A Method for Requiring Block Size for Spectrum Measurement of Ocean Surface Waves

Spectrum measurement of field data of ocean surface waves is essential for the performance research, analysis and design of ship structure, and laboratory testing. There are two problems in measuring a power spectrum of wave data. One is how to estimate the upper bound of the mean-square error of a spectrum measurement of a block of wave data at a given wind speed (sea state). The other is how to quantitatively require the lower bound of the block size according to a given wind speed and a given degree of the mean-square error of spectrum measurement of a block of wave data. This paper presents four theorems and two corollaries with respect to the solutions to the above two problems. Theorems 1-4 describe the general solutions to the above problems based on two general spectral models of wave data. Corollaries 1 and 2 show the specific solutions to the problems based on the Pierson-Moskowitz spectrum. With Corollaries 1 and 2, the effect of large wind speed on the spectrum measurement of wave data is analyzed. The significance of the present results is examined and demonstrated by spectrum measurement of real field data of ocean waves