Seismic vibration produced by wind turbines in the Eskdalemuir region Release 2.0 of Substantial Research Project Presented to FMB Issue Date: 15/05/2014 Document No: FMB_203_FINAL_V5R This report has been prepared by Xi Engineering Consultants Ltd with full cooperation and significant input from the Ministry of Defence. It has been prepared on behalf of, and under the guidance of, the Eskdalemuir Working Group, whose members are the Scottish Government (chair), the Ministry of Defence, UK Government's Department for Energy and Climate Change, Renewable UK, Scottish Renewables and the Aviation Investment Fund Company Ltd. Xi Engineering Consultants Conference House, 152 Morrison Street, Edinburgh, EH3 8EB T +44 (0)131 200 6170 E [email protected] xiengineering.com Registered address: Xi Engineering Consultants Ltd, 5th Floor, 125 Princes Street, Edinburgh, EH2 4AD, Company no. SC386913 Document Summary Xi Engineering Consultants Ltd has conducted substantial analysis of the mechanisms and underlying physics of wind turbine seismic vibration propagation in line with the ‘Eskdalemuir Working Group agreed scope for the substantial research Stage One - dated 16 August 2013’. The aim of this research is to re-examine the Styles Model which is used by the Ministry of Defence to safeguard the detection capabilities of the Eskdalemuir seismic array with respect to seismic vibration produced by wind turbines. The research presented here builds on initial Phase 0 work (reported in Doc Ref FMB_102_TRP0_V6) which found that based on studies of Nordex N80 wind turbines there was the prospect for head room in the Eskdalemuir budget. This report supersedes Release 1.0 of Substantial Research Project (Final version released on 3/3/2014). The work presented in this document is valid for a three bladed, up-wind, horizontal axis wind turbine mounted on a tubular steel tower with a gearbox transmission under normal operating conditions. On-tower and far-field seismic vibration data has been analysed from four turbine types, which vary in maximum production capacity from 0.66 MW to 2.5 MW systems. In analysing these turbines it has become apparent that the Nordex N80 is not representative of all the turbines within the Eskdalemuir Consultation zone. As the Phase 0 work calculated the budget headroom based on N80 turbines it can be confirmed that the Phase 0 calculated headroom no longer remains valid. It has been found that the seismic vibrations produced by certain turbines are dominated by vibrations related to blade pass frequencies, whilst other turbines produce seismic vibrations related to their structural resonances. A physics-based algorithm was developed which was based on the conservation of energy approach in which seismic amplitude varied linearly with the swept area of the rotor and the cube of wind speed. The physics-based algorithm was fitted to empirical data from Craig, Clyde and Dun Law wind farms using a very conservative approach. The utilisation factor used from the 2005 Styles model has been re-analysed using recent concurrent wind data sets for an 80 m hub height from 14 locations across the consultation zone. Based on the same principles as the 2005 budget, the analysis of this more recent data provides a revised utilisation factor of 82%. However as the approach to the algorithm has changed a utilisation factor of unity has been used until, and if, further work is conducted in this area. The MoD released revised tables with all wind turbines which have been allocated budget and those subject to MoD objection up to and including the end of December 2013. The cumulative rms amplitude of the December 2013 MoD tables was calculated with the physics-based algorithm, frequency-distance weighting function and utilisation factor and found to be 0.1938325 nm. This figure is significantly less than the 0.336 nm threshold. An assessment has been made of the developable land for wind turbines between 10 km and 50 km from the array. A method for understanding the physical implications of an exclusion zone has been developed, and different scenarios investigated. Xi Engineering Consultants Ltd recommend that the 2005 model currently used by the MOD to safeguard Eskdalemuir seismic array be replaced by the physics-based algorithm presented in this report. FMB_203_FINAL_V5R Page 2 15/05/2014 All rights reserved ©2014 Xi Engineering Consultants Ltd. This report has been written by Xi Engineering. All rights are reserved. No part of this report may be reproduced, modified, stored in a retrieval system or transmitted in any form or by any means without prior written permission. Matters relating to this document should be directed to: Dr Mark-Paul Buckingham E: [email protected] Managing Director T: 0131 200 6171 Principal contacts at client’s organisation Tim French E: [email protected] Chair of Eskdalemuir Engineering Group, Fund Management Board subgroup FMB_203_FINAL_V5R Page 3 15/05/2014 All rights reserved ©2014 Xi Engineering Consultants Ltd. Contents 1 Introduction .................................................................................................................................................... 8 2 Technical background ..................................................................................................................................... 9 2.1 Eskdalemuir seismic array ..................................................................................................................... 9 2.2 Previous studies .................................................................................................................................. 11 2.3 Ground vibration produced by wind turbines ..................................................................................... 13 3 Field survey ................................................................................................................................................... 14 3.1 Introduction ........................................................................................................................................ 14 3.2 Methodology ....................................................................................................................................... 14 3.2.1 Dun Law wind farm ......................................................................................................................... 14 3.2.2 Craig wind farm ............................................................................................................................... 16 3.2.3 Clyde wind farm .............................................................................................................................. 18 3.2.4 St Seine-l’Abbaye wind farm ........................................................................................................... 20 3.3 Data processing ................................................................................................................................... 22 3.3.1 Variations to the primary processing methodology ....................................................................... 22 3.4 Statistical analysis ............................................................................................................................... 27 3.5 Results ................................................................................................................................................. 28 3.5.1 Dunlaw ............................................................................................................................................ 28 3.5.2 Craig wind farm ............................................................................................................................... 30 3.5.3 Clyde wind farm .............................................................................................................................. 36 3.5.4 St Seine-l’Abbaye ............................................................................................................................ 40 3.6 Azimuth effect ..................................................................................................................................... 41 3.6.1 Dun Law .......................................................................................................................................... 41 3.6.2 Craig wind farm ............................................................................................................................... 43 3.6.3 Clyde ............................................................................................................................................... 44 3.7 Discussion ............................................................................................................................................ 48 3.7.1 Comparison of far-field spectra from different wind turbines ....................................................... 48 3.7.2 Characterisation of far-field seismic vibrations .............................................................................. 49 3.7.3 Relationship between on-tower and far-field seismic vibrations ................................................... 49 3.7.4 Azimuth effect interpretation ......................................................................................................... 50 4 Broadband operational noise ....................................................................................................................... 51 4.1 Introduction .......................................................................................................................................