Abstract
Experimental measurements of the surge drift motion of a soft-moored barge in random waves are compared to the results of an improved numerical simulation employing a quadratically nonlinear Volterra functional polynomial and a complete surge motion quadratic frequency response function (QFRF). Assuming that the surge drift oscillation system is linear and driven by second-order drift force, experimentally determined viscous and wave drift damping coefficients and theoretically calculated wave drift force QFRF's are used to obtain the QFRF's for surge drift oscillation from an equation of motion in bi-chromatic waves. The required wave drift damping coefficients are bi-frequency dependent, and are determined from free oscillation tests in bi-chromatic waves. Surge drift motion QFRF's determined from an extensive series of bi-chromatic wave tests are compared to analytical predictions based on bi-chromatic, as well as mono-chromatic, wave damping coefficients. In this way the effect of bi-frequency wave damping on drift motion, which is often approximated or neglected, is treated in a manner that is consistent with Volterra theory. As such, this study appears to be the first to experimentally determine bi-frequency wave drift damping coefficients from free oscillation tests in bi-chromatic waves. The simulation of surge drift motion in random waves is achieved directly through a double inverse Fourier transform of the predicted motion QFRF's with a given wave spectrum. A more conventional time domain simulation based on the solution of an equation of motion using only mean wave, drift damping and drift force excitation time series synthesized from the complete force QFRF is carried out for comparison. Finally, a similar time domain computation is performed, but, with drift force excitation synthesized from a force QFRF whose oscillatory terms are estimated from mean drift force only. Statistical parameters derived from large ensembles of model test data are compared with those of the simulations.
Krafft, Martin Joseph (1992). Wave drift damping and motion of a moored vessel in random seas. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1307084.