10.2 Shell Plate Replacement, Lapped Patch Plates & Insert Plates

Engineering Principles of Shell Plate Remediation

The cylindrical shell of an aboveground atmospheric storage tank is subjected to primary circumferential membrane tensile stress (hoop stress) generated by hydrostatic product pressure:

$S = \frac{2.6 \cdot D \cdot H \cdot G}{t}$

Because tensile stresses reach their maximum in the lowest shell courses, any shell imperfection—such as localized thinning, deep gouges, mechanical distortion, or crack-like flaws—can trigger catastrophic brittle rupture or ductile plastic collapse. When shell defects exceed allowable limits established in API 653 Section 4.3, mechanical remediation is mandatory.

API 653 Section 9 governs three primary mechanical repair methods for cylindrical shells:

1. Complete or Partial Shell Plate Replacement (Section 9.2)
2. Lapped Patch Plates on the Shell (Section 9.3, Shell Repairs Using Lap-welded Patch Plates)
3. Flush-Welded Insert Plates (Section 9.2, Removal and Replacement of Shell Plate Material)

Selecting between these methods depends on base metal thickness, design hoop stress, fatigue environment, and whether the repair is temporary or permanent.

+-------------------------------------------------------------------------+
|                     SHELL REPAIR METHOD COMPARISON                      |
|                                                                         |
|   [ Full Shell Replacement ]        [ Flush Insert Plate ]              |
|   +-----------------------+         +--------------------+              |
|   | Existing Shell Course |         |                    |              |
|   |                       |         |   +------------+   |              |
|   +=======================+         |   | 2" min rad |   |              |
|   | Replacement Plate     |         |   |   Insert   |   |              |
|   | (Full-pen butt weld)  |         |   +------------+   |              |
|   | Min 12" or 12t        |         | Full-pen butt weld |              |
|   +=======================+         +--------------------+              |
|                                                                         |
|                    [ Lapped Patch Plate ]                               |
|                    +--------------------+                               |
|                    |  Base Shell Plate  |                               |
|                    |                    |                               |
|                    |   +============+   |                               |
|                    |   | Lap Patch  |   |                               |
|                    |   | Fillet-weld|   |                               |
|                    |   +============+   |                               |
|                    |  3/16" <= t <= 1/2"|                               |
|                    +--------------------+                               |
+-------------------------------------------------------------------------+

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Shell Plate Replacement (API 653 Section 9.2)

When widespread corrosion, extensive cracking, or severe mechanical distortion affects an entire section of a shell course, the defective material must be cut out and replaced with new plate material.

Minimum Dimensional Requirements (Section 9.2.2.1)

To prevent intense thermal restraint and severe weld distortion, replacement shell plates must conform to minimum dimensional constraints:

$\text{Minimum Replacement Dimension} = \max(12\text{ in.}, 12t)$

Where $t$ represents the nominal thickness of the replacement shell plate (in inches). For metric units, the minimum dimension is $\max(300\text{ mm}, 12t)$.

• Engineering Rationale: Welding introduces high localized heat inputs. If a replacement plate is smaller than $12\text{ in.}$ or $12t$, the heat-affected zones (HAZ) of the opposing perimeter welds overlap. This thermal superposition creates high triaxial residual tensile stress, prevents elastic strain relief, and promotes severe localized shell peaking or flat-spotting.

Material Matching and Toughness Criteria

Replacement plate material must strictly satisfy original design standards or current API 650 requirements:

• Tensile and Yield Properties: Must equal or exceed the minimum yield strength ($S_y$) and ultimate tensile strength ($S_u$) of the existing shell course.
• Weldability: Chemical composition must provide equivalent or superior weldability, verified by Carbon Equivalent ($CE$) calculations: $CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15} \le 0.43\%$
• Impact Toughness: If the shell thickness exceeds $1/2\text{ in.}$ ($12.5\text{ mm}$) and operates at or below the Minimum Design Metal Temperature (MDMT), replacement plates must be Charpy V-notch impact tested per API 650 Section 4 to prevent brittle fracture.

Joint Geometry

All replacement shell plates must be joined into the surrounding shell using full-penetration, full-fusion butt welds. Complete joint penetration (CJP) ensures 100% joint efficiency ($E = 1.0$), matching the original structural continuity of the cylindrical shell.

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Lapped Patch Plates on the Shell (API 653 Section 9.3)

Lapped patch plates consist of a plate fillet-welded over a defect on the external or internal surface of the shell. While common in field maintenance, lapped patches introduce severe eccentric bending moments, high stress concentrations at fillet toes, and crevice corrosion risks between the overlapping plates.

[!CAUTION] Under API 653 Section 9.3, Shell Repairs Using Lap-welded Patch Plates, lapped patch plates on the shell are considered non-standard repairs. They require explicit owner-user authorization and must be rigorously evaluated by a Storage Tank Engineer if intended as permanent installations.

+-------------------------------------------------------------------------+
|               API 653 SECTION 9.3 LAPPED PATCH CONSTRAINTS              |
+-----------------------+-------------------------------------------------+
| Parameter             | Mandatory Code Requirement                      |
+-----------------------+-------------------------------------------------+
| Owner Approval        | Explicit owner-user authorization mandatory     |
| Minimum Thickness     | 3/16 in. (4.8 mm / 0.1875 in.)                  |
| Maximum Thickness     | 1/2 in. (12.5 mm / 0.500 in.)                   |
| Plate Thickness Limit | Cannot exceed thickness of host shell plate     |
| Corner Geometry       | Rounded corners, minimum 2-in. (50 mm) radius   |
| Minimum Dimension     | 4 in. (100 mm)                                  |
| Maximum Dimension     | 48 in. (1200 mm) vertical / 72 in. horizontal   |
| Clearance to Seams    | Minimum 1 in. (25 mm) or 8t from existing welds |
| Shell Stress Limits   | Prohibited where design stress > allowable      |
+-----------------------+-------------------------------------------------+

Technical Limitations on Lapped Shell Patches:

1. Thickness Range ($3/16\text{ in.} \le t_{patch} \le 1/2\text{ in.}$): The minimum thickness is $3/16\text{ in.}$ ($4.8\text{ mm}$) to ensure structural robustness and prevent weld blow-through. The maximum thickness is $1/2\text{ in.}$ ($12.5\text{ mm}$) because thicker plates generate unacceptably high eccentric bending moments across the fillet-welded lap.
2. Relative Thickness Constraint: The patch thickness cannot exceed the thickness of the shell plate to which it is attached. A patch stiffer than the host shell transfers excessive local shear stress into the thinned host plate.
3. Corner Radius (Minimum $2\text{ in.} / 50\text{ mm}$): Square, 90-degree corners act as intense notch stress raisers. Thermal expansion and cyclic hoop flexure induce rapid fatigue crack initiation at sharp corners. Consequently, all rectangular or trapezoidal patch plates must have corners rounded to a minimum 2-inch radius.
4. Clearance from Existing Seams: Lapped patch plates must maintain a minimum clearance of $1\text{ in.}$ ($25\text{ mm}$) or $8t$ (whichever is greater) from existing vertical or horizontal butt welds. Lapped patches cannot cross shell weld seams unless specifically engineered and accepted under Section 9.3.
5. Critical Zone Exclusion: Lapped patch plates on the shell are strictly prohibited from crossing or touching the shell-to-bottom corner weld.

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Flush-Welded Insert Plates (API 653 Section 9.2)

Flush-welded insert plates (also known as flush patches) represent the preferred, permanent engineering method for repairing isolated shell defects. Unlike lapped patches, insert plates preserve the concentric geometry of the shell, eliminate eccentric bending, and do not create trapped crevice corrosion pockets.

+-------------------------------------------------------------------------+
|                   FLUSH INSERT PLATE JOINT & TAPERING                   |
|                                                                         |
|   Existing Shell Plate                     Replacement Insert Plate     |
|   Thickness = t_shell                      Thickness = t_insert         |
|   =========================\               /=========================   |
|                            \  Full-Pen    /                             |
|                             \ Butt Weld  /                              |
|                              \          /   Uniform Taper (>= 1:4)      |
|                               \        /---------\_ _ _ _               |
|   =============================\======/=============================   |
|                                                                         |
+-------------------------------------------------------------------------+

Geometry and Corner Radii (Section 9.2.2.1)

• Rectangular Insert Plates: Must have corners rounded to a minimum radius of 2 inches (50 mm) to eliminate stress risers, unless the insert plate terminates on an existing horizontal or vertical shell weld joint.
• Circular Insert Plates: Circular insert plates must have a minimum diameter of 12 inches (300 mm). Circular geometry provides optimal, uniform stress distribution with no sharp directional discontinuities.

Edge Preparation and Alignment

Insert plates must be prepared with standard welding bevels (e.g., $60^\circ$ included angle V-groove or U-groove with a $1/16$ to $1/8\text{ in.}$ root face). Plate misalignment (offset) must satisfy API 650 Section 7.2.1:

• For plate thickness $\le 5/8\text{ in.}$ ($16\text{ mm}$): maximum offset is $1/16\text{ in.}$ ($1.6\text{ mm}$).
• For vertical joints, maximum offset cannot exceed $10%$ of plate thickness or $1/8\text{ in.}$ ($3.2\text{ mm}$).

Thickness Transition Taper (1:4 Rule)

When an insert plate is thicker than the adjacent host shell plate by more than $1/8\text{ in.}$ ($3.2\text{ mm}$), the thicker plate edge must be machined or ground with a smooth, continuous taper.

• Taper Ratio: The length of the taper must be at least four times the difference in plate thickness ($L \ge 4 \cdot \Delta t$), forming a transition slope no steeper than 1:4 ($14^\circ$).
• Rationale: An abrupt thickness step creates a severe stress concentration factor ($K_t > 2.5$) under membrane hoop tension. A 1:4 taper ensures smooth stress flow lines across the joint.

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Weld Spacing Rules: API 653 Figure 9.1

When installing replacement plates, insert plates, or penetrations, new weld seams must be carefully spaced away from existing shell welds. Overlapping or closely spaced welds cause thermal interaction that multiplies residual tensile stresses, hardens the heat-affected zone, and promotes cracking.

API 653 Figure 9.1, Acceptable Details for Replacement of Shell Plate Material, governs the minimum acceptable clearances between weld toes. (Do not reach for API 653 Table 9.1 here — that table is Hot Tap Connection Sizes and Shell Plate Thicknesses and has nothing to do with weld spacing.) Figures 9.2 through 9.5 give the companion door-sheet details for riveted, lap-weld, and butt-weld shell seams:

+-------------------------------------------------------------------------+
|                     API 653 FIGURE 9.1 WELD CLEARANCES                  |
|                                                                         |
|                     Existing Vertical Shell Butt Weld                   |
|                                    | |                                  |
|                                    | |                                  |
|                  <--- >= 12" ----->| |                                  |
|                                    | |                                  |
|   +======================+         | |                                  |
|   |                      |         | |                                  |
|   |  New Insert Plate    |         | |                                  |
|   |  (or Door Sheet)     |         | |                                  |
|   |                      |         | |                                  |
|   +======================+         | |                                  |
|              |                     | |                                  |
|              v >= 6"               | |                                  |
|   -----------+---------------------+-------------------------------+    |
|              |                     | |                                  |
|              | Existing Horizontal Shell Butt Weld                      |
|              |                                                          |
+--------------+----------------------------------------------------------+

Minimum Weld Toe Clearances (API 653 Figure 9.1):

1. Vertical Butt Weld to Vertical Butt Weld: Minimum $12\text{ in.}$ ($300\text{ mm}$) or $5t$ (whichever is greater).
   • Rationale: Vertical welds carry full primary membrane hoop stress. A 12-inch buffer ensures independent stress relaxation.
2. Vertical Butt Weld to Horizontal Butt Weld: Minimum $3\text{ in.}$ ($75\text{ mm}$) or $5t$ (whichever is greater).
   • Rationale: Horizontal welds carry secondary longitudinal (axial) stress, permitting tighter clearance.
3. Insert Plate Butt Weld to Existing Vertical Weld: Minimum $12\text{ in.}$ ($300\text{ mm}$) or $5t$.
4. Insert Plate Butt Weld to Existing Horizontal Weld: Minimum $6\text{ in.}$ ($150\text{ mm}$) or $5t$, OR the insert plate may cross the horizontal seam at $90^\circ$ and terminate directly on the seam, provided the existing seam is ground flush and inspected.
5. Fillet Weld Toe to Butt Weld Toe: Minimum $3\text{ in.}$ ($75\text{ mm}$) or $5t$.

Material Group Classifications (API 650 Table 4.4a):

• Groups I, II, III, IIIA: Lower-strength carbon steels (e.g., ASTM A283C, A285C, A36). Weld spacing rules are standard ($12\text{ in.}$ vertical, $6\text{ in.}$ horizontal).
• Groups IV, IVA, V, VI: High-strength, normalized, or quenched-and-tempered steels (e.g., ASTM A537 Class 1/2, A516-70, A737). These materials exhibit higher hardenability. Strict adherence to Table 9.1 clearances and controlled preheats ($> 200^\circ\text{F} / 93^\circ\text{C}$) are mandatory.

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Non-Destructive Examination (NDE) Requirements (Section 12.1)

Following the welding of replacement shell plates or insert plates, comprehensive NDE must be executed to certify weld integrity:

• Volumetric Examination (100% RT or UT): All new full-penetration vertical and horizontal butt welds joining replacement plates or insert plates must be 100% radiographically examined (RT) or ultrasonically examined (UT) in accordance with ASME Section V and API 650 Section 8.1.
• Surface Examination (MT or PT): Magnetic particle (MT) or liquid penetrant (PT) examination must be performed on:
  • The root pass of all butt welds.
  • Cavities resulting from back-gouging.
  • The finished weld cap and weld toes to verify freedom from undercutting, toe cracks, and porosity.
• Fillet Weld Examination: All attachment fillet welds on lapped patch plates must be 100% examined by MT or PT.
• Visual Examination (VT): 100% visual inspection by the Authorized Inspector per ASME Section V Article 9.