During customer days at the production plant in Rostock in January 2013 Liebherr presented its new subsea crane, type RL-K 7500. With this new knuckle boom crane the company extends its range of offshore cranes for subsea operations. The RL-K 7500 is able to lift up to 260t in the air and handle loads down to a water depth of 3,400m. The maximum rope diameter is 100mm, the maximum overturning moment of the crane is 75,000kNm. Depending on customers’ specifications, the boom configuration allows for working radii of up to 50 m.
One advantage of the crane’s knuckle boom is that loads can be lowered to the water surface with a short rope length between crane boom head and water line.
Based on sophisticated Motion Reference Units (MRUs) the Active Heave Compensation system compensates this movement. In order to meet the high power demands of the AHC the hydraulic hybrid drive system Pactronic, originally developed for Liebherr mobile harbor cranes, was specially adapted for subsea applications. Pactronic is characterized by an additional energy storage device. A hydraulic accumulator supplements the fluid pump in delivering power to the system. It serves as a pressure storage reservoir incorporating a gas in conjunction with a hydraulic fluid. Energy is stored in this compressed gas to be released upon demand.
The AHC system is fully integrated in the Litronic system. One of its most important features is its self-learning function. It automatically recognizes the vessel’s motion and adjusts itself according to this information. Hence, the system does not need to be manually adjusted and is independent of weather conditions. Up to 70% of the power required to operate the AHC system is currently obtained from Pactronic. This means that the hybrid drive system provides a maximum power of almost 4 MW.
In order to ensure optimized rope guidance, the crane has a patented horizontal winch shifting system that guarantees the ideal fleet angle of the rope under all load conditions in both hoisting and lowering operations. A further innovation is the vertical winch frame lifting system, which is also patented. Two hydraulic cylinders allow for the adjustment of the lifting height without moving the drum of the hoisting winch. Due to this design, wear of the main wire rope is minimized, intended to extend its service life and reduce life cycle costs. With the RL-K 7500 Liebherr offers an innovative crane concept for subsea applications.
The RL-K 7500 can be rated both as general purpose offshore crane and as heavy lift crane, being able to hoist loads weighing up to 300 t. It can thus be installed on board drill vessels and also heavy lift vessels.
(As published in the January/February 2013 edition of Marine Technologies - www.seadiscovery.com)