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Effects of sea severity on wave-overtopping for FPSO fixed in head seas
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A series of experiments applying regular linear, nonlinear waves (Stokes 5th order-like) and random waves on a fixed FPSO model were performed in the wave-flume tank. Statistical information was gathered for wave overtopping of extreme waves on the fixed deck. This obtained statistical data of wave overtopping includes the wave elevation exceeding the freeboard, wave loads affected by the wave-overtopping and the nonlinear behavior of the waves. A systematic analysis, which centers on the wave height and wave steepness, was done for the waves and wave effects on the forces. Wave elevation time series was also measured at some distance from the ship model without the effects of the model. Horizontal and vertical forces on the model were measured using a dynamometer. Sensitivity to the wave nonlinearity parameter and deck clearance is discussed. The measured forces (Fx, Fz) increased almost linearly with the wave steepness as well as with the wave period (wave height) at the given steepness. The band widths of spectra of the measured random waves were calculated and these were narrow banded. The observation of the probability density function (histogram) did not indicate a Gaussian distribution. The skewness and kurtosis values also indicated high nonlinearity in the measured waves. The horizontal and vertical force LTF derived from the random input and output wave data over-estimated the LTF derived from the regular wave test data by about 30-60%. The probability of exceedance gave better agreement with that of wave elevations and the Rayleigh linear theory nearly over-estimated the exceedance for the vertical and horizontal forces. The nonlinearity of waves and higher nongaussianity can be due to the wavemaker limitations at the wave flume tank and equipment inadequacies which could not be improved at the time of the experiments. The probability of wave overtopping was taken for varying significant waves heights. It was observed that the waves overtopped for a significant wave height of 12 m. It was predicted using a ratio of the freeboard height and standard deviation that approximately 17 to 18 waves would overtop an FPSO in a duration of 30 minutes for seas of Hs 12 m. Overtopping of the waves at the bow of the FPSO model was observed using photographs for some high waves.
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Includes bibliographical references (leaves 82-84).
Issued also on microfiche from Lange Micrographics.
Bakrawala, Murtaza A. (2003). Effects of sea severity on wave-overtopping for FPSO fixed in head seas. Master's thesis, Texas A&M University. Available electronically from
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