LKAB Minerals supplied high density MagnaDense for Hywind Scotland, the first commercial floating wind park (illustration: LKAB)

LKAB Minerals supplied high density MagnaDense for Hywind Scotland, the first commercial floating wind park.

The design concept is based on a floating spar-type platform which is anchored to the seabed. The floating spar’s turbine is much taller than the hull and has a moving rotor that harnesses the strong offshore winds in the North Sea. The waves, currents and winds are strong external forces and keeping the very heavy and tall floating wind turbines upright in open sea is a big challenge. A lot of weight is needed to achieve a low centre of gravity in the structure. This solution involved the use of LKAB Minerals.

MagnaDense as a loose ballast.  In addition, MagnaDense was also used to produced high density concrete with a density of 4 t/m³. This was used as a counterweight which aided the installation process and added to the overall weight of the structure.

The high density concrete was produced and placed within the substructure at the construction site, the Navantia wharf in Spain, before being transported to Stord, Norway. After completion, the foundations where transported to the Hardangerfjord, offshore Stord. After uprighting and water ballasting of the foundations, the MagnaDense loose bulk ballast was installed in the floating foundations. Following this operation the tower and turbine were mated to the substructure before the tow out to its final destination in the Scottish waters. Each structure was ballasted with approximately 5,000 tonnes of the heavy aggregate and an additional 1,100 tonnes of concrete ballast.

The five turbines of this first commercial floating wind farm, installed during 2017, is now producing electricity of the Scottish coast. With the supplied >27,000 tonnes of high-density MagnaDense as a loose ballast, the centre of gravity of the floating foundation was lowered and keeps the wind turbine upright. It was of major importance to stabilise the foundation, ensuring it was not affected by sea waves and currents.

Statoil’s Hywind pilot project offers a fresh and unique concept to the energy market; the first commercial floating wind farm of its kind in the world.

The Hywind tower with its 176 metres above the water surface and the blades with a span of 75 metres is impressive and much larger than conventional and typical wind turbines in use. The sheer size means that it will also be able to generate more power, up to 6 MW/each, enough to power 20,000 households in total. Key to producing energy and utilising as much of the capacity as possible, is strong and constant winds. A placement further out from the coast makes this possible. Here the waters are deeper and the floating design was needed to realise the ambitious goals for the project.

In the case of Hywind Scotland, MagnaDense offered the best price/quality for the chosen design. Given its high specific density and very good compaction properties as a loose bulk material, it results in more weight in less volume. The fact that the high density material also has excellent properties as a concrete aggregate (EN-12620 certified) for producing high quality and high density concrete made it ideal for the bottom ballast that functions as a counterweight.

MagnaDense aggregate is manufactured from the natural iron oxide, magnetite. It is chemically inert in marine conditions and does not leach, supporting the sustainability aspects of the Hywind project. In fact and proving the point, LKAB Minerals has even bought back material that had been in use for 22 years in Shell’s Draugen Floating Loading Platform, which was commissioned at an oilfield 150 km offshore from the Norwegian coast. The reclaimed material was tested and compared with the original shipping analysis that proved the material chemically to be unchanged. With this assurance, the material was again offered to the market and reused as a concrete aggregate.

As a value-added service and to support the financial sustainability of the project, LKAB Minerals offered her customer the generous opportunity to return the over-supply of MagnaDense upon completion of the project. This ensured that costs were controlled and prevented any spoil of unused product.

Tags: LKAB Minerals, Statoil