9.3 Bottom Pitting Evaluation & Remaining Thickness Before Next Inspection

Morphologies and Drivers of Tank Bottom Pitting

Pitting corrosion represents the primary failure mode of aboveground storage tank bottoms. Unlike broad uniform metal loss, which thins the plate evenly, pitting forms localized galvanic cells that penetrate deeply into the plate wall. An API 653 Authorized Inspector must distinguish between two fundamentally different pitting regimes:

+-------------------------------------------------------------------------+
|                    BOTTOM PLATE PITTING COMPARISON                      |
+-----------------------+-------------------------------------------------+
| Parameter             | Top-Side (Internal) Pitting                     |
+-----------------------+-------------------------------------------------+
| Primary Drivers       | Stagnant water bottoms, BS&W sludge, acid salts,|
|                       | Microbiologically Influenced Corrosion (MIC)    |
| Morphology            | Steep, narrow, conical or hemispherical craters |
| Distribution          | Concentrated in low spots, water draws, sumps   |
| Visual Visibility     | Readily accessible to VT after floor cleaning   |
+-----------------------+-------------------------------------------------+
| Parameter             | Bottom-Side (Soil-Side) Pitting                 |
+-----------------------+-------------------------------------------------+
| Primary Drivers       | Contaminated sand cushions, differential        |
|                       | aeration (clay lumps), CP shielding, moisture   |
| Morphology            | Broad saucer depressions or sharp holiday pits  |
| Distribution          | Under entire floor, near perimeter, under rocks |
| Visual Visibility     | Invisible to VT; detected strictly by MFL/UT    |
+-----------------------+-------------------------------------------------+

1. Top-Side (Product-Side) Pitting

Top-side pitting occurs when water entrained in stored hydrocarbons drops out by gravity and pools on the tank floor. This layer creates an active electrolyte containing dissolved oxygen, chlorides, and sulfur compounds. Underneath accumulated sludge banks, anaerobic colonies of Sulfate-Reducing Bacteria (SRB) such as Desulfovibrio proliferate. These bacteria reduce sulfate ions ($SO_4^{2-}$) to highly corrosive hydrogen sulfide ($H_2S$), creating rapid localized galvanic cells between the iron sulfide ($FeS$) corrosion product and the base metal. Pitting penetration rates can exceed 30 to 50 mpy ($0.030-0.050\text{ in./yr}$).

2. Bottom-Side (Soil-Side) Pitting

Soil-side pitting occurs on the underside of the floor plate in contact with the foundation cushion. When native soils, clay balls, or rocks touch the steel, oxygen cannot diffuse as readily as it does through coarse sand, creating differential aeration cells. The oxygen-starved region beneath a clay ball becomes an active anode, while the surrounding well-aerated steel acts as a cathode, driving rapid localized dissolution. Furthermore, if water infiltrates between an impermeable secondary liner and the tank floor, cathodic protection currents cannot penetrate through the dielectric membrane, shielding the steel and causing severe unmitigated corrosion.

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API 653 Acceptance Criteria for Widely Scattered Pitting

Under API 653 Section 4.4.5.3, widely scattered pitting on bottom plates may be accepted without mechanical repair provided specific fitness-for-service conditions are satisfied:

1. Remaining Thickness at Pit Base: The projected minimum thickness at the bottom of the deepest pit must equal or exceed the required $MRT_{threshold}$ ($0.10\text{ in.}$ for bare bottoms, or $0.05\text{ in.}$ with an RPB or lining) at the completion of the next operating run ($O_r$).
2. Pitting Density and Clustering: Pits must be widely scattered. If pits coalesce into clusters, they form an effective localized thin area (LTA) that undermines the structural load-bearing capacity of the plate.
3. Annular Plate Restrictions: Within the annular plate ring, pitting cannot reduce the plate thickness below the minimum allowable values of API 653 Table 4.4.

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Magnetic Flux Leakage (MFL) Floor Scanning and Limitations

To detect localized pitting across thousands of square feet of tank flooring, non-destructive testing relies on Magnetic Flux Leakage (MFL) floor scanners.

                 Direction of Travel --->
         [ Permanent Magnet ]=====[ Permanent Magnet ]
                  |                         |
                  v   Magnetic Flux Lines   v
    ========================+====+========================  <-- Tank Floor Plate
                            |    |  Magnetic Flux Leakage
                            \____/  Detected by Hall-Effect Sensors
                         Localized Pit

Operating Principle

An MFL scanner utilizes high-strength neodymium permanent magnets to drive magnetic flux lines through the carbon steel floor plate until near-saturation is achieved. When the magnetic field encounters an abrupt volume reduction (such as a top-side pit, bottom-side pit, or gouge), the magnetic flux can no longer be contained within the steel and "leaks" into the air above the plate. Arrayed Hall-effect sensors or inductive coils detect this leakage flux, converting it into a proportional electrical signal that maps defect locations.

Critical MFL Limitations and Dead Zones

While MFL provides high-speed scanning (up to $1\text{ m/s}$), an API 653 inspector must account for severe operational limitations:

1. Perimeter Dead Zones: Due to scanner chassis geometry and magnetic attraction forces, MFL drive heads cannot scan within 1 to 2 inches (25 to 50 mm) of lap weld seams, sketch plate laps, column bases, and the shell-to-bottom corner weld. This uninspected dead zone overlaps directly with the Critical Zone, necessitating supplemental manual ultrasonic testing (UT).
2. Qualitative Nature: MFL signals correlate with total volumetric metal loss rather than true physical pit depth. A broad, shallow depression can generate the same voltage amplitude as a sharp, deep pit.
3. Mandatory Ultrasonic Proving: MFL is strictly a screening tool. Every critical indication exceeding the calibration threshold (e.g., $>20%$ or $>40%$ nominal wall loss) must be quantitatively verified and sized using manual or automated Ultrasonic Testing (UT) depth gages.

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Statistical Extrapolation: Extreme Value Analysis (EVA)

In large storage tanks, comprehensive 100% floor coverage is rarely achievable. Sump assemblies, internal heating coil supports, roof support columns, and structural piping create physical obstructions that prevent scanner access.

To establish integrity confidence across uninspected areas, API 653 and asset integrity engineers utilize Extreme Value Analysis (EVA) based on the Gumbel Extreme Value Distribution.

Mathematical Formulation

In any plate subset, the maximum pit depths ($x$) follow a double exponential cumulative probability distribution:

$F(x) = \exp\left(-\exp\left(-\frac{x - u}{\alpha}\right)\right)$

Where:

• $F(x)$: Probability that the deepest pit depth in a sampled area is less than or equal to $x$.
• $u$: Location parameter (the characteristic mode of maximum pit depth).
• $\alpha$: Scale parameter (a measure of statistical dispersion or variability).

Estimating Maximum Probable Pit Depth in Unscanned Zones

If an inspector samples $N$ accessible floor plates and records the deepest pit on each plate, $u$ and $\alpha$ can be determined via linear regression on a Gumbel probability plot:

$y = -\ln(-\ln(F)) = \frac{x - u}{\alpha} \implies x = u + \alpha \cdot y$

To predict the absolute deepest pit ($x_{max}$) expected across the total floor area (comprising $T$ total plates, where $T > N$), the return period reduced variate is calculated as:

$y_T = -\ln\left(-\ln\left(1 - \frac{1}{T}\right)\right) \approx \ln(T)$

$x_{max} = u + \alpha \ln(T)$

If the statistically projected deepest pit depth $x_{max}$ leaves a remaining thickness ($t_{nominal} - x_{max}$) that falls below $MRT_{threshold}$ at the end of the operating run, the inspector must mandate proactive plate repairs or interval reduction, even if no through-wall hole was physically observed in scanned zones.

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Bottom Repair Methodologies: Overlay, Patch Plates and Replacement

When bottom plate evaluation reveals pitting or thinning exceeding allowable limits, API 653 Section 9.10 governs acceptable remediation techniques:

+-------------------------------------------------------------------------+
|                   API 653 BOTTOM REPAIR METHODOLOGIES                   |
+--------------------+----------------------------------------------------+
| Repair Technique   | Key Engineering Requirements & Limits (Section 9.10)|
+--------------------+----------------------------------------------------+
| 1. Weld Overlay    | - Minimum remaining plate thickness >= 0.10 in.    |
|                    | - Restores isolated top-side pitting only          |
|                    | - Low-hydrogen electrodes (E7018); ground smooth   |
+--------------------+----------------------------------------------------+
| 2. Welded Lap      | - Minimum patch thickness: 3/16 in. (0.1875 in.)   |
|    Patch Plates    | - Minimum 2-in. corner radius (no sharp corners!)  |
|                    | - Minimum dimension >= 4 in.; 1-in. defect overlap |
|                    | - PROHIBITED in 3-in. Critical Zone                |
|                    | - >= 3-in. clearance from shell corner weld        |
|                    | - >= 2-in. or 5t clearance from bottom seams       |
+--------------------+----------------------------------------------------+
| 3. Complete Bottom | - Slotted shell replacement (6 to 12 in. slot)     |
|    Replacement     | - Full bottom removal & replacement                |
+--------------------+----------------------------------------------------+

1. Weld Overlay Remediation

Weld overlay involves depositing weld metal directly into localized top-side pit depressions to restore base plate thickness.

• Thickness Constraint: To prevent weld arc blow-through and severe localized thermal stress, weld overlay is strictly restricted to areas where the remaining base metal thickness is at least 0.10 in. (2.5 mm).
• Finishing: The deposited weld metal must be ground smooth and flush with the surrounding plate surface to prevent stress concentrations and ensure an acceptable profile for protective lining application or future ultrasonic scanning.

2. Welded Lap Patch Plates (API 653 Section 9.10)

Welded lap patch plates represent the most prevalent bottom repair method.

• Thickness Requirements: Patch plates must be at least $3/16\text{ in.}$ ($0.1875\text{ in.} / 5\text{ mm}$) thick or equal to the nominal thickness of the host bottom plate.
• Geometry and Corner Radius: Patch plates may be circular, oval, or rectangular with rounded corners having a minimum radius of 2 inches (50 mm). Sharp 90-degree corners are strictly prohibited because they act as intense stress raisers, initiating fatigue cracks under cyclical bottom flexure.
• Overlap and Dimensions: The patch plate must extend at least $1\text{ in.}$ ($25\text{ mm}$) beyond the boundary of the corroded area in all directions, and maintain a minimum dimension of at least $4\text{ in.}$ ($100\text{ mm}$).
• Clearance from Critical Zone: Lap patches are strictly prohibited in the 3-inch Critical Zone and must terminate at least $3\text{ in.}$ ($75\text{ mm}$) radially inward from the shell-to-bottom corner fillet weld.
• Clearance from Existing Seams: Patches must maintain at least $2\text{ in.}$ ($50\text{ mm}$) or $5t$ clearance from existing three-lap or two-lap joints, or cross the seam at an angle $\ge 45^\circ$ and extend $6\text{ in.}$ beyond the weld seam.

3. Tank Bottom Replacement Options

When pitting is pervasive across multiple sketch plates, localized patching becomes uneconomical, necessitating complete floor replacement:

• Slotted Shell Installation (Double Bottom): A horizontal slot is cut through the lowest shell course approximately 6 to 12 inches (150 to 300 mm) above the existing floor. New annular and sketch plates are slid through the slot, a new sand cushion or concrete slurry with an impermeable liner is installed over the old bottom, and a new shell-to-bottom corner weld is deposited. This avoids the immense labor of removing the old bottom and provides an immediate secondary containment interstitial space.
• Complete Bottom Removal: The old floor plates are cut out with torches, the foundation is re-graded and compacted, cathodic protection anodes are replaced, and a brand new single- or double-bottom assembly is welded.

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Documentation and Baseline Thickness Mapping

API 653 Section 13 mandates that all bottom plate inspection findings, non-destructive testing scans, and repair dossiers be maintained in the owner-user's permanent tank integrity file throughout the operational life of the vessel.

A complete bottom evaluation dossier must include:

1. Floor Layout Grid Map: An alphanumeric coordinate drawing numbering every individual floor plate (e.g., Row A through Z, Plate 1 through N), recording plate nominal thicknesses, weld seam types, and sump locations.
2. MFL Calibration Logs: Dynamic calibration records documenting scanner sensitivity adjustments on a reference plate containing machined flat-bottom holes (FBHs) at known depths (e.g., $20%, 40%, 60%, 80%$ wall loss).
3. UT Proving Verification Tables: A cross-referenced inspection log matching MFL defect indications with verified manual ultrasonic thickness readings, identifying exact pit depths and differentiating top-side from bottom-side metal loss.
4. Extreme Value Analysis Report: Statistical probability calculations documenting projected maximum pit depth in uninspected areas.
5. As-Built Repair Drawings: Complete weld maps detailing the exact dimensions, plate thicknesses, heat numbers, and weld seam clearances of all installed lap patches, insert plates, or weld overlays.

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New Bottoms Supported by Grillage (API 650 Annex I)

The Body of Knowledge lists new bottoms supported by grillage, API 650 Annex I, excluding calculations as a topic the inspector must understand, and the exclusion list keeps API 650 Annex I except for I.1 and I.2 outside the exam — so what is needed is the concept and the inspection consequences, not the design arithmetic.

What a Grillage-Supported Bottom Is

Instead of resting the new bottom directly on a sand or asphalt cushion, a grillage raises it on a grid of structural supports — typically steel beams, channels, or a system of parallel runners laid across the foundation — with an open, ventilated interstitial space beneath. The old bottom (or a membrane liner) below the grillage becomes the release prevention barrier.

Why Owners Choose It

• The underside becomes inspectable and ventilated. The single biggest cause of tank floor replacement is soil-side corrosion the inspector cannot see. An open, drained, ventilated space eliminates the trapped-moisture electrolyte that drives differential aeration and MIC.
• Leak detection becomes direct. Any product passing the new bottom drains across the barrier below to a perimeter observation point rather than migrating into the subgrade.
• Cathodic protection is no longer required for the new plate, because the plate is no longer in contact with an electrolyte.

What It Costs, and What the Inspector Must Check

A grillage converts the bottom from a fully supported membrane into a spanning structural element, and that changes the inspection focus completely:

The trade the inspector must understand: grillage buys visibility and drainage of the underside at the cost of introducing bending stress and hard line contacts into a plate that was designed to be uniformly supported. Where API 653 elsewhere assumes a bottom bedded on a cushion — the settlement criteria of Annex B, for instance — those assumptions have to be re-examined for a grillage-supported floor.