Maximize aboveground storage tank (AST) reliability with Equity Engineering. We combine API 653 and API 579-1/ASME FFS-1 principles to evaluate shell distortions, foundational settlement, and corrosion. Our quantitative engineering evaluations help justify continued operation of tanks, ensuring your storage assets remain safe and operational while reducing maintenance costs.
Estimate Remaining Life of Storage Tanks
Shell distortion in aboveground storage tanks (ASTs) is a common concern, especially when considering the tank’s flexible design and large size. Foundation settlement, operational upsets, high wind or seismic events, and inadequate repair techniques can also lead to significant physical or environmental damage. Regular inspections only identify localized and widespread shell distortion. At Equity, we conduct Level 3 fitness-for-service (FFS) assessments to address corrosion and distortion and determine whether the tank can remain in service.
- Reduce the risk of tank failure
- Increase operational safety
- Improve business continuity
Types of Shell Distortion
Shell distortion is a relatively common occurrence in ASTs due to the large size and flexible nature of the design.
Localized Shell Distortion
- Bulges
- Flat spots
- Peaking
- Banding
Widespread Shell Distortion
- Out-of-roundness
- Plumbness
- Global buckling
What is Level 3 FFS for Tanks?
FFS assessments are conducted on in-service tanks to assess the structural integrity and determine whether the tanks are fit for continued operation or if the flaw or damage will cause a failure. Equity’s team of mechanical and structural engineers combines the principles of API 653 and API 579 when conducting a Level 3 FFS assessment to improve safety, reduce risk, optimize construction, maximize equipment availability, and reduce future maintenance and inspection costs.
Case Study: FFS Optimizes Remaining Life of AST Shells
After a routine inspection of a 245’ (74.6m) diameter carbon-steel, internal floating roof AST, a North American-based refinery identified a region of local metal loss on the tank bottom and shell near the corner joint. Given the location of the damage, Equity conducted a Level 3 assessment in accordance with API 579 to evaluate the FFS of the localized thinning on the AST. Currently, API 653 Tank Inspection, Repair, Alteration, and Reconstruction does not offer rules for localized thinning across this junction. Even if the local thinning was identified exclusively on the shell near the top of the first course, an API 579 Level 3 assessment would still have been required due to the large tank diameter.
Result: The evaluation indicated two options to extend the remaining life of the tank:
- Option 1: reduce the fill height to about 75% to satisfy the required load cases using a future corrosion allowance (FCA) and extend the remaining life by 10 years
- Option 2: adjust the fill height to 85%, which will extend the remaining life by 1-2 years
Using the information Equity provided, the refinery made an informed decision while also meeting operational requirements.
Equity’s FFS Assessment Workflow
Our mechanical and structural engineering team combines the appropriate level of technology with their extensive industry experience to provide practical and cost-sensitive recommendations to challenging FFS problems. Our team offers many related services to help improve the reliability of and extend equipment life, from both a reactive and proactive approach, including:
- Brittle fracture screening and minimum pressurization temperature (MPT) evaluation
- Volumetric metal loss
- Crack-like flaw assessments
- Distortion and tank edge settlement
- Creep and fatigue life estimation
- High-temperature hydrogen attack (HTHA)
- Mechanical vibration of piping systems or equipment
- Fire damage assessments
Equity Software: Protect Aboveground Tank Safety with Tank FEA
Equity Software has four new automated finite element analysis (FEA) webtools for conducting automated FEA of nozzles, aboveground storage tanks, local thin areas, and pipeline dents.
The Tank FEA webtool is a fast, reliable, and systematic way to assess edge settlement issues and ensure the longevity and safety of your storage tanks. This easy-to-use webtool delivers advanced stress assessments through automated axisymmetric FEA, ensuring compliance with API 653 while also addressing safety-critical flaws like ratcheting, collapse, and local damage. Use Tank FEA to help you reduce downtime and preserve tank safety.
Equity’s Role in Advancing FFS Methodologies
Our engineers pioneered the advancement of FFS technologies and were the lead investigators of the international standard API 579-1/ASME FFS-1 Fitness-for-Service. Today, the document is the FFS standard of choice for other industries such as pulp & paper, biofuels, food processing, and non-commercial nuclear.
Equity was the lead investigator of API 571 Damage Mechanisms Affecting Fixed Equipment in the Refining and Petrochemical Industry. Understanding the effects and potential for future damage progression is critical in completing an effective FFS evaluation.
