Fully Quantitative Risk-based Inspection (RBI) of Atmospheric Storage Tanks (AST)

Table of Content

• RBI Assessment Process
• Calculation of Risk for an Equipment Item or AST
• Determination of Probability of Failure (POF)
• Development of Corrosion Systems or Corrosion Loops and Identification of Damage Mechanisms
• Inspection History Data Population
• Atmospheric Storage Tanks Bottom Corrosion
• Combined Atmospheric Storage Tank Floor Corrosion Rate
• Determination of Consequence of Failure (COF)
• Selection of Consequence Calculation Type
• Determination of Environmental Consequences
• Inspection Planning
• Risk Mitigation
• Cost Benefits
• Conclusion

Inspection and maintenance departments in several organizations experience inspection and maintenance backlogs. Above storage tanks are always seen as the “ugly duck”. That is why, not much effort and budget are allocated to inspect and maintain them. The question arises whether Risk-based inspection (RBI) can help to solve these backlogs.

The RBI methodology is a systemic approach that focuses on loss of containment due to material deterioration which can optimize the in-service period on the basis of the condition and repairs done. It may or may not help you to postpone major shutdowns but assists you in the decision-making process and gives you confidence. RBI provides a practical assignment of resources to assess and maintain equipment technical integrity based on risk levels. Furthermore, the American Petroleum Institute’s Inspection Standard for AST Tanks, API 653 has identified the value of RBI for storage tanks in terms of determining inspection strategies as well as inspection intervals. It emphasizes that these RBI assessments on ASTs must be reviewed for any changes in risk, at least every 10 years, or more often, as changes occur with respect to tank design or foundation as well as the assumptions taken.

RBI Assessment Process

Quantitative programs are model-based approaches where numerical values are calculated, and more discreet input data is used. The advantages of a quantitative approach are:

A. Calculates, with some precision, when the risk acceptance limit is reached or exceeded. 

B. Discrimination between equipment risk allows prioritization of mitigation.

C. Trending and monitoring risk exposure over time as well as other metrics.

D. Benchmarking of reliability management such as POF trending and comparisons.

Quantitative methods are more systematic, consistent, and documented, and they are easier to update with inspection results than qualitative approaches. A quantitative approach generally uses a software program to calculate risk and develop inspection program recommendations. Quantitative RBI outlines a methodology for prioritizing equipment risk in a risk matrix or ISO-risk plot in addition to calculating discrete risk values for prioritization from higher to lower risk. POF and COF are combined to produce an estimate of risk for equipment. Equipment items are ranked based on risk with POF, COF, and risk calculated and reported separately to aid identification of major contributors to risk, or risk drivers.

Read More About Risked Based Inspection

Calculation of Risk for an Equipment Item or AST

API RP 581 has a comprehensive methodology to calculate the risk of aboveground storage tanks (ASTs). The origin of methodology was initially created from AST Risk Assessment Manual RAM (API, 2002) for the AST Committee of API and later encouraged by the RBI Committee of API. The initial scope was mainly covering tank floor thinning. The methodology was later extended to include a quantitative method for shell thinning, as well as susceptibility analysis (supplement analysis) for shell brittle fracture and cracking.

The illustration below shows how risk is calculated for an equipment item in a quantitative RBI analysis, the same approach for AST is available in API RBI 581. The equation of risk is simple, the product of the probability of failure (PoF) and the consequence of failure (CoF) is called as Risk. CoF can be expressed in terms of the environmental or safety consequence effects and the economic impacts. On the other hand, PoF is the product of the Generic Failure Frequency (GFF), the statistical frequency of failure for a given type of tank, based on API and other sources, and the damage factor DF. Age, damage type, damage rate and inspection effectiveness have a significant impact on DF calculation.

Determination of Probability of Failure (POF)

Splitting Tank into RBI Components

RBI Component is any part of the equipment that is designed and fabricated to a recognized code or standard. For example, a pressure boundary may consist of components (cylindrical shell sections, formed heads, nozzles, AST shell courses, AST bottom plate, etc.). API 581 added tank bottom edge (TANKBOTEDGE) component and API 620 tank components to Table 3.1. Suggested Component Generic Failure Frequencies.

TANKBOTEDGE refers to the near shell region of the tank bottom and is considered to extend 24 to 30 inches inside the shell. This is consistent with most annular ring dimensions. This component type can be used for tanks with or without an annular ring. TANKBOTTOM refers to the entire tank bottom, or if a TANKBOTEDGE is modeled, it refers to the remaining part of the tank bottom that does not include the edge component. If the component is a Tank620 TANKBOTEDGE, use the minimum thickness for an annular ring or the critical zone (for tanks without annular rings), whichever is applicable, in accordance with API STD 653.

Development of Corrosion Systems or Corrosion Loops and Identification of Damage Mechanisms

The aim of establishing the corrosion systems for corrosion loops is to address AST's deterioration mechanisms that pose a threat to the integrity of the tanks. Corrosion loops are normally prepared on the process flow diagrams (PFDs) by identifying potential damage mechanisms for all components of the tank in accordance with API 581 and API 571 which can be identified based on fluid stream, the material of construction, operating conditions, and % of corrosive ingredients like H2S, CO2, etc.

The purpose of a corrosion loop is to graphically represent the identification of those damage mechanisms that develop over a period, gradually weakening the system boundary and integrity of components until failure occurs. A clear understanding of expected and possible damage mechanisms for equipment is crucial to conducting the risk assessment and applying suitable inspection methods to mitigate the risk posed by them. Identification of these damage mechanisms is carried out in accordance with NACE (National Association of Corrosion Engineers), API 571, API 572, API 574, API 579, API 580, and API 581. To aid a structured approach to the tanks under assessment, the corrosion loop for the tank is subdivided into the following sub-loops. The damage mechanisms include floor corrosion (where Cathodic protection and drainage issues are important), internal corrosion (where the contents of the tank, the presence of species such as Sulphate sulphate-reducing bacteria and temperature-controlled corrosion rates), and non-corrosion related mechanisms such as differential settlement.

Inspection History Data Population

Magnetic Flux Leakage (MFL) inspection results interpretation for corrosion rate and measurement thickness of tank bottom for assessment can be calculated as per the below table.

Atmospheric Storage Tanks Bottom Corrosion

If the inspection history is not available and tanks are relatively new, then the corrosion rate calculations model is available from API RP 581.

A: Product Side Corrosion: The product side corrosion of the tank bottom depends on the following parameters.

• Measured Or Estimated Corrosion Rate (mpy)
• Product-Side Condition
• Operating Temperature
• AST Steam Coil Heater
• Water Draws

B. Soil Side Corrosion: The soil side corrosion of the tank bottom depends on the following parameters.

• Measured Or Estimated Corrosion Rate (mpy)
• Soil Condition Soil (Ω-Cm)
• AST Pad Material
• AST Drainage
• Cathodic Protection 
• Bottom Type
• Operating Temperature (°C: °F)

Combined Atmospheric Storage Tank Floor Corrosion Rate

The internal and external corrosion rates are estimated by multiplying the base corrosion rate by the respective adjustment factors. This will produce two separate corrosion rates that are combined as described below. It is assumed that the soil-side corrosion will be localized in nature while the product-side corrosion will be either generalized or localized. Note that to avoid understating the risk, it is recommended that the combined corrosion rate should not be set lower than 2 miles per year.

Option-1

If the internal corrosion is generalized in nature, the corrosion areas will likely overlap such that the bottom thickness is simultaneously reduced by both internal and external influences. In this case, the internal and external rates are additive.

Option-2

For pitting and localized corrosion, the chances are low that internal and external rates can combine to produce an additive effect on wall loss. In this case, the user chooses the greater of the two corrosion rates as the governing rate for the proceeding step.

Determination of Consequence of Failure (COF)

Development of Inventory Groups

Inventory groups are developed to determine the mass of fluid that could realistically be released in the event of a leak or rupture from pressure boundary components. The inventory groups are determined based on an assessment and review with operation and process personnel with consideration given to the presence of control valves that can be remotely activated to isolate the plant in case of an emergency. These drawings are mostly highlighted, and color coded on PFDs.

Selection of Consequence Calculation Type

The COF results are presented in terms of either area or financial loss. Financial-based COF is provided for all components, while area-based COF is provided for all components apart from AST bottoms, PRDs, and heat exchanger bundles. The COF associated with ASTs is concerned primarily with the financial losses due to loss of containment and leakage through the AST bottom as well as leakage and/or rupture of an AST shell course. However, safety/area-based consequences can also be addressed for the shell courses following the Level 1 or Level 2 consequence analysis methods. Detailed procedures for calculating the financial COF for both bottom plates and shell courses are provided in API RP 581.

Determination of Environmental Consequences

Environmental consequences for AST bottoms are driven by the volume and type of product spilled, the property impacted, and the cost associated with cleanup.

A. Dike Area:

A release of petroleum products is contained within a diked area or other secondary containment system such as a RPB, spill catch basin, or spill tank.

B. On-Site Soil:

A release of petroleum products is limited to contaminating on-site surficial soils. On-site refers to the area within the physical property boundary limits of the facility. 

C. Off-Site Soil:

A release of petroleum products contaminates off-site surface soils. Off-site refers to the property outside of the physical property boundary limits of the facility. 

D. Subsurface Soil:

A release of petroleum products contaminates subsurface soils. Subsurface impacts may or may not be contained within the physical property boundary limits of the facility. 

E. Ground Water:

A release of petroleum products contaminates groundwater. Groundwater refers to the first encountered phreatic water table that may exist subsurface at a facility.

F. Surface Water:

A release of petroleum products contaminates off-site surface water. 

The environmental sensitivity is given as Low, Medium, or High and determines the expected cost factor per barrel of spilled fluid for environmental cleanup in a worst-case scenario. Environmental consequences for AST bottoms are driven by the volume and type of product spilled, the property impacted, and the cost associated with cleanup.

Inspection Planning

The outcome of an RBI assessment is an inspection plan. API RP 581 guides to follow targets for different parameters which are vital to calculate risk. A target is defined as the maximum level acceptable for continued operation without requiring a mitigating action. Once the target has been met or exceeded, an activity such as inspection is triggered. Several targets can be defined in an RBI program to initiate and define risk mitigation activities, as follows.

A. Risk Target: The risk target may be expressed in area (ft2/year) or financial ($/year).

B. POF Target: A frequency of failure or leak (#/year). 

C. DF Target: A damage state that reflects an unacceptable failure frequency factor.

D. COF Target: A level of unacceptable consequence in terms of consequence area (CAf) or financial consequence (FC).

E. Thickness Target: A specific thickness, often the minimum required thickness, tmin.

F. Maximum Inspection Interval Target: A specific inspection frequency is considered unacceptable, triggering the inspection planning process.

Risk Mitigation

The major objective of RBI is to give direction to the management for the decision-making process of prioritizing resources to manage risk. Inspection affects the uncertainty of the risk associated with pressure equipment primarily by improving knowledge of the deterioration state and predictability of the POF. Although inspection does not reduce risk directly, it is a risk management activity that may lead to risk reduction.

Impending failure of pressure equipment is not avoided by inspection activities unless the inspection triggers risk mitigation activities that change the risk. RBI can be a good tool in determining the most appropriate line of action to mitigate risk. The possible mitigations are focusing on inspections to increase confidence in the actual damaged state of the equipment, fitness for services requirements, any repairs or replacements needed, lining installation, quality of coating, and corrosion inhibition requirements. 

Cost Benefits

The primary objective of an RBI assessment is not usually to reduce inspection costs, however, it is normally a side effect of the optimization of inspection planning. When the inspection program is optimized, based on an understanding of risk, one or more of the following cost-reduction benefits may be achieved:

A. Elimination of ineffective, unnecessary, or inappropriate inspection activities.

B. Elimination or reduction of inspection for low-risk items.

C. Deployment of online or non-invasive inspection methods may be substituted for invasive methods that require equipment shutdown.

D. Replacement of less effective, frequent inspections with more effective, infrequent inspections.

Conclusion

The fully quantitative Risk-Based Inspection (RBI) approach for Atmospheric Storage Tanks (ASTs) offers a structured and systematic method to manage and mitigate the risks associated with tank integrity. By leveraging detailed numerical data and advanced modeling techniques, RBI enables organizations to prioritize inspection and maintenance activities based on calculated risk levels. This approach not only improves decision-making processes but also optimizes inspection intervals, potentially reducing unnecessary shutdowns and associated costs.

The methodology aligns with the American Petroleum Institute's standards, ensuring compliance and enhancing the reliability of inspection strategies. By breaking down ASTs into specific components and focusing on detailed corrosion and damage mechanisms, RBI provides a comprehensive understanding of potential risks. This allows for the development of targeted inspection plans and effective risk mitigation activities, ultimately leading to enhanced safety, environmental protection, and operational efficiency. As organizations continue to adopt and refine this approach, the benefits of increased confidence in equipment integrity and optimized resource allocation become increasingly evident.

Author: Ashfaq Ali