Research On Direct Calculation Of Ultra Large Container Ship's Structure Strength

In recent years,the international trade keeps increasing with the rapid development of global economy,which leads to more demands for container transportation.In addition,the container ship size is driven towards more large under the major container shipping companies seeking scale effect.Due to ultra large container ship's characteristics,including large opening,low stiffness and high speed,the yielding and fatigue strength are serious.More challenges are brought to structures with the large-sized trend,which makes the traditional hull girder theory inappropriate.Therefore it's necessary to carry out some research on direct calculation of ULCS's structure strength.In this paper,the ship motion and wave load of a ULCS are analyzed based on 3D linear potential flow theory and equivalent design wave method,which is then compared with classification society rules and determines the design wave parameters used for strength assessment.The yielding strength and characteristics of structural response under bending and torsional loads are discussed,and also the region of high stress are recognized.Finite element fine mesh model are taken for the areas where stress is concentrated and then those unsatisfactory structures are improved.Based on 3D linear frequency domain hydro-elastic method,taking into account the effect of springring,the modal analysis is firstly done by FE method and the elastic effect of wave load is discussed later.In order to evaluate springing's effect on fatigue life,some hatch coners with different locations and types are selected and their fatigue damage are calculated according to spectral analysis method,compared with the results of rigid method.Finally,a method for fast modeling of local structure fine mesh is put forward and a corresponding program is developed.With the help of this program,several optimization plans are established to improve the fatigue strength of a biarc negative coner and also the results are analyzed.This study finds that it's necessary for structure assessment of ULCS to calculate the wave load directly and consider both bending and torsional loads.Moreover,fine mesh model should be taken for areas where stress is concentrated,such as longitudinal hatch coaming end and hatch coner.It's very likely to come up with springing when the ship on the sea.And the influence of asymmetric modal must be considered.The characteristics of high-frequency stress response at hotspots will make the fatigue damage greatly increase and the fatigue life reduce by 30%~40%,somewhere even reaching up to more than 50%.So the effect of springing can't be ignored for fatigue assessment.A method for fast modeling of local structure fine mesh and a customized program developed in this paper are helpful to improve efficiency during structural design and supply reference to similar problems.For biarc negative coner,there must be an optimal radius ratio of arc and the fatigue life reach the maximum at this point.