Multiple extended tuned mass dampers (METMD) system has been developed for reducing the multiple resonant responses of the platforms to random ocean wave loads. The METMD system is composed of several ETMD subsystems that cover a specific frequency bandwidth, and their average frequency is tuned to one of the resonant frequencies of the platform. The offshore platform and the METMD system are simplified as an m+1 DOFs system to be theoretically analyzed. The total mass ratio of the METMD system to the platform is 0.14 and the frequency ratio of the exciting frequency to the platform's natural frequency varies in [0.5, 1.5]. The theory analysis shows that:
the platform has the better vibration control effect when the nondimensional frequency bandwidth, which is defined as the ratio of the frequency range to the controlled platforms natural frequency, is in [0.35, 0.6];
the damping coefficient of ETMD systems is in [0.05, 0.15] and
the optimal number of the ETMDs is 5 when frequency bandwidth equals to 0.45 and damping coefficient is 0.1.
The FEM simulation shows that the displacements decrease ratio of the whole platform under four different wave forces are 38.71%, 33.65%, 44.7% and 35.18%, respectively. The METMD has a better vibration control effect on the mega-platforms' vibration under the random wave force loads.
The platforms are built in severe ocean environment. They have to suffer all kinds loads, such as earthquake load, wave load, wind load, ice load and the loads caused by machines and equipment set on the platform. So the platforms vibrate very severely under the associated action of these loads. The light vibration of the platform can make the workers on it feel panic, such as Pinghu platform's severe vibration driven by the work of the natural gas compressor (Huang, 2001).