Case study: Discovering the seismic effects of wind farms

March 2006

PDF article available

Güralp Systems' strong-motion and rapid deployment instruments were used in a recent study of the ground movements caused by wind turbines.

The UK Government Department of Trade and Industry has set targets for a significant increase in the use of renewable energy sources, particularly wind farms. These farms are operational in several locations around the country and offshore. The moors of southern Scotland, being both windy and sparsely-populated, are an ideal location for new windfarms.

However, to date, no wind turbines have been permitted within 80 km of the Güralp-operated Eskdalemuir seismic array, because it was thought that vibrations from the turbines would increase the background noise level there.

To determine the possible effect of building wind turbines near the array, a group at Keele University, led by Professor Peter Styles, performed experiments on two existing Scottish wind farms.

Studying a single turbine

CMG-DM24S12AMS.

The group used a Güralp DM24S12AMS acquisition and monitoring system to study vibrations in and around a wind turbine in Ardrossan, west Scotland. This is a complete digitizing and recording station with an on-board PC, which can gather data from up to 12 CMG-5U single-axis accelerometers and 6 additional CMG-5TD triaxial digital instruments.

The DM24S12AMS system with connections.

For this experiment, four CMG-5U instruments were fixed to glass plates on the inside wall of the wind turbine tower, with their sensitive axes pointing North, South, East and West directions. Two more uniaxial instruments were installed on the floor in a vertical configuration.

CMG-5U (left); 5Us installed inside the tower (right).

The central DM24S12AMS module was installed in the centre of the tower base and connected either to mains power via a 3-phase adapter, or to a 12V battery pack.

CMG-5TD

A CMG-5TD sensor was also deployed, buried in a shallow sand-filled pit a short distance from the tower. This sensor recorded ground movements, whilst the 5U instruments monitored the motion of the tower itself.

All data streams were automatically recorded by Scream! on the hard disk of the DM24S12AMS' built-in laptop computer. The data was downloaded later by connecting this computer to a local network.

The experiment was repeated at a second tower nearby.

Results

This experiment demonstrated that wind turbines do generate vibrations in the ground, at predictable frequencies related to the speed at which the blades rotate and the normal modes of vibration of the towers.

It also showed that wind turbines cause ground movements at a level proportional to the power output of the turbine. The scientists suggest that this could be improved by changing the design of the turbine foundations, so that they take into account seismic coupling as well as structural stability.

Studying an entire wind farm

CMG-6TD.

The first experiment used Güralp Systems' strong motion instruments to measure high-frequency structural vibrations near a turbine. At longer distances, these high frequencies are attenuated, so to study the effects of distant wind farms, the group used CMG-6TD medium motion velocity sensors. Velocity instruments are much more sensitive to ground motion at low frequencies than accelerometers.

In this experiment, 7 CMG-6TD instruments loaned from the sensor pool of SeisUK at the University of Leicester were deployed in a straight line at distances ranging from 1 to 17 km from the wind farm at Dun Law in the Scottish Borders, with an additional 3 forming a triangular array at a distance of 6 km.

Cross-section of a typical pit installation.

Each instrument was set to record data in RE-USE mode, saving all the streams it generated into its 3 Gb of Flash memory. This allowed the CMG-6TDs to run for approximately 55 days between visits to the site. At each visit, data was downloaded onto a 40 Gb Firewire disk with the flush command. This was repeated for the 4-month duration of the experiment.

Results

This experiment demonstrated that seismic disturbances from wind farms travel as seismic surface waves which can be detected many kilometres away. The group also found that the ground motion generated by a group of turbines is proportional to the square root of the number of turbines: thus, a wind farm of 25 turbines causes 5 times as much ground motion as a wind farm with a single turbine.

The Keele group was able to use these results, together with a separate infrasound experiment, to suggest that it would be possible to build wind farms on the Scottish Southern Uplands with a capacity of 1.6 GW and suggested that turbines of current design could be sited as close as 17.5 km from the array.

The Ministry of Defence accepted these results, and has now reduced the total exclusion zone around Eskdalemuir from 80 km radius to 10 km.

Read the full report from Prof. Styles' group.

Installation ideas

Strong-motion monitoring

The CMG-DM24S12AMS makes “plug and go” strong motion experiments simple to perform.
It can be connected to 12 CMG-5U single component instruments.
5U instruments can be installed in any orientation, either vertically or horizontally.
Up to 6 CMG-5TD or other Güralp digital instruments can also be connected.
The DM24S12AMS and 5TD include external triggering capabilities, so that the entire array can be triggered from any sensor.
Data can be stored on the DM24S12AMS' built-in PC or transferred over a local network using Scream!.
The DM24S12AMS can be powered either from the mains or a 12 V DC power supply. It can also recharge connected batteries when mains power is available.

Temporary installations

The CMG-6TD is designed for easy installation, and is ideal for short-term experiments in medium-noise sites.
Robust and waterproof, it produces good results buried in a plastic bag in a shallow pit.
6TD instruments can be supplied with up to 4 Gb of Flash memory.
Downloading data from a 6TD is as easy as plugging in a FireWire disk.
A set of five 6TD sensors is delivered in a smart case with power and USB hubs, to make data download even easier.



Home Products New installations Customer support About us Latest news The company Addresses and directions Local distributors Vacancies Live seismic activity	Printable version Case study: Discovering the seismic effects of wind farms March 2006 PDF article available Güralp Systems' strong-motion and rapid deployment instruments were used in a recent study of the ground movements caused by wind turbines. The UK Government Department of Trade and Industry has set targets for a significant increase in the use of renewable energy sources, particularly wind farms. These farms are operational in several locations around the country and offshore. The moors of southern Scotland, being both windy and sparsely-populated, are an ideal location for new windfarms. However, to date, no wind turbines have been permitted within 80 km of the Güralp-operated Eskdalemuir seismic array, because it was thought that vibrations from the turbines would increase the background noise level there. To determine the possible effect of building wind turbines near the array, a group at Keele University, led by Professor Peter Styles, performed experiments on two existing Scottish wind farms. Studying a single turbine CMG-DM24S12AMS. The group used a Güralp DM24S12AMS acquisition and monitoring system to study vibrations in and around a wind turbine in Ardrossan, west Scotland. This is a complete digitizing and recording station with an on-board PC, which can gather data from up to 12 CMG-5U single-axis accelerometers and 6 additional CMG-5TD triaxial digital instruments. The DM24S12AMS system with connections. For this experiment, four CMG-5U instruments were fixed to glass plates on the inside wall of the wind turbine tower, with their sensitive axes pointing North, South, East and West directions. Two more uniaxial instruments were installed on the floor in a vertical configuration. CMG-5U (left); 5Us installed inside the tower (right). The central DM24S12AMS module was installed in the centre of the tower base and connected either to mains power via a 3-phase adapter, or to a 12V battery pack. CMG-5TD A CMG-5TD sensor was also deployed, buried in a shallow sand-filled pit a short distance from the tower. This sensor recorded ground movements, whilst the 5U instruments monitored the motion of the tower itself. All data streams were automatically recorded by Scream! on the hard disk of the DM24S12AMS' built-in laptop computer. The data was downloaded later by connecting this computer to a local network. The experiment was repeated at a second tower nearby. Results This experiment demonstrated that wind turbines do generate vibrations in the ground, at predictable frequencies related to the speed at which the blades rotate and the normal modes of vibration of the towers. It also showed that wind turbines cause ground movements at a level proportional to the power output of the turbine. The scientists suggest that this could be improved by changing the design of the turbine foundations, so that they take into account seismic coupling as well as structural stability. Studying an entire wind farm CMG-6TD. The first experiment used Güralp Systems' strong motion instruments to measure high-frequency structural vibrations near a turbine. At longer distances, these high frequencies are attenuated, so to study the effects of distant wind farms, the group used CMG-6TD medium motion velocity sensors. Velocity instruments are much more sensitive to ground motion at low frequencies than accelerometers. In this experiment, 7 CMG-6TD instruments loaned from the sensor pool of SeisUK at the University of Leicester were deployed in a straight line at distances ranging from 1 to 17 km from the wind farm at Dun Law in the Scottish Borders, with an additional 3 forming a triangular array at a distance of 6 km. Cross-section of a typical pit installation. Each instrument was set to record data in RE-USE mode, saving all the streams it generated into its 3 Gb of Flash memory. This allowed the CMG-6TDs to run for approximately 55 days between visits to the site. At each visit, data was downloaded onto a 40 Gb Firewire disk with the `flush` command. This was repeated for the 4-month duration of the experiment. Results This experiment demonstrated that seismic disturbances from wind farms travel as seismic surface waves which can be detected many kilometres away. The group also found that the ground motion generated by a group of turbines is proportional to the square root of the number of turbines: thus, a wind farm of 25 turbines causes 5 times as much ground motion as a wind farm with a single turbine. The Keele group was able to use these results, together with a separate infrasound experiment, to suggest that it would be possible to build wind farms on the Scottish Southern Uplands with a capacity of 1.6 GW and suggested that turbines of current design could be sited as close as 17.5 km from the array. The Ministry of Defence accepted these results, and has now reduced the total exclusion zone around Eskdalemuir from 80 km radius to 10 km. Read the full report from Prof. Styles' group. Installation ideas Strong-motion monitoring The CMG-DM24S12AMS makes “plug and go” strong motion experiments simple to perform. It can be connected to 12 CMG-5U single component instruments. 5U instruments can be installed in any orientation, either vertically or horizontally. Up to 6 CMG-5TD or other Güralp digital instruments can also be connected. The DM24S12AMS and 5TD include external triggering capabilities, so that the entire array can be triggered from any sensor. Data can be stored on the DM24S12AMS' built-in PC or transferred over a local network using Scream!. The DM24S12AMS can be powered either from the mains or a 12 V DC power supply. It can also recharge connected batteries when mains power is available. Temporary installations The CMG-6TD is designed for easy installation, and is ideal for short-term experiments in medium-noise sites. Robust and waterproof, it produces good results buried in a plastic bag in a shallow pit. 6TD instruments can be supplied with up to 4 Gb of Flash memory. Downloading data from a 6TD is as easy as plugging in a FireWire disk. A set of five 6TD sensors is delivered in a smart case with power and USB hubs, to make data download even easier.