57774801 rbi methodology
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RISK BASED INSPECTIONMETHODOLOGY
INTRODUCTION
Risk Based Inspection (RBI) provides for the first time the
requisite information to prioritise inspection activities on
the basis of a risk benefit approach.
RBI examines each of the equipment items in the process
by calculating the risk of operation as the product of theLikelihood of Failure (LoF) and the Consequence of
Failure (CoF) if such a failure should occur. High risk
items can receive more attention, while low risk items can
receive a lesser, more appropriate level of inspection.
DNV AND RBI
DNV facilitated and supported the development of the
Risk Based Inspection methodology for refineries and
petrochemical plants for the American Petroleum Institute
(API), producing the Base Resource Document API 581and software. Similarly, DNV has become involved in a
Joint Industry Project (JIP) to develop an industryaccepted offshore RBI methodology
DNV has applied
the RBI
methodology on
numerous chemical
and petrochemical
facilities worldwide,
both onshore and
offshore.
INTRODUCTION TO RBIMETHODOLOGY
The RBI process is an integrated method of analysis to:
Identify the driving parameters for both likelihoodand consequence of failure (leaks or loss of
containment).
Risk rank all pieces of equipment included in thestudy using safety or business interruption as the risk
measure.
Provide an inspection plan guideline and detailedequipment inspection plans.
The basis of all RBI studies is:
Identification of the system to be analysed. Identification of the type of hazard involved. Separate calculation of the failure consequence and
likelihood.
Calculation of the risk.The Key Concepts of RBI
RISK = Likelihood of Failure XConsequence of
FailureGeneric
Failure
Frequency
Item Technical
Module
SubFactor
X
Age
Damage
type/rate
Inspectioneffectiveness
Damage area
Item repair
Other repairs
Injury
Business interruption
Environment clean up
SYSTEM DEFINITION
A quantitative RBI study, undertaken by DNV, comprises
a comprehensive scenario based approach. For eachequipment item it considers:
Equipment design data. The operating conditions (normal and upset) and fluid
properties.
The material of construction, the damage mechanismsand the severity of damage mechanism.
The unique equipment item inspection history.The methodology includes:
Pressure vessels, piping, heat exchangers, pumps,
compressors, furnaces, reactors and storage tanks.
LIKELIHOOD OF FAILURE (LOF)
The probability of each equipment item failure is based on
DNVs extensive Generic Failure Frequency (GFF)
database. A modification factor is then applied for each
specific damage mechanism. This factor is based on the
strength of the component, the operating load, the active
damage mechanism(s) and the inspection
history/effectiveness. The methodology makes use of
probabilistic and statistical methods to allow for the
uncertainties in the parameters involved.
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DET NORSKE VERITAS
Typical damage mechanisms included:
Thinning (general and localised). External (thinning and cracking) Stress Corrosion Cracking High Temperature Hydrogen Attack (HTHA) Fatigue Brittle FractureTechnical supplements can be used to establish damage
mechanism rates /severities if actual data is not available.
CONSEQUENCE OF FAILURE (COF)
The failure of pressure containing equipment and
subsequent release of hazardous materials can lead to
many undesirable effects. Four basic risk categories are
considered:
Flammable (jet fire, pool fire, explosion) Toxic Environmental Business InterruptionFour standard release sizes are considered for each
equipment item. Consequence models have been
developed based on DNVs consequence software
PHAST. This is used to determine the discharge rate,
dispersion and consequence of each release. Consequenceis measured in terms of safety hazard zone, toxic impact,
major environmental damage or business interruption.
Consequence effect areas are calculated for each incident.
Event trees are then used to determine the probability ofeach incident outcome. The methodology can also account
for detection and mitigation measures (e.g. isolationvalves, fire monitors).
RISK ANALYSIS
Risk is the product of the likelihood of failure and the
consequence.
Risk = LoF x CoF
Within the RBI methodology the risk can be expressed in
terms of damage area or more frequently financial risk.
The risk ranking identifies those equipment items with thehighest risk and whether the risk is dominated by
likelihood of failure (a good candidate for inspection) or
consequence (a good candidate for mitigation devices) or
both. The figure below shows ranking in the form of a risk
matrix.
5
4
3
2
1
High RiskMedium-High R isk Med. High Risk
Medium Risk Low Risk
Likelihood
Category
Consequence Category
RISK BASED INSPECTION PLANNING
Risk based inspection planning is a matter of acquiring
enough information about each equipment item by
inspection that the equipment replacement date can be
predicted. The risk based approach is unique in that it
allows a degree of flexibility as to the level of inspectionthat is appropriate to give the required knowledge and how
this knowledge is accumulated, for example - by one or
two major internal inspections or by more frequent, less
intrusive inspections. The figure below shows the use of
RBI tools to evaluate alternate inspection strategies.
0
1
10
100
1000
0 3 6 9 12 15 18 21 24 27 30
Year
RiskLevel
No
Inspections
High Level
Inspections
Low LevelInspections
"Average"
"Below Average"
"Above Average"
TOOLS FOR RBI
The fundamentals of Risk Based Inspection apply
universally to all types of petrochemical processes, either
onshore or offshore installations. There are however, some
differences on the specifics of the process and the type of
installation. DNV has developed proprietary software tools
for RBI. ORBITTM
Onshore is available for the onshorerefinery-, petrochemical- and chemical industries.ORBIT Offshore, Analysis of Topside Systems, has
been developed specifically for the offshore industry.