Windtech: Addressing shallow water challenges

NETHERLANDS: In July turbine manufacturer Siemens was awarded the turnkey contract to engineer, procure and construct the largest Dutch near-shore, shallow-water wind project, Westermeerwind.

Mammoet’s multi-purpose pontoon Schelde, capable of launching ships, will be equipped with stabilisers and a crane (pic: BM4Wind)
Mammoet’s multi-purpose pontoon Schelde, capable of launching ships, will be equipped with stabilisers and a crane (pic: BM4Wind)

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The project, sited in the IJsselmeer lake, along the northern and western Noordoostpolder dykes, will use 48 Siemens direct-drive SWT-3.0-108 turbines with hub heights of 95 metres. The deal includes a comprehensive 15-year operations and maintenance contract.

The project layout consists of three turbines rows placed from 500 to 1,100 metres from the dyke, electrically split into six 33kV cable strings (4x8, 1x10 and 1x6 turbines). These six export cables individually feed power to shore through the dyke, where it is collected in a Siemens-designed sub-station and delivered at 100kV level to the Tennet high-voltage network in Ens.

For project permission, the export cables are required to be submerged 2-2.5 metres into the seabed to avoid damage by fishing nets and vessel anchors. Siemens will use a water-jetting tool for this task. This is only one of the many technical challenges the company has had to face, says Willie Wienholts, Siemens' Westermeerwind project manager. "Apart from installing turbines in only 3-7 metres water depth, we have had to cope with sluice gate passage constraints and the lake's busy shipping traffic and recreational usages," he said.

This combination of technological and logistical challenges required Siemens and its partners to turn innovative ideas into novel dedicated solutions for turbine pre-assembly and installation.

Stabilising legs

The sluice gates prevent the delivery of large components and assemblies, and even smaller-size jack-up barges and self-propelled vessels," says Siemens' construction manager for the project, Lars Jakobsen. "Our installation partners, civil engineering contractor Ballast Nedam and heavy-lift specialist Mammoet, decided that the best installation equipment solution would be a barge offering limited draught and equipped with stabilising legs."

The BM4Wind Ballast Nedam/Mammoet joint venture will deliver feeder (transport) barges, tugs and additional supply and service vessels. The installation barge, Schelde, will be fitted with a crawler-type crane to install the monopiles and turbines, with an anchor system for relocating to new positions. Prior to the installation work, the hull will remain in the water, jacked and elevated only high enough to provide a stable working platform.

Ballast Nedam is responsible for the design, delivery and installation of the monopile foundations. Each one measures five metres in diameter with an average length of 40 metres and a mass of between 250 and 300 tonnes.

Flange connection

An unusual feature of the design is the direct flange connection between tower and monopile, which eliminates a traditional transition piece, implying that the pile-ramming operation is performed directly on a pile's flange. The monopiles, manufactured by the Dutch SIF group, will be delivered to the site on transport barges.

Mammoet is responsible for the transport and installation of the towers and turbines. "Individual feeder barges will carry a complete tubular steel tower in two sections and a fully assembled nacelle by water from Amsterdam harbour via the sluice gates to the construction site," said Jakobsen.

Another strategic choice is that the full rotor star will be pre-assembled, rather than adopting the current trend for single blade installation. This is the only available technical solution for the time being as there is a restriction on the boom height that can be applied on the installation barge/crawler crane solution. Rotor pre-assembly will either be conducted at the lake or onshore. "The lake assembly options incorporate multiple interconnected barges, again fitted with stabilising legs. Both options are still being evaluated," said Jakobsen.


Westermeerwind is an independent venture, though it simultaneously forms part of a 429MW onshore/near shore overall project called Windpark Noordoostpolder. This further comprises two separately owned onshore sub-projects, with 26 and 12 Enercon 7.5MW turbines respectively, all with hub heights of 135 metres.

Residents of the neighbouring councils - Noordoostpolder, Urk and Lemsterland - will be offered an opportunity to invest in the project around one year after the expected commissioning in early 2016.

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