Massive mast

Dutch crane builder Huisman ltrec has supplied a 3,000 short ton (2,700 metric tonne) capacity heavy lift mast crane for the new Sapura 3000 offshore construction vessel.

The US$ 200 million vessel is jointly owned by SapuraCrest Petroleum of Malaysia and international offshore engineering contractor Acergy (formerly Stolt Offshore). The heavy lift derrick and pipelay combination vessel is designed to be the most advanced deepwater construction ship in the Asia Pacific waters. In addition to the main 90 m boom mast crane, Huisman also supplied a pair of 40 tonne capacity pedestal-mounted cranes, six pipe davits rated at 50 tonne-metres and all the other lifting, handling and pipe laying equipment.

Huisman, which also builds the PTC platform twin-ring containerised cranes for Mammoet, was also partly responsible for the design of the the automation and drive system of the ship itself, all of which are electrically driven.

Traditionally, cranes and winches were driven by hydraulics or direct current, mainly to generate the high torque required. However, Huisman says that evolution in electrotechnology makes frequencycontrolled asynchronous three-phase motors a viable and preferable solution.

For the power supply and to suppress harmonic distortion, Huisman uses a 3,000 kVA 12 pulse transformer with rectifier. This creates a direct current bus of 930 Volts from of the ship's net. Virtually all drive systems are fed from this bus system.

A key feature of the crane is that the boom has free continuous rotation. The direct current is generated in the fixed crane pedestal and transferred through slip rings to the main winch frame inside the pedestal. All large winches for main, auxiliary and topping hoist are on this huge winch frame that rotates along with the slewing movement of the boom. The frequency converters that control the drive motors of these winches are also on the rotating winch frame.

Movement of the rotating winch frame is synchronized using absolute encoders, rather than being mechanically coupled to the boom or mast head. As the boom rotates on an enormous slew ring, a large external slip ring unit is used to transfer signals. This slip ring also transfers 2 x 400 Amps at 930 V DC from the DC bus in the pedestal to the various frequency-controlled motors that are located on the slew platform and boom. There are four pull-in winches and three sling hoists. Various auxiliary supplies and signals are transferred to the crane cabin, which also rotates with the slew platform.

The entire crane is controlled by an advanced programmable logic controller (PLC) system, to which hundreds of inputs and outputs are connected through remote input/output (I/O) and Profibus.

Less important connections with rotating parts are controlled through wireless ethernet connections. These include the SCADA (Supervisory Control and Data Acquisition) user interface in the crane cabin, CCTV cameras and spoken communication.

The SCADA system records operational data that can be transmitted to Huisman for remote diagnostics.