SITE SPECIFIC WIND TURBINE FOUNDATION CERTIFICATION

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1 SITE SPECIFIC WIND TURBINE FOUNDATION CERTIFICATION Mary Prabha, Nishikant Dash, Stefan Baars, GL Renewables Certification, 1st Floor, No.- 494, U.P Royal Building, 4th Cross, Sampige Road, Malleswaram, Bangalore , Republic of India ABSTRACT Site specific wind turbine foundation certification is an important part of the complete certification process of wind turbines. Certification of wind farms, wind turbines and their components are considered to be state-of-the-art and mandatory in most regions around the world. Certification facilitates in building the confidence of wind turbine. It also differentiates between different wind turbines. Furthermore, certification ensures safety and thereby, enabling exports. Site specific wind turbine foundation certification is necessary and mandatory as part of Site- Specific Design Assessment module in the Project Certification. The benefit of certification from GL Renewables Certification (GL RC) to wind turbine manufacturers, banks and insurers are to create a safe, reliable and successful onshore as well as offshore wind farm projects. The modules to obtain Type and Project Certificates are explained in detail according to the Guideline for the Certification of Wind Turbines, Edition A wind-turbine tower structure and its foundation receive a wide spectrum of millions upon millions of fatigue load cycles during the turbine's 20 year expected life span. The foundations of wind turbines are subjected to highly dynamic loading and these loads are extremely eccentric in nature. The wind's aerodynamic and mechanical forces concentrated at the top of the tower create a huge overturning moment at the tower base. Therefore, the wind-turbine foundation must be able to resist this large concentrated force. On an average in India, the cost of a wind turbine foundation amounts to a fraction of 5 to 10 percent of the total cost of onshore wind turbine. Nevertheless, the subcontractors who produce these foundations on behalf of plant operators are often under extreme pressure to cut costs. Therefore, the site-optimized foundation design makes the projects more competitive. And, it is really important to note that wind turbine foundation failure can cause complete failure of wind turbine system. Hence, site-specific foundation certification becomes necessary and essential for continuous and safe functioning of wind turbine as the life expectancy of wind turbine is 20 years. Moreover, the foundations should also suit the Indian site conditions. This paper focuses on the complete process of certification of site-specific wind turbine foundation considering the soil parameters, extreme and fatigue load cases and safety concept. 1. INTRODUCTION Certification of wind turbines has a history of more than thirty years. It has been applied differently in scope, requirements and depth in different countries, e.g. Denmark, Germany and Netherlands on the basis of their own rules and regulations. These countries are still leading in the development and application of certification rules. But during recent years a number of other countries such as India, China, USA and Japan as well as many banks that fund the projects realized the necessity of a thorough evaluation and certification of wind turbines and wind farms. Wind turbines are being installed in India since the late 1980s. India is one of the top five wind turbine markets in the world. In order to streamline the growth of industry, The Ministry for New and Renewable Energy (MNRE), made the requirement for Type Certification of wind turbines mandatory in India. The Centre for Wind Energy Technology (C-WET) which is under MNRE prepares and issues the Revised List of Models and Manufacturers (RLMM) of wind turbines periodically. Certification as per GL RC (e.g. [1]) is widely recognized by C-WET. To suit the Indian conditions, some type-certified wind turbines may undergo major/minor changes. Especially in case of wind turbine foundations which are designed site-specifically, they need to suit the Indian conditions. Many wind turbines in India are coming up in areas with high seismic vulnerability. Therefore, safety level of such foundations is an important requirement to

2 be followed. This criteria, makes the certification of wind turbine foundations in India very important in the present scenario. GL RC is an internationally renowned and operating certification body for wind turbines. GL RC is market leader in this area of expertise. As independent certification body, GL RC carries out numerous examinations, certifications and expertises. The company is also actively involved in the development of national and international standards and offers complete range of services for wind turbine and wind farm certifications including third party inspections and any kind of expertise reporting. Other certification bodies also use GL RC s Guideline for the Certification of Wind Turbines (Edition 2010) [1] and Guideline for the Certification of Offshore Wind Turbines (Edition 2005) [2] as their benchmark for certification. Both guidelines are regularly updated by Technical Notes which are available on GL RC s homepage ( Furthermore, GL RC is accredited to carry out certification in accordance with all relevant national and international standards in the field of wind energy [8]. GL RC s Guidelines (e.g. [1,2]) are used to carry out Type Certification and Project Certification of wind turbines. The scope consists of the examination of structural integrity, safety and compliance with these requirements. In the field of wind energy the focus lies on complete wind turbines or components such as rotor blades, gearboxes, generators, towers or foundation. Type Certification is performed for the complete wind turbine and Project Certification is performed for the complete wind farm. Furthermore components can be certified to enable sub-suppliers entering the market [8]. An independent assessment of wind turbines will ease approvals from regulation authorities worldwide. Financiers and insurers also provide favorable business conditions for the certified wind turbines as the certification improves product confidence and reduces the risk on their investment. In addition to this, certification of wind turbines influences customers in the selection of Original Equipment Manufacturers (OEM s). Certification of wind turbines helps successful market entry and market growth through increased competitive advantage. GL RC provides Type, Project and Component Certificates of wind turbines. In particular, Project Certification gives the possibility to assess technical integrity of wind turbine due to site specific demands (e.g. cold climate or wake effects in wind park configuration) to ensure reliability. Minimizing risks and building up confidence to investors, insurers, operators and authorities are the main aspects of a third party assessment within the Project Certification [8]. The benefit of Certification from GL RC is that the various certification processes and guidelines can be applied almost simultaneously, ensuring safe, reliable and successful wind projects onshore and offshore. The different modules in a Project certification are described in a flowchart, as shown in Figure 1. Figure 1: Modules of Project Certification

3 Site specific wind turbine foundation certification is an important and compulsory part of Site- Specific Design Assessment module in Project Certification. Site specific design assessment is useful for customers even without a Project Certification. As most foundations are designed sitespecifically, site specific foundation certification is important and can be done independently from a Project Certification as per the customer s requirements. 2. WIND TURBINE FOUNDATIONS AND THEIR IMPORTANCE Foundation is needed to support and absorb loads from the wind turbine. The choice of foundation type is entirely dependent on soil conditions and water table location prevailing at the planned site of a wind turbine. A safe and economic foundation design is an important part of all construction projects. As explained previously, wind turbine foundations design is very important as they are designed for significant dynamic and eccentric loads. Wind turbines are usually supported by either a slab foundation or a pile foundation. This further, depends on geotechnical conditions and foundation method of developing resistance to overturning forces. When the top soil is strong enough to support the loads from wind turbine, a slab foundation is preferred. But when the load carrying capacity of the top soil is not sufficient enough to support the loads from wind turbine, a pile supported foundation is preferred. This is practiced as loads are transferred to larger depths where stronger soils are present. The groundwater conditions along with the water table location below the foundation base should also be considered while determining the load carrying capacity of the top soil for the wind turbine foundation. In India in many cases, the foundation of a wind turbine is designed to meet the local site-specific requirements considering the type of soil and seismic zone in which they are founded. Figure 2, shows a picture of Wind turbine foundation construction in India. Figure 2: Wind turbine foundation construction in India 3. CERTIFICATION OF WIND TURBINE FOUNDATIONS Generally certification of wind turbine foundations can be performed in the following cases: 1. As part of the Type Certification of the wind turbine which includes design assessment and manufacturing evaluation of the foundation. 2. As part of the Project Certification of a wind farm as site-specific foundation examination. 3. Site-specific foundation design assessment in addition to an existing type certificate of a turbine, without applying for a complete project certificate. In India, the foundation design is mostly modified to suit the local site requirements and Indian conditions. Hence, the site-specific foundation design assessment becomes very important in the present scenario. However in many cases in India, site-specific foundation certification is not practiced or followed.

4 4. SITE SPECIFIC WIND TURBINE FOUNDATION CERTIFICATION Sometimes the type certification of a wind turbine does not include the foundation certification (optional according to GL RC s guideline) and, type certification ends at the tower bottom. The reason is that often standard type foundations are not planned by the designer but only site specific solutions are planned. The foundation designs for the specific sites are based on site conditions and local requirements. In India, the local site-specific requirements considering the type of soil, seismic zone should be evaluated while designing the foundation of a wind turbine. Other factor such as quality of available workmanship is also considered which could lead to an increase in cost by delays during the construction phase. The wind turbine foundation is critical as the loads from the tower are transferred to the soil through the foundation. Hence, wind turbine customers and financiers should obtain site specific certification of the foundation. However in India in many cases, the site-specific foundation design is verified by structural design consultants or educational and research institutes, but not certified by an accredited certification body like GL RC. Therefore, it is very important to consider GL RC for site specific foundation certification. Even when a customer does not require the complete project certificate, a site specific assessment of the foundation can be achieved. Even though site specific wind turbine foundation certification is an important part of the Project Certification, site specific wind turbine foundation certification can be done with/without the complete project certification. 5. PROCESS OF SITE SPECIFIC FOUNDATION CERTIFICATION The process of certification of a site-specific wind turbine foundation is described below as shown in Figure 3. For the assessment of the design of the foundation, the manufacturer should submit a full set of documents related to foundation to the certifying authority like GL RC. During the assessment, it will be verified that the wind turbine foundation is suitable for the intended site, soil conditions and that the requirements for the structural integrity of the foundation are met with due consideration of the external conditions. Figure 3: Modules of Site Specific Foundation Certification 5a. Load Assumptions The load assumptions required for site-specific foundation certification could be the following: a) in case where the site is covered by the type certified load assumptions, the tower bottom loads from the already certified load assumptions can be used and b) in cases when the site is not covered by the type certificate loads, a new load calculation must be done. If necessary or if the load calculation is not clear in some cases, a load calculation (with reduced scope) is done for the comparison of the site with the type class loads. Additionally for Indian site conditions, earthquake loads must be considered in areas with risk of occurrence of earthquake.

5 As a requirement for C-WET to consider earthquake loads for sites in India, GL RC also has provision to consider the same. 5b. Documentation The set of documents that the manufacturer submits for certification of foundation may include drawings, site specific geotechnical investigation report along with detailed design calculations, load calculation report, and construction instructions. The drawings should be true-to-scale design drawings and should contain all the necessary information and technical requirements. These drawings should include the representation of the foundation, geometry, along with detailed presentation of the reinforcement (reinforcement drawings). The material specifications and soil requirements for the intended site should also be mentioned in the drawings. The specification of turbine type and type class on the drawings is recommended. For site-specific certifications, a soil investigation report is very important. The report should contain statements on the ground with strata, faults, disturbances and Inclusions. The groundwater conditions along with soil and rock properties and their parameters should be included in the soil investigation report. The boundaries of the investigated areas and their boundary conditions should also be mentioned in a soil investigation report. The dynamic properties of soil such as the dynamic shear modulus should be provided in the soil investigation report in order to calculate the rotational spring stiffness. The detailed design documentation of the foundation submitted should include the loads for which the foundation was designed and the material parameters and relevant verifications. The load assumptions must be clearly stated in the documentation. An instruction for construction should be provided, specifying all steps and procedures for construction. The scope of documentation for certification may differ depending on the case and, will be determined by GL RC. 5c. Design assessment of foundation The loads on the foundation should be determined considering the different load cases. These load cases are described in GL RC s Guideline for the Certification of Wind Turbines (Edition 2010) [1] which is based on IEC [3]. The analysis should be performed with the least favourable of all combinations of actions according to the GL Guideline for the Certification of Wind Turbines (Edition 2010) [1]. Partial safety factor for the loads on soil should be applied on the characteristic loading according to the GL RC s Guideline for the Certification of Wind Turbines (Edition 2010) [1]. Additional loads resulting from imperfections and tilting of tower and foundation should also be considered as 3 mm/m tilting of the foundation for consideration of unevenly subsidence of ground. For the dynamic analysis, the distance of the natural frequency for the overall structure from the excitation frequencies is decisive in avoiding resonance. In the assessment of the expected natural frequencies, a parameter study is needed for the dynamic soil parameters; this should be defined so that a range of possible soil types and soil properties is covered. The required minimum values for the shear modulus G of the soil may be specified as dynamic values in relation to the Poisson s ratio ν. Static strength analysis and fatigue analysis of the structural steel components and reinforced concrete will be performed. The design will be verified applying the safety concept as described in GL RC s Guideline for the Certification of Wind Turbines (Edition 2010) [1]. Generally for the analysis of reinforced concrete and pre-stressed concrete, Eurocode 2 EN 1992 [5] should be applied. Relevant partial safety factors γ M are to be considered for concrete and steel components. For the components of steel in the foundation such as the embedded steel section in foundation, the relevant parts will be verified according to Eurocode 3 EN 1993 [7]. Exceptions from the above mentioned standards shall be agreed with GL RC. The safety level of the above

6 mentioned standards shall be fulfilled in any case. In India, the reinforced concrete designs for the foundations are done as per IS 456:2000 [4]. For components of reinforced concrete or pre-stressed concrete, detailed fatigue analysis should be provided for the concrete, the reinforcing steel and the pre-stressing steel with the loads of group F as described in GL RC s Guideline for the Certification of Wind Turbines (Edition 2010) [1]. The load spectra and corresponding mean values or Markov matrices will be used for verification of the fatigue strength. CEB-FIP Model Code 1990 [6] or equivalent shall be applied for fatigue verification. Slab foundations should be verified against tilting (eccentricity of load / gaping joint), bearing capacity failure, sliding, buoyancy and settlement of the soil. In pile foundations analysis of the external bearing capacity of piles should be performed for all loads as described in GL RC s Guideline for the Certification of Wind Turbines (Edition 2010) [1] and Eurocode 7 EN 1997 [9]. The maximum static pile loads should be determined and verified with the pile capacity. 6. CONCLUSION Wind turbine foundations are very important as these are designed for significant dynamic loads compared to building foundations. The design of wind turbine foundation is quite complex as the loading direction changes with wind direction and nacelle orientation. Vertical and shear forces along with significant overturning moments must be resisted within tolerances for foundation settlement and tilt which further increases the complexity in wind turbine foundation design. Failure of the wind turbine foundation leads to complete failure of the wind turbine system. Hence site-specific foundation certification is important and necessary also to meet Indian site conditions. Site specific wind turbine foundation certification can be done in addition to the type certificate and with/without the complete project certification as per the customer s requirements. The process of certification of a site-specific wind turbine foundation should be followed properly for continuous and safe functioning of the wind turbine for its expected life span. 7. REFERENCES 1) GL Renewables Certification, Hamburg, Germany: Guideline for the Certification of Wind Turbines, Edition ) GL Renewables Certification, Hamburg, Germany: Guideline for the Certification of Offshore Wind Turbines, Edition ) IEC : Wind turbine generator systems Part 1: Safety requirements, second edition February ) IS 456:2000 Indian Standard Plain and Reinforced Concrete Code of Practice. 5) DIN EN 1992, Eurocode 2, Design of concrete structures. 6) Comite Euro-International du Beton, CEB-FIP Model Code ) DIN EN 1993, Eurocode 3, Design of steel structures. 8) Woebbeking, M.: Type and Project Certification, Proceedings, Wind Power Shanghai, ) Eurocode 7, Geotechnical design Part 1 and 2.