IN DEPTH: DolWin Alpha platform

The enormous yellow box looks too big to be moved. It is the size of a large apartment block — ten storeys high, 62 metres long and 42 metres wide — and weighs in at 9,300 tonnes, more than 1,100 double-decker buses.

And yet it sits on a barge about to be floated 30km downriver and then 75km out to sea, where it will be lifted on top of a 3,500-tonne steel foundation.

It is a monumental undertaking but one that is key if Germany is to achieve its ambition of having 25GW of wind power in the North Sea by 2030.

The yellow box at marine contractor Heerema’s fabrication yard in Zwijndrecht, South Holland, is the 800MW DolWin Alpha, the first of a series of giant offshore converter stations destined for German waters. For transmission system operator TenneT, these platforms are central to its €4.5bn ($6bn) investment that would bring 6.2GW of offshore wind onto the grid by 2017, 10GW by 2020 and 14GW by 2023.

DolWin Alpha, which is being delivered by power technology giant ABB under a $700m deal with TenneT, will be the biggest facility ever installed in the offshore wind sector. And it needs to be.

The platform has the unprecedented job of channelling energy from a trio of vast wind farms being built in the high winds and heaving waves of the German Bight, converting their alternating current (AC) output to direct current (DC), stepping up the voltage from 155kV to 320kV, and sending the electricity 75km to shore and on to the Dörpen West receiving station near Emden.

“These wind farms’ distance from shore is too far and their capacity too large to connect them with AC cables [to the onshore grid] — the energy losses would be too high for them to be economic — so you have to have DC cables to transport production to shore,” states TenneT chief executive Mel Kroon.

“We have interconnectors connecting Holland and the UK, for instance, with converter stations at either end, but this is the first time we are building such a thing offshore and this is a real challenge on many levels, in terms of the technologies involved and of the maintenance and operation aspect.”

Due to be towed out to site later this month, DolWin Alpha will be hoisted by Heerema's 14,000-tonne-capacity crane vessel Thialf on to its six-legged steel jacket foundation in 25 metres of water.

At the heart of the platform is an HVDC Light voltage source converter (VSC) from ABB. Based around innovations in insulated-gate bipolar transistors and extruded polymer cables, the VSC technology has the ability to smooth often large fluctuations in wind power levels without additional compensation or grid reinforcement before sending the electricity to shore.

“HVDC Light is a relatively new technology — it was first applied around 15 years ago — but it has had a very steep development and is now in the fourth generation of development,” says Hanspeter Fässler, ABB Power Systems’ senior vice-president for grid technology. “Functionality has been greatly enhanced and losses further reduced.”

Power losses from the DolWin Alpha converter station — which is the first commercial installation of a 320kV HVDC Light system — have been brought down to under 1% through advances in the VSC technology.

DolWin Alpha, designed by Heerema with help from Dutch engineering consultancy Iv-Groep, was fabricated from 35 steel deck sections in a stacked “pancake” assembly. The “air insulated” rooms housing the main electrical equipment are open and spacious to cut the chance of sparking off fires, with oxygen levels in the transformer room reduced by 15%. Main electrical equipment onboard is cooled with desalinated seawater.

“This is much like some oil and gas platforms and yet very different,” says Wim Matthijssen, chief operating officer of Heerema Fabrication Group, which has a 50-year history of constructing jackets and topsides for the offshore oil industry.

“Oil and gas platforms are very compact, filled with equipment, piping and so on. But these platforms [for the offshore wind industry] are quite open and airy by comparison.

“We are still learning. If compared to BorWin1 [the 400MW HVDC transmission link built around the Heerema-supplied BorWin Alpha converter station and installed off Germany in 2010], the DolWin platform shows real progress from a relatively young industry.”

An unmanned installation, DolWin Alpha features a 24-person living quarters and helipad for maintenance activities, and a free-fall lifeboat for emergency evacuation.

DolWin Alpha left Heerema’s Zwijndrecht yard in late May, stopping in Schiedam to be fitted with its crane and helideck before heading out to site. Once energised this autumn, the platform will be a hitching post for production from the DolWin1 cluster — German developer Trianel’s two-phase 400MW Borkum West 2 wind farm, Dong’s 277MW Borkum Riffgrund 1 and Windreich’s 400MW MEG Offshore 1. Enough spare capacity will remain for a fourth development to feed in to DolWin Alpha at a later date.

The platform is also expected to serve as a model for TenneT’s other offshore converter stations, with the 900MW DolWin2 and 900MW DolWin3 platforms slated for switch-on in 2015 and 2017. The 576MW HelWin1, 690MW HelWin2, 800MW BorWin2, 864MW Sylwin1, 108MW Riffgat and 111MW Nordergrunde platforms are due to be up and running around the same time as well, giving Germany a total offshore grid capable of handling more than 6.2GW of wind.

“DolWin Alpha will form the fundamental basis for our future [offshore converter station] projects,” notes TenneT Dolwin1 technical project director Frank Bohm.

“BorWin1 was the mother of all projects for us, but DolWin Alpha will be part of a generation of platforms that influences all that come after it.”