Industry Best Practice for Wind Turbine Foundations
Industry best practice, an often-used term in EPC construction contracts, but what does it mean and how do you ensure your EPC contractor is designing and constructing your wind turbine foundations to industry best practice?
Onshore wind turbines are multi-million dollar assets and the turbine type certification process, which draws on decades of global knowledge, offers comfort to project owners and financiers that turbine technology risk is minimized. Unfortunately, the same rigor is not always applied to the design of the very foundations which support these assets over their operating life.
Wind turbine foundations are unique structures in the civil structural engineering world. They differ from typical civil engineering structures in that: the dead-load to wind-load ratio is much lower; they experience highly variable dynamic loading with many more cycles; there is a significant stress discontinuity at the tower interface; the bearing pressure distribution is highly eccentric; and the soil-structure interaction is more complex. Because of these differences wind turbine foundations often fall outside of the scope of national building codes and standards which are typically developed for and based on research related to buildings and bridges.
In Australia, there are five main areas from which the requirements for civil engineering structures may be determined:
- Australian Government Legislation (Federal, State and Territories)
- The National Construction Code (NCC)
- Australian Standards
- Local Authority Rulings (State, Territories and Local Council Planning departments, etc.)
- Common industry practice including published codes of practice
Key Australian standards that are used to design civil engineering structures such as wind turbine foundations include:
- AS/NZS 1170: Structural design actions
- AS 3600: Concrete Structures
- AS 5100 Bridge Design Standards
The NCC and the above Australian Standards are often used to inform the design of wind turbine foundations. However, they do not address many of the differences between wind turbine foundations and typical civil engineering structures, nor do they contain explicit references to the design and construction of wind turbine foundations.
The differences between turbine foundations and typical civil engineering structures and the impact of these differences on the behaviour of the structure and the additional design considerations required can be overlooked by engineers who are not experienced with the intricacies of wind turbine foundations particularly if they use the NCC and Australian Standards as the basis of design.
An experienced engineer will, hopefully, turn to international standards including the IEC 61400 series of standards for wind turbines published by the International Electrotechnical Commission. IEC 61400-6 (currently in draft form), is an international standard developed to provide guidance for the design of onshore wind turbine towers and foundations. IEC 61400-6 is intended to build upon and complement the IEC 61400-1 Wind Turbine Design Requirements standard, which provides design basis information for wind turbines such as load combinations, design principles and load factors, the IEC 61400-6 standard will provide a set of technical requirements for the geotechnical and structural design of onshore wind turbine towers and foundations. Further searching should find DNVGL-ST-0126 Support structures for wind turbines, published by DNV GL in recognition of the existing bias of existing national standards towards buildings and bridges and the lack of a single source standard. DNVGL-ST-0126 Support structures for wind turbines specifically address the design of support structures for wind turbines, including foundations.
However, IEC 61400-6 and DNV GL-ST-0126 are arguably not required to be complied with by local contractors and designers unless explicitly called out in the contractual employer’s requirements. Moreover, these documents do not explicitly detail the engineering design, construction, testing and documentation requirements that an owner should include in their wind farm EPC contract employer’s requirements to ensure the EPC contractor is designing and constructing their wind turbine foundations to industry best practice.
So, to help owners answer the question ‘How do you ensure your EPC contractor is designing and constructing your wind turbine foundations to industry best practice?’DNV GL, leveraging its global expertise, has developed bespoke EPC Employer’s Requirements for utility-scale wind projects in Australia.