Section 2 Design basis evaluation

2.1 Module requirements

2.1.1 The purpose of this module is to check that the design basis is properly documented and provides a sufficient and sound basis to be developed into a detailed design that is certifiable to IEC 61400-22 Wind turbines – Part 22: Conformity testing and certification and IECRE OD-502 Project Certification Scheme.

2.2 Guidance on input requirements

2.2.1 The applicant’s design basis, or bases, should address the following:

design life;
design methodologies and principles, and key assumptions;
codes and standards (or any other published and validated work upon which the project will be based) for design, manufacture, transportation, installation and commissioning;
site conditions design parameters (external (wind, marine), ground conditions);
principles for material selection;
assumptions made for parameters influencing the loading, but not covered by the WT type certificate and/or modifications to the controller;
WT type including details of its type certification and any deviations;
relevant statutory requirements;
a description of the support structure concept;
an overview of methods and environmental conditions for transportation and installation;
any factors which could affect manufacturing, transportation or installation;
commissioning philosophy;
operations, inspection and maintenance philosophy;
details of grid connection (mandatory for IEC 61400-22 Wind turbines – Part 22: Conformity testing and certification, while an optional item for IECRE OD-502 Project Certification Scheme projects); and
any other requirements specified by the client or applicant.

2.2.2 Where appropriate to the site and the type of asset(s) under consideration, the design basis for geotechnical aspects should include:

location of foundations, foundation type and configuration;
summary description of soil and seabed conditions;
design soil parameters for each purpose (e.g. capacity, stiffness, installation);
methodology for foundation capacity;
methodology for foundation response (stiffness and damping) and settlement;
foundation installation methodology;
scour philosophy to include likelihood and scour allowances or mitigation measures;
cyclic loading effects;
seismic earthquake effects including magnitude;
seabed hazards, assessment of impact on structure/foundation and mitigation measures;
foundation acceptance criteria and definition of load and resistance or material factors and any other performance requirements (e.g. limits on settlement or rotation);
installation tolerances; and
other interfaces to be considered (e.g. jack-up footprint–foundation interaction).

2.2.3 The structural design basis should also address the following, including a description of how the respective aspects have been selected or established:

design parameters for the external conditions including wind, wave, current, seismic, boat-impact, dropped object and other accidental conditions;
philosophy for corrosion management;
marine-growth thickness and hydrodynamic coefficients;
all design load cases, including load combinations, load factors, load reduction factors, extreme loads and accidental loads;
air-gap requirements for blade clearance (in both the operating and parked conditions) and for any local structure not designed for wave loading;
for floating offshore structures, which hydrodynamic parameters were considered and how they were calibrated in the numerical model;
inter-array effects, such as wake effects (in the absence of a more detailed methodology, Annex D, IEC 61400-1 Wind turbines – Part 1: Design requirements can be used to determine the wake and wind farm turbulence);
WT availability assumptions;
the interface between the substructure and its foundation; and
the adequacy of the transition piece and its (grouted or bolted) connections (if applicable).

2.3 Evaluation methodology

2.3.1 Regardless of the modules in LR’s scope as certification body, the design basis evaluation will consider all project phases and the operational phase of the wind farm.

2.3.2 LR’s design basis evaluation will confirm that appropriate codes and standards have been selected, and that the considerations described in Ch 2, 2.2 Guidance on input requirements have been incorporated, as appropriate for the asset(s) being evaluated.

2.3.3 As part of this evaluation LR will confirm that the parameters reviewed during the site condition assessment have been correctly incorporated in the design bases.

2.3.4 The design basis evaluation will include assessment of the design methodologies and acceptance criteria for the ultimate limit states (ULS), service limit states (SLS), fatigue limit states (FLS), accidental limit states (ALS), natural frequencies, ship impact analysis and cyclic loads for the support structure and foundations, and mooring systems as applicable.

2.3.5 Evaluation of the design basis for electrical aspects will be limited to grid connection aspects, where required, noting that this is an optional evaluation under IECRE OD-502 Project Certification Scheme, including confirmation that the client has conducted, or is planning to conduct, power system studies according to relevant National or International Standards to show that the project is capable of meeting the grid in-feed requirements at the point of common coupling with the existing grid system. Such calculations will cover all conceivable operational and wind level scenarios that could impact the grid and will include predicted voltage fluctuation levels, fault levels, MVA flows, voltage and frequency excursion limits prior to disconnect, transformer in-rush currents and harmonic generation levels. The design basis evaluation for electrical aspects will also include the proposed earthing practices to be followed, confirming that they are acceptable to the grid operator, and consideration of lightning protection and energy dissipation.

2.3.6 If cables are required to be evaluated as part of the certification scope (as an ‘other installation’), the design basis evaluation will entail confirmation that the proposed codes and standards, and the proposed methodology, for the design and analysis of the cable system are suitable for the main properties of the cable (and ancillaries as applicable) including strength, protection and fatigue.