In this paper, extreme value of the section load on a ship navigating in irregular waves is predicted and analyzed. Design sea state method is introduced to determine the limit sea conditions and cruising sea conditions. The load of the ship navigating these sea conditions are calculated by the COMPASS-WALCS software. An extreme value analysis method of the irregular load is put forward to give the most possible extreme value and design extreme value. The rationality of the extreme value analysis method is verified by comparison with experiment data. Finally, the numerical calculation results and the test results are comprehensively compared and analyzed.
Ships navigate and work under different sea conditions throughout their service life, and they are subjected to wave force during most of its service time. The wave force in rough sea will bring great danger to ship structure. Therefore, it is of great significance to predict the maximum wave load during service time to design ship structure. The sea condition is random and thus the wave load is also random. To determine the maximum wave load, the probability method of extreme value analysis is usually adopted. In present paper, both the numerical result of wave load and experimental data is used as the input of the extreme value analysis. In summary, the numerical methods for solving wave load are developed from two-dimensional slice theory to three-dimensional theory, from linear to nonlinear theory, from rigid body boundary conditions to hydro-elastic boundary conditions. The three-dimensional potential flow theory with hydro-elastic boundary conditions is adopted here to predict the irregular random wave load on ship and this is realized by utilizing the COMPASS-WALCS software.
After obtaining the irregular random wave loads in time domain, the extreme value in long term and short term need to be predicted through probability method. The ship structure need to be designed strong enough to resist this extreme load. To predict the extreme value within a certain period of time, the probability density function need to be fitted from the time history curve of wave load. For linear wave load, the spectral analysis method is most often used and the load is assumed to conform to the Rayleigh distribution. While the nonlinear wave load is assumed to follow the Weibull distribution, its probability density function is fitted through statistical analysis of peak load in histogram. Based on the probability density functions of different sea conditions, the short-term and long-term prediction will be performed, and the short-term analysis lasts for a few hours and long-term analysis several years. Ren and Dai (1997) , Qin (2003), Xu (2008), Li et al. (2013) and Tang (2014) have given reasonable short-term and long-term forecasting methods for nonlinear loads. Chen and Shen (1996) compared the long-term value of wave bending moment with the design load. Gu et al. (1998) studied the influence of sea state on long-term prediction. IACS (1992) also stipulated the conditions for long-term extreme value analysis in the direct calculation of wave load. Luo (2016) investigated the extreme value of wave loads of a flat ship. Jiao et al. (2019) carried research on short-term prediction of wave loads and slamming loads based on experimental data of a ship.