API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-33
The risk matrix is grouped into four areas: high risk, medium high risk, medium risk, and low risk. If an
exchanger has been identified as high risk prior to the turnaround, it would require a more rigorous
inspection than has been used on that bundle in the past. For example, if the bundle were determined to be
a HIGH risk on the risk matrix and past inspections for that bundle were usually effective, it is very likely that
a highly effective inspection would be required at the upcoming shutdown. The benefits of the different levels
of inspection are discussed in Section 6.8.
Inspection Planning Based on Risk Analysis
The inspection target date is the date at which the calculated risk using Equation (5.55) exceeds the risk
target, Risk . An inspection is required prior to the target date to maintain a risk level below the risk target.
tgt
The target date for the next inspection is calculated using the inspection adjusted Weibull parameters.
Use of Risk Target in Inspection Planning
The risk target is a function of the owner-user’s corporate philosophy for making risk decisions. Some
companies are more risk adverse than others, and this will have a direct impact on the inspection planning
results.
Equation (5.71) is used to calculate the target POF for a bundle as a function of the COF and using the
target risk:
P
tube
f , tgt
Risk
= (5.71)
C
tgt
tube
f
A target inspection date is calculated using Equation (5.56). The target date is the date when the bundle risk
reaches the target risk.
tube
A user defined P is used in place of the calculated
f , tgt
P
,
required for inspection planning.
tube
f tgt
if a lower risk or probability of bundle failure is
The target inspection time is calculated using Equation (5.72). The target time is the number of years from
the installation date when the bundle risk reaches the target risk.
( ) 1
tube
ln 1
,
β
tinsp = ηtgt ⋅ − ⎡
⎣ −P ⎤
f tgt ⎦
(5.72)
The target inspection date is calculated using Equation (5.73) using t insp
and the installation date. The target
date is the date when the bundle risk reaches the target risk.
Target Inspection Date = Bundle Installation Date + t insp
(5.73)
Bundle target characteristic life, η tgt
, is calculated using the P
,
shown in Equation (5.74).
t
plan
η
tgt
=
−ln ⎡
⎣1
−P
tube
f , tgt
⎤
⎦
1
β
Determine Inspection Recommendation
tube
f tgt
and the bundle age at the plan date as
(5.74)
Once a decision has been made to inspect per Equation (5.74), an economic decision can be made as to the
appropriate level of inspection with similar techniques as described in Section 4.9.1 by comparing the cost of
the various inspection techniques to the reduction in risk expected for the level of inspection. Note that no
tube tube
inspection is required if Pf , plan
≤ P .
f , tgt
The target Uncertainty, AU tgt
% is the level of uncertainty associated with an inspection required to remain
tube
below the P at the Plan Date from Equation (5.75).
f , tgt