Abstract: We study the integral sliding mode H_∞ damping control problem for a vibration system of offshore steel jacket platforms subject to wave force. By considering the unmodeled dynamics and control delay, we first establish a nonlinear time-delay system of the simplified offshore platform. Then, we propose a delay-dependent integral sliding mode control scheme and present a sufficient condition of the existence of the delay-independent integral sliding mode H_∞ controller. The controller gain can be obtained by solving a nonlinear minimization problem numerically. Finally, based on a simplified offshore platform system, we demonstrate the effectiveness of the delay-independent integral sliding mode H_∞ control scheme. The simulation results show that the delay-dependent integral sliding mode H_∞ controller can compensate for the effects of the unmodeled dynamics and control delays on the system, reduce the oscillation amplitudes of the displacement and acceleration of the offshore platform significantly, and improve the safety and reliability of the system.