2.3 Shell Courses, Welds, Attachments & Wind Girders

2.3 Shell Courses, Welds, Attachments & Wind Girders

API 653 Core Principle: Tank shell integrity is governed by membrane hoop stress equilibrium, precise weld joint efficiency factors, and strict dimensional tolerances. Deviations from true cylindrical geometry—such as out-of-plumbness, vertical weld peaking, and horizontal weld banding—create localized bending stresses that compound cyclic hydrostatic fatigue and accelerate brittle fracture risks.

The cylindrical shell of an aboveground atmospheric storage tank is an engineered pressure vessel containing massive hydrostatic forces. Evaluating shell plates, weld seams, stiffening girders, and external structural attachments under API 653 requires rigorous verification of thickness grading, dimensional tolerances, and weld joint structural soundess.

Shell Course Architecture & Thickness Grading

Tank shells are constructed from multiple stacked horizontal rings of rolled steel plates known as shell courses.

Course Numbering & Hydrostatic Gradient

• Numbering Convention: Standard industry convention numbers the bottommost shell ring connected to the tank bottom as Course 1. Subsequent courses are numbered sequentially ascending toward the roof (Course 2, Course 3, ..., Course $N$).
• Hydrostatic Head Mechanics: Hydrostatic pressure exerted by the liquid is a direct function of liquid depth: $P = \rho g H = \gamma H$ where $\rho$ is product density, $g$ is gravitational acceleration, and $H$ is liquid height above the point of interest. The circumferential membrane tensile stress (hoop stress) in a thin-walled cylinder is: $\sigma_h = \frac{P \cdot D}{2t}$ where $D$ is tank diameter and $t$ is shell thickness.
• Plate Thickness Grading: Because $P$ is maximum at the bottom and decreases linearly to zero at the liquid surface, hoop stress is greatest in Course 1. To optimize material efficiency and economy, shell plates are graded in thickness: Course 1 has the greatest nominal thickness (often 0.75 in. to 1.75 in. in large tanks), with each successive course rolled from progressively thinner plate, reaching the code-mandated minimum nominal thickness (typically 3/16 in. to 5/16 in. per API 650 Section 5.6) at the top course.

Shell Alignment Profiles

During plate fit-up, adjacent courses may be aligned in one of three configurations:

1. Flush on Inside: The internal surfaces of all shell courses align along a common vertical plane. This is mandatory for external and internal floating roof tanks to ensure a smooth, unobstructed path for rim seal shoes and wipers.
2. Flush on Centerline: Centerlines of plate thicknesses align, producing small internal and external offsets at course transitions.
3. Flush on Outside: The external surfaces align, used occasionally for uninsulated fixed-roof tanks.

Vertical vs. Horizontal Weld Seams

The structural mechanics governing vertical and horizontal shell welds differ fundamentally due to biaxial stress fields:

Vertical Weld Seams

• Stress Regime: Vertical seams carry the full circumferential hoop stress ($\sigma_h$). In a pressurized cylinder, hoop stress is twice the longitudinal axial stress ($\sigma_h = 2\sigma_L$).
• Weld Requirements: Vertical welds must be full-penetration, full-fusion double-welded butt joints (or single-welded butt joints with permanent backing strips if permitted by historical construction codes).
• Joint Efficiency ($E$): Because vertical seams resist the primary containment stress, weld quality directly impacts shell minimum thickness calculations ($t_{\min}$). API 653 Table 4.2 assigns joint efficiencies based on the original construction standard and the degree of radiographic examination:
  • Full Radiography per API 650: $E = 1.00$
  • Spot Radiography per API 650: $E = 0.85$
  • Un-radiographed / Unknown historical joints (e.g., early API 12C): $E = 0.70$ or $0.35$ for lap-welded joints.

Horizontal Weld Seams

• Stress Regime: Horizontal seams carry longitudinal axial stresses generated by shell dead weight, roof dead and live loads, wind overturning moments, seismic action, and internal vacuum. These stresses are typically less than 15% to 20% of hoop stress.
• Weld Requirements: Modern API 650 requires complete fusion and penetration butt welds, although partial penetration was permitted under certain early historical codes. Joint efficiency is generally not a limiting factor in hydrostatic pressure calculations for horizontal seams.

Shell Dimensional Distortions & Inspection Tolerances

Dimensional distortion induces high secondary bending stresses at weld seams. API 653 Section 10.5 establishes strict evaluation criteria:

1. Out-of-Plumbness (API 653 Section 10.5.2 & API 650 Section 7.5.2)

• Definition: The tilt or leaning deviation of the top of the shell relative to the bottom of the shell.
• Tolerance Limit: The maximum out-of-plumbness shall not exceed 1/200 of the total tank height ($H/200$). For example, in a 40-ft high tank, the maximum permissible tilt is: $\text{Plumbness Limit} = \frac{40 \times 12\text{ in.}}{200} = 2.4\text{ inches (61 mm)}$
• Measurement Method: Plumbness is measured at 8 to 16 equally spaced shell stations using an optical theodolite, total station, laser plummet, or a mechanical plumb bob suspended inside the shell in calm wind.

2. Peaking at Vertical Welds (API 653 Section 10.5.4)

• Definition: An angular out-of-round distortion forming a sharp localized "peak" (ridge or valley) along a vertical weld joint.
• Tolerance Limit: The maximum allowable peaking deviation is 1/2 in. (13 mm).
• Measurement Tool: Measured using a horizontal 36-inch (900 mm) sweep board curved cut to the true nominal radius of the tank. The sweep board is held horizontally centered across the vertical weld, and the gap between the board and plate is gauged.

3. Banding at Horizontal Welds (API 653 Section 10.5.5)

• Definition: An inward or outward circumferential bulge, crease, or "hula-skirt" distortion occurring along a horizontal weld seam.
• Tolerance Limit: The maximum allowable banding deviation is 1/2 in. (13 mm).
• Measurement Tool: Measured using a vertical 36-inch (900 mm) straight edge placed centered vertically across the horizontal weld.

4. Flat Spots & Dents

• Evaluation: Flat spots on rolled plates reduce buckling resistance against wind and vacuum. Dents with sharp gouges or creases act as severe stress concentrations requiring ultrasonic thickness mapping and magnetic particle testing to ensure no micro-cracking exists.

Wind Girders & Shell Stiffening Rings (API 650 Section 5.9)

Thin cylindrical shells have immense tensile hoop strength but relatively poor resistance to external compressive loads, such as high cross-winds and internal vacuum:

Top Wind Girders

• Function: Open-top tanks and external floating roof tanks require a structural ring beam at or near the top of the upper shell course. The top wind girder maintains shell roundness against wind pressures, preventing the top course from buckling oval and jamming floating roof seals.
• Design: Can be formed from rolled structural angles, structural channels, or fabricated plate girders welded continuously to the shell. Often configured with handrails and grating to double as an inspection walkway.

Intermediate Wind Girders

• Function: On tall tanks with thin upper shell courses, additional horizontal stiffening rings (intermediate wind girders) are installed at intermediate elevations.
• Spacing: Spacing is determined using the API 650 transformed shell height calculation, ensuring that unstiffened vertical shell spans do not exceed the critical elastic buckling threshold.

Mandatory Wind Girder Drainage Holes

• The Corrosion Trap: Wind girders oriented horizontally form continuous catch basins for rainwater, windborne salts, and corrosive atmospheric contaminants.
• Code Requirement: API 650 Section 5.9 requires stiffening rings that may trap liquid to be provided with adequate drain holes; industry practice sizes them at a minimum diameter of 3/4 in. (19 mm) and places them at all low points around the circumference.
• Inspection Focus: Inspectors must verify that drain holes are open and flowing. Plugged drain holes allow standing water to pool against the tank shell, leading to severe crevice corrosion, pitting, and potential shell perforation at the girder-to-shell fillet weld.

Shell Attachments & Welded Accessories

Structural accessories welded to the tank shell must be maintained with the same diligence as primary pressure-retaining components:

1. Stairways, Platforms, and Ladders: Spiral stairways, straight ladders, and gauging platforms must be inspected for corroded structural stringers, loose bolting, and cracked attachment welds.
2. Corrosion Under Structural Attachment Clips: Many tanks feature structural clips welded to the shell for insulation support, piping brackets, or conduit runs. If these clips are not seal-welded all around, water penetrates the unsealed gap behind the clip, initiating hidden corrosion under brackets.
3. Weld Spacing to Shell Seams (API 653 Section 9.2 and Figure 9.1): Welded attachments must maintain minimum clearance distances from existing vertical and horizontal shell welds. (API 653 Section 9.3 is Shell Repairs Using Lap-welded Patch Plates, not a weld-spacing clause.) Welding too close to shell seams creates overlapping heat-affected zones (HAZ), generating high residual stress and localized embrittlement.
4. Grounding Lugs: Electrical grounding lugs provide critical discharge paths for static electricity and lightning protection. Connections must be uncorroded, tightly bolted, and demonstrate an electrical resistance to earth of less than 10 ohms per API RP 2003.

The Critical Shell-to-Bottom Corner Weld

The junction attaching the Course 1 shell plate to the bottom sketch plates or annular ring is the most structurally critical and highly stressed welded joint on the entire tank:

Stress Profile & Biaxial Restraint

The bottom corner weld (comprising inner and outer continuous fillet welds) resists immense radial shear and moment restraint. As the tank is filled, the shell expands outward under hoop tension, while the flat bottom plates are anchored to the foundation by product weight. This produces an intense cyclic bending moment at the shell-to-bottom corner joint, subjecting the weld toes to cyclic plastic strain.

Failure Modes

• Fatigue Cracking: Initiates at the inner weld toe or annular plate heat-affected zone due to cyclic hydrostatic filling and emptying.
• Lamellar Tearing: In thick bottom or annular plates with poor through-thickness ductility, welding stresses and bending loads can tear inclusions inside the base metal.
• Under-Bottom Corrosion: Soil-side moisture can corrode the bottom projection plate immediately adjacent to the outer corner weld.

Inspection Protocols

• Visual Examination (VT): 100% inspection of inner and outer fillet welds for undercut, porosity, cracking, and excessive leg dimensions.
• Magnetic Particle Testing (MT) or Dye Penetrant (PT): Mandatory for detecting surface-breaking fatigue tears along the weld toes.
• Specialized Corner Vacuum Box Testing: The inner corner fillet weld is tested using a 90-degree corner vacuum box with a clear viewing window, evacuated to 3 to 5 psi vacuum with bubble solution applied.
• Ultrasonic Thickness (UT) Scanning: Verifies remaining thickness of the Course 1 shell plate and adjacent bottom annular plate.

Shell Distortion Limits, Inspection Tools & Remedial Actions