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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
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Fixed Offshore Platforms: Load and Resistance Factor Design. API RP 2A-LRFD-S1.
Washington, D.C., Feb. 1.
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BSH. 2007. Design of Offshore Wind Turbines.
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Accessed Dec. 11, 2010.
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