MARITIME: Compensation For Active Heaving
Recent Western Cape Business News
WHEN a ship is heaving in rough seas, positioning loads accurately on the seabed poses a challenge to even the most experienced marine crane operators. With active heave compensation (RAHC) from Rexroth, represented locally by the Hytec Group, a ship’s movements are recorded and integrated into a closed-loop control process, which in turn compensates for ninety five percent of a ship’s motion, with the aid of secondary-controlled drives.
Secondary-controlled drives, from Rexroth, have been proving themselves in a wide range of applications for over 20 years, and are ideal suited for active heave compensation. More than one hundred crane and winch systems are currently in service, positioning loads from five tons, to over two hundred and fifty tons safely on the seabed.
In order to keep the crane load in position, the drive winds up the winch cable during a downward movement of the ship. In this process step the drive acts as a motor and consumes energy to perform its function. However, when the ship moves upwards, the drive unwinds the cable, and operates as a pump in generator mode. This recovers lost energy, which is then stored by the hydraulic system for the next downward motion. The system successfully recovers up to seventy percent of the drive’s operational energy requirements. This represents a considerable energy saving, which makes the system highly efficient, the company claims.
The RAHC system provides a number of advantages over competitor products currently on the market. The system affords highly precise movement compensation, while at the same time being energy-efficient. The system is also compact and requires minimal installation space. Rexroth engineers have paid particular attention to ensuring the RAHC system provides high levels of safety for personnel, equipment, materials and the environment. Standard operating and safety functions of the system include, hoisting and lowering in speed control, active heave compensation, active rope tensioning, monitored freefall mode, constant tension mode, automatic overload protection system mode, and a manual overload protection system.
Rexroth’s modular system structure sets no power limits on the RAHC system – whether for a 5-ton winch or a 500-ton crane with hoisting and slewing, Rexroth can custom develop a RAHC solution which meets the exact operating requirements of the client.
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