Page 530 - Marine Structural Design

506 Part IVShuctural Reliability

It can be seen fiom the above figure that thickness measurements and renewal for the
components with POD of less than 80% should be carried out in order to guarantee the annual
reliability index over the lowest limit of safety level during the first 20 service years. They
may be done in Special Survey No.3 during the first 20 service years, but should be
implemented in Annual Survey if the FPSO is required to keep in service over 20 service years.
Figure 28.4 shows the time-variant reliability with the risk of C1 and L3 combination. From
this figure, it is found that the annual reliability index is always greater than the lower limit of
safety level and thickness measurement may not be necessary during the first 20 service
years, but the thickness measurement and then renewal for the components with POD of less
than 80% in Intermediate Survey should be carried out if the FPSO is required to keep in
service over 20 service years.
From the above example, we conclude that the inspection pIanning is dependent on the
consequence of failure (lower limit of safety level), corrosion rate, ship age and probability of
detection (POD). The requirements of inspection gradually more demanding with the increase
of the consequence of failure (lower limit of safety level), corrosion rate and ship age and with
the decrease of POD. The latter usually makes thickness gauging and judgement more difficult.

28.5 Risk Based ‘Optimum’ Inspection

This Sub-section is based on Xu et a1 (2001). Experience with in-service inspections of ship
and offshore structures have adequately demonstrated that there are two categories of
damages:
those could have been or were anticipated (natural, predictable)
0 those could not have been anticipated (human caused, unpredictable)
A substantial amount (if not a majority) of damages falls in the second category -
unpredictable and due to the ‘erroneous’ actions and inaction’s of people.
Quantitative inspection analyses (e.g. probability or risk based inspection methods and
programs) can help address the first category of defects by providing insights of when, where,
and how to inspect and repair. However, such an analysis cannot be relied upon to provide
information that addresses the second category of defects. Expert observation and deduction
(diagnostic) techniques must be used to address the second category of defects.
Such recognition techniques lead to the development of the ‘optimum’ inspection method (Xu
et al, 2001). The overall objective of the ‘optimum’ inspection method is to develop an
effective and efficient safety and quality control system in the life-cycle management of the
structural systems.
Inspection Performance
Inspection performance is influenced by the vessel, the inspector, and the environment.

The vessel factors can be divided into two categories: design factors and condition/
maintenance factors. Design factors, including structural layout, size, and coating, are fixed at
the initial design or through the redesign that may accompany repair. Conditiodmaintenance
factors reflect the change in a vessel as it ages, including the operation history and
characteristics of individual damagesldefects (crack, corrosion, bucking), its size, and its
location.