What are the API 653 settlement limits for storage tanks?

API 653 Section 12.3 provides settlement evaluation criteria. The maximum allowable differential settlement between adjacent measurement points is typically limited to 1 inch per 10 feet of arc length for rigid bottom tanks and 2 inches per 10 feet for flexible bottoms. Planar tilt is limited to 1/100 to 1/200 of the tank diameter depending on diameter, and uniform settlement is generally acceptable up to 6-12 inches if uniform and the connected piping can accommodate it.

How is out-of-plane settlement index calculated?

The out-of-plane settlement index measures the deviation of each measurement point from the best-fit tilted plane of the tank bottom. After removing the planar tilt component from the settlement data, the remaining deviations represent true localized distortion. The out-of-plane index is the maximum deviation divided by the arc length between adjacent measurement points, expressed in inches per 10 feet.

What foundation types are used for storage tanks?

The three common tank foundation types are ringwall (concrete ring supporting the shell with compacted fill center), mat/slab (full concrete pad under the entire tank bottom), and gravel pad (compacted crushed stone ring or full pad). Ringwall foundations are most common for large tanks, slab foundations for heavy or critical-service tanks, and gravel pads for small field tanks or where soil conditions are favorable.

How often should tank settlement be monitored?

API 653 recommends settlement monitoring during hydrostatic test (before, during fill, at full, and after drainage), then annually for the first 5 years after construction, and every 5 years thereafter for tanks on stable foundations. Tanks showing active settlement should be monitored quarterly or semi-annually. Settlement rate exceeding 1 inch per year typically warrants engineering investigation and more frequent monitoring.

Tank Settling / Foundation Calculator | API 653 Settlement Evaluation

Tank Geometry

Tank Diameter

Tank Shell Height

Estimated Shell Weight

lbs

Total weight of shell, roof, and appurtenances

Maximum Product Weight

lbs

Weight of stored product at maximum fill level

Foundation

Foundation Type

Soil Bearing Capacity

psf

Allowable bearing: 1500-3000 psf (clay), 3000-6000 psf (sand), 6000+ (rock)

Survey Date

Previous Survey Date (optional)

Settlement Readings (8 Points Around Circumference)

Enter settlement readings at 8 equally-spaced points (every 45°) around the tank shell perimeter, starting from North (Point 1) and proceeding clockwise. Enter elevation or settlement values in inches. Positive values = downward settlement.

Point 1 (N, 0°)

Point 2 (NE, 45°)

Point 3 (E, 90°)

Point 4 (SE, 135°)

Point 5 (S, 180°)

Point 6 (SW, 225°)

Point 7 (W, 270°)

Point 8 (NW, 315°)

Understanding Tank Settlement

Settlement Types
Uniform settlement is generally acceptable if piping can accommodate it. Differential and tilting settlement cause shell distortion, bottom plate stress, and seal problems on floating-roof tanks.

API 653 Criteria
Section 12.3 provides maximum allowable differential settlement based on tank diameter and bottom flexibility. The out-of-plane index is the key metric for localized distortion.

Survey Practice:
Settlement is measured by surveying benchmarks welded to the tank shell at equally-spaced points around the circumference. A minimum of 8 points is standard; 16 points may be used for large or critical tanks.

Learn the Theory

Understand tank settlement types, API 653 evaluation criteria, foundation design, and remediation methods

Read Engineering Guide →

Results

Overall Assessment

Uniform Settlement -

Max Differential Settlement -

Planar Tilt -

Tilt Direction -

Max Edge Settlement -

Out-of-Plane Index -

Settlement Rate -

Foundation Bearing Stress -

Formula

S_diff = |S_i - S_i+1| / L × 10

S_diff = Differential settlement (in/10 ft)

S_i = Settlement at point i (in)

L = Arc length between adjacent points (ft)

OOP = S_measured - S_plane

Tilt = (S_max - S_min) / D

Standards & References

API 653
Tank Inspection, Repair, Alteration, and Reconstruction
API 653 Section 12.3
Foundation and Settlement Evaluation
API 650 Appendix B
Foundation Design Recommendations
API 650 Appendix E
Seismic Design of Storage Tanks
ACI 376
Code Requirements for Design and Construction of Concrete Structures for Containment of Refrigerated Liquefied Gases

Engineering Notes

8 Points minimum: API 653 recommends a minimum of 8 equally-spaced measurement points; 16 points for D > 150 ft
Differential limit: 1"/10 ft arc for rigid bottoms; 2"/10 ft for flexible bottoms
Planar tilt limit: Typically D/200 to D/100 depending on tank service and age
Settlement rate: > 1"/yr warrants investigation; > 2"/yr requires corrective action
Floating-roof tanks: Excessive tilt causes seal binding and uneven gap
Piping flexibility: Uniform settlement acceptable if piping connections can accommodate movement

Tank Settling / Foundation Calculator

Tank Geometry

Foundation

Settlement Readings (8 Points Around Circumference)

Understanding Tank Settlement

Learn the Theory

Results

Formula

Standards & References

Engineering Notes

Related Calculators

API 650 Tank Wall Thickness

Tank Volume

Corrosion Rate

Hoop Stress