Risk-Informed Approaches to Safety Regulation - Structural Integrity of Offshore Wind Turbines - page 141

influence, for example, estimates of wind effects associated with a 50- or
100-year MRI. For typical building occupancies, ASCE Standard 7-10
specifies a 700-year MRI wind speed. Similar MRI estimates are needed for
wave and current effects or for combined wind, wave, and current effects.
Note that the MRI is insufficient to establish the structural reliability. The
associated load factor also plays a key role; for example, the probability of
exceedance of some load level, 1.6 W, with W determined on the basis of a
50-year MRI wind speed, is about the same as the probability of exceeding
1.0 W when W is defined on the basis of a 700-year wind speed. This is also
the reason why the IEC-based offshore wind turbine design procedure,
which begins with a 50-year wind speed basis and applies load factors of
1.25 or 1.35 when verifying ultimate limit states, might yield the same reli-
ability as the use of an alternative factored load that begins with a 100-year
wind speed (as in API RP 2A) and applies a load factor of 1.0.
Whereas a typical MRI for an offshore oil and gas platform design is
100 years, a 50-year MRI is commonly used for offshore wind turbines
in Europe. Although the combination of the MRI and an associated load
factor can lead to similar reliability levels with either the 50- or the 100-year
MRI, the 50-year MRI used for offshore wind turbines in Europe partly
reflects the thinking that consequences of a turbine failure typically do
not lead to loss of life or grave environmental effects (see Chapter 4). The
selection of MRI for the design-basis event of a facility is not sufficient to
determine the risk for that facility.
Finally, to account explicitly for economic consequences or the con-
sequences of an unreliable energy supply, approaches similar to those
presented briefly in this appendix may be used to establish appropriate
alternative design MRIs, rather than an approach based on engineering
judgment with regard to structural performance.
Time-Domain Methods
Computer-intensive time-domain methods similar to those recently
developed by Simiu and Miyata (2006) and Long et al. (2007) can allow
rigorous estimates of ( a ) combined load effects, with any mean recur-
rence interval, from Monte Carlo simulations of simultaneous time his-
tories of wind, wave, current, and storm surge effects; and ( b ) attendant
uncertainties in those estimates. Such methods will help to sharpen sig-
nificantly estimates of combined load effects used for allowable stress