Attachment Holland:DAW 1-112 Exh. Pet.-ML-3 ICE THROW RISK ASSESSMENT FOR THE PROPOSED KINGDOM COMMUNITY WIND POWER PROJECT Client Green Mountain Power Corp. Contact Charles Pughe Document No 41404/AR/02A Classification Client’s Discretion Status Final Date 17 November 2010 Authors: Y Boucetta / P Heraud Checked by: MLeblanc Approved by: B Ait-Driss Holland-DAW_1-112-000 Page 1 Attachment Holland:DAW 1-112 Garrad Hassan America, Inc. Document : 41404/AR/02A IMPORTANT NOTICE AND DISCLAIMER This report is intended for the use of the Client on whose instructions it has been prepared, and who has entered into a written agreement directly with Garrad Hassan America, Inc. (“GH”). GH’s liability to the Client is set out in that agreement. GH shall have no liability to third parties for any use whatsoever without the express written authority of GH. The report may only be reproduced and circulated in accordance with the Document Classification and associated conditions stipulated in this report, and may not be disclosed in any public offering memorandum without the express written consent of GH. This report has been produced from information relating to dates and periods referred to in this report. The report does not imply that any information is not subject to change. Key To Document Classification Strictly Confidential : Recipients only Private and Confidential : For disclosure to individuals directly concerned within the recipient’s organisation Commercial in Confidence : Not to be disclosed outside the recipient’s organisation GH only : Not to be disclosed to non GH staff Client’s Discretion : Distribution at the discretion of the client subject to contractual agreement Published : Available to the general public © 2009 Garrad Hassan America, Inc. Holland-DAW_1-112-000 Page 2 Attachment Holland:DAW 1-112 Garrad Hassan America, Inc. Document : 41404/AR/02A CONTENTS Page 1 INTRODUCTION 1 2 ASSESSMENT SUBJECT 2 3 ICE THROW ASSESSMENT METHODOLOGY 4 4 DATA SOURCES AND OTHER INPUTS 6 4.1 Wind climate during icing events 6 4.2 Control methodologies 6 4.3 Assessment guidelines and data 7 5 RESULTS OF ICE THROW ASSESSMENT 9 5.1 Wind turbine icing 9 5.2 Technical feasibility of icing during operation 9 5.3 Individual risk 10 5.4 Control mitigation 16 6 CONCLUSIONS 17 REFERENCES 18 LIST OF TABLES 19 LIST OF FIGURES 20 Holland-DAW_1-112-000 Page 3 Attachment Holland:DAW 1-112 Garrad Hassan America, Inc. Document : 41404/AR/02A 1 INTRODUCTION Garrad Hassan America, Inc. (“GH”) has been contracted by Green Mountain Power Corporation (the “Client”) to undertake an assessment of the risk of ice fragments shed from wind turbines striking members of the public in the vicinity of some turbines from the proposed Kingdom Community Wind power project (the “Project”). The results of this work are presented in this report. 1 Holland-DAW_1-112-000 Page 4 Attachment Holland:DAW 1-112 Garrad Hassan America, Inc. Document : 41404/AR/02A 2 ASSESSMENT SUBJECT The proposed Project site is located Southwest of Orleans, Vermont. The site elevation is approximately between 680 m to 800 m. The Project consists of 21 wind turbines. As there is still uncertainty regarding the wind turbine models of the Project, The Ice Throw Analysis has been performed for three models of wind turbine, namely Siemens SWT-101, Vestas V112 and GE 2.75-100 model. The worst case scenario with the Vestas V112, the turbine model with the largest rotor diameter, is presented in this report. The key parameters of the wind turbine model are summarized in Table 2.1. Wind turbine model Vestas V112 RatedPower 3MW Rotor diameter 112 m Hubheight 84m Cut-in wind speed 3 m/s Cut-out wind speed 25 m/s Nominal rotor speed 17.7 rpm Nominal tip speed 103.7 m/s Table 2.1 Wind turbine parameters This assessment is focused primarily on the area surrounding each turbine as presented in Figure 2.1. Of particular interest is the risk of ice throw on the surrounding parcels from wind turbines 1, 5, 6, 8, 15, 19, 20 and 21, which are the closest to non-participating landowner property lines. 2 Holland-DAW_1-112-000 Page 5 Attachment Holland:DAW 1-112 Garrad Hassan America, Inc. Document : 41404/AR/02A Figure 2.1 Proposed locations and property line 3 Holland-DAW_1-112-000 Page 6 Attachment Holland:DAW 1-112 Garrad Hassan America, Inc. Document : 41404/AR/02A 3 ICE THROW ASSESSMENT METHODOLOGY The assessment methodology used herein is based on that developed by GH in conjunction with the Finnish Meteorological Institute and Deutsches Windenergie-Institut as part of a research project on the implementation of wind energy in cold climates (WECO) primarily funded by the European Union and also supported in part by the United Kingdom Department of Trade and Industry [1]. The guidelines for safety assessments in relation to ice throw were developed by GH in the WECO project and that work was summarized in a series of conference papers [2, 3, and 4]. These guidelines have been applied to the Project site by considering the proposed turbine type, the terrain of the site and surrounding area, and assumptions for human presence in the surrounding area. The overall approach is presented schematically in Figure 3.1 and is based on a staged approach: Determine the periods when ice accretion on structures is technically possible, based on historical climatic observations. Within those periods, determine when the wind speed conditions are within the operational range of the wind turbines. Within the resultant periods, if applicable, exclude those periods when the wind turbines will be shut down automatically by the wind turbine control system or by remote operators. Based on an estimate from the above of the amount of icing, use guidelines to arrive at probability of fragments landing at the distances from the turbines which are of interest. Where information is available, estimate probability of members of the public being present within the distances from the turbine which are being considered. Arrive at combined probability of members of the public being hit by ice fragments. Compare that probability to a suitable benchmark risk such as natural hazards. It is our professional opinion that this methodology is sound and provides an appropriate analysis of the Project. 4 Holland-DAW_1-112-000 Page 7 Attachment Holland:DAW 1-112 Garrad Hassan America, Inc. Document : 41404/AR/02A Historical reference station data Temperature Hours when icing Humidity conditions are Adjust to site Cloud cover present Cloud base Precipitation Observed evidence Validation from area (possibly anecdotal) Subset of hours when wind speed is Concurrent wind Adjust to site in turbine operating speed range Likelihood of unplanned shutdown due to ice (mainly sensor icing) Subset of hours Specification of when turbine is turbine and "ice" operating control system Shutdown by ice prevention / detection control system Assess risk of ice throw at distance of interest Estimate probability of public presence Estimate risk of Revise control people being struck strategy by ice fragments Evidence of fragment Acceptable risk? size and mass No Yes No further action Figure 3.1 Ice throw risk assessment procedure 5 Holland-DAW_1-112-000 Page 8 Attachment Holland:DAW 1-112 Garrad Hassan America, Inc. Document : 41404/AR/02A 4 DATA SOURCES AND OTHER INPUTS 4.1 Wind climate during icing events Climatic data recorded at the site during icing events have been provided to GH by the client. The data was recorded for each 10-minute period from sensors mounted on the meteorological tower (mast 808) from June 2003 through May 2008. A joint wind speed and wind direction table of the icing period (November to March) was derived from these measurements and used as the base meteorological input for this study. 4.2 Control methodologies Ice detectors are typically mounted to the nacelle of a turbine or nearby meteorological tower and monitored by the wind farm control system, triggering an automatic or remote manual shutdown of the wind farm in the event that icing conditions are detected. It is also generally accepted in the wind industry that any ice build up on the blades of an operating turbine will lead to additional vibration. This is caused by both mass and aerodynamic imbalances. All machines, including the Vestas V112, are equipped with vibration monitors, which will shut the machine down during these periods. Depending on the ice throw risk, it may be appropriate to implement a winter operating protocol that will curtail the operating of wind turbines in the event of icing and when extreme weather conditions present unsafe conditions for the general public. This typically involves operator or automatic system shut down under one or more of the following circumstances: The installed ice monitoring device(s) and heated wind sensors (installation subject to reliability testing) detect unsafe conditions are present due to icing conditions; Ice accretion is recognized by the remote or on site operator; Air temperature, relative humidity and other meteorological conditions at the site are conducive to ice formation; Air temperature is several degrees above 0 degrees C after icing conditions; and Any other weather conditions which appear unsafe. During any of these events, a typical operating protocol will provide that turbines which present a safety risk to the public are to be placed in Pause mode, in which the units are inoperative. 6 Holland-DAW_1-112-000 Page 9 Attachment Holland:DAW 1-112 Garrad Hassan America, Inc. Document : 41404/AR/02A 4.3 Assessment guidelines and data The guidelines produced in the WECO project were based on a combination of numerical modeling and observations. The numerical modeling involved Monte-Carlo simulations of a range of scenarios of ice building up on a wind turbine and being shed from the rotor blades.