A question mark hangs over the long-term stability of Europe’s shallow-water turbines, after research linked to the Horns Rev 1 wind farm found that high-powered currents were causing the stone “armour” around the base of monopile foundations to collapse.
Exposed from behind this layer of shielding stone, turbines could potentially be dangerously destabilised by the effect of scour – the wave- and tide-driven sediment that can eat away at the seabed around fixed structures.
A team from the Technical University of Denmark (DTU) carried out model tests designed to mirror the impact of offshore conditions on the three-layer cover of “scour protection” placed around the 80 turbines at Horns Rev 1, which, three years after installation, had sunk by as much as 1.5 metres.
Experiments in the current flume at the DTU testing facility in Lyngby established that “horseshoe vortices” – twisting flows created by a change in water pressure at the surface of the monopile – were working their way around the stones and carrying off soil from around the foundation, causing the stone armour to sink into the sea floor.
“There has been a great deal of research done in the area of scour at Horns Rev, but scour protection of monopiles has not been studied nearly so much,” says Anders Wedel Nielsen, a hydraulic research engineer who led the DTU team, and is now working at Danish environmental research consultancy DHI.
“Although there was an indication that it was due to hydrodynamics, from the basic first models we and others were working with you had to add waves to get sufficient force to move [the scour protection stones]. But it turned out that the power of [horseshoe vortices] was much stronger than expected.”
Horseshoe vortices are well-known as a main culprit in scour around unprotected piles. It turns out they also undercut scour protection.
“All the near-shore, shallow-water wind farms off Europe are exposed to similar scour and horseshoe vortex issues,” says Nielsen. “Certainly Egmond [in the Dutch North Sea] and, I believe, Arklow [off Ireland] have seen problems related to these things.”
At Horns Rev 1 – in five- to 15-metre-deep waters in the Danish North Sea – the 4.2-metre-diameter monopile bases were encased in a 50cm-deep “filter layer” of 10cm sea stones overlaid by two layers of 40cm quarry rocks.
At the time this scour protection was laid in, suggests Nielsen, developer Elsam (which later became part of Dong Energy) wanted to use a finer grade of stones for the bottom layer – but found that these were washed away before there was time to place the larger rocks on top.
“If you try to dump all three layers at once over the side of a barge, you get the heaviest stones on the bottom and the smallest stones on top – which is the opposite of what you want,” explains Nielsen.
The first survey of the monopiles at Horns Rev was undertaken in 2005 – at which point new stones were heaped about the base of the turbines.
The loss of scour protection has serious implications.
“If you get some large scour holes around a monopile [due to the stone armour sinking] you can potentially have some problems… that could cause fatigue,” states Nielsen. “Of course, this sort of thing has not been a problem at Horns Rev at all. The integrity of the structures has not been threatened at any point, so far as I know. But it is a consideration that must be taken into account.”
A range of physical tests on monopile models – part of an ocean-energy research project underwritten by Norwegian utility Statkraft and supported by the Danish Council for Strategic Research and DHI – were run in two differently sized current flumes that were fitted with recirculation pumps to flow water at more than 60cm per second.
Flow visualisation using coloured dyes showed that the horseshoe vortices caused current to stream around a monopile “in a very similar way” to those passing around an unprotected foundation, coursing through gaps between the stones and transporting away the sediment.
Despite the shortcomings of current scour protection, the practice of dumping stones remains “a relatively cheap, easy to handle” way to reduce scour and creates a “flexible, resilient structure” that generally adjusts to shifts in flow dynamics to shield the monopile from damage.
“If you were to use a stiff metal plate, for instance, as an alternative form of protection you could get a situation where you start to have erosion underneath it and then you just have a gap [at the base of the foundation] which would present a big problem [for turbine stability].”
Nielsen says there are a number of ways to potentially minimise the problems posed by horseshoe vortices. One is to devise a system to lay finer-grade stones as a filter layer under the top scour-protection cover, although there is no proven method for successfully doing so.
Another, he says, “is to accept that you’ll have some sinking and to prepare for it – including burying the turbine cables so they will not be exposed even if the scour protection sinks”.
“You can estimate how much sinking you might anticipate with a given pile diameter and different types of stone,” he notes. If this is known, then replacement or topping up of the scour protection can be factored in to long-term operations-and-maintenance plans.
A third option is to model for installing unprotected monopile foundations. “For some wind farms where scour is light, it might be most economic to skip the scour protection altogether.”
One of Nielsen’s ongoing projects at DHI is exploring predictive models that would help forecast the likely long-term impact of scour on unprotected monopile foundations. It may be more economic, for instance, to simply fill in the scour holes when they become problematic.
Another potential solution is to not use monopiles at all.
The DTU research found that the wider the monopile, the lar-ger the horseshoe vortices are – causing more scour and greater sinking of the protective cover.
The tripod and quattropod jackets generally favoured for deep-water wind farms suffer much less scour because their legs are much narrower than monopiles. They are also much stiffer, making them less likely to become destabilised by scour.
Of course, in the deeper-water sites now being developed with jacket foundations, the impact of scour is naturally diminished: current velocity drops away the greater the water depth and with it the force of the horseshoe vortices.
Nielsen says that through “fine grain” pre-construction modelling of a development site, shallow offshore wind farms can be designed to minimise the effects of horseshoe vortices and scour.
“This research was carried out really for the next generation of wind farms in relatively shallow water in order to optimise the layout of the development and the overall design – and so to hopefully save some money for the developer,” says Nielsen.
“With proper planning, scour and the sinking of scour protection doesn’t need to be a threat.”
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