A Risk-Informed Approach to Performance Assurance - Structural Integrity of Offshore Wind Turbines - page 74

control of facility performance, and to provide engineers with more
flexibility in designing with nontraditional systems and materials and
in achieving innovative design solutions. One common feature of most
recent proposals for PBE is their distinction among levels of performance
for different facility categories where life safety or economic consequences
of damage or failure differ. Current codes generally make such distinc-
tions by simply stipulating a higher design load, a step that may not lead
to better performance and indeed may be irrelevant for dealing with cer-
tain low-probability events where effective design requires other consid-
erations in addition to strength. The design objectives in PBE are often
displayed in a risk matrix such as that illustrated in Figure 4-2, in which
one axis describes severity of hazard (e.g., minor, moderate, severe) and
the second identifies frequency of occurrence. The severity of the incident
(consequence) can also be thought of in terms of performance objectives
(continued function, life safety, collapse prevention). PBE might require
that a critical facility remain functional under an extremely rare event (sus-
taining minor damage) and provide continued service without inter-
ruption under a rare event. Current prescriptive design codes for offshore

Severity of Incident (or Consequences)

Frequency
Occurrence
Likelihood

Serious
( 3 )

Major
( 4)

Catastrophic
(5 )

Incidenta l
(1 )

Mino r
( 2 )

F requen t
(5)

High Risk

O ccasiona l
( 4 )

Seldo m
(3 )

Remot e
( 2 )

Low Risk

U nlikel y
( 1 )

FIGURE 4-2 Example risk matrix driven by safety or environmental
consequences. (S OURCE : TRB 2008, Figure 2-5).