Due to the underactuation of crane systems and the wave-induced ship motions, the payload swing is unavoidable and may put relevant operations in danger. Thus, the compensation system is an essential part of offshore crane to ensure human safety and increase operation efficiency. In this paper, a combined ship motion compensation (CSMC) approach is proposed based on the direct ship motion compensation (DSMC) proposed by Barge Master. The principle and structure of CSMC was introduced. The dynamic models of DSMC and CSMC were established in a uniform manner. Numerical results were included to demonstrate that: first, CSMC can achieve comparable performance as DSMC for payload positioning; second, comparing to DSMC, the maximum total power consumption of CSMC can be reduced by about 80% at sea state 4–6.

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.

When laying down a long slender beam from a near-vertical orientation, to a horizontal position on a flat surface, the payload may slip and move suddenly in unintended and unpredictable ways. This occurs during crane operations when the movements of the overhead trolley and lowering of the hoist cable are not properly coordinated. The payload's unintended sliding can potentially cause damage and injure people. This paper presents static and dynamic analyses of slender-beam payload lay-down operations that establish a structured method to predict the safe conditions for lay-down operations. Also, a new method to measure the friction coefficient of surface-to-line contact is proposed. Lay-down experiments are carried out to verify the theoretical predictions.