Page 449 - Marine Structural Design

Chapter 23Basics of structural Reliability 425

First excursion from safe
domain to failure domain

\

LSF: g(x(t))=xl -x2(t)

Time t

Figure 23.5 Time Variant Reliability

P/ (t) = 1 - P[N(~) o n g(x(o)) > 01 (23.31)
=

(23.32)
where, N(t) is the number of out-crossing in the interval [O,t]. Out crossings are the excursions
from safe domain to failure domain.
For marine structural analysis, stochastic load processes are often replaced by time-invariant
random variables to represent the lifetime loads. This also applies to cases of simultaneous
random loads. The combined extreme load needs to be determined appropriately since the
respective maximum values of different load processes do not necessarily occur
simultaneously. This depends upon the application of methods of load combination as
described in the above subsection.

23.8 Reliability Updating
Reliability methodology can be used as a tool to reassess structural integrity. Such
reassessment is required, e.g. when the inspection results are available or design conditions are
changed. New measures of structural reliability are achieved based on new information. The
information can be grouped in two classes
Sampling of quantities
Observations
To demonstrate the reliability updating, a component with load S and strength R is taken as an
example. The failure probability is
m
PI = P[R - S I = (.)A (sb (23.33)
01
-m