Offshore Codes and Standards

During the early 1990s, the wind energy industry—through the International Electrotechnical Commission (IEC)—began to establish international standards for land-based wind turbines. Technical Committee 88 (TC88) was established to develop and manage a suite of applicable

standards for wind turbines. The primary standard for structural design requirements is IEC 61400-1 Edition 3 (IEC 2005). It defines design classes, external (environmental) conditions for each design class, design load cases, fault conditions that must be included in the design,

procedures for assessing all 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 the detailed characteristics of wind are so important to unsteady aerodynamic load distributions along the rotating blades, it is crucial that this part of the external conditions be defined consistent with analytical theory used for rotor load estimation.

In 2000, IEC/TC 88 began to develop an offshore wind turbine standard, Offshore Requirements for Wind Turbines, IEC 61400-3 (IEC 2010). It 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 guidelines for offshore oil and gas structures, including the American Petroleum Institute (API), International Standards Organization (ISO), Det Norske Veritas (DNV), and Germanischer Lloyd (GL) guidelines, 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 different standards and assessed the suitability for wind turbines (Frandsen et al. 2005). The RECOFF study concluded that for most “support structure” requirements, standards such as API and ISO could be used. The crucial deficiency, however, was the manner in which dynamic loads were estimated. Wind turbines are uniquely subjected to both wind and wave stochastic loading, and both loading sources are nearly equal in importance with respect to dynamic excitation of the wind turbine. IEC 61400-3 is the only international standard that specifically considers these offshore wind turbine issues. It is less mature than other international standards and guidelines, but because it is based on earlier standards, it represents an integrated version of all the work that preceded it. Also, because it is part of a series of international standards that consider the broader wind industry’s needs (such as verification testing for performance, structural design compliance, power quality, gearbox design requirements, and small turbines), it is the best available standard for the issues of structural safety for offshore wind turbines.

Codes such as the IEC 61400-3 and API RP 2A-LRFD (1997) have some overlapping design requirements for wave and current loading conditions. A direct comparison of the IEC and API standards shows, however, that other specific differences should be integrated. Examples of the differences include the following:

• The IEC uses a 50-year return period for defining extreme environmental design conditions, and API RP 2A-LRFD (1997) uses a 100-year return period for defining design conditions for high-consequence platforms.

• API RP 2A-LRFD (1997) prescribes three levels of design wave height based on the platform type and its failure consequence—IEC requires that designs use the measured site wave- height statistics and wind environment and adjusts component safety factors based on the consequence of that component failing.

• API RP 2A-LRFD (1997) is a basis for the design of offshore structures subject to wave, wind, current, and earthquake loading conditions; it does not, however, consider the scope and range of all conditions required for designing wind turbine support structures. Similarly,

IEC 61400-3 lacks some of the detailed provisions given by API RP 2A-LRFD with respect to some offshore engineering practices.

These are some examples of the code differences, but the greatest challenge is to develop a full understanding of the conflicting requirements and the real differences in safety levels for these codes so that a guideline can be developed to clarify what BOEM will require from each developer. This comparison must also evaluate the similarities and differences in the failure consequence for the types of facilities. These consequence-of-failure issues should include human life safety, environmental impact, energy supply reliability, and economic factors.

Ultimately, BOEM must determine the areas where existing standards are applicable and resolve any conflicts that may exist in establishing equivalent safety requirements for the design of offshore wind turbine systems.

The best approach will most likely use IEC standards with API RP 2A-LRFD (1997) and other standards to fill critical gaps. MMI Engineering compared structural reliability (Dolan et al. 2009) and concluded that the IEC offshore wind standard and the API RP 2A-LRFD (1997) standard supplied similar levels of safety for offshore wind turbines in three reference sites in U.S. waters (Dolan et al. 2009). Although this study gave needed assurance that API and IEC could be combined to address both the offshore foundations and substructure (API 1997; API RP 2A-LRFD) as well as the wind turbine itself (IEC 61400-3), it was not comprehensive enough to fully address tropical storms that frequent the Atlantic coastline and the need for specifications for turbines to withstand their impact. The need for this work has been recognized by IEC/TC-88 and will be examined by a follow-on maintenance team that will revisit IEC 61400-3 begining in 2010.