The heave motion estimation for active heave compensation system in offshore crane

Active heave compensation is widely applied to decouple lifting loads from the ship motions due to wave excitation. This work presents a method of heave velocity estimation in active heave compensation system, which depends on the measured signals from an inertial measurement unit (IMU). The method is achieved by the motion transformation from IMU to the top of offshore crane and a heave filter which is a combination of a highpass filter and an integrator. Although the optimal cutoff frequency of the heave filter can be derived by analysing the error sources, it is difficult to use in the practical applications. The suboptimal or applied cutoff frequency of the heave filter in this work is determined by limiting the amplitude response error, and depends proportionally to the estimated dominant frequency of wave motion by the FFT. Because the heave filter introduces the phase error which impacts the estimation significantly, a lowpass filter is utilized in the modified heave filter for phase correction, and the lowpass cutoff frequency depends on the applied cutoff frequency and the dominant frequency of wave motion. Hence, all parameters for filtering process can be determined adaptively online. Active heave compensation system is simulated in the active heave compensation experimental facility and the compensation efficiency can reach 79.6%, verifying the feasibility of heave motion estimation.