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3 Standards and Practices This chapter addresses Task I of the committee’s charge—“Standards and Practices” (see Box 1-2). It provides background on and a summary of the applicable regulations, standards, recommended practices, and guidelines that have been used in the offshore wind industry, and it describes the state of maturity of each of these documents. The terms “regulations,” “stan- dards,” and “guidelines” are discussed in Box 3-1. In its review of standards and practices, this chapter discusses technical terms related to risk assessment, strength analysis, and other areas. Deﬁ- nitions of these terms can be found in the glossary, and some are discussed further in Appendix A. INTERACTIONS BETWEEN NONSTRUCTURAL FAILURES AND WIND TURBINE STRUCTURAL INTEGRITY Although the committee’s charge is limited to structural integrity (see Chapter 1), malfunction or failure of nonstructural components and systems during operation can result in structural overload or failure. This interaction is dealt with through the deﬁnition of “design load cases” (DLCs) in standards and guidelines. Such cases specify the combination of loads that a facility must be designed to resist or withstand. Although the committee has not reviewed the DLCs in detail, it notes that DLCs nor- mally include the structural loads placed on the turbine as a result of fail- ure or malfunction of ancillary systems such as control systems, protection systems, and the internal and external electrical networks. In such DLCs, failures in ancillary systems are normally postulated as occurring under unfavorable wind and wave conditions. For example, in International Electrotechnical Commission (IEC) 61400-3, DLC 2.3 involves both an 38
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Standards and Practices 39 BOX 3-1 Regulations, Standards, and Guidelines The use of various terms to describe technical guidance is com- mon among engineering disciplines. Some terms have speciﬁc and generally accepted deﬁnitions, and others are less precise. The fol- lowing describes the terms used throughout this document and the class of documents to which they refer, with some background on how these documents are typically developed. Regulations. Regulations are sets of requirements promul- gated by government authorities. Although they may be inter- national and implemented by way of treaties (for example, International Maritime Organization regulations applicable to international shipping), regulations are generally established at the national and state levels. Rules and regulations devel- oped by the various U.S. federal agencies are codified in the Code of Federal Regulations. Standards. A standard is a document that has been developed in accordance with a protocol. Diverse interests are represented, there is a process for resolving opposing opinions, and the ﬁnal version is adopted by a consensus vote of the constituencies involved. Examples of organizations that follow a recognized standards development process are the International Organiza- tion for Standardization, the International Electrotechnical Com- mission (IEC), the American National Standards Institute, the American Wind Energy Association, and the American Petro- leum Institute (API). Standards may be international, national, or industry-speciﬁc in scope, and the term “standard” may not be present in the title. In this report, “standard” refers to any docu- ment developed according to a recognized process and subject to a vote of constituencies to establish a consensus before becoming (continued on next page)
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40 Structural Integrity of Offshore Wind Turbines BOX 3-1 (continued) Regulations, Standards, and Guidelines ﬁnal. Examples of standards referred to in this report are IEC 61400-3 and API RP 2A. Guidelines. A guideline is a document that has been developed by a group or a company and that is not subject to a formal pro- tocol or a vote of constituencies. These documents are typically vetted through an internal quality process and may be peer reviewed, but they are ultimately the product of the group or company, and no consensus is required for their completion or use. In this report, “guideline” refers to any document devel- oped by a group or company for which no recognized protocol or consensus vote is necessary. Examples of guidelines referred to in this report are Guideline for the Certification of Offshore Wind Turbines, developed by Germanischer Lloyd; Design of Offshore Wind Turbine Structures, developed by Det Norske Veritas; and Guide for Building and Classing Offshore Wind Tur- bine Installations, developed by the American Bureau of Ship- ping (ABS 2010). extreme operating wind gust and loss of the electrical network. Other examples require consideration of yaw misalignment that might result from mechanical or electrical failure and consideration of what emergency procedures might be needed to cope with structural damage caused by nonstructural triggers such as overspeeding, brake failures, and lubrication defects. In sum, the standards and guidelines that will likely be used in the structural design of offshore wind turbines for the United States and that will inform the work of certified verification agents (CVAs) consider how nonstructural components can trigger structural failures in offshore wind turbines.
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Standards and Practices 41 INTERNATIONAL ELECTROTECHNICAL COMMISSION Background on Land-Based Wind Turbines: Historical Perspective During the early 1990s, the wind energy industry—through IEC—began to establish international standards for land-based wind turbines. There were at least two motivations for establishing international standards: • The existing European design standards (e.g., in Denmark, Germany, and the Netherlands) were insufﬁcient in that they did not result in reliable performance over the 20-year design life of the turbines. Many wind turbines experienced breakdowns in major components, such as gearboxes and blades, after less than 10 years of operation, leading to excessive downtimes. • The industry wanted to make sure that all wind turbines complied with the same standard so that price competition could take place on a uni- form basis (excluding substandard wind turbine designs). The United States saw the IEC standards activities of the 1990s as a way to provide a fair and uniﬁed approach to the emerging world wind energy market and has participated in the development of the IEC standards since their inception. Technical Committee 88 (TC 88) was established to develop and manage a suite of applicable standards for wind turbines. Description of Relevant Standards The primary standard for wind turbine structural design requirements is IEC 61400-1 Ed. 3 (IEC 2005). This standard deﬁnes design classes, external (environmental) conditions for each design class, DLCs, fault con- ditions that must be included in the design, procedures for assessing static and dynamic loads, electrical requirements, and methods for assessing the site-speciﬁc suitability of the turbine. Perhaps the most important part of the standard is a detailed deﬁnition of the turbulent wind environment. Because understanding the minute characteristics of wind is so important in assessing unsteady aerodynamic load distributions along the rotating blades, it is crucial that this part of the external conditions be deﬁned in a manner consistent with the analytical theory used for rotor load estimation.
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42 Structural Integrity of Offshore Wind Turbines In 2000, TC 88 began to develop an offshore wind turbine standard, Design Requirements for Offshore Wind Turbines, IEC 61400-3 (IEC 2010a). It was intended to address requirements for offshore wind turbines that were not previously covered. The standard defers to IEC 61400-1 for the wind turbine aspects of the design requirements and relies on existing mature standards for setting general support structure requirements. The IEC offshore committee surveyed structural standards and guidelines for offshore oil and gas structures, including those developed by the American Petroleum Institute (API), the International Organization for Standardization (ISO), Det Norske Veritas (DNV), and Germanischer Lloyd (GL), and attempted to use them as the basis for the new IEC 61400-3 requirements. A European-funded project, “Requirements for Offshore Wind Turbines” (RECOFF), included formal comparisons of these various standards and assessed their suitability for wind turbine design. The RECOFF study concluded that, for the vast majority of sup- port structure requirements, standards such as those of API and ISO could be used. However, the crucial deﬁciency was the manner in which dynamic loads were estimated. Offshore wind turbines are subject to wind and wave stochastic loadings that are nearly equal in importance with respect to dynamic excitation of the wind turbine. IEC 61400-3 is the only international standard that speciﬁcally addresses these issues. It is less mature (less fully developed) than other international standards and guidelines for land-based wind turbines, but it is based on earlier standards and therefore represents an integrated version of all the work that has preceded it. Because it is part of a series of international stan- dards that address the broader wind industry’s needs, such as veriﬁca- tion testing for performance, structural design compliance, power quality, gearbox design requirements, and small turbines, it is the best available standard for addressing the issues of structural design for off- shore wind turbines. The IEC certiﬁcation standard for type and project certiﬁcation is IEC 61400-22, Wind Turbines—Part 22: Conformity Testing and Certiﬁcation (IEC 2010b). This standard deﬁnes requirements for both type certiﬁcation and project certiﬁcation. The IEC 61400-22 certiﬁcation standard is a fur- ther development of the previous certiﬁcation standard, IEC WT 01 (IEC 2001), in particular with regard to requirements for project certiﬁcation.
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Standards and Practices 43 Turbine Type Certiﬁcation Process There are few legal requirements for structural design in land-based U.S. wind energy installations, and no single agency has full responsibility. The structures must meet local and state building codes, and the electrical sys- tems must meet electrical standards. These codes and standards are inad- equate for deﬁning wind turbine design requirements, and there is no overarching permitting process that addresses structural design. How- ever, this approach does not appear to have impeded the industry or become a detriment to public safety. Instead of relying on statutory regulations, the process is commercially driven. Owners and operators choose to require type-certiﬁed wind turbines for their projects. The type certiﬁcation process is outlined in Figure 3-1. The turbines are usually certiﬁed to IEC or other European standards. Recognizing that the offshore certiﬁcation process is unique, TC 88 has begun to draft a second edition of its wind turbine certiﬁcation process, IEC 61400-22 (IEC 2010b). The new edition will rely on IEC 61400-3 for offshore technical requirements while deﬁning the certiﬁcation process. Both IEC 61400-3 and WT 01 Ed. 2 assume that the turbine will be certi- ﬁed to a set of design classes speciﬁed in IEC 61400-1 Ed. 3, whereas the support structure is designed to site-speciﬁc conditions. The IEC standards development process assumes that multiple parties will be responsible for different aspects of the project and offers guidance for each phase of the project. It allows for the use of other standards for the support struc- ture, such as API RP 2A-LRFD-S1 (API 1997), DNV guidelines, and GL (Optional) (Optional) Design Type Testing Manufacturing Foundation Characteristic Evaluation Evaluation Design Evaluation Measurement Final Evaluation Report Type Certificate FIGURE 3-1 Type certiﬁcation process under IEC 61400-22.
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44 Structural Integrity of Offshore Wind Turbines Windenergie Group speciﬁcations (though the latter two guidelines are heavily inﬂuenced by the API offshore standards for their offshore support structure guidance). However, some of the speciﬁcations of API RP 2A are not adequate for the design of offshore turbines, for which dynamic time- dependent behavior must be determined as accurately as possible by using, for example, modern time-domain analysis methods. Foundation designs are integrated into the type certiﬁcation for some turbines. Where this is the case, the foundation design must be evaluated for the external conditions for which it is intended. Poor geotechnical inves- tigation and foundation design have led to delays and cost overruns at European wind farms (Gerdes et al. 2006). Project Certiﬁcation Technical design requirements (IEC 61400-3) typically are separated from certiﬁcation procedures (IEC 61400-22). The latter standard deﬁnes the certiﬁcation process and relies on technical standards such as IEC 61400-3 to specify the design requirements. The overall certiﬁcation qual- ity system needed to implement the full process from design through manufacturing, installation, continuous monitoring, and decommission- ing requires management procedures. Project certiﬁcation is covered under IEC 61400-22 (see Figure 3-2). According to this standard, the pur- Type Certificate Design Basis Site-Specific Site Manufacturing Transport / Site Assessment Surveillance Installation / Assessment Commissioning Surveillance Project Periodic Certificate Monitoring FIGURE 3-2 Project certiﬁcation process under IEC 61400-22.
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Standards and Practices 45 pose of project certiﬁcation is to determine whether type-certiﬁed wind turbines and their integrated foundation designs conform to the exter- nal conditions, applicable construction and electrical codes, and other requirements of a speciﬁc site. Under this process, the external physical environmental conditions, grid system conditions, and soil properties unique to the site are evaluated to determine whether they meet the requirements deﬁned in the design documentation for the wind turbine type and foundations. Wind turbines and their support structures are mass produced, as opposed to the customized design approach typically applied for offshore oil and gas installations. Final permitting of wind power plants results in the installation of many turbines of the same design type (hence the term “type certiﬁcation” for a turbine that meets a generic design class, rather than site-speciﬁc environmental conditions). Although it is likely that the same design has operated in other sites, a new installation must integrate the environmental and physical conditions of the site into the engineering evaluation of suitability for the site. IEC recognizes that offshore turbines will be designed and tested long before most projects are even conceived. Thus, the IEC standards require and give guidance for evaluating the suit- ability of a type-certiﬁed turbine for speciﬁc site conditions. API STANDARDS Background on Oil and Gas Facilities: Historical Perspective API standards were developed with a focus on offshore facilities for oil and gas and include, among other items, wind–wave–current models, analysis approach, and structural and foundation design parameters. API RP 2A is the primary standard used by the offshore oil and gas industry for the structural design of ﬁxed offshore structures, which are the most similar to traditional offshore wind structures, but API has additional standards for offshore ﬂoating structures, including API RP 2T, API RP 2FPS, and API RP 2SK. These standards represent more than 60 years of design experience. Although they were primarily developed to address the offshore oil industry in the Gulf of Mexico, the API series has become a comprehensive set of standards that is used internationally. In sup- port of the recommended practices, additional documents such as 2MET
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46 Structural Integrity of Offshore Wind Turbines (Oceanographic and Meteorological) and 2GEO (Geotechnical) have been developed to address conditions applicable to both ﬁxed and ﬂoat- ing structures. API has been engaged with ISO in developing an ISO series of offshore standards using many of the API standards as their base documents. More than 80 percent of the ISO 19900 series has been published. API has restructured about 50 percent of its offshore series to match the ISO struc- ture and incorporate the ISO standards. This integration provides for a single international set of offshore standards with U.S.-speciﬁc criteria attached to the universal core technical requirements. Description of Relevant Standards The API Series 2 standards are comprehensive and cover all aspects of off- shore design: planning requirements, installation requirements, ﬁxed and ﬂoating platform structural requirements, operations throughout the life of the system, and decommissioning requirements. For structural design, API RP 2A-WSD, the commonly applied standard for ﬁxed offshore plat- forms, uses an elastic component design methodology prescribing load development procedures, structural design methods, extreme load condi- tions, material and component safety factors, and the character and return periods for design-level extreme events for both sea states and wind conditions. The standard focuses mainly on sea states rather than wind because that is the primary source of platform loads (usually about 70 per- cent of the total load on a ﬁxed platform). Detailed wind conditions are frequently characterized on the basis of a quasi-static load deﬁnition, which is generally sufﬁcient for a statically responding facility. For dynam- ically sensitive facilities, wind loading is usually developed by using an offshore-speciﬁc wind spectrum model. IEC AND API DIFFERENCES Standards such as IEC 61400-3 and API RP 2A have some overlapping design requirements for wave and current loading conditions. However, a direct comparison of the IEC and API standards indicates differences that should be assessed in any effort to use these standards together for
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Standards and Practices 47 the U.S. offshore wind industry. The following are examples of differences between the IEC and API standards: • IEC uses a 50-year return period for the deﬁnition of extreme envi- ronmental design conditions, while API RP 2A uses a 100-year return period for the deﬁnition of design conditions for high-consequence platforms. • The probability of exceedance of load levels (or, equivalently, the return period of the wind–wave–current loading), for example at a 50- or 100-year return level, constitutes only one element determining the fail- ure probability, or the probability of acceptable performance, of a facil- ity. Equally important are the inherent safety factors accounting for knowledge uncertainties (due to incomplete or otherwise limited infor- mation concerning a phenomenon) and material factors, load combi- nation requirements, parameters inherent in interaction equations, and so on. These aspects are often disregarded in risk discussions but can affect failure probabilities more than could a factor of two or three in the return period of the loading. Therefore, a careful assessment is needed to determine the overall failure probability in either or both of the standards. • The deﬁnitions of DLCs are different. IEC requires the structure to be veriﬁed for normal and abnormal conditions together with speciﬁc load cases in close association with the wind turbine’s operational sta- tus. API requires the structure to be veriﬁed for operational conditions, normally a 1-year storm, and extreme conditions, which are deﬁned primarily by using environmental conditions. • API RP 2A prescribes three levels of design based on consequence. These levels are characterized by decreasing loads for decreasing con- sequence. In contrast, IEC keeps the load level constant while adjust- ing component safety factors on the basis of the consequence of that component failing. • API RP 2A provides a basis for the design of offshore structures sub- ject to wave, wind, current, and earthquake loading conditions in addition to loads from drilling, production, and ongoing personnel activities. API RP 2A does not address the scope and range of all con- ditions relating to the design of wind turbine support structures such
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48 Structural Integrity of Offshore Wind Turbines as blade–wind–tower interaction and presence or absence of yaw con- trol. Similarly, IEC 61400-3 lacks some of the detailed provisions given by API RP 2A with respect to certain offshore engineering practices. It is important for the industry to develop a full understanding of the differences in the requirements and overall performance levels inherent in these codes. This comparison should seek to clarify the relative levels of structural reliability inherent within each code when applied to a wind tur- bine project at a speciﬁc location and to evaluate the similarities and dif- ferences in the consequences of failure (either loss of function or collapse of the structure) for the types of facilities. One ﬁnal issue is that ﬂoating platforms for wind turbines are explicitly not covered by IEC 61400-3. Research will be necessary to deﬁne all issues that may affect the design of such a structure. Such issues are likely to include hydrostatic and hydrodynamic stability, coupled aerodynamic loading from the rotor and wave loading, station keeping, and electrical distribution system connections for a highly compliant support structure. ISO STANDARDS As described previously, the ISO 19900 series of standards addresses off- shore platforms for the oil and gas industries. These standards were based on existing API standards for ﬁxed steel and ﬂoating structures and on a Norwegian standard, the leading offshore concrete standard. The over- sight groups (work groups under ISO TC 67/SC 7) for these ISO standards are establishing an ongoing updating and maintenance process now that the ﬁrst version of the standards has been published. To meet industry needs while the European Union standards requirements were developed, the load and resistance factor design (LRFD) version of API RP 2A was adopted as an interim ISO standard. An international committee structure with considerable U.S. and API leadership and engagement developed the second version of the Fixed Steel Platform standard (ISO 19902), as well as a suite of accompanying general offshore standards: ISO 19903 (Fixed Concrete Structures), ISO 19904 (Floating Systems), ISO 19905 (Jackups), and ISO 19908 (Arctic Structures). A full description of the ISO and API work programs is given by Wisch et al. (2010). This ISO series harmonizes international practices into a single, integrated suite of standards. The ISO
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Standards and Practices 51 AMERICAN BUREAU OF SHIPPING ABS has been at the forefront of developing guidelines for the offshore oil and gas energy sector since the industry’s formative years, but it is a new- comer to the offshore wind ﬁeld. The ABS Guide for Building and Class- ing Offshore Wind Turbine Installations (ABS 2010) was developed by harmonizing ABS experience from offshore oil and gas platforms with the guidelines provided in the IEC 61400 series of documents. Requirements on the following subjects are speciﬁed in the guide for the support struc- ture of a bottom-founded offshore wind turbine: • Classiﬁcation, testing, and survey; • Materials and welding; • Environmental conditions; • Load case deﬁnitions; • Design of steel and concrete structures; • Foundations; and • Marine operations. Requirements with regard to the survey during construction and instal- lation and the survey after construction are generally in accordance with established ABS rules for offshore structures. Alternative survey schemes are also acceptable to account for the uniqueness of offshore wind tur- bines, such as serial fabrication and installation. Design environmental conditions and DLCs required by the ABS guide are generally in agreement with those required by IEC 61400-3 but have a number of amendments, mainly to account for the effects of tropical hurricanes in U.S. waters. The principle of site-specific design is addressed in the definition of the DLCs in the guide. Envi- ronmental conditions with a baseline return period of 100 years are required to be considered for the extreme storm conditions (DLCs 1.6, 6.1, and 6.2). Furthermore, the omnidirectional wind condition is required for turbines subject to tropical hurricanes, cyclones, and typhoons (DLC 6.2). The established ABS rules and guides for offshore structures, as well as API RP 2A, have been discussed to provide a technical basis for the devel- opment of support structure and foundation design criteria. The guide specifies a set of design criteria for steel support structures by using a
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52 Structural Integrity of Offshore Wind Turbines working stress design approach, which is still accepted as a common design practice in the United States. Allowable stress levels are deﬁned for various design conditions, including normal, abnormal, transport, and installation on site, as well as earthquake and other rare conditions. Equivalent LRFD criteria are also speciﬁed as an acceptable alternative. The requirements for electric service platforms are addressed in the ABS Rules for Building and Classing Offshore Installations. This document, the ﬁrst edition of which was published in 1983, is used in the veriﬁcation of bottom-founded structures worldwide. GERMAN STANDARDS AND PROJECT CERTIFICATION SCHEME The Federal Maritime and Hydrographic Agency (Bundesamt für Seeschifffahrt und Hydrographie, or BSH) is the agency in Germany that decides on the approval of offshore wind farm development projects in the North Sea and the Baltic Sea. It carries out the application procedure for offshore wind farms in the German Exclusive Economic Zone, which is the area outside the 12–nautical mile zone where most of the German offshore wind farms will likely be installed. Part of the approval procedure is to examine whether all installations and structural components have been certiﬁed according to the BSH standard Design of Offshore Wind Turbines (BSH 2007), which was issued in June 2007. This standard covers development, design, implementation, operation, and decommissioning of offshore wind farms within the scope of the Marine Facilities Ordinance and regulates the various structural components of an offshore wind farm. It refers to another BSH standard, Standard for Geotechnical Site and Route Surveys—Minimum Require- ments for the Foundation of Offshore Wind Turbines, issued in August 2003. To develop these standards, BSH established a steering committee that included technical experts in relevant ﬁelds and representatives of three classiﬁcation and certiﬁcation societies (SGS, DNV, and GL). BSH requirements for project certiﬁcation are set forth for each of the following phases: Phase I. Development, Phase II. Design,
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Standards and Practices 53 Phase III. Implementation, Phase IV. Operation, and Phase V. Decommissioning. The certiﬁer or registered inspector company is to be selected from a preapproved list of BSH-preapproved offshore wind energy certiﬁcation companies. The list currently consists of SGS, DNV, GL, and DEWI Off- shore. Companies can apply for approval as offshore wind energy certiﬁ- cation companies. For a given project, one certiﬁcation company could cover one phase (e.g., design certiﬁcation) and others could cover other phases. For exam- ple, a second company could cover implementation (manufacturing, transport, and installation), and a third could cover operation. BSH is the ﬁnal approval authority for all ﬁve phases. It reviews the design and certiﬁcation documentation itself in determining whether to grant ﬁnal approval of a project phase. In the process, BSH is often sup- ported by individual external technical experts with speciﬁc knowledge of that phase—for example, a geotechnical expert for Phase I and a wind tur- bine expert for Phase II. ONGOING STANDARDS DEVELOPMENT AND RELATED RESEARCH: NATIONAL AND INTERNATIONAL American Wind Energy Association Development of Offshore Recommended Practices In October 2009, the American Wind Energy Association (AWEA), in conjunction with the National Renewable Energy Laboratory, initiated an effort to develop a set of recommended practices for assessing the local, national, and international standards and guidelines that are being used for all wind turbines in the United States and to make recommendations on their use and applicability. The effort is aimed at three major areas where current standards (and related guidelines and other such docu- ments) are ambiguous or have signiﬁcant gaps when applied in the United States. One of these areas is offshore wind energy. The offshore wind energy group will address all areas that are rele- vant to the Bureau of Ocean Energy Management, Regulation, and
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54 Structural Integrity of Offshore Wind Turbines Enforcement (BOEMRE) project application and approval process. These areas include structural reliability; manufacturing, qualiﬁcation testing, installation, and construction; safety of equipment; operation and inspec- tion; and decommissioning. The AWEA initiative has enlisted expert stakeholders from the offshore industry community to develop a consensus set of good practices in the use of standards for planning, designing, constructing, and operating off- shore wind energy projects in U.S. waters. The group plans to prioritize its recommendations by using international standards whenever possible, followed by national standards, classiﬁcation society standards, and com- mercial standards and guidelines. The AWEA recommended practices will apply to all bottom-ﬁxed structures installed on the outer continental shelf (OCS) or in near-shore locations (e.g., state waters) but will not necessarily be sufﬁcient to ensure the structural integrity of ﬂoating offshore wind turbines. The AWEA offshore group was divided into three subgroups. Each of the groups is working independently, but all are expected to deliver a ﬁnal guideline by the end of 2011. The three subgroups are discussed below. Group 1, Structural Reliability, is addressing design issues relating to structural reliability of offshore wind turbines. Because many wind turbines targeted for installation in the United States may have already been designed and type-certiﬁed to IEC design classes (see Chapter 3), one focus of the work is establishing the appropriate interfaces between the existing IEC standards and other standards governing the structural reliability of the integrated turbine system. The group will recommend standards and practices that provide a methodology for establishing turbines at speciﬁc U.S. sites, taking into account the unique metocean and subsurface conditions. Group 2, Fabrication, Construction, Installation, and Qualiﬁcation Test- ing, is developing recommended practices for the safe and orderly deployment of offshore wind turbines during the construction and installation phases. Any manufacturing issues unique to offshore wind turbines will be addressed, as will issues relating to the establish- ment of adequate infrastructure. IEC’s TC 88 is not addressing much of this phase of deployment, so this group will probably not need to mix and match existing standards as will Group 1. However, it will
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Standards and Practices 55 have to identify applicable standards from other industries and adapt them to cover these activities. Qualiﬁcation testing will be treated as an overarching activity that may be applied to any project phase. Group 3, Operation, Maintenance, and Decommissioning, is developing recommended practices for operation and inspection. The recom- mendations are not likely to include extensive turbine component inspection; owner–investor wind farm maintenance systems are generally more comprehensive than periodic inspections that could be carried out by BOEMRE or other federal agencies, and the con- sequences of failure in a secondary component are generally limited to economic risk to the wind farm itself. However, in-service struc- tural inspection of the tower and the substructure or below the waterline will be necessary over the ﬁeld service life. Conservatively, the design life of the substructure is 20 years, but designs could allow repowering scenarios where foundations could be reused. In any case, foundation and substructure design should consider removal and disposition of the system when it is no longer serviceable. IEC Floating Wind Turbine Initiative There is strong interest worldwide in the development of new technol- ogy for deeper water. Such technology may include floating support structures for wind turbines. Only one ﬂoating wind turbine has been deployed to date, by Statoil in Norway in 2009, but technology develop- ment is accelerating, and permits for prototypes in U.S. waters will soon be sought (Maine Public Utilities Commission 2010). In May 2011, IEC TC 88 approved a project to develop an IEC technical speciﬁcation for the design of floating wind turbines. The forecast publication date is January 2013 (IEC 2011). Bureau Veritas Guidance for Floating Offshore Wind Turbines In January 2011, Bureau Veritas issued guidelines for the “Classiﬁcation and Certiﬁcation of Floating Offshore Wind Turbines.” The guidelines specify the environmental conditions under which ﬂoating offshore wind turbines may serve, the principles of structural design, load cases for the platform and mooring system, stability and structural division,
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56 Structural Integrity of Offshore Wind Turbines and design criteria for the top structure. The guidelines cover ﬂoating plat- forms supporting single or multiple turbines with horizontal or vertical axes.2 The committee was not able to review these guidelines for this report. BOEMRE Research Program Under its Technology Assessment and Research (TA&R) Program,3 BOEMRE carries out research in support of operational safety and pol- lution prevention on the OCS. The renewable energy element of the pro- gram has sponsored work on offshore wind inspection methodologies, comparisons of offshore wind standards, experience with offshore wind accidents, CVAs, and other topics. For example, BOEMRE held a work- shop in October 2010 that reviewed the expected activities of CVAs.4 It recently awarded a project to ABS covering design standards for offshore wind farms. The project focuses on governing load cases and load effects for offshore wind turbines subject to revolving storms on the U.S. OCS and on calculation methods for breaking wave slamming loads inﬂicted on offshore wind turbine support structures. AREAS OF LIMITED EXPERIENCE AND MAJOR DEFICIENCIES IN STANDARDS Generally, standards embody the collective experience of an industry, but they tend to lag the knowledge base because of the time needed for the consensus-driven standards development process to incorporate the lessons learned. The standards for offshore wind are still immature, and several shortcomings are expected when the ﬁrst projects are installed in U.S. waters. Third-party assessments (e.g., by CVAs) can overcome the shortcomings by relying on good engineering judgment to determine adequate safety. Examples of deﬁciencies in offshore wind standards that were identiﬁed during this study are described below. • Type-certiﬁed wind turbine designs may not meet the extreme wind gust criteria for some high-intensity hurricanes in the United States. 2 Bureau Veritas press release, Jan. 12, 2011. 3 Information on projects carried out under the TA&R program for renewable energy can be found on the BOEMRE website at http://www.boemre.gov/tarprojectcategories/RenewableEnergy.htm. 4 http://www.boemre.gov/tarprojects/633/af.pdf.
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Standards and Practices 57 Although turbines should always be type-certiﬁed to the expected site wind conditions (under Class S in IEC 61400-1 and 61400-3), the cur- rent standard does not speciﬁcally address hurricanes in the estimation of peak wind and wave heights, duration of sustained high winds, or extreme directional wind changes. In addition, IEC 61400-3 DLC 6.2 allows dependence on yaw system backup power for 6 hours, which may not be sufﬁcient to ensure safe hurricane ride-through. • Monopile substructures for wind turbines exceed the diameters and experience base of the oil and gas industry. Extrapolating current prac- tice to the larger sizes can introduce unintended effects. Monopiles up to 5 m in diameter are in use today. In 2010, hundreds of offshore wind turbine installations were discovered to have excessive tilt due to fail- ure of the grouting connection at the tower transition piece. This raises issues concerning vertical tilt tolerances and transition piece grouting practices in the current standards. • The behavior and possible degradation of soil strength under combined dynamic loading from the wind turbine and waves are not well described in the current standards. Moreover, the empirical cyclic degradation methods speciﬁed are not appropriate. [A recent paper (Andersen 2009) provides a good description of cyclic degradation of clays under shallow foundations.] • Offshore wind turbines in the Great Lakes will encounter freshwater ice, which may induce first-order loading from numerous new DLCs. Research and speciﬁcation development for ice loading in the Great Lakes are needed, because the loads cannot be estimated from prior wind energy experience in the Baltic Sea. • Extreme wave loads may result from breaking waves at some shallow- water sites. The magnitude of the loading will depend on the type of sub- structure used and in some instances could be a controlling factor in design. Standards require analysis of this condition to estimate (a) the wave characteristics and (b) the turbine response to the waves, for which models have not yet been validated for some substructure types. • Gravity-based substructures are used frequently but are more poorly documented in the standards than are steel substructures, which are more commonly used by the offshore oil and gas industry. However, design of shallow-water, steel substructures for oil and gas structures
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58 Structural Integrity of Offshore Wind Turbines is mainly concerned with preventing plastic collapse, while design of offshore wind turbines is more concerned with preventing failure due to resonance and fatigue. • Offshore wind turbines are expected to increase in size from about 3 MW per turbine today to possibly 10 MW over the next decade. The scaling up of turbine size may introduce effects not anticipated or cov- ered by any of the current standards. • Signiﬁcant experience has been gained since the current IEC offshore wind standards were written. The experience has improved the knowl- edge base with respect to design requirements for turbine support struc- tures and has led to reﬁnements in design methodologies. Much of this experience has not yet been incorporated into the standards. Moreover, the causes of recent technical failures in foundations and grouted con- nections and the design requirements to avoid such failures are still being analyzed, so they are likewise not reﬂected in current standards. • Floating wind turbine systems are not addressed adequately in any of the current standards. IEC is considering a proposal to write a techni- cal speciﬁcation on ﬂoating wind turbine systems (IEC 2010a). (Bureau Veritas has just released guidelines for the “Classiﬁcation and Certiﬁca- tion of Floating Offshore Wind Turbines,” but the committee was not able to review them for this project.) FINDINGS FOR TASK I: CHAPTER 3 Findings for Chapter 3 appear below. They address Task I of the statement of task. Chapter 4 also addresses Task I. A full set of recommendations for Task I appears at the end of Chapter 4. 1. Regulations in most countries—notably in continental Europe—take a prescriptive approach, regulating in detail the design, construction, and operation of offshore wind turbines to achieve acceptable levels of safety, environmental performance, and reliability. 2. The starting points for most of the offshore wind energy regulations and guidelines (for example, those of DNV, GL, ABS, BSH, AWEA, and the Danish Energy Agency) are IEC 61400-1 (Wind Turbines—Part 1: Design Requirements) and IEC 61400-3 (Wind Turbines—Part 3: Design Requirements for Offshore Wind Turbines). The IEC standards do not
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Standards and Practices 59 cover all aspects of the design and construction of offshore wind turbines. 3. Nongovernmental organizations and private companies that establish and maintain technical rules and guidelines for the design, con- struction, and operation of ships and offshore structures—commonly known as classiﬁcation societies—have developed guidelines. The most comprehensive industry guidelines for offshore wind turbine design, fabrication, installation, and commissioning have been developed by classiﬁcation societies such as DNV, GL, and ABS. These standards are more comprehensive than are the IEC standards in the sense that they cover both the load and resistance sides, whereas the IEC standards cover explicitly only the load side. However, there are still deﬁciencies that must be overcome. For instance, the European society guidelines do not explicitly address environmental site conditions for the United States (e.g., storms and hurricane conditions for the Gulf of Mexico and the East Coast). Only the GL rules deal with the design and certiﬁcation of wind turbine mechanical and electrical components (e.g., the gear- box, the generator, and the control systems). 4. Methodologies for strength analysis differ among the various standards and guidelines and are not always fully delineated. Some standards are based on strength or limit states design, while others are based on allow- able stress design. The philosophies underlying these methods are fun- damentally different, making it difﬁcult to compare such standards against one another to ensure consistent safety levels, especially when the standards are applied to novel concepts. There is a need for a clear, transparent, and auditable set of assumptions for strength analyses. 5. As discussed in Chapter 1, although regulations (MMS 2009) pro- mulgated by the U.S. Department of Interior’s BOEMRE require that detailed reports for design, construction, and operation of offshore wind turbines be submitted for BOEMRE approval, they do not spec- ify standards that an offshore wind turbine must meet. Rather, a third party (CVA) is charged with reviewing and commenting on the ade- quacy of design, fabrication, and installation and submitting reports to BOEMRE indicating the CVA’s assessment of adequacy. Moreover, when a general level of performance such as “safe” is identiﬁed, no guid- ance is provided on how to assess whether this level of performance has
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60 Structural Integrity of Offshore Wind Turbines been met. Hence, the BOEMRE regulations and accompanying guid- ance lack the clarity and speciﬁcity needed for the development of off- shore wind energy on the OCS. 6. As discussed in Chapter 2, states and private companies are developing plans for offshore wind energy projects in state waters and on the OCS. Well-deﬁned U.S. regulations for development on the OCS are needed (a) to provide a resource for states as they develop requirements for projects in state waters and (b) to supply industry with sufﬁcient clarity and certainty on how projects will be evaluated as companies seek the necessary ﬁnancing. Further delays in developing an adequate national regulatory framework are likely to impede development of offshore wind facilities in U.S. waters. Moreover, developments in state waters could proceed in the absence of federal regulations, possibly leading to inconsistent safety and performance across projects. The United States urgently needs a set of clear and speciﬁc standards and regulatory expectations to avoid these negative outcomes, facilitate the orderly development of offshore wind energy, and support the stable eco- nomic development of a nascent industry. REFERENCES Abbreviations ABS American Bureau of Shipping API American Petroleum Institute AWEA American Wind Energy Association BSH Bundesamt für Seeschifffahrt und Hydrographie DNV Det Norske Veritas GL Germanischer Lloyd IEC International Electrotechnical Commission MMS Minerals Management Service ABS. 2010. Guide for Building and Classing Offshore Wind Turbine Installations. Houston, Tex. Andersen, K. H. 2009. Bearing Capacity Under Cyclic Loading—Offshore, Along the Coast, and on Land. Canadian Geotechnical Journal, Vol. 46, No. 5, pp. 513–535. API. 1997. Supplement 1: Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms: Load and Resistance Factor Design. API RP 2A-LRFD-S1. Washington, D.C., Feb. 1.
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Standards and Practices 61 BSH. 2007. Design of Offshore Wind Turbines. DNV. 2009. Offshore Substations for Wind Farms, DNV-OS-J201. Høvik, Oslo, October. DNV. 2010a. Design of Offshore Wind Turbine Structures, DNV-OS-J101. Høvik, Oslo, October. DNV. 2010b. Design and Manufacture of Wind Turbine Blades, Offshore and Onshore Wind Turbines, DNV-DS-J102. Høvik, Oslo, November. Gerdes, G., A. Tiedemann, and S. Zeelenberg. 2006. Case Study: European Offshore Wind Farms—A Survey for the Analysis of the Experiences and Lessons Learnt by Develop- ers of Offshore Wind Farms, Final Report. Prepared by Deutsche WindGuard, Deutsche Energie-Agentur GmbH (dena), and the University of Groningen. http:// www.offshore-power.net/Files/Dok/casestudy-europeanoffshorewindfarms.pdf. GL. 2005. Guidelines for the Certiﬁcation of Offshore Wind Turbines, 2nd ed. IEC. 2001. IEC System for Conformity Testing and Certiﬁcation of Wind Turbines—Rules and Procedures. IEC WT 01 Ed. 1. Geneva. IEC. 2005. Wind Turbines—Part 1: Design Requirements. IEC 61400-1 Ed. 3. Geneva. IEC. 2010a. Wind Turbines—Part 3: Design Requirements for Offshore Wind Turbines. IEC 61400-3 Ed. 1. Committee Draft 88/257/CD. Geneva. IEC. 2010b. Wind Turbines—Part 22: Conformity Testing and Certiﬁcation. IEC 61400-22 Ed. 1. Geneva. IEC. 2011. Work program for IEC TC 88 project IEC/TS 61400-3-2, Wind Turbines— Part 3-2: Design Requirements for Floating Offshore Wind Turbines. http://www. iec.ch/dyn/www/f?p=103:38:0::::FSP_ORG_ID,FSP_APEX_PAGE,FSP_LANG_ID, FSP_PROJECT:1282,23,25/txt/3kyo11.pdf,IEC/TS%2061400-3-2%20ED.%201.0. Maine Public Utilities Commission. 2010. Request for Proposals for Long-Term Contracts for Deep-Water Offshore Wind Energy Pilot Projects and Tidal Energy Demonstration Projects. http://www.maine.gov/mpuc/electricity/rfps/standard_offer/deepwater2010/. Accessed Dec. 11, 2010. MMS. 2009. Renewable Energy and Alternate Uses of Existing Facilities on the Outer Con- tinental Shelf. Federal Register, Vol. 74, pp. 19638–19871. http://www.mms.gov/ offshore/RenewableEnergy/PDF/FinalRenewableEnergyRule.pdf. Wisch, D. J., A. Mangiavacchi, P. O’Connor, and W. R. Wolfram. 2010. Strategy and Struc- ture of the API 2 Series Standards, 2010 and Beyond. OTC 30831. Presented at Offshore Technology Conference, Houston, Tex., May.