Sarens goes to sea

The plan for Thorntonbank is to install 60 wind turbines, providing 300MW of power. The first phase of the project, taking place this summer, will see six turbines installed. Each is mounted on top of a 2,800t flask-shaped concrete foundation, and rises 120m from the sea bed.

The seabed at Thorntonbank was prepared by contractor Dredging, Environmental and Marine Engineering (DEME). Loose sand was dredged from the seafloor, and retained for use as ballast in the foundations. Once the sand had been removed, DEME prepared a flat surface on the bedrock, to tolerances of 5cm, ready to receive the gravity based foundations.

The foundations were built at Halve Maan, a fabrication yard near Ostend. The 42m high, 2,800t hollow concrete foundations are shaped like a vase, wider at the base than at the top. They were transported from the fabrication yard to the harbour at Zeebrugge on 112 axles lines of Kamag self propelled modular transporters. From there, the crane barge Rambiz carried them out to Thorntonbank, and placed them on the seabed.

The sea at Thorntonbank is around 25m deep. The base of the conical foundation is 23.5m across, and the conical section stands 17m high. A cylinder on top reaches between 38.5m and 44m high, depending on water depth. The iron and concrete walls of the foundations are 500mm thick.

The sand that had been retained from the original dredging works was mixed with water, and pumped into the top of the foundations. The sand sinks to the bottom of the foundations, holding them firmly in place on the sea bed. With the foundations in place, it was time for Sarens to install the wind turbines on top.

Each wind turbine consists of two tower sections. The nacelle is mounted on top of the tower, and then the rotor and three blades are installed in one piece. To reach the heights needed, Sarens would use a 750t capacity Liebherr LR 1750 crawler, mounted on a 40m-high modular tower, carried by a jack up barge, the Barge Buzzard.

Work started at the dock in Zeebrugge in June with the mobilisation of a 600t Terex-Demag CC2800-1. This crane loads and unloads wind turbine sections as they arrive at the staging area. Next, Sarens brought in a 550t Liebherr LG 1550 pedestal crane, to install the LG 1750 on top of the Sarens modular lift tower (SMLT). The crawler mounts on the tower with four custom-made connectors.

In mid-June, the Barge Buzzard arrived at the quay. Bart Wauters, Sarens project manager, explains, “The Sarens modular tower has a footprint of 12m by 12m and is 40m high. The tower is assembled of three 12m units and one 4m unit. It is stick-built; every pipe is transported separately and needs to be bolted on site. The heaviest pipe is 17t. You can consider it being similar to a lattice-boom section, but it is too big to be transported over roads.”

The LG 1550 worked with two more cranes, a DC 1100 and an AC 200, to install the LR 1750 on top of the tower. Wauters says, “The crane was lifted on to the tower in different parts: the undercarriage, then the upper, then the superlift boom. The 63m main boom was lifted in two parts. In order to lift the boom to this height, it took two cranes, one holding each end of the boom. The third was for access in a man-basket to introduce pins and make all of the connections.”

This is Sarens' first job with the LR 1750. Wauters says, “There was a test lift of the crane on 5 July in Zeebrugge, before it was mounted on the platform. It was delivered brand new, and its offshore lifts will be the first lifts from the crane. This is the first lift of this design. We haven't installed 5MW turbines before. With 2MW turbines, you can use a normal crane on a conventional barge; something like the CC2800.”

The Barge Buzzard carried the LR 1750 out to Thorntonbank, where it will stay throughout the installation process. Two barges travel between Thorntonbank and two staging areas, in Ostend and Zeebrugge. At Zeebrugge, the LG 1550, used to install the SMLT and the LR 1750, is first used to prepare the wind turbines, lifting the blades for fitting on the hubs. It then travels out with the transport barge, to act as a tailing crane for the crawler, and to lift the assembled rotors and blades into position, ready to be lifted to the topof the tower.

Wauters says, “We have five people working permanently on the Buzzard: two crane operators, two lift supervisors and a mechanic. Then on the other barge is a tailing crane operator. Then we have four crane operators on the shore: two in Zeebrugge, two in Ostend, and one supervisor who moves around. Our personnel stay on the barge for two or three weeks, depending on their rotation. The Buzzard has accommodation for 48.

“The wind turbine comes offshore in four sections: tower sections one and two, the nacelle and the complete rotor (with three blades). The rotor is assembled on shore in Zeebrugge horizontally, and the complete rotor rests on a frame on the barge. The LG 1550 acts as a tailing crane on the transport barge to upend the rotor.”

In its working configuration, the LR 1750 at Thorntonbank carries 150t of counterweight, at an extended radius. Wauters says, “On the back of the LR 1750 is another small electrical crane, which lowers ballast hooks and the counterweight to the barge before the barge sails to the next move. This has to be done to lower the complete centre of gravity to guarantee the stability of the barge.”

A superlift boom is fitted, but does not carry its own counterweight. Instead, Wauters explains, “The superlift forces go from the backmast, and are pulled out over the deck of the barge. On the barge there are a set of connection eyes welded, and a set of connection cables that carry the superlift counterweight forces.

“These welded eyes are installed on two positions of the barge. The crane lifts the heavy parts, towers and nacelle, from one side of the barge, and then booms up and slews 180 degrees. The superlift cables then connect to the deck, and the crane booms out to the required radius to install.

“The installation radius is 22m. The heaviest part, the nacelle, is 320t. The hub plus rotor blades in the complete rotor weighs 130t. We have scheduled six weeks to install six turbines, including weather delays.

“All wind turbine assembling and bolting is done by Repower, the turbine manufacturer," Wauters says. Riggers climb up the stairs inside the tower to bolt the sections together.

Moving the Sarens modular lift tower and the LR 1750 between job sites is a complicated task. Wauters says, “The most secure configuration is the barge jacked up, with the crane in its sea-fastened condition. It can weather heavy conditions there. Bringing the crane from lifting configuration to sea condition takes six hours.

“Once the platform is jacked up, the only limiting factor of the weather is the wind: during lifting operations wind speeds are limited to less than 9 m/s. The wind forecasts are followed carefully, and there is an actual reading of wind speed on the crane.

“There is a complete procedure for bringing the crane from working mode to sailing mode, and back again. You have to bring down the counterweight on the back of the crane, and there are another 20 points or so. The superlift boom needs to be tensioned with cables to the barge, the main hook linked with cables to the barge, and the runner hook connected too.”