Innovative Subsea Crane Enables Stable Heavy Lifts On Board Specialist Vessels

GERMANY – ASIA – As the size of everything from container vessels to oil recovery rigs gets ever larger, so too does the equipment designed to handle their manufacture and maintenance and the past few weeks have seen crane maker Liebherr undertake works previously unheard of at its test stand in Rostock. Four giant cranes were required in tandem to lift a new RL-K 7500 subsea crane, weighing in at 665 tonnes, onto its transportation device bound for Asia before its outfitting aboard a specialist vessel.

Another model of Liebherr’s series of knuckle boom cranes, an RL-K 4200, was recently shipping to Daewoo Shipbuilding & Marine Engineering Co. Ltd. (DSME), together with the RL-K 7500. The giant Korean rig and shipbuilder plans to equip several of its ships with a set of Liebherr cranes consisting of one RL-K 7500 and three RL-K 4200.

Since it was the first time that four cranes had ever been involved in a single lift at the test stand in Rostock a lot of planning and preparation work was required, just the assembly of the mobile cranes took approximately two days. Besides the crane’s dead weight a further challenge of the lift, which itself took around an hour and a half, was the loosening of about 100 bolts on the slewing ring.

The new subsea crane with knuckle boom represents a convenient solution for subsea operations. Together with the crane’s prototype the above mentioned RL-K 7500 is already the fourth model to be completed. It features impressive technical figures, being able to lift up to 300 tonnes in the air and handle loads down to a water depth of 3,600 metres. 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 radius of up to 50 metres.

One vital 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. 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.

The Heavetronic® – Liebherr Active Heave Compensation-System (AHC) is based on sophisticated Motion Reference Units (MRUs), i.e. sensors designed in house to recognise and predict the vessel’s motion, the AHC system 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 harbour cranes, was specially adapted for subsea applications. Pactronic® is characterised 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 revolutionary AHC system is fully integrated in the well proven Litronic® system. One of its most important features is its self-learning function. It automatically recognises 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 optimised rope guidance, the crane has a patented horizontal winch shifting system. It guarantees the ideal fleet angle of the rope under all load conditions in both hoisting and lowering operations whilst 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 innovative design of the winch unit, which is based on many years of experience and profound knowledge of Liebherr crane technology, wear of the main wire rope is minimised, thus significantly extending its service life and reducing life cycle costs.

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.

With the RL-K 7500 Liebherr offers an innovative crane concept for subsea applications which can be rated both as general purpose offshore crane and as a heavy lift crane. 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.

Photo: The four cranes lift the giant subsea unit and inset the RL-K 7500 subsea crane in operation.