Majid Kashani - MSc Student , Sharif University of Technology
Ali Akbar Golafshani - Associate Professor , Sharif University of Technology
Amin Gholizad - Assistant Professor , University of Mohaghegh Ardabili

Offshore platforms, located at severe environmental conditions, are generally subjected to two types of environmental loading; normal-condition loads such as wave-induced hydrodynamicforces and extreme-condition loads like seismic excitations [1]. Among classified categories of environmental loads of offshore platforms, those of great importance are wave-induced hydrodynamic forces in normal conditions and seismic excitations in extreme conditions.Adjustable parameters of a vibration control system for offshore platforms are generally designed by considering only one type of environmental loadings; either normal-condition loadsto mitigate fatigue damage or extreme-condition loads to guarantee collapse prevention. So, it isimportant to investigate the effectiveness of vibration control system, designed for the more influential type of external loading, when subjected to another type. Also it is ideal for thesystem to have an acceptable performance in both normal and extreme conditions.Vibration control of offshore platforms has been studied by many researchers such as Vandiverand Mitone [2], Kawano and Venkataramana [3], Kawano [4], Abdel-Rohman [5], Lee [6],Suneja and Datta [7], Suneja and Datta [8], Terro et al. [9], Vincenzo and Roger [10], Ou et al. [11], Ou et al. [12], Suhardjo and Kareem [13], Wang [14], Mahadik and Jangid [15], Li et al. [16], Zribi et al. [17], Patil and Jangid [18] and Ou et al. [19]. All of them have found it effectiveto use control mechanisms for mitigation of vibrations induced by different environmental loads. But it should be noted that none of the previous works has investigated the effectiveness ofvibration control systems, considering both normal-condition and extreme-condition loadings.The purpose of this paper is to compare the seismic performance of dampers optimized for wave loading with that of dampers designed for seismic excitation, and to present methods for enhancing the seismic performance of dampers optimized for wave load, such as using hybriddamping systems, in which one damper is optimized for wave loading and the other is designedfor quake excitation. It is intended to improve seismic effectiveness of vibration control system while maintaining its effective performance in fatigue damage mitigation by using hybrid damping systems.

Offshore Jacket Platform, Friction Damper Device (FDD), Tuned Mass Damper (TMD),Hybrid Damping System (HDS), Seismic Performance