Study On Model Conversion And Dynamic Characteristics For Fixed Offshore Structures

The ocean contains rich source of energy, including oil and gas resources, as well as wind energy resources. It has been a big issue to all human being to explore and develop these resources. Offshore structures, such as offshore platforms and offshore wind turbines are the fundamental facilities for operation and production on the sea. It is very important to ensure the safty of these structures and avoid the occurance of severe accidents. To solve these problems, three topics are focused in this thesis, including model reduction and structure modification for fixed platforms, fundamental frequency study for jacket-type offshore wind turbines. And following works are done.Traditional model reduction schemes could only yield the eigen-properties of the reduced model approximating to those of the full-order model. In this thesis, a model conversion technique for structural dynamic systerms is proposed, which is capable of converting a physically realizable, high-order model into a predefined, physically realizable, low-order model. Upon the completion of the conversion, the resulting target model preserves or approximates the dynamic characteristics—the chosen structural frequencies and their mode shapes—of the high-order model. The mathematical kernel of the proposed model conversion technique is the cross-model cross-mode (CMCM) method. Two numerical examples are demonstrated in this paper. The numerical results indicate the final target model can match perfectly with the chosen dynamic characteristics of the source model.A 12-DOF shear building (SB) model with specific stiffness and mass coefficients was proposed by the IASC-ASCE Structural Health Monitoring Task Group as the identification model for several benchmark problems on damage detection. The present thesis addresses improperness, including the parameter and modeling errors, of this model for the intended purpose of damage detection. In turn, a model calibration procedure was utilized to calibrate the stiffness and mass matrices of the SB identification model. A physically meaningful SB model that preserves the first three vibration modes (two lateral and one torsional modes) of a full-order finite-element model was obtained, and it was sequentially employed as the baseline model for implementing the damage detection diagnosis. Numerical results indicate that the 12-DOF shear building model is an over-simplified identification model, through which only idealized damage situations for the benchmark structure can be detected.A new modification method for the structural dynamic characteristics is presented. The modification procedure is to reassign the one or more natural frequencies of the original structure by adding mass and stiffness terms. Upon the completion of the modification, the dynamic characteristics of modified structures are satisfactory accrording to the design requirements. The method extends the cross-model cross-mode (CMCM) method originally developed for model updating, with the complete modal data replaced by the target frequencies. Numerical study results indicate the modifaction method is applicable to the modification of structural frequency. The frequencies of modifed structures match perfectly with the target frequencies. And the required modal information is less with no need of target mode shape values.The newly proposed structural modification method is applied to an offshore fixed platform in Bohai to reassign the first natural frequency. Perfect results can be obtained by both of adding a tuned mass damper (TMD) to the deck and modifying the stiffness of beams in the top floor of the original jacket platform.For systematically investigating the influence on the fundamental frequency of a jacket-type offshore wind turbine due to its three major components (RNA, tower and jacket substructure), we proposed a simplified model (base-moveable flagpole model) that is fully characterized by 8 parameters. The base-moveable flagpole model was further simplified into a SDOF system. Through analytical derivation and systematic numerical studies, the quantitative knowledge related to the fundamental frequency of a jacket-type offshore wind turbine under a certain combination of its three major components was obtained.