certification of offshore wind farms_schwartz and argyriadis

8/22/2019 Certification of Offshore Wind Farms_Schwartz and Argyriadis

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Certification of Offshore Wind Farms

Silke Schwartz1

and Kimon Argyriadis2

Germanischer Lloyd WindEnergie GmbH, Steinhöft 9, 20459 Hamburg, Germany,1

tel:+49 (0)40 31106 -5550 fax: -1720, email: [email protected]:+49 (0)40 31106 -138, fax: -1720, email: [email protected]

Abstract

The paper presents the general way through the certification with emphasis on the load

assumptions. An overview of the state of the art considering the external conditions for the

load assumptions and methods to improve the instantaneous calculation approach are given.

The differences between offshore site data and generic requirements in regulations are

presented. Since for the machinery of the wind turbine the influence of the wave loads is

small, the certification is similar to the certification for onshore turbines for different classes

with respect to wind park influence. A comparison of the machinery loads for a selective windturbine on- and offshore will be drawn up. The influence of wave loading on machinery

components is analysed.

For tower and foundation, a generic approach is not possible, detailed knowledge of the site

conditions, such as water depth ranges, soil conditions, park influence, wave scatter diagram,

extreme wind and wave conditions and currents is necessary. Due to lack of available data,

this paper gives different methods to obtain sea states out of the correlation between wind

speed and wave as well as spectral correlation, including shallow water influence. A method

used to combine extreme waves and wind gusts is analysed and the influence of braking

waves, the variation of soil conditions and water depths are discussed. Further special

offshore influences and boundary conditions like scour, corrosion, maintenance, availabilityare pointed out.

Keywords: Certification, Offshore, sea spectra, load assumptions, scour

1 Introduction

In the growing market for wind energy and the limited available space onshore, the

development of offshore wind farms becomes more and more important. The particular

requirements offshore became apparent when planning the first offshore wind farms in the

UK, Denmark, the Netherlands, Sweden and Germany. For reliable predictions of the

aerodynamic and hydrodynamic loading for the design of wind turbines the experiences from

onshore wind technology and the knowledge of the construction of offshore structures (e.g.:oil platform) as well as results from research projects leading to the publication of

Germanischer Lloyd (GL) Regulations for the Certification of Offshore Wind Energy [1] in

1995 should applied.

The Regulations were developed in the JOULE 1 Offshore study [2] by merging the GL

Regulations for the Certification of Wind Energy Conversion Systems [3] and the Rules for

Offshore-Installations [4]. In the meantime it was possible to apply these guidelines in reality

and to prove and compare the requirements in the regulations with the conditions at offshore

sites. Experience in offshore certification according to GL classes as well as site

specifications offshore exist through several projects. Further site measurements within the

scope of the EU funded research project OWTES (Offshore Wind Turbine at Exposed Sites)

admit to review the requirements in the Regulations and new perceptions for loadassumptions are discussed.



2 Certification of offshore wind farms

While onshore wind turbines are generally installed with a Type Certificate offshore wind

turbines need in general additional monitoring, verification and certification. All of these

activities can be included in the Project Certificate. Additionally to a Type Certificate which

can issued for offshore wind turbines in the same way as for the onshore (including design

assessment, prototype testing (onshore), quality management) the Project Certificate maycomprise the following tasks:

• Type certification of wind turbine

• Site assessment

• Site specific design assessment of the foundation

• Risk evaluation

• Monitoring of fabrication, transport and installation

• Commissioning witnessing

• Periodic monitoring

• Monitoring of decommissioning

From the range of tasks the appropriate modules will be put together, on the basis of the

relevant regional requirements and the preferences of the owner/investor.

In Type Certification the main task is the evaluation of the design documentation including

parallel independent analysis of loads and structural details. Regarding offshore projects and

the influences on offshore wind turbines it became evident that a Type Certification of the

whole structure as in the onshore case is not possible. The influences of site specific

parameters, like water depths, wave heights and soil conditions, on the support structure

(foundation and tower) are significant, resulting in site specific designs. In contrary it can be

shown that the influence of the site parameters (except wind) on the machinery design (above

yaw bearing) is limited, allowing type classification.

Site Assessment is one of the mandatory modules in Project Certification. It comprises the

assessment of the environmental conditions and the validation of the design assumptions on

the basis of the site specific conditions. The influence from following items is considered in

the assessment:

• Site data (water depth, tidal ranges, etc.)

• Environmental conditions at the site (wind, waves, currents, temperature, sea ice,

seismic activity etc.)

• Influence of the turbines to the site and to themselves (scour, wake influence)

• Geotechnical data

• Electrical conditions at the site

In many cases the machinery of the turbines to be installed are derived from onshore designs.

In the certification procedure it has to be shown that the measures taken to comply with the

requirements of the marine environment are sufficient.



3 Requirements for the load assumptions

3.1 Site data

The site data shall include the co-ordinates for all turbines considered. Special care has to be

taken to provide accurate measurements of the water depth for each location referenced to a

clearly specified datum (usually LAT = Lowest Astronomical Tide).

Water depth is a driving factor for the foundation loading. It has decisive influence on the

turbine dynamics, on the support structure loads and in the case of scour and sand movement

to the pile penetration depth and load carrying capacity. For gravity foundations the danger of

settlements and the appropriate soil preparation have to be taken into account. Thus additional

analysis should be performed regarding the soil stability and the occurrence of moving sand.

The possibility of soil changes during the whole lifetime of the project has to be considered.

Additionally, the tidal range at the site has to be documented.

3.2 Environmental data

It is of prime importance to obtain appropriate wind, wave and current data for the site. Otherdata are to be evaluated as applicable (temperature of air and sea, ice, seismic activity, etc.).

Correlation of wind and waves

The 50 year return period shall be used for extremes. In the case of combined actions, their

joint probability should be used. It is considered that the extreme wind speed occurs in the

same 50-year storm as the extreme wave. Although during this storm the two extremes are not

correlated. The same may be the case regarding the extreme water level. It is expected that the

extreme storm surge occurs during the 50-year storm and thus together with the extreme

wave. But the 50-year storm is not correlated to the highest astronomical tide. If any statistical

information on the extreme water level exists this may be considered, alternatively the values

have to be added resulting in a conservative approach.Wind

The external wind conditions for an offshore site are given, in the same way as onshore, with

annual average wind speed, wind distribution, the turbulence intensity and the wind profile.

The wind climate offshore is different from the wind conditions onshore with lower

turbulence intensities, wind shear and usually higher mean wind speeds. It has to be stated

that at high wind speeds turbulence intensity increases with increasing wind speed.

Additionally, the influence of wakes from neighbouring turbines on turbulence intensity gets

more importance than for onshore sites.

Since it is difficult to get measurements at most projected offshore sites, special attention has

to be paid on the numerical modelling of the wind conditions at the site. The different winddirections and the distance to shore have to be considered as well as the influence of the sea

roughness due to different fetch and water depths. It is recommended to verify the models

with the most nearby placed offshore measurements to get confidence in the data. A long term

correction according to nearby onshore data is possible.

Waves

Wave heights and sea states are highly dependent on site conditions like fetch, time and water

depth. The waves are described by long term distributions of the sea state in the scatter

diagram while the short term description is given by wave spectral representations. The sea

state statistics are given via scatter diagrams presenting the probability of the different

significant wave heights (Hs) and zero crossing periods of the waves (Tz) for the given site.To reduce the number of calculations these scatter diagrams are “lumped” to some



representative mean values. An example of a scatter diagram for a North Sea site is given in

Figure 1.

WAVE HEIGHT / PERIOD SCATTER TABLE

HEIGHT PERIOD (s)

(m) 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 TOTALS

10.5-11.0 0.1 0.1 0.210.0-10.5 0.1 0.1 0.1 0.3

9.5-10.0 0.2 0.2 0.2 0.6

9.0-9.5 0.2 0.3 0.2 0.1 0.8

8.5-9.0 0.3 0.4 0.3 0.2 1.2

8.0-8.5 0.1 0.4 0.6 0.4 0.2 1.7

...

1.5-2.0 4.6 32.6 50.1 29.7 9.6 2.1 0.4 0.1 129.2

1.0-1.5 0.1 10 55.4 68.6 33.7 9.2 1.8 0.3 179.1

0.5-1.0 0.5 22.7 85.3 76.1 28.2 6 0.9 0.1 219.8

0.0-0.5 3 41.5 75.5 37.3 8.6 1.3 0.1 167.3

TOTALS 3.6 78.8 248.9 233.3 101.9 27.4 5.4 0.8 0.1 700.2

Figure 1: Wave scatter diagram

Sea spectra

The formulations of wave frequency spectra in marine applications are the Pierson-

Moskowitz spectrum (PM-spectrum) for a fully developed sea and the JONSWAP spectrum

(Joint North Sea Wave Project) for a developing sea. It is formulated as a modification of the

PM-spectrum for a developing sea state in a fetch limited situation. For finite water depths the

self-similar spectral shape (TMA-Spectrum [9]) was developed. This spectrum is an extension

of the JONSWAP Spectrum and water depth dependent. In offshore industry the PM-

spectrum is widely used for fatigue analysis while the JONSWAP spectrum is used for

extreme load analysis.

In many cases the scatter data are not available or the scatter diagram does not include anyinformation about the wind speed to be used in combination with the wave height. In this case

theoretical assumptions of the wind wave correlation have to be built up. Using the one-

parametric wave spectra, or the relations derived for the JONSWAP spectrum in conjunction

with the TMA-formulation a relation may be derived for wind speed to significant wave

height.

The JONSWAP spectrum is given in [8]:

( )

−−

−−

⋅⋅=⋅

−⋅

⋅=

22

2

1exp

,

2

1exp4

5

2

)(

4

5exp

p

p

p

p

PM

pS nf

gnf S

ωσ

ωω

η

ωσ

ωω

η γ ωγ

ω

ω

ω

αω

with

? = wave frequency

ωp = spectral peak frequency

g = acceleration of gravity

a ˜ 0.0081 generalized Phillips´ constant

σ = spectral width parameter (s a ˜ 0.07 for ω < ωp; s b ˜ 0.09 for ω > ωp)

γ = Peak shape parameter (Pierson-Moskowitz spectrum for γ = 1)

nf = Normalizing factor to ensure same Hs to PM-spectrum, i.e.( )[ ] 1803.0

135.0065.05−

+= γ nf



For the North Sea data the parameters γ , s a and s b are not constant but show considerable

scatter. Average values from the JONSWAP data were: γ = 3.3; s a = 0.07; s b = 0.09. In the

data, the peak shape parameter γ varied between about 1 and 6 and was approximately

normally distributed with a mean of 3.3 and a standard deviation of 0.79. In different

geographical areas the JONSWAP Spectrum form also appears to be capable of representing

the observations rather well, provided that the parameters γ , s a and s b are chosen inaccordance with the local data [15].

For not fully developed sea states the influence of the time and fetch x the wind is acting may

be considered with [16]:

The dimensionless time of wind: ( ) timeug ⋅= / θ

and the fetch: ( ) xug ⋅= 2 / ξ

with g the acceleration of gravity and u the hourly mean wind speed at 10 m above the sea

surface.

The dimensionless peak frequency is

⋅⋅=⋅= −− 7

3

3.08.16;84.2;16.0max

2θξ

πω ν

gu p

the value of the Phillips constant is 32

028.0 να ⋅=

the peak period ν1

⋅=g

uT p

and the significant wave height isg

u H

s

2

3

5

0094.0 ⋅⋅=−

ν

As stated above the P-M and the JONSWAP spectra were developed for deep water situations

and the TMA-spectrum [9],[6] for finite water depth. Its general validity was checked against

measurements (Texel, Marsen, Arsloe). The influence of the water depth on the spectrum maybe seen in figure 2. The resulting significant wave height from the calculation using the TMA-

JONSWAP formulation and the measurements from a site are shown in figure 3.

)(S)(S dk JONSWAP,TMA, ωΦ⋅=ω ηη and

g

dd ω=ω

with

F k = transformation factor

? d = dimensionless depth dependent frequency

d = water depth

1)(

)2(5.01)(

5.0)(

dk

2

ddk

2

ddk

=ωΦ

ω−⋅−=ωΦ

ω⋅=ωΦ

2for

1for

1for

d

d

d

>ω>ω≤ω



0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1 1.5 2 2.5Omega

JONSWAP

TMA, d=10 m

TMA, d=15 m

TMA, d=20 m

TMA, d=30 m

TMA, d=40 m

Figure 2: JONSWAP and TMA wave spectra

In the case shown in figure 3 not only the water depth is limited but the distance to the coast is

relatively small. The result will be that the sea state is not fully developed and the wave

heights are limited by the finite water depth. From the external data considered it is clear that

the importance of water depth, fetch and time should be taken into account.

0.0 5.0 10.0 15.0 20.0 25.0 30.0

wind speed v [m/s]

H s [ m ]

TMA full developed

TMA fetch limited

JONSWAP fetchlimited

Measurement

Measurement 2

Figure 3: Wave height comparison

A further problem in the absence of reliable data is the combination of probabilities when a

wave scatter diagram has to be combined with a wind distribution. In most projects examined

a wave scatter diagram for the possible project site existed as well as a separate wind

distribution. It is assumed that that shallow water and current effects are included in the wave

scatter diagram. For the combination a method was proposed by Matthies [10] resulting in ascatter diagram for the combined case. A more simple method is to correlate the cumulative



probability distribution functions of the wind speed and the significant wave height. From the

pairs of same probability of exceedance the pairs of wind speed and significant wave height

are derived. In this process caution has to be taken for the existence of wind driven sea and

swell waves. In this case the analysis has to be performed for two different distributions of

wave heights according to their overall probability of occurrence.

Direction of wind and waves

Directionality of wind, wave and current may be considered if accurate data is available. In

this case not only the mean wind and wave direction have to be considered, but the

distribution of the wind and wave misalignment, too. Generally, if no detailed information is

available, it is recommended to apply wind and wave data as unidirectional, acting from the

same direction for the turbines lifetime. In the regulations the wave direction is always the

direction of the mean wind speed. In general this applies for open sea conditions and extreme

waves. It is site specific to verify if this assumption is correct. If sufficient evidence exists that

any of the environmental forces are directional, it may be possible to position the structure in

the most advantageous direction.

Breaking wavesBreaking waves may lead to high loads due to trigger ringing or dynamic amplifications on

the structure. They can be produced by shoaling (wave moving from deep water into shallow

water), by wind-wave, wave-wave and wave-current interaction. Shoaling is the most

important type for offshore wind turbines. In shallow water the empirical limit of the wave

height is approximately 0.78 times the local water depth. This has been found to compare well

with data, although some investigators have found a weak dependence on beach slope. In

deep water, waves can also break with a theoretical limiting steepness of 1/7 (0.14 times

wavelength ?). Three types of breaking wave may occur in shoaling waters, when the water

depth decreases and the wave height increases. The difference between the types is not very

sharp and their occurrence depends on the deep water wave height, the length ratio and the

seabed slope. Galvin [11] characterises breakers with the following ratio:

Br > 5 spilling breaker

5 > Br > 0.1 plunging breaker( )2

0

or m

HB

⋅λ= and

0.1 > Br surging breaker

with:

H0 = wave height in deep water

λ0 = wave length in deep water

m = seabed slope

Further information may be found in [12]. Surging breakers are usually not important for

offshore structures. Spilling breakers may be analysed as a limited height regular wave. For

plunging breakers, few data are available on the velocities and accelerations of water particles

in waves. Due to the sudden immersion of a member the impact force on it lead to a dynamic

response of the member, which may increase the force on the member considered (slap force).

There are different engineering methods to calculate wave slap. The physics of the forces are

identical for vertical and horizontal members (slam) so methods to calculate slamming forces

are used. In the offshore study [1] and the Atkins report [12] a method for calculating the slap

force is described. The formula for the slamming/slap force is similar to the drag formula

applying a slap coefficient instead of the drag coefficient. The dynamic amplification of themember as a function of the eigenfrequency, angle to the water surface and wave celerity



should be considered. In the Danish recommendation for offshore wind turbines [13] it is

stated that the maximum particle velocity in the plunging breaking wave is given by the

expression d gu 25.1max = , and shall be applied as the velocity in a monotonous velocity

profile for the entire wave above the still water level. Below the still water level a velocity

profile is applied, as in conventional wave theory.

A solution used is to calculate the load using regular wave theory and apply a “corrected”

drag coefficient for the part of the pile affected by the plunging breaking wave. The

application of an increased drag coefficient due to slap or the corrected drag coefficient for

high water particle speed results in high drag coefficients in the magnitude of 3 – 3.7.

4 Load assumptions

4.1 Extreme load

From the experience in offshore and onshore load analysis it is generally known that design

driving load cases for the machinery are depending on the control system of the turbine and

the extreme wind speed. Aerodynamic loading dominates the loading on the machinery. Forthe support structure the extreme 50-year storm case is often the design driver for the ultimate

loading, with the portion of wave to wind load varying with the water depth. In the following

the problem to implement the analysis for the 50-year storm case is addressed.

50 year storm case

The assumptions for the extreme wind speed and the extreme wave height gain significance in

this relation. The extreme wave height to be combined with this wind speed is assumed to be

site specific. Limiting value for the wave height is very often the breaking wave height

HB ≈ 0.78 d. The problem is not only to find the corresponding extreme wave and the

respective wind speed, the analysis procedure to combine the loading implies some problems,

too.

To take account of the turbine elastic behaviour and the nature of wind loading time domain

simulations are commonly performed based on stochastic wind speed time series. In this case

turbine dynamic behaviour and dynamic amplification of wind loads is correctly represented,

but several realisations are necessary due to the stochastic nature of gusts. An often used

alternative is to apply a constant wind speed and then multiply the loads with a dynamic

amplification factor as defined in building standards, or derived by separate analysis.

In offshore industry it is recognised that, especially in shallow waters, the wave particle

kinematics non-linearity may play an important influence. In this case an analysis has to be

performed on the significance of the wave non-linearity and the structure elastic behaviour.

Since offshore structures placed in shallow waters are dominated by non-linear extreme waveaction and not by elastic response the quasi static approach is often used.

In combination with the analysis method using stochastic wind speed time series a stochastic

wave time series could be used. This calculation would result in a correct phasing of wind

speed and wave elevation during a storm (provided the simulation length and number is big

enough). The turbine elastic behaviour is represented in the load analysis. A major problem is

that only linear wave kinematics can be used for the stochastic wave train. Some methods

proposed in recent time to include wave kinematics non-linearity have found only scientific

application and are still too complicated for engineering use. The alternative form would be to

use the dynamic amplification factor method for the wind load and nonlinear single wave

kinematics for the wave load. The problematic point is to calculate the dynamic amplificationfactor for the wind loading and to separate wind from wave loading. The influence of the



different analysis methods are shown in table 1 for the mud line overturning moment Mxy.

The example is based on a generic 2 MW wind turbine for a water depth of 20 m. It has to be

stated, that the principal load direction of the wind and wave loads may be different.

Wave only H = 12.1 m

Mxy [MNm]

Wind only v = 50 m/s, I =11%, ve = 63 m/s

Mxy [MNm]

Linear (Airy, Wheeler stretched) 56 Constant wind, stiff structure 47Non-linear (Stream function) 95 Turbulent wind, elastic structure 61Difference non-liner/linear 1.7 Difference dynamic/static 1.3

Table 1: Mud line overturning moment for a generic 2MW offshore wind turbine in 20 m water depth.

In the analysis it can be seen that the wind and wave loads at the mud line are of the same

order of magnitude. At the tower top and for the machinery the wave loads do not have

significant influence, except in the cases of resonance between the wave period and its higher

harmonics and the support structure eigenfrequency. In the present case the dynamic

amplification of the linear wave load is below 10% of the maximum load.

It is clear, that a simultaneous combination of the extreme gust wind speed and the extreme

wave will result in a very conservative approach. It is assumed that the extreme wave and the

extreme gust occur during the same 50-year storm but their appearance is not correlated. This

can be shown in table 2, presenting results from stochastic analysis.

Wind only Wave only Combined wind and wave Wind + wave Quadratic addition61 59 80 120 85

Table 2: Mud line overturning moment for different combinations. Stochastic, dynamic analysis

In the offshore industry the 1-minute averaged wind speed is usually used in combination

with the extreme wave for the global structure analysis. In [2] a reduced wave height was

derived to be combined with the extreme gust wind speed (5s-average), having the same

probability of occurrence as the combination of maximum wave height with 1-minuteaveraged wind speed. According to the offshore study [1] the extreme wave height is taken as

the most probable highest of 1000 waves during a storm assuming a Rayleigh distribution.

This is a conservative approach and for different sites an extra investigation may lead to

different results [7] considering the wave spectral broadness and the storm duration.

To comply with the requirements of the dynamic analysis and the wave kinematics non-

linearity the present approach includes several realisations of the 50-year storm:

• Dynamic simulation using stochastic wind and waves. This approach uses linear wave

theory, but takes full account of the structure dynamics.

• Simulation using constant (1-minute averaged) wind speed, with correction fordynamic amplification and deterministic non-linear extreme wave (Hmax = 1.86 Hs). In

this case the extreme wave influence for wave load dominated structures is

considered.

• Simulation using constant (5-second averaged) wind speed, with correction for

dynamic amplification and deterministic non-linear reduced extreme wave

(Hmax = 1.32 Hs). In this case the extreme non-linear wave influence for wind load

dominated structures is considered.

The result for the overturning moment Mxy and the shear force Fxy at the mud-line is shown in

table 3.



Wind & Wave 5-s av. & reduced wave 1-min. av. & max. wave Turbulent & stochasticFxy [MN] 2.8 5.1 2.6Mxy [MNm] 86 120 80

Table 3: Mud line load for different combinations of 50-year storm case.

In the present analysis the dynamic amplification is applied as a factor on wind speed whereas

the amplification due to wave load is included through the support structure elasticity insimulations using nonlinear waves. Usually not only a single wave but a train with 4 to 5

waves with extreme height is simulated. In this case the dynamic amplification due to wave

loads (and the nonlinear kinematics) is included in the analysis, but due to the train of

consecutive extreme waves used a conservative result may be received. A less conservative

way could be to use a wave train of 4-6 nonlinear waves with linear changing wave height

until the maximum wave height is reached.

4.2 Fatigue load

For the fatigue analysis special care has to be applied in consideration of all aspects of

offshore wind farms. Different water depths and soil conditions in park configurations maylead to different eigenfrequencies for the individual turbines. Furthermore different tidal

ranges, corrosion allowances, scour influences, higher probabilities of idling load cases due to

worse availability, wake influences etc. have to be considered for each turbine resulting in a

high number of fatigue analyses.

It is essential to develop methods to detect the turbine with the highest fatigue loading in an

offshore wind farm configuration. In the offshore industry fatigue loads on offshore structures

may be calculated using frequency domain analysis [14]. For wind loading time domain

simulations are used to cover the non linear behaviour of the structure. In this context the

combination of the two methods was tried.

A successive consideration of aerodynamic and hydrodynamic loading was carried andcompared with a simultaneous simulation in the time domain. The investigation was carried

out for a 2 MW generic offshore turbine located at 8 m water depth. In table 4 the equivalent

loads for the fatigue analysis for a successive and a simultaneous calculation are compared.

The loading out of the successive investigation is combined with a quadratic superposition.

Wind only Wave only Wind+Wave Wind+Wave Deviation[kN] or[kNm]

[kN] or[kNm]

Simultaneous[kN] or [kNm]

Quadratic[kN] or [kNm]

%

Thrust force mud line 131 340 364 364 0.0

Thrust force MSL 77 62 91 99 8.2

Thrust force tower top 48 10 48 49 2.1Tilting moment mud line 3829 2117 4220 4375 3.5

Tilting moment MSL 3336 857 3333 3444 3.2

Tilting moment tower top 2709 683 2707 2793 3.1

Thrust force hub 48 3 48 48 0.0

Torque hub 208 0 207 208 0.1

Tilting moment hub 549 0 549 549 0.0

Blade root flap moment 474 1 474 474 0.0

Table 4: Fatigue load spectra for different section N = 108, S/N-curve exponent m = 4

It can be seen that the successive consideration with the quadratic superposition is a good and

conservative approach to obtain loading on the turbine. It can Also be seen that the influence

of the wave loading on the machine and blade loading is neglectable in the present case.Design of the main bearing may be influenced by an increased thrust at the hub, found in



some cases. This increased thrust force is a factor of the dynamic amplification of the wave

forces and depends heavily on first and second natural frequencies of the support structure. In

the example presented a relatively stiff (T ≈ 2 s) structure was chosen resulting in minimal in-

fluence in thrust force.

The method for the superposition of wind and wave generated equivalent loads is a relatively

simple solution to gather acceptable and comparable load spectra of the combined loading.

General experience shows that a superposition of wave loads obtained from a frequency

domain analysis and wind loads from a time domain analysis has to be performed very

carefully. The different approaches and the discretisation errors of the wave elevation may

lead to considerable errors for sections near the water line. But this method allows extensive

parameter variations to be investigated i.e. covering the whole range of water depths, soil

properties etc. within a wind farm. This way highly loaded wind turbines in a farm may be

detected for which more precise time domain analysis need to be carried out.

5 Generic Design

It is a general view that a generic design may help manufacturers in the development of offshore wind turbines. The turbine classes defined are applicable for wind conditions. Wave

heights are site specific and the influence of the different external conditions especially on the

support structure raises doubts on the applicability of this procedure.

In the following an analysis was performed to compare loading on the machinery and the

supporting structure according the standard GL onshore and offshore classes [3][4] and to site

specific approaches. The data were selected from existing projects GL Wind is involved.

The environmental conditions are between class 1 and 2 according to the GL-Regulations for

offshore wind turbines. To generalise the results, mean values from the different projects and

those derived for generic offshore wind turbine designs are shown. The wind conditions

according to the Regulations and the site assumptions are shown in table 5.

10-minextreme

[m/s]

5-secextreme

[m/s]

Mean windspeed[m/s]

Windshear

Turbulenceintensity

[%]

Hmax[m]

Meansea level

[m]GL offshore class I 50 60 10 0.11 I = 12+park 10 14GL offshore class II 42.5 51 8.5 0.11 I = 12+park 7 8Site abc (mean) 46 56 ≈ 10 0.11 I15 ≈ 9+park ≈ 10 ≈ 14

Site z 37 44 ≤ 8 0.11 I = 12+park ≈ 7 ≈ 8

GL onshore class I 50 70 10 0.16 20 - -GL onshore class II 42.5 59.5 8.5 0.16 20 - -

Table 5: Environmental conditions for generic classes compared to site data

The wave and current parameters applied in the investigations were site data. In the case the

generic turbine classes do not provide any information on the parameters to be applied and no

simplified method exists to derive these, the site data were applied. The results of this analysis

are shown in tables 6 and 7. To get a realistic approach on the applicability two cases with

different support structures and slightly modified turbine parameters were analysed. The first

case was analysed according to GL class 2 and compared with site z which showed

environmental data equal or less severe than class 2. In the second case the analysis was

performed for a class 1 turbine and the results were compared to the mean of the site specific

analysis.

As it can be seen in table 6 the fatigue machinery loads (from blade tip to yaw bearing) of the

generic offshore wind turbine are less or equal to the loads of the equivalent onshore windturbine. Similar results are achieved for most of the extreme loads considered. From the



results it can be seen that the generic approach gives a good approximation of the averaged

site specific machinery loads. Significant differences were seen in the tower top loads due to

the different support structure dynamics (site / generic). The loads at the mud line showed

large scatter. This makes clear that the support structure has to be site specifically designed.

Generic model a Generic model bGL2 onshore

GL2 offshore

GL2 offshore

site z

GL2 onshore

GL1 offshore

GL1 offshore

mean of sites abc

Blade root M edge 1.07 1.03 1.12 1.14

Blade root M flap 1.27 1.10 1.13 1.07

Hub fixed Fx 1.46 0.88 0.99 1.21

Hub fixed Mx 1.38 0.82 1.24 1.17

Hub fixed My 1.47 0.97 1.04 1.30

Hub rotating Myz 1.10 1.07 0.96 1.11

Tower top Fx 1.74 0.59 1.00 1.18

Tower top Mx 1.34 0.79 1.18 1.20

Tower top My 1.70 0.83 1.04 1.27Mean 1.39 0.90 1.08 1.18

Table 6: Comparison of fatigue machinery loads from generic classes and site specific analysis

Generic model a Generic model b

GL2 onshore

GL2 offshore

GL2 offshore

site z

GL2 onshore

GL1 offshore

GL1 offshore

mean of sites abc

Blade root M edge 1.11 1.09 0.99 0.91

Blade root M flap 0.86 0.98 0.73 1.10

Hub fixed Fx 0.89 1.14 1.03 0.98

Hub fixed Mx 0.88 1.14 0.99 0.96

Hub fixed My 1.07 0.89 1.15 0.92Hub rotating Myz 1.02 0.98 1.02 1.22

Tower top Fx 0.83 2.14 1.09 0.94

Tower top Mx 0.81 1.42 1.23 1.04

Tower top My 1.00 0.98 1.08 0.94

Mean 0.94 1.20 1.03 1.00

Table 7: Comparison of extreme machinery loads from generic classes and site specific analysis.GL-coordinate system.

From the parametric analysis performed following points have been identified:

• “Marinized” onshore turbines are initially developed according to standard classes.

This leads to a conservative estimate of the loading for the machinery (at least for thetypical sites now in development).

• The support structure is largely influenced by wind, wave loading, water depth and

soil parameters.

• A generic approach for the machine is possible; site specific design of the substructure

is required.

• Existing classes I and II apply for offshore conditions.



6 Further influences on offshore wind turbine loading

6.1 Scour

Scour is the removal of seabed soils by currents and waves. Such erosion can be due to a

natural geological process or can be caused by structural elements interrupting the natural

flow regime above the sea floor. From observations, sea floor variations can usually becharacterized as some combination of the following.

• Local scour. Steep sided scour pits around structure elements.

• Global scour. Shallow scoured basins of large extent around a structure, possibly due

to overall structure effects, multiple structure interaction, or wave-soil-structure

interaction.

• Overall seabed movement of sand dunes, ridges, and shoals that would also occur in

the absence of a structure. Such movements can result in sea floor lowering or rising,

or repeated cycles of these.

Scour can result in removal of vertical and lateral support for foundations, causing

undesirable settlements of shallow foundations and overstressing of foundation elements.

Where scour is possible it shall be taken into account in design and/or its mitigation shall be

considered. In case that scouring may occur, the foundation has to be protected by suitable

means, alternatively, the foundation has to be considered partly unsupported. If no other data

are available for the specific site conditions, the scour depth at pile foundations may be

estimated as 2.5 × d (d = pile diameter) for design purposes. Indicative analysis using the

generic turbine model for a water depth of 20 m with scour equal to 1 diameter showed

considerable influence on loading.

Load type blade flap[kNm] hub thrust[kN] hub tilt[kNm] tower top tilt[kNm] bottom overturning[kNm]

Change ratio 2.5% 7% 3% 2.5% 68%Table 8: Change of fatigue loading due to scour of 1d.

The changes of the loads due to scour are not only due to the modified wave kinematics but

also to the changes in eigenfrequencies of the support structure. In the present example the

basic natural frequency of the support structure changed by 10%. Due to the relevance of

scour criteria laid down for the design shall be verified by regular surveys. Countermeasures

shall be taken in case of exceedance of the limits established in the design.

6.2 Soil variationThe soil conditions in a wind farm may vary considerably. Knowledge of the soil conditions

existing at the construction site is necessary to develop a safe and economic design. During

analysis of the data from different projects a variation of the first natural frequencies of the

offshore wind turbines of up to 10% was encountered. Thus geophysical surveys should be

performed in the first stage with a geotechnical investigation following during main analysis.

Structure–foundation interaction should in general be modelled non-linearly. For piled

foundations the non-linear behaviour of axial and lateral pile-soil support should be modelled

explicitly to ensure load deflection compatibility between the structure and pile-soil system.

For pile analysis the effects of geometrical and material non-linearities should also be

accounted for within the structure-pile-soil system. The effect of cyclic loading which may

cause a reduction of shear strength and bearing capacity of the soil shall be investigated.



In practice it is very difficult to perform time domain simulations considering the foundation

non-linearity. In this case the p-y curves are approximated by linear springs which differ with

respect to the applied load. This means that different natural frequencies are derived for

extreme and fatigue load analysis and for different load cases. The procedure is not straight

forward since loads are a functions of the eigenfrequencies which in their turn depend on the

overall loads.

6.3 Other topics

Availability

In offshore conditions, more emphasis shall be put on reliability, extended remote control and

longer maintenance periods. Whenever a severe fault occurs in an onshore turbine a service

team can be at the site within hours. However, if this happens at an offshore location and

weather conditions do not allow immediate access the turbine may remain in the faulty

conditions for a long period. This situation has to be considered in the load analysis. The

reduced turbine availability will result in a greater percentage with the turbine in the idling

mode. Since aerodynamic damping is very low in this case the dynamic amplification of wind

and wave loads may become considerable. The influence on fatigue loading may not be

neglected and has to be considered during design. Additionally longer periods with no grid

connection or the turbine idling in a fault mode may lead to the occurrence of extreme storms

combined with unfavourable yaw and pitch conditions.

Corrosion Control

An important design modification for offshore siting is proper corrosion protection. Even if

coatings have a high quality corrosion of the pile at the splash zone has to be considered.

Often a corrosion allowance is applied ranging between 0.2 mm/year to 0.5 mm/year. The

amount is a factor of the protection system applied and the site temperature, salinity, humidity

etc. This allowance should be considered in load analysis by using the most onerous situation

for extreme load cases and an intermediate “mid life” situation for fatigue analysis.

7 Conclusion

A Type Certification of the whole offshore wind turbine structure as in the onshore case,

independent of the site, is not possible. Since the influence of the wave loading on the

machinery is of minor importance, standardised machinery designs may be developed. The

machinery may be certified according to generic classes, as defined in GL´s Regulation for

the Certification for Offshore Wind Turbines. The influence of the site hydraulic and soil

conditions on the support structure is significant, resulting in site specific designs.

To comply with the increased requirements for certification of offshore wind farms a Project

Certification may be carried out. In this it has to be shown that the wind turbines, includingfoundation, comply with the requirements of the site environment. Thus one essential part of

the Project Certification is the site assessment.

In the site assessment effort has to be made to get a description of the joint distributions of

wind and waves and, if possible, their directional distribution, including misalignment.

Possible combinations of the different environmental phenomena have to be considered in a

realistic manner. The alternative of the simple addition of the loads from different extreme

situations or the simple addition of the loads from wind and waves would result in an extreme

conservatism.

The structural design of the offshore wind turbine machinery and support structure has to take

into account both wind loads and wave loads. The structural response of the structure whichmay result from wind and waves has to be analysed and considered in the definition of the



load conditions. For the foundations currents and the non-linear wave loads shall be included

in the analysis. The experience from the wind industry as well of the offshore industry is used,

but was adapted to the special shallow water conditions occurring at the project sites.

8 References

[1] Germanischer Lloyd, Regulations for the Certification of Offshore Wind EnergyConverter Systems, Hamburg 1995.

[2] Matthies et al, “Study of Offshore Wind Energy in the EC JOULE I (JOUR 0072)”,

Verlag Natürliche Energie 1995.

[3] Germanischer Lloyd, Regulations for the Certification of Wind Energy Conversion

Systems, Hamburg, 1999.

[4] Germanischer Lloyd, Rules for Classification and Construction - Offshore Technology,

Part 2 Offshore Installations, Hamburg, 1999.

[5] H. Söding, V. Bertram, “Ships in a Seaway”, Handbuch der Werften XXIV. Band, 1998.

[6] V. Müller, “Flachwassereinflüsse”, TU Hamburg-Harburg, AB Meerestechnik 1.

[7] K. Kokkinowrachos, “Offshore-Bauwerke” in Handbuch der Werften XV. Band, 1980.

[8] CD ISO 19901-1 “Petroleum and natural gas industries — Specific requirements foroffshore structures — Part 1: Metocean design and operating conditions“, 2002

[9] E. Bouws, H. Günther, W. Rosenthal, C.L. Vincent, “Similarity of the Wind Wave

Spectrum in Finite Depth Water”, Journal of Geophysical Research, vol.90, 1985.

[10] Matthies H.G., Meyer M., Nath C., “Offshore-Windkraftanlagen: Kombination der

Lasten von Wind und Wellen“, Proceedings of the DEWEK 2000.

[11] Galvin CJ, „Waves on Beaches”, Academic publishing, New York 1972.

[12] Dept. of Energy, “Fluid loading on fixed offshore structures, guidance on design and

construction”, WS Atkins engineering sciences, 1987

[13] Danish Energy Agency, “Recommendation for technical approval of offshore wind

turbines”, December 2001.

[14] S. Schwartz, K. Argyriadis, “Analysis of the Fatigue Loading of an Offshore Wind

Turbine Using Time and Frequency Domain Methods”, Proceedings of the EWEC 2001

[15] W. Blendermann, “Umgebungsbedingungen und Lastannahmen in der Meerestechnik”

Institut für Schiffbau der Universität Hamburg, 1995.

[16] H. Söding, „Bewegungen und Belastungen von Schiffen in Seegang“, Institut für

Schiffbau der Universität Hamburg, 1982.

certification of offshore wind farms_schwartz and argyriadis

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