Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 511

1-Engineering drawings
2-Inspection records
￿ Damage
￿ Modification
￿ Degradation
3-Geotechnical data

Structure modeling
and foundation

Metocean data, Hm, Hs
and storm return period

Pushover analysis

Wave and current
load

Platform capacity

Limit state function

Load cov and
CDF

Capacity cov
and CDF

FORM

Probability of
failure

FIGURE 7.22 Procedure for calculation of probability of failure. CDF = cumulative distribution
function; cov = coefficient of variation; FORM = first-order reliability method.

7.9.4 Offshore Structure Acceptance Criteria
In order to establish the assessment acceptance criteria, it is necessary to estab-
lish the relationship between the probability of failure of the representative
structures and their reserve strength, defined in terms of RSR.
In the reliability analysis procedure described above, the structural capacity
of the platform, R, is assumed to be a random variable with a log-normal dis-
tribution. The mean value of R is estimated from an ultimate strength (or push-
over) analysis, as described. By arbitrarily moving the mean structural capacity
to represent a range around the calculated capacity, it is possible to estimate the
probability of failure of a range of designs of the same structural type. This
allows the relationship between RSR and probability of failure to be