FITNESS-FOR-SERVICE FOR ABOVEGROUND STORAGE TANKS

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 E2G, we conduct Level 3 fitness-for-service (FFS) assessments to address corrosion and distortion and to 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. E2G’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, EG 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 E2G provided, the refinery made an informed decision while also meeting operational requirements.

E2G'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:

E2G'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. E2G 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.

Recommended Reading

Fitness-for-Service Expertise

Our mechanical and structural engineering team has completed thousands of FFS assessments for pressure vessels, piping systems, and storage tanks. As pioneers of FFS technologies and lead investigators on many international standards, our team combines their extensive experience with state-of-the-art technology to help extend the lifecycle of your existing assets and create safe operating environments.

Our experts:

  • Have 200+ years of combined field experience
  • Pioneered development of FFS
  • Are primary authors on WRC 528 and WRC 562
  • Are lead investigators on API 579 and API 571
  • Provide emergency onsite support within 24 hours
  • Are a committee leader – API 579 Part 3 brittle fracture screening task group