Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
MARINE BOARD Chair: Michael S. Bruno, Stevens Institute of Technology, Hoboken, New Jersey Vice Chair: Thomas M. Leschine, University of Washington, Seattle Steven R. Barnum, Hydrographic Consultation Services, Suffolk, Virginia Jerry A. Bridges, Virginia Port Authority, Norfolk Mary R. Brooks, Dalhousie University, Halifax, Nova Scotia, Canada James C. Card, Maritime Consultant, The Woodlands, Texas Stephen M. Carmel, Maersk Line Limited, Norfolk, Virginia Edward N. Comstock, Raytheon Company, Sudbury, Massachusetts Stephan Toni Grilli, University of Rhode Island, Narragansett Douglas J. Grubbs, Crescent River Port Pilots Association, Metairie, Louisiana Frederick J. Harris, General Dynamics, San Diego, California Judith Hill Harris, City of Portland, Maine John R. Headland, Moffatt & Nichol Engineers, New York, New York John M. Holmes, Port of Los Angeles, San Pedro, California Ali Mosleh, University of Maryland, College Park George Berryman Newton, QinetiQ North America, Marstons Mills, Massachusetts Patrick Ernest OâConnor, BP America, Inc., Houston, Texas Robert W. Portiss, Tulsa Port of Catoosa, Oklahoma Peter K. Velez, Shell International Exploration and Production, Inc., Houston, Texas John William Waggoner, HMS Global Maritime, New Albany, Indiana TRANSPORTATION RESEARCH BOARD 2011 EXECUTIVE COMMITTEE OFFICERS Chair: Neil J. Pedersen, Administrator, Maryland State Highway Administration, Baltimore Division Chair for NRC Oversight: C. Michael Walton, Ernest H. Cockrell Centennial Chair in Engineering, University of Texas, Austin (Past Chair, 1991) Vice Chair: Sandra Rosenbloom, Professor of Planning, University of Arizona, Tucson Executive Director: Robert E. Skinner, Jr., Transportation Research Board
SPECIAL REPORT 305 Structural Integrity of Offshore Wind Turbines Oversight of Design, Fabrication, and Installation Committee on Offshore Wind Energy Turbine Structural and Operating Safety TRANSPORTATION RESEARCH BOARD OF THE NATIONAL ACADEMIES Transportation Research Board Washington, D.C. 2011 www.TRB.org
Transportation Research Board Special Report 305 Subscriber Categories: Energy; bridges and other structures Transportation Research Board publications are available by ordering individual publi- cations directly from the TRB Business Ofï¬ce, through the Internet at www.TRB.org or national-academies.org/trb, or by annual subscription through organizational or indi- vidual afï¬liation with TRB. Afï¬liates and library subscribers are eligible for substantial discounts. For further information, contact the Transportation Research Board Business Office, 500 Fifth Street, NW, Washington, DC 20001 (telephone 202-334-3213; fax 202-334-2519; or e-mail TRBsales@nas.edu). Copyright 2011 by the National Academy of Sciences. All rights reserved. Printed in the United States of America. NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competencies and with regard for appropriate balance. This report has been reviewed by a group other than the authors according to the pro- cedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. This study was sponsored by the Bureau of Ocean Energy Management, Regulation, and Enforcement, U.S. Department of the Interior. Cover design by Debra Naylor, Naylor Design, Inc. Cover photo: Middelgrunden offshore wind turbines in the strait of Ãresund, outside Copenhagen harbor, Denmark. (Photo by Tore Johannesen, iStockphoto.) Typesetting by Circle Graphics. Library of Congress Cataloging-in-Publication Data National Research Council (U.S.). Committee on Offshore Wind Energy Turbine Structural and Operating Safety. Structural integrity of offshore wind turbines : oversight of design, fabrication, and installation / Committee on Offshore Wind Energy Turbine Structural and Operating Safety, Marine Board, Transportation Research Board of the National Academies. p. cm. â (Transportation research board special report ; 305) 1. Offshore structuresâ Design and constructionâSafety measuresâGovernment policyâUnited States. 2. Wind turbinesâDesign and constructionâSafety measuresâGovernment policyâ United States. 3. Wind power plantsâUnited StatesâSafety measures. 4. Electric power-plants, OffshoreâUnited StatesâSafety measures. I. Title. TC1665.N38 2011 621.4â²53âdc22 2011004767 ISBN 978-0-309-16082-7
The National Academy of Sciences is a private, nonproï¬t, self-perpetuating society of distinguished scholars engaged in scientiï¬c and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. On the authority of the charter granted to it by the Congress in 1863, the Academy has a man- date that requires it to advise the federal government on scientiï¬c and technical matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meet- ing national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Charles M. Vest is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, on its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Insti- tute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academyâs purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. Charles M. Vest are chair and vice chair, respectively, of the National Research Council. The Transportation Research Board is one of six major divisions of the National Research Council. The mission of the Transportation Research Board is to provide lead- ership in transportation innovation and progress through research and information exchange, conducted within a setting that is objective, interdisciplinary, and multimodal. The Boardâs varied activities annually engage about 7,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. The program is supported by state transportation departments, federal agencies including the compo- nent administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation. www.TRB.org www.national-academies.org
Committee on Offshore Wind Energy Turbine Structural and Operating Safety R. Keith Michel, Herbert Engineering Corporation, Alameda, California, Chair Bruce R. Ellingwood, Georgia Institute of Technology, Atlanta George M. Hagerman, Jr., Virginia Coastal Energy Research Consortium, Virginia Beach Jan Behrendt Ibsoe, ABS Consulting, Inc., Houston, Texas Lance Manuel, University of Texas at Austin Walt Musial, National Renewable Energy Laboratory, Golden, Colorado Robert E. Sheppard, Energo Engineering, Houston, Texas Emil Simiu, National Institute of Standards and Technology, Gaithersburg, Maryland Susan W. Stewart, Pennsylvania State University, State College David J. Wisch, Chevron Energy Technology Company, Houston, Texas Staff Madeline G. Woodruff, Study Director
Preface Although many of the worldâs largest wind farms are located in the United States, these installations are entirely land based. Land-based wind resources are plentiful but are located principally in the central regions of the country, remote from the major population centers where electricity demand is growing but transmission line access and capacity are limited. There are obstacles to installing an enhanced transmission system capable of connecting land-based wind farms to the highly pop- ulated areas, particularly with regard to permitting. Costs related to installation and maintenance are signiï¬cantly higher for offshore wind farms than for those located on land. However, offshore wind farms offer a number of advantages that could offset these higher costs. Offshore installations can be located close to coastal metropolitan areas, reducing transmission infrastructure requirements. The intensity of offshore wind energy is also greater, allowing the offshore wind tur- bine to operate at greater efficiencies than a comparable land-based installation. There are currently offshore wind projects planned along the U.S. East Coast, the Gulf of Mexico, and the Great Lakes. To date, most off- shore wind farms have been located in the waters of the European and Scandinavian nationsâGermany, Denmark, and the United Kingdom being the most important. These countries have been the leaders in both technological and regulatory development related to offshore wind power generation. The international standards for offshore wind turbine design and certiï¬cation established by the International Electrotechnical Com- mission (IEC) are formally recognized in European national regulations. Some of these national regulations also recognize the guidelines and reg- ulations developed by classiï¬cation societies. vii
viii Structural Integrity of Offshore Wind Turbines In the United States, where offshore wind energy has been much less of a focus, regulatory development has lagged. As a result, permitting of sites in U.S. waters is proceeding without a clear set of national regula- tions for the design, fabrication, installation, and commissioning of off- shore wind turbines. The Minerals Management Service (MMS), which has been renamed the Bureau of Ocean Energy Management, Regula- tion, and Enforcement (BOEMRE), is responsible for the orderly, safe, and environmentally responsible development of offshore renewables on the outer continental shelf. BOEMRE requested that the Transportation Research Boardâs (TRBâs) Marine Board conduct a study to guide the agency in the regulation and technical oversight of the nascent offshore wind energy industry in the United States. A study committee consisting of 10 members from academia, national research centers, and industry was appointed by the National Research Council (NRC). Members have expertise in structural engineering, wind energy, regulation, third-party verification in offshore platforms and wind turbines, and oceanography. Biographical sketches of the committee members appear at the end of this report. The report represents the con- sensus opinion of the committee members and presents the committeeâs ï¬ndings and recommendations on the standards and practices that could be used in oversight of U.S. offshore wind installations, the role of third- party reviewers and BOEMRE in overseeing of the design and construc- tion of offshore wind turbines, the necessary qualiï¬cations of third-party reviewers, and the selection process for identifying and approving third- party reviewers. The committee met three times over a 5-month period. These face- to-face meetings were supplemented by numerous conference calls. The committee listened to presentations from a wide range of stakeholders, including state and federal regulators, standards development organi- zations, wind farm developers, turbine manufacturers, and research scientists and engineers with expertise in the wind energy industry. The committee also reviewed various studies and workshop proceedings sponsored by BOEMRE. These resources proved invaluable as the com- mittee discussed alternative approaches to oversight processes and for- mulated the ideas that are presented in this report.
Preface ix ACKNOWLEDGMENTS The committee acknowledges John Cushing, Lori Medley, and the other staff members of BOEMRE who provided the committee with insight into the responsibilities and workings of BOEMRE and into the various studies on offshore wind energy conducted under the auspices of BOEMRE and its predecessor, MMS. The committee also acknowledges the government and industry representatives, listed below, who took time from their busy schedules to present background information and their own ideas and opinions to the committee at its meetings, and to the others who assisted the committee by providing relevant publications and answering questions by telephone and e-mail. The following individuals made presentations at the ï¬rst committee meeting, June 28â29, 2010: â¢ John Cushing, BOEMRE, U.S. Department of the Interior; â¢ Malcolm Sharples, President, Offshore Risk and Technology Consult- ing, Inc.; â¢ Fara Courtney, Executive Director, U.S. Offshore Wind Energy Collaborative; â¢ Grover Fugate, Executive Director, Rhode Island Coastal Resources Management Council; â¢ Elmer âBudâ Danenberger, MMS (retired); â¢ Kenneth Richardson, Vice President for Energy Projects, American Bureau of Shipping; â¢ Jan Behrendt Ibsoe, Vice President for Global Renewable Energy, ABS Consulting (committee member); â¢ William Holley, Technical Advisor for the U.S. National Committee of the IEC, Technical Committee 88, Chief Consulting Engineer, Wind Systems, GE Energy; and â¢ John Dunlop, Senior Project Engineer, American Wind Energy Association. The following individuals made presentations at the second commit- tee meeting, August 10â11, 2010: â¢ Thomas Laurendine, Assistant Vice President, Liberty International Underwriters (formerly with MMS);
x Structural Integrity of Offshore Wind Turbines â¢ Jeff Shikaze, Program ManagerâRenewable Energy, Canadian Stan- dards Association (CSA); â¢ Richard McNitt, Business Development Manager, CSA International; â¢ Peter Vickery, Principal Engineer, Applied Research Associates, Inc., IntraRisk Division; and â¢ Tom McNeilan, General Manager, Fugro Atlantic (on behalf of the Offshore Wind Development Coalition). The report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by NRCâs Report Review Committee. The pur- pose of this independent review is to provide candid and critical com- ments that will assist the institution in making the published report as sound as possible and to ensure that the report meets institutional stan- dards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain conï¬dential to pro- tect the integrity of the deliberative process. The committee thanks the following individuals for their review of the report: C. P. âSandyâ Butterï¬eld, Boulder Wind Power Inc., Boulder, Colorado; Vice Admiral James C. Card (retired), The Woodlands, Texas; Kent Dangtran, Dangtran OTC, LLC, Cypress, Texas; John Headland, Mof- fatt & Nichol Engineers, New York, New York; Mary Hallisey Hunt, Strate- gic Energy Institute, Georgia Institute of Technology, Atlanta, Georgia; Alberto Morandi, American Global Maritime Inc., Houston, Texas; John Niedzwecki, Zachry Department of Civil Engineering, Texas A&M University, College Station, Texas; James Schneider, Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, Wisconsin. Although these reviewers provided many constructive comments and suggestions, they were not asked to endorse the committeeâs ï¬ndings or recommendations, nor did they see the ï¬nal draft of the report before its release. The review was overseen by Lawrence T. Papay, PQR, LLC, and C. Michael Walton, University of Texas at Austin. Appointed by NRC, they were responsible for making certain that an independent exami- nation of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered.
Preface xi Responsibility for the final content of this report rests entirely with the authoring committee and the institution. Madeline Woodruff, Senior Program Officer in NRCâs Division on Engineering and Physical Sciences, served as study director and assisted the committee in the preparation of its report under the supervision of Stephen R. Godwin, Director, Studies and Special Programs, TRB. Suzanne Schneider, Associate Executive Director of TRB, managed the report review process. Norman Solomon edited the report, Janet M. McNaughton handled the editorial production, Juanita Green managed production, and Jennifer J. Weeks prepared the manuscript for pre- publication web posting, under the supervision of Javy Awan, Director of Publications, TRB. Amelia Mathis and Claudia Sauls provided assis- tance with meeting arrangements and communications with the com- mittee. The committee extends its sincere gratitude to the diligent and capable staff of the National Academies. Without their efforts and sup- port, producing a report with the depth and quality of this study in a relatively short time would not have been possible.
Acronyms and Abbreviations Acronyms and abbreviations used in the report are listed below. A glossary provides pertinent deï¬nitions. AASHTO American Association of State Highway and Transportation Ofï¬cials ABS American Bureau of Shipping ACI American Concrete Institute ACP Alternative Compliance Program AISC American Institute of Steel Construction ALARP as low as reasonably practicable ANSI American National Standards Institute API American Petroleum Institute ASCE American Society of Civil Engineers ATC Applied Technology Council AWEA American Wind Energy Association BOEMRE Bureau of Ocean Energy Management, Regulation, and Enforcement BSH Bundesamt fÃ¼r Seeschifffahrt und Hydrographie (German Federal Maritime and Hydrographic Agency) BV Bureau Veritas CFR Code of Federal Regulations CMS Conditioning Monitoring System COP construction and operations plan CVA certiï¬ed veriï¬cation agent DLC design load case DNV Det Norske Veritas EA environmental assessment EIS environmental impact statement EPACT Energy Policy Act of 2005 ESP electric service platform xiii
xiv Structural Integrity of Offshore Wind Turbines FERC Federal Energy Regulatory Commission GAP general activities plan GL Germanischer Lloyd GOM Gulf of Mexico GW gigawatts IEC International Electrotechnical Commission IMO International Maritime Organization ISO International Organization for Standardization kV kilovolts LRFD load and resistance factor design MMS Minerals Management Service MRI mean recurrence interval MW megawatts NBS National Bureau of Standards NDT nondestructive testing NEPA National Environmental Policy Act NRC National Research Council NREL National Renewable Energy Laboratory NTL notice to lessees OCS outer continental shelf OEM original equipment manufacturer QA quality assurance QC quality control PBD performance-based design PBE performance-based engineering PE professional engineer PEIS programmatic environmental impact statement PTC production tax credit SAP site assessment plan SCADA supervisory control and data acquisition SFPE Society of Fire Protection Engineers TA&R Technology Assessment and Research program TC technical committee USACE United States Army Corps of Engineers USCG United States Coast Guard USDOE United States Department of Energy USDOI United States Department of the Interior USGS United States Geological Survey WSD working stress design
Glossary A Array. A group of wind turbines conï¬gured in a grid layout. Array losses. See turbine-to-turbine interference. B Bearing. A device to allow constrained relative motion between two or more parts, typically rotation or linear movement. C Capacity. The rated continuous load-carrying ability of generation, transmission or other electrical equipment, expressed in megawatts (MW). The âsizeâ of a power plant is usually characterized by its net power generation capacity in MW. Certiï¬cation. See Box 1.3 Class (or wind turbine class). Classiï¬cations deï¬ned by IEC for wind turbines based on three parameters: the average wind speed, extreme 50-year return 10-min averaged gust, and turbulence. Classiï¬cation. See Box 1.3 Classiï¬cation society. Industry associations and companies that supply services (such as certiï¬cation) to the industry, evaluating the design, fabrication or installation with reference to its own rules or guidelines rather than an externally developed standard or guideline. Code of Federal Regulations (CFR). Rules and regulations deï¬ned by the U.S. federal government having the force of law. Title 30, part 250 (30 CFR 250) covers âOil and Gas and Sulphur Operations in the Outer Continental Shelf.â Part 285 (30 CFR 285) covers âRenewable Energy Alternate Uses of Existing Facilities on the Outer Continental Shelf.â xv
xvi Structural Integrity of Offshore Wind Turbines Composite (tower or rotor). Engineered materials made from two or more constituent materials with significantly different physical or chemical properties that remain separate and distinct on a macroscopic level within the ï¬nished structure. Condition monitoring. A process that involves a system of sensors and monitoring equipment used to remotely monitor specific proper- ties of a mechanical or structural system (e.g., fluid temperatures or material strain) for the purpose of determining its ability to operate normally. D Deepwater. A water depth range for offshore facilities; typically beyond 500 feet (152 m) though there is no deï¬nitive water depth range. Design basis. The extreme conditions under which the wind turbine is designed to operate. E.g. 50- or 100-year extreme wind and wave load- ing events. Also includes potential fault conditions of the turbine. Developer. The entity in a wind project that designates and arranges for the building of an infrastructure on land or an offshore site in order to productively exploit wind energy. Analysis of the landâsea and wind resource characteristics are crucial in the development process. Direct drive. A mechanism that takes the power coming from a motor without any reductions (such as a gearbox). Distribution system. The part of the electrical grid infrastructure that moves electricity between local destinations either on the power gener- ation side or the demand side (transmission systems transfer electricity over longer distances). The wind farm electric power distribution sys- tem consists of each turbineâs power electronics, the turbine step-up transformer and distribution wires, the electric service platform (ESP), cables to shore, and the shore-based interconnection system. Downwind turbine. Refers to a horizontal axis wind turbine in which the hub and blades point away from the wind direction, the opposite of an upwind turbine. Drivetrain. The transmission system of the wind turbine that converts the low speed shaft rotational power from the rotor to electrical power via either a gearbox and generator assembly or a direct drive mechanism.
Glossary xvii E Electric service platform (ESP). An offshore platform serving as a col- lection and service point for a wind farm, also called a transformer platform. Environmental Impact Statement (EIS). A document required by the National Environmental Policy Act (NEPA) for certain actions âsig- niï¬cantly affecting the quality of the human environment.â It is a tool for decision making, describing the positive and negative environ- mental effects of a proposed action and listing one or more alternative actions that may be chosen instead of the action described in the EIS. Exploratory leases. Acting under the authority granted to MMS through the Energy Policy Act of 2005, the agency initiated the Interim Policy, which allows for exploratory leases in November 2007 in advance of the ï¬nal regulatory framework in order to jumpstart the review and potential authorization of the renewable energy development process. The limited leases authorize a term of 5 years for activities on the OCS associated with renewable energy resource data collection and tech- nology testing. F Federal waters. Refers to U.S. territorial waters regulated by the U.S. fed- eral government, as opposed to areas regulated by state authorities. Typically this is the region beyond 3 nautical miles from shore, with the exception of parts of the gulf coast. G Gear-driven. Using a mechanical system of gears or belts and pulleys to increase or decrease shaft speed. Goal-based standards (also known as performance-based standards). A hierarchical standard in which the starting point is a set of high-level performance objectives supported by a series of minimum perfor- mance criteria that are necessary to support this objective and, ï¬nally, a choice of methods by which satisfaction of these criteria can be demonstrated. These methods may be prescriptive in nature; rational alternative means and methods are permitted, provided that their acceptability can be veriï¬ed by either analysis or tests.
xviii Structural Integrity of Offshore Wind Turbines Guidelines. See Box 3.1. Gravity base (or gravity-based) foundation. A type of foundation that relies on mass and a larger base dimension to provide stability and resist overturning. H Helical stage. A cylindrical gear wheel that has slanted teeth that follow the pitch surface in a helical manner. Horizontal axis turbine. A ânormalâ wind turbine design, in which the shaft is parallel to the ground and the blades are perpendicular to the ground Hydrokinetic. Referring to devices that extract energy from moving water such as rivers, ocean currents, and waves. I Interconnection system. The electrical system of cabling, typically oper- ating at medium voltage, that connects the turbines to one another as well as to the facility substation. J Jacket. A type of offshore structure consisting of a vertical framing system with multiple legs and a piled foundation. Jackup rig. A floating barge fitted with supporting legs that can be lowered to the seabed. L Limit states design. A method of proportioning structural members, com- ponents, and systems such that the design strength, deï¬ned as the prod- uct of a nominal strength and a resistance factor, equals or exceeds the required strength under the action of factored load combinations (also denoted load and resistance factor design, or LRFD, in the United States). Load and resistance factor design (LRFD). See limit states design. M Marine spatial planning. A tool that brings together multiple users of the ocean, including energy, industry, government, conservation, and recreation, to make informed and coordinated decisions about how to use marine resources sustainably.
Glossary xix Memorandum of understanding (MOU). A document that deï¬nes an agreement between two governmental agencies regarding how they will interact in an area of shared oversight. For example, there is an MOU between the former MMS and the Federal Energy Regulatory Com- mission (FERC) that clariï¬es the roles each organization has in the oversight of energy projects in the OCS. Monopole. A turbine foundation structure composed of a large steel tube driven into the seabed. Multi-pile. See jacket. N Nacelle. The portion of a wind turbine that sits atop the tower protect- ing the mechanical and electrical components (i.e., the drivetrain, controller, and brake) from the elements. O Outer Continental Shelf (OCS). Refers to all submerged lands, its subsoil, and seabed that belong to the United States and are lying seaward and outside of the statesâ jurisdiction, the latter defined as the âlands beneath navigable watersâ in Title 43, Chapter 29, Sub- chapter I, Section 1301 of the U.S. code, The United States OCS has been divided into four leasing regions: Gulf of Mexico, Atlantic, Pacific, and Alaska. P Performance-based design. A design approach that identifies an appropriate structural system and design parameters based on the desired levels of performance (or performance targets) of the facil- ity of which the structure is part; often used in seismic and blast- resistant design. Pitch. The angle between the edge of the blade and the plane of the bladeâs rotation. Blades are turned, or pitched, out of the wind to con- trol the rotor speed. Planetary stage. An outer gear that revolves about a central sun gear of an epicyclic train. Power electronics. The application of solid-state electronics for the con- trol and conversion of electric power.
xx Structural Integrity of Offshore Wind Turbines Prescriptive. A regulatory environment in which particular activities and schedules and parameters are prescribed a priori rather than derived from performance targets. Prevailing wind. The predominant direction from which the wind blows. Production tax credit (PTC). A federal incentive program that is designed to help level the playing ï¬eld of energy production where other forms of energy are subsidized. At the time of press, the PTC is currently offered to wind projects in service by December 31, 2012, over the ï¬rst 10 years of operation, at a value of 2.2 cents/kWh (which increases with inï¬ation). Project certification. A process to verify that the wind turbine and its support structures meet the site-specific conditions. Use of a type- certified wind turbine is a prerequisite. R Recommended practices. A type of standard or guideline developed by a standards-development body. Regulations. See Box 3.1. Return period. The average interval of time between recurrences of an event such as an earthquake or storm of a certain size or intensity, used in risk analysis. A storm of a given intensity that has a return period of 10 years would have a 1-in-10 probability of being exceeded (in intensity) in any given year. Risk-informed basis. An integrated decision paradigm in which tradi- tional deterministic engineering evaluations are supported by insights derived from probabilistic risk assessment (PRA) methods that take into account uncertainties due to randomness, modeling, and com- pleteness. Decisions may be based on both qualitative and quantita- tive factors and consider traditional engineering information and the risk signiï¬cance of the decision. Rotor. A complete system of blades that supplies all the force driving a wind generator. The rotor has three blades manufactured from fiberglass-reinforced epoxy, mounted on a hub. The blades are pitch-regulated to continually control their angle to the wind and are designed to optimize energy production and to generate minimal noise.
Glossary xxi S SCADA (supervisory control and data acquisition). The wind farm monitoring system that allows the owner or the turbine manufacturer, or both, to be notiï¬ed of faults or alarms, remotely control turbines, and review operational data. Scour. The effect of ocean waves and currents displacing seabed material around the base of ï¬xed structures Shallow water. A water depth range for offshore facilities; typically less than 200 feet (61 m), although there is no definitive water depth range. Siting. The process of determining a suitable location for a wind project development. Standards. See Box 3.1. State waters. U.S. territorial waters regulated by state authorityâs gov- ernment, as opposed to areas regulated by the federal government, typically within 3 nautical miles of shore. Stationkeeping (nautical). Maintaining a ï¬xed position in the water rel- ative to other vessels or to a stationary object or given location. Step-up transformer. Equipment designed to increase the voltage of an electric power system. Substation. A part of an electric system in which transformers are used to step up or step down the voltage in utility power lines for transition between long-distance transmission and local production or distri- bution lines. Switchgear. A device within an electric system used to control the ï¬ow of electricity from one part of the system to another. T Transformer. An electrical device used to transfer power from one cir- cuit to another using magnetic induction, usually to step voltage up or down. Transition piece. The connector between the foundation and the tower, e.g., ï¬tted around the section of the monopole that protrudes above the waterline. Tripod. An offshore jacket structure with three legs. Turbine spacing. The distance between wind turbines within an array.
xxii Structural Integrity of Offshore Wind Turbines Turbine-to-turbine interference. The aerodynamic losses experienced in a wind turbine array as the upstream turbines affect the energy cap- ture of the turbines downstream within the array. Type certiï¬cation. Obtained by the wind turbine manufacturer (from an independent body) to demonstrate that a wind turbine generator system or installation (facility) meets speciï¬ed standards for key ele- ments such as identiï¬cation and labeling, design, power performance, noise emissions, and structural integrity. U Upwind turbine. A horizontal axis wind turbine in which the hub and blades are in front of the tower in the direction of the incoming wind (the opposite of a downwind turbine). Yaw control is required to main- tain the upwind orientation. V Veriï¬cation. See Box 1.3. W Wind farm. A set of wind turbines or one or more turbines, when con- sidered together with the rest of the equipment involved in transfer- ring electricity from the turbines to shore. Wind resource. The average wind speed and direction at a range of heights on a site; required to determine the viability of a wind turbine. Wind shear. Changes in wind velocity with elevation. Wind turbine generator. A rotating machine that produces electricity from the wind. Working stress design. A method of design in which structures or mem- bers are proportioned for prescribed working loads at stresses that are well below their ultimate values. The allowable stresses are determined by applying safety factors to the ultimate values. Y Yaw. To rotate around a vertical axis, such a turbine tower. The yaw drive is used to keep an upwind turbine rotor facing into the wind as the wind direction changes.
Contents Executive Summary 1 1 Introduction 5 Study Charge and Scope 10 Committee Approach 12 Organization of the Report 14 2 Offshore Wind Technology and Status 17 Wind Technology 17 Status of Offshore Wind Installations 28 Offshore Wind Energy for the United States 32 3 Standards and Practices 38 Interactions Between Nonstructural Failures and Wind Turbine Structural Integrity 38 International Electrotechnical Commission 41 API Standards 45 IEC and API Differences 46 ISO Standards 48 Classiï¬cation Society Guidelines 49 Det Norske Veritas 49 Germanischer Lloyd 50 American Bureau of Shipping 51 German Standards and Project Certiï¬cation Scheme 52
Ongoing Standards Development and Related Research: National and International 53 Areas of Limited Experience and Major Deï¬ciencies in Standards 56 Findings for Task I: Chapter 3 58 4 A Risk-Informed Approach to Performance Assurance 62 Risks to Human Life and the Environment Posed by Structural Failure of Offshore Facilities 63 Regulatory Options and Policy Considerations 66 Seeking the Right Regulatory Balance 68 Regulatory Evolution in the Oil and Gas, Marine, and Civil Infrastructure Industries 68 Transition from Prescriptive to Performance-Based Regulations 76 Risk Mitigation Through Performance-Based Engineering 77 Alternative Approaches to Regulating the U.S. Offshore Wind Industry 80 Goal-Based Standards for Offshore Wind Turbines 82 Overview of Projected BOEMRE Role 89 Implementation: Capacity and Expertise 91 Findings for Task I: Chapter 4 92 Recommendations for Task I: Chapters 3 and 4 92 5 Role of Third-Party Oversight and Certiï¬ed Veriï¬cation Agents 96 Background 96 Offshore Oil and Gas: History of Use of CVAs 97 Current BOEMRE Regulatory Proposals for Offshore Wind Turbines and Use of CVAs 102 Scope of Reviews 102 CVAs and Goal-Based Standards 104 Summary 106 Findings and Recommendations for Task II 106
6 Qualiï¬cations Needed by Certiï¬ed Veriï¬cation Agents 109 Survey of Qualiï¬cations for Other Third-Party Reviews 109 U.S. Regulations for Offshore Wind Turbine CVA Qualiï¬cations 114 Evaluation of Accreditation Approaches 115 Offshore Wind Turbine CVA Qualiï¬cations 117 Filling the Experience Gap 121 Findings and Recommendations for Task III 123 7 Summary of Key Findings and Recommendations 127 Finding: Safety and the Environment 127 Findings and Recommendations: Standards and Practices (Task I) 128 Findings and Recommendations: Role of the CVA (Task II) 131 Findings and Recommendations: CVA Qualiï¬cations (Task III) 133 Findings and Recommendations: Implementation 135 Appendices A Risk-Informed Approaches to Safety Regulation 137 B Text of Pertinent Regulations 148 Study Committee Biographical Information 160