Trial for ice-defying gravity base

An ice-defying gravity-base solution (GBS) is being cued up for its first outing as a foundation for offshore wind turbines this year in the Baltic Sea.

STX Finland’s hybrid steel and concrete concept was trialled with marine contractor Terramare off Gävle, Sweden, as a substructure for shipping lane markers.

The company is now aiming to have a pair of its foundations topped with 3MW turbines in the water this year as a stepping stone to a full-scale offshore wind farm in 2015.

The STX design breaks from the convention of building the full GBS quayside and floating it out for installation, instead opting for a lightweight steel “skeleton” that can be lowered into place by crane vessel, then ballasted with aggregate and injected concrete.

By using a “shape-stiffened” structure, steel content is cut by 30% compared to traditional designs, streamlining handling, transportation and installation of the foundation. It also trims fabrication lead time, as the thinner steel plates are easier for mechanised welding machines to produce at speed.

“Rather than starting from the engineering premise of designing a really cool foundation, we have approached it with the aim of getting a wind farm using these foundations up and running in the shortest possible time by reducing working capital and simplifying logistics,” says the head of STX Finland Windenergy, Per Stenius.

“Typical of most of the other [GBS] projects out there is that they involve building a greenfield factory, but — in the Baltic particularly — the feed-in tariffs won’t allow for that, there is no room for new investment. So the cost is too high.”

A key feature of the flask-shaped concept, which is fitted with an “ice cone” to prevent the structure being crushed during the Baltic’s ice-locked winter, is that it has no bottom plate under the caisson base, meaning it can be tailored to a range of seafloor landscapes in water depths of 30-60 metres.

“If you have a bottom plate, you have to make sure it fits perfectly on the seabed. Without it, you save weight, you save cost and your foundation becomes much more flexible at the point of installation,” notes Stenius.

Weighing 300-600 tonnes at load-out, depending on the size of turbine it will shoulder, the foundation, unlike many competing designs, does not need to be towed by tugs to a wind farm. Rather, it can be transported three at a time on flat-bottomed seagoing vessels that can travel in waves as high as three metres at speeds up to 18.5km/h, accelerating the installation process.

This would be a boon to Baltic offshore projects because the summer installation weather window can be as short as three months.

STX is tightening its focus on getting serial production of the GBS down to less than one week per unit. The focus now is on smaller and smaller components — “how can we make assembly even faster, even higher-quality?” Stenius says.

Markets being targeted for the GBS are Estonia, Finland and Sweden — which the company calculates could together ultimately see more than 16GW of offshore wind farms.

First hopes, however, are pinned on a demonstrator competition run by the Finnish government being a springboard for a wind project in 2015.