WORKING AT HEIGHTS - Wind Energy Network

metmasts featureOffshore meteorological stationsa hidden challengeAccounting for a smallproportion of investment andeffort spent on a project,the meteorological stationis easily overlooked. Butdesigning and installinga ‘met mast’ demands ahighly integrated approachand understanding of thecomplex structural design.For project developmentteams it captures many of thechallenges encountered indeveloping the project.Capital riskThe mast, which can cost anythingbetween £4 and £10 million, is normallyinstalled before the project has reachedthe financing milestone of full planningconsent. This timing means that theinstallation happens before ‘financialclose’, putting the developer’s capital atrisk if consent is refused.For example, Cirrus Shell Flat wind farmdevelopment was shelved in 2008, sixyears after a met mast was installed.Given all this, whybother with a mast inthe first place?Met masts are installed earlyin the project developmentphase, with the objectiveof taking accurate windmeasurements in order toincrease confidence in thewind speed predictions.Best in classAlthough they are not the only means ofunderstanding the wind speed at a site,the consensus view is that a mast is thebest option for decreasing the uncertaintyin wind speed predictions. The figurebelow shows possible measurementalternatives and how the uncertainty levelscompare.The measured dataalone gives an improvedunderstanding of the site’swind regime. But the valueof the data can be increasedby comparing them tomeasurements made over amuch longer time period at aweather station nearby. Theprocess known as ‘measurecorrelate-predict’involvesidentifying correlationsbetween the long-term dataand the measured on-sitedata.In essence, understandingthe correlation enablessynthesis of the long termrecord for the specific site– effectively providing a statistical view ofwind speed over a much longer time span.Ultimately, the technique increases thecertainty of wind speed predictions, andby inference, power production, over thelife of the project.Structural design challengesIt is tempting to assume that the metmast structure is fairly simple, especiallywhen compared to a highly dynamic windturbine.While this might be true to some extent,the mast’s offshore foundation is not astatic structure and dynamic loads needto be considered. Lattice towers and theirfoundations must be robust enough towithstand the wind and waves at the sitewithout comprising the quality of windmeasurements.The highest possible level of certaintyabout wind farm production is animportant element of securing finance atthe best available terms, a potent driver ofthe cost of energy.102www.windenergynetwork.co.uk