Shell out-of-plane settlement (cosine-curve fit) and localized bottom bulge/depression evaluation
◉ API 653 ANNEX B STYLE CHECK · NOT A REGULATORY SUBMISSION TOOL
⚠ Indicative tool only. Formulas are sourced directly from API 653 Annex B (Evaluation of Tank Bottom Settlement) — B.3.2 (shell out-of-plane settlement), B.3.3 (localized bottom bulge/depression), and B.3.4.4 (edge-settlement angle interpolation). The edge-settlement allowable limits themselves (Figures B-10 and B-11) are given only as graphs in the source, not formulas — this tool does not digitize those curves, so allowable edge settlement values must be entered manually (read off the graphs or from another source) rather than computed. Confirm this Annex is still current in your edition of API 653 before relying on this for a real assessment.
Identification
Research entries stay private to you for practice/testing. Switch to Official once this is a real assessment your company should see.
Shell Out-of-Plane Settlement (B.2.2, B.3.2)
Enter elevation survey readings around the tank shell (at least 8 points, equally spaced per API 653 Fig. B-1). The tool fits an optimum cosine curve by least squares, computes each point's out-of-plane deflection Si, and checks it against the allowable limit.
Leave at 1 for immediate neighbors (standard case). Per B.2.2.4's note, increase this if you took more measurement points than needed and want S evaluated using a wider-spaced subset (e.g. 2 = every other point).
#
Angle °
Measured Elev/Settlement ft
Predicted (cos. curve)
Ui
Si
|Si|/Smax
Cosine Curve Fit
a (mean elevation)—
b (amplitude)—
c (phase, rad)—
R² (valid if ≥0.9)—
Result Not evaluated
Arc length L (between S-calc points)—
Allowable deflection Smax—
Maximum |Si| observed—
Localized Bottom Bulge / Depression (B.3.3)
Applies to bottoms with single-pass lap-welded joints. BB = 0.37 × R.
Result Not evaluated
Allowable—
Measured—
Surplus / deficit—
Edge Settlement (B.2.3, B.3.4)
Allowable edge settlement (Bew for welds parallel to the shell, Be for welds perpendicular) comes from Figures B-10/B-11 in the source — graphs only, no formula given, so enter the values you've read off those figures (or from elsewhere) rather than expecting this tool to compute them. The angle-interpolation formula (B.3.4.4) itself is exact and is computed here.