Risk Strategies for Pressure Retaining Equipment

by Michael Hurley, 7/11/2024 1 Comments

Tags: API 580 API 581 Damage Mechanisms Mechanical Integrity Risk Based Inspection

What are equipment/inspection strategies in relation to mechanical integrity (MI) and risk based inspection (RBI)? It is a simple answer, but can be complicated to produce a documented set. API RP 581 has gone as far as the RAGAGEP can go since it is up to the Owner/User to define them as they are responsible for their risk. Knowing strategy/effectiveness tables are likely to be used without modification therefore AOC created a set that can be used as inspection recommendations out of the box.

Risk Strategies for Pressure Retaining Equipment

What are equipment/inspection strategies in relation to mechanical integrity (MI) and risk based inspection (RBI)?

It is a simple answer, but can be complicated to produce a documented set. API RP 580 version 4, Section 13.4 states:

"The results of a risk assessment are normally used as the basis for the development of an overall inspection strategy for the group of items included. The inspection strategy should be designed in conjunction with a monitoring or mitigation program so that all equipment items will have resultant risks that are acceptable. For the development of an inspection strategy, users should consider the following:
risk criteria and ranking;
risk drivers;
equipment history;
number and results of inspections;
type and effectiveness of inspections; and
equipment in similar service and remaining life." ⁽¹⁾

This section of the document and the definition of inspection plan, per section 3.1.27:

"inspection plan

Strategy defining how and when an asset or component will be inspected, repaired, and/or maintained.

NOTE For the purposes of this document, the inspection plan is the product of an RBI analysis that details the scope, extent/locations, methods, and timing of specific inspection/monitoring activities for determining the condition of a specific piece of equipment. It is the essential part of a risk-based management plan." ⁽¹⁾

This alludes to the strategy being written at the individual equipment or for only equipment in "equipment in similar service and remaining life." Regardless of hierarchy, strategies are there for one main reason, to translate the risk result into discreet actions. We at AOC believe that strategies should be separated from individual equipment plans and written at a higher level (i.e. unit/plant/organization). Therefore we believe API 580 strategies and API 581 effectiveness tables are the same concept and are separate for each component of the individual equipment plan which is the inspection recommendation.

AOC believes that equipment strategies or effectiveness tables need to be simple, easily understood, and prescriptive for the individual company, plant, or facility. Therefore let's discuss the minimum information for both the API 580 type strategies and API 581 type effectiveness tables for an individual entity. AOC believes an API 580 type strategy should have the minimum information:

Equipment component type to which it applies.
The damage mechanism to which it applies.
The minimum extent for each inspection to be used by risk level. This might be more than one.
The percentage of components that can be grouped by risk level using the rules from API.
The maximum inspection frequency for each inspection extent per risk level considering any remaining life.
The API effectiveness level corresponding to the minimum extent for each inspection to be used by risk level.
Rule(s) to allow for the inspection extent(s) elimination.
Notes with additional information that apply to the above bullets.

An API 581 type effectiveness table should have all of the same parts as the above, with the exclusion of the maximum inspection frequency. Please note that RAGAGEP has little to offer on this topic. The most specific content comes from API 581 which are shown starting in 2.C.5.2 and continuing to 2.C.11.1. Note that the title for each table contains the word "example". This is about as far as the RAGAGEP can go since it is up to the Owner/User to define them as they are responsible for their risk. Below is an excerpt from API 581 Version 3, 2nd Addendum for general thinning:

Table 2.C.8.1 - LolE Example for General Thinning ⁽²⁾
Inspection Category Inspection
Effectiveness
Category Intrusive Inspection Example ^{a, b, c, d} Non-Intrusive Inspection Example ^{a, b, c, d}
A Highly Effective For the total surface area:
> 50% visual examination (partial internals removed)

AND

> 50% of the spot ultrasonic thickness measurements For the total surface area:
100% UT/RT of CMLs

OR

For selected areas:
10% UT scanning

OR

10% profile radiography
B Usually Effective For the total surface area:
> 25% visual examination

AND

> 25% of the spot ultrasonic thickness measurements For the total surface area:
> 75% spot UT

OR

> 5% UT scanning, auotomated or manual

OR

> 5% profile radiography of the selected area(s)
C Fairly Effective For the total surface area:
> 5% visual examination

AND

> 5% of the spot ultrasonic thickness measurements For the total surface area:
> 50% spot UT or random UT scans (automated or manual)

OR

random profile radiography of the selected area(s)
D Poorly Effective For the total surface area:
< 5% visual examination without thickness measurements For the total surface area:
> 25% spot UT
E Ineffective Ineffective inspection technique/plan was utilized. Ineffective inspection tecnhique/plan was utilized.
^a Inspection quality is high.
^b Inspection points (CMLs, scans, etc.) are set up by knowledgeable individuals.
^c That the number of CMLs and area for scanning (UT or profile radiography) is one that will detect damage if occurring.
^d Percentage refers to percent of established CMLs examined (e.g. for spot UT) or the percent surface area examined.

Table 2.C.8.1 - LolE Example for General Thinning ⁽²⁾
A	Highly Effective	For the total surface area: > 50% visual examination (partial internals removed) AND > 50% of the spot ultrasonic thickness measurements	For the total surface area: 100% UT/RT of CMLs OR For selected areas: 10% UT scanning OR 10% profile radiography
B	Usually Effective	For the total surface area: > 25% visual examination AND > 25% of the spot ultrasonic thickness measurements	For the total surface area: > 75% spot UT OR > 5% UT scanning, auotomated or manual OR > 5% profile radiography of the selected area(s)
C	Fairly Effective	For the total surface area: > 5% visual examination AND > 5% of the spot ultrasonic thickness measurements	For the total surface area: > 50% spot UT or random UT scans (automated or manual) OR random profile radiography of the selected area(s)
D	Poorly Effective	For the total surface area: < 5% visual examination without thickness measurements	For the total surface area: > 25% spot UT
E	Ineffective	Ineffective inspection technique/plan was utilized.	Ineffective inspection tecnhique/plan was utilized.
^a Inspection quality is high. ^b Inspection points (CMLs, scans, etc.) are set up by knowledgeable individuals. ^c That the number of CMLs and area for scanning (UT or profile radiography) is one that will detect damage if occurring. ^d Percentage refers to percent of established CMLs examined (e.g. for spot UT) or the percent surface area examined.

This is good, but if we review the recommended intrusive inspection, effectiveness level C, we begin to see that it is not prescriptive enough for inspection planning. The recommended extent of "> 5% visual examination" is open to interpretation:

Can I just look in the manways, and large nozzles to satisfy this requirement?
Do I physically have to go in?
What locations do I need to examine?

Are these the discreet actions (inspection recommendations) that could be executed directly? I am certain that some people who have been employed as RBI professionals for many years would say yes. As the author, I say no, because no group of RBI practitioners would answer the above questions exactly the same. Imagine passing just this on to a group of experienced API inspectors and how many answer variations would you receive then? In addition, it is typical that Owner/Users exercise these without revision and use them for inspection recommendations in various RBI software solutions.

The implication of not creating a succinct set of strategies/effectiveness tables points to poorly understanding their use beyond mere inspection recommendations. These documents provide for consistency with:

Inspection Recommendations – The same inspection techniques and extents are used through several inspection cycles providing for data trending.
Inspection Recommendations – The inspection techniques and extents from the strategies/effectiveness tables are meant to provide the minimum inspection to maintain or drive down a component's risk. Understand that end of life situations cannot be handled by using RBI strategies/effectiveness tables but must handled by using Fitness For Service (FFS) analysis.
Inspection Execution – The inspection techniques and extents recommended along with other supporting detail need to allow for adequate budgeting and execution by the Owner/User or a contractor.
Reviewing Inspection Results – The strategies/effectiveness tables are used to rate the effectiveness of each inspection result. Effectiveness is simply, how likely was the inspection to find the damage mechanism it was targeting.

We must always keep in mind that consistency needs to be maintained across plans and RBI assessments. API 580 states in section 16.2 that:

"The required data shall be captured and maintained such that the RBI assessment can be recreated or updated at a later time by others who were not involved in the original RBI assessment."

Therefore, it is important to be congnizant of those who come after and to provide enough information and detail in the recommended inspection strategies so that inspection effectiveness can be consistently evaluated across assessments.

Knowing that strategy/effectiveness tables are likely to be used without modification, dictates that we at AOC create a set that can be used as inspection recommendations out of the box. AOC provides two examples at the end of this paper for thinning. One is an API 580 compliant strategy, and the other is an API 581 effectiveness table. These are examples of actual product provided to clients. Take them and compare them to your own written documentation and comment below to let us know what you think. Or reach out to us directly and one of our RBI professionals will get back to you.

Thinning (Internal Corrosion) Vessel Components – API 580 Strategy

The colored columns of the table are the minimum requirements by risk category for the applicable component and damage mechanism. The table below provides a list of the minimum inspection techniques, extents, grouping (similar service) percentage, maximum inspection frequency, elimination of internal visual inspection, and minimum confidence for each specified extent by risk category. Rows 1 and 2 below are separate required minimum inspection extents.

Minimum Risk Based Recommendations for Internal Corrosion
	Internal Corrosion Risk Category
	Low	Medium	Medium High	High
Minimum Internal Visual Inspection Extent^{2, 3, 5, 7} Note: The primary aim of visual inspection is to identify, measure, and deal with locally thinned areas. These areas should be used to locate CMLs AND	>=30% Visual Coverage of the entire internal surface area of the vessel. This includes the internal visual examination of 10% of the nozzles, manways, and supports/appurtenances.	>=50% Visual Coverage of the entire internal surface area of the vessel. This includes the internal visual examination of 25% of the nozzles, manways, and supports/appurtenances.	>=70% Visual Coverage of the entire internal surface area of the vessel. This includes the internal visual examination of 50% of the nozzles, manways, and supports/appurtenances.	>=90% Visual Coverage of the entire internal surface area of the vessel. This includes the internal visual examination of all nozzles, manways, and supports/appurtenances.
Minum UT Examination Extent General or Localized^{2, 3, 6, 7}	2 UT readings per head, 2 UT readings per 20' shell band, 1 reading on 25% of nozzles (Min. 1 Inlet and 1 Outlet nozzle),	2 UT readings per head, 4 UT readings per 20' shell band, 1 reading on 25% of nozzles (Min. 1 Inlet and 1 Outlet nozzle),	4 UT readings per head, 4 UT readings per 20' shell band, 1 reading on 50% of nozzles (Min. 1 Inlet and 1 Outlet nozzle),	4 UT readings per head, 8 readings per 20' shell band, 1 reading on 100% of nozzles (Min. 1 Inlet and 1 Outlet nozzle).
% Vessels from Group^{1, 3} Grouping (similar service) is only available within the facility's boundary.	Minimum of 10%	Minimum of 25%	Minimum of 50%	High risk vessels cannot be grouped.
Maximum Inspection Frequency (Yr.), or 1/2 remaining life, whichever is less	15	15	10	5
Eliminate internal visual inspections, if allowed, when this criteria is met in addition to the following: CR <= .005 in/year Corrosion Type = General No other internal DMs exist	5 years of inspection history from the RBI analysis date. During this period at least 1 internal for general corrosion and 2 internals for localized corrosion	5 years of inspection history from the RBI analysis date. During this period at least 1 internal for general corrosion and 2 internals for localized corrosion	10 years of inspection history from the RBI analysis date. During this period at least 2 internal for general corrosion and 3 internals for localized corrosion	Internal visual inspection cannot be eliminated for High risk vessels.
Inspection Confidence/Effectiveness	D POD⁷ ≈ 30%	C POD⁷ ≈ 50%	B POD⁷ ≈ 70%	A POD⁷ ≈ 90%
¹ Vessels can be grouped and representative inspection results from a vessel can be used if the group of vessels are in the same service (process environment), do not have a High criticality rating, process conditions do not exceed the key variables, are constructed of the same materials, the only failure mode is general internal corrosion or localized corrosion with predictable locations. ² This is separate inspection which must be graded for confidence individually. ³ If damage is found, continue testing likely locations by doubling the percentage until no further damage is seen. ⁴ The proposed inspection is the best generic recommendation based on the API codes and practices. The Owner/User may substitute an equivalent inspection technique and extent for the proposed inspection provided that it offers the same probability of detection (POD) as the generic recommendation. ⁵ The visual inspector must be an API Authorized Pressure Vessel or Tank Inspector according to API 510 or API 653. These credentials need to be verified prior to the start of work. ⁶ The Technician must hold certifications for UT and/or RT examination that conform to ASNT SNT-TC-1A. These credentials need to be verified prior to the start of work. All NDE shall be performed per ASME V requirements and the NDE providers company procedures. ⁷ POD = Probability Of Detection. The POD shown is the average of the ranges shown in API RP 581, Table 2.C.2.1-Inspection Effectiveness Categories. ⁸ The inspection must return the following minimum information: Damage Rate/Corrosion/Pitting Rate Damage Extent - Was damage found? - Good description of the damaged area and its location and dimensions. Proper documentation of the inspection technique used - Enough to repeat it. Inspection Extent - Good description of the total amount of the asset examined as a percentage.

Thinning (Internal Corrosion) Vessel Components – API 581 Effectiveness Table

Minimum Risk Based Recommendations for Internal Corrosion
	Internal Corrosion Risk Category
	Low	Medium	Medium High	High
Minimum Internal Visual Inspection Extent^{2, 3, 5, 7} Note: The primary aim of visual inspection is to identify, measure, and deal with locally thinned areas. These areas should be used to locate CMLs AND	>=30% Visual Coverage of the entire internal surface area of the vessel. This includes the internal visual examination of 10% of the nozzles, manways, and supports/appurtenances	>=50% Visual Coverage of the entire internal surface area of the vessel. This includes the internal visual examination of 25% of the nozzles, manways, and supports/appurtenances.	>=70% Visual Coverage of the entire internal surface area of the vessel. This includes the internal visual examination of 50% of the nozzles, manways, and supports/appurtenances.	>=90% Visual Coverage of the entire internal surface area of the vessel. This includes the internal visual examination of all nozzles, manways, and supports/appurtenances.
Minum CML UT Location Extent General or Localized^{2, 3, 6, 7}	2 UT readings per head, 2 UT readings per 20' shell band, 1 reading on 25% of nozzles (Min. 1 Inlet and 1 Outlet nozzle)	2 UT readings per head, 4 UT readings per 20' shell band, 1 reading on 25% of nozzles (Min. 1 Inlet and 1 Outlet nozzle)	4 UT readings per head, 4 UT readings per 20' shell band, 1 reading on 50% of nozzles (Min. 1 Inlet and 1 Outlet nozzle)	4 UT readings per head, 8 readings per 20' shell band, 1 reading on 100% of nozzles (Min. 1 Inlet and 1 Outlet nozzle)
% Vessels from Group^{1, 3} Grouping (similar service) is only available within the facility's boundary.	Minimum of 10%	Minimum of 25%	Minimum of 50%	High risk vessels cannot be grouped
Maximum Inspection Frequency (Yr.), or 1/2 remaining life, whichever is less	Maximum frequency calculated per API 581 methodology	Maximum frequency calculated per API 581 methodology	Maximum frequency calculated per API 581 methodology	Maximum frequency calculated per API 581 methodology
Eliminate internal visual inspections, if allowed, when this criteria is met in addition to the following: CR <= .005 in/year Corrosion Type = General No other internal DMs exist	5 years of inspection history from the RBI analysis date. During this period at least 1 internal for general corrosion and 2 internals for localized corrosion	5 years of inspection history from the RBI analysis date. During this period at least 1 internal for general corrosion and 2 internals for localized corrosion	10 years of inspection history from the RBI analysis date. During this period at least 2 internal for general corrosion and 3 internals for localized corrosion	Internal visual inspection cannot be eliminated for High risk vessels
Inspection Confidence/Effictiveness	D POD⁶ ≈ 30%	C POD⁶ ≈ 50%	B POD⁶ ≈ 70%	A POD⁶ ≈ 90%
¹ Vessels can be grouped and representative inspection results from a vessel can be used if the group of vessels are in the same service (process environment), do not have a High criticality rating, process conditions do not exceed the key variables, are constructed of the same materials, the only failure mode is general internal corrosion or localized corrosion with predictable locations. ² This is separate inspection which must be graded for confidence individually. ³ If damage is found, continue testing likely locations by doubling the percentage until no further damage is seen. ⁴ The proposed inspection is the best generic recommendation based on the API codes and practices. The Owner/User may substitute an equivalent inspection technique and extent for the proposed inspection provided that it offers the same probability of detection (POD) as the generic recommendation. ⁵ The visual inspector must be an API Authorized Pressure Vessel or Tank Inspector according to API 510 or API 653. These credentials need to be verified prior to the start of work. ⁶ The Technician must hold certifications for UT and/or RT examination that conform to ASNT SNT-TC-1A. These credentials need to be verified prior to the start of work. All NDE shall be performed per ASME V requirements and the NDE providers company procedures. ⁷ POD = Probability Of Detection. The POD shown is the average of the ranges shown in API RP 581, Table 2.C.2.1-Inspection Effectiveness Categories. ⁸ The inspection must return the following minimum information: Damage Rate/Corrosion/Pitting Rate Damage Extent - Was damage found? - Good description of the damaged area and its location and dimensions. Proper documentation of the inspection technique used - Enough to repeat it. Inspection Extent - Good description of the total amount of the asset examined as a percentage.

REFERENCES

"American Petroleum Institute (2023), API RP 580 - Elements of a Risk-Based Inspection Program, Fourth Edition," https://store.accuristech.com/api/standards/api-rp-580?product_id=2572356.
"American Petroleum Institute (2016), API RP 581 - Risk-Based Inspection Methodology, Third Edition, Includes Addendum 1 (2019) and Addendum 2 (2020)," https://store.accuristech.com/api/standards/api-rp-581?product_id=1911658.

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Thinning (Internal Corrosion) Vessel Components – API 580 Strategy

Thinning (Internal Corrosion) Vessel Components – API 581 Effectiveness Table

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Table 2.C.8.1 - LolE Example for General Thinning ⁽²⁾
Inspection Category	Inspection Effectiveness Category	Intrusive Inspection Example ^{a, b, c, d}	Non-Intrusive Inspection Example ^{a, b, c, d}
A	Highly Effective	For the total surface area: > 50% visual examination (partial internals removed) AND > 50% of the spot ultrasonic thickness measurements	For the total surface area: 100% UT/RT of CMLs OR For selected areas: 10% UT scanning OR 10% profile radiography
B	Usually Effective	For the total surface area: > 25% visual examination AND > 25% of the spot ultrasonic thickness measurements	For the total surface area: > 75% spot UT OR > 5% UT scanning, auotomated or manual OR > 5% profile radiography of the selected area(s)
C	Fairly Effective	For the total surface area: > 5% visual examination AND > 5% of the spot ultrasonic thickness measurements	For the total surface area: > 50% spot UT or random UT scans (automated or manual) OR random profile radiography of the selected area(s)
D	Poorly Effective	For the total surface area: < 5% visual examination without thickness measurements	For the total surface area: > 25% spot UT
E	Ineffective	Ineffective inspection technique/plan was utilized.	Ineffective inspection tecnhique/plan was utilized.
^a Inspection quality is high. ^b Inspection points (CMLs, scans, etc.) are set up by knowledgeable individuals. ^c That the number of CMLs and area for scanning (UT or profile radiography) is one that will detect damage if occurring. ^d Percentage refers to percent of established CMLs examined (e.g. for spot UT) or the percent surface area examined.