Risk-Based Inspection Technique - Offshore Structures: Design, Construction and Maintenance - page 593

TABLE 8.23 Seismic Load

ISO Seismic Zone Score

≥ ≤

0 2 0

3 3 4

4 4 7

5 5 10

2 = 0.2 g, etc.). Typical offshore platforms that are designed for wave only can
normally withstand ground accelerations of 0.15 to 0.2, provided that they were
initially designed for a reasonable wave height and have good joint detailing.
Ground accelerations higher than this start causing problems, with an ISO
Zone 4 or larger requiring specific earthquake design features for ductility (special
kl/r values, extra-heavy cans, redundant framing schemes, etc.).
Obviously, it is important to define the earthquake zone. Table 8.23 pre-
sents scores between 0 and 10 for seismic load. Higher scores are used for
platforms existing in higher earthquake zones to penalize them for design
inadequacy. In an earthquake zone, seismic load is a major design considera-
tion; therefore, the overall weight in the total score is 8.
This factor does not have any interdependence with other factors or parameters
in the likelihood category. It is formulatedwith the earthquake zone and earthquake
design (yes or no) parameters from the design category of the data sheets.

Likelihood Categories
Most factors provide a score between 0 and 10. Factors like bracing-legs and
year-load location have values ranging from 1 to 10, and scores for appurte-
nances range from 0 to 12. A value of 0 indicates that the factor has the least
effect on increasing the probability of failure. A value of 10 indicates that the
factor has a maximum detrimental effect on the likelihood of failure.
The weights indicate how strongly the effect assessed by each factor affects
the overall likelihood of failure of the platform. The total score is calculated by
the formula iw i s i , where w i is the weight of the i factor and s i is the score. This

score can mathematically range from 15 to 2255, but for practical purposes
scores range from about 40 to 350.
The total weighted scores are converted to a likelihood category according
to the ranges in Table 8.24 . These ranges have been developed from the ranges
presented in the DNV paper.
After scores for all the platforms in the fleet have been obtained, the
cumulative percentage can be calculated and, using statistical data, the

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