Settlement assessment in a storage tank: Application of API 653 Annex B in API 650 Tanks

Introduction

Settlement in a flat-bottom storage tank represents a significant threat to structural integrity, operational stability, and industrial safety. These deformations, caused by irregular soil conditions, differential consolidation, unbalanced loads, or seismic events, can compromise critical tank components such as the bottom, shell, and seals. In light of this, it is essential to have regulatory and digital tools that enable the technical and systematic evaluation, classification, and management of these settlements.

API 650 sets forth the design and construction guidelines for welded tanks, ensuring structural performance under standard conditions. However, during the asset’s service life, API 653 becomes the key reference for inspection, maintenance, and repair. Within this standard, Annex B is especially relevant as it provides the procedure for settlement evaluation and acceptability criteria.

This article presents a technical and detailed review of Annex B, offering professionals a practical guide for applying regulatory criteria in managing settlement in a storage tank built in accordance with API 650.

Regulatory fundamentals and the relationship between API 650 – API 653 – API 579  

What is API 650 and why is it important in design?

API 650 is the internationally accepted standard for the design and construction of welded storage tanks for liquids. It defines geometric parameters, materials, allowable stresses, welding procedures, testing, and seismic considerations. Its objective is to ensure safe structural behavior from the fabrication stage. While it addresses typical loads, it does not explicitly consider differential settlements that may occur over time.

What does API 653 establish regarding inspection?

API 653 governs the inspection, repair, alteration, and reconstruction of an atmospheric steel storage tank built in accordance with API 650. This standard focuses on maintaining the integrity of the asset throughout its service life and addresses phenomena not considered in the original design, such as settlements caused by ground consolidation. Annex B provides the methodology for identifying, classifying, and evaluating different types of settlements through measurement curves and acceptability criteria.

Integration with API 579-1/ASME FFS-1 for advanced analysis

When settlements exceed the limits defined by API 653 or present complex geometries, the analysis must be escalated to a Fitness-For-Service (FFS) assessment under the API 579-1/ASME FFS-1 standard. This standard allows for the evaluation of the tank’s remaining structural capacity by applying stress modeling, nonlinear simulations, and plastic deformation analysis. Cases such as severe localized depressions or shell buckling require this type of advanced evaluation to support safe and well-founded operational decisions.

New provisions in API 653 Annex B

API 653 Annex B has undergone significant updates in recent years, driven by technological advancements, accumulated operational experience, and the need for greater regulatory clarity. These modifications are documented in Addendum 1 (April 2018) and the official Errata of March 2020, aiming to strengthen the acceptability criteria and expand the valid methods for evaluating settlements in API 650 tanks.

Recent regulatory updates

Among the most notable additions are clarifications regarding out-of-plane settlements and adjustments to localized settlement limits for large-diameter tanks. These updates enable a more flexible approach, aligned with modern operational realities, without compromising technical rigor. Cross-referenced standards were also incorporated, recommending the use of API 579 when geometric limits are exceeded but there is no visible evidence of structural damage.

Adjustments to evaluation criteria

The acceptable parameters for height differences between adjacent points were revised, based on tank diameter and the longitudinal extent of the settlement. These adjustments enable more accurate evaluations and reduce the likelihood of unnecessary interventions by integrating a risk-based approach. The distinction between permanent and seasonal settlements was also introduced as a criterion to determine whether a settlement should be corrected or simply monitored.

Newly accepted measurement methodologies

The standard now officially recognizes the use of high-resolution technologies such as 3D laser scanners, automated total stations, and continuous-reading strain sensors. These tools allow for accurate profiling of the tank bottom and improved detection of dishing or out-of-plane settlements, which might go unnoticed using traditional methods. The use of digital capture systems is also encouraged to generate automated curves, feed predictive analysis models, and reduce interpretation errors.

A representative example is HMT LLC, a company that utilizes technologies such as 3D laser scanners and automated ultrasonic testing to capture millions of data points per tank. These solutions enable the creation of detailed models of deformation and tilt, facilitating accurate technical evaluations in accordance with the guidelines set forth in API 653 Annex B.

Interface with API 579 for Out-of-Tolerance Scenarios

When settlements exceed the geometric limits established in Annex B, but no immediate structural failures are observed, it becomes mandatory to assess the tank’s condition under the Fitness-For-Service (FFS) methodology of API 579. This integration strengthens decision-making and allows tanks with complex settlements to remain in operation, provided structural modeling demonstrates that integrity is not compromised. The analysis can be performed at Level 1 (conservative), Level 2 (analytical), or Level 3 (finite element), depending on the complexity of the settlement.

Types of settlements and their classification according to Annex B 

Annex B of API 653 provides a technical classification for the types of settlement that can affect the stability and functionality of flat-bottomed storage tanks. Each type of settlement has different structural implications and requires specific evaluation methods.

Types of settlement and their associated risks according to API 653 Annex B

• Uniform settlement: This occurs when the entire base of the tank descends evenly. Although it does not cause significant internal deformation, it can affect external connections and drainage systems. It is recommended to monitor the situation and verify that piping and connections are not compromised.

• Inclined settlement: This occurs when one side of the tank settles more than the other, creating a tilt in the vertical axis. API 653 states that this inclination should not exceed 1:120 (8.3 mm/m or 1 in/10 ft). The calculation is made by measuring the elevation difference between two opposite points on the tank edge and dividing it by the total diameter.

• Localized sSettlement: This consists of isolated depressions beneath the tank base. They are represented through radial settlement curves and must be evaluated based on the tank’s diameter and the length of the affected area. API 653 sets limits based on these proportions. A localized settlement can induce significant bending stress on the tank shell.

• Dishing-type settlement: This refers to a bowl-shaped (concave) deformation in the tank bottom. It is typically caused by poor foundation conditions or backfill consolidation. It increases the stress due to hydraulic pressure and may concentrate tension at a critical point on the bottom plate.

• Out-of plane settlement: This involves asymmetric waviness or distortions. Detection requires structural modeling or three-dimensional scanning. Consequences include localized plastic deformation or shell buckling, and the analysis may need to be escalated to API 579 in cases of uncertainty.

Measuring techniques, equipment and monitoring

Accurate settlement measurement is essential for assessing the structural condition of tanks in accordance with Annex B of API 653. The methodology is based on topographic surveys conducted along the perimeter and internal radii of the tank.

Procedure Recommended by API 653

Measurement points should be placed every 3 to 5 meters (or 10 to 15 feet) along the edge of the tank bottom, as well as along internal radii if localized or dishing-type settlement is suspected. Measurements should be repeated at intervals defined by the criticality of the asset, following seismic events, structural modifications, or significant foundation fill operations.

Recommended Equipment

• High-precision optical level: Useful for small to medium-diameter tanks.
• Robotic total station: Enables automated measurement with high accuracy and error control.
• 3D laser scanning: An advanced technology that enables millimetric variation detection, bottom surface modeling, and the visualization of out-of-plane deformations, bottom surface modeling, and visualization of out-of-plane deformations.

IoT technology applications for continuous monitoring

Displacement and deformation sensors are now integrated into IoT platforms, enabling real-time data collection. When connected to predictive analysis software, these systems support maintenance decisions based on actual data and deformation trends.

Laser Scanning in Action: Real-Time Tank Monitoring with Leica C10

The practical use of 3D laser scanning for tank deformation monitoring is illustrated in the video below. Using the Leica C10 survey-grade laser scanner, this method captures up to 50,000 data points per second, generating highly accurate point clouds that allow engineers to assess:

• Shell out-of-roundness
• Verticality deviations
• Radial displacements
• Tilt monitoring at different fill levels

This approach also enables the unfolding of tank walls into flat plots, where colored heat maps (blue/yellow) clearly indicate inward or outward deviations. Advanced analysis even allows predictive insights into possible failure scenarios, such as buckling or spill risk due to ground settlement.

Watch the full demonstration in the following video, courtesy of: ttp://www.youtube.com/@jamesrye886

Data analysis and settlement curves

The graphical representation of settlements is a key tool for assessing the severity and distribution of deformations in flat-bottom tanks. API 653 recommends using settlement curves generated from both perimeter and radial measurements.

Graphical method and differential calculation

• Collect elevation data at equidistant points along the tank perimeter.
• Plot a base (theoretical) curve using the average or reference elevation.
• Overlay the actual field-obtained curve.
• Identify local deviations, slope changes, and critical zones.
• For radial settlements, plot curves from the center to the edge (useful for identifying dishing-type patterns).

Comparison parameters

• The maximum allowable difference between adjacent points varies according to the tank diameter (typically 1/120 to 1/240 of the diameter, per API 653).
• If deformation induces shell buckling or compromises structural connections, a detailed evaluation is required.
• Complex cases must be escalated to FFS analysis under API 579-1, especially when additional stresses, plastic distortion, or uneven base-to-soil contact are present.

In combination with digital tools, these curves can be integrated into predictive analysis models to anticipate settlement evolution and optimize asset management. 

Acceptability criteria according to API 653

Annex B of API 653 establishes specific criteria to determine whether a settlement is structurally acceptable, based on its type, magnitude, and affected length.

Acceptable Settlement Limits

• Tilted settlement: The vertical axis of the tank must not deviate more than 1:120 (8.3 mm/m or 1 in/10 ft).
• Localized settlement: The limit depends on the tank diameter and the length of the depression. For tanks <60 m (200 ft), the maximum deviation between adjacent points must not exceed 13 mm (0.5 in) if the distance is less than 3 m (10 ft).
• Dishing or out-of-plane settlements: These must be evaluated graphically and verified to ensure they do not exceed the limits that would cause deformation in the tank bottom or shell.

Recommendation when limits are exceeded

When settlements exceed the criteria of Annex B, a structural evaluation is recommended according to API 579-1/ASME FFS-1, at the following levels:

• Level 1: Conservative assessment using simplified calculations.
• Level 2: Detailed analysis with elastic modeling.
• Level 3: Finite element analysis under real loading conditions.

This approach enables safe decisions regarding repair, continuous monitoring, or decommissioning.

Corrective actions and rehabilitation techniques

When settlements fall outside the normative limits, corrective actions must be applied to restore the tank to a safe operating condition. These actions must comply with API 653 and, for structural reconstruction aspects, with Chapter 10 of API 650.

• Re-Leveling and Foundation Adjustment: This involves lifting the tank using hydraulic jacks to correct inclinations or depressions, followed by replacement or reinforcement of the foundation material. This operation requires detailed planning and continuous monitoring during execution.

• Bottom Plate Repair by Welding: If settlement has caused damage to bottom plates, partial replacements or insert sections may be implemented. The type of repair will depend on the depth of the damage, accessibility, and compliance with the minimum thickness requirements of API 653.

• Shell and Bottom Plate Reinforcement: When distortion has induced excessive stresses in the tank wall, internal or external reinforcements may be installed, designed according to API 650 – Chapter 10. This includes horizontal stiffeners, compression rings, or localized reinforcements in high-stress areas.

Each corrective measure must be supported by structural engineering and an integrity analysis based on the type of settlement.

Case studies and practical application

Tilted tank – successful intervention

In 2019, a storage terminal in Louisiana (USA) identified a progressive inclined settlement in an 80-foot diameter API 650 tank. The inclination ratio was 1:100, exceeding the 1:120 limit established by API 653. A re-leveling plan using hydraulic jacks was implemented, followed by redesign and compaction of the foundation.

This type of intervention has been addressed by specialized firms such as Becht, which have documented similar scenarios in alignment with API 653 Annex B. After completing geometric and structural verification, the tank was returned to service, demonstrating that controlled corrective maintenance can help avoid costly replacements and ensure continued safe operation.

3D scanning project and API 579 evaluation

At a refinery in the Persian Gulf, a 120-foot tank showed dishing-type settlements causing out-of-plane deformations. 3D laser scanning (LiDAR) technology was used to capture the bottom surface with high precision. The data were integrated into a finite element model and evaluated using API 579 Level 3. The result allowed the tank to remain in service under continuous monitoring. This case, presented at the 2022 Middle East Storage Conference, demonstrated how digitalization and advanced modeling improve operational decisions in complex settlement scenarios.

Recommendations and best practices 

To ensure the structural integrity of a storage tank subject to settlement, it is essential to adopt a comprehensive management approach:

• Maintaining historical records of settlement measurements facilitates comparative analysis and the prediction of long-term deformation patterns.
• Additional inspections are recommended after disruptive events such as earthquakes, floods, or nearby excavation work.
• Technical personnel should be trained in interpreting settlement curves, using API standards, and applying FFS evaluations.
• The incorporation of digital tools such as leveling sensors, 3D laser scanning, and AI-based monitoring platforms, and AI-based monitoring and predictive analysis platforms enable early detection of anomalies before they develop into critical failures.

These practices, aligned with the provisions of API 653 and API 579, ensure safer, more cost-effective, and more sustainable operation of storage assets.

Conclusions

The inclusion of new provisions in Annex B of API 653 marks a turning point in the evaluation of settlements in API 650 tanks. It is no longer just about applying geometric formulas or traditional inspections but about integrating risk-based criteria, high-precision digital tools, and advanced structural evaluation methodologies as defined in API 579.

Adopting these updated normative practices not only helps prevent severe structural failures but also optimizes resources, extends tank service life, and supports informed decision-making without unnecessary repairs. Furthermore, this modern approach fosters operational sustainability by reducing uncontrolled emissions, increasing asset availability, and ensuring compliance with increasingly stringent regulations.

In this context, Annex B becomes a key technical instrument for integrity engineering, tailored to the current challenges of the industry. Its systematic application—combined with continuous monitoring capabilities, predictive analysis, structural modeling, and FFS evaluation in accordance with API 579-1/ASME—ensures maintenance decisions are based on evidence, safety, and long-term cost-effectiveness.

References

1. American Petroleum Institute. (2014). API Standard 653 – Tank Inspection, Repair, Alteration, and Reconstruction (5th ed., reaffirmed 2020). Washington, DC: API.
2. American Petroleum Institute. (2020). API Standard 650 – Welded Tanks for Oil Storage (13th ed.). Washington, DC: API.
3. American Petroleum Institute & American Society of Mechanical Engineers. (2021). API Standard 579-1 / ASME FFS-1 – Fitness-For-Service (2021 ed.). Washington, DC: API and ASME.
4. Engineering Equipment and Materials Users’ Association (EEMUA). (2020). EEMUA Publication 159 – Users’ Guide to the Inspection, Maintenance and Repair of Above Ground Flat Bottomed Storage Tanks (5th ed.). London: EEMUA.
5. American Water Works Association. (2021). AWWA D100-21 – Welded Carbon Steel Tanks for Water Storage. Denver, CO: AWWA.
6. American Petroleum Institute. (n.d.). API Technical Report 5805 – Settlement Evaluation Case Studies.
7. American Petroleum Institute. (n.d.). API 653 Interpretations Database. Retrieved from https://my.api.org/Standards/Search