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

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Figure 5.14 presents normalized DELs at the chosen output locations on the jacket for the six different cases. The results show a significant variation in DEL when natural frequency is varied, in some cases up to 65%. The loads tend to increase when the natural frequency is either lower or higher than the original value, and the scale of the increase is much larger than for other parameters. These results show the importance of designing a support structure with a fundamental natural frequency in the right range. If the frequency is too low then the structure may experience resonance with the rotational frequency of the rotor. If the frequency is too high then the structure will be overdesigned and the high stiffnesses will lead to increases in member loading. Figure 5.14: Normalised DELs at pile head, with variation in natural frequency Tide height The baseline fatigue loads were calculated with the water level at mean sea level (MSL) of 50m. In order to test the influence of tide height on jacket structure loads, four additional cases were considered with water levels at LSWL, LAT, HAT and HSWL (values taken from [55]). The total water level variation between LSWL and HSWL is 5.66m. Figure 5.15 presents normalized DELs at the chosen output locations on the jacket for the five different cases. The results show that tide height has only a minimal effect on fatigue loading, with a maximum of 1% difference from the baseline. A study into tidal effects on monopile support structure fatigue loading [68] has shown that for fatigue load calculations using a water level of MSL + 10% of tidal range gives accurate damage equivalent loads. The mean difference from the baseline results across the calculated DELs is small but positive, which indicates that the above conclusion derived for monopiles is also appropriate for jacket structures. 66
Figure 5.15: Normalised DELs at pile head, with variation in tide height 5.4.2 Extreme load case parameter study For the design of the UpWind reference jacket support structure, a comprehensive set of extreme load cases was simulated to calculate the driving loads on the structure [54]. The worst case wind and wave combination was found to come from DLC 6.1 (idling during 50 year storm). Therefore DLC 6.1 is investigated in this analysis. The 50 year wind speeds and wave conditions used for this load case can be found in [55]. Irregular waves are modelled using a Jonswap spectrum with a peakedness parameter (gamma) equal to 3.3. A constrained wave is used in GH Bladed to model the 50 year individual wave height. This approach ensures that the irregularity of the background sea state and the nonlinearity of the extreme wave are both modelled in the simulation. First, a baseline extreme load set was performed with parameters as described above, with multiple wind seeds and wind-wave misalignment as required by the IEC 61400-3 offshore standard [69]. The driving load combinations were identified, and subsequent load sets performed with these load combinations. The IEC standard states that for storm load cases a range of wave periods and water levels must be considered [69]. This can lead to a huge number of simulations. The constrained wave period and tide height parameters are varied individually, to determine the effect on the extreme loading: Results in this section are presented in terms of the absolute maximum load from the Bladed simulation. The results are normalized so safety factors are not required. The extreme loading on a jacket structure is generally driven by the axial forces in the pile heads, so for the purposes of this section extreme loads are reported at the pile head only. The output locations on the Bladed model are shown in Figure 5.16. 67
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Project UpWind Contract No.: 019945
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Acknowledgement The presented work
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Summary The overall aim of the UpWi
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Table of Contents Acknowledgement..
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Task 4.1 Integration of support str
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Sequential coupling The sequential
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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 58 and 59: Guideline, DLC 1.5). Furthermore, t
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 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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