Passive heave compensation (PHC) system is widely applied in offshore equipment because of its superiority in energy conserving and reliability. However, it has poor adaptability to changing sea conditions and the compensation accuracy is low. Hydraulic transformer (HT), working as a pressure-flow control element, can potentially solve the problems mentioned above. In this paper, an HT based PHC (HTPHC) system is proposed for the first time, and a compensation algorithm based on higher-order sliding mode (HOSM) together with a prediction algorithm for the heave motion of the vessel is derived to get good compensation effect using the new PHC system. The prediction algorithm is proved to be effective according to the measured data of sea trials, and reduces the difficulty of designing and parameter tuning process compared with the existing ones. The effectiveness of the proposed control algorithm is evaluated with simulation, moreover, the effectiveness can still be maintained under changing sea conditions which is also verified by simulation.

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