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
- Flat spots
Widespread Shell Distortion
- 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:
- 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
E2G's Role in Advancing FFS Methodologies
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.
- 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