Investigating the Effects of Combined Tuned Liquid Damper (CTLD) on Dynamic Behavior of Offshore Jacket-Type Platforms

In the present paper, the efficiency of combined tuned liquid damper (CTLD) in controlling the dynamic responses of offshore jacket platforms under the earthquake and sea wave excitation is investigated. This type of damping system consists of one or more tanks containing a fluid, generally water or oil, which can be installed on the topside (superstructure) of the platform. During the excitation, hydrodynamic action induced by the sloshing of the water in the tank acts as a resistant force against the vibration and controls the structural response. In fact, due to the oscillation of the structure, the fluid inside the tank begins to oscillate in the opposite direction. During this process, most part of the fluid has a wave-like oscillatory motion, while the part adjacent to the tank’s floor experiences a rigid-type displacement and exerts impact pressures to the tank’s walls. In order to attain maximum decrease in the structural response, the oscillation frequency of the fluid inside the tank should be near the natural frequency of the structural free vibration which can be determined by performing a modal analysis. Hence, one of the objectives of the present study is to adjust the frequency of fluid’s oscillation based on the natural frequency of the jacket structure. In other words, the aim is to find a frequency range in which the maximum decrease can be achieved in the amplitude of structural responses. In this research, using the FE software ANSYS, a jacket-type platform having dimensions appropriate for the Persian Gulf climate (case study: SPD1 platform) was modeled and then dynamically analyzed by the modal and time-history approaches subjected to the records of El Centro and Tabas earthquakes as well as 10 cases of wave loading with different height and period. The CTLD system was optimally designed and after the verification of FE results, the dynamic responses of the jacket-type platform with and without CTLDs were compared.