Risk-Informed Approaches to Safety Regulation - Structural Integrity of Offshore Wind Turbines - page 140

for failures that are benign to 7 × 10 −7 /year for failures that are sudden and
lead to widespread damage or collapse. In seismic situations, the accept-
able probabilities are conditioned on the design-basis event; for ordinary
building structures, this conditional probability (given occurrence of the
design-basis event) is 10 percent for total or partial collapse.
ASCE Standard 43-05 Standard 43-05 (ASCE 2005) addresses seismic
design criteria for nuclear facilities. Like ASCE Standard 7-10, it adopts
a uniform risk approach to earthquake-resistant design rather than a
uniform hazard approach. Table 1-2 of this standard stipulates target
performance goals in terms of the annual probability of failure for facil-
ities requiring different levels of protection. For facilities requiring con-
finement of highly hazardous materials with high confidence, the target
probability is 10 −5 /year or less, and the structure must be designed to
remain essentially elastic under such conditions.
CRITICAL APPRAISAL OF EXISTING RISK-INFORMED
ANALYSIS AND DESIGN PRACTICES FOR APPLICATION
TO OFFSHORE WIND TURBINES
Component Versus System Reliability Analysis
Most codified reliability-based design for civil infrastructure has focused
on individual buildings, bridges, and other industrial facilities for which
the hazard can be identified at a point (e.g., Ellingwood 2007). A distin-
guishing and essential feature of risk-informed decision tools for wind tur-
bine farms in coastal and offshore environments is their ability to account
for the spatial correlation in the intensity of the hazard (such as from a hur-
ricane) over geographic scales on the order of tens of kilometers within the
region affected (Vickery and Twisdale 1995); multiple wind turbine units
experience correlated risks under such conditions. In addition, the
presence (or lack) of advanced warning systems and the effect on risk-
mitigation options should be considered (Lakats and Paté-Cornell 2004).
Design MRIs of Joint Wind Effects
MRIs of design wind effects for strength design have typically been spec-
ified with consideration for knowledge uncertainties. Such uncertainties