The results of experimental measurements of the slowly varying surge motion of a soft moored rectangular barge in the bi-frequency domain are compared to numerical predictions based on a Volterra model and linear diffraction theory. Theoretical calculations were made using experimentally determined mono- and bi-frequency domain wave drift damping coefficients for the prediction of off-diagonal quadratic frequency response functions. Preliminary results indicate that predictions based on monochromatic wave drift damping underestimated experimental results, while those employing bi-chromatic wave drift damping provided better agreement.


The response of large moored vessels and floating offshore structures to irregular seas is dominated by large amplitude, low frequency quadratic nonlinear motions. As the use of such soft mooring systems for deep water drilling, production and storage facilities is rapidly increasing, the development of more accurate numerical prediction tools, of which experimental verification is an integral part, becomes ever important. Dalzell (1976) conducted the first model tests to determine the quadratic frequency response function (OFRF) for added wave resistance in random seas. In addition, Dalzell developed a cross-bi-spectral method to estimate the OFRF for low frequency, second-order added resistance (drift force). OFRF I for added resistance was theoretically analyzed by Dalzell and Kim (1976) and applied to the prediction of surge drift motion of a moored ship in random head seas by Kim and Breslin (1976). Pinkster (1980) measured and calculated the OFRF for the drift force of vessels in head seas using Dalzell's method. Slow drift motion of a moored vessel in random waves has usually been predicted in the time domain by solving an, equation of motion using drift force and damping coefficients: estimated from theoretical or experimental studies using monochromatic waves. For instance, one such extensive theoretical and experimental time: domain investigation was conducted by Wichers (1987).

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