Mooring system plays an important role in station keeping of floating offshore structures. Coupled analysis on mooring-buoy interactions has been increasingly studied in recent years. At present, chains and wire ropes...Mooring system plays an important role in station keeping of floating offshore structures. Coupled analysis on mooring-buoy interactions has been increasingly studied in recent years. At present, chains and wire ropes are widely used in offshore engineering practice. On the basis of mooring line statics, an explicit formulation of single mooring chain/wire rope stiffness coefficients and mooring stiffness matrix of the mooring system were derived in this article, taking into account the horizontal restoring force, vertical restoring force and their coupling terms. The nonlinearity of mooring stiffness was analyzed, and the influences of various parameters, such as material, displacement, pre-tension and water depth, were investigated. Finally some application cases of the mooring stiffness in hydrodynamic calculation were presented. Data shows that this kind of stiffness can reckon in linear and nonlinear forces of mooring system. Also, the stiffness can be used in hydrodynamic analysis to get the eieenfrequencv of slow drift motions.展开更多
In the last two decades, the damage detection for civil engineering structures has been widely treated as a modal analysis problem and most of the currently available vibration-based system identification approaches a...In the last two decades, the damage detection for civil engineering structures has been widely treated as a modal analysis problem and most of the currently available vibration-based system identification approaches are based on modal parameters, namely the natural frequencies, mode shapes and damping ratios, and/or their derivations, which are suitable for linear systems. Nonlinearity is generic in engineering structures. For example, the initiation and development of cracks in civil engineering structures as typical structural damages are nonlinear process. One of the major challenges in damage detection, early warning and damage prognosis is to obtain reasonably accurate identification of nonlinear performance such as hysteresis which is the direct indicator of damage initiation and development under dynamic excitations. In this study, a general data-based identification approach for hysteretic performance in form of nonlinear restoring force using structural dynamic responses and complete and incomplete excitation measurement time series was proposed and validated with a 4-story frame structure equipped with smart devices of magneto-theological （MR） damper to simulate nonlinear performance. Firstly, as an optimization method, the least-squares technique was employed to identify the system matrices of an equivalent linear system of the nonlinear structure model basing on the exci- tation force and the corresponding vibration measurements with impact test when complete and incomplete excitations; and secondly, the nonlinear restoring force of the structure was identified and compared with the test measurements fi- nally. Results show that the proposed data-based approach is capable of identifying the nonlinear behavior of engineering structures and can be employed to evaluate the damage initiation and development of different structure under dynamic loads.展开更多
By the biological construction of a bird neck,a bionic bird-neck multilevel rigid-flexible structure is proposed and some biometric properties are explained.The proposed structure can flexibly deform in six directions...By the biological construction of a bird neck,a bionic bird-neck multilevel rigid-flexible structure is proposed and some biometric properties are explained.The proposed structure can flexibly deform in six directions,which inspires the study of its mechanical properties for flexible deformations.First,the structural configuration and composition are determined based on the study of the anatomical characteristics of the woodpeckers.Since the skeletons and muscles have very different values for the elasticity modulus and the deformation is mostly dependent on the muscle tension,the bionic structure consists of rigid units and bio-syncretic components.For combined deformations,the mechanical model is established by the connectivity matrix to describe the connection of each level.Second,based on the principle of minimum potential energy,an integral form-finding method is proposed for flexible combination deformations.All of the integral forms obtained with the theoretical analysis are compared with the results with Finite Element Analysis.The structural parameters of the bionic structure were then tightly fixed to the actual shape of the bird’s neck and the corresponding overall form took on an"S"shape,which perfectly matched the construction of the bird’s neck.In addition,for the pre-deformation form,by analyzing the potential energy of the bionic structure,due to the adjustable dynamic stiffness property,an explanation is provided for the significant dynamic stability of the bird neck in bending.This study not only proposes a bionic rigid-flexible structure with high spatial accessibility but also explains biological properties of a bird neck based on the study of its mechanics characteristics.Based on the modeling and the mechanical properties of the bionic structure in flexible spatial combination deformations,the multi-steady state,and the variable dynamic stiffness,the bird-neck bionic rigid-flexible structure has significant applications such as aeronautical deployable systems,manipulator positioning,and dynamic stability fields.展开更多
基金Supported by the National Natural Science Foundation of China under Grant No.（51079034）.
文摘Mooring system plays an important role in station keeping of floating offshore structures. Coupled analysis on mooring-buoy interactions has been increasingly studied in recent years. At present, chains and wire ropes are widely used in offshore engineering practice. On the basis of mooring line statics, an explicit formulation of single mooring chain/wire rope stiffness coefficients and mooring stiffness matrix of the mooring system were derived in this article, taking into account the horizontal restoring force, vertical restoring force and their coupling terms. The nonlinearity of mooring stiffness was analyzed, and the influences of various parameters, such as material, displacement, pre-tension and water depth, were investigated. Finally some application cases of the mooring stiffness in hydrodynamic calculation were presented. Data shows that this kind of stiffness can reckon in linear and nonlinear forces of mooring system. Also, the stiffness can be used in hydrodynamic analysis to get the eieenfrequencv of slow drift motions.
基金The authors gratefully acknowledge the support provided through the National Natural Science Foundation of China (NSFC) under grant No. 50608031the Hunan Provincial Natural Science Foundation of China under grant No.08JJ1009the Key Project of Chinese Ministry of Education (No. 108102)
文摘In the last two decades, the damage detection for civil engineering structures has been widely treated as a modal analysis problem and most of the currently available vibration-based system identification approaches are based on modal parameters, namely the natural frequencies, mode shapes and damping ratios, and/or their derivations, which are suitable for linear systems. Nonlinearity is generic in engineering structures. For example, the initiation and development of cracks in civil engineering structures as typical structural damages are nonlinear process. One of the major challenges in damage detection, early warning and damage prognosis is to obtain reasonably accurate identification of nonlinear performance such as hysteresis which is the direct indicator of damage initiation and development under dynamic excitations. In this study, a general data-based identification approach for hysteretic performance in form of nonlinear restoring force using structural dynamic responses and complete and incomplete excitation measurement time series was proposed and validated with a 4-story frame structure equipped with smart devices of magneto-theological （MR） damper to simulate nonlinear performance. Firstly, as an optimization method, the least-squares technique was employed to identify the system matrices of an equivalent linear system of the nonlinear structure model basing on the exci- tation force and the corresponding vibration measurements with impact test when complete and incomplete excitations; and secondly, the nonlinear restoring force of the structure was identified and compared with the test measurements fi- nally. Results show that the proposed data-based approach is capable of identifying the nonlinear behavior of engineering structures and can be employed to evaluate the damage initiation and development of different structure under dynamic loads.
基金the National Natural Science Foundation of China(12122208,11972254 and 11932015).
文摘By the biological construction of a bird neck,a bionic bird-neck multilevel rigid-flexible structure is proposed and some biometric properties are explained.The proposed structure can flexibly deform in six directions,which inspires the study of its mechanical properties for flexible deformations.First,the structural configuration and composition are determined based on the study of the anatomical characteristics of the woodpeckers.Since the skeletons and muscles have very different values for the elasticity modulus and the deformation is mostly dependent on the muscle tension,the bionic structure consists of rigid units and bio-syncretic components.For combined deformations,the mechanical model is established by the connectivity matrix to describe the connection of each level.Second,based on the principle of minimum potential energy,an integral form-finding method is proposed for flexible combination deformations.All of the integral forms obtained with the theoretical analysis are compared with the results with Finite Element Analysis.The structural parameters of the bionic structure were then tightly fixed to the actual shape of the bird’s neck and the corresponding overall form took on an"S"shape,which perfectly matched the construction of the bird’s neck.In addition,for the pre-deformation form,by analyzing the potential energy of the bionic structure,due to the adjustable dynamic stiffness property,an explanation is provided for the significant dynamic stability of the bird neck in bending.This study not only proposes a bionic rigid-flexible structure with high spatial accessibility but also explains biological properties of a bird neck based on the study of its mechanics characteristics.Based on the modeling and the mechanical properties of the bionic structure in flexible spatial combination deformations,the multi-steady state,and the variable dynamic stiffness,the bird-neck bionic rigid-flexible structure has significant applications such as aeronautical deployable systems,manipulator positioning,and dynamic stability fields.
