Residual Ultimate Strength Research Of Damaged Ship Structures

The ship hull structure is composed by various plates and stiffened panels by the welding process.During the ship operation term,fatigue cracking will be produced in the welding regions due to periodic alternating loads caused by the waves.In addition,occasional accidents caused by man-made operational mistakes,such as ship grounding,collision and cargo falling,will cause plastic local damage to ship deck and bottom structures.Therefore,it is essential to assess the residual ultimate strength of stiffened plate structures with initial damages for the security of aged ships.For some special ships,such as large,ultra-large container ships,the torsional stiffness of hull girders is significantly reduced due to the large deck opening.As a result,it should not only evaluate the ship ultimate bearing capacity to resist the longitudinal bending moment but also the torsional ultimate strength.In order to improve economic efficiency of ship construction,high tensile steels have been widely used and the applied plate thickness of ship structures becomes much thinner.In this way,the rigidity of the ship components is decreased conversely.At the same time,the use of high-strength steels has little influence on improving the anti-fatigue ability and crashworthiness of the ship structures.In view of the above problems in the ultimate strength of hull structure,the following researching work has been done in this thesis:(1)The empirical formulas for predicting the ultimate strength of the intact stiffened plates under uniaxial compression have been systematically summarized.The buckling and post-buckling behavior of the stiffened plates was analyzed by using nonlinear finite element method.The influence of different parameters including the plate thickness,stiffener size and longitudinal spacing on the buckling failure of the stiffened plates was studied.(2)Hull structures may be involved in local dent damage due to heavy object falling,collision,grounding and so on.In this paper,various forms of dent shape have been summarized for comparison.The preset dent is introduced to the intact plate structures by using the dent shape equations.Several dent parameters including the dent shape,dent size,dent depth and dent location are considered to investigate the dent effects on the ultimate strength of stiffened panels.Corresponding to the numerical analysis results,the reduction factor of ultimate strength of stiffened plates with dent damage has been proposed.(3)When the hull structure reaches the ultimate limit state under severe torsional moment,the hull girders will both undergo the torsional shear stress and the warping stress.In this thesis,the structure collapse failure behavior of complex hull girders under twisting load has been analyzed by the model of simplified box girders.The effect of transverse members including transverse frame and transverse bulkhead on the resisting capacity of ship structures to torsional load was studied.The influence of stiffeners on the torsional ultimate strength of hull girder is analyzed by changing different thickness and cross section size.(4)The residual ultimate strength of box girder with inclined cracks under the action of torsional load is analyzed.The nonlinear finite element method is employed to analyze the influence of main crack parameters including the crack size and crack angle on the torsional failure modes of closed box girders.Based on the numerical results,a simplified formula for predicting the residual torsional ultimate strength of cracked box girders has been proposed by curve fitting method.(5)Based on the IACS common rules,a simplified box girder model with large deck opening is designed to study the effect of cracks on the torsional ultimate strength of ship hull girders.Three kinds of initial deflections are compared to analysis the effects of initial imperfection on the ultimate torsional strength of box girders with large openings.By changing the position and size of the cracks,the influence of one-side cracks and two-side cracks on the torsional failure behavior of stiffened box girders with large openings has been analyzed.The influence of the number of side and bottom stiffeners and the thickness of stiffener webs on the residual ultimate strength of box girders has been also considered.The main innovations in this thesis include the following aspects:(1)By using the nonlinear finite element method,the influence of plate thickness and yield strength variation of high tensile materials have been considered.A modified simple formula for predicting the ultimate strength of stiffened panel under uniaxial compression has been proposed.The prediction error for the ultimate strength of stiffened plates has been reduced from 15% to less than 5%.The effects of the relationship between the stiffness of the stiffener and the plate elements on the failure modes of the stiffened panel have been analyzed,and a relative stiffness factor κ of the stiffened panel has been proposed.(2)By means of collecting the shape forms of dent damage,a new dent function has been given considering the lateral deformation of the stiffeners caused by the dent.By changing the dent parameters including the dent depth,dent size,dent angle and dent location,an equation as the function of relative dent length and depth has been given to analyze the residual ultimate strength of damaged stiffened panels.(3)The large torsional buckling deformation of box girders has been analyzed,and the failure modes of box girders under ultimate torsional moment have been deeply studied.The equivalent torsional stiffness expression of box girder considering the influence of transverse bulkhead has been given by using the energy method.The torsional failure mechanism of ship hull girders with large opening has been investigated.It is found that torsional shear deformation of box girders mainly occurs in the area of side shell plates.The roles of stiffeners and transverse frames playing on the responding of box girders to large torsional deformation have been qualitatively analyzed.The critical height of transverse frame equivalent to the bulkhead has been given.(4)A stiffened box girder with large deck openings has been proposed to analyze the influence of cracks on the ultimate strength of ship hull girders.By separately considering the influence of cracks located in the bottom plate section and side plate section on the ultimate strength of simplified box girders,the concept of effective residual section(ERS)has been proposed.By investigating the correlativity between the pre-existing cracks and the torsional failure region of intact box girders,the concept of initial damage of the failure zone(IDFZ).Based on the two new concepts of ERS and IDFZ,combined with the theory of net section yielding,new formulas for predicting the residual torsional strength of box girders with one-side crack and two-side crack damage have been proposed.