Report on the Research Project OWID Offshore Wind Design Parameter

Report on the Research Project OWID Offshore Wind Design Parameter T. Neumann a, S. Emeis b and C. Illig c a DEWI German Wind Energy Institute, Ebertstr. 96, Wilhelmshaven, Germany b Institute for Meteorology and Climate Research, FZ Karlsruhe (IMK- IfU), Garmisch-Partenkirchen, Germany c DEWI-OCC Offshore and Certification Centre GmbH, Am Seedeich 9, Cuxhaven, Germany supported by Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU-Project 0329961a 1 ) and ENERCON, GE-Wind Energy, Multibrid, Repower Offshore wind energy can play an important role to support the aim of producing 20% of the total electricity demand by renewable energies by 2020. Most of the projected German areas for the offshore use however lie far away from the coast in water depths up to 40m. For this situation no real experience is given. Even the Danish offshore north sea wind farm Horns Rev erected in 2002 cannot be compared directly as it has a much shorter distance to the coast line of about 14-20km. The FINO1 platform which is installed in 2003 about 45km off the island Borkum is equipped with a met mast with a height of about 100m and records the long term meteorological and oceanographic conditions in the North Sea. Within the project OWID the FINO-data is used to reduce incomplete knowledge when adapting wind turbines to the maritime conditions. We start with a thoroughly evaluation of the acquired FINO1 data with the focus on the mechanical loads a future wind turbine is exposed to. In addi- 1 Design Parameters and Load Assumptions for Offshore WEC in the German Bight on Basis of the FINO-measurements

tion to the undisturbed wind field the disturbed wind stream within the wake field is simulated by CFD models as we think that the major part of the load origins in the wake fields. Both undisturbed disturbed wind fields are used to calculate the loads on a realistic offshore wind turbine with regard on the lay out and the life time. To keep the project close-to-reality manufacturers of 5MW wind turbines are included by not only giving financial support but also supplying realistic models of multi-megawatt wind turbines. The project shall result in proposals to improve the applicable guidelines to minimise risks in the planning, building and operating phase of offshore turbines and to set up a reliable basis for financing and insurance of the scheduled projects in the German Bight. 1 FINO1 Platform Fig. 1 shows a photo of the FINO1 platform shortly after its erection in 2003. Scientists regularly visit the platform to work on technical and biological research projects. Because of the distances to be covered and the difficult boarding manoeuvres from ship to platform, the preferred means of transport is by helicopter. Fig. 2 shows the position of the platform in the area of the first approved German offshore wind projects, indicating also the approximate travelling times. In Fig. 1 the positions of the sensors for wind speed and wind direction have been marked with arrows. A series of identical cup anemometers has been arranged on the south-east side facing away from the platform which measures the long-term wind speed at eight different height levels (33.5 to 101.5 m). The platform thus provides the most complete continuous wind measurement and meteorological data set in the offshore area world wide. On the north-west side of the mast, classic wind vanes are installed at 33.5, 50, 70 and 90m height to determine the wind direction. High-resolution ultrasonic anemometers (USA) are installed at the intermediate levels (40, 60 and 80m). They not only provide data on short-term

Fig.1: The FINO1 Platform in the North Sea /1/

fluctuations of the wind flow (turbulence), but by means of the 3-D wind vector are also capable of recording the horizontal wind direction and vertical wind flows. Since they are arranged at a position of 180 to the cup anemometers they can provide an additional measurement of the wind speed at the lower levels when the wind is coming from the north-west and the cup anemometers are therefore lying in the wake of the met mast. Other sensors are measuring the meteorological standard parameters such as temperature and humidity and irradiation at different levels. Fig.2: Location of the FINO1 platform 45 km in front of the island Borkum Set up of the measurements The reduced accessibility of the FINO1 station requires a high reliability and stability of the technical system. Therefore components with well known performance and proven offshore capability have been used. For instance classical hair hygrometers were taken to measure the humidity instead of capacitive sensors. For the data processing and acquisition stand alone data loggers have been used where it was possible. Only the high frequency data measurements are realised with a PC-dependent solution.

Low Frequency Data Table 1 gives an overview of the measured parameters, stored as 10min averages (1min internal). The position of the wind speed and wind direction sensors can be seen in Fig. 1. Parameter Heights (Sensor) Wind Speed 100,90,80,70,60,50,40,33m (Cup Anemometer) Wind Direction 90,70,50,33m (Vane) Temperature (PT100) 100,70,50,40,33m Air Humidity *) 100,50,33m Air Pressure *) 100,20m Global Irradiation *) and 33m UVA Tab. 1: Meteorological Parameters FINO-data is published as raw data, therefore includes sensor calibration, however effects of the platform or the met mast on the data are not corrected. Bars of the Lightning protection cage V(90m) / v(100m) Tower shadow South wind direction Fig 3: The influence of the mast and the lightning protection cage for the top anemometer can be clearly seen by a comparison of the wind speeds at 90m and 100m.

High Frequency Data The sonic anemometers at 40, 60 and 80m height allow a high frequency analysis of the turbulence structure of the wind and can also be used to study the influence of the mast effect on the wind stream, which is quite substantial as can be seen in Fig. 3 For a study of the interaction of combined wave and wind forces on the platform structure it is equipped with strain gages and acceleration sensors. Detailed analysis of this data is done within the GIGAWIND Project. In Tab. 2 the high frequency parameters measured at the FINO1- platform are listed. Parameter Heights (Sensor) Wind Speed u,v,w 80, 60, 40m Acceleration Mast u,v 100, 50m Acceleration Jacket u,v 7m, -4m, - 15,5m Strain Gages Jacket- -5 to -17, -25m Construction Tab. 2: High Frequency Parameters of the FINO1-Plattform sampled at 10 Hz Hydrographical Measurements The basic set up of the hydrographical measurements can be seen in Fig. 4. The sea state is measured by two different ways, a 3D wave buoy and a WAMOS system that is processing radar reflection from the sea surface. For the meteorology sea surface temperature is crucial, it is measured in 3 and 6m beneath LAT.

Fig. 4 Schematic set up of the hydrographical measurements which consist of sensors for the sea state, sea level, the water current and physical properties of the sea water Relevant Standards and Guidelines At present external wind conditions in the offshore regime are defined in guidelines by GL, IEC and DNV. DNV* Design of Offshore Wind Turbine Structures, Ed. June 2004 IEC 61400-3* Design Requirements for Offshore Wind Turbines, Working Draft, July 2005 GL Guideline for the Certification of Offshore Wind Turbines, Ed. 2005 *in connection with IEC 61400-1 Design Requirements and DNV/ Risoe Guidelines for Design of Wind Turbines (onshore focused)

WEA Class I II III S V ref [m/s] 50 42,5 37,5 V ave [m/s] 10 8,5 7,5 A I 15 [-] 0,18 site specific a 2 B I 15 [-] 0,16 a 3 C I 15 [-] 0,145 a 3 Fig.5: According to the GL-Offshore guideline overall wind conditions are defined according to standard or site specific classes together with standardised turbulence parameters. It is an usual approach within the guidelines to define a reference and average wind speed for certain classes together with parameters for different turbulence regimes. While the DNV and IEC guidelines still refer to the onshore related IEC-61400-1, the GL-Offshore already introduced a special subclass C with a lower assumption for the turbulence as can be seen from figure 8. For the standard classes a Rayleigh distribution of the wind speed is assumed, for the site specific class a Weibull distribution is used, based on individual measurements. For the FINO1 site Weibull parameters A=11.1m/s and k=2.16 have been found by an analysis of the data up to January 2005 as can be seen in figure 6. Fig 6: Wind speed distribution as measured at the FINO1-Platform. The right hand site shows the result of a long term correlation /2/.

Normal Wind Profile (NWP) Preliminary results shall be presented for two parameters of the normal wind conditions. The first one is the Normal Wind Profile (NWP) that is defined quite similar in the above mentioned guidelines. The mean wind speed V at height z is consistently given as a potential law V(z)=V hub (z/z hub ) a (1) and V hub is the wind speed at hub height z hub. Fig. 7 shows the onshore wind profile (a=0,2) according to IEC-61400-1 in comparison with the offshore profiles defined by the GL-guideline and IEC-61400-3 (a=0,14). In addition mean wind profiles as measured at the FINO1 platform during 2004 have been plotted. As could be expected, the onshore NWP according to IEC-61400-1 exaggerates the measured wind profile, while the assumption in the GL and IEC-61400-3 seems to reproduce the right trend as for the measured mean wind profile it looks like a conservative assumption. The strong dependence of the profile on the atmospheric stability however is neglected. When T water is less than T sea, the measured profile exaggerates the profile of the GL and IEC-61400-3 drastically. It must be mentioned that these observations are preliminary and must be assured by a more detailed analysis during the OWID project. 100 80 FINO1-Profile: all data 2004 FINO1-Profile: T sea <T air height z (m) 60 40 IEC-1, alpha = 0.2 20 GL offshore / IEC-3, alpha = 0.14 0 5 6 7 8 9 10 11 12 13 14 15 wind speed v (m/s) Fig. 7: Measured wind profiles at the FINO1-platform in comparison with the IEC-61400-1 and 3 and the GL-Offshore Guideline

Normal Turbulence Model Turbulence intensity is one important parameter for the definition of the external load assumptions, normal turbulence is described within the Normal Wind Turbulence Model (NTM). It defines a monotone decline of the turbulence intensity with increasing wind speed. In the GL-guideline and IEC 61400-1 Ed. 2 it is defined as σ 1 = I 15 (15m/s+aV hub )/(a+1) (2) with the class parameters a and I15. This leads to the functional behaviour as shown in figure 8 for the turbulence classes A and C. The turbulence definition in the IEC-61400-1 Ed. 3 is analytically different but reveals similar values for the considered cases. Again measured turbulence intensities at the FINO1 platform have been plotted in Figure 7 for comparison. The values have been collected during January, July and December 2004 at the 100m level. 100.0% 90.0% 80.0% Jul 04 Jan 04 Dez 04 GL-A GL-C 70.0% Turbulenzintensität 60.0% 50.0% 40.0% 30.0% 20.0% 10.0% 0.0% 0 5 10 15 20 25 30 35 Windgeschwindigkeit (100m) Fig. 8: Turbulence intensities according to GL-Offshore Guideline and measured at the FINO1 platform within 2004 The intensities in the definition of turbulence class C as well as for turbulence class A seem to be an upper limit for the measured values. As a rough picture intensities are scattered around an average value of 5-8% for wind speeds above 8m/s, which is only half the value as given by class C. In contrast to the turbulence class model the lowest measured values can be found for wind speeds in between 8-10m/s and they are slightly in-

creasing for higher wind speeds. The reason for this behaviour hasn t been clarified up to now, one major effect will certainly stem from the influence of waves which produce a larger surface roughness for higher wind speeds. Again it must be stated that a further analysis of these effects is necessary and will be carried during the project. Extreme Wind Conditions Extreme wind conditions are given by are defined in order to represent extreme overall loads, extreme load changes and extreme inhomogeneities of the load distribution. They are standardised as Extreme wind speed model (EWM) Extreme Operation Gust (EOG) Extreme Direction Change(EDC) Extreme Coherent Gust (ECG) Extreme Coherent Gust incl. Direction Change (ECD) Extreme Wind Shear (EWS) A thorough analysis of the FINO1 data will be carried out in order to verification of extreme load conditions for the offshore case. In Table 3 examples for the data analysis and the connected extreme wind parameter is given. Assessment of FINO-Data regarding Loads Load Parameter to be assessed Data to be assessed from FINO-Data Link to standards 1 mean extreme overall load extrapolated maximum wind speed for a 10min interval EWM 2 mean vertical load shear on the rotor maximum value of vertikal wind speed and wind direction shear EWS 3 turbulence intensity as a function of height evaluation of the variance of the three wind components as a function of wave height, wave age and sea surface spectra NTM 4 short term extreme loads gust parameters depending on height EOG,EDC 5 time constant for the occurence of extreme loads derivation of the maximum wind speed increase during a gust EOG, EDC Table 3: Examples for an assessment of extreme wind conditions

Outlook The starting point for OWID is the measured undisturbed wind conditions at the FINO1 platform. For large offshore wind farms, wind conditions will be severely influenced by the wake fields of the wind turbines. This extra turbulence may dominate the design conditions for certain situations. Therefore in a next step wind park effects will be simulated by using Computational Fluid Dynamics (CFD) methods /3,4/. Both undisturbed and disturbed (park effect) wind conditions are taken as input for modelling a 5MW wind turbine and study the effect on the layout. Together with an evaluation of life time effects this will be the basis for proposals to improve standard wind conditions in the guidelines. References /1/ T. Neumann, et al.: Errichtung der ersten deutschen Offshore Wind Messplattform in der Nordsee. DEWI-Magazin Nr. 23, August 2003 /2/ V. Riedel, et al: Das erste Messjahr auf der FINO1-Plattform in der Nordsee Auswertung und Analyse des Windprofils und Abschätzung des statistischen Langzeitmittels. DEWI-Magazin Nr. 26, February 2004 /3/ Barthelmie, R. (editor): Proceedings of the ENDOW Workshop 'Offshore Wakes: measurements and modelling', Risoe National Laboratory, Roskilde, 2002. /4/ Riedel, V.: Adaptation and Verification of the Phoenics CFD-Software for Wind Resource Assessment in Complex Terrain, Diploma Thesis, German Wind Energy Institute/University of Oldenburg, 2003.