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A graph, plotting the maximum achievable effectiveness of available design with current technology as a function of cost, would in general yield a curved line such as the one shown in Figure 1. In addition, this curve shows the saturation effect that is usually encountered as the highest levels of performances are approached.

Points above the line cannot be achieved with currently available technology and they represent currently unachievable designs, although some of these points may be feasible in the future when further technological advances will be made. Points inside the envelope are feasible, but are dominated by designs whose combined cost and effectiveness lie on the envelope. Considering the starting point D0 for the design inside the envelope, there are alternatives that reduce costs without decreasing any aspect of effectiveness design point D1 or that increase some aspects of the effectiveness without decreasing others or without increasing costs design point D2.

For these reasons, the projects represented by the points on the envelope are called the cost-effective solutions. The process of finding the most cost-effective design is additionally complicated by the influence of uncertainty. The exact outcomes achieved by a particular system design cannot be known in advance with certainty, so the cost and the effectiveness of a design are better described by a probability distribution than by a point.

Concept B represents an intermediate situation. The main structural system, consists in all the structural elements that form the offshore wind turbine.

In general, the following segments can be identified: a. Although non directly influencing the load bearing capacity of the main structural system, parts of the auxiliary structural system have an influence to the structural loads e. The previous scheme of Figure 2 can be appreciated with reference to Figure 3, where the main parts of an offshore wind turbine structure are shown, with explicit reference to the foundation structure.

Some of the substructures have been further developed in numerical F. Support structure The support structure consists in the tower, the substructure and the foundations. The typical tower dimensions vary with the hub height and the nacelle-motor assembly.

In general, the tower height is determined as a tradeoff between the cost for an increase in tower height and an eventual additional gain in energy production, as a consequence to an increase in wind speed with the altitude. The tower weight is strongly influenced by the height and the optimization adopted e. As a consequence, weights may vary significantly: typical towers for 3 MW power plants weight tons www. The gravity based foundation is a gravity base serving as the foundation of the tower.

The gravity base is designed with the objective of avoiding lifting between the bottom of the gravity base and the seabed. This is achieved by providing sufficient ballast such that the bottom plate of the gravity base always remains in compression under all environmental conditions. The mono-pile foundation consists of a welded steel pile which transfers the loading on the wind turbine to the supporting soils by means of lateral earth pressure.

As a consequence, a certain depth is required to achieve the required capacity. Depending on the specific site conditions, the mono-pile is either driven when the soil conditions allow it or drilled in the sea-bed when a rock is encountered. The tripod foundation consists of a 3-leg structure, made of cylindrical steel tubes with driven steel piles.

With respect to the mono-pile, it ads stiffness and strength. Currently, a known application of a tripod foundation is on a demonstrator offshore wind turbine project. The jacket foundation consists of a 3-leg or 4-leg structure, made of cylindrical steel tubes with driven steel piles, with either vertical or inclined pile sleeves.

The decision on what type of foundations to use is based on technical primary the water depth and the soil condition and economical factors. The main parts of an offshore wind turbine structure for different foundations partially adapted from IEC " Fig. Basic Macroscopic F. The foundations of Figure 3 cover a water depth range, in real word applications, from a few meters and up to 45 meters, on a tripod or jacket quattropod foundations www.

For deeper waters other solutions such as floating support platforms may be considered. Jonkman and Buhl cross-studied in simulation the response of a floating support platform a barge with catenary moorings having a turbine installed on land as a reference, concluding that the barge was susceptible to excessive pitching during extreme wave conditions, yet the load excursions reduce with decreasing severity in the waves.

Finally, a relatively novel type of foundation called bucket foundation Ibsen and Brincker, is currently under testing in which the stability of the foundation is ensured by a combination of earth pressures on the skirt and the vertical bearing capacity of the bucket. Among the advantages indicated by the authors is that the steel weight is about half as compared to a traditional pile foundation, it is much easier to install and it can easily be removed when the wind turbine is taken down.

Rotor-nacelle assembly Rotor-nacelle assembly consists in the nacelle, the rotor and the blades. Generally, these elements are standardized and manufactured optimized as a whole and tested in demonstrator projects before their commence in projects. The three-bladed concept is the most common concept for modern wind turbines, although concepts based on one and two blades are still present. However, they are not considered for mainstream applications due to specific disadvantages e.

From a structural point of view, the rotor-nacelle assembly is beside the point of the offshore wind structure design, however certain characteristics e. At present commercial power plant turbines vary in power output, with the upper end consisting of units from 3 to 5MW see Table 2. In this sense, a power plant already tested in the field presents the most likely choice.

In one case Gerdes et al. The performance requirements of safety and robustness, are identified as follows: a. As a consequence, the structural characteristics stiffness, inertia, etc. As a consequence, a check of the degradation due to fatigue and corrosion phenomenon is required; c. In fulfilling the above, the conception and design of offshore wind turbines, in particular the structural design, has to be framed within rules, dictated by International Codes and Standards.

Generally speaking, these documents, originate from different entities, identified as activities of: a. Edition with supplement Edition Third edition Det Norske Veritas. October These Codes and Standards are combined with area or country-specific structural codes, for the design and verification of the structural parts. Therefore, the following sections can be identified: a.

Seabed section; consists in the parts of the structural system in the seabed. The soil characteristics have an influence to the foundation system, that in accordance to the vertical and horizontal extension, has an influence on its part to the underwater segment, described in sequence; b. Underwater section; this section is formed by the part of the substructure exposed to the water; c. Air exposed section; this section is formed by: i. In Figure 5, the most relevant loads agent on the offshore wind turbine are shown, as relevant to the different sections.

With reference to the diagram in Figure 6, the external conditions can be divided in environmental wind, marine and other and electrical. The environmental conditions are site-specific and in general, need to be assessed, along with the seismic, topographic and soil conditions.

In general, the major concern is the wind and the marine conditions, since they contribute the most to the loads agent on the structural system.

Wind is most relevant to the structural integrity of the motor-nacelle assembly. The electrical conditions refer to the network conditions.

IEC indicates the necessary checks to be performed on the electrical network, which include grid compatibility conditions of a wind farm. External conditions adapted from IEC , draft Normal wind conditions Wind conditions Extreme wind conditions Waves Sea currents Marine conditions Water level Marine growth Seabed movement and scour Air temperature Environmental conditions Humidity Solar radiation Rain, hail, snow and ice Chemically active substances Mechanically active substances Other conditions Salinity causing corrosion Lighting Seismicity causing earthquakes Water density Water temperature Traffic Electrical conditions " Fig.

The required data are those obtained by anemometric measurements, usually synthesised in a wind-rose diagram, which shows the frequency of winds blowing from particular directions. Starting from these data, it is possible to define the design wind force for the structural analysis. In particular, it is required to estimate the maximum average wind speed with a prefixed return period 50 years. This can be obtained by means of extreme values analysis and the consequent estimation of a p.

In addition to obtaining the extreme values, it is necessary to evaluate the long term wind conditions, useful for fatigue and deformation analysis other than the efficiency assessment of the eventual wind farm. For the above mentioned reasons, and aiming at the statistical quantification of the turbulence fluctuation, along with models providing the vertical profile of the mean wind speed, an eligible spectral model is implemented.

In the end, the induced forces on the structural system are estimated on the basis of appropriate formulas. The same Standard recommends the following equation for estimating the extreme wind speed with a recurrence period of 50 years: 0. Among the various analytic-numerical techniques to represent these components, the most used in structural design is the one based on a spectral model.

The wake effect is relevant when considering a wind farm, consisting in many turbines in several rows. In this case, the presence of a wind turbine will influence the wind flow locally, and the turbulence in the wake behind the turbine will be different from that in front of the turbine.

The DNV- Guidelines for Design of Wind Turbines state that wake effects need to be considered, for wind turbines installed behind other turbines with a distance of less than 20 rotor diameters.

Marine Conditions Marine conditions include waves, sea currents, water level and marine growth. Marine conditions, similar to wind, are divided into normal and extreme. The action of the wave motion affects the structure in contact with water, as a consequence of the alternative motion of fluid particles, induced by the fluctuating perturbation of the liquid surface, or, in shallow water conditions, as a consequence of the breaking waves.

In order to determine the wave loads relevant to the structural analysis, three phases can be identified: a. Regarding the point a. The significant wave height is a measure of the intensity of the wave climate as well as of the variability in the arbitrary wave heights.

Regarding the point b. For the fatigue analysis of the structure subject to the wave action, it is necessary to define an appropriate spectral density of the surface elevation. The characteristic spectral density of the specific sea-state S f can be defined by means of the parameters HS and TP, after selecting an appropriate mathematical model for the S f function.

IEC , indicates two spectral types: - the Jonswap spectrum for a developing sea; and - the Pierson-Moskowitz PM spectrum for a fully developed sea. In general, the sea state is characterised by a distribution of the energy spectral density, depending on the geographic direction of the wave components: this is obtained by multiplying the one-dimensional spectrum S f by a function of directional dispersion, symmetric to the principal direction of the wave propagation Finally point c.

The wind velocity generated sea surface current velocity is determined on the basis of appropriate site-specific measurements. This is apparent from the fact that the transition zone changes, and consequently, the loads agent on the structure vary. IEC , indicates that a constant water level equal to the mean sea level the level of the sea taken over a long period, taking account of all tidal effects but excluding meteorological effects may be assumed for ultimate load cases in normal wave conditions.

The marine growth, which is site specific and depends on conditions such as salinity, oxygen content, pH value, current and temperature, adds weight to the structural components and may increase the hydrodynamic forces on the components. Therefore, the potential for marine growth has to be addressed by increasing the outer diameter of the structural member in question in the wave load calculations.

Seabed movement and scour Seabed movement is mostly present in the form of sand waves. Sand waves are caused by tidal currents in marine non-cohesive bottoms and generally form regular patterns, formed after a slow process that may cover several years.

They can be found in places where the upper soil layer consists of loose material that can be transported by sea currents. Scour is the phenomenon where a hole around the structure, in correspondence to the seafloor, is created by the sand particles transported away by the flowing water.

Scour is strongly affected by the presence of the structures, since the latter cause local increase of the current and wave motions. Scour is caused by currents, waves or their combination, as well as from sea screws caused by ship manoeuvres. Seabed movement and scour have an affect the lateral loading capacity of the foundation, the natural vibration of the structure, and ultimately, it can be the cause of power production disruption being the power cable from the structure over to the seafloor.

Other environmental conditions Other environmental conditions can affect the integrity and safety of an offshore wind turbine, by thermal, photochemical, corrosive, mechanical, electrical or other physical actions, while their effects may increase when combined. Regarding the power output, a change in power is linear to the air density, thus, in order to avoid losses in the energy supply, a different fixed blade pitch angle must be selected at lower air density, and the rotor speed may also have to be adapted, something that has a direct consequence on the loads from the rotor.

Air density depends fundamentally on two factors: the air temperature including the differences between summer and winter and the altitude. The latter has practically no influence, since offshore wind turbines are placed at more or less standard altitudes. It should be evaluated the possibility to control the humidity for internal zone compartments for corrosion protection. Regarding ice and snow accumulation, DNV OS-C, requests that ice accretion from sea spray, snow, rain and air humidity shall be considered, where relevant, while snow and ice loads may be reduced or neglected if snow and ice removal procedures are established.

Furthermore, when determining wind and hydrodynamic loads, possible increases of cross-sectional area and changes in surface roughness caused by icing shall be considered, where relevant. So may occur from waste in-sea waste materials e. Modern offshore wind turbines are equipped with outer lightning protection by means of multiple receptors located in the rotor blades and the lightning rod at the weather mast.

Underwater seismic activity, together with volcanic eruption, may lead to seismic sea waves tsunamis. Risk analysis for the evaluation of the occurrence of an extreme event should be carried out, as indicated in a next paragraph. It is anticipated that the design values of the external forces e. For further specifications on the identification and application of the loads, contingency scenarios and load cases, together with a concise application, the reader is directed to Bontempi et al.

Contingency scenarios are defined, that account for the various situations in which the turbine can be found during its lifetime. In the above defined contingency scenarios, appropriate coefficients for the combination of loads and materials are introduced.

In general, partial safety factors for loads vary with the design situation e. The partial safety factors for the materials are also defined varying from 1. Vessel impact loads These loads account for the load from a vessel impacting the structural system. Vessel impact loads can be divided in two classes: a. Normal impact loads. The pollution degree may be reduced within certain areas of the equipment by the use of encapsulation, conformal coating, etc.

Reduction of pollution degree within the overall equipment may also be achieved by the use of enclosures providing protection in accordance with IEC Steps may be taken to control and reduce the pollution degree at the creepage location by design features or the consideration of the operating characteristics of the component or equipment.

Pollution degree 2 can be achieved by reducing the possibilities of debris accumulation filtered ventilation and condensation or high humidity at the creepage locations. Continuous application of heat, through the use of heaters or continuous energizing of the equipment when it is in use, can be used to control condensation.

Continuous energizing is considered to exist when the equipment is operated without interruption every day and 24 hours per day or when the equipment is operated with interruptions of duration which do not permit cooling to the point that condensation occurs. Pollution degree 1 can be achieved by potting, moulding the equipment or circuit or by enclosing it in an IP 67 enclosure.

Overvoltage category IV shall be applied to determine clearance spacings in general locations within a wind turbine. Surge protectors that are relied upon to protect control and protection circuits and other circuits relied upon for the safe operation of the turbine shall include monitoring circuits or other automatic means to indicate a surge protective device has failed.

Use of surge suppression protection may be used to locally reduce the overvoltage category to below IV for specific equipment or circuits. Surge protective devices, if used, shall be installed in close proximity to the equipment being protected. These protective device functions include: short circuit, overcurrent, ground fault, over temperature and arc fault. Semiconductor switching devices without additional air gap disconnect contacts are not suitable to meet the requirements of Where lighting or other electrical systems are necessary for safety during maintenance, auxiliary circuits shall be provided with their own disconnect devices, such that these circuits may remain energized while all other circuits are de-energized.

The choice and installation of the equipment of the earthing arrangement earth electrodes, earthing conductors, main earthing terminals and bars shall be made in accordance with IEC A risk-based approach shall be applied during the lightning protection system design with respect to the risk of damage and personnel safety. The protection of electrical systems within the turbine shall follow a lightning protection approach as set out in IEC , with the design incorporating a combination of bonding, shielding and surge protection devices.

Design requirements of the lightning protection system are provided in IEC Where there is a probability of rodents or other animals damaging cables, armoured cables or conduits shall be used. If a capacitor bank is connected in parallel with an induction generator i. Alternatively, if capacitors are fitted, it shall be sufficient to show that the capacitors cannot cause self-excitation. The limits of the protection shall be so designed that any lightning electromagnetic impulse transferred to the electrical equipment will not exceed the limits governed by the equipment insulation levels.

The interior of a turbine is not considered to be a conditioned space. If the converter is installed within a conditioned space, it shall be evaluated for that environment. The pollution degree shall be chosen with respect to ingress of moisture from humidity and condensation due to extended durations of de-energization.

The environmental service conditions defined in IEC may be modified to accommodate the specific end application within the turbine. Converter controls and protection shall additionally comply with the applicable requirements in Clause 8. If operation of the wind turbine may result in twisting of flexible cables, such as the connecting cables between rotating parts nacelle and parts of the fixed structure tower or foundation , the operational conditions of use shall not cause damage to the conductors or their insulation.

The evaluation shall address service life, electrical and environmental operating conditions of the subassembly. Controls that prevent damage to conductors or their insulation including rotational limits shall be considered part of the control system. Where multiple cables are grouped or tied together, the assembly loading shall be distributed such that the individual cables are not subjected to loads that exceed their individual ratings.

Cable size and temperature rating shall be evaluated based upon the operating temperature of the assembly including representative size and number of cables fully twisted, carrying maximum normal current and including electrical terminations similar in distance in the end application. The slip rings shall be rated for normal operating and abnormal overload conditions to which they may be subjected.

The normal rating of a slip ring is based upon the circuit electrical loads. The overload rating shall be based upon source circuit fault current capacity and may account for overcurrent protection if provided. Slip rings used in safety critical power and control circuits shall be provided with means to address failure and wear. The conductors and components shall comply with their applicable standard with regards to: operating temperature range, environmental conditions, electrical isolation, electrical impulse withstand and short circuit withstand capabilities as necessary for the electrical and environmental conditions within the turbine.

Vertical power transmission equipment shall be constructed with sufficient mechanical strength to withstand the foreseeable mechanical forces deflection, movement and loading for its use as determined from the results of the design load cases for the specific component within the specific turbine. Sections of vertical power transmission conductors and components shall be suitably attached to the tower or to components of the tower intended to support the assembly.

The evaluation of these vertical power transmission systems shall account for the following conditions: a static loading on system components; b expected deflection and forces on the transmissions assemblies and support structure resulting from bending of the tower under anticipated extreme conditions; c expected force direction and magnitude of displacement of the assembly; d component fatigue, loosening of fasteners; e degradation, wear, deformation and creep of polymeric electrical insulating materials; f loss of electrical conductivity or electrical isolation; g operation for the intended turbine life span or specified maintenance period for the assembly.

The assembly may be evaluated by testing, analysis or a combination of the two. Scaled testing may be used to represent the complete system. The mechanical and structural suitability of the power transmission assembly and supporting members may be addressed via analysis per the design load case evaluation. The design load cases shall specifically include and address the forces on the assembly, subassemblies and components including insulating and conducting materials.

Consideration of short circuit conditions for vertical power transmission systems should include electrical, thermal and mechanical effects on components as well as installed systems.

Pitch and yaw motor drive and converter controls shall additionally comply with the applicable requirements in Clause 8. The turbine generator shall be rated for continuous operation duty S1 according to IEC The combination of an electrical machine powered by a frequency converter drive shall have coordinated electrical and isolation ratings.

Low voltage transformers within the wind turbine shall either be fully enclosed, including all terminations, or they shall be located in dedicated areas behind barriers or panels as required in recognized national or international standards and codes.

High voltage HV transformers within the wind turbine shall comply with the requirements for restricted accessibility and lock out as defined in HV switchgear shall be located in areas of the turbine only accessible by authorized personnel.

Cautionary markings shall be posted on the turbine access, hatch or door to wear appropriate personnel protection equipment for the hazards inside the area beyond the access, hatch or door. HV switchgear installed in areas accessible to normal maintenance personnel shall be metal enclosed and specified for internal arc containment classification IAC.

HV switchgear shall be tested, rated and marked for the IAC fault current with a duration of not less than 1 s. Pressure relief vents shall output without obstruction and without risk to personnel or equipment. As required by SF6 switchgear shall be installed in areas with sufficient ventilation so as to avoid hazard to personnel in the event of a leak.

Electrical cabinets and enclosures shall be provided with doors or covers that are securely fastened. Wiring associated with the control systems shall be mechanically protected from damage due to hub rotation, service personnel interaction and unintended impacts within the hub. In addition to these conditions, account has to be taken of the seismic, topographic and soil conditions at the wind turbine site. It shall be shown that the site-specific conditions do not compromise the structural integrity.

The demonstration requires an assessment of the site complexity, see For assessment of structural integrity, two approaches may be used: a a demonstration that all these conditions are not more severe than those assumed for the design of the wind turbine, see If any conditions are more severe than those assumed in the design, the structural and electrical compatibility shall be demonstrated using the second approach.

The partial safety factors for loads in 7. To obtain the slope of the terrain, planes are defined that fit the terrain within specific distances and sector amplitudes for all wind direction sectors around the wind turbine, see Figures 10 and The fitted planes do not need to pass through the tower base. Accordingly, the terrain variation from the fitted plane denotes the distance, along a vertical line, between the fitted plane and the terrain at the surface points.

The resolution of surface grid and its original source map used for terrain complexity assessment should not exceed 50 m. Similarly, TVI 30 is the mean value of the twelve sector-wise standard deviations of terrain variation normalized by the circle radius, weighted by the energy inflow.

For both indices, three complexity categories low L , medium M and high H are defined, see Table 5. If not, the site is assessed as complex and assigned one of the three complexity categories, L, M or H, depending on the highest category TSI or TVI for any of the circle areas. When there are no site data, it may be estimated by an appropriate flow model.

Alternatively, it may be estimated from the complexity of the site. Values shown in Table 6 may be assigned depending on the complexity category defined according to the procedure described in The design values for the Kaimal and Mann models can be obtained from Annex C. When there are no measured site data, the values in Table 7 may be used for C CT depending on the complexity category of the site.

Simulation models shall be validated against representative data. COV of the annual maximum wind speed can approximately be obtained assuming a Gumbel distribution and assuming that for example 50 year and year return values of the wind speed, V 50 and V , are available. All measurements, except air density, shall be available as function of wind direction, given as a 10 min average.

Attention should be given to wakes from significant structures and orographic obstacles within a distance from the wind turbine of 20 times the characteristic length of the structure or the orographic obstacle.

The influence can be neglected if the bottom edge of the rotor is at least four times higher than the height of the structure or the orographic obstacle. In regions prone to hurricanes, cyclones and typhoons, the extreme wind speed shall be evaluated by appropriate methods, for example as given in Annex J. For cold climate, additional parameters should be derived for the position of the wind turbine. Icing condition may be assessed according to Annex L.

Consideration should be given to surrounding terrain and vegetation. In this case using the average wind shear may not be sufficient. Where seasonal variations contribute significantly to the wind conditions, the monitoring period should be long enough to include these effects the minimum to capture seasonal effects would be 12 months.

During data evaluation, a quality check and filtering should be performed and documented. A measure-correlate-predict MCP procedure may be performed to extend the data. A long-term assessment is normally required for the estimation of the extreme wind speed, long-term mean wind speed as well as air density, but only if the available long term source is appropriate and sufficiently reliable. Alternative methods can be used. These methods representativeness of the measurements for the site.

The assessment of the suitability of the wind turbine at a site in a wind farm shall take into account the deterministic and turbulent flow characteristics associated with single or multiple wakes from upwind machines, including the effects of the spacing between the machines, for all ambient wind speeds and wind directions relevant to power production. The increase in loading generally assumed to result from wake effects may be accounted for by the use of an added turbulence approach, or by using more detailed wake models.

In either case, the wake model shall include adequate representation of the effect on loading of ambient turbulence and discrete and turbulent wake effects. For fatigue calculations, the effective turbulence intensity I eff may be derived according to Annex E.

The added turbulence for fatigue and ultimate loads may be assumed to be the same. No earthquake assessment analysis is required for sites already excluded by the applicable local seismic code due to their weak seismic action.

For locations where the seismic load cases described below are critical, the engineering integrity shall be demonstrated for the wind turbine site conditions. The assessment may be based on Annex D. The evaluation of load shall take into account the combination of seismic loading with other significant, frequently occurring operational loads. The seismic loading shall depend on ground acceleration and response spectrum requirements as defined in local codes.

If a local code is not available or does not give the ground acceleration and response spectrum, an appropriate evaluation of these parameters shall be carried out. The ground acceleration shall be evaluated for a year return period. The earthquake loading shall be superposed with operational load that shall be the largest of a mean loads during normal power production determined at V r , b loads during emergency stop at V r , and The partial safety factor for load for all load components shall be 1,0.

The material safety factor for steel can be set to 1,0. The seismic load evaluation may be carried out through response spectrum methods, in which case the operational load is added using the SRSS square-root-sum-of-squares or equivalent load combination arising from the seismic loading.

The seismic load evaluation may be carried out through time-domain methods, in which case sufficient simulations shall be undertaken to ensure that the operational load is representative of the time averaged values referred to above. The number of tower natural vibration modes used in either of the above evaluations shall be selected in accordance with a recognized seismic code.

The evaluation of the resistance of the structure may assume elastic response only, or ductile energy dissipation. However, it is important that the latter is assessed correctly for the specific type of structure in use, in particular for lattice structures and bolted joints. The acceleration response spectrum at the engineering bedrock and seismic response evaluation method are described in Annex D.

The response spectrum method shall not be used if it is possible that seismic action will cause significant loading of structures other than the tower. The assessment can be performed separately for the fatigue load suitability and the ultimate load suitability. If the turbine has been designed with the wind speed distribution in 6. The list represents a set of minimum requirements. Local and national grid compatibility requirements need to be anticipated at the design stage.

This can be done by additional multiplication with a turbulence structure correction parameter C CT as defined in Where there are no site data for the wind shear, it shall be calculated taking topography and roughness of the surrounding terrain into account. As an alternative, for an air density greater than the one specified in 6. This may be done by additional multiplication with a turbulence structure correction parameter C CT as defined in Alternatively, the wind turbine site central estimate of extreme 3 s average wind speed at hub height with a return period of 50 years shall be less than V e For Class S turbines, both the extreme 3 s average wind speed and the extreme 10 min average wind speed shall be assessed.

If the average site air density is different from the one specified in 6. Alternatively, it can be demonstrated that the ambient site-specific extreme turbulence does not exceed the ETM model used for DLC 1. For determination of the site-specific turbulence, the site-specific conditions, the frequency of the wake situations and the wind farm layout shall be accounted for. The calculations shall account for variations of wind conditions with mean wind direction and speed as well as for wake effects, vertical wind shear, mean wind flow angle, etc.

For the fatigue loading, a comparison of damage equivalent moments and damage equivalent load of the load duration distribution of the driving torque is sufficient for verification of components. For ultimate loading a comparison of contemporaneous loads is not required. Turbulence structure shall be based on site-specific values.

As an alternative, the longitudinal turbulence intensity may be increased by the C CT factor. In the case of wake effects, it shall be verified that structural integrity is not compromised.

This may be performed using a wake model, for example see Annex E, with the turbulence model adjusted to the site-specific parameters. Fatigue load calculations shall be performed, if one of the criteria in Ultimate limit state analyses shall be performed if one of the criteria in As a minimum, the following ultimate design load cases shall be assessed: DLC 1. If the design load cases for the standard classes are adequate, no further evaluations need to be performed.

Annex B provides definitions of the aforementioned ultimate and fatigue load cases for sitespecific conditions. If relevant, other load cases in design situations 1 , 6 , and 7 in Table B. Design situations 2 , 3 , 4 , 5 , and 8 in Table B. The installation of a wind turbine shall be performed by personnel trained or instructed in these activities.

The site of a wind turbine facility shall be prepared, maintained, operated and managed so that work can be performed in a safe and efficient manner. This should include procedures to prevent unauthorized access where appropriate.

The operator should identify and eliminate existing and potential hazards. Checklists of planned activities shall be prepared, and logs of completed work and results of that work should be kept.

When appropriate, installation personnel shall use approved eye, foot, hearing, and head protection. All personnel climbing towers, or working above ground or water level, should be trained in such work and shall use approved safety harnesses, safety climbing aids or other safety devices. When appropriate, a buoyancy aid should be used around water. All equipment shall be kept in good repair and be suitable for the task for which it is intended.

Cranes, hoists and lifting equipment, including all slings, hooks and other apparatus, shall be adequate for safe lifting. The critical wind speeds and precaution measures shall be included in the installation manual. Wind turbines are often sited on hilly terrain. Therefore, heavy equipment shall be set down in such a manner that it cannot shift.

A suitably-sized, level lay-down area is preferred for all handling and assembly operations. Where this cannot be provided, all heavy equipment shall be securely blocked in a stable position. Where there is risk of movement caused by the wind with risk of consequent damage, blades, nacelles, other aerodynamic parts and light crates shall be secured with ropes and stakes, or ground anchors.

Inspection shall be carried out to confirm proper lubrication and pre-service conditioning of all components. In this case, the energization of such equipment shall be carried out in accordance with a written procedure to be provided by the wind turbine supplier. All elements where motion rotation or translation may result in a potential hazard shall be secured from unintentional motion throughout the erection process.

Fasteners identified as critical shall be checked and procedures for confirming installation torque and other requirements shall be obtained and used. All hoisting equipment, slings and hooks shall be tested and certified for safe load. The design shall incorporate provisions for safe access for inspection and maintenance of all components. The requirements of Clause 10 also cover electrical measurement equipment temporarily installed in the wind turbine for the purpose of measurements.

When appropriate, operation and maintenance personnel shall use approved eye, foot, hearing and head protection. All personnel climbing towers, or working above ground or water level, shall be trained in such work and shall use approved safety belt, safety climbing aids or other safety devices.

External events detected as faults but not critical for the future safety of a wind turbine, such as loss and reinstatement of the electrical load, may allow automatic return to normal operation after completion of the shutdown cycle.

Guards designed to protect personnel from accidental contact with moving components shall be fixed, unless frequent access is foreseen where they may be movable. Guards shall a be of robust construction, b not be easy to by-pass, and c where possible, enable essential maintenance work to be carried out without their dismantling.

Maintenance procedures shall require safety provisions for personnel entering any enclosed working space, such as hub or blade interior, that ensure any dangerous situation will be known by standby personnel to immediately initiate rescue procedures if necessary. These can include, but are not limited to, preloading of fasteners, changing of lubrication fluids, checking other components for proper setting and operation and proper adjustment of control parameters.

The wind turbine site should be refurbished to remove hazards and prevent erosion. All unscheduled automatic shutdowns should be recorded. The manual shall require that where there is a fire or apparent risk of structural damage to the wind turbine or its components, no one should approach the wind turbine unless the risk is specifically evaluated. The manual shall also provide for unscheduled maintenance. The maintenance manual shall identify parts subject to wear and indicate criteria for replacement.

LTC is for temperatures below the normal environmental conditions 6. For ice type definitions, see ISO For simplicity, cold climate conditions will be treated here as IC and LTC conditions unless stated otherwise.

The impact of cold climate on the structural integrity or safety systems of the wind turbine shall be assessed.

As a consequence of LTC effects in cold climate conditions, minimum ambient temperature conditions shall be assumed which are intended to represent many different sites and do not give a precise representation of any specific site.

The effect of the actual air density at the site 37, as influenced by temperature and altitude, shall be considered during the site suitability analysis. In IC, the effect of ice accretion on blade aerodynamic coefficients and on the blade mass distribution needs to be considered.

In LTC, low ambient temperatures Ice accretion on rotor blades often occurs for temperatures higher than minimum ambient temperatures In cold climate conditions, altered turbine performance resulting to a shift in turbine operational point shall be assessed.

Higher loading may be a consequence of aforementioned effects. Higher loading during icing may also be a consequence of imbalances resulting from altered rotor aerodynamics and additional ice mass. Further guidance can be found in Annex L. One air density according to Alternatively, the minimum temperature for DLC 8. If operation of the iced turbine is not considered in the load assumptions, measures have to be taken to prevent operation of the iced turbine.

These measures need to be redundant; a single failure shall not lead to unintended operation of an iced turbine. It shall be assured that the oil temperature in the gearbox has reached a temperature to avoid damage before power can be transmitted. For a cold start procedure, see Remaining conditions may be stated by reference to the appropriate wind turbine class. Table A. Corrections for different altitudes can be applied.

For other design situations, the analysis is performed for all sectors using aggregated wind conditions. The aggregation of wind conditions shall be carried out such that proper fatigue and extreme loads are maintained, or the design parameters shall be chosen conservatively. Consideration should be given to the potential variation in wind shear, density and other parameters that can affect fatigue loading on the wind turbine. Wake effects, earthquake loading and icing shall be considered in load cases as specified for other conditions, if appropriate.

When a wind speed range is indicated in Table B. The range of wind speeds may be represented by a set of discrete values, in which case the resolution shall be sufficient to assure accuracy of the calculation. As stated in 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.

Design load cases DLC 1. For DLC 1. As an alternative to this analysis, the appropriate characteristic values of all load components relevant for each specific turbine component determined from the simulation may be directly extrapolated. Table B. When site-specific values are not available, it may be assumed to be identical to the ECD defined in 6. EDC s Site-specific extreme direction change. When site-specific values are not available, Equations 21 and 22 in 6.

When site-specific values are not available, Equations 18 and 19 in 6. When site-specific values are not available, Equations 13 through 17 in 6. EWS s Site-specific extreme wind shear. When site-specific values are not available, Equations 27 and 28 in 6. NWP s Site-specific wind profile model. When site-specific wind speed profile is not available, Equation 9 in 6. F Fatigue see 7. Annex E provides guidance on the use of appropriate wake models for DLC 1. If an added wake turbulence model is used, the maximum centre-line wake turbulence intensity shall be applied.

The use of a meandering wake model e. If there is no information available, the number of events suggested in 7. Parameters for the models have been selected to satisfy the general turbulence requirements given in 6.

The model assumes that the isotropic von Karman [2] energy spectrum is rapidly distorted by a uniform, mean velocity shear. The turbulent velocity fluctuations are assumed to be a stationary, random vector field whose components have zero-mean Gaussian statistics. The first model is recommended. When this parameter is zero, the isotropic model is recovered. As this parameter is increased, the longitudinal and lateral velocity component variances increase while the upward velocity component variance decreases.

The resulting turbulent eddy structure is stretched in the longitudinal direction and tilted relative to the plane. Assuming that the random velocity field generated by the model is convected past the turbine at the hub-height wind speed, the velocity component spectra observed at a point may be computed by integrating the spectral tensor components.

Mann [4] carried out such integrations and compared the results to the Kaimal spectral model. The scale parameter may be found by equating the asymptotic, inertial-sub-range longitudinal spectra. This procedure is detailed in Mann [4]. The Mann model parameters derived herein to represent the Mann sheared turbulence model are based on conforming to the external conditions defined in Clause 6 and whereby the resulting wind spectra are equivalent to the Kaimal spectra.

The Mann model turbulent wind field needs to be then generated based on the three model parameters derived from the measured spectra. The longitudinal scale has been chosen to approximate the original Kaimal spectrum and, for the lateral and upward scales, to satisfy the spectral requirements in 6.

Table C. Kaimal, J. Wyngaard, Y. Izumi, and O. Cote, Spectral characteristics of surface-layer turbulence, Q. Mann, The spatial structure of neutral atmospheric surface-layer turbulence, J. Fluid Mech. Mann, Wind field simulation, Prob. One approach is the dynamic simulation in time domain and the other approach is the response spectrum method RSM. In time domain approach, the ground acceleration at the surface is estimated from the response spectrum at the engineering bedrock, or the time history of the ground motion specified in the local building codes.

The site-specific parameters of the response spectrum may be based on local building codes []. Special care is required when response spectrum method is used for wind turbine support structures due to its low damping ratio [8].

In Annex D, the design response spectrum is first presented in Clause D. An example of time domain simulation is described in []. In general, the response spectrum is given by local codes. The acceleration response spectrum may be defined according to Equation D. T C can be taken as 0,3 s to 0,5 s for stiff and hard soil conditions, 0,5 s to 0,8 s for intermediate soil conditions and 0,8 s to 1,2 s for loose and soft soil conditions.

These parameters can be determined by local design codes as shown in references [2], [3], [4]. The design response spectrum can be written as in Equation D. These parameters are described in Clause D. Local design codes usually define typical soil types and their amplification factor. Those valued in local design code may be used for the seismic load evaluation of the wind turbine.

The bottom of the tower can be fixed to the ground for the load estimation of tower. More complicated model may be needed for the load estimation of the foundation, which is out of scope of Annex D. Takei and T. Witcher, Seismic analysis of wind turbines in the time domain, Wind Energy, No. Prowell, A. Elgamal, C. Uang, J. Luco, H. Romanowitz, and E. Duggan, Shake table testing and numerical simulation of a utility-scale wind turbine including operational effects, Wind Energy, DOI: The approach is applicable for regular as well as irregular wind farm layouts.

It is acceptable to adjust the formulas for other layouts and other than uniform distribution. It should be taken into consideration for each neighbour affecting wind turbine, the sector disturbed and their associated probability of occurrence conditioned on hub height mean wind speed.

Wake effects from wind turbines "hidden" behind other machines need not be considered, for example in a row only wakes from the two units closest to the machine in question are to be taken into account. Dependent on wind farm configuration, the number of nearest wind turbines to be included in the calculation of I eff is as given in Table E.

Table E. Inside large wind farms, wind turbines tend to generate their own "wind farm ambient" turbulence. IEC Figure E. It describes the changes in the mean flow field over the wind farm as well as the changes in the turbulence intensity and turbulence structure compared to ambient conditions.

The recommended modelling of the wake deficit is based on the thin shear layer approximation of the Navier-Stokes equations in their rotational symmetric form with the pressure term disregarded. The empirical filter functions, accounting for the near field pressure influence i.

F 2 and the lack of equilibrium between the mean shear flow field and the turbulence field in the near and intermediate wake regime i. The initial wake deficit to be developed downstream using Equations E. Modelling of the meandering process consequently includes considerations of a suitable description of the "carrier" stochastic transport media i.

The stochastic modelling of wake meandering is established by considering a cascade of wake deficit releases "emitted" at consecutive time instants in agreement with the passive tracer analogy. The propagation of each "emitted" wake deficit is subsequently modelled, and the collective description of these constitutes the wake meandering model in space and time.

Adopting Taylor's hypothesis, the downstream advection of each wake deficit "release" is dictated by a suitable advection velocity often taken as the ambient mean wind speed. As for the dynamics in the lateral and vertical directions, each considered wake cascade-element is successively displaced according to the large scale lateral and vertical turbulence velocities at the instantaneous wake deficit position.

Thus, this turbulence contribution is considered independent of the ambient turbulence. With a length scale comparable with the characteristic size of the wake deficit, the basic DWM split in scales implies that the wake induced turbulence meanders with the wake deficit.

The induced small scale turbulence is consequently formulated in the meandering frame of reference. Although violating the second order statistics i.

Rotational symmetry of the induced wake turbulence intensity is assumed, resulting in a scaling coefficient that for a given downstream distance only depends on the radial coordinate. This is a complicated process that calls for simplification. Wakes including associated small scale wake induced turbulence from the turbines in question are treated individually, and their correlated meandering is then subsequently modelled. Referring to ambient wind speed conditions, two different approaches are applied for the wind regimes corresponding to below and above rated power, respectively.

The parameter i includes all upstream turbines relative to the spatial position x for a given mean wind direction. Note that with the recipe formulated in Equation E. However, since the induced turbulence contributions are small scale this does not compromise the present application. Wake meandering a pragmatic approach.

Wind Energy, 11, , p. Energy Eng. Wind Energy. The undisturbed wind field is the site-specific mean wind speed consisting of a deterministic part i. The resulting inflow wake effects to be superimposed consist of stochastically moving wake deficits, V hub U , and associated wake induced turbulence contributions, respectively, and added according to the recipe formulated in Equation E.

Note that the wake deficit meandering is by far the dominating wake effect. Frequently, there is insufficient data even at a single point within a wind farm to carry out the evaluation. However, the extended data record can be synthesized by extrapolation based on a long-term record for another location. The MCP methods are a means to create that extended record.

The following explanation is taken from [2]. Further information can be found in [3], [4] and [5]. One version is described here, based upon the concurrent hourly data from the wind turbine site and a nearby reference meteorological station Met.

These data are cross-plotted and used to derive sector-wise linear regression equations; the sectors being consistent with those used by the Met. The data sets used for deriving the regression equations should be as long as possible, at least conservatively covering the conservative part of any seasonal variations.

Station record sector by sector, for a period sufficiently long to eliminate short-term variations, probably at least seven years. The result is an hourly mean record for the site, which may be processed into a probability distribution for site assessment. The minimum recommended length of data set is ten years. It is also possible to apply the method of independent storms MIS , a derivative of the Gumbel method, which utilizes more than one data point per year from a data set, also described by Cook [1].

This method can be used for data sets that are as short as four years. MIS selects individual storms' peak wind speeds by application of thresholds and time filters to ensure that all values are from independent events. The sector-specific regression coefficients are applied to a table of the maximum hourly wind speed at the Met.

Station, by year for basic Gumbel and by storm event for MIS, and by sector. A similar table is therefore built up for the wind turbine site. The maximum value in each year for the candidate site is extracted for use in a Gumbel analysis. The use of the coefficients is appropriate here since they have been formed from hourly mean data and are being applied to hourly mean data.

In this method, there is no assumption that the maximum value at the candidate site occurs in the same sector as the maximum at the reference site. By using the sector-specific regression coefficients, the maximum at the candidate site can be more accurately determined, taking account of the inter-site relationships. The gust factors should be estimated from the site-measured data, or by theoretical methods. Users of these methods for estimation of extreme wind speed should note that the resulting accuracy depends significantly on data quality and actual local conditions compared to the reference station.

As a result, correlations from a regression analysis may be poor. In cases where existing local codes are recognized as applicable by authorities having jurisdiction, the extreme values obtained using such codes should be considered as minimum values for design in preference to MCP.

Reference documents [1] N. Harris, Gumbel re-visited a new look at extreme value statistics applied to wind speeds, Journal of Wind Engineering and Industrial Aerodynamics, 59 p. Harris, The accuracy of design values predicted from extreme value analysis, Journal of Wind Engineering and Industrial Aerodynamics, 89 p. Assuming that local stresses are related to the loading so that the stress progressively increases with increased loading, the strength of a structural component can be defined in terms of an ultimate load that causes failure.

Given the service loading, the adequacy of the structure can be assessed by comparing the extreme values of the loading with the ultimate load resistance, applying suitable factors of safety.

For wind turbines, loading depends on the turbulent wind inflow for a variety of wind conditions. Thus, it is necessary to analyse the extreme values of the loading on a statistical basis in order to determine a suitable characteristic load.

For a given wind condition, it is reasonable to model the short-term load response as a stationary random process. Given that loads can be represented as such processes, methods are described in the following for the extraction of data for extrapolation and load extrapolation.

Convergence criteria also are proposed and an alternative for estimating the long-term loads using the inverse first-order reliability method IFORM is given. The methods have been tested for a 3-bladed horizontal-axis upwind turbine. More information and guidance can be found in [1]. When extracting, the designer should consider the effect of independence between peaks on the extrapolation and minimize dependence when possible.

If the method chosen for extrapolation is sensitive to independence assumption e. A simple approach to ensure independence is to assume that the global extreme in each tenminute simulation or local extremes from intervals no shorter than three response cycles are independent and thus require a minimum time separation between individual response extremes of three response cycles defined by three mean crossings over the block size.

If a systematic statistical approach is desired, the designer may test for independence using standard estimation techniques e.

Peak over threshold methods may also be employed, but the designer should be careful that truncation errors and correlation introduced by the threshold do not influence the shape of the empirical distribution dramatically. Data may be extracted by choosing the global individual response extremes from each simulation or some subset created by breaking the simulation into blocks of equal time or ensuring a minimum time separation between extremes. Two different cases are regarded for aggregation of simulated short-term distributions of extremes for a specific observation period T into an empirical distribution of the long-term extremes for the same period: extrapolation from global extremes, and from local extremes.

The extreme load response, s r , of the desired return period, T r , is obtained from Equation G. Tr The practical implementation of these formulas would typically be to use discrete wind speed values. Strictly, n should be a random number for which a distribution dependent on V should be assumed.

However, n has for wind turbine applications limited variation compared to its mean value. Consequently, replacing n by its mean value conditional on V , as implicitly done above, is sufficiently accurate. The approximation may be accepted if, when applying the formulas proposed in the following, one uses an s-value representative of the wind speeds that contribute most to the specific load response under consideration.

Based on the approximation, one has the following expression: Vout G. One method for accomplishing this is to compute a number of simulations, where the number of simulations per bin is determined by the Weibull or appropriate distribution of wind speed.

One potential disadvantage of this method is that loads that are dominated by high wind speeds may have very few simulations from which to extract large extreme values in the tail of the empirical distribution. To address this issue, additional long-term distributions can be calculated using additional simulations for the low probability wind speed bins. The total simulation time per bin should follow the original wind speed distribution.

But, a number of new long-term empirical distributions can be formed using randomly bootstrapped data from all bins, in which a large number of simulations are available.

Once a number of long-term distributions are formed, they can be averaged to form a single aggregate long-term distribution that can be used for extrapolation to lower probability levels.

Some loads are dominated by wind speeds near rated while others are dominated near cut-out or other wind speeds. It is important that the designer examine the dominant wind speeds closely to ensure that a sufficient number of simulations are carried out to ensure stability of the method.

If the extremes are obtained using any other method e. In the procedure that involves aggregation before fitting, empirical long-term distributions for the loads following aggregation of all wind speed bins can be established by making use of similar convergence criteria as proposed above for short-term distributions.

The appropriate fractile at which to impose the convergence criterion should be higher than the fractile corresponding to any apparent "knee" often observed in the empirical long-term distribution to ensure that convergence is checked closer to the tail of this empirical distribution. Note that m will be equal to the number of simulations if global maxima are used. For example, blade flap moments away from tower for outboard sections might be problematic at certain wind speeds; i.

The recommended number of simulations is determined by calculating a confidence interval for the resulting empirical distribution. This interval may be estimated using bootstrapping methods [ 3 ], the binomial estimation method [ 4 ], or it may be inherently estimated as a part of the extrapolation method employed. Note that bootstrap resamplings will be composed of repeated values from the original sample since, for each resampling, data are sampled randomly with replacement.

The process is repeated so as to form a large number, N b , of bootstrap resamplings. From these N b estimates, constituting the set, l 1 , l 2 , l 3 , l 4 , l 5 , , l Nb , confidence intervals can be found in the usual manner by ordering the data. These can then be used for the numerator of Equation G. A minimum number of 25 bootstrap resamplings may be sufficient to determine a reasonable estimate of confidence bounds.

However, a larger number closer to 5 will lead to more reliable estimates. This saving is simplified by tabulating parameters for calculating a binomial confidence interval that will result for most common situations.

The number of simulations is of the order of 15 to 35 for each wind speed bin. The parameters in Table G. This estimate can then be inserted into Equation G. In this method, turbulence and wind turbine response simulations are carried out for NTM conditions.

The wind speed s that yields the highest load is are then identified. The convergence criteria for IFORM should be the same as for the other extrapolation methods, except that the designer need only estimate confidence intervals for the load distributions from identified important wind speeds often only one. The desired fractile of the load distribution for this bin is derived and depends on the target probability level.

To reach the appropriate fractile, extrapolation may be required. For this kind of macroscopic view of fatigue, there is general agreement that an increment of damage results from each hysteresis cycle displayed in the local stress-strain diagram.

Thus, each local maximum of the load time history is paired with the local minimum that completes a full cycle rain-flow cycle counting, see [1] or [2]. Each of these cycles is characterized by the paired extreme values or equivalently by the range and midpoint values, i. In this expression, it has been further assumed that the local stress at the failure location is linearly related to the loading. Thus, the desired minimum level of reliability may be expected when the damage sums to unity.

For the life of a wind turbine, there will be many cycles of varying sizes resulting from a broad range of wind conditions. Therefore, for design purposes, a load spectrum should be estimated. The largest cycles for this spectrum will be estimated from a smooth fit to the data obtained from simulations or testing of a duration that is significantly shorter than the turbine lifetime.

For each wind condition, it may be assumed that the load is modelled by a stationary random process. This restriction is eliminated later when the issue of varying midpoint levels is addressed through the use of an equivalent cyclic range. Utilizing these results, and considering the requirement from 7. The formulation up to this point has ignored the effect of the variability in the midpoint levels for each load cycle. One simple way of dealing with this variability is to define damage equivalent load cycles with a fixed midpoint value.

In this case, the damage done by the equivalent cycles is exactly the same as that done by the cycles with varying midpoints. Thus, failure will occur on average for the same number of constant amplitude cycles for the equivalent cyclic range, S eq , as for cycles at any given cyclic range and midpoint value.

Typically, M 0 is chosen to give R values the ratio of maximum load to minimum load for the equivalent load cycles that are in the middle of the range of values observed directly in the load data. Often, an acceptable value is the mean load considering all operating wind speeds.

Fortunately, in most cases where the S-N curves are defined analytically e. Care should be taken, however, as the range becomes large.

Depending on the midpoint value, the maximum or minimum load value for the given cycle can get close to the static strength, in which case the simple, high-cycle S-N curve may not be applicable. Also, for larger range values, the local stress or strain may transition from a compression-compression or tension-tension dominated case to a tension-compression case, which could have a different analytical S-N curve representation.

It is important to utilize the proper S-N relation in determining the equivalent cyclic range. For a given load time history, the rain flow cycles are first identified. Then a set of equivalent constant-midpoint cycles is computed considering the proper S-N relation for each cycle.

The distribution of these equivalent cycles is then estimated giving a new short-term equivalent load spectrum.

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IEC outlines the minimum design requirements for fixed offshore wind turbines and is not intended for use as a complete design specification or instruction manual. Several different parties may be responsible for undertaking the various elements of the design, manufacture, assembly, installation, erection, commissioning, operation and maintenance of an offshore wind turbine and for ensuring that the requirements of this document are met.

The division of responsibility between these parties is a contractual matter and is outside the scope of this document.

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