Kinematics and force analysis of a novel offshore crane combined compensation system

Offshore cranes are widely used in offshore industry. However, because of the wave motion of the ship and the flexibility of the rope, the sway of payload is unavoidable and may put relevant operations in danger. Thus, compensation system is an essential part of offshore crane to ensure human safety and increase operation efficiency. In this article, a novel offshore crane combined compensation approach named four-post combined compensation is proposed based on the three-post direct ship motion compensation, aiming to overcome its shortcomings. The structure and principle of four-post combined compensation is introduced. The mathematical models of four-post combined compensation and three-post direct ship motion compensation are established uniformly and based on which we carry out numerical simulations and comparative analysis of three-post and four-post schemes. Numerical simulation results are included to demonstrate that comparing to three-post scheme, four-post scheme has the following advantages: first, the maximum actuator force requirement can be reduced significantly; second, the work load is distributed more evenly among cylinders; third, the main support shaft can prevent the platform from tipping over; the actuators always push the platform and do not have to be bi-directional, which might result in lower cost and risk.