| This study was performed under the financial support of the National High-tech Research Development Program named Key Problems Study of the New Ice-Resistant Platform (No. 2001AA602015). The research subjects are the existing steel jacket offshore platforms in the Bohai Sea, Liaodong Gulf. The main goals are to find the optimal mitigation strategy to reduce ice-induced vibrations and to provide the instructive suggestions for the other flexible structures vibration mitigation under environmental loads.In section one, a summary of vibration induced by environmental loads of flexible structures are presented, especially for the ice-resistant offshore platforms as well as state-of-art of ice-induced vibration.In section two, analysis are conducted on ice forces features,dynamic characteristic of platform and features of dynamic deck response of offshore platform based on field measurements. Therefore, effective measures have to be introduced to reduce the vibration level down to acceptable limits. In others words, field observations are the basis of carrying out dynamic analysis and ice-induced vibration of offshore platforms.In section three, commonly used vibration mitigation strategies, like vibration dissipation, top isolation and dynamic vibration absorption that have already been implemented in civil engineering are introduced. According to the understanding of field observations as well as characteristics of environmental load, the geometric features of platforms, features of dynamic response, mitigation objectives, it is believed that adding an auxiliary device is an optimal choice.In section four, based on the mechanism of the Tuned Mass Damper (TMD) and the fact that the interaction force generated between the relative position of the TMD and structure, time-domain and phase analysis for the vibration reduction of the TMD system is presented. Due to the stochastic characteristic of environmental excitation the motions between the structure and the TMD cannot lock in when out of phase, causing the sharp vibration reduction. In order to analyze the vibration reduction of the TMD from the mechanism, it is critical to conduct the analysis from the relative movement between structure and itself. The evaluation criteria for the performance of the TMD are given based on relative movement analysis between structure and itself. Experimental study and simulation analysis conducted demonstrated the reliability of the proposed criteria.In section five, to investigate the feasibility that the TMD can indeed mitigate the vibrations induced by ice forces on offshore steel jacket platforms, a large scale TMD device was designed and manufactured with respect to an offshore oil platform. Large scale testing is expensive and the requirements for model similarity in model scale testing are hard to meet. Thus, an experimental technique that focuses on the control device itself and emulates the behavior of the structure with a simulator is presented. This novel experimental system consists of a virtual part and a tangible part called semi-experimental testing system (also can be called Hybrid Experimental Testing system). The virtual parts include scaled external load and a scaled simplified platform model. The tangible parts include the TMD device and a moving table, which is controlled by a hydraulic actuator.In section six, to assess the effectiveness of the TMD to mitigate ice-induced vibrations of platforms, the method of measurement of the equivalent damping force of the TMD based on semi-simulation experimental testing system is presented. The interaction force between the TMD and the structure can be measured directly by this system. In this chapter, the principle and method to measure force is introduced. The force comparison between experimental and calculated values is made under sinusoidal displacement inputs. Experimental results show that the force measured from load cell is almost equal to the inertial force of the TMD. Therefore, the feasibility of using this system to measure equivalent damping forces of the TMD was confirmed. The vibration reduction performance of the TMD was carried out by incorporating its equivalent damping force. Experimental results show that the TMD is quite effective to mitigate ice-induced vibration.In section seven, to study the physical principle of TLD (Tuned Liquid Damper) device and provide references for evaluating the performance of TLD. A small scale rectangular TLD device was manufactured. A method to measure the equivalent damping force of the TLD device based on the semi-simulation experimental testing system is presented. Harmonic displacement input was used to identify the resonant frequency of the TLD device as well as the damping force. The experimental force results agree with the theoretical. So the feasibility of the testing system was verified.In section eight, H_∞method has been used extensively in many control systems designed for structural applications due to its stability and robustness. H_∞control of the critical modes of vibration of an offshore platform under ice loads is studied. The control is applied to a platform via an active tuned mass damper (AMD) located at the top of the platform. An algorithm combining the H_∞method together with a balanced reduction scheme in modal space is used for control design. The solution for ice-induced vibration response of the system is derived in terms of the pseudo-excitation method. With the derived solution, extensive parametric studies can be carried out. The optimal parameters of H_∞, controllers for achieving the maximum vibration response reduction of the platform can be identified. The effectiveness of H_∞controllers for this particular application is evaluated in this study. The results show that the ice-induced vibration response of the platform can be considerably reduced if the parameters of H_∞controllers are selected appropriately.At last, some concluding remarks are stated. Suggestions and directions on the further research and applications of structural control for offshore platforms are presented.
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