| The marine umbilical is one of advanced and key equipments during offshore hydrocarbon resource extraction,which connects the manifold and the surface floater performing the remote control function through the hydro pressure,optical and electrical transmission.Functional components and strengthening components are helically gathered to compose the umbilical.So,the basic mechanical model of the cross-section of an umbilical is an unbonded multi-layer and multi-component helically wound structure.Meanwhile,the global configuration of an umbilical is a kind of wave type with large length during operation.With the consideration of the complicated structural characteristic in geometric bi-scale of the local section and global configuration,there are many challenges about the analysis and design of the umbilical that need to be addressed under the complicated environmental loads such as wind,wave and current.Therefore,these above functions are ensured by the accurate analysis and efficient design of mechanical properties of umbilicals.Umbilicals are subjected to the large tension,torsion and bending loads during the operation.However,the recent analytical theory is difficult to describe the mechanical behaviors exactly and the huge complicated numerical model is needed to perform the simulation,which cause the low efficiency and accuracy.Moreover,traditional design process is uncoupled to two parts,the analysis of the local cross-section and the hydrodynamic response analysis of the global configuration.Hundreds of load cases need to be analyzed and checked,which influences the design efficiency.In addition,the cross-sectional layout design of umbilical with a number of functional components just relies on the artificial experience,which shows lack of the quantized theory and design method.Under the support from the National High Technology Research and Development Program of China(863 Program)a Research on the Integrated Design and Ancillaries of Umbilicals in Subsea Production System"(2014AA09A224-3),the thesis focuses on four key mechanical problems of the structural analysis and design of the umbilical.Firstly,the tension-torsion semi-analytical analysis method combining two-dimensional numerical simulation of the core structure and three-dimensional stiffness theory model is presented.Secondly,the fast equivalent analysis methodology with geometric bi-scale of the umbilical is established for the mechanical properties of the local cross-section and the deformation of the global configuration.Thirdly,the integrated optimization design method with geometric bi-scale of the local section and the global configuration is presented.Finally,the multidisciplinary optimization model of the cross-sectional layout of the umbilical is established.Detailed research contents are summarized as the following description.(1)The mechanical behavior analysis of the umbilical under tension and torsionUmbilical is often subjected to the tensile and torsional loads in the axial direction during the installation and operation.There is the coupling effect between tension and torsion as well as the radial shrinkage phenomenon of the inner core structure because of the helically wound structural characteristics of the umbilical.There are a few gaps between the components of the core,and the material of the sheath involves nonlinear constitutive relationship.Therefore,it takes the challenge to analyze the mechanical behavior of tension and torsion due to the nonlinear deformation.The current analytical theories only involve the single tension or torsion behavior of the umbilical,and the introduced index macro Poisson’s ratio is difficult to describe the nonlinear characteristic accurately.Firstly,concerning the actual deformation the simplified structure model of some helical curve beams winding an inner cylinder is established for the tensile armor layer.The tension-torsion stiffness matrix between the load and displacement with the radial stiffness of the core structure is derived through the curve beam theory.Secondly,through the two-dimensional numerical model of the core structure of the umbilical,the radial stiffness is analyzed exactly,which is substituted to the above tension-torsion stiffness formula to obtain the sectional tension and torsion mechanical properties of the umbilical.Therefore,the semi-analytical analysis method of umbilical tension-torsion behavior is established to solve the problem that it is difficult to describe the radial deformation by the previous theoretical methods.Finally,taking the umbilical used in the South China Sea as an example,with the consideration of the core nonlinear mechanical characteristics the semi-analytical method is adopted to explain the tension nonlinearity of the umbilical.Obtained tensional stiffness is compared with the experimental result,and the error is lower than 5.76%,which verifies the correctness and feasibility of the presented method.(2)The fast equivalent analysis with geometric bi-scale of the umbilicalCompared with axial tension and torsion behaviors,it is difficult to analyze the bending behavior due to the contact and friction between inner components,which needs to establish the numerical model with the length of several pitches.Moreover,it is significantly different with respect to the geometric scales between the local cross-section and the global configuration,which causes great challenge to analyze the mechanical properties exactly based on the detailed global numerical model with a lot of degrees of freedom.It makes the structure optimization design infeasible.Specially,the umbilical is the form of one-dimensional periodic structure.The novel numerical implementation of the asymptotic homogenization method is introduced to establish fast equivalent analysis methodology with the geometric bi-scale of the umbilical and predict the equivalent mechanical properties of the cross-section and the deformation of global configuration.Finally,the umbilical in Chapter 2 is taken as an example,and the effective unit cell model is established under the periodic boundary condition.The cross-sectional tensile,bending,torsional and coupled stiffnesses of the umbilical are obtained with an one-dimensional homogeneous beam,which are compared with the results from references to validate the correctness of the results through the fast equivalent analysis methodology.Meanwhile,the displacement of a catenary global configuration is analyzed based on the homogenous beam with these equivalent sectional mechanical properties.It is found that the error is below 3.10%comparing with the result of the detailed numerical simulation,which verifies that the asymptotic homogenization method is feasible to perform the fast equivalent analysis methodology with geometric bi-scale of the umbilical.(3)Integrated optimization design with geometric bi-scale of the umbilicalThe cross-sectional analysis and load time-history analysis of the global configuration under environmental loads are uncoupled in traditional design methodology of the umbilical.The global dynamic analysis involves hundreds of load cases and the entire design process requires repeated iterations to verify the design,which restrict the design efficiency.The multi-objectives integrated optimization methodology with geometric bi-scale of the umbilical is introduced to perform the parameters of the local cross-section and the global configuration in a loop.The umbilical design requires that the tension and bending curvature responses of global configuration should be smaller.Meanwhile,larger tension stiffness and smaller bending stiffness of cross-section are desired.Considering constraints of design variables and allowable strength,the optimization model is established with the maximum tension strain and the maximum bending moment as the objectives.Moreover,the RBF(Radial Basis Function)neural network is adopted to establish the approximate surrogate model and NSGA-Ⅱ(Non-dominated Sorting Genetic Algorithm Ⅱ)is utilized to solve the optimization problem.Finally,the optimization model is implemented on the umbilical with lazy-wave global configuration.Pareto optimal solutions are obtained and the effectiveness of the optimization strategy is verified through numerical simulation corresponding to the same solutions.The properties of the optimized umbilical are compared with the initial design.It is found that the fatigue life of the umbilical is improved considerably.(4)Cross-sectional layout optimization design of the umbilicalWith more functions of the umbilical being integrated,the kind and number of functional components gradually increase.The different cross-sectional layout will cause the unique mechanical properties of the umbilical.Meanwhile,the generated heat in electric cable could influence the physical and mechanical properties of materials during electric power transmission.So,the cross-sectional layout design with multiple functional components has to be treated as a multidisciplinary optimization problem.The specification only performs compacted and symmetric criterias,which are not described by the mathematical theory and detailed design method.In this paper,cross-sectional compactedness,balance,symmetry and temperature distribution are considered and are quantized through introducing corresponding indices.With these mechanical,geometric and thermal properties as the objectives,the multidisciplinary optimization design methodology of the cross-sectional layout of the umbilical is established.PSO(Particle Swarm Optimization)algorithm is adopted in order to carry out the optimization problem and obtain Pareto optimal solutions.Finally,the above umbilical case is designed through the optimization design strategy and the optimal sectional layout design is obtained,which validate the effectiveness and efficiency of the multidisciplinary optimization methodology.In a summary,these above research achievements are applied to develop the first integrated and fast design software for marine umbilicals,and to guide the engineering application of the first umbilical used in the South China Sea.It can provide a valuable theoretical fundament and design methodology for the advanced design of marine umbilicals. |
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