Study On Prediction Method And Nonlinear Dynamics For Ship Surf-Riding/Broaching

Surf-riding describes a phenomenon where ship sailing in severe longitudinal waves,acted by excessive wave-induced surging forces,accelerates to wave celerity and is finally captured by waves.Broaching happens when ship experiences large yawing moment and a sudden change of heading angle,and finally causes a loss of course-keeping ability.Surf-riding/Broaching may cause severe loss of stability even in longitudinal waves,thus,such phenomena have been recognized as stability failure modes in the IMO second-generation intact stability criteria.The second-generation intact stability criteria are expected to be formally finalized in 2019,and researchers are currently working on the establishment and validation of the direct stability assessment(DSA)method.Based on the unified theory and the fundamental principle of IMO's direct stability assessment procedure,a weakly nonlinear 6-DOF model considering both maneuvering and seakeeping motion is developed.Modules of this model are separately validated.The model is applied to both a 42 m and ITTC A2 purse seiners for numerical simulation to reproduce surf-riding/broaching phenomenon in following and quartering seas.The simulation results show qualitative and quantitative agreement with published model test results and the model is demonstrated to be effective in surf-riding/broaching prediction.Then,influences of the factors involved in surf-riding/broaching prediction are discussed,among which the propeller thrust reduction effect in waves is found to be a key factor for surf-riding/broaching prediction.Taking propeller thrust reduction effect into account,the threshold of surf-riding/broaching rises and agreement of simulation results with published experiment results is improved.To evaluate the risk of surf-riding/broaching phenomenon in actual seas,probability assessment of surf-riding/broaching is conducted following the IMO's direct stability assessment procedure.As a result,adopting current 6-DOF model,the consistency between Level 2 criteria and direct stability assessment result is guaranteed.Furthermore,probability of surf-riding/broaching occurrence in irregular waves is statistically analyzed.The nonlinear mechanics of surf-riding/broaching is finally studied by using a simplified 4-DOF maneuvering model.In the light of bifurcation analysis,critical conditions for ship's motion including periodic surging motion,surf-riding and broaching are discussed.