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

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From these results it can be clearly seen that the loads on the jacket are dominated by the wind. This is due to the small member diameters in the jacket at the water level, which results in a relatively low contribution from the hydrodynamics. At the pile head the contribution from the waves is proportionally greater, as expected, but even at this depth the relative contribution from the waves is still small. The DEL histograms also show the effect of aerodynamic damping. The damage equivalent loading from combined wind and waves is lower than the sum of the damage calculated from the wind and waves separately. This is due to the damping of the wave-induced motion resulting from the rotor thrust. Wind-wave misalignment The baseline fatigue loads were calculated with codirectional wind and waves. In order to test the influence of wind-wave misalignment on jacket structures, three additional cases were considered with the wind-wave misalignment varied. The waves were considered to approach the structure at six different angles from the wind, from -30° to 120° in 30° intervals. A different proportion of time was then assigned to each wind-wave misalignment angle for the three cases. Case 3 models a uniform distribution between all six misalignment angles, and Cases 1 and 2 are intermediate weightings. A graphical display of the cases considered is shown in Figure 5.9. Proportion of time spent at each misalignment [-] 1.2 1 0.8 0.6 0.4 0.2 0 Codirectional Case 1 Case 2 Case 3 -30 0 30 60 90 120 Wind/w ave misalignment [deg] Figure 5.9: Misalignment cases considered Figure 5.10 presents normalized DELs at the chosen output locations on the jacket for the four different cases. At the pile head the damage equivalent load increases by up to 2.5% when misalignment is taken into account. This is due to the additional forcing from the waves approaching from a different angle. However the DEL also increases by nearly 2% at the upper joint, which is located 15m above the water level. This may be due to the fact that jackets are relatively soft in the torsional mode, which can be excited by large misalignments. The reduced aerodynamic damping may also have an effect. 62
Figure 5.10: Normalised DELs with variation in wind-wave misalignment Availability The baseline fatigue loads were calculated assuming 100% availability. For monopile support structures it has been shown that in some cases, depending on site conditions and structure diameter, a lower availability can lead to design driving loads due to the loss of aerodynamic damping [67]. In order to test the influence of availability on jacket structures, four additional cases were considered with availability varied from 95% to 80% in 5% intervals. The reduction in availability was calculated by replacing a proportion of the DLC 1.2 simulations (power production) with DLC 7.2 simulations (idling with fault). Figure 5.11 presents normalized DELs at the chosen output locations on the jacket for the five different cases. It can clearly be seen that a reduction in availability leads to a reduction in fatigue loading across the whole structure. This is because the largest loading contribution comes from the wind, so it is more damaging for the machine to be operating. 63
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Project UpWind Contract No.: 019945
Page 3 and 4:
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
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 58 and 59: Guideline, DLC 1.5). Furthermore, t
Page 60 and 61: Table 5.28: Output locations for da
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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