Environmental Engineering Reference
In-Depth Information
the U.S. offshore wind industry. The following are examples of differences
between the IEC and API standards:
IEC uses a 50-year return period for the definition of extreme envi-
ronmental design conditions, while API RP 2A uses a 100-year return
period for the definition of design conditions for high-consequence
platforms.
The probability of exceedance of load levels (or, equivalently, the return
period of the wind-wave-current loading), for example at a 50- or
100-year return level, constitutes only one element determining the fail-
ure probability, or the probability of acceptable performance, of a facil-
ity. Equally important are the inherent safety factors accounting for
knowledge uncertainties (due to incomplete or otherwise limited infor-
mation concerning a phenomenon) and material factors, load combi-
nation requirements, parameters inherent in interaction equations, and
so on. These aspects are often disregarded in risk discussions but can
affect failure probabilities more than could a factor of two or three in
the return period of the loading. Therefore, a careful assessment is
needed to determine the overall failure probability in either or both of
the standards.
The definitions of DLCs are different. IEC requires the structure to be
verified for normal and abnormal conditions together with specific
load cases in close association with the wind turbine's operational sta-
tus. API requires the structure to be verified for operational conditions,
normally a 1-year storm, and extreme conditions, which are defined
primarily by using environmental conditions.
API RP 2A prescribes three levels of design based on consequence.
These levels are characterized by decreasing loads for decreasing con-
sequence. In contrast, IEC keeps the load level constant while adjust-
ing component safety factors on the basis of the consequence of that
component failing.
API RP 2A provides a basis for the design of offshore structures sub-
ject to wave, wind, current, and earthquake loading conditions in
addition to loads from drilling, production, and ongoing personnel
activities. API RP 2A does not address the scope and range of all con-
ditions relating to the design of wind turbine support structures such
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