Taking a Closer Look at Faulting and Seismic Hazards for Wind Farms
Across much of the world earthquake activity is a common occurrence. Although some places have more seismic activity than others, hazards from earthquakes may have a wider reaching long-term impact on wind farms than that expected by many in the design community. Even though a site may not have recent seismic activity, this does not preclude it from future seismic shaking or ground rupture. Within areas that have active seismic motion, seismic and fault hazard investigations are an important part of project development. For those not familiar with these types of investigations there are various review components which can be obscure without a guideline for interpretation. Even for technical specialists, variable code requirements and local geologic differences make individual project reviews difficult to achieve if someone is not familiar with a standard guidance document. The intent of this article is to discuss these considerations during wind farm seismic evaluations.
By Daniel E. Kramer, Petralogix Engineering, and Garret Hubbart, Neil O. Anderson & Associates, USA
By providing a detailed approach that is consistent with a standard guidance document for seismic hazard evaluations, potential risk avoidance and/or tolerance levels could be designed for and incorporated into a project during its initial development. Overall, this will reduce project costs and the long-term potential for structural failure, excessive maintenance, or repair due to seismic hazards. We recognise that an attempt for such a unified document has been made by some (e.g. ASCE/AWEA and their ‘Recommended practice for compliance of large land-based wind turbine support structures’ (ref. 1)). Documents like these become an excellent resource for all; however, such documents may be relatively short on detail and simplified on the area of seismic hazard consideration. We suggest that a more detailed discussion and application of specific code be compiled to aid in this area of design.
A Uniform Guidance Document
At present, a uniform guideline model that adequately addresses all necessary considerations for the assessment of seismic risk does not exist for wind farm development. This type of standard should be developed based on ‘Guideline models’, ‘Empirical relationships’, ‘Site-specific faulting and soil conditions’ and ‘Design guidelines for wind turbines’ and then incorporated into a single approach that is applied to all projects. If this were done, a uniformed approach for ‘appropriate due diligence’ that is focused on seismic probability factors and review techniques could be achieved.
General Fault Rupture Consideration
Guidelines for evaluating the hazard of surface fault rupture exist that are well developed (e.g. ref. 2) and these should be used for close review of any active seismic settings where faulting and fault rupture could occur. For brevity, we have included only the following points to consider: ‘As a practical matter, fault investigation should be directed at the problem of locating existing faults and then attempting to evaluate the recency of their activity’. The referenced guideline further discusses (1) the opinion on recency of motion along a fault, indicating a correlation of higher probability of future movement along a fault (over the long term) which has demonstrated historically recent motion, and (2) the consideration of the potential for faulting along older, previously inactive faults through reactivation. These are both important points to reflect on during a discussion of seismic hazard and their significance is demonstrated throughout this article.
Empirical Relationships Consideration
During review of any site, an often overlooked consideration is the project area’s historic earthquake activity profile. Quality data exists for most sites within the USA (ref. 3). Most countries have similar resources, and data for the last 100+ years is widely accessible via a longitude and latitude centre point. When this data is combined with examination of local fault zone widths, lengths and motion types (i.e. reverse, normal and strike slip motion) an estimation of maximum ground displacement can be achieved through comparison against known empirical relationships (ref. 4) (Figure 1).
Exploration and Characterisation Techniques
As part of the investigatory process, various techniques should be explored to confirm and identify soil types, groundwater conditions, voids, faulting, land-sliding potential, site-specific movement-motion clues and so on. A short tagline listing of some exploration and characterisation techniques includes: Direct field evidence, Indirect field evidence, Deterministic approaches, Probabilistic approaches, Deaggregation, Associated shaking risks, Empirical relationships, Site seismic histories, Qualitative hazard analysis, Quantitative hazard analysis, Assessment of model uncertainties and Acceptable risk tolerance.
Each of these techniques has individual components which are required to complete them in whole. For instance, direct field evidence as stated includes observations from trenches (Figure 2), borings and aerial photograph reviews. Indirect field evidence may be in the form of geophysical investigations or probability analysis. Much of this effort could be related back to a standardised or semi-standardised guideline for assessment of wind turbine projects. In defining terms such as the ‘taglines’ listed above, if a unified approach and model were to be accepted by the wind industry for sites, then additional certainty and congruency could be added to project design reviews.
Areas of Seismic Significance
In general, there are particular areas that are impacted by seismicity and areas which are relatively free from seismic motion. However, ground motions with peak horizontal accelerations of 0.3g or less are not uncommon for many areas and it is likely that under these conditions the governing lateral design loading to consider is wind rather than seismic. However, as accelerations increase above this level structural damage to associated wind farm facilities (substations, roads, power and water lines, etc.) becomes noticeable. At some point the design load consideration shifts and seismicity becomes the governing design element. But for the majority of the USA and world it is less of an issue, specifically when considering the turbine towers themselves.
The USGS map (Figure 3) shows the potential acceleration for the USA. It shows in particular that most sites in the Midwest are likely to have very little impact from seismicity. Areas around St Louis and the New Madrid Fault, as well as the Pacific West region, tend to have the most potential for significant site accelerations.
Worldwide Seismic Considerations
It is obvious from Figure 3 that there are many areas in the USA which will be affected in the future by seismic forces. These areas encompass a huge part of the potential wind energy projects within the USA. Likewise, it can be assumed that worldwide there are similar zones of development that are going to need the same level of review for seismic hazards. As companies continue to expand and develop their project portfolios into South America, New Zealand, Japan, Central Europe, Russia, Indonesia, Africa and China the issue of seismic risk and faulting will need to be better unified and studied. In the future, there will be sites which have failure and/or damage due to seismic forces that is significant. The well-studied sites, whose project team members used a thorough approach, will probably be better off than those that did not undergo rigorous study and incorporation of geologic details into turbine design.
Discussion
Faulting and seismic hazards can have deleterious effects on projects. If not properly characterised from an early stage, a great deal of expense may need to be incurred later in a project’s life when time is critical. By taking a proactive approach, the wind energy community may be able to have better outcomes on projects that are affected by these challenges.
Unified and fully accepted criteria, which incorporate a single approach, will help to simplify this hazard assessment process. If soil assessment and underlying geological conditions are not compared against design parameters then an accurate picture will not be developed for the site’s seismic hazards. In each of the risk assessment models previously listed the base geology and soil conditions come second to the structure and its general design. In some cases, the soil response and behaviour may not even be considered. This is not an appropriate methodology.
Conclusion
Measures can be taken to ensure stability of structures, safety to critical infrastructure grids, and safety to employees and the public. Starting from a unified approach to assessment (which integrates soil behaviour and potential seismic motions with empirical data), a comprehensive evaluation process could be developed for all renewable wind energy facilities. Seismic hazard studies when not properly performed or directed could yield results in which a particular project component (i.e. substation, tower, foundation or collection line) is damaged, endures catastrophic failure, or requires continual maintenance and/or repair. Therefore, such studies are an important part of any long-term wind power project.
Further Reading
1. ASCE/AWEA RP2011, Recommended practice for compliance of large land-based wind turbine support structures, American Wind Energy Association.
2. California Geological Survey, 2002, Note 49, Guidelines for evaluating the hazard of subsurface fault rupture.
3. United States Geological Survey, 2014, ANSS Advanced National Seismic System – Earthquake catalog inventory. Available at http://earthquake.usgs.gov/monitoring/anss/.
4. Wells, D.L. and Coppersmith, K.J., 1994, New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bulletin of the Seismological Society of America 84(4), 974–1002.
5. Kramer, S.L., 1996, Geotechnical earthquake engineering. Prentice-Hall International Series, p. 112.
6. USGS, 2014, 2008 NSHM figures. Available at http://earthquake.usgs.gov/hazards/products/conterminous/2008/maps/.
Biography of the Authors
Daniel Kramer is a Certified Engineering Geologist and Professional Geophysicist for Petralogix Engineering, Inc. with a career emphasis in seismicity, hydrology and structural geology. Daniel has extensive experience in performing fault, landslide and hydrological investigations and has led numerous geophysical and geological field explorations for large energy projects throughout the USA.
Garret Hubbart is a Professional Geotechnical Engineer for Neil O. Anderson & Associates, with a career emphasis on geotechnical design and site characterisation. Garret has overseen and provided expert evaluations for a variety of large wind development projects across the USA.
