Study On Asymmetrical Slamming And Structural Response Of Ultra-Large Container Ships

In recent years,container ships are developing towards large scale in order to improve the economy,which makes the structural safety become more prominent.Slamming phenomenon of this ship occurs easily due to higher speed and larger bow-flare shape.One the one hand,ship slamming under oblique waves has obvious asymmetry,which can induce whipping,and thus,cause hidden danger to structural safety.However,the current researches mainly focus on the symmetrical case due to the complexity of asymmetric slamming,and thus,asymmetric slamming should be further studied.On the other hand,the torque generated by asymmetric slamming has negative effect on the large opening structure of container ships.However,the degree of influence is not clear due to the lack of research.This paper discusses the mechanism and load characteristics of asymmetric slamming through numerical and experimental methods.Based on the characteristics of ULCS(Ultra-Large Container Ship),the asymmetric slamming loads and torsional whipping responses under oblique waves are studied.Some related research work has been done as follows:1)For the problem of asymmetric water entry slamming,the slamming mechanism of simple two-dimensional sections under two asymmetrical factors(inclination angle and transverse velocity)was studied.It is used to calculate the local asymmetric slamming load of very large container ships.A gas-liquid two-phase flow model was established based on the N-S equations,which is solved by the CFD(Computational Fluid Dynamics)method.The VOF algorithm is used to track the free surface.In this paper,fuzzy tracking of free surface and the discontinuity of pressure at the interface of air and water are corrected.In order to make a comparative analysis,a BEM method is presented to solve the asymmetric water entry problem.The applicability of these two methods is discussed by comparing the published results,and then the slamming characteristics of ship section entering water with heel angels and transverse velocity are comprehensively discussed.Finally,the slamming problem of the elastic body is studied based on the CFD-FE method,and the elastic effect on the slamming pressure is further clarified.2)At present,there exist some certain difficulties in the numerical study of the slamming problem of 3D ship bow.In order to clarify the slamming characteristics of 3D bow,the corresponding free-drop tests were carried out.In there,related technology of asymmetric free-drop experiment is introduced in detail,and the self-designed angle device is used to adjust the water entry attitude of the test model so as to realize the experimental requirements of asymmetric water entry.The reliability is discussed through the uncertainty analysis of the experiments.Based on the experimental data,the basic characteristics of the 3D asymmetric slamming pressure are analyzed,and the effects of the heel angle and the longitudinal angle on the slamming pressure are further discussed.Finally,the applicability of the 2D numerical method in predicting the 3D bow slamming pressure is discussed by comparing the 2D numerical results and 3D experimental data.3)Whipping response under asymmetric slamming was studied based on the three-dimensional nonlinear time-domain hydroelasticity method,where the torsional whipping is mostly concerned.The influence of elastic deformation on the flow field is included based on the velocity potential theory.The boundary element method combined with slice theory is used to calculate the asymmetric slamming force.A unified dynamic equation of fluid load and structural response is finally established.The applicability of the method is verified by convergence analysis and numerical comparison.Finally,taking a 21kTEU container ship as an example,the elastic effect and asymmetry on the slamming force is discussed,and the characteristics of torsional whipping under asymmetric slamming are analyzed.4)Combined with hull motion and water entry slamming theory,the slamming loads of ship hull was studied,and a prediction method for local asymmetric slamming load of hull was proposed.Firstly,the motion of the hull is predicted based on the three-dimensional nonlinear hydroelastic method,and then,the velocity at the concerned position is predicted by the incident wave model.In order to account for the influence of wave inclination and particle velocity of wave fluid,a wave reference system was introduced to transform hull slamming into oblique water entry problem.Finally,slamming pressure was predicted by CFD method.Compared with the published test data,the applicability of this method is verified.Three-dimensional and local elastic effects are evaluated in detail,and then the characteristics of asymmetric slamming loads at the bow are analyzed in oblique wave cases.Finally,the effects of different wave directions on the slamming loads are discussed.5)Based on the related results of asymmetric slamming loads,the structural strength of ULCS under slamming pressure was evaluated.The design value of slamming load is firstly determined by long and short term analysis,and then the structural response under asymmetric slamming load is calculated by the finite element method.Firstly,the dynamic response mechanism of large open structure under asymmetric slamming load is studied,and the effects of load rise time,peak value and slamming duration on the structural response are discussed.Then,taking a 21kTEU container ship as an example,the direct finite element analysis is carried out on the two structural models:local structure and whole ship,and the differences between the two methods are compared.Finally,the influence of asymmetric slamming load(torque)on the structure is discussed in detail.