Final report for WP4.3: Enhancement of design methods ... - Upwind

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Guideline, DLC 1.5). Furthermore, the rotor start positions shall vary from 0 deg to 90 deg (in 30 deg steps). Fatigue Load cases An extreme load analysis shall be carried out for the fatigue load cases. 5.4 Design load case parameter analysis for jacket structure The design of offshore wind turbine support structures is strongly driven by parameters relating to the external conditions at the site at which the turbine is to be located, as well as the properties of the structure and turbine itself. For the purposes of offshore wind turbine support structure design it is important to understand the relative importance of the various external conditions applied to the structure in the design process. These external conditions include the following: • Wind speed • Wind direction • Wave height • Wave period • Wave direction • Tidal levels • Current velocity and profile • Current direction • Turbulence intensity If all possible combinations of these factors were accounted for in the design of the structure, the number of simulations required would be huge and exceed practical limits for the designer. In order to achieve an economic design it is therefore important to know which of these factors are the most significant in determining the loading on the structure, so that the amount of computational time and effort can be minimised whilst maintaining accuracy of results. The effects of changing the above parameters on monopile support structures are reasonably well known. However, as suitable shallow water sites in European waters become more limited jacket support structures are increasingly being used as a preferred solution for offshore wind turbines. Therefore, this section uses the UpWind reference jacket support structure [54] to test the relative influence of a number of key design load case parameters affecting offshore wind turbine jacket support structure design. General considerations In order to account fully for the dynamics of an offshore wind turbine the combined wind and wave loading on the whole structure must be modelled in a non-linear time-domain simulation. This ensures that the aerodynamic damping of wave-induced motion is properly captured, which can be an important effect. The importance of integrated analysis for jacket structures has also been shown in [52], in which a significant interaction between rotor rotation and local jacket brace vibration was demonstrated. The GH Bladed software tool [9] is used to perform integrated load calculations and analysis. The UpWind 5MW reference wind turbine [52] is used for the analysis, mounted on the UpWind reference jacket support structure [54] shown in Figure 5.5. This structure was optimized using the UpWind 50m design basis [55], so the external conditions from this document are used as a baseline. Local joint flexibilities are not included in the model, as it has been shown that for jacket structures modelling flexibility in the joints does not typically affect the magnitude of forces and moments [66]. A full description of the geometry, mass and stiffness of the jacket support structure can be found in [54]. 58
Figure 5.5: Jacket support structure implemented in GH Bladed 5.4.1 Fatigue load case parameter study The fatigue load cases investigated are DLC 1.2 and DLC 6.4, which cover the full range of normal operation and idling conditions experienced by the wind turbine over its lifetime. DLC 7.2 is also considered for the availability study. The wind speeds and sea state parameters used for the simulations can be found in [55]. Irregular waves are modelled using a Jonswap spectrum with a peakedness parameter (gamma) equal to 1. Firstly a baseline fatigue load set was performed. Subsequent load sets were run with the following parameters varied individually, to determine the effect on the fatigue loading: • Wind/wave misalignment • Availability • Wind class • Structural natural frequency • Tide height In this section results are presented in terms of damage equivalent loads (DELs), with a specified S-N slope and frequency. Usually for welded steel details a bi-linear S-N curve is used with inverse slopes m1=3 and m2=5. If the fatigue load is wind dominated (i.e. more stress ranges with high frequency and low amplitude) then m2=5 is most relevant. If wave load is dominating (i.e. more stress ranges with low frequency and high amplitude) then m1=3 is most relevant. In the case of jacket structures the wind load is normally dominating; however for the purposes of this study it is important that the wave loads are still properly represented. A single m value is required for the below comparisons, so m=4 is used for all DELs as a compromise. A reference frequency of 0.0158Hz is used, equivalent to 1e7 cycles in 20 years. Lifetime-weighted DELs are derived using a rainflow cycle counting algorithm with application of Miner’s rule, based on the appropriate annual wind speed distribution. For jacket structures joints are generally the weakest point in structure due to concentration of stresses at the welds. During the optimization of the structure the lowest fatigue lives were found to be in the upper joint at the top level of bracing [54]. Other loads on the jacket which give a significant contribution to the fatigue damage are the tower base overturning moment and the pile head axial forces. For the purposes of this section, therefore, damage equivalent loads are reported at the pile head, the upper joint and the tower base. A list of output locations and load components is given in Table 5.28. The output locations on the Bladed model are shown in Figure 5.6. 59
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Project UpWind Contract No.: 019945
Page 3 and 4:
Acknowledgement The presented work
Page 5 and 6:
Summary The overall aim of the UpWi
Page 7: Table of Contents Acknowledgement..
Page 10 and 11: Task 4.1 Integration of support str
Page 12 and 13: Sequential coupling The sequential
Page 14 and 15: analysis. In the new multibody code
Page 16 and 17: Figure 2.4: 2nd global bending mode
Page 18 and 19: 3.2 Comparison of deterministic loa
Page 20 and 21: Figure 3.5: Time series of axial fo
Page 22 and 23: • Time domain simulation in ADCoS
Page 24 and 25: • A supplementary load vector for
Page 26 and 27: The following results are found: Fi
Page 28 and 29: Figure 4.5: Output positions in the
Page 30 and 31: It is directly visible that there i
Page 32 and 33: ated eigenfrequency is shifted into
Page 34 and 35: Water levels For the calculation of
Page 36 and 37: guidelines for the inclusion of cli
Page 38 and 39: damaging if the frequency of the ex
Page 40 and 41: uncertain parameters carefully. Thr
Page 42 and 43: Table 5.3 shows annual reliability
Page 44 and 45: Steel substructure for offshore win
Page 46 and 47: Table 5.16 shows the required FDF v
Page 48 and 49: A Fracture Mechanical (FM) modellin
Page 50 and 51: Figure 5.4: Annual reliability indi
Page 52 and 53: Further, the effect of possible ins
Page 54 and 55: Rainflow evaluation The load cases
Page 56 and 57: For DLC 7.2 it has to be considered
Page 60 and 61: Table 5.28: Output locations for da
Page 62 and 63: From these results it can be clearl
Page 64 and 65: Figure 5.11: Normalised DELs with v
Page 66 and 67: Figure 5.14 presents normalized DEL
Page 68 and 69: Figure 5.16: Output locations for e
Page 70 and 71: UPWIND WP4: Offshore Support Struct
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UPWIND WP4: Offshore Support Struct
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Final report for WP4.3: Enhancement of design methods ... - Upwind

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