WIND TURBINE FOUNDATION SYSTEMS SUPPLY CHAIN CHALLENGES AND MATERIALS SELECTION

IntroductionThe highest grade of anchor bolts currently specified in a wind turbine foundation is made from ASTM A722 steel of 150 ksi Ultimate Tensile Strength. The ASTM A722 high strength steel is manufactured by only one steel mill in North America and the other option from Europe is considerably more expensive. The single steel mill in North America that provides this material has only finite capacity. Further adding to this supply problem is the use of ASTM A722 threaded rods for Ground Anchor systems and Segmental Bridge construction. Creating additional capacity takes time and investment – time that the industry may not have as its growth proceeds at such a rapid pace. There may be some Engineering reasons for this material to be the sole option typically specified in a wind turbine foundation design when the loading requires it. However, there are other materials that can be used in place of ASTM A722 on most, if not all, projects that provide all the necessary mechanical properties required. More must be done to encourage use of alternate materials in wind turbine foundation designs. The wind energy industry is one of the more innovative and it should continue this through all areas of a project so as to avoid any future growth impediments.

As the wind energy industry continues to grow at a geometric rate in the United States and Canada, supply chain issues are becoming more prevalent. The recent passage of the three (3) year Renewable Energy PTC extension will foster continued rapid installation of wind energy in the United States. As wind energy becomes an even more vital part of securing America’s energy future it is important to eliminate obstacles which could hinder those efforts. As a point of focus, products for securing wind turbines to their foundations have a limited number of suppliers in the market. Of these products, typically referred to as anchor bolts, there is one particular grade (ASTM A722) that will cause a supply constraint in the wind industry and impede successful growth if no additional options are considered for specification into wind projects. The purpose of this presentation is to address this constraint and provide options to overcome it.

ASTM A722 Wind Industry Usage (Tons)

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1 – 20% Wind Energy by 2030: Increasing Wind Energy's Contribution to U.S. Electricity Supply, prepared by the U.S. Department of Energy

Imran Khan – Dayton Superior Corporation – Product Manager – 7777 Washington Village Dr, Suite 130, Dayton, OH 45459 imrankhan@daytonsuperior.com – www.daytonsuperior.com

Wind Energy GrowthAs you can see by the chart in Figure 1, the growth in wind energy installations has been at a geometric pace. This rate of installations is expected to continue its increase due to the three (3) year extension of the Production Tax Credit (PTC) passed late last year and an overwhelming need for alternative energy solutions. The AWEA and other industry groups have set out to aid in reaching a goal of having 20% of US energy production using wind power by the year 2030. This ambitious goal requires an increase in the number of wind installations up to 16,000 MW per year starting in 2018 and carrying through 2030.1 With the current environment and demand for alternative energy options fostering its growth wind energy has a strong advantage over other existing and currently more prolific forms of energy production. An additional source of influence is the desire for the United States to develop energy production alternatives to reduce or eliminate the need for purchasing energy production materials – such as oil – from other nations.

Wind Power Growth Effects on Supply ChainIt would be expected that such sustained growth in wind installations would also create stresses on the capacity of the supply chain for components and manufacturing of all materials needed to install a wind turbine. The anchor bolts are high strength steel threaded rods used with heavy hex nuts and washers to secure the massive wind turbine towers to their foundations. There is a limited number of manufacturers of these anchor bolt products and an even more limited supply of sources for the raw steel used in manufacturing the anchor bolts. The most specific area of constriction in the supply chain relates to ASTM A722 150 ksi steel. This particular steel product is manufactured by a special method whereas a hot rolled bar of steel is stretched to 80% of its Ultimate Tensile Strength and then stress relieved to achieve the high strength properties required in post-tensioned applications such as those in the wind industry. Due to limitations in the availability of equipment there is only one steel mill in North America that is capable of producing ASTM A722 steel. Currently the relative usage of ASTM A722 steel in anchor bolts compared to the other steels used is approximately 34% or just under 4000 tons for approximately 1700 out of 4900 total foundations. As the size of the wind turbine nacelles increases due to the need for larger MW capacity wind turbines, this further compounds the constraint.

Other Industries’ Effects on Steel AvailabilityThere are other industries that use ASTM A722 and this usage is not expected to decrease, but rather increase as with the wind industry. Ground anchor applications are typically governed by the suggested requirements listed in the Post-Tensioning Institute’s (PTI) Recommendations for Prestressed Rock and Soil Anchors. Ground anchors used in construction projects are four (4) to seven (7) times longer than those used to anchor wind projects. There is also a considerable amount of ASTM A722 used in segmental bridge construction which is also an application that continues to grow. The growth in these applications is expected to be further accelerated in the near term due in part to the most recent infrastructure spending that is part if the economic stimulus package. Additionally this growth can be expected to continue well beyond any effects created by the stimulus package due to the significant improvements required in the highway and bridge sectors in their current deteriorating state. Finally, there are many projects in the wind industry that require ground anchors to secure foundations due to the soil conditions of the project installation sites. These ground anchors are the very same as those referenced in the PTI Recommendations. Growth in the usage of ground anchors and segmental bridge construction are thus expected to have a significant impact on the availability of ASTM A722 for use in wind turbine anchor bolts.

Currently Used Alternate MaterialsThe most often used substitute for ASTM A722 steel is 4140 Quench and Tempered (Q&T) steel. This particular steel is not processed per the requirements of ASTM A722, but rather meets the Yield and Tensile strength properties call out by the specification. 4140 Q&T has been used as a substitute for ASTM A722 steel for a number of years mostly in ground anchors of sizes above 1-1/2” diameter because ASTM A722 is not domestically available in sizes larger than 1-1/2” diameter. The availability of this material is also limited due to limitations in the size of heat treat ovens at most steel mills. There is only one mill in North America that is capable of producing a 4140 Q&T steel bar over 24 feet in length. Currently this steel mill has reached its maximum output capacity and thus there is a constriction in supply of 4140 Q&T to the ground anchors market. This affects the wind industry because a large number of foundations requiring ground anchors utilize sizes above 1-1/2” diameter and, as mentioned, those sizes are only available to be manufactured using 4140 Q&T.

Need for a SolutionThe issues addressed in this presentation are provided to give focus to a potential supply chain shortcoming that will affect the ability of the wind energy industry to continue its opportunity to secure America’s energy future. As we move forward in the growth of wind energy supply we face an often overlooked portion of the supply chain which provides, literally, the foundation for the industry’s success and is one of the biggest threats to impeding its growth. It is imperative that the Engineering and Development community in the wind industry take a closer look to identify and allow acceptance of additional suitable steels used to manufacture anchor bolts for securing wind turbines to their foundations. If this need is not addressed now our success in reaching the ambitious goals that are needed for our energy independence will be jeopardized.