Last September the test bed in Rostock was the stage of a single lift of the RL-K 7500 subsea crane. The same crane was then loaded and shipped to the South Korean customer.

On December 21, a new Liebherr RL-K 7500 started its long journey to South Korea. after waiting since the middle of September for its shipment at the pier of the Liebherr factory in Rostock. Mid-December, the transport ship arrived at the Liebherr factory, and the RL-K 7500 was loaded aboard together with another model of Liebherr’s range of knuckle boom cranes, the RL-K 4200. Both cranes are currently on the way to Daewoo Shipbuilding & Marine Engineering Co. LTd. (DSME). DSME plans to equip several of its ships with a set of Liebherr cranes consisting of one RL-K 7500 and three RL-K 4200. The two other RL-K 4200 will follow in the first quarter of 2015.

The new subsea crane with knuckle boom represents a solution for subsea operations. Together with the crane’s prototype, the RL-K 7500 is the fourth model to be completed. It is capable of lifting up to 300 t in the air and handle loads down to a water depth of 3,600 m. The maximum rope diameter is 82 mm, the maximum overturning moment of the crane is 75,000 kNm. Depending on customers’ specifications, the boom configuration allows for working radii of up to 50 m.

The crane’s knuckle boom allows loads to be lowered to the water surface with a short rope length between crane boom head and water line. Furthermore, the crane is equipped with a number of state-of-the-art hydraulic, electric and electronic features and functions, some of which were submitted for patent by Liebherr, e.g. Heavetronic, Pactronic and the innovative winch system.

Heavetronic – Liebherr Active Heave Compensation-System (AHC), based on Motion Reference Units (MRUs), i.e. sensors designed in-house to recognize and predict the vessel’s motion, compensates this movement. In order to meet the high power demands of the AHC the innovative 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.

Aiming to ensure optimized rope guidance, the crane has a patented horizontal winch shifting system. Liebherr said it guarantees the ideal fleet angle of the rope under all load conditions in both hoisting and lowering operations.

Another notable feature 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 of the winch unit, wear of the main wire rope is minimized, thus extending its service life and reducing life cycle costs, Liebherr said.

Easy adjustment to customer requirements regarding load and water depth can be made by exchanging the lebus shells and by adapting the width of the winch to the actual rope diameter. Consequently, the customer is not obliged to change the complete hoisting winch.

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 heavy lift vessels. The knuckle boom is designed for use in hazardous areas. The crane can additionally be equipped with an Arctic temperature package allowing for operation at temperatures down to -40 degrees Celsius.