hub height wind turbines height of the centre of the swept area of the wind turbine rotor above the mean sea level 3.15 hummocked ice crushed ice and ice floes piled up into ridges whe
Symbols and units
A C Charnock’s constant [-] d water depth [m] f p wave spectral peak frequency [s –1 ] g acceleration due to gravity [m/s 2 ]
The thickness of sea ice, denoted as \$h\$ in meters, is a critical measurement in understanding ice conditions The term \$h_N\$ represents the thickness of sea ice with a recurrence period of \$N\$ years, also measured in meters Additionally, \$h_m\$ refers to the ice thickness that corresponds to the long-term mean value of the annual maximum ice thickness observed during winters with ice, measured in meters.
H N individual wave height with a recurrence period of N years [m]
H sN significant wave height with a recurrence period of N years [m]
H redN reduced wave height with a recurrence period of N years [m] k wave number [-]
K max accumulated freezing degree-days [°C] s sea floor slope [°] p(V hub ) probability density function of hub height wind speed [-]
S η single sided wave spectrum [m 2 /Hz]
R d design value for component resistance [-]
R k characteristic value for component resistance [-]
S d design value for load effect [-]
S k characteristic value for load effect [-] t time [s]
T z mean zero-crossing wave period [s]
U ss sub surface current velocity [m/s]
U bw breaking wave induced surf current velocity [m/s]
V N expected extreme wind speed (averaged over 10 min), with a recurrence period of N years [m/s]
The variable \( V_{redN} \) represents the reduced extreme wind speed, averaged over three seconds, with a recurrence period of \( N \) years, measured in meters per second (m/s) The sea surface elevation, denoted as \( \eta \), is referenced relative to the still water level (SWL) in meters (m) The von Karman’s constant is represented by \( \kappa \), while \( \lambda \) indicates the wavelength in meters (m) The wave direction is given by \( \theta_w \) in degrees (°), with the mean wave direction denoted as \( \theta_{wm} \) and the current direction as \( \theta_c \) Additionally, \( \sigma \) represents the standard deviation of sea surface elevation in meters (m), and \( \tau \) indicates temperature in degrees Celsius (°C).
Abbreviations
ECD extreme coherent gust with direction change
EWLR extreme water level range
EWM extreme wind speed model
NWLR normal water level range
NWP normal wind profile model
RWM reduced wind speed model
General
The engineering and technical requirements to ensure the safety of the structural, mechanical, electrical and control systems of an offshore wind turbine are given in the following clauses
This specification outlines the requirements for the design, manufacturing, installation, and operational manuals of offshore wind turbines, along with the associated quality management processes It also incorporates safety procedures established in the various practices related to the installation, operation, and maintenance of these turbines.
Design methods
This standard requires the use of a structural dynamics model to predict design load effects
Such a model shall be used to determine the load effects for all relevant combinations of
– 18 – 61400-3 © IEC:2009 external conditions and design situations as defined in Clause 6 and Clause 7 respectively A minimum set of such combinations has been defined as load cases in this standard
The support structure design for an offshore wind turbine must be tailored to the specific external conditions of the site, as outlined in Clause 12 These conditions should be clearly summarized in the design basis.
The rotor-nacelle assembly, originally designed according to the standard wind turbine class outlined in IEC 61400-1, 6.2, must be evaluated to ensure that the unique external conditions of the offshore site do not jeopardize its structural integrity.
The demonstration will compare the loads and deflections for specific offshore wind turbine site conditions against those from the initial design, considering reserve margins and environmental impacts on structural resistance and material selection Additionally, the calculations will factor in site-specific soil properties affecting the dynamic characteristics of the offshore wind turbine, along with potential long-term variations in these properties due to seabed movement and scour.
The design process for an offshore wind turbine, as shown in Figure 2, highlights the essential components and relevant clauses of the standard This iterative process includes load and load effect calculations for the entire wind turbine, which consists of the integrated support structure and rotor-nacelle assembly The structural design is considered complete once its integrity is confirmed through the limit state analyses outlined in section 7.6.
Full-scale testing data can enhance confidence in predicted design values and validate structural dynamics models and design scenarios For guidance on measuring mechanical loads during full-scale testing, refer to IEC 61400-13.
The adequacy of the design must be verified through calculations and/or testing When utilizing test results for this verification, it is essential that the external conditions during testing accurately represent the characteristic values and design situations outlined in the standard Additionally, the chosen test conditions, including test loads, should consider the applicable safety factors.
Design basis for offshore wind turbine
Support structure design RNA design
RNA design (e.g IEC 61400-1, standard wind turbine class)
Design situations and load cases (7.4)
Load and load effect calculations (7.5)
Figure 2 – Design process for an offshore wind turbine
Safety classes
An offshore wind turbine shall be designed according to one of the following two safety classes:
• a normal safety class that applies when a failure results in risk of personal injury or other social or economic consequence;
• a special safety class that applies when the safety requirements are determined by local regulations and/or the safety requirements are agreed between the manufacturer and the customer
Partial safety factors for normal safety class wind turbines are specified in 7.6 of this standard
Partial safety factors for wind turbines in the special safety class must be mutually agreed upon by the manufacturer and the customer Offshore wind turbines that meet the criteria for this special safety class are classified as class S turbines, as outlined in section 6.2.
Quality assurance
Quality assurance shall be an integral part of the design, procurement, manufacture, installation, operation and maintenance of offshore wind turbines and all their components
It is recommended that the quality system complies with the requirements of ISO 9001
Rotor – nacelle assembly markings
The following information, as a minimum, shall be prominently and legibly displayed on the indelibly marked rotor – nacelle assembly nameplate:
• hub height operating wind speed range, V in – V out ;
• IEC wind turbine class (see IEC 61400-1);
• rated voltage at the wind turbine terminals;
• frequency at the wind turbine terminals or frequency range in the case that the nominal variation is greater than 2 %
General
The external conditions described in this clause shall be considered in the design of an offshore wind turbine
Offshore wind turbines face various environmental and electrical conditions that can impact their loading, durability, and overall operation To guarantee safety and reliability, it is essential to consider environmental, electrical, and soil parameters during the design process, which must be clearly documented in the design specifications.
Environmental conditions encompass wind, marine factors such as waves, sea currents, water levels, sea ice, marine growth, seabed movement, and scour, along with other environmental elements Additionally, electrical conditions pertain to the network conditions.
Account shall be taken of the soil properties at the site, including their time variation due to seabed movement, scour and other elements of seabed instability
Wind conditions are crucial for ensuring the structural integrity of the rotor-nacelle assembly, while marine conditions can also impact this integrity depending on the support structure's dynamic properties Even if marine conditions are deemed negligible during the design phase, it is essential to demonstrate structural integrity by considering the specific marine conditions at each offshore wind turbine installation site.
Other environmental conditions also affect design features such as control system function, durability, corrosion, etc
External conditions are classified into normal and extreme categories Normal conditions involve recurring structural loading, whereas extreme conditions pertain to rare design scenarios Design load cases must include critical combinations of these external conditions alongside wind turbine operational modes and other design situations.
The normal and extreme conditions to be considered in design are prescribed in the following subclauses.
Wind turbine classes
When designing offshore wind turbine installations, it is crucial to consider the external conditions specific to the intended site type According to IEC 61400-1, wind turbine classes are categorized based on wind speed and turbulence parameters, primarily aimed at addressing most onshore applications.
The classification of offshore wind turbines based on wind speed and turbulence parameters is essential for designing the rotor-nacelle assembly effectively.
Class S wind turbines are designated for specific external conditions or safety requirements as specified by the designer or customer.
To fully define the external conditions for designing an offshore wind turbine, it is essential to consider not only wind speed and turbulence intensity, which categorize wind turbine classes, but also several other critical parameters, particularly marine conditions The specific values for these additional parameters are detailed in sections 6.3, 6.4, 6.5, and 6.6.
The design lifetime shall be at least 20 years
The manufacturer must detail the models and essential design parameters in the design documentation If the models specified in Clause 6 are utilized, it is adequate to provide a statement of the parameter values Additionally, the design documentation should include the information outlined for reference in Annex A.
The abbreviations added in parentheses in the subclause headings in the remainder of this clause are used for describing the wind conditions for the design load cases defined in 7.4.
Wind conditions
An offshore wind turbine shall be designed to safely withstand the wind conditions adopted as the basis of design
The wind regime for offshore wind turbines is categorized into normal and extreme wind conditions Normal wind conditions are expected to occur more than once a year during typical operations, while extreme wind conditions are characterized by a recurrence period of either 1 year or 50 years, crucial for load and safety assessments.
The support structure design for an offshore wind turbine must be grounded in wind conditions that accurately reflect the specific site This assessment should adhere to the guidelines outlined in Clause 12.
The design of the rotor-nacelle assembly must consider wind conditions that are either specific to the site or defined by the models and parameters in IEC 61400-1 It is essential to demonstrate that these site-specific external conditions do not jeopardize the structural integrity of the assembly.
1 are used as the basis of design of the rotor – nacelle assembly, the following exceptions to the models and parameter values may be assumed:
• the inclination of the mean flow with respect to a horizontal plane is zero;
The wind profile, denoted as V(z), represents the average wind speed as a function of height, z, above the still water level For standard wind turbine classes, the typical wind speed profile follows the power law.
V = (3) where, for normal wind conditions, the power law exponent, α, is 0,14
The extreme wind speeds averaged over three seconds (V e50, V e1) and the extreme wave heights (H 50, H 1) are considered uncorrelated, leading to a conservative combination Consequently, the reduced extreme wind speeds (RWM) will be utilized alongside the extreme wave heights.
Marine conditions
Offshore wind turbines must be engineered to endure the specific marine conditions established for their design, which encompass factors such as waves, sea currents, water levels, sea ice, marine growth, scour, and seabed movement Additionally, other pertinent external conditions related to the offshore environment are outlined in section 6.5.
The support structure design for an offshore wind turbine must consider environmental factors, particularly the marine conditions specific to the turbine's location.
The designer shall consider the influence of marine conditions on the rotor – nacelle assembly
The rotor-nacelle assembly of offshore wind turbines is typically designed for a variety of marine environments rather than a specific site Designers often consider generic marine conditions that are at least as severe as those expected during the turbine's operation Through appropriate analysis, it may be demonstrated that the marine environment has a negligible impact on the structural integrity of the rotor-nacelle assembly, depending on the dynamic properties of the support structure and the assumed design conditions.
Marine conditions for load and safety in offshore wind turbines are categorized into two types: normal marine conditions, which are expected to occur more than once a year during standard operations, and extreme marine conditions, characterized by a recurrence period of either 1 year or 50 years.
Waves exhibit irregular shapes and vary in height, length, and speed, often approaching offshore wind turbines from multiple directions at once To accurately represent the characteristics of a real sea, it is essential to describe sea states using a stochastic wave model.
The stochastic wave model characterizes the sea state as a combination of numerous small frequency components, each representing a periodic wave with distinct amplitude, frequency, and propagation direction, while maintaining random phase relationships A design sea state is defined by a wave spectrum, \( S_\eta \), along with significant wave height, \( H_s \), peak spectral period, \( T_p \), and mean wave direction, \( \theta_{wm} \) Additionally, the wave spectrum can be enhanced with a directional spreading function when necessary.
Standard wave spectrum formulations are provided in Annex B
1 The normal range of water levels is, however, defined in this standard as the variation in water level with a recurrence period of 1 year, refer to 6.4.3.1
Periodic waves can serve as an abstraction of real sea conditions for design applications A deterministic design wave is defined by its height, period, and direction.
When designing an offshore wind turbine, it is essential to consider the correlation between wind conditions and wave patterns This relationship should be analyzed through the long-term joint probability distribution of relevant parameters.
The joint probability distribution of the parameters is influenced by local site conditions, including fetch, water depth, and bathymetry Consequently, this distribution should be established based on appropriate long-term measurements, supplemented by numerical hindcasting techniques when necessary.
The relationship between normal wind conditions and waves involves analyzing mean wind and wave directions, as their multi-directional distributions can significantly impact the loads on support structures This influence varies based on the directionality of the wind and waves and the axi-symmetry of the structure Designers may sometimes justify a conservative approach by assuming that wind and waves are co-directional and acting from a single worst-case direction These assumptions regarding wind and wave directions are evaluated for each design load case as outlined in section 7.4.
When taking account of the wind and wave misalignment, particular care shall be taken to ensure that the directional data and wind turbine modelling techniques are reliable, refer to 7.5
Wave models are categorized into stochastic sea state representations and regular design waves The stochastic sea state models will utilize a wave spectrum that is suitable for the specific site planned for the offshore wind turbine.
The significant wave height, peak spectral period, and direction for each normal sea state must be chosen alongside the corresponding mean wind speed, utilizing the long-term joint probability distribution of metocean parameters relevant to the expected site.
For accurate fatigue load calculations, designers must ensure that the quantity and resolution of normal sea states considered are adequate to capture the fatigue damage linked to the complete long-term distribution of metocean parameters.
For ultimate load calculations, normal sea states are defined by the expected significant wave height, \$H_s\$, conditioned on a specific mean wind speed, except as noted in section 7.4.1 Designers must consider the range of peak spectral periods, \$T_p\$, relevant to each significant wave height Design calculations should utilize peak spectral period values that produce the maximum loads on the offshore wind turbine.
The height of the normal deterministic design wave, denoted as \$H_{NWH}\$, is assumed to be equal to the expected value of the significant wave height, \$H_{s,NSS}\$, which is conditioned on a specific mean wind speed.
Other environmental conditions
Environmental conditions, aside from wind and marine factors, can significantly impact the integrity and safety of offshore wind turbines through various actions such as thermal, photochemical, corrosive, mechanical, and electrical influences Additionally, the interaction of these climatic parameters may amplify their effects.
The following other environmental conditions, at least, shall be taken into account and the action taken stated in the design documentation:
• rain, hail, snow and ice;
Climatic conditions should be defined using either representative values or limits of variable conditions When selecting design values, it is essential to consider the probability of simultaneous occurrence of these climatic conditions.
Offshore wind turbines are designed to operate effectively despite variations in climatic conditions that fall within a normal range, typically corresponding to a recurrence period of one year or more.
Unless correlation exists, other extreme environmental conditions according to 6.5.2 shall be combined with the normal wind conditions according to IEC 61400-1 and normal marine conditions according to 6.4
The normal other environmental condition values that should be taken into account, are:
• ambient air temperature range of −10 °C to +40 °C;
• relative humidity of up to 100 %;
• water temperature range 5 of 0 °C to +35 °C
When additional external conditions are specified by the designer, the parameters and their values shall be stated in the design documentation and shall conform to the requirements of IEC 60721-2-1
The extreme other environmental conditions that shall be considered for design of an offshore wind turbine are temperature, lightning, ice, and earthquakes
The extreme air temperature range for offshore wind turbines in the standard wind turbine classes should be at least −20 °C to +50 °C
The provisions of lightning protection required in IEC 61400-1 may be considered as adequate for offshore wind turbines in the standard wind turbine classes
No minimum ice requirements are given for offshore wind turbines in the standard wind turbine classes Ice build.up on the wind turbine parts shall be considered from
• moisture and debris at temperatures around and below 0 °C;
• spray of the wave crest at temperatures below 0 °C
No minimum earthquake requirements are given for offshore wind turbines in the standard wind turbine classes For consideration of earthquake conditions and effects see IEC 61400-1.
Electrical power network conditions
The normal conditions at the offshore wind turbine terminals to be considered are listed below
Normal electrical power network conditions apply when the following parameters fall within the ranges stated below:
• voltage – nominal value (according to IEC 60038) ± 10 %;
5 The ambient air and water temperatures shall be 1-hour average values
• voltage imbalance – the ratio of the negative-sequence component of voltage not exceeding
• auto-reclosing cycles – auto-reclosing cycle periods of 0,1 s to 5 s for the first reclosure and 10 s to 90 s for a second reclosure shall be considered;
Electrical network outages are expected to happen approximately 20 times annually A typical outage is defined as lasting up to 6 hours, while an outage extending up to 3 months is classified as an extreme condition.
General
The load-carrying components of offshore wind turbine structures must be verified for integrity to ensure an acceptable safety level It is essential to confirm the ultimate and fatigue strength of structural members through calculations and/or tests, demonstrating the structural integrity of the offshore wind turbine while maintaining the required safety standards.
The structural analysis shall be based on ISO 2394
Calculations must be conducted using suitable methods, with detailed descriptions included in the design documentation These descriptions should provide evidence of the validity of the calculation methods or reference relevant verification studies Additionally, the load levels used in strength verification tests must align with the safety factors appropriate for the characteristic loads as specified in section 7.6.
Design methodology
It is essential to ensure that limit states are not surpassed in the design of wind turbines According to ISO 2394, model testing and prototype tests can serve as alternatives to calculations for verifying the structural integrity of the design.
Loads
Loads described in 7.3.1, through 7.3.6, shall be considered for the design calculations
Gravitational and inertial loads are static and dynamic loads resulting from gravity, vibration, rotation and seismic activity
Aerodynamic loads are static and dynamic loads that are caused by the airflow and its interaction with the stationary and moving parts of wind turbines
Airflow in wind turbines is influenced by several factors, including average wind speed, turbulence across the rotor plane, rotor rotational speed, air density, and the aerodynamic design of turbine components Additionally, the interactive effects of these elements, such as aeroelastic effects, play a crucial role in determining overall performance.
Actuation loads in wind turbines arise from their operation and control, encompassing various categories such as torque control from generators or inverters, yaw and pitch actuator loads, and mechanical braking loads Accurate calculation of these loads is crucial for assessing the system's response and performance.
6 Six hours of operation is assumed to correspond to the duration of the severest part of a storm
When evaluating actuator forces, it is essential to consider the range of loading, particularly for mechanical brakes Factors such as friction, spring force, and pressure, which are affected by temperature and aging, must be accounted for to ensure accurate assessment of response and loading during braking events.
Hydrodynamic loads are dynamic loads which are caused by the water flow and its interaction with the support structure of an offshore wind turbine
Hydrodynamic loads are influenced by various factors, including the kinematics of water flow, water density, water depth, the shape of the support structure, and their interactive effects, which encompass hydroelastic effects.
Offshore wind turbine support structures must maintain a minimum height clearance above hydrodynamic loads, calculated based on the highest crest elevation expected over a 50-year recurrence period This clearance, known as the air gap, takes into account factors such as the highest astronomical tide, positive storm surge, extreme wave crest height, and the motion of the support structure The air gap is defined as 0.2 times the significant wave height (H s50), with a minimum requirement of 1 meter.
Hydrodynamic loads arising from wave “run-up” should be considered, particularly for the design of appurtenances
Offshore wind turbines experience both static and dynamic loads from sea ice Static loads arise from temperature variations and fluctuations in water levels beneath fast ice covers In contrast, dynamic loads result from the movement of ice floes induced by wind and currents, as well as their failure upon contact with the turbine's support structure.
The significance of ice loads in the design of support structures for offshore wind turbines is influenced by the unique location and features of the site, whether at sea or in lakes For detailed guidance on ice load calculations, refer to Annex E.
Other loads such as wake loads, impact loads, ice loads, etc., may occur and shall be included where appropriate
Where relevant, earthquake loads shall be considered according to IEC 61400-1 In addition, hydrodynamic loads from waves resulting from sub-sea earthquakes (tsunamis) may need to be considered
Hydrostatic loads acting on the support structure because of internal and external pressures and resulting buoyancy shall be taken into account where appropriate.
Design situations and load cases
This subclause describes the design load cases for an offshore wind turbine and specifies a minimum number to be considered
For effective design, the lifespan of an offshore wind turbine should be characterized by a range of design scenarios that encompass the critical conditions the turbine is likely to encounter.
The load cases for offshore wind turbine design must be established by combining operational modes and specific design situations, including assembly, erection, and maintenance conditions, with external factors It is essential to consider all relevant load cases that have a reasonable probability of occurrence, along with the performance of the control and protection systems The design load cases used to ensure the structural integrity of the turbine should be calculated through this comprehensive combination.
• normal design situations and appropriate normal or extreme external conditions;
• fault design situations and appropriate external conditions;
• transportation, installation and maintenance design situations and appropriate external conditions
If correlation exists between an extreme external condition and a fault situation, a realistic combination of the two shall be considered as a design load case
In every design scenario, multiple design load cases must be evaluated, with a minimum requirement to consider those outlined in Table 1, which details the design load cases based on wind, marine, electrical, and other external conditions Furthermore, for offshore wind turbines installed in areas prone to sea ice, the design load cases specified in Table 2 must also be taken into account.
To ensure the reliability of offshore wind turbines, it is essential that the turbine controller can initiate a shutdown before reaching the maximum yaw angle or wind speed during design load cases with a deterministic wind model Furthermore, it must be demonstrated that the turbine can effectively shut down under turbulent conditions that mirror the same deterministic wind changes.
Other design load cases shall be considered, if relevant to the structural integrity of the specific wind turbine design
For each design load case, the appropriate type of analysis is stated by “F” and “U” in Table 1
The analysis of fatigue loads, denoted as "F," is essential for evaluating fatigue strength, while "U" pertains to the assessment of ultimate loads, focusing on material strength, blade tip deflection, and structural stability.
Design load cases labeled as “U” are categorized into normal (N), abnormal (A), and transport and erection (T) types Normal design load cases, which are anticipated to occur frequently during a turbine's lifespan, reflect a turbine in a standard condition or one that has experienced minor issues In contrast, abnormal design situations are rarer and typically involve significant faults that trigger system protection mechanisms The classification of the design situation—N, A, or T—dictates the application of the partial safety factor γf to the ultimate loads, as detailed in Table 3.
Table 1 – Design load cases Desi gn si tuati on DLC W ind condi ti on W aves W ind and w ave direct ionalit y
S ea currents Wa te r level Other condit ions T ype of anal ys is
Pa rt ia l safety factor 1 1 NTM V in < V hub < V out RNA
COD, UNI NCM M S L For ex trapol at ion of ex tr em e l oads on t he RNA
NS S J oi nt prob di st ri but ion of H s , T p , V hub
COD, M U L No cu rr e n ts NW LR or ≥ MS L F * 1 3 E T M V in < V hub < V out
COD, UNI NCM M S L U N 1 4 E CD V hub = V r – 2 m /s , V r , V r + 2 m /s
M IS , w ind di rec ti on change
COD, UNI NCM M S L U N 1 6a NTM V in < V hub < V out
COD, UNI NCM NW LR U N
1) P ow er produc ti on 1 6b NTM V in < V hub < V out
COD, UNI NCM NW LR U N
Table 1 – Design load cases (continued) Desi gn si tuati on DLC W ind condi ti on W aves W ind and w ave direct ionalit y
S ea currents Wa te r level Other condit ions T ype of anal ys is
Pa rt ia l safety factor 2 1 NTM V in < V hub < V out
COD, UNI NCM M S L Cont rol s ys tem faul t or l os s of el ec tr ic al net w ork
COD, UNI NCM M S L P rot ec ti on sy ste m o r prec edi ng int ernal el ec tr ic al faul t
COD, UNI NCM M S L E xt e rnal or int ernal el ec tr ic al faul t i nc ludi ng los s of el ec tr ic al net w ork
2) P ow er pro- duc ti on pl us oc currenc e of f aul t 2 4 NTM V in < V hub < V out
Control, protection, and electrical system faults, including the loss of the electrical network, are critical issues in COD and UNI No currents NW LR or ≥ MS.
COD, UNI No cu rr e n ts NW LR or ≥ MS L F * 3 2 E OG V hub = V in , V r ± 2 m /s and V out
3) S tart up 3 3 E D C 1 V hub = V in , V r ± 2 m /s and V out
M IS , w ind di rec ti on change
Table 1 – Design load cases (continued) Desi gn si tuati on DLC W ind condi ti on W aves W ind and w ave direct ionalit y
S ea currents Wa te r level Other condit ions T ype of anal ys is
Pa rt ia l safety factor 4 1 NW P V in < V hub < V out
COD, UNI No cu rr e n ts NW LR or ≥ MS L F * 4) Norm al s hut dow n 4 2 E OG V hub = V r ± 2m /s and V out
COD, UNI NCM M S L U N 5) E m ergenc y shut dow n 5 1 NTM V hub = V r ± 2m /s and V out
COD, UNI NCM M S L U N 6 1 a E W M Turbul ent w ind m odel V hub = k 1 V ref
ESS H s = k 2 H s50 MI S , MU L E C M E W L R U N 6 1b E W M S teady w ind m odel V ( z hub ) = V e50
MI S , MU L E C M E W L R U N 6 1c RW M S teady w ind m odel V ( z hub ) = V red50
MI S , MU L E C M E W L R U N 6 2 a E W M Turbul ent w ind m odel V hub = k 1 V ref
M IS , M U L E C M E W LR Los s of el ec tri cal net w ork U A 6 2b E W M S teady w ind m odel V ( z hub ) = V e50
M IS , M U L E C M E W LR Los s of el ec tri cal net w ork U A 6 3 a E W M Turbul ent w ind m odel V hub = k 1 V 1
M IS , M U L E C M NW LR E xt rem e yaw m is al ignm ent U N 6 3b E W M S teady w ind m odel V ( z hub ) = V e1
M IS , M U L E C M NW LR E xt rem e yaw m is al ignm ent U N
6) P ark ed (s tanding s till or i dl ing) 6 4 NT M V hub < 0, 7 V ref
NS S J oi nt prob di st ri but ion of H s , T p , V hub
COD, M U L No cu rr e n ts NW LR or ≥ MS L F *
Table 1 – Design load cases (continued) Desi gn si tuati on DLC W ind condi ti on W aves W ind and w ave direct ionalit y
S ea currents Wa te r level Other condit ions T ype of anal ys is
Pa rt ia l safety factor 7 1 a E W M Turbul ent w ind m odel V hub = k 1 V 1
MI S , MU L E C M N W L R U A 7 1b E W M S teady w ind m odel V ( z hub ) = V e1
MI S , MU L E C M N W L R U A 7 1c RW M S teady w ind m odel V ( z hub ) = V red1
7) P ark ed and faul t condi ti ons 7 2 NTM V hub < 0, 7 V 1
NS S J oi nt prob di st ri but ion of H s , T p , V hub
COD, M U L No cu rr e n ts NW LR or ≥ MS L F * 8 1 To be s tat ed by t he m anuf ac turer U T 8 2a E W M Turbul ent w ind m odel V hub = k 1 V 1
COD, UNI E C M NW LR U A 8 2b E W M S teady w ind m odel V hub = V e1
COD, UNI E C M NW LR U A 8 2c RW M S teady w ind m odel V ( z hub ) = V red1
8) Trans port , a sse m b ly , m ai nt enanc e and repai r 8 3 NTM V hub < 0, 7 V ref
NS S J oi nt prob di st ri but ion of H s , T p , V hub
COD, M U L No cu rr e n ts NW LR or ≥ MS L No gri d duri ng in st a lla tio n peri od
The following abbreviations are used in Table 1:
ECD extreme coherent gust with direction change (see IEC 61400-1)
ECM extreme current model (see 6.4.2.5)
EDC extreme direction change (see IEC 61400-1)
EOG extreme operating gust (see IEC 61400-1)
ESS extreme sea state (see 6.4.1.5)
EWH extreme wave height (see, 6.4.1.6)
EWLR extreme water level range (see 6.4.3.2)
EWM extreme wind speed model (see IEC 61400-1)
EWS extreme wind shear (see IEC 61400-1)
MSL mean sea level (see 6.4.3)
NCM normal current model (see 6.4.2.4)
NTM normal turbulence model (see IEC 61400-1)
NWH normal wave height (see 6.4.1.2)
NWLR normal water level range (6.4.3.1)
NWP normal wind profile model (see IEC 61400-1)
NSS normal sea state (see 6.4.1.1)
RWH reduced wave height (see 6.4.1.7)
RWM reduced wind speed model (see 6.3)
SSS severe sea state (see 6.4.1.3)
SWH severe wave height (see 6.4.1.4)
V r ± 2 m/s sensitivity to all wind speeds in the range shall be analysed
* partial safety factor for fatigue (see 7.6.3)
When interpreting the wind speed range in Table 1, it is essential to focus on the wind speeds that create the most challenging conditions for wind turbine design This range can be represented by discrete values, ensuring that the resolution is adequate for accurate calculations Additionally, the design load cases are defined with reference to the wind and marine conditions outlined in Clause 6.
7 In general a resolution of 2 m/s is considered sufficient
In calculating the loads on the RNA, it is generally assumed that waves are co-directional with the wind, except for design load cases (DLC 1.4 and 3.3) that involve a transient change in mean wind direction Both the wind and waves are considered to act from a single direction (uni-directional).
For most design load cases, it is generally acceptable to assume that the wind and waves are co-directional when calculating the loads on the support structure However, this assumption does not apply to design load cases 1.4 and 3.3, which involve a transient change in mean wind direction, nor to situations where the wind turbine is parked or idling.
The multi-directionality of wind and waves can significantly impact the loads on support structures, particularly when these structures are non-axisymmetric For certain design load scenarios, as shown in Table 1, load calculations may be simplified by assuming wind and waves come from a single worst-case direction However, it is essential to verify the structural integrity by applying these calculated worst-case loads to the appropriate directional orientations of the support structure.
According to IEC 61400-1, the mean or extreme yaw misalignment for each design load case must be specified Yaw misalignment refers to the horizontal deviation of the wind turbine rotor axis from the direction of the wind.
In this design scenario, an operational offshore wind turbine is linked to the electric load, with a focus on addressing rotor imbalance The design calculations will incorporate the maximum mass and aerodynamic imbalances, including deviations in blade pitch and twist, as specified during rotor manufacturing.
In addition, deviations from theoretical optimum operating situations such as yaw misalignment and control system tracking errors shall be taken into account in the analyses of operational loads
DLC 1.1 and 1.2 outline the load requirements due to atmospheric turbulence (NTM) and stochastic sea states (NSS) that offshore wind turbines experience during their operational lifespan.
DLC 1.1 analysis is essential for determining the ultimate loads on the RNA The calculations for this DLC will utilize statistical extrapolation from the load response results obtained through multiple simulations of stochastic sea states and turbulent inflow across various mean wind speeds For each sea state, the significant wave height will be considered as the expected value conditioned on the corresponding mean wind speed.