Active Heave Compensation (AHC)

Idea have developed a retrofit Active Heave Compensation (AHC) kit for shipboard cranes. AHC systems compensate for the motion of a vessel at sea by extending and shortening the lifting cables. An AHC retrofit kit is cheaper than a new crane with AHC, and can be installed much more quickly.

Active Heave Compensation is essential with lifting activities that can be carried out at depths exceeding a kilometre. Because of its length, a cable in these circumstances behaves so elastically that a small motion of the ship can put the load into a rising and falling oscillation. A second application of the AHC on shipboard cranes is to extend the weather window, and a third is to expand the range of work which can be carried out, for example to cover jobs which were previously only possible using a jack-up platform.

An AHC consists of two additional sheaves with a hydraulic cylinder placed between them, a motion sensor and PLCs running powerful software in a redundant configuration. The control software translates the crane's slewing angle, the jib apex angle and the ship's motions in real time into a control signal for the hydraulic cylinder. It is a tough challenge to devise a control system which will remain stable under all possible circumstances. Both the ship and the crane must be extensively modelled. Idea are among the handful of firms with the required expertise in house.

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