A sliding mode control design with the flexible parameters of the sliding surface for offshore container crane is investigated in this study. The offshore operations of handling containers are performed between a mega container ship, and a smaller ship, called the mobile harbor (MH), which is equipped with container crane. The concept of the MH is to load and unload container in the open sea, and transfer them to local harbor, thereby minimizing the port congestion and also eliminating the need of expanding outwards. The control objective during the offshore operation is to keep the payload in the desired tolerance in the presence of the MH motion. A new control strategy, which is a combination of a sliding mode control (SMC) and compensation mechanism, is proposed as well. Here, the sliding surface is design in such a way that the sway motion of the payload is incorporated into the trolley dynamics. Included in the proposed control law is the flexible control gains, obtained by a fuzzy real time inference, which automatically strengthen or weaken the positioning subsystem and the anti-swing subsystem. The control law guarantees the asymptotic stability of the closed-loop system. Simulation and experimental results are provided to prove the effectiveness of the proposed control system for offshore container cranes.