Final report for WP4.3: Enhancement of design methods ... - Upwind
Final report for WP4.3: Enhancement of design methods ... - Upwind
Final report for WP4.3: Enhancement of design methods ... - Upwind
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Figure 5.13 presents normalized DELs at the chosen output locations on the jacket <strong>for</strong> the four different<br />
cases. The results show that the fatigue loads decrease by up to 20% when the wind speed distribution is<br />
reduced to Class III. This scale <strong>of</strong> reduction is seen across the whole structure. These results demonstrate<br />
that one <strong>of</strong> the main drivers <strong>for</strong> jacket fatigue loads is the wind loading, and that the local wind speed<br />
distribution is an important factor in jacket support structure <strong>design</strong>.<br />
Figure 5.13: Normalised DELs at pile head, with variation in wind class<br />
Structural natural frequency<br />
The baseline fatigue loads were calculated with a structural natural frequency <strong>for</strong> the first tower mode<br />
equal to 0.29Hz. This is equivalent to 1.44P, where 1P is the rotational frequency at rated speed<br />
(12.1rpm). In order to test the influence <strong>of</strong> natural frequency on jacket structures, the stiffness <strong>of</strong> the support<br />
structure was artificially <strong>for</strong>ced by changing the Young’s modulus <strong>of</strong> steel. Five cases were considered<br />
in addition to the baseline case, shown in Table 5.29.<br />
Table 5.29: Structural natural frequencies considered<br />
Frequency<br />
(multiple <strong>of</strong> 1P)<br />
Frequency (Hz) E_steel (N/m2)<br />
1.09 0.219 1.1E+11<br />
1.18 0.237 1.3E+11<br />
1.26 0.254 1.5E+11<br />
1.35 0.273 1.8E+11<br />
1.44 0.291 2.1E+11<br />
1.53 0.309 2.4E+11<br />
65
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