14.2 Tank Settlement Types: Uniform, Tilt, Out-of-Plane & Edge Settlement (API 653 Annex B)

Introduction to Tank Foundation Settlement Dynamics

Aboveground atmospheric storage tanks are among the heaviest industrial structures erected directly upon geotechnical subgrades without deep driven piles. A large crude storage tank (e.g., $280\text{ ft}$ in diameter by $60\text{ ft}$ tall) transmits an immense hydrostatic fluid load—exceeding $150,000\text{ tons}$—over a relatively thin steel footprint. Because foundation designs typically utilize concrete ringwalls with compacted gravel cores, asphalt-capped pads, or stabilized earthen berms, foundation settlement over decades of operation is virtually inevitable.

While modest, uniform foundation movement is acceptable, non-uniform settlement generates severe secondary stress fields, distorts the cylindrical shell, misaligns floating roofs, and imposes intense localized plastic strain on bottom plates and nozzle connections. API Standard 653 Annex B (Evaluation of Tank Bottom Settlement) provides the governing engineering framework for measuring, categorizing, and evaluating foundation settlement.

+-------------------------------------------------------------------------+
|               FOUR FUNDAMENTAL TYPES OF TANK SETTLEMENT                 |
|                                                                         |
|   1. UNIFORM SETTLEMENT                     2. PLANAR (RIGID BODY) TILT |
|      (No Shell Stress; Piping Hazard)          (Flat Plane Rotation)    |
|        +-------------+                           +-------------+        |
|        |             |                           |\             \       |
|        |    TANK     |                           | \    TANK     \      |
|        +-------------+                           +--\-------------+     |
|        |=============| Foundation                |===\=============|    |
|        vvvvvvvvvvvvvvv Even Drop                 v    vv    vvv   vvvv  |
|                                                  Differential (Planar)  |
|   3. OUT-OF-PLANE SETTLEMENT                4. EDGE SETTLEMENT          |
|      (Differential Shell Warping)              (Localized Floor Creep)  |
|        +---~~~-------+                           +-------------+        |
|        |  (Wavy)     |                           |   |     |   |        |
|        |             | Shell                     |   |TANK |   | Shell  |
|        +-~~-----+----+ Buckling                  +---+     +---+ Wall   |
|        ====\___/======                           |   |     |   |        |
|            Out-of-Plane Local Low Spot           +---|~~~~~|---+        |
|                                                     Edge Settlement     |
+-------------------------------------------------------------------------+

Geotechnical Mechanics of Settlement

Settlement is driven by three primary geotechnical mechanisms:

1. Primary Consolidation: The slow expulsion of pore water from saturated cohesive clay strata beneath the tank pad under sustained hydrostatic load, taking years or decades to stabilize.
2. Secondary Compression (Creep): Long-term plastic adjustment and reorientation of soil particles under constant effective overburden stress.
3. Differential Soil Bearing and Void Formation: Variations in soil bearing capacity across a footprint (e.g., half the tank founded on bedrock and half on soft alluvial silt, or localized subgrade erosion caused by underground piping leaks or rainwater ingress beneath the bottom).

1. Uniform Settlement & 2. Planar (Rigid Body) Tilt

Uniform Settlement

Uniform settlement occurs when the entire tank foundation—including the perimeter shell footing and the internal sub-base—subsides downward into the subgrade by an identical distance across its entire footprint.

• Structural Shell and Bottom Impact: Because the vessel descends as a pure rigid body, uniform settlement induces zero differential stress, zero membrane shear, and zero bending distortion in the cylindrical shell or bottom plates. The tank retains its true circular geometry and vertical plumbness.
• Piping and Appurtenance Hazards: While the tank structure itself is undamaged, uniform settlement poses severe hazards to external connected infrastructure. Rigidly anchored product piping, firewater deluge lines, foam risers, electrical conduits, and drain connections that tie the settling tank to stationary pipe racks, pump pads, or containment dikes are subjected to immense vertical shear forces and bending moments. Unmitigated uniform settlement can shear nozzle attachment necks, crack nozzle reinforcing pad fillet welds, or rupture external valves.
• Operational and Drainage Consequences: Uniform settlement can lower the tank bottom below surrounding drainage swales or water tables, causing external water to pool against the shell-to-bottom corner weld, accelerating external corrosion, or preventing gravity emptying of bottom sumps.

+-------------------------------------------------------------------------+
|               PIPING STRAIN INDUCED BY UNIFORM SETTLEMENT               |
|                                                                         |
|          Settled Tank Shell Course           Stationary Pipe Rack       |
|                 |                                      |                |
|                 |     Nozzle Neck                      |                |
|                 |    +------------+                    |                |
|                 |====|            |==========+         |                |
|                 |    +------------+          |         |                |
|                 |            Bending Moment  |         |                |
|                 v                   M        v         v                |
|           [DOWNWARD SHIFT]                   ====================       |
|                                                Stationary Manifold      |
|                                              (Zero Vertical Movement)   |
+-------------------------------------------------------------------------+

Planar Tilt (Rigid Body Tilt)

Planar tilt occurs when the tank foundation tilts as a completely flat, planar surface around a neutral axis of rotation. The settlement profile around the circumference follows a pure, single-cycle trigonometric cosine wave:

$U_i = a + b \cos(\theta_i - \beta)$

• Shell Stress Characteristics: Because the foundation remains a flat plane without warping, planar tilt does not induce secondary out-of-plane bending stresses or membrane distortion in the shell plates. The circular cylinder remains circular; it simply operates at an angle to the gravitational vertical.
• Hydrostatic Liquid Level Redistribution: Tilting shifts the liquid gravity head toward the low side. For a tilt angle $\alpha$, the effective liquid head on the low side increases, reducing designed shell freeboard and increasing circumferential membrane hoop tension ($S = 2.6 D H G / t$) on the lower shell courses.
• Floating Roof Binding and Derailment: Planar tilt is exceptionally hazardous to external and internal floating roofs:
  • The cylindrical shell tilts relative to gravity, while the floating roof always floats perfectly horizontal on the liquid surface.
  • This geometric divergence causes the horizontal floating roof to contact the tilted shell wall, squeezing the perimeter rim seal assembly on the high side while opening an evaporative gap on the low side.
  • Severe tilt causes floating roof rim shoe jamming, puncturing vapor fabric seals, bending roof support legs, and derailing vertical rolling ladders from their track assemblies.

3. Out-of-Plane Settlement (Differential Shell Settlement)

Unlike planar tilt, out-of-plane settlement (often called differential shell settlement) occurs when points around the shell circumference settle non-uniformly, deviating from a true flat tilted plane. The shell circumference is forced into a wavy, distorted profile.

+-------------------------------------------------------------------------+
|            OUT-OF-PLANE DIFFERENTIAL SETTLEMENT PHENOMENOLOGY           |
|                                                                         |
|            Planar Tilt Baseline (Fitted Cosine Plane)                   |
|            - - - - - - - - - - - - - - - - - - - - - - - - - - -        |
|                     \                               /                   |
|                      \  Out-of-Plane               /                    |
|                       \ Deflection (Si)           /                     |
|                        v                         v                      |
|            ~~~~~~~~~~~~~\_______________________/~~~~~~~~~~~~~~~~~      |
|                      Circumferential Low Spot / Dip                     |
|                                                                         |
|   Structural Consequences:                                              |
|   - Circumferential Shell Bending Moments (M_circ)                      |
|   - In-Plane Membrane Shear Stresses (Tau_xy)                           |
|   - Upper Shell Course Ovalization & Localized Buckling                 |
|   - Floating Roof Binding & Rim Seal Lock-up                            |
+-------------------------------------------------------------------------+

Mechanics of Shell Distortion and Buckling

Because a thin cylindrical shell possesses high membrane stiffness but very low bending rigidity, differential foundation settlement forces the cylinder to twist and warp:

1. In-Plane Membrane Shear: When a localized section of the foundation subsides, the adjacent shell plates must carry the unsupported weight of the shell above by acting as a deep beam. This transfers huge shear stresses ($\tau_{xy}$) across vertical and horizontal butt-welded shell seams.
2. Circumferential Bending and Ovalization: The differential downward deflection pulls the top of the shell inward or pushes it outward, turning the circular cross-section into an ellipse or multi-lobed oval. Shell out-of-roundness disrupts wind girder stability and jams floating roof pontoon travel.
3. Localized Shell Buckling: In thin upper courses (where thickness is typically $5/16\text{ in.}$ to $3/8\text{ in.}$ / $8\text{ to }10\text{ mm}$), out-of-plane settlement introduces axial compressive membrane stresses. When compressive stress exceeds the critical elastic buckling threshold (per Timoshenko cylinder formulas), the shell plates buckle, forming prominent diagonal diamond wrinkles or vertical wave indentations.
4. Nozzle Rotation: Nozzles located near differential settlement troughs experience severe rotational angular deflection, twisting connected flange faces and causing catastrophic gasket blowouts.

4. Edge Settlement, Bottom Settlement Near the Shell & Bottom Settlement Remote from the Shell

Edge Settlement (API 653 Annex B.2.3)

Edge settlement is one of the most critical structural defects evaluated under API 653. It is defined as a localized, steep depression of the bottom plate occurring immediately adjacent to the shell-to-bottom corner weld, extending radially inward over a relatively short distance ($R$).

+-------------------------------------------------------------------------+
|               CROSS-SECTION OF PERIMETER EDGE SETTLEMENT                |
|                                                                         |
|      Tank Shell Wall Course                                             |
|            |   |                                                        |
|            |   |                                                        |
|            |   |                                                        |
|            |   | Inner Fillet Weld                                      |
|            |   | v                                                      |
|     =======+---+===------------------                                   |
|     Outside  |     \                 \                                  |
|     Annular  |      \                 \  Original Floor Level           |
|     Plate    |       \                 +----------------------          |
|     Projection       \                 |                                |
|                       \_______________/   Depth (B)                     |
|                       <------ R ------>                                 |
|                       Radial Settled Width                              |
|     +----------------+                                                  |
|     | Concrete       | Soil Washout / Subgrade Settlement               |
|     | Ringwall       |                                                  |
|     +----------------+                                                  |
+-------------------------------------------------------------------------+

• Physical Causes: Edge settlement typically develops in tanks with concrete ringwalls when the internal soil core consolidates or settles away from the rigid, unyielding concrete wall. Alternatively, in tanks on earthen foundations without ringwalls, rainwater cascading off the shell erodes the exterior earthen shoulder, or excessive soil bearing pressure causes foundation edge shear failure.
• The Critical Zone Threat: The region extending radially inward $3\text{ inches}$ ($75\text{ mm}$) from the shell-to-bottom corner weld is designated as the Critical Zone (API 653 Section 3 definitions and Annex B.2.3). Under edge settlement:
  • The bottom plate acts as a cantilever beam loaded by full liquid head, sagging downward over the settlement trough.
  • This sharp localized curvature creates intense plastic bending strains combined with membrane hoop and radial tension.
  • Severe tension concentrates at the toe and root of the shell-to-bottom fillet weld. In older tanks constructed from low-toughness steels, this stress concentration can trigger sudden brittle fracture or ductile weld toe tear-out.
• Measurement (B.2.3.1 and B.2.3.2): Figure B.6 defines the measured settlement $B$ and the radial width $R$, and shows the straight-edge technique for locating the breakover point where the settled area begins. Figure B.7 gives the correction for cone-up and cone-down bottoms: $B$ must be measured from a projection of the unsettled bottom, never from a level line run from the breakover point to the shell.
• Weld Seam Orientation Factor: If a bottom plate weld seam (especially a three-plate lap joint) traverses the edge settlement zone, the risk escalates dramatically. Bending a lap weld across its longitudinal axis opens the fillet weld root, causing rapid seam rupture.

Bottom Settlement Near the Tank Shell (API 653 Annex B.2.4)

API 653 carries a third bottom-settlement category that candidates routinely collapse into edge settlement. Annex B.2.4 covers bottom settlement near the tank shell — a depression or bulge close to, but not at, the shell-to-bottom junction. Figure B.8 illustrates it and defines the measured quantities the same way the remote case does: $R$ is the radius of the inscribed circle in the bulged or depressed area, and $B$ is the depth of the depression or the height of the bulge.

• Evaluation route (B.2.4.2): use the equation given in B.3.3 — the same $B_B = 0.37R$ relationship used for localized settlement remote from the shell — or, alternatively, carry out a rigorous stress analysis of the deformed profile.
• Why it is not edge settlement: edge settlement (B.2.3) is measured from the shell inward to a breakover point and is judged against the $B_{ew}$ / $B_e$ curves of B.3.4. Settlement near the shell under B.2.4 is an inscribed-circle depression or bulge and is judged with the B.3.3 equation. Picking the wrong route — B.3.4 curves for a B.2.4 bulge, or the 0.37R equation for a true edge settlement — is a classic exam error.

Bottom Settlement Remote from Shell (Bulges and Depressions)

Bottom settlement occurring well away from the shell (beyond the 3-inch Critical Zone) is classified as internal bottom depressions or bulges (API 653 Annex B.2.5, Localized Bottom Settlement Remote from the Tank Shell).

• Mechanics: These are localized, dish-shaped depressions or mounds formed by localized soil consolidation, organic decay, or sub-bottom void formation. Provided the curvature is smooth and gradual, bottom plates can undergo substantial plastic stretching without through-wall rupture.
• Operational Risks:
  • Stagnant Water Pockets: Depressions create localized low spots where water, sediment, and sludge accumulate. Stagnant acidic water pools beneath hydrocarbon inventory, causing severe accelerated localized pitting and Microbiologically Induced Corrosion (MIC).
  • Floating Roof Support Leg Punctures: When the floating roof lands on its support legs during an outage or draining, an uneven bottom can cause a leg to land squarely on an elevated ridge or concentrated slope, transmitting high concentrated point loads that can punch through a thinned bottom plate.

Survey Procedures and Measurement Station Layout

To quantitatively evaluate tank settlement in accordance with API 653 Annex B, systematic geometric elevation surveys must be conducted prior to hydrotesting, during initial water filling, during out-of-service turnarounds, or whenever structural distortion is suspected.

+-------------------------------------------------------------------------+
|               CIRCUMFERENTIAL SURVEY STATION SPACING LAYOUT             |
|                                                                         |
|                                 Point 1 (0 deg)                         |
|                                        o                                |
|                                  .  '     '  .                          |
|                           o                         o Point 2           |
|                       Point 8 \                     /                   |
|                              \       TANK       /                       |
|                     Point 7 o |   DIAMETER (D)  | o Point 3             |
|                              /                   \                      |
|                       Point 6 /                     \                   |
|                           o                         o Point 4           |
|                                  '  .     .  '                          |
|                                        o                                |
|                                 Point 5 (180 deg)                       |
|                                                                         |
|       Spacing Between Points: L <= 32 ft (10 m)  [API 653 12.5.2]       |
|       Number of Points: N = D / 10 (Minimum N >= 8)                     |
+-------------------------------------------------------------------------+

Circumferential Station Layout Rules (API 653 12.5.2, Annex B.2.1, Figures B.1 and B.2)

The governing clause is API 653 12.5.2, Initial Settlement Survey, not Annex B: a settlement survey required by 12.5.1 shall initially be taken with the tank empty, using an even number of elevation measurement points $N$ uniformly distributed around the circumference. That baseline matters — in the absence of an initial survey, the tank shall be assumed to have been initially level. Annex B.2.1 then covers measurement practice, and Figure B.1 and Figure B.2 show the minimum recommended measurement locations on the tank shell and bottom plate respectively. Circumferential elevation points are surveyed on the lower exterior shell projection or bottom annular ring extension immediately adjacent to the shell wall:

1. Number of Survey Points ($N$): The minimum number of equally spaced measurement stations around the shell circumference is determined by the formula:

$N = \frac{D}{10}$

Where:

• $D$ is the tank diameter in feet.
• Regardless of diameter, the absolute minimum number of survey points is $N \ge 8$.
• All values of $N$ shall be rounded to the next higher even whole number — a 130-ft tank gives $N = 13$, which rounds to 14, not to 16. The even-number rule exists so that stations fall on opposed diameters; Figure B.1 also notes there must be at least 4 equally spaced diametrical measurement lines.

1. Maximum Spacing ($L$): The arc length distance ($L$) between adjacent circumferential measurement stations must not exceed $32\text{ ft}$ ($10\text{ m}$):

$L = \frac{\pi D}{N} \le 32\text{ ft}$

1. Elevation Benchmarks: All elevation readings must be established using a high-precision optical surveyor's level, digital barcode level, or total station referenced to a permanent, unyielding external benchmark. The benchmark must be physically located well outside the zone of geotechnical influence of the tank foundation (typically at least $100\text{ to }200\text{ ft}$ away from the tank shell).

Internal Floor Survey Layout (Annex B.2.1, Figure B.2)

When the tank is out of service and decontaminated, an internal floor survey is executed: Annex B.2.1 warns that bottom measurements are easy to get wrong. Where the bottom is distorted or corroded beyond the shell, shell readings taken near bottom lap welds can produce significant elevation errors, and repaired, replaced, or slotted-in bottoms may not sit parallel to the original bottom — in those cases surveying the weld between the first and second shell courses gives more consistent results. Likewise, a bottom that is not in firm contact with the soil can badly over- or under-state edge and bottom settlement; when a reading sits near the allowable, repeat it with the bottom forced down onto the foundation or take a second set where contact is firm.

• Radial Survey Lines: Elevation survey lines originate from the tank center and extend outward radially to each exterior shell measurement station ($N$ radial lines).
• Measurement Intervals: Points are recorded at regular radial intervals (typically every $5\text{ to }10\text{ ft}$) along each line, supplemented by concentrated elevation grids over visible depressions, annular plate laps, and bottom sumps.