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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.