Excessive acceleration is one of the stability failure modes involving large-amplitude roll of a ship. The applicabilityof the vulnerability criteria of excessive acceleration for a ship with moonpool is compared and analyzed. The influence of the roll damping of the moonpool on the assessment of the ship long-term failure probability is studied by comparing the ship lateral acceleration and the long-term failure probability that are calculated by using the roll damping coefficients of different initial roll angles. The catenary mooring model is used to constrain the sway of the ship, and the six degrees of freedom (6-DoF) of the ship with moonpool in beam waves is simulated by using the Realizable turbulence model. The feasibility of using computational fluid dynamics (CFD) method to directly evaluate the stability of the ship with moonpool under excessive acceleration is validated by comparing with the experimental results. The results show that the Level 1 vulnerability criterion is not suitable for the sample ship, whereas for the sample ship with moonpool, the equivalent linear roll damping coefficient from the free roll attenuation test with an initial roll angle of 10° is recommended to be used in the Level 2 vulnerability criterion of the excessive acceleration. The accuracy of the direct numerical simulation of the excessive acceleration based on CFD method is acceptable for the ships with moonpool.

The second-generation intact stability criteria of International Maritime Organization (IMO) has entered the trial period. This paper focuses on the surf-riding/ broaching stability failure mode in irregular waves. It reproduce the process of the surf-riding/broaching phenomenon through massive repeated numerical simulations with different speeds, wave directions, wave spectrum truncation ranges and metacentric heights ( GM) by adopting a six degrees of freedom (6-DoF) numerical modeling. Hilbert transform is used to calculate the instantaneous wave velocity to identify the ‘speed increase’ state, and the probability of ‘speed increase’, broaching, excessive yawing and rolling are quantitatively studied. It is found that the occurrence of ‘speed increase’ is closely related to the surf-riding, and its probability estimates increase with the increase of the ship speed. Most ships capsize after experiencing a relatively long and continuous duration of ‘speed increase’, and the wave spectrum frequency truncation has a great impact on the probability estimates of ‘speed increase’. However, the probability estimates of broaching after ‘speed increase’ increase sharply with the increase of the wave angle for the ships at the same speed. The total duration of broaching can account for 30% to 50% of the total duration of ‘speed increase’ at high speed when the wave angle reaches 30 degrees. With the increase of the ship speed, the proportion of the duration of broaching in the total time of excessive yawing gradually increases to 60% to 80%, indicating that the yawing at high speed is mainly caused by broaching. The same as the variation of the probability of broaching after ‘speed increase’, the probability estimates of excessive rolling also increases with the increase of wave angle. The quantitative probability estimates of the ‘speed increase’, broaching and excessive yawing and rolling can provide technical support for the application of the second generation intact stability criteria of IMO.