rbi methodology

Donegani Anticorrosione S.r.l 1

RBI Methodology

API 580 – 581

Risk Based Inspection


Introduction

Although the term ‘RBI’ has been adopted relatively recently, the practice of planning plant inspection based on the risk of failure is not new. Most operators base the scope and depth of their inspection plans on the rate of in-service damage and the consequence of failure, within a traditional time-based inspection programme. (Same operators can rightly claim that their plant inspections have always been risk-based).

The term ‘RBI’ refers to a structured, team-based assessment process with the aim of targeting inspection by considering the Risk of Failure (RoF), which is a combination of the Likelihood of Failure (LoF) and Consequence of Failure (CoF).


RBI – Risk Based Inspection

• Is based on the norms API 581 & RP 580• Developed by API (American Petroleum

Institute) in collaboration with:

- DOW

- SHELL

- FINA

- TEXACO

- EXXON

- ………….


Objective of RBI Methodology

• Increase safety

• Increase the productivity of inspection

• Decrease the losses of production

• Reduce the direct costs of inspection

• Change the frequencies of inspections in base to the risk

• Prevent catastrophic damages


RISK

Definition of the Risk:

Risk (RoF) = Likelihood of Failure (LoF) x Consequence of Failure (CoF)


Risk Based Inspection

Consent of:

• Define the critical state for every single ITEM

• Create a priority for the Inspections basing on the critical state

• Focus the attention on the expected damages


RBI Methodology

RBI Methodology developed by different companies, varied in the details, but it is founded however on fundamental step:

• Background to RBI• Assessment Personnel• Damage Mechanism Assessment• Risk Assessment• Inspection Planning and Risk Mitigation• Updating RBI Assessment


RBI Methodology Step 1.Background to RBI

Step 2.Assessment Personnel

Step 3.Damage Mechanism

Assessment

Step 4.1.Likelihood of failure-LoF

Step 4.2.Consequence of failure-

CoF

Step 5.

Inspection Planning

Step 6.Updating RBI Assessment

Step 4.3Risk Assessment


Background to RBI

RBI study is designed around the systematic, inductive assessment process of identifying historical, active or potential damage mechanism in each equipment. The process is a ‘bottom up’ procedure where one damage mechanism id considered at a time. Once the feasible damage mechanisms have been identified, their probabilities (likelihood) of leading to component failure are determined and their effects on the components are recorder and analysed to yield a calculated failure consequence.


Assessment Personnel

All RBI systems have the following common

elements:

• Assessment preparation• Information Gathering• Damage Mechanism Review• Risk Assessment• Inspection Plans• Inspection Review.


Assessment preparation

Since RBI is a team-based approach, the first step in the introduction of an RBI programme is to identify and assemble a competent Study Team. The RBI team should include staff with knowledge of plant operation, process hazards, risk assessment and inspection techniques.


Typical RBI team

• Inspection engineer(s);

• Material and corrosion engineer(s);

• Maintenance engineer(s);

• Process engineer(s);

• Operations engineer(s);

• Safety engineer(s);

• Team leader, typically a RBI specialist(s).


Information Gathering

Are necessary the following information:• Operational manual• P&ID• Sketches of project of the equipment• Pipe class• Property of the fluids• Design and operating conditions• Special instructions and History inspection• ……………


Damage Mechanism Assessment

The Study Team will need to review the historical and generic date relevant to each item of equipment. This will include consideration of previous inspection reports (if available) to determine the known or expected damage mechanisms.

The review should consider damage mechanisms which may lead to failure, their expected rates of deterioration and the general location where damage may arise.

Wherever quantitative rates of damage are not available, an element of expert judgement will need to be exercised by the Study Team.


Risk Assessment

The failure likelihood and the failure consequence are calculated on the basis of quantitative Likelihood Factors (LF) and Consequence Factors (CF).


Likelihood of Failure (LoF)

The answers to the LF questions are assigned a series of values and associated weighting and the combination of all these determines the Likelihood of Failure (LoF), derived for each equipment in turn.

The LoF formula is specifically of the following form:

LoF = LF2 Σ (LFi)

LF2 = Failure likelihood (within a given period of time)

LFi = represents several factors that are considered to contribute to the overall Likelihood of Failure.


LFi Factors

• LF1: Current condition

• LF2: Failure likelihood

• LF3: Inspection Effectiveness

• LF4: Shutdown Frequency

• LF5: Operational Stability

• LF6: Operation in Relation to Design

• LF7: Recurring Repairs


Consequence of Failure (CoF)

The answers to the Consequence Factors (CF) questions are assigned a series of values and associated weightings and the combination of these results is the Consequence of Failure (CoF), derived for each item in turn.

The CoF formula is specifically of the following form:

CoF = (FM x CF1 x CF2) + (CF3 x CF6) +

CF4 + (CF5 x CF7) + CF8 + CF9


FM Factor

In addition to other consequence factors the consequence calculation also consider the failure mode of an assigned damage mechanism. Each damage mechanism knowledge base is assigned one of three failure modes, i.e. ‘Leakage’, ‘Leakage or Rupture’, ‘Rupture’.


CFi Factors

• CF1: Product Loss

• CF2: Pressure Factor

• CF3: Explosion/Fire Damage Potential

• CF4: Toxicity

• CF5: Effect of Failure on Production

• CF6: Location of Component

• CF7: Time Taken to Rectify Failure

• CF8: Threat Personnel/Environment

• CF9: Engineered Safety Systems


Weight of Factors


Risk Matrix

The resultant Likelihood of Failure (LoF) is plotted on the vertical axis, and the Consequence of Failure (CoF) is plotted on the horizontal axis, as an assessment point on a dimensionless five-by-five Risk Plot. The Lof axis is grouped into five relative Likelihood Categories, ranging from 1 to 5 (approximately 10-2/yr to 10-7/yr), and the CoF axis is grouped into five relative Consequence Categories, ranging from A to E. In this way, the Risk of Failure (RoF) is considered to increase with increasing LoF and CoF.


Risk Matrix

Like

lihoo

d

1

2

3

4

5

B C D EA

Consequence

Risk


Inspection Planning

After the risk assessment, the Study Team develops action plan based on all of expected damage mechanisms. The inspection plane consider the inspection interval and the scope of inspection.

The proposed inspection plane are designed to provide an assurance that the ‘residual risk’ of the equipment is in accordance with the ‘ALARP’ principle, i.e. the aim of assessing and managing equipment is to reduce the RoF to As Low As is Reasonably Practicable (‘ALARP’).


Risk Mitigation

The RBI methodology systematically leads the Study Team through the process of risk mitigation, by considering several mitigation action. The impact of each mitigation action can be evaluated, for subsequent collective appraisal of the benefits of the proposed action, using the ALARP principle. An appropriate strategy to manage the integrity of each equipment is therefore developed.


Updating RBI AssessmentRBI Study can be used to justify extending the periods between inspections. The RBI process relies on a feedback loop to update the inspection plan, such that it is relevant to the actual condition of the plan. The feedback stage provides continuing confidence that damage dose not exceed expectations and ensures that plan inspections take account of operational experience. Once an inspection plan has been developed during a session, future inspection of the equipment should be carried out to confirm that damage rates are within expectations and to identify any unexpected damage. The inspection should be carried out by competent staff and the results recorded and re-evaluated.


FeedBack of the Study

Start RBI Inspection Plan Rev.0

Plant Operation

Shutdown/Inspection

Inspection Reporting

Inspection Plan Rev.1

Maintenance Shutdown

Plant Operation

Maintenance Shutdown

Base RBI Assessments

Review RBI Assessments


Risk vs TimeNo Inspection Plan

60 months

120 months

180 months


Risk vs Time With Inspection Plan

60 months

120 months

180 months


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Fax. 0321677444

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rbi methodology

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