Numerical And Theoretical Research On The Structural Response Characteristics Of A Leg Mating Unit

Float-over installation is a new efficient topside installation method which is more and more popular in recent years. The barge-topside system will undergo complex motions induced by the wave, the current and the swell. Leg Mating Unit(LMU) is widely used to absorb the impact between the topside and the substructures. LMU is now the key device in non-mechanical float-over installation. To break the foreign company’s monopoly of LMU technology, Offshore Oil Engineering Co., Ltd and Shanghai Jiaotong University carried out a joint- project on the research and development of the first large-scale LMU in China. The content of this thesis is part of the research project.The two main functions of LMU are the impact buffering and the control of the topside vertical position, both of which are directly related to the load-displacement curve of the rubber stack. Therefore the rubber stack is the most important part of LMU. This thesis probed several frequently-used types of LMUs and discussed the design requirements, the nonlinear characteristic, the accuracy requirement and the influencing factors of the rubber stack.At present few articles discussed in depth the load-displacement characteristic and the influencing factors of the rubber stack. Hence it is necessary to take a research on this aspect. The difficulties in the research of the rubber absorber are the nonlinear constitutive relation, the incompressibility, the large deformation and the contact&friction. The rubber constitutive relations and tests were introduced and the numerical methods appropriate for this highly nonlinear problem were discussed, including implicit and explicit FEM, the contact algorithms and the simulation of incompressibility. The impact load and impact time are analysed and it is confirmed that the absorbing process is quasi-static and the viscidity and the effect of strain rate could be ignored. Subsequently, a series of rubber stacks were investigated using FEM, through which the influences of layer numbers, the diameter ratio and the friction condition were obtained and the Poisson’s ratio with a value 0.499 was proved to be adequate to simulate the incompressibility. The study also revealed the relations of the buffering effect with the stiffness of the rubber stack. Some recommendations about the design of the rubber stack were put forward. The mesh mapping method, the CEL method and the SPH method were used to survey the influence of the element distortion in the numerical calculation and no adverse effect was found.On the basis of the qualitative rules obtained above, the theoretical research on the load-displacement relation was carried out for the further direct guiding on the design of the rubber stack. In the previous theoretical researches, rubber was regarded as a linear elastic material in general. Only the initial stiffness was obtained in most instances and the response under finite strain was not predicted. On account of this, based on the continuum mechanics and the Mooney-Rivlin type constitutive relation, the load-displacement relationships of the circular and annular rubber layers bonded between rigid plates were derivated theoretically. Three boundary conditions were considered for the annular rubber layers. The detailed derivation and the final analytical formulae were presented. A series of rubber stacks were calculated using these formulae and the results were compared with the FEM calculation. It is proved that these formulae have high accuracy and extensive range of application. These formulae were also compared with the previous formulae and the characteristic of the rubber layers was discussed, based on which some conclusions of the numerical researches were verified.There are complex contact and friction interactions between the internal structures of LMU. Multiple nonlinear factors exist. Consequently, the structural response analysis is also an important and difficult issue. Nonlinear FEM is the main numerical method for this problem. However, none knows how far the FEM results deviate from the reality, given various error sources in such a complex problem. The first LMU scale model experiment was performed and the measurement results were compared with the implicit and explicit FEM analysis. These two FEM methods were proved available to simulate the complex interactions between the components, the stress distribution and the stiffness characteristics of the test model. Both of them can be used in the structural analysis, the strength verification and numerical calculation of the load-displacement curve of the actual LMU.In addition, a simplified method for the preliminary structural analysis of the LMU was proposed in this thesis. The simplified mechanical model and the loading&boundary conditions were discussed. The first domestically developed LMU was analysed using this simplified method and the explicit quasi-static method. Through the comparison of the results, the simplified method was proved to be applicative in the preliminary structural analysis of LMU. Moreover, the original structure and rubber stack design were optimized based on the FEM calculation. Finally, the strength verification and performance evaluation were performed for the optimized LMU. The domestically developed LMU was proved to meet the design requirements. The LMU is structural safe with excellent performance.The main innovations are:(1) The influences of layer numbers, the diameter ratio and the friction condition on the load-displacement of the rubber stack were obtained through the numerical research which can provide a preliminary reference to the design of LMU.(2) Based on the continuum mechanics and the Mooney-Rivlin type constitutive relation, the load-displacement relationships of the circular and annular rubber layers bonded between rigid plates were derivated theoretically. These formulae have extensive range of application and provide technical support for the determination of the design parameters.(3) Through the comparison of the results of the nonlinear FEM and the measurement results of LMU scale model experiment, the nonlinear FEM was proved to be applicative in this high nonlinear problem.(4) A simplified method for the preliminary structural analysis of the LMU was proposed(5) Structural assessment and optimizing were performed for the first domestically developed LMU.