Aboveground storage tanks (ASTs) are vital assets in the oil and gas industry, holding crude oil, refined products, chemicals, and water at tank farms, refineries, and terminals around the world. API 653, the industry standard for tank inspection, repair, alteration, and reconstruction, provides the framework for maintaining these assets safely and reliably. This checklist guide covers the key inspection elements that operators and inspectors must address to comply with API 653 and protect against leaks, structural failures, and environmental contamination.
Why Tank Inspection Matters
Storage tanks can hold millions of gallons of hazardous materials. A tank failure can result in massive environmental spills, fires, explosions, and significant financial losses. API 653 establishes minimum requirements for maintaining the integrity of welded or riveted aboveground storage tanks built to API 650 or its predecessors. Regulatory agencies such as the EPA, state environmental agencies, and local fire marshals often reference API 653 as the applicable standard for tank inspection programs. Compliance with this standard is not optional for most operators. Beyond regulatory requirements, a disciplined inspection program extends tank service life, reduces unplanned downtime, and prevents the enormous costs associated with emergency repairs or environmental remediation.
External Visual Inspection
External inspections can be conducted while the tank is in service and provide valuable early warning of developing problems without requiring the tank to be taken out of service.
- Inspect the tank shell exterior for corrosion, pitting, paint deterioration, staining, or bulges that indicate internal issues
- Check shell-to-bottom junction for signs of leakage, wet spots, or staining on the foundation
- Examine the roof condition including floating roof seals, drain systems, pontoons, and fixed roof vents and fittings
- Verify that all tank appurtenances including nozzles, manholes, gauging hatches, and vents are in good condition
- Assess the condition of external coatings, insulation jacketing, and heat tracing systems where installed
Foundation and Settlement Inspection
The tank foundation supports the entire weight of the tank and its contents. Foundation problems can lead to shell distortion, floor stress, and ultimately tank failure.
- Measure tank shell settlement using precision leveling equipment and compare against API 653 settlement criteria
- Inspect the ringwall foundation for cracking, spalling, erosion, or displacement
- Check for evidence of undermining, washout, or vegetation growth around the foundation perimeter
- Verify that the tank pad drainage system is functioning and directing water away from the foundation
- Document and trend settlement readings over time to identify progressive settlement patterns
Shell Thickness Inspection
Shell plate thickness measurements are the primary means of determining the remaining strength of the tank shell and calculating the next inspection interval under API 653.
- Perform ultrasonic thickness measurements on each shell course at predetermined grid patterns or areas of suspected corrosion
- Calculate minimum required shell thickness using API 653 formulas based on product specific gravity, fill height, and material properties
- Identify and document areas of localized corrosion, pitting, or lamination that may require repair or closer monitoring
- Compare current thickness readings against previous inspection data to determine corrosion rates
- Establish the next inspection interval based on calculated remaining life for each shell course
Floor Inspection
Floor inspection requires the tank to be taken out of service and cleaned. It is typically the most comprehensive and critical part of an API 653 internal inspection.
- Conduct visual inspection of the entire floor surface after cleaning for corrosion, pitting, and perforation
- Perform magnetic flux leakage (MFL) scanning of the tank floor to detect underside corrosion and thinning
- Measure floor plate thickness at representative locations and at areas identified by MFL scanning
- Inspect floor-to-shell welds and annular plate condition including weld undercut and edge corrosion
- Check for floor settlement or depressions that could trap water and accelerate underside corrosion
Roof Inspection
The type of roof, whether fixed cone, dome, or floating, determines the specific inspection requirements. Each roof type presents unique integrity concerns.
- For floating roofs, inspect pontoons, deck plates, seals, drain systems, and anti-rotation devices for damage and wear
- For fixed roofs, examine roof plates, rafters, columns, and support structures for corrosion and structural adequacy
- Check roof fittings including pressure-vacuum vents, gauge hatches, sample ports, and foam chambers
- Inspect internal floating roof support legs, deck seals, and rim vents for correct function and condition
Appurtenances and Accessories
Tank nozzles, valves, manholes, and other appurtenances are potential leak paths and must be inspected as part of the overall tank evaluation.
- Inspect all nozzle reinforcement pads, welds, and flange faces for corrosion, distortion, or leakage
- Verify that all shell and roof manholes have functional gaskets and proper bolting
- Check overflow protection systems and high-level alarms for correct setpoints and operability
- Inspect tank mixers, heating coils, and internal structures for corrosion and mechanical integrity
- Verify that cathodic protection systems for the tank bottom are operational and providing adequate protection
Leak Detection and Containment
Secondary containment and leak detection systems provide the last line of defense against environmental releases from storage tanks.
- Inspect secondary containment dikes, berms, or walls for structural integrity and sufficient capacity
- Verify containment area drain valves are closed and locked during normal operations
- Check tank bottom leak detection systems including release prevention barriers and monitoring wells
- Confirm that spill response equipment and materials are available and accessible near the tank farm
Best Practices
- Maintain a tank integrity database that tracks all inspection data, thickness readings, corrosion rates, and repair history for every tank
- Use risk-based inspection (RBI) methodology per API 580/581 to prioritize inspection resources on highest-risk tanks
- Schedule internal inspections during planned turnarounds to minimize production impact and coordinate with cleaning requirements
- Engage API 653 certified inspectors for all assessments and ensure inspection reports meet the documentation requirements of the standard
- Implement routine operator rounds that include external tank inspection as a daily or shift-based activity
- Establish repair and alteration procedures that comply with API 653 Section 9 requirements including welding, materials, and NDE
How Checksheets Helps
Checksheets provides API 653-aligned inspection checklist templates that guide inspectors through every required element of external, internal, and in-service tank inspections. Inspectors can record thickness readings, annotate photographs directly on tank diagrams, and generate detailed inspection reports from the field. The platform automatically calculates corrosion rates and next inspection dates based on entered data, helping operators stay ahead of their inspection schedules. Integration with asset management systems ensures that inspection findings flow directly into maintenance planning workflows. With Checksheets, tank farm operators can manage their entire fleet of storage tanks from a single platform with full audit trail and compliance documentation.