Prediction And Prevention Of Ship Transverse Stability Failure Events In Longitudinal Seaway Based On The Unified Model

In adverse sea states,the ship tends to sail in longitudinal waves to avoid the stability failure caused by beam sea.However,recently ships,especially some specific types of ships,become prone to transverse stability failure problems in longitudinal waves such as parametric roll and surf-riding/broaching.The International Maritime Organization（IMO）is dedicated in the enforcement of the second generation intact stability criteria on five stability failure modes including parametric roll and surf-riding/broaching,which will bring great challenges and opportunities to the shipbuilding industry in China.Thus in this paper,the researches on numerical simulations,model experiments and mitigation methods of the main transverse stability failure problems in longitudinal waves,parametric roll and surf-riding/broaching,are conducted in order to yield a better understanding on these phenomena and contribute to the Chinese proposal on IMO second generation stability code.Firstly based on the unified theory,the 6-DOF weakly nonlinear time domain program coupling maneuvering and seakeeping models,Safe-Stab（Safety Assessment of Ship Dynamic Stability in Waves）,is developed.Different parts of the program are verified separately.Model experiments are vital for the understanding on the mechanism of parametric roll and surf-riding/broaching as well as the validation of numerical models.In this research,the free running model experiments on parametric roll of the KCS（KRISO Container Ship）and a10000TEU containership and surf-riding/broaching of a 42m purse seiner are conducted in towing tanks.In the model experiments of parametric roll,it is found that parametric roll occurs while T_roll/T_e is around 2.When the ship speed gets too small or too large,T_roll/T_e becomes far away from 2 and parametric roll disappears.Bilge keel is very important for the reduction of parametric roll steady amplitude.Moreover,the practical non-ergodicity of parametric roll in irregular waves are investigated.It is found that sufficient test time or enough repeated tests are needed for the statistical post-processing of parametric roll to overcome the practical non-ergodicity.Parametric roll experiments in short crest irregular waves are conducted.It is found that the roll motion in short crest irregular waves includes parametric roll motion as well as conventional roll motion.In the model experiments of surf-riding/broaching,different cases with various speeds and wave heights are conducted.Results show that pure surf-riding and broaching after surf-riding occur as Froude number grows larger than 0.35-0.4 and wave height is large enough.The developed Safe-Stab program is applied to the numerical simulations of the head sea parametric roll of the KCS and the 10000TEU containership and the surf-riding/broaching of the 42m and ITTC A2 purse seiners.The simulation results are compared with model experiments to validate the program.From the comparison on parametric roll results between numerical simulations and model experiments,it is found that the program can predict exactly the occurrence of parametric roll and its steady amplitude.The comparison on surf-riding/broaching results of the two purse seiners are conducted qualitatively and quantitatively.Results show good agreements between numerical simulations and model experiments.Thus the program is well validated.Based on the fully validated program,the influencing factors of parametric roll and surf-riding/broaching are investigated.The parametric roll responses of a 3100TEU containership in regular and irregular head seas are simulated.It is found that as the ship speed becomes higher,T_roll/T_e becomes closer to 2 and the steady amplitude of parametric roll grows larger.the steady amplitude of parametric roll also gets large when the wave height is large causing the large variation on roll restoring moment.The practical non-ergodicity of parametric roll is confirmed by the numerical simulation.It is further concluded that 8 repeated tests with each lasting for 1800s are needed for the statistical post-processing of parametric roll to overcome the practical non-ergodicity.Moreover,the practical non-ergodicity cannot be fully eliminated by repeated tests.Through the simulation on surf-riding and broaching of the purse seiners,the influence of rudder emersion on broaching is investigated numerically.Results show that rudder immersed depth decreases dramatically and rudder inflow velocity is reduced by wave orbital velocity when surf-riding occurs,which cause the loss on rudder effectiveness.The loss on rudder effectiveness is the key factor for the emergence of broaching motion.Moreover,it is found that the influence of rudder emersion takes effect only when Froude number is higher than 0.36.Parametric roll and surf-riding/broaching are the main causes for the capsizing of ships sailing in longitudinal waves and bring a significant number of tragic losses.Therefore,it is of great practical significance to find ways for the prevention and mitigation of parametric roll and surf-riding/broaching.Based on the time-frequency analysis method,namely Incremental Real-time Hilbert-Huang Transform（IR-HHT）,a detection scheme is proposed for the early detection of parametric roll.Parametric roll with different ship speeds in both regular and irregular head seas is detected successfully through the detection scheme based on IR-HHT technique.At the time when parametric roll is detected,the roll amplitude is still small and operations for the stabilization of parametric roll can be implemented promptly.Moreover,the rudder parametric roll stabilization is proved to be effective though numerical simulations and model experiments in both regular and irregular waves.Results show that rudder anti-roll moment can stabilize parametric roll motion especially when control system is activated based on detection results obtained from the detection scheme and during the onset stage of parametric roll.The prevention of surf-riding by adjusting the center of buoyancy（COB）and installing bilge keel in design stage is attempted based on numerical simulation.It is concluded that the threshold of surf-riding is affected by the adjustment of the ship’s COB.For the hull forms with COB closer to stern,the threshold for surf-riding is generally lower and surf-riding is easier to occur comparing to those with COB closer to stem.This is mainly due to the differences on wave resistance and wave surging force caused by adjusting of the ship’s COB.Moreover,the bilge keel has no effect on the threshold of surf-riding in following waves.However,in quartering waves especially when the heading angle is larger than 10 deg,the bilge keel has large influence on the range of capsize due to broaching and capsize on the wave crest.For the vessel without bilge keels,broaching and capsizing can easily occur even in relatively low Froude number due to lack of enough roll damping to compensate for the roll moment.