Renewables - OE February 2011

February 2, 2011

Cape Wind clears another hurdle
The Cape Wind offshore wind power project in January received a final permit from the US Environmental Protection Agency to begin construction of the first US offshore wind farm. The agency's approval came a day after the US Army Corps of Engineers issued Cape Wind a permit allowing construction of the controversial project.

The Obama administration has promoted Cape Wind in its push to increase the use of renewable energy. Last year, Interior secretary Ken Salazar announced the department's approval of the project, which will comprise 130 3.6MW wind turbines installed off the coast of Massachusetts. Opposition from Cape Cod residents and indigenous groups delayed the project for years and forced the developer to scale down and reconfigure original plans.

Cape Wind president Jim Gordon said receipt of the last permit approval was the culmination of decade's effort by his company and 17 federal and state agencies. ‘American drive, competitiveness and ingenuity is entering the race with Europe and Asia for leadership in this burgeoning new global industry,' he added.

The company said it will now focus on securing financing for the project. Gordon has said he hopes to see the wind farm generating power by the end of 2012.

The state announced last year a proposed marine terminal in the Massachusetts port of New Bedford that would be the first North American facility designed for the staging and assembly of offshore wind turbines. Also last year, Massachusetts company Mass Tank and Germany's EEW entered an agreement to open a new manufacturing facility to supply monopole foundations and transition pieces for the wind farm. (by Russell McCulley)

Class society guidance Windfall Three of the leading classification bodies announced studies, standards or guidance for the offshore wind farm sector within weeks of each other at the end of 2010. ABS released specific design guidance for fixed wind farm structures in US tropical storm waters while DNV and Bureau Veritas launched new standard initiatives relating to the development of floating facilities. David Morgan reports.

ABS in Houston hailed its Guide for Building and Classing Offshore Wind Turbine Installations as the first to address design considerations for bottom founded wind turbine support structures in US Outer Continental Shelf (OCS) tropical storm prone areas such as the Gulf of Mexico and East Coast. Previous guides have been primarily based on experience from European coastal waters, says ABS managing principal engineer Qing Yu, the new guide's principal author.

As well as the unique environmental conditions on the US OCS, the guide takes into account the wellestablished International Electrotechnical Commission (IEC) 61400 series of standards for wind turbines, ABS' offshore rules and guides and the American Petroleum Institute's Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms (API RP 2A).

‘The direct application of design criteria in existing standards, such as IEC 61400-3, is not sufficient for the offshore wind turbines in US waters,' explains Qing Yu. ‘We incorporate additional requirements based on calibration studies that use regional and site-specific conditions of US waters,' he adds.

The ABS guide incorporates refinements to the design environmental conditions and design load cases required by IEC 61400-3 to account for the effects of tropical hurricane conditions. Yu points out the principles of site-specific design are more directly addressed in the definition of the design load cases. ‘Omnidirectional wind conditions are required for the design load case in the event an offshore wind turbine loses its connection to the electrical power grid during a tropical hurricane.'

The guide's strength design criteria for the turbine steel support structures are based on the commonly accepted working stress design (WSD) approach. ‘Allowable stresses are defined for a variety of design conditions including normal, abnormal, transport and installation, as well as earthquake and other rare conditions,' explains Yu. ‘In addition, equivalent load and resistance factor design (LRFD) criteria are defined as an alternative to the WSDbased criteria.'

Floating forward

Business may be booming in the fixed offshore turbine sector, but the market for floating variants is still very much in its infancy even though the deep waters around Japan, China, Southern Europe and the USA are seen as significant untapped markets for such structures.

‘However, floating wind turbines introduce new risks and technological challenges related to stability, station keeping, power transmission and structural strength,' says Johan Sandberg, the business development leader for wind at Norway's DNV. ‘In addition, economic aspects are likely to be challenging in the early phases. One barrier to the growth and development of this industry has been the lack of a design standard.'

In a bid to speed up progress in this area DNV and its industry partners have launched a joint industry project with the aim of developing a common design standard for wind farm floaters. By sharing best practice in all aspects of the design process – including safety philosophy, loads and load effects, materials, code formats, floating stability and mooring – the JIP is looking to develop a fully fledged, global standard as well as provide guidance for construction and in-service inspection work.

‘A lot will be learned from the traditional offshore industry,' says Sandberg. ‘The experience of the JIP partners, ranging from designers and developers to operators and turbine manufacturers, is very important.' In DNV's experience, ‘co-operation to create unified rules results in faster industrial progress', explains Sandberg, adding that the resulting common framework ‘supports the fast-moving development of specific technology and business' which is especially valuable to emerging industries.

DNV is contributing research, experience and riskbased methodologies from three of its core industries – wind energy, maritime and offshore oil & gas – using its existing offshore wind turbine standard for fixed installations and recently developed guideline on offshore floating structures as a basis. ‘It is still possible for other companies to join the project,' notes Sandberg.

Gallic guidance

Meanwhile, France's Bureau Veritas has issued its own guidelines for the classification and certification of floating offshore wind turbines. Guidance Note NI 572 covers floating platforms supporting single or multiple turbines with horizontal or vertical axes, and specifies the environmental conditions under which these turbines may serve, the principles of structural design, load cases for the platform and mooring system, stability and the structural division and design criteria for the top structure.

Echoing DNV's sentiments, Maxime Pachot, BV's offshore wind turbine manager, says: ‘There is growing demand for offshore wind turbines which can be safely installed in very deep water locations. They will use one or more types of floating platform to mount the turbine and may need a service life equivalent to offshore oil & gas projects. Both operators and authorities need to know these platforms are safe and will be up to the job. Although this is a new way of generating energy out at sea, it builds on proven technology and experience.'

These guidelines, covering three categories of floater – ballast, TLP and buoyancy – marry BV experience with and rules for offshore floating units and moorings with the internationally accepted standards for wind turbine topsides set out in IEC 61400-3. ‘They will help field developers choose the right system and the right pathway for approval to meet local and international regulations and their own industrial requirements,' says Pachot.

BV is a partner in the EU HiPRWIND project which brings together 19 companies with the aim of developing enabling technology elements for deepwater offshore wind. (by David Morgan)


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