Living Energy · No. 9 | December 2013 5756 Living Energy · No. 9 | December 2013

Grid Access

Tall as a redwood tree, the newly installed HelWin1 transformer platform rises up out of the North Sea. It was erected recently with utmost precision and marks a signifi cant stage in link-ing offshore wind power to the grid.

Text: Onno Groß

of the Sea

Mammoth Pillars

Author Onno Groß, a veteran journalist on maritime topics, is based in Germany’s “Gateway to the World,” the Port of Hamburg.

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Living Energy · No. 9 | December 2013 5958 Living Energy · No. 9 | December 2013

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mately 85 kilometers off the coast. Siemens was commissioned to con-struct the platform by grid operator TenneT in mid-2010. With the Italian cable specialist Prysmian, the consor-tium is implementing a total of four North Sea grid connection projects: HelWin1 and HelWin2 off Heligoland, BorWin2 off Borkum, and SylWin1 off Sylt. The substructure and platform were constructed at the Nordic Yards

s anyone who has ever stood in a forest in front of a giant red-wood tree knows, it is virtually

impossible to escape the lure of these mighty giants. Firmly rooted in the earth, the trunk may have a diameter of several meters. In California, a road has actually been hewn through one of these arboreal mammoths. The pilings of the newly erected trans-former platform HelWin1 in the North Sea rival the redwoods in height and stability. The platform, which lies just off the Heligoland archipelago, fills a significant gap in the grid conne c tion of offshore wind farms. “The steel structure and complex construction made the HelWin1 proj-ect a real technical challenge. Now, the platform has been raised safely,” comments Marc Becker, lead project manager for all offshore grid connections at Siemens, with visible relief. His elation at the success of the weather-dependent venture is audible. What has been achieved is much more, after all, than the pioneering installation of a colossus in the North Sea, which had to be completed within a narrow time frame. Becker also relates other fac-tors, such as the discovery of unex-ploded ammunition from World War II and a complex approval procedure. It is clear that the project faced a number of initial hurdles that had to be negotiated.The HelWin1 converter platform serves as a mainland link for the energy generated by the offshore wind farms “Nordsee Ost” (North Sea East) and “Meerwind” (Sea Breeze). The alternating current power generated by the wind turbines is transformed into low-loss direct cur-rent for transmission onto land, known in technical jargon as high-voltage direct current (HVDC). Siemens is one of the leading suppliers of this technology, the market for which is set to double its present vol-ume of €3 billion globally over the next five years. Particularly stable and versatile HVDC platforms were developed for the German North Sea wind farms being erected approxi-

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shipyard in Wismar and at Warne-münde on the Baltic Sea.

Pilings as Tall as Big BenThe installation of the HelWin1 plat-form northwest of Heligoland began in April 2013, and started with the anchoring of the substructure, which weighs 1,000 tonnes. Ten steel pilings, which formed the structure’s jacket, had to be rammed into the seabed.

The acoustic emission was minimized with the aid of an impact shell (coffer-dam) designed to protect marine mammals from the pile-driving noise. At up to 3.2 meters in diameter and boasting a wall thickness of 8 centime-ters, the pilings are true steel giants, whose length of up to 100 meters makes them as tall as the famous London landmark Big Ben. The platform stands at 22 meters

above sea level, perched significantly higher than the maximum recorded wave height of 19.32 meters. The second step of the construction phase in-volved the platform’s transportation to its final position 20 kilometers off the coast of Heligoland. HelWin1 was converted into a floating vessel for this purpose, before being towed out into the North Sea by three tugboats. The outer doors were welded shut,

while inside, all the bulkheads were closed in order to create something resembling an ocean-going “ship.”

World War II LegacyThis special form of transport was se-lected in order to ensure that as much of the fabrication as possible took place on land, thus avoiding the exor-bitant costs incurred by construction at sea. Accordingly, the complete HVDC

1. Offshore wind turbine plants generate medium-voltage AC power

2. Wind energy generated by the wind farm tur-bines transformed to higher AC power at the substation platform

3. HVDC platform converts the alternating current from several substation platforms to direct cur-rent for transmission

4. Subsea cables, some more than 100 kilome-ters in length, trans-port the low-loss direct current onto land

5. A converter station on land transforms the direct cur-rent back into alternating current for feeding into the high-voltage grid and for further transmission

A Crucial Link

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As the main land conversion and transmission node for energy gathered by offshore farms “Nordsee Ost” and “Meerwind”, HelWin1 plays a crucial role in the integration of renewables into Germany’s grid.

In order to transport the platform to its final location 20 kilometers beyond Heligoland, HelWin1 had to be converted into a floating vessel towed by three tugboats.

would have affected the acoustic measurement equipment that is needed for piling operations, with the result that the latter had to be retrieved from the water and later replaced. This took a week.

The Art of Picking a Weather WindowThe platform was eventually posi-tioned precisely above the pilings with a deviation of just a few millimeters. In a second step, it was connected to the pilings within the jacket, a phase whose completion depended on the weather. “The process of threading the platform onto the legs can only be performed when the wave swell is less than 70 centimeters,” explains Randolph Schmid, Siemens Project Manager Offshore. The final phase involved the platform being hoisted

Siemens is currently testing three dif-ferent methods in connection with the platforms’ installation. The self-float-ing transport used for HelWin1 and BorWin2 is one option, while, in the case of the SylWin1 project, the plat-form lies on a towed pontoon and is positioned in situ. Finally, the plat-form used in the HelWin2 project is lowered onto the prefabricated jacket structure via a large floating crane. Besides pioneering spirit, offshore installations require technical back-ups to safeguard against malfunctions and sufficient margins of tolerance. “These days, when we assess a project, we evaluate it on the basis of the transport and installation processes. We have learned from experience and now know which challenges are in-volved,” emphasizes Becker. Installing an offshore platform depends on a series of controllable factors, but the weather cannot be influenced so easi-ly. All components must be robust, as the costs at sea are ten times as high as on land, while the cost in terms of time is five times as high. The learn-

ing processes involved in cooperationwith the sanctioning authorities are another factor.

A New HVDC Platform Each YearIn the wake of the successful anchor-ing of HelWin1, 100 engineers and em-ployees are finalizing the installation. The platform must be fully functional in time for the commissioning in No-vember 2014. “With the installation of our platform at sea, we have success-fully mastered the most critical part of this project and are now on the final stretch for commissioning in 2014,” states Karlheinz Springer, CEO of the Power Transmission Division within the Energy Sector of Siemens AG. With a transmission rate of 576 megawatts, HelWin1 will eventually supply electric-ity to over 500,000 households on the German mainland. “We are certainly confident of our ability to execute new offshore projects,” says Becker. “We’re now familiar with the installation pro-cess, we have reliable suppliers, and we know how to cope with heavy

platform was constructed in the ship-yard. An impressive seven stories high and with a surface area of 75 by 50 me-ters, it is as big as half a soccer field. Transporting the 11,000-tonne platform from the Baltic Sea around the tip of Denmark ultimately took seven days, and a further four were planned for the installation. Although the envi-ronmental conditions in the North Sea, which is 23 meters deep at this point, are a more predictable aspect of the project, the ambitious schedule was threatened by the discovery of two World War II torpedoes some dis-tance away from the platform. “The seabed surrounding Heligoland is full of unexploded ammunition, which first had to be removed,” explains proj-ect manager Marc Becker. Although the discovery was not made directly in the construction area, the detonation

swell – in short, the expertise is now on hand.”Europe remains the most important market for offshore HVDC technology. Germany has developed clear expan-sion targets for renewable energies: Network operator TenneT will be issu-ing one invitation to tender for plat-forms with a capacity up to 900 mega-watts each year up to and including 2020, in order to comply with the grid expansion obligation.Wind power installation is also mak-ing rapid progress in Great Britain. In Germany, 33 plants amounting to a capacity of over 2,250 megawatts have been approved to date, while a further 8,000 megawatts are planned by 2020, and a total of 25,000 megawatts are envisaged by 2030. In England, instal-lation of up to 51,000 megawatts is an-ticipated. Many more mammoth off-shore pillars will thus be required in future. p

Onno Groß, a science journalist from Hamburg, writes regularly on maritime issues for newspapers and magazines such as National Geographic and Handelsblatt.

onto the pilings via hydraulic power. As it was raised up with excruciating slowness, centimeter by centimeter, the faces of all Siemens participants lit up with delight and relief. The window of opportunity had been chosen with pinpoint accuracy: one day’s journey to the location, two days’ positioning, and two days for the threading. Rais-ing the platform required a further day. Says Schmid: “It’s primarily a matter of checking the weather fore-cast and not losing one’s nerve.” The August deadline for the installation of HelWin1 had been agreed a year be-fore, and Siemens was therefore able to stick to the schedule. Although the completion was preceded by a steep learning curve, it did result in HelWin1 overtaking BorWin2 on the installa-tion timetable.

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