Proposed API 581 Inspection Plan Optimization Example

, 2/4/2021 12:00:00 AM Be the first to comment

Tags: API 581 Mechanical Integrity Process Safety Management Risk Analysis Risk Based Inspection Risk Management Technology

This article deals with a proposal for a risk analysis option that allows for individual damage mechanism risk calculation. This is an example of how it can be applied and how it domonstrates value.

Proposed API 581 Inspection Plan Optimization Example

In a previous post, Proposed API 581 Inspection Plan Optimization, I presented an optimization to API 581(1). In this post I work out an example to illustrate its point. I created an example that gives us probability of failure (POF), consequence of failure (COF), and risk using an example already published by Trinity Bridge in 2011 (2) as a guide. This example's assumptions are:

  • The data quality is good
  • The RBI Analysis Date was 05/01/2008
  • The Planned Date was 05/01/2028
  • The asset's In Service Date was 01/01/1972
  • Two damage mechanisms exist: internal thinning and internal cracking
  • The only inspection performed was for thinning on 04/04/2004, API 581 Level B effectiveness
  • No inspections past the RBI Analysis Date were assumed for typical inspection planning purposes as this example shows the risk evaluated "as is" and projected for 20 years
  • The Risk Target was assumed = 40 ft2/yr
  • The COF (Cf) was simplified to COF Personnel, ft2, only
  • POF was generated for three damage factor cases with FMS = 1 and gfftotal = 3.06E-05
    • POF for combined damage factor, Pf(t) = gfftotal ⋅ Df(t) ⋅ FMS
    • POF for thinning damage factor, Pf(Thinning) = gfftotal ⋅ Df(Thinning) ⋅ FMS
    • POF for cracking damage factor, Pf(Cracking) = gfftotal ⋅ Df(Cracking) ⋅ FMS
  • Risk was calculated three ways
    • With POF for combined damage factor, R(t) = Pf(t) ⋅ Cf
    • With POF for thinning damage factor, R(Thinning) = Pf(Thinning) ⋅ Cf
    • With POF for cracking damage factor, R(Cracking) = Pf(Cracking) ⋅ Cf

These three risk values were projected over a 20 year plan period and compared to the risk target of 40 ft2/yr. We will not address the specific steps to calculate these risk values in this example, but below is a presentation in tabular format of the results.

Table (1): Risk Values Projected Over 20 Years
RBI Analysis Date with 1B effectiveness inspection ThinningPlan Date with 1B effectiveness inspection ThinningPlan Date with 1B effectiveness inspection ThinningPlan Date with 1B effectiveness inspection Thinning
Date in Service1/1/1972
Last Thin Inspection (B) Date4/4/2003
Thinning Init. Thick (in) at last insp. date0.75
Corrosion Allowance (in)0.13
Planned Period (yrs)01216
COF Personnel ft217115.04817115.0517115.0517115.05
Thinning Age (yrs)5.0796.087.0821.09
Thinning Rate (in/yr)0.0110.010.010.01
tmin (in)0.6970.700.700.70
Frac Wall Loss0.1580.1720.1860.381
Thinning Factor195082361
Frac Wall Loss Lower0.1400.160.180.35
Frac Wall Loss Upper0.1600.180.20.4
Thinning Factor Lower6.0002070300
Thinning Factor Uper20.00070110400
Cracking Age (yr)36.35637.35738.35852.367
Severity Index1.0001.0001.0001.000
Best Cracking Factor1.0001.0001.0001.000
Calc Cracking Factor52.07653.65555.23877.797
Combined Damage Factor70.775103.755137.115438.842
Generic Failure Freq3.06E-053.06E-053.06E-053.06E-05
POF Combined DF, failures/yr0.0021660.0031748960.0041957330.013428554
Risk Combined DF, ft2/yr37.0754.3384977971.81016454229.8303547
POF Thinning Factor, failures/yr0.0005720.0015330.0025050.011048
Risk Thinning Factor, ft2/yr9.7926.2442.88189.09
POF Cracking Factor, failures/yr0.0015940.0016420.0016900.002381
Risk Cracking Factor ft2/yr27.2728.1028.9340.74
Risk Target, ft2/yr40.0040.0040.0040.00
Time to Target (yr) Combined from analysis date1.00
Time to Target (yr) Thinning from analysis date2.00
Time to Target (yr) Cracking from analysis date16.01

The table above only presents the relevant columns and values that are needed for illustration. The yellow highlight shows:

  • The calculated risk value (ft2/yr) for each risk case when it met or past the Risk Target.
  • The time (yr) for each risk case to meet or pass the Risk Target.

The results show for each risk case:

  • The combined damage factor case exceeded the Risk Target almost immediately, on or before 05/01/2009. The time to target for this case was one year or less.
  • The individual thinning damage factor case exceeded the risk target just shortly after the combined, on or before 05/01/2010. The time to target for this case was two years or less.
  • The individual cracking damage factor case did not exceed the risk target until just on or before 05/01/2024. The time to target for this case was 16 years or less.

So here we are in 2008, we have run the risk analysis to determine where we are and how to plan the inspections for the future. Here are the outcomes:

  • Using the traditional 581 planning approach with the 20 year period, it is likely that both the cracking and thinning inspections would be due on or before 05/01/2009 to get past the Plan Date of 05/01/2028.
  • Using the proposed individual damage mechanism risk approach, the thinning inspection would be due on or before 05/01/2010, and the cracking inspection would be due on or before 05/01/2024 to get past the Plan Date of 05/01/2028.

There is obvious value here and it allows the Owner/User to be more flexible in their inspection planning process. However, there are literalists who believe that if they follow the methodology exactly, they will not be penalized if an incident occurs. Experience tells us that this is a false assumption. As API 581 is open to interpretation, for the literalist if it is not written down, it is precluded from being done. This is a narrow interpretation with which many of you will disagree.

My personal belief is that API recommended practices, such as 581, should be interpreted by the Owner/User. I also believe that if alternatives are not provided in these same recommended practices, governments and regulatory agencies will take them literally. This will invalidate the Owner/User argument that if it is not explicitly precluded in the recommended practice, then it can be done. Finally, this will stop the Owner/User from interpreting these recommended practices to their benefit. Do any of you believe that there is only one way of "doing things"?

Let's continue the conversation! I am interested in your feedback on this proposal. Please comment below or contact me directly to start the conversation.

  1. API RP 581, Risk-Based Inspection Methodology, Third Edition, April 2016. API Publishing Services, 1220 L Street, N. W., Washington, D. C. 20005
  2. Lynne Kaley, "API RP 581 Risk-based Inspection Technology Demonstrating the Technology Through a Worked Example Problem”, Inspectioneering Journal, March/April 2009.

Be the first to comment


There are no comments for this article.

Add your comment

Related Services

Risk Based Inspection (RBI) Implementation and Planning

AOC has delivered thousands of sustainable Risk Based Inspection (RBI) programs earning the trust of owner operators.

Damage Mechanism Review / Corrosion Study

One of the most important steps in an RBI project is the corrosion study or damage mechanism review.

End-of-Life, Remaining-Life, and Fitness-for-Service Assessments

When evaluation of inspection results suggest that an asset is near its end of useful life, Fitness for Service evaluations can determine if the asset us suitable for continued operation.

Related Tools

API 580 Work Process Quiz

How well do you know RBI? Take this short quiz to test your knowledge of the API 580 risk-based inspection (RBI) work process.

RBI Potential Savings Calculator

Create mechanical integrity (MI) program value rather than it being seen as a necessary cost to minimize.

Related Training

RBI/MI Overview

A high level overview introducing Mechanical Integrity and Risk Based Inspection

API 580 RBI Overview

What impact does Risk Based Inspection (RBI) have on my organization?

API 580 Training

Is your Risk Based Inspection (RBI) program aligned with the API 580 Recommended Practice? Are you ready for certification?

API 581 Overview

What's actually going on inside all of that fancy software? An introduction to the API 581 methodology.

API 581 Training

A deep dive into quantitative Risk Based Inspection (RBI) as outlined in API 581.

Related Knowledge

Proposed API 581 Inspection Plan Optimization

A proposal for a risk analysis option that allows for individual damage mechanism risk calculation in API 581

How to Incorporate the New PHMSA Underground Gas Storage Requirements

This is a practical approach to incorporating the new PHMSA gas well rules into your integrity program with the rest of your surface and subsurface assets.

RBI Value: Both Transitioning And From The Start

A look at how RBI adds value whether you are just starting out or transitioning from a traditional methodology.

Plant Management 101: Common Work Processes

A dysfunctionality found in many refineries, chemical plants, and other production facilities, is a lack of common asset management work processes.

Hidden Benefits of Risk Based Inspection (RBI)

What are the hidden benefits of implementing Risk Based Inspection?

Asset Value Management vs Asset Performance Management

A look at how the financial sector's concept of Asset Value Management can be applied to the petrochemical industry.

Qualitative Risk Assessment of a Commercial Refrigeration System

A case study presenting the methods used to calculate qualitative risk for a critical refrigeration system and the results of the assessment.

Risk Based Inspection Seen Through the Eyes of the Experts

Our experts consider why organizations implement RBI, common challenges, and strengths of an RBI champion.

Qualitative RBI of a Commercial Refrigeration System

Refrigeration system owner/users who are maintaining their assets as prescribed may be unaware of the damage mechanisms that could affect the reliability of their assets and safety of their personnel.

Dynamic Risk - Mechanical Integrity

Things are always changing. Including your risk profile.