the SIWEH in terms of a transfer function. A statistical analysis of simulated low-frequency surge motions is performed, and the calculated extreme response is compared with the predictions of three theoretical models, namely the Gaussian, Exponential, and three-parameter methods. If the surge response has a non- Gaussian distribution, Stansberg's three-parameter model predicts extreme amplitudes reasonably well. The low-frequency drift force on a slender fixed circular cylinder is calculated using both Pinkster's approximate method and in the time domain using the Morison equation. Pinkster's approximate method underestimates the force, but it is computationally more efficient. The time domain approach is also used to estimate the low-frequency drift force on a moored slender circular cylinder and the results are compared to those for a fixed cylinder. It is observed that cylinder motion tends to reduce the drift force significantly.
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