Seismic monitoring and multiphysics modelling of ground-borne vibrations from small wind turbines

Abstract

Wind energy is planned to play a major role in UK and Scottish Governments achieving renewable energy targets. The Southern Uplands of Scotland are a prime resource for wind and also home to the Eskdalemuir seismological station, a component of the International Monitoring System of the Comprehensive Nuclear-Test Ban Treaty.

Previous work demonstrated that large wind turbines generate vibrations at frequencies significant to
Eskdalemuir which are transferred into the ground and can be detected at many kilometres. In order to protect its capabilities, a 50 km consultation zone is enforced around Eskdalemuir for all new wind turbine developments, regardless of size.

In this thesis, an integrated approach combines multiphysics modelling and seismic monitoring to characterise the vibrations from small wind turbines (<50 kW) to assess their effect on Eskdalemuir. Four wind turbines, differing in power, hub height and tower structure, have been monitored using a combination of accelerometers and seismometers attached to the tower and buried in the ground at distances up to 200 m from the turbine.

Surface waves are shown to be the predominant wave type originating from the turbines; however, body waves are also present. The waves attenuate at a rate inversely proportional to the distance from the turbine, confirming that the sensors lie within the near-field radiation zone of the tower. Wind speed is shown to affect the tower vibration amplitude differently for each turbine.

Visualisation of the bending modes and radiation patterns in the ground have been obtained through multiphysics modelling and this, together with seismic monitoring, has permitted the frequency peaks in the monitored spectrum to be identified as originating from either the turbine or an alternative source.

Importantly, it has been shown that the algorithm currently used to assess wind farm vibrations around
Eskdalemuir may not be suitable for small wind turbines.