Assessment for Probability of Ship Capsizing due to Pure Loss of Stability in Quartering Seas (2nd Report)

Abstract

The authors had proposed a theoretical method for calculating probabilities of capsizing due to pure loss of stability for a ship running in quartering seas in their previous report. In this report, their method was verified by the Monte Carlo simulation in time domain. That is, the capsizing probabilities predicted by the proposed method are slightly larger than those given by the Monte Carlo simulation. We used a simplified numerical model to obtain time series those are sufficient for statistical analyses. The model treated two degrees of freedom, namely, surge and roll and took the non-linear restoring moment due to a wave and a roll angle into account as well as wind forces, wave exciting forces and so on. Resistance, propeller thrust and roll damping moments were predicted by model experiments. Repeating numerical runs with common wind and wave spectra and different phases, we obtained the probability of capsizing of a small trawler in quartering seas. In addition, all simulated capsizings were occurred when the ship was situated on a crest of a wave. Immediately before the capsizings, the ship suffered synchronized roll motion or not as the case may be. This capsizing mode coincides with the mode assumed in the proposed theoretical method for capsizing probabilities.