| Compared with normal container ships,ultra large container ships(ULCS)are considered to be one of the most representative high-tech products in the modern shipping industry for its particular characteristics.Ultra large container ships are characterized by larger deck opening,higher speed,more requirement of stability as well as more employment of high strength steel.When ships sail in rough sea,the motion states and the loading conditions faced by ships are of diversity,with the motions and the loads greatly coupling.Under this situation,the loads imposed on the hull girder is not a simple bending or torsion,but combined loads as a result of the interaction between ships and waves.The torsional strength of ultra large container ships is considerably decreased by the large deck openings,which means that when conducting the examination of the structure strength of ultra large container ships,the ultimate torsional strength should also be evaluated in addition to the ultimate longitudinal strength to study the additional effect of torsional moments on the ultimate strength of hull girders.The ultimate capacity of ultra large container ships under combined loads needs to be investigated.Experiments are the most convincing measurements of the ultimate strength of hull girders among massive structure strength evaluation methods.However,it is not feasible to conduct ultimate strength experiments with true ships due to the huge cost.As an alternative,similar scale models are employed in experiments.It is of great significance to design a similar scale model of the true ship in the ultimate strength experiments,in which the the progressive collapse behaviors of scale models could represent those of true ships.In view of the above issues involved in the ultimate strength evaluation of ultra large container ships under combined loads,the following researching work has been done in this thesis:(1)Similarity Design method of the model test of ultra large container ships under combined loads of torsion and bending was proposed,in which the geometry similarity,buckling strength similarity as well as ultimate strength similarity are taken into account.Height of neutral axis,section modulus as well as moments of inertial and polar inertial moments of neutral axis are incorporated in geometry similarity.Ultimate bending strength,ultimate torsional strength as well as ultimate strength with combined bending and torsion are included in the strength similarity to ensure that the whole collapse process of the similar scale model is in good similarity with that of true ship.With this design method,similar scale models of true ships can be designed to be used in the model experiments of ultimate strength of hull girders under bending,torsion and the combination of bending and torsion.(2)Model experiment of ultimate bending strength of the hull girder regarding an ultra large container ship was conducted,giving detailed illustrations of experiment procedure and experiment results.And failure mechanisms of the hull girder of ultra large container ships under pure bending were comprehensively investigated with nonlinear finite element analysis.The accuracy and reliability of nonlinear finite element method was verified through the comparison of the location of neutral axis,ultimate strength and progressive collapse orders between numerical model and test model.(3)Model experiment of ultimate torsional strength of the hull girder regarding an ultra large container ship was also conducted,having detailedly illustrated the experiment procedure and experiment results.The collapse mechanisms of the hull girder of an ultra large container ship under pure torsion were comprehensively studied with nonlinear finite element analysis.Transverse forces and axial forces will be induced in torsional process of the hull girder as a result of the nonlinear collapse behaviors,causing the model to be imposed on a non-pure torsional state and hydro-cylinders to be damaged by the induced transverse forces.Corresponding to this,a scheme of pure torsional moment application was proposed to apply the pure torsional moment on the test model without damaging the hydro-cylinders.(4)Ultimate strength characteristics are summarized with comparisons of the bending and torsional ultimate strength of conventional container ships and ultra large container ships.Shear failures and warping failures are the main collapse behaviors involved in the torsional failure process.Shear stress and warping stress are normally developed together when the hull girder with a large deck opening is under torsion.As a result,two failure modes of the hull girder of ultra large container ships were discussed in this paper.(5)Collapse behaviors of the hull girder of ultra large container ships under three combined loads of vertical bending,horizontal bending and torsional moment were numerically investigated,which could represent the extreme loading conditions when the ships sail in extremely rough sea.A three dimensional hull girder ultimate strength envelope which could describe the collapse behaviors of ultra large container ships was proposed.And a simple formula which takes initial imperfections into account that can be used to predict the safety zone for ultimate strength of hull girders under three combined loads of vertical bending,horizontal bending as well as torsional moment was proposed.The innovations of this paper mainly focus on the following points:(1)Similarity Design method of the model test of ultra large container ships under combined loads of torsion and bending was proposed with geometry similarity of typical cross section,strength similarity of local components as well as the failure mechanisms of hull girders under combined loads of torsion and bending.Compared with those designed with geometry similarity only,scale models designed with the method proposed in this paper could achieve better consistency of progressive collapse process with true ships,increasing the capacity of evaluating and predicting the ultimate strength and failure mechanisms of true ships with model tests.(2)Ultimate pure bending strength experiment and ultimate pure torsional strength experiment were conducted with the same test section,in which the similar scale model was designed with the similarity design method proposed in this paper.Ultimate bending strength and ultimate torsional strength characteristics of ultra large container ships were detailedly illustrated with both experimentally and numerically obtained results,providing the literature with experimental data on ultimate pure bending strength and ultimate pure torsional strength of ultra large container ships.(3)Systematic investigations of the ultimate strength interactions of ultra large container ships under combined loads were conducted,giving an empirical formula including initial imperfections which could represent the three dimensional hull girder ultimate strength envelope of ultra large container ships and evaluating the ultimate strength of ultra large container ships under combined loads,which could offer references and instructions for the design and manufacture of ship structures. |
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