Recently the transformer station for the offshore wind farm Global Tech I was successfully installed.
The offshore platform is a core element of the North Sea wind farm: The power from its 80 wind turbines is gathered here and transformed from 30 to 155 kilovolts. The transmission grid operator will subsequently draw power from the platform via export cable to its own offshore transformer station, where it will be converted from alternating to direct current for transmission to shore. In this way transmission losses are minimised. The Global Tech I platform, located amid its array of turbines occupying 41 square kilometres, will also serve as its logistical base. Around 30 technicians working in shifts around the clock will maintain reliable power generation. Thomas Maetzel, Commercial Managing Director of Global Tech I Offshore Wind GmbH: “The transformer station is an important milestone in the construction of our wind farm. We are proud that together with our partners Alstom Grid and Keppel Verolme we were able to develop this innovative platform concept and successfully complete its installation.” It is the first time a German offshore wind farm has deployed a floatable platform that autonomously installs itself using an environmentally friendly method of installation with suction cans.
Environmentally compatible installation with suction cans in the seabed
The wind farm’s internal transformer station was delivered complete for use by the consortium Alstom Grid GmbH and Keppel Verolme B. V. The steel structure of the platform was constructed in Rotterdam by Keppel Verolme and the electrical equipment installed by Alstom Grid directly at the Dutch shipyard. Two seagoing tugboats took three days to tow the floating, closed steel body of the transformer station from Rotterdam to the construction zone. In total four tugboats were needed for positioning. The legs of the support structure, which were affixed to the body and towered above it during the voyage, were then descended to the seabed. The suction cans are mounted as the four feet of the supporting legs. With each a height of 9.5 metres and a diameter of 11 metres these steel cylinders were first pressed into the seabed under the weight of the platform itself, 9,000 metric tonnes. In the next step, vacuum pumps drew out the seawater in the cylinders from above, thereby producing a negative pressure that pulled in the seabed from beneath the suction cans. This is an environmentally sound method, because no ramming of piles is necessary yet the support structure is buried deeply in the seabed, securely anchoring the transformer station. In a third step, the station was jacked up 20 metres above the sea surface and locked into place. Arjen Schampers, Technical Managing Director of Global Tech I Offshore Wind GmbH: “We adapted a method from the oil and gas industry to install our transformer station and this is the first time this technique has been used in the North Sea. Even when offshore wind power is pioneering work, this example clearly shows there is still much technological experience we are able to call upon in this new industry.”
Logistical base and efficient power generation
The closed body, termed the topside of transformer station, consists of seven distinct decks each with surface area of 46 by 46 metres. These include the cable deck, the main and intermediate decks and the working deck. The cable deck is on the lowest level and includes diesel generators that serve to supply the entire wind farm with auxiliary power for its own use in cases of interruption of the grid connection. Above this are the main and intermediate decks. The central control room of the transformer station with protection and control systems and communications technology is housed on the main deck. Due to their extensive sizes, some of the high-voltage equipment, such as the four transformers, switchgear and reactive-power compensation chokes, are spread across both the two levels of main and intermediate decks. They are located inside the closed steel body of the platform in order to protect them from the aggressive, salt-laden atmosphere. In addition, redundancy is provided for all the high and medium voltage equipment. For example, should one transformer fail, it would neither impact the performance of the wind farm nor restrict its output. The working deck is the uppermost level and is constantly exposed to the weather. An offshore crane is installed here, a containerised spare parts store and a helicopter landing pad. The accommodation section for up to 34 operation, service and repair technicians, who must man the platform around the clock, is also located here. As well as cabins for sleeping they will also have communal areas and a fitness room. Accommodating the service teams on the platform allows economic operation of the offshore wind farm, which lies at a distance of 180 kilometres from Bremerhaven.
Installation of the 80 tripod foundations began in September 2012 and so far 32 have been completed. Since the beginning of this year, work to lay cables between the installed tripods has been underway. According to the current planning status, installation of the wind turbines will commence this summer. Completion of the wind farm is presently expected by spring 2014.
Press Release, May 9, 2013