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TALKING POINT

Terry Bolsher considers the planning, supply problems and geotechnics of wind energy

There is a vocal element of society that talks about the countryside damage caused by erecting wind turbines.

Yet the fact remains, as stated by a panel of UN scientists and 160 governments world wide, that burning fossil fuels is causing immense damage to the environment.

But over 80% of people do openly support the use of wind turbines to help meet energy demands with only 5% objecting. Already, there are almost 150 wind farms with almost 2000 wind turbines. These generate 2200MW of power to help meet energy needs across the UK and well over 250 more sites are in planning for a further 4500 turbines.

If we are to catch up with certain other European countries we must fully embrace this technology as part of an overall package of more environmentally friendly solutions and learn from those that have long since made this decision.

Countries like Denmark and Germany, with approximately 20% and 30% respectively of their power demands met by wind energy, have already found solutions to problems we are presently grappling with.

Many turbine manufacturers are based in those countries and supply UK projects, leaving us to solve the problems in the ground. But with world wide product demand high, many have an order book in excess of two years manufacturing capacity resulting in supply problems for the UK's needs.

The UK planning process is constrained by many factors, including nding a site with the correct topography that is exposed to consistent wind ow and close to the National Grid.

Consultation is needed with the Ministry of Defence, the Royal Society for the Protection of Birds and local authorities for land based wind farms generating up to 50MW, and with national and devolved government for proposals generating greater than 50MW or for offshore farms.

Engineering considerations come a long way down the list, and planning approval may x the hub height of wind turbines, resulting in the foundation being below natural ground level.

This can lead to floatation loads on the base that increase the complexity and cost of the foundation design unnecessarily, all for the sake of 1m to 2m on the hub height of the turbine.

Wind loadings and the resulting overturning moments create a wide spectrum of axial loads on piles. The European standards applied by turbine manufacturers require either exhaustive fatigue stress analysis of the forces in the pile reinforcement or the compliance with a particular load case that does not allow any tensile forces in the pile.

This means using more dead load in the form of a larger or deeper base. Typically, precast concrete pile section sizes are 300mm2, 350mm2 or 400mm2 and predominantly now the larger two sizes. Section size is mainly decided by the structural design to resist a combination of axial compression or tension loading and a co-existent bending moment.

Load testing on driven precast concrete piles is carried out mainly using a pile driving analyser, although occasionally static load tests are specied. Where 100% reliance is placed on dynamic tests, the third parties involved in approvals - there can be several layers involving the developer, turbine manufacturer and nancers - will expect to see the ultimate design resistances mobilised under the test blows and subsequent CAPWAP analyses.

Another design consideration is that shaft friction on piles degrades with reversible cyclic loads. The European convention is to rely only on 80% of the ultimate or proven value. With this in mind, piles are designed to global geotechnical factors of safety of two and a half on compression and three on tension giving long term results, which accord with established European partial factor design.

Understanding the complexity of the loadings induced by an operating wind turbine is an area where we should and can use proven experience.

We must not keep trying to 're-invent the wheel' and should take note of the experience generated from the installation of the thousands of wind turbines in Denmark and Germany so we can make sure that the dream of environmentally friendly clean energy does not turn into a nightmare.

Terry Bolsher is managing director of Aarsleff Piling.

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