Section
3 Integrated loads analysis evaluation
3.1 Guidance on module requirements
3.1.1 The purpose of the integrated loads analysis (ILA) is to confirm that the
site-specific loads and load effects on the integrated WT structure, including the
rotor nacelle assembly (RNA) plus the support structure and supporting soils (where
applicable), have been derived in conformity with the design basis.
3.1.2 Loads analysis defines the site-specific loads and load effects on the
WT and supporting structure, foundations and RNA. The loading conditions report
should demonstrate that the loading calculations comply with the design basis.
Ideally the design basis evaluation must therefore be completed before starting this
module, however this may not always be practicable. The final ILA evaluation should
not be undertaken before the design basis evaluation has been completed.
3.1.3 If the loads and load effects in the design basis are less onerous than
those assumed for the WT type certification, and the support structure and the WT
characteristics are identical, no further loads analysis needs to be undertaken. For
offshore sites, it is recommended that such an analysis is undertaken to account for
the different load conditions that arise from the site-specific environmental
conditions and the project- and site-specific support structure concept.
3.2 Guidance on input requirements
3.2.1 Where deemed necessary following the considerations in Ch 2, 3.1 Guidance on module requirements 3.1.3, loads analyses should be completed by the applicant. The applicant
shall provide full documentation to the certification body of the load calculations
and a comparison with the loads assumed for the type certificate.
3.2.2 The effect of upstream conditions should be considered when they can be reasonably
foreseen. Examples of when upstream wind conditions could influence a wind farm
are:
- wind farms which are being developed as a series of zones; and
- wind farms which may be influenced by other upwind wind farms.
3.2.3 Estimations of conditions within the wind farm should be made, and consideration as
to how the primary conditions of wind speed and turbulence intensity vary within the
wind farm array should be provided at a selection of WT locations.
3.2.4 The loading analysis should consider all the load cases described in
Section 7.3, IEC 61400-3 Wind turbines – Part 3: Design requirements for offshore
wind turbines; as a minimum, the design load cases specified in Table
1 of that standard should be considered. In addition, if the site condition
assessment reports have indicated that sea ice may occur at the site, then the
design load cases as specified in Table 2 of that standard should also be
considered.
3.2.5 The fatigue load cases are considered to be adequately covered by IEC
61400-1 Wind turbines – Part 1: Design requirements and IEC 61400-3-1 Wind
energy generation systems – Part 3-1: Design requirements for fixed offshore
wind turbines. However, for ultimate strength load cases, additional
requirements are specified in Section 7.6, IEC 61400-3-1 Wind energy generation
systems – Part 3-1: Design requirements for fixed offshore wind
turbines.
3.2.6 Type certification usually assumes a Rayleigh wind speed probability
distribution (reference 3.1.1 of IEC 61400-1 Wind turbines – Part 1: Design
requirements). Other probability distributions can also be used, provided
the applicability is demonstrated by means of data which is appropriate for the wind
farm site and that a good fit is proven between measurements and the theoretical
distribution. If the distribution selected to represent the wind farm site is
different from that used for type certification, then the effect of this on the
calculated fatigue life of the WT facility should be assessed and documented.
3.2.7 Loading conditions for fixed structures should generally follow the
requirements of IEC 61400-3-1 Wind energy generation systems – Part 3-1: Design
requirements for fixed offshore wind turbines; accidental conditions,
including vessel impact and dropped objects, and post-accident conditions should
also be taken into account.
3.2.8 To account for the facility being unmanned, the return period of the
extreme environmental actions defined in ISO 19902 Petroleum and natural gas
industries – Fixed steel offshore structures, ISO 19903 Petroleum and
natural gas industries – Fixed concrete offshore structures and API RP
2A-WSD Recommended Practice for Planning, Designing and Constructing Fixed
Offshore Platforms – Working Stress Design may be reduced by a factor of two
(i.e. 50-year return period environmental actions may be used instead of 100-year
environmental actions).
3.2.9 Loading conditions for floating structures should generally follow the
requirements of IEC TS 61400-3-2 Wind energy generation systems – Part 3-2:
Design requirements for floating offshore wind turbines, supported by
DNVGL-ST-0119 Floating wind turbine structures, considering the following:
- the coupled analysis of the WT and floating support structure to
investigate the response motions of the overall systems, especially in
operating conditions, and its effects in the global dynamics;
- tank (hydrostatic) testing conditions should be included in the
design load cases; and
- time-domain simulations must be of sufficient duration to give
stable response statistics; in general, continuous three-hour simulations
are necessary.
3.2.10 Loading conditions for floating structures should also include the
following accidental/fault conditions in the design load cases defined in Paragraph
7.4.9, IEC 61400-3-1 Wind energy generation systems – Part 3-1: Design
requirements for fixed offshore wind turbines, and Paragraph 7.4 as per IEC
TS 61400-3-2 Wind energy generation systems – Part 3-2: Design requirements for
floating offshore wind turbines:
- accidental flooding (caused by, for example, collision,
equipment failure or human error) of any single compartment below the
waterline; accidental flooding of more than one compartment may need to be
considered, depending on the consequences of the collision load cases;
- the failure of any single mooring line;
- vessel impact and dropped objects; and
- post-accident conditions.
3.2.11 For floating structures, the load cases specified in Pt 3, Ch 10, 4
Design aspects of LR’s Rules and Regulations for theClassification of
Special Service Craft, July 2019 should be considered where wind, waves and
current are acting, unless suitable site-specific combinations have been derived. To
account for the facility being unmanned, the return period of the various
environmental actions defined in the LR Rules may be reduced by a factor of two
(e.g. use a 50-year return period rather than a 100-year period).
3.2.12 A Campbell diagram should be produced to identify the natural frequencies of the
system which are at risk from resonance. A Campbell diagram is a plot of natural
frequency against rotor operational speed. A Campbell diagram should take account of
the following dynamic effects:
- centrifugal stiffening affecting the natural frequency of blade modes; and
- transformation of the blade natural frequencies from the rotating frame to
the stationary frame (e.g. by multi-blade coordinate transform).
It is recommended that the Campbell diagram covers natural frequencies at least up to
five times the blade passing frequency. Additionally, it is recommended that the
uncertainty in natural frequency, due to manufacturing tolerances, variations in
soil conditions, lifetime stiffness variation and weight due to marine growth and
corrosion, is taken into account.
3.2.13 Wind veer is a variation in wind direction with height above sea level.
Its occurrence can cause larger loading on the support structure at the blade
passing frequency which may not be captured by other design load cases. For WTs with
a support structure natural frequency in the range of blade passing frequencies
bounded by the minimum and maximum generator speeds, it is recommended that the WT
and support structure loading are evaluated at the wind speeds where the natural
frequency is closest to the blade passing frequency with appropriate wind veer for
fatigue and extreme cases to assess the risk of resonance.
3.2.14 The mass and stiffness of the structure and the soil may change
considerably during a WT’s lifetime. Scour, corrosion, marine growth, soil settling
and sand movement may influence the WT’s natural frequencies, which needs to be
considered in the ILA by applying the most adverse conditions. Mean values may be
applied for fatigue analysis if 19 there are no resonant operational modes. If the
WT may operate within the resonance range of the support structure, within ±5 per
cent of the support structure’s natural frequency, suitable limits of permissible
vibrations need to be defined within the ILA (and vibration monitoring systems need
to be provided).
3.2.15 If the certification body is required to undertake an independent ILA ( see
Ch 2, 3.3 Evaluation methodology), the following inputs
will also be required:
- structure and geometry of the RNA including blade aerodynamics, structural
and mass distributions of blades, electrical drivetrain properties and
details of mechanical and electrical actuation systems;
- controller data file in a format compatible with the WT simulation tool in
use by LR;
- description of the WT controller philosophy and details of any shutdown
triggers and procedures; and
- details of the site-specific support structure including
thicknesses, section diameters, joint locations, secondary steel locations
and masses, model test report(s), hydrodynamic coefficients and the
anchoring system (as applicable).
3.3 Evaluation methodology
3.3.1 LR’s evaluation should not commence until the applicant has provided their ILA
including their comparison with the loads considered in the WT type
certification.
3.3.2 LR’s evaluation will consider:
- the combinations of external conditions and design situations (e.g. normal,
fault, transport, installation);
- the respective partial load safety factors;
- the calculation methods (e.g. simulation procedure, number of simulations
and combinations of wind and wave loads), if applicable;
- the design driving load cases defined with reference to the site conditions
and the operation and safety system of the WT; and
- any difference between the site-specific loads and the loads assumed for the
type certificate.
3.3.4 Timing of LR’s independent ILA, if undertaken, is important, and should be agreed
with the applicant for each project to support the project’s design process; it will
typically be undertaken after LR’s review of the applicant’s initial ILA, and should
be timed so that the results can be compared with a specific revision or iteration
of the applicant’s ILA. LR will normally undertake a single independent ILA, unless
the applicant requires additional iterations to align with their own ILA
iterations.
3.3.5 When undertaking an independent ILA, LR will develop a model of the
integrated WT support structure (WT, tower, substructure and foundations) with
associated project site conditions in the WT simulation tool in use by LR, once
there is sufficient level of detail in the project design parameters, in accordance
with the chosen scenario and iteration. LR will provide a specification for the
necessary basic input data required for integrated WT structure model, and use the
required inputs from the site conditions evaluation and design bases. For foundation
modelling, LR will derive their own pile–soil interaction (PSI) model, based on the
provided inputs, aligning this with the designer's foundation model in order to
limit the potential for differences in the analysis results.
3.3.6 When undertaking an independent ILA, the following processes will be
performed during the set-up and loads analysis:
- set up an independent model;
- modal analysis of the full WT structure;
- establish a reduced set of load case simulations;
- independent parallel calculations of loads, taking account of
the site-characteristic variations of wind and wave direction, as specified
in the design basis;
- comprehensive end-to-end checks of all parameters, model
choices and output time series for sufficient load cases to capture the
driving events for extreme load components;
- review and independent verification of fatigue load cases (for
instance to establish a list which is sufficient to calculate at least 90
per cent of the fatigue damage); and
- post-processing of time series code checks, for example to
prepare ultimate driving load components, extrapolation of extreme loads,
local stress time series, damage equivalent loads, Markov matrices (via
rainflow cycle counting) or fatigue damage accumulation of local stress time
series.
3.3.7 LR will make initial sensitivity checks, before the full set of load
calculations, on loads used for design (FLS and ULS) – in particular investigating
the consequences of variability of structural frequencies for the selected design
positions. Where the wind farm comprises a range of soil conditions, LR would
typically select two positions, representing the softest and stiffest locations, but
these can be adjusted if there is evidence to demonstrate that other locations in
the wind farm are more critical. This enables LR to optimise its approach by
rationalising (and reducing) the number of different load case simulations essential
to characterise the wind farm and which will enable the evaluation of the design of
the structures.
3.3.8 The independent ILA will be based on the approved design basis and with
the identified load cases that are necessary to demonstrate compliance with the
requirements of IEC 61400-1 Wind turbines – Part 1: Design requirements, IEC
61400-3-1 Wind energy generation systems – Part 3-1: Design requirements for
fixed offshore wind turbines, IEC TS 61400-3-2 Wind energy generation
systems – Part 3-2: Design requirements for floating offshore wind turbines
and other standards referenced in the project design basis.
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