Ice-induced Vibration Mitigation Of Ice-resistant Offshore Jacket Platform

The vibrations of flexible ice-resistant offshore jacket platforms are very serious and can cause various problems. Therefor, it is necessary. to discuss the proper vibration control methods. In civil engineering, the vibration control methods have become mature increasingly both in theory and applications.Based on the ice-induced vibration characteristics and the measured response data from field measurement of the prototypes, and combined with the principles of vibration isolation and vibration absorption, the author analyzed the feasibilities and the application scopes of vibration mitigation methods,i.e., icebreaking cone, vibration isolation cone, top isolation and TMD (Tuned Mass Damper).The field measured data show that the platforms of Bohai Sea are flexible structures, and their base frequencies are very close to the broken frequencies of ice plates. The ice-induced vibration occurs significantly under the action of sea ice. Most of the vibration energy is concentrated in a natural frequency, which is usually the first-order frequency of the platforms. Furthermore, the first mode of vibration is translational. Therefore, the offshore platform can be simplified as a single degree of freedom model in order to illuminate the vibration mitigation principles clearly, while the structure can be simplified as a model, which has one or two degrees of freedom when the vibration mitigation devices are installed on it. This paper introduces the mechanical models and dynamic equations of vibration mitigation strategies.The physical meanings and working principles of different mechanical models are discussed in details. Especially the phase explanation of TMD principle is discussed. For three different typical jacket platforms, i.e., NP1-29, JZ9-3WHPB, JZ20-2MUQ, the damping effect of vibration control strategies are simulated under under steady state ice-force and random ice-force respectively. By comparing the numerical results with various vibration mitigation strategies, the effects of each strategie are discussed. Finally, the application scopes of each strategy are determined.