3.1 Tank Bottom Types, Annular Plates & Internal Bottom Inspection

3.1 Tank Bottom Types, Annular Plates & Internal Bottom Inspection

Key Inspection Concept: The tank bottom is the primary containment boundary most vulnerable to invisible, severe underside soil-side corrosion. API 653 establishes rigorous mechanical criteria separating standard sketch plate areas from the high-stress Critical Zone, mandating specialized non-destructive examination (NDE) methods to prevent catastrophic bottom ruptures and environmental releases.

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1. Tank Bottom Configurations and Slopes

Aboveground atmospheric storage tank bottoms must provide reliable product containment while accommodating substantial foundation settlement, cyclic hydrostatic loading, and thermal expansion. Tank floors are assembled from carbon steel plates (minimum nominal thickness typically 1/4 in. or 6 mm per API 650) arranged in two primary geometric groupings: central rectangular plates and perimeter sketch plates.

Floor Topography: Cone-Up vs. Cone-Down

Atmospheric storage tank floors are rarely constructed dead flat, except in very small diameter tanks (typically under 30 ft or 9 m). Flat bottoms suffer from uneven localized ponding, irregular plate waviness, and unpredictable sediment accumulation. Large-diameter tanks utilize engineered slopes, typically designed with a gradient of 1 in. vertical rise or fall per 10 to 12 ft of horizontal run (approximately a 1:120 to 1:100 slope, or 0.8% to 1.0%):

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2. Annular Plate Rings: Engineering, Function & API Requirements

The Mechanics of Annular Plates

When a storage tank is filled with liquid product, hydrostatic pressure pushes the cylindrical shell courses radially outward. However, the tank bottom plates resting on the foundation restrain the bottom edge of the lowest shell course. This mechanical restraint creates an intense rotational bending moment and high shearing stress at the shell-to-bottom corner weld joint.

To prevent fatigue cracking and plastic tearing of standard lap-welded floor plates, modern tanks utilize an annular plate ring. The annular ring consists of a continuous, circular band of heavy, butt-welded radial plates installed directly beneath the vertical shell courses. These plates act as an elastic cantilever beam, distributing high bending moments and seismic overturning stresses smoothly into the foundation.

Code Triggers: When Are Annular Plates Mandatory?

Per API 650 Section 5.5 (and, for in-service evaluation, API 653 Section 4.4), a dedicated butt-welded annular plate ring is required when any of the following conditions are met:

1. First Shell Course Design Stress: The product design stress in the bottom shell course exceeds 23,200 psi (160 MPa).
2. High-Strength Material Groups: The bottom shell course is fabricated from materials belonging to API 650 Groups IV, IVA, V, or VI (such as quenched and tempered or normalized fine-grained micro-alloyed steels).
3. Shell Plate Thickness: The nominal thickness of the bottom shell course exceeds 1.0 in. (25 mm) for older designs or specific thickness-stress thresholds established in API 650 Table 5.1a/b.
4. Seismic Overturning Conditions: Seismic design calculations under API 650 Annex E mandate annular rings of specified width and thickness to resist shell uplift.

Dimensional and Joint Requirements

API 650 and API 653 establish strict geometric dimensions for annular plates:

• Radial Width: The annular plate must have a minimum radial width of 24 in. (600 mm) measured from the inside surface of the tank shell to the interior lap weld connecting the annular ring to the standard floor sketch plates.
• Exterior Projection (Chime): The annular plate must project outward at least 2 in. (50 mm) beyond the outside surface of the shell plate.
• Butt-Welded Radial Joints: All radial joints between adjacent annular plates must be complete penetration, complete fusion butt welds, backed by permanent or removable ceramic/steel backing strips. Lap joints between adjacent annular plates are strictly forbidden because radial lap welds create severe stress risers perpendicular to the shell bending moments.

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3. The Critical Zone: Definition, Mechanics & Repair Limitations

Exact Definition under API 653

API 653 Section 3 definitions and 9.10.1.2: The Critical Zone is defined as the portion of the bottom plate or annular plate ring within 3 in. (75 mm) of the inside edge of the tank shell, measured radially inward from the toe of the shell-to-bottom fillet weld.

   Shell Plate (Vertical)
         |  |
         |  |
         |  |   Shell-to-Bottom Inside Fillet Weld Toe
         |  |  |
_________|  |__|___________________________________
  Bottom |  |  |                                   
  Chime  |  |  |<------ 3.0 inches (75 mm) ------->|
  Proj.  |  |  |         THE CRITICAL ZONE         |   Interior Floor Plates
_________|__|__|___________________________________|_______________________
               Foundation / Concrete Ringwall

Structural Significance of the Critical Zone

During hydrostatic filling, shell settlement, and cyclic operational cycles, the steel in the Critical Zone undergoes severe cyclic plastic strain and localized bending. A "plastic hinge" forms in this 3 in. band as the bottom plate bends upward to match the shell's outward radial deflection. Any mechanical flaw, notch, metallurgical discontinuity, or severe corrosion pit in this region can rapidly initiate brittle fracture or fatigue crack propagation through the bottom plate.

Strict Repair Limitations

Because the Critical Zone operates under triaxial stress states, API 653 Section 9.10.1.2 enforces rigid repair restrictions:

1. Prohibition of Lap Patch Plates: Standard lap-welded patch plates are strictly prohibited inside the Critical Zone. Lap welds create severe stress concentrations, eccentric loading, and uninspectable root crevices.
2. Tombstone Patch Exception (API 653 9.10.1.2): The only lap patch permitted within the Critical Zone is a specialized "tombstone" patch plate that extends completely through the shell-to-bottom joint and up into the shell. This requires cutting the shell-to-bottom weld, full engineering review, preheating, and 100% MT/PT and vacuum box examination.
3. Permissible Permanent Repairs: Permanent repairs within the Critical Zone require either:
   • Full Plate Replacement: Removing and replacing the damaged annular plate section using full-penetration butt welds.
   • Butt-Welded Insert Plates: Installing a butt-welded insert plate that is flush with the surrounding metal, with all corners rounded to a minimum radius of 2 in. (50 mm) and welds fully radiographed or ultrasonically examined.
4. No Attachments or Welds: No mechanical attachments, pipe supports, or heating coil brackets may be welded within the Critical Zone.

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4. Bottom Plate Weld Seams and Assembly Rules

Standard interior tank floors consist of rectangular plates joined by lap welds. The engineering integrity of these seams is governed by strict layout rules:

• Minimum Overlap: Plates must overlap by at least 5 times the nominal plate thickness or 1.0 in. (25 mm), whichever is greater, for full-fillet welded lap joints.
• Three-Lap Joints (Three-Plate Laps): The intersection where three bottom plates overlap is a primary leak path due to the thickness step-down. API 650 Section 5.1 requires the upper plate corner to be chamfered or notched before welding to allow a smooth, liquid-tight fillet weld transition.
• Weld Spacing from Shell Welds: Three-plate laps must be spaced at least 12 in. (300 mm) from one another, at least 12 in. from the tank shell, and at least 12 in. from any annular plate radial butt weld.

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5. Internal Bottom Inspection Methodology

When a tank is taken out of service, cleaned, and gas-freed, the inspector must deploy complementary non-destructive examination (NDE) methods to assess the floor's physical integrity and remaining corrosion allowance.

Visual Examination (VT)

Visual examination is the fundamental starting point of every out-of-service inspection:

• Topography and Waviness: Inspecting for floor plate buckling, ridges, sharp peaks, and depressions caused by foundation settlement or excessive under-plate pressure.
• Floating Roof Support Leg Contact: Checking for localized gouging, indentations, mechanical galling, or punching shear where floating roof support legs land on bottom plates. Reinforcing landing pads (minimum 3/8 in. or 10 mm thick) must be verified.
• Coating/Lining Condition: Checking for blistering, disbondment, holidays, and under-film rust jacking on internal protective linings.
• Weld Seams: Examining all lap and butt welds for cracking, undercut, porosity, and severe grooving corrosion along the heat-affected zone (HAZ).

Magnetic Flux Leakage (MFL) Scanning

Magnetic Flux Leakage (MFL) is the primary high-speed screening tool for volumetric inspection of tank bottoms:

• Operating Principle: Powerful permanent neodymium magnets or electromagnets induce a near-saturation magnetic flux field within the carbon steel plate. Where localized metal loss exists—either on the top-side or bottom-side (soil-side)—the magnetic field is forced to "leak" out into the air above the plate.
• Detection: Arrays of solid-state Hall effect sensors or induction coils measure the leaked flux vector, converting field disruptions into digital flaw maps.
• Key Limitation: MFL cannot measure absolute remaining wall thickness directly. It is a relative screening technology sensitive to pit geometry, volume, and scan speed. All MFL indications exceeding a pre-established screening threshold (e.g., 20% or 30% metal loss) must be proved up using ultrasonic testing.

Ultrasonic Testing (UT) Prove-Up

Every significant MFL indication is validated using high-precision Ultrasonic Testing:

• Manual Straight-Beam A-Scan: A high-frequency dual-element transducer is coupled to the plate over the identified indication. The time-of-flight between the initial pulse and the backwall reflection determines precise remaining wall thickness to within ±0.005 in. (0.1 mm).
• Distinguishing Top-Side vs. Bottom-Side: Straight-beam UT immediately differentiates top-side pitting (visually verifiable) from soil-side corrosion (blind to visual VT).
• Automated Ultrasonic Crawlers (B-Scan and C-Scan): Continuous mapping crawlers generate cross-sectional thickness profiles (B-scans) and plan-view color-coded depth maps (C-scans) across severely pitted plates, critical zone bands, and annular rings.

Vacuum Box Bubble Leak Testing

Vacuum box testing is mandatory for verifying the pressure tightness of all newly constructed or repaired floor fillet welds, lap joints, and insert plate seams per API 650 Section 8.6 and API 653 Section 12.1.8:

1. Bubble Solution Application: A continuous film of foaming bubble-test solution is applied along a section of the weld seam.
2. Box Placement & Evacuation: A rectangular box with a transparent viewing window and a soft sponge-rubber perimeter gasket is placed over the wetted weld. An air ejector or vacuum pump evacuates the box to a partial vacuum of 21 to 35 kPa (3 to 5 psi, or 6 to 10 in. Hg).
3. Inspection Dwell Time: The inspector views the weld through the window for a minimum dwell time (typically at least 5 to 10 seconds). Continuous streams of expanding bubbles indicate through-thickness pinhole leaks or porous weld fissures.

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Comparison of Tank Bottom NDE Methods