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General

Within the scope of the site assessment, records of the conditions prevailing at the installation site shall be checked for plausibility. This includes the features listed in the following sections. Additional conditions that are necessary to be considered and are not listed in these sections should also be stated. Locations at which special events, e.g. cyclones and floodings, may be expected shall be considered in consultation with GL Wind.

Furthermore, in the case of confirmation of an existing certificate, a comparison shall be undertaken with the conditions assumed during the certification.

Determination of the wind conditions

The basic parameters listed below for the wind at the turbine site shall be determined:

• air density rho
• reference wind speed V_ref
• annual average wind speed V_ave
• wind speed distribution
• wind direction distribution (wind rose)
• turbulence intensity I₁₅ for V_hub = 15 m/s
• wind shear (see also Section 4.2.3.1.2 of [2])
• inclination of the average wind direction from the horizontal plane

The wind conditions can be determined at the intended site by measurements. The site conditions shall be correlated with long-term records of local meteorological stations.

Determination of other environmental conditions

The environmental conditions shall be determined for the comparison with the assumptions made in the design of the wind turbine.

Determination of the electrical power network conditions

The electrical conditions shall be determined at the grid connection point between the wind turbine and the existing electrical grid at the intended site, in order to ensure compatibility between the turbine and, where necessary, all electrical equipment located between the turbine and the grid. This shall include the following items at least: 

• normal supply voltage and fluctuations
• normal supply frequency and fluctuations
• voltage symmetry
• symmetrical and asymmetrical faults
• number and type of the electrical grid outages and their average duration
• Special features of the electrical grid at the site as well as requirements of the local grid operator shall be taken into account. These may be:
  • auto-reclosing cycles
  • short-circuit impedance at the connection points of the wind turbine
  • harmonic voltage distortion from the turbine’s power system

Site specific support structure (tower, substructure and foundation)

The support structure carries the machinery part of the wind turbine and consists of tower, substructure and foundation. The assessment of the support structure is performed on the basis of the site specific load assumptions, which were assessed before. The following tasks are to be solved:

• Derivation of the soil parameters from the soil investigation report
• Eigenfrequencies of the complete system (considering soil stiffness as well as stiffness and mass distributions for support structure, nacelle and rotor blades)
• Investigation of the conformity of eigenfrequencies as considered in load simulations with those derived by variations of measured water depth / soil stiffness and support structure within the wind farm
• Dents and buckling
• Punching shear
• Loss of static equilibrium, e.g. overturning of the foundation
• Ultimate strength
• Fatigue strength
• Acceptable deformations
• Corrosion protection
• Consideration of inspectability and maintenance for the selection of fatigue material safety factors
• Connections (e.g. bolted, welded and grouted joints)
• Scour (scour protection, acceptable scouring, intervals fort he examination of scour protection / scour depth)
• Verification of „ship friendly design of the substructure in case of collision“: It is to be agreed upon with the authority, whether this verification is mandatory

Verifications of the ultimate and fatigue strengths are to be carried out for all load carrying structures and connections (welded connections, bolted connections and grouted connections). For the soil the general bearing capacity, base failure and in some cases dynamic-cyclic strength and deformations are to be verified.

GL Wind carries out parallel calculations and examination of drawings. The submitted drawings and specifications have to make clear statements concerning manufacturing tolerances, qualitiy stages / - statements as well as manufacturing procedures. For welded structures the welding instructions (WPS = welding procedures specification or pWPS = preliminary welding procedures specification) are to be submitted.

The successful completion of the assessment is concluded with a Certification Report for the support structure.

Assessment of the foundation / soil properties

The foundation (soil properties) at the intended site shall be assessed in accordance with the local situation (subsoil, building codes) by a geotechnical report as a rule.

 

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