Risk-Based Inspection Technique - Offshore Structures: Design, Construction and Maintenance - page 573

The investigators arrived at the matrix by identifying the following:

If the parameters were related to each other, that is, the first factor correlated
to the second parameter or not. For example, the design year of the platform
implies that certain factors of safety and certain detailing practices were fol-
lowed in the platform

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s design, so, in this case, the year of the platform
design correlates with the design practice in that year so that it will be cor-
related with the design load, as described in Chapter 7 .

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The weight of the correlation between two factors. For example, the number
of legs and the bracing system together strongly affect redundancy and pos-
sibility of damage to the platform. In contrast, the effect of marine growth
on the loads is considered small.

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For various reasons, not all strong interactions were covered. In some
cases, when the interaction was reconsidered during construction, it was
decided that it was either too complex to describe adequately or the interac-
tion was not as strong as originally believed. In other cases, such as in the
case of corrosion, the effect of locality, which is captured in the calculation,
was determined to be more important than the interaction with the type of
structural system, and as a result it was omitted. Furthermore, two other para-
meters were already bound to the bracing systems and leg factors; tying on a
third would make this factor overpower most of the other factors that affect
likelihood.
In the case of the interaction between location and grouted piles, where the
initial assessment ranked it as low, a factor was developed for a reason other
than the physical interaction of piles with the local load environment. Instead,
the factor for grouted piles included location to reflect design, detailing and
construction practices that would lessen the impact of retrofitting a platform
by grouting its piles.

Likelihood Calculation for Strength
The definition of the parameters mainly depends on the characteristics of the
fleet of fixed platforms to be maintained, as some factors exist in some loca-
tions but don

t exist in other locations. The discussion of the factors here is
just an example of the effect of each factor, which should be tailored to
each location and region world-wide. This is done by collecting the data for
the whole structure and then a comprehensive study is required to evaluate
the role of each factor. For example, one owner may have an old platform con-
structed in the year 1995, while another owner may have a platform from 1960,
so the weight and the score will be different. The numbers in Tables 8.1 and 8.2
are just a guideline and cannot be mentioned in any standard. The normal
procedure for qualitative risk assessment is that a team, consisting of the
owner, the operator and the engineering group with expertise, hold a meeting

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