Research On Whipping Responses Of Ultra Large Container Ships In Waves

In recent years,ultra large container ships(ULCS)have received extensive attention from international maritime community because of their property of low energy consumption and scalization.Due to their own structural characteristics,such as large principal dimensions,relatively high speeds,large proportion of high strength steel in ship structure,relatively low structural stiffness compared to their dimensions,and the large flare of bow region,when ULCS are advancing in moderate to high sea conditions the bow area will frequently slam on the incident waves,inducing whipping response of the ship hull and thus endangering the total strength and the fatigue strength.Even though the main classification societies around the world have released rules or guidances on the effects of slam-induced whipping on the structural strengths of ULCS,the rules or guidances were designed to provide general suggestions on the analysis process without stipulating specific calculation methods and corresponding softwares,and some strength estimation methods still had their shortcomings,thus causing difficulty for structural design and safety estimation of ULCS.Based on this background,this paper conducts theoretical and experimental research on whipping responses of ULCS in regular or irregular wave conditions,establishing an engineering applicable system for prediction of whipping responses and structural responses estimation of ULCS to meet the pressing demands from the industry of ULCS whipping responses analysis.The main contents of this paper are as follows:Firstly,in order to further reveal the physics of fierce hull-wave interaction,the wave load experiments of segmented ship model are carried out in the wave tank for a targeted ULCS in moderate to severe seas.The regular wave tests analyze the characteristics of ship motions and whipping responses and their dependences on wave height.The time history of bow region slamming pressure is analyzed for guidance of local strength estimation.For the tests in irregular waves,the statistical parameters of motions and whipping responses are extracted to evaluate their dependence on ship velocity and wave height.Statistical analysis and analysis on the relations between the low frequency components and total whipping responses are carried out to analyze the whipping characteristics.Secondly,the current nonlinear 3D time domain whipping response prediction method is improved.The 2D generalized Wagner method is adopted to calculate slamming pressure in bow region,and more cross sections in the bow and stern regions are adopted to better simulate the 3D effect in these regions whilst balancing the requirement of time efficiency.To improve the commonly adopted method to obtain the instantaneous wet surface of the hull,this paper adopts a better method that is consistent with the linear incident waves to better consider nonlinear effects.The boundary value problem to solve radiation force in waves adopts the body surface and free surface conditions that take into account the ship velocity effect,and the retardation function method is used to reflect the wave memory effect.The advantage of the proposed nonlinear method against current momentum theory based method is revealed by comparison of their simulation results.By comparing the the time history or statistical parameters of structural responses between numerical and experimental results,the validity of the proposed numerical method and the characteristics of ULCS whipping in head seas are revealed.Thirdly,the momentum impact method in head seas is extended to the oblique wave conditions,based on which a new numerical method suitable for nonlinear slamming-induced whipping responses analysis in oblique seas is proposed.To avoid the numerical problems caused by the fact that yaw and sway motions do not have corresponding recovering forces,the so-called automatic rudder model is added in the numerical framework.This paper adopts the 3D FEM to perform the modal analysis and then extracts the modal results from the 3D FEM results.The structural responses are numerically solved by modal superposition method.The advantage of the new method against current head seas momentum theory based method is revealed through comparison of their simulation results.By comparing the numerical result and the experimental result,the proposed 3D nonlinear method to estimate the whipping responses in oblique seas is validated.The whipping response characteristics of ULCS in oblique sea conditions are analyzed which may serve as reference for future theoretical and experimental research.Since some aspects of current classification society rules have inadequacies,this paper researches the effects of whipping responses on the structural strengthes of ULCS based on the direct computation approach.First the design wave loads obtained from the direct computation and rules are compared and analyzed,then the influences of whipping on yielding and buckling strengthes are analyzed based on the 3D FE model of the targeted ULCS.The formulas of different classification societies on the ultimate strength estimation are compared,and the different estimation results are consequently analyzed.By analyzing the effects of springing on fatigue damage,the collection of short-term sea states in which the springing effects can be safely omitted is obtained.With reference to the characteristics of whipping responses,the group of short-term sea states in which only the contribution of whipping to fatigue damage needs to be considered is accordingly obtained,further revealing the contribution of whipping to fatigue damage.Several representative short-term sea states are picked out to further clarify the details of whipping effects on the fatigue damage.This is a comprehensive and detailed research on the fatigue damage analysis,serving as reference for futher improvement of fatigue damage analysis of classification society rules.