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

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Figure 5.16: Output locations for extreme loads Constrained wave period The baseline extreme loads were calculated with a constrained wave period of 10.87s. This is the lower bound of the range specified by the IEC standard [69], which is generally considered to be conservative. In order to test the influence of the wave period on the extreme jacket loading, three additional cases were considered in addition to the baseline case with wave periods varying from 9.3s to 14.0s. Normalised axial force 1.2 1.15 1.1 1.05 1 0.95 0.9 0.85 0.8 9.3s 10.87s 12.44s 14.0s Mbr 1 End 1 Mbr 2 End 1 Mbr 3 End 1 Mbr 4 End 1 Figure 5.17: Normalised extreme loads at pile head, with variation in constrained wave period Figure 5.17 presents normalized extreme loads at the pile heads for the four different cases. The results confirm that shorter wave periods result in higher extreme loads, with load increases of up to 10% resulting from a reduction in wave period to 9.3s. This is an important result, because it enables designers to perform preliminary extreme load calculations at a single wave period with greater confidence that they 68
are capturing the worst case loading. This significantly reduces the number of simulations required in the preliminary extreme load analysis. It is important to note, however, that for detailed design using the most conservative load will not necessarily result in the most cost effective support structures. Rather, the load with a return period of 50 years should be used. This implies that a statistical approach should be used as a basis for the rules in the standard. Tide height The baseline extreme loads were calculated with the tide height at HSWL, equal to +3.29m. This is the upper bound of the range specified by the IEC standard [69], which is generally considered to be conservative. In order to test the influence of the tide height on the extreme jacket loading, three additional cases were considered in addition to the baseline case with tide heights varying from LSWL to HSWL+2.13m. Normalised axial force 1.1 1.05 1 0.95 0.9 0.85 0.8 LSWL HAT HSWL HSWL+2.13m Mbr 1 End 1 Mbr 2 End 1 Mbr 3 End 1 Mbr 4 End 1 Figure 5.18: Normalised extreme loads at pile head, with variation in tide height Figure 5.18 presents normalized extreme loads at the pile head for the four different cases. The results confirm that higher water levels result in higher extreme loads, although the load increases are smaller in magnitude than when the wave period is varied. Again, this is an important result because it enables designers to perform extreme load calculations at a single water level with greater confidence that they are capturing the worst case loading. 5.4.3 Summary and key findings The fatigue loading on the structure is found to be dominated by the wind, with a relatively low contribution from the hydrodynamics. This is reflected in the small changes in DEL when marine parameters are varied (tide height, wind-wave misalignment) compared to the large changes in DEL when wind parameters are varied (wind class). The small hydrodynamic influence is also shown in the fact that a decrease in availability leads to a reduction in loading. The parameter which has the most effect on fatigue loading is the structural natural frequency. This demonstrates the importance of placing the natural frequency in the right range when designing a jacket support structure. The parameter which has the most effect on the extreme loading on the structure is the wave period of the 50 year maximum wave. Conservative load results are given when this parameter is set to the lower bound of the range given in the standard. 69
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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
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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 66 and 67: Figure 5.14 presents normalized DEL
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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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