Since the early start of the wind turbine industry in the ‘80s independent certification by third parties has played an important role to minimise the number of errors in design and to provide confidence between the manufacturers, developers, owners, finance, insurance and the authorities.
Ensuring the quality of wind turbines
Wind turbines should generally have as a minimum type approval according to a recognized certification system, such as IEC WT 01, and there may also be additional local national requirements applicable to the chosen project specific approval system. DNV offers type certification services according to a certification scheme defined by the International Electrotechnical Commission (IEC) in the document IEC WT 01 and also according to other various national schemes.
The following modules of the IEC WT 01:2001 scheme are mandatory:
- Design evaluation
- Type testing
- Manufacturing evaluation
- Final evaluation.
The following modules of the IEC WT 01:2001 scheme are optional:
- Foundation design evaluation
- Type characteristic measurements.
Design evaluation
The design evaluation is normally carried out as a review of the design documentation submitted by the manufacturer.
In addition to the documentation review, independent analyses will usually be carried out for critical parts of the wind turbine.
Independent load calculations are performed using an aeroelastic program different from the one used by the manufacturer. For this purpose DNV uses two advanced aeroelastic programs, namely FLEX5 and HAWC.
The DNV Nauticus program is used for the independent gear rating calculations. This program has been developed on the basis of the DNV classification note 41.2.
If required, FEM analysis may be carried out for critical components. DNV has access to several commercial FEM program systems.
Type testing
Type testing is performed in order to verify the wind turbine performance with respect to power production performance and power quality. Furthermore, type tests and measurements are carried out in order to verify the load calculations and the blade design. Type testing will normally comprise of:
- Power performance
- Functional testing
- Load measurements
- Dynamic and static full-scale blade testing.
Normally, an accredited laboratory carries out the main parts of these type tests and DNV reviews test plans and test reports. If some type tests are not carried out by accredited laboratories DNV will verify these through completion of audits and surveys.
Manufacturing evaluation
DNV carries out a survey of the manufacturing of one wind turbine sample to be covered by type certification according to IEC WT 01. Wind turbine manufacturers normally operate a certified quality system according to the ISO 9000. For such certified quality systems DNV performs an audit to confirm that the work instructions, purchase specifications, quality control procedures and records etc. follow the design covered by certification. Extended audits are relevant for quality systems that are not certified.
Repeated surveys of manufacturing of wind turbines can be carried out by DNV but this is normally outside the type certification.
Foundation design evaluation
The foundation design evaluation module is carried out as a review of the design documentation received from the client.
Type characteristic measurements
The type characteristic measurements module comprises of one or more of the following elements:
- Power quality tests
- Acoustic noise measurements.
The measurements shall be carried out by accredited testing laboratories. Otherwise DNV will verify that the test is carried out to IEC/ISO 17200 or IEC/ISO 17025, as applicable.
Issue of the type certificate
When the wind turbine subject to type certification is found to comply with the applicable code/standards/requirements, a final verification report and a type certificate will be issued.
Maintenance of the type certificate
The validity period of a wind turbine type certificate is specified in the certificate and is typically between one and five years.
Annual surveys of randomly chosen specimens of each type of turbine are carried out during the validity period of the type certificate for the purpose of verification of the built wind turbine in relation to the design initially approved by DNV.