Final Report 4.1

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UPWIND 3. Requirements and levels of load mitigation In this Chapter, requirements for load mitigation are defined. This includes a definition of design ranges for offshore support structures and their dynamic behaviour. Based on these requirements, three different levels of load mitigation are introduced, which shall be further elaborated later on. 3.1 Design ranges for offshore support structures In the design of offshore support structures, the first eigenfrequency of the structure is an important factor to consider as it describes the dynamic behaviour of the offshore wind turbine. As for every dynamic system, if an excitation frequency gets close to this structural eigenfrequency, resonance occurs and the resulting response will be larger than in the quasistatic case. This leads to higher stresses in the support structure and, more importantly, to higher stress ranges, which is an unfavourable situation with respect to the fatigue life of the offshore wind turbine. Therefore it is important to ensure that the excitation frequencies with high energy levels do not coincide with the eigenfrequency of the support structure. In the offshore environment, wind turbines are excited by wind and waves. Here for waveinduced fatigue loading sea states with a high frequency of occurrence have the largest impact. These sea states are generally characterized by relatively short waves with significant wave heights of Hs around 1 m to 1.5 m and a zero-crossing period of Tz around 4 s to 5 s [17]. The excitations from the wind are in general connected to rotational frequency effects of the rotor. Due to the rotation of the rotor, aerodynamic loads are concentrated around the rotor frequency and multiples of the blade passing frequencies. Rotational-sampling effects like the 1P frequency are generated due to mass imbalances in the blades or 3P frequency effects generated due to tower shadow effects. Thus, the ratio between the rotor speed, or more precisely the rotor speed range, and the fundamental eigenfrequency f0 of the support structure is an important design driver for the support structure design since resonance frequencies must be avoided. In general, three design solutions exist depending on the ratio between the fundamental eigenfrequency f0 and either the rotor frequency 1P or the blade passing frequency 3P: � soft-soft, i.e. f0 < 1P � soft-stiff, i.e. 1P < f0 < 3P � stiff-stiff, i.e. 3P < f0 In practice, soft-stiff designs are most common. Sometimes soft-soft designs are used for tall towers, but the impact of the wave energy can become critical in several cases. Stiff-stiff designs are rare, as the necessary material for achieving such stiff structures imposes high costs. Offshore wind turbines nowadays operate with variable rotor speed, hence the frequency ranges depends on the rotational speed. This enables further design ranges: � Very soft, hardly realizable due to strength requirements and exposure to excessive dynamic wave excitation (unless a compliant design with an eigenfrequency below the significant wave excitation is chosen) � Soft-soft design in the resonance range of the rotor speed requires an exclusion window for stationary operation of the rotor speed, soft-soft designs are subject to quite significant wave excitation Page 30 of 146
UPWIND � Classical soft-stiff design range, proven to be suffering from significant wave excitation � Blade resonance range with excessive excitation from cyclic aerodynamic loading, design impossible without a large exclusion window of the rotor speed � Stiff-stiff, design is considered uneconomical due to the high consumption of material required for the stiffness Figure 3.1 illustrates these options applied for the Upwind 5 MW reference turbine (see Appendix A). At state of the art offshore wind farms, mostly the second and third design ranges shown in Figure 3.1 are found. The reason is that most of the structures are supported by monopiles. For such structures it is difficult to achieve the stiff-stiff region due to economical constraints. The very soft region is critical due to high wave loading. Therefore most of the structures are placed into the soft-stiff region, where the structures are out of any rotational-dependent resonance, economic in material consumption and where the wave impacts are lower. For future larger turbine types with 5 MW rated power and larger water depths, monopile structures in the softstiff design region are difficult to design, as certain limits in pile diameter and wall thicknesses are reached. Therefore the soft-soft design region, in the 1P rotor frequency range, could be an option. To avoid resonances, different operational control concepts like a rotational speed window can be used as described later in Chapter 4. 0 0 (1) (2) (3) (4) (5) Rotor frequency ± 10% Rotor freq. Blade passing frequency ± 10% Blade passing frequency 6,9 12 20,7 36 [rpm] 0,10 0,22 0,31 0,66 rated rotor speed (i.e. maximum stationary rotor speed) Figure 3.1: Design ranges for the fundamental eigenfrequency of the support structure of a variable-speed wind turbine at the example of the Upwind 5 MW design 3.2 Critical load effects for certain support structure types Depending on the type of the support structure, different loading events can be critical. An important difference can be found for bottom-mounted and floating structures, but also for single-piled and braced ones. The Section below deals with steel-type structures only and will point out certain aspects of some exemplary structures. For state of the art offshore wind farms, monopiles are by far the most widely used support structure types. Monopiles consist of a single tubular pipe that transfers the loads mainly laterally into the sea bed. This layout makes the structure relatively sensitive to the uncertainties of the soil conditions. On the other hand, monopiles might be applied in a range from soft to relatively stiff soil conditions. However, monopiles are not the best suited concept for very soft or very stiff soil conditions or when boulders occur in the sea bed. In the presence of bedrock, drilled and subsequently grouted monopiles can be applied, or a combination of drill and drive. [Hz] Page 31 of 146
Page 1 and 2: Project UpWind Contract No.: 019945
Page 3 and 4: Acknowledgement UPWIND The presente
Page 5 and 6: Summary UPWIND The objectives withi
Page 7 and 8: Table of Contents UPWIND Acknowledg
Page 9 and 10: 1. Introduction 1.1 The UpWind proj
Page 11 and 12: UPWIND As support structures and fo
Page 13 and 14: Load types Operational Aerodynamic
Page 15 and 16: UPWIND directional changes, is more
Page 17 and 18: UPWIND Another effect of such tides
Page 19 and 20: UPWIND Figure 2.3: Polar distributi
Page 21 and 22: UPWIND Figure 2.5: Relative change
Page 23 and 24: UPWIND Corrosion is another importa
Page 25 and 26: UPWIND 2.2.1 Aerodynamic damping Ae
Page 27 and 28: UPWIND same as in state 1, changes
Page 29: UPWIND 2.2.4 Soil damping For offsh
Page 33 and 34: UPWIND As a result of the large str
Page 35 and 36: UPWIND The next level of load mitig
Page 37 and 38: UPWIND One of the most obvious adva
Page 39 and 40: UPWIND structural stability of the
Page 41 and 42: UPWIND components. Moreover, the co
Page 43 and 44: 4.4 Park configuration UPWIND In ad
Page 45 and 46: UPWIND shown that such robust conce
Page 47 and 48: UPWIND eigenfrequency was well dist
Page 49 and 50: Figure 5.4: Turbine generator speed
Page 51 and 52: Figure 5.6: Concept of an extended
Page 53 and 54: UPWIND Figure 5.8: Non-lifetime wei
Page 55 and 56: UPWIND Figure 5.10: Relative change
Page 57 and 58: UPWIND In terms of the dynamic cont
Page 59 and 60: UPWIND industry, especially in civi
Page 61 and 62: ω ω d 0 � 1 �1 � μ� UPWI
Page 63 and 64: UPWIND called optimal adjustment. T
Page 65 and 66: Figure 5.19: Detail of simulation r
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Page 69 and 70: UPWIND 6. Load mitigation concept a
Page 71 and 72: Figure 6.3: Detail of simulation re
Page 73 and 74: UPWIND by keeping the power output
Page 75 and 76: UPWIND the transients are becoming
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Page 79 and 80: UPWIND tower side-to-side damping t
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UPWIND Figure 6.14: Non-lifetime we
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UPWIND In order to translate a coll
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UPWIND activated IPC. It shows that
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UPWIND In Table 6.4, the results ar
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df with � const du UPWIND If then
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Figure 6.22: Lower-toggle bracing g
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SetSpeed + SpeedError - MeasuredSpe
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7. Design methodologies UPWIND The
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UPWIND sufficient pile penetration
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UPWIND Global buckling check Under
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UPWIND iteration of the design cycl
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8. Design demonstration UPWIND In t
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V [m/s] Table 8.1: Lumped scatter d
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UPWIND 8.1.2 Reference turbine The
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UPWIND The support structure consis
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Elevation 82.76 m + MSL 77.76 m + M
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UPWIND For all simulations, lumped
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Table 8.6: Ultimate loads at mudlin
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UPWIND monopile and transition piec
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UPWIND of the larger misalignments
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UPWIND Figure 8.14: Relative change
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UPWIND 8.2.3 Design optimization an
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UPWIND as therefore the cost contri
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UPWIND downwind configuration and o
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UPWIND bottom-fixed offshore wind t
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Verlag, Hannover, Germany, 2009. UP
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Table A.3: Data of multi-member sup
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Operating conditions Idling DLC 6.4
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Operating conditions Power producti
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DLC2.3 - ULTIMATE Operating conditi
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DLC 6.2a - ULTIMATE UPWIND Operatin
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UPWIND Figure C.2: Ultimate utiliza
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UPWIND Figure C.4: Ultimate utiliza
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