Risk-Based Inspection Technique - Offshore Structures: Design, Construction and Maintenance - page 585

TABLE 8.15 Cathodic Protection

Criteria Score

Grade 1: < 10% consumed 0

Grade 2: < 50% consumed 3

Grade 3: > 50% consumed 7

Grade 4: 100% consumed 10

No data (assume worst) 10

platform failure, as there is a high probability of unpredicted local failure during
the gap between inspections. On the other hand, because underwater survey is a
more detailed inspection, so it is less likely that an undetected local failure or
deficiency in strength or an increased load will occur. A well-inspected platform
is defined as a platform that has been checked for flooded members and for
which several joints have been cleaned/inspected; it should be able to go longer
without an inspection update than a platform that was inspected with only a
swim-by.
When a platform is inspected, the data obtained are used to recompute the
risk and the platform is re-ranked. In this way, if two platforms have the same
likelihood of failure prior to the inspection of one, the inspected platform

'

s like-
lihood drops, as does its overall risk. The platform with lowered risk is then a
lower inspection priority while the uninspected platform becomes a higher
inspection priority.
The quality of the inspection also affects the ranking. Two platforms with the
same risk prior to being inspected — one with a Level II inspection and the other

with a Level III will have the same risk immediately after the inspection.

While not strictly true, it is assumed that both levels of inspection will be able to
determine the current condition of the platform with the same level of confidence.
However, the Level III or close visual inspection is assumed to be able to detect
impending problems with more confidence than the Level II. Four years after
the inspection, the score of the platform with the Level II inspection increases to
8 while the score for the Level III platform increases to 5, indicating the system
considers that the predictive capabilities of the Level III inspection are significantly
better than those of the Level II inspection.
Level I inspections, defined in API RP2A as above-water walk-around
inspections, have been excluded because they should be performed every
year for all platforms.
As shown in Table 8.16 , the input data will be for the year of last Level II,
III or IV inspection. The overall weight for the inspection history factor in the
total score is 5.

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