Study guide
2.3 Shell Courses, Welds, Attachments & Wind Girders
API 653 Aboveground Storage Tank Inspector Exam
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1 Chapter 1: API 653 Scope, Inspector Certification & Roles
1.1 Scope of API 653, Tank Types & Construction Standards 1.2 Inspector Roles, Responsibilities & Certification Requirements 1.3 Inspection Planning, Safety, Confined Space Entry & Preparation
2 Chapter 2: Tank Shell & Roof Components Inspection
2.1 Fixed Roof Types, Structural Supports & Inspection Practices 2.2 Floating Roofs, Seals, Drainage & Operational Issues 2.3 Shell Courses, Welds, Attachments & Wind Girders 2.4 Tank Appurtenances, Nozzles, Manways & Piping Connections
3 Chapter 3: Tank Bottom & Foundation Inspection
3.1 Tank Bottom Types, Annular Plates & Internal Bottom Inspection 3.2 Foundation Types (Concrete Ringwall, Earth, Asphalt) & Inspection 3.3 Leak Detection Systems, Secondary Containment & Under-Tank Monitoring
4 Chapter 4: Corrosion & Damage Mechanisms in Storage Tanks
4.1 Atmospheric Corrosion, Condensation & External Deterioration 4.2 Corrosion Under Insulation (CUI) & Piping/Nozzle Interfaces 4.3 Soil-Side Corrosion, Under-Bottom Pitting & Microbiologically Induced Corrosion (MIC) 4.4 Product-Side Deterioration: Chemical Attack, Ethanol SCC, Concentration Cell & Sulfuric Acid Corrosion
5 Chapter 5: Materials, Toughness & Brittle Fracture Prevention
5.1 Tank Steels, Material Specifications & Historic Standards 5.2 Brittle Fracture Assessment: Figure 5.1 Decision Tree & Exemption Curves (API 653 Section 5) 5.3 Charpy V-Notch Impact Testing & Minimum Design Metal Temperature (MDMT)
6 Chapter 6: Cathodic Protection & Interior Linings
6.1 Cathodic Protection Principles & Systems for Tank Bottoms (API RP 651) 6.2 CP Criteria, Surveys, Potential Measurements & Interference (API RP 651) 6.3 Tank Bottom Interior Linings, Surface Prep & Inspection (API RP 652)
7 Chapter 7: Corrosion Rates, Remaining Life & Inspection Intervals
7.1 Determining Corrosion Rates (Short-Term vs. Long-Term) 7.2 Remaining Life Calculations & Limiting Components 7.3 Establishing In-Service & Out-of-Service Inspection Intervals (API 653 Section 6)
8 Chapter 8: Tank Shell Minimum Thickness Evaluation
8.1 Shell Minimum Thickness Calculations (tmin for Hydrostatic & Operating Levels) 8.2 One-Foot Method, Variable Design Point & Reconstructed-Tank Shell Thickness 8.3 Joint Efficiency (E) Selection, Missing Data & Radiography Factors 8.4 Evaluation of Pitting, Localized Thin Areas (LTA) & Critical Length (L)
9 Chapter 9: Bottom Plate & Annular Ring Thickness Evaluation
9.1 Minimum Bottom Thickness Formulations (MRT, MRTbottom, MRTlinear) 9.2 Annular Plate Evaluation, Stress Criteria & Critical Zones 9.3 Bottom Pitting Evaluation & Remaining Thickness Before Next Inspection
10 Chapter 10: Tank Repairs & Alterations Standards
10.1 Classification: Repair vs. Alteration vs. Reconstruction (API 653 Section 9) 10.2 Shell Plate Replacement, Lapped Patch Plates & Insert Plates 10.3 Shell Penetrations, Door Sheets & Nozzle Additions 10.4 Hot Tapping Requirements, Limitations & Pre-Engineering (API 653 Section 9.15 & API Publ 2201)
11 Chapter 11: Welding Procedures, Qualifications & Quality Control
11.1 Welding Procedure Specification (WPS) & Procedure Qualification Record (PQR) per ASME IX 11.2 Welder Performance Qualification (WPQ) & Continuity per ASME IX 11.3 Essential, Non-Essential & Supplementary Essential Variables 11.4 Preheating, Post-Weld Heat Treatment (PWHT) & Weld Flaw Remediation
12 Chapter 12: Nondestructive Examination (NDE) Methods
12.1 Visual Examination (VT) & Magnetic Particle Testing (MT) per ASME V 12.2 Liquid Penetrant (PT) & Ultrasonic Thickness Measurement (ASME V Article 6 & SE-797) 12.3 Radiographic Testing (RT): Acceptance Criteria, Film Interpretation & ASME V/API 650
13 Chapter 13: Hydrostatic Testing & Post-Repair Examination
13.1 Hydrostatic Test Triggers, Exemption Criteria & Major Alterations (API 653 Section 12.3) 13.2 Leak Testing: Vacuum Box, Tracer Gas, Diesel & Air Pressure Tests 13.3 NDE Examination Requirements for Shell, Bottom, Roof & Nozzle Welds (API 653 Section 12.1)
14 Chapter 14: Relief Devices, Tank Settlement & Nameplate Records
14.1 Pressure & Vacuum Relief Devices, Venting Principles (API RP 576 & API 2000) 14.2 Tank Settlement Types: Uniform, Tilt, Out-of-Plane & Edge Settlement (API 653 Annex B) 14.3 Evaluating Settlement Limits, Cosine Curve Analysis & Remedial Action 14.4 Tank Marking, Nameplates, Inspection Reports & Recordkeeping (API 653 Section 13)
2.3 Shell Courses, Welds, Attachments & Wind Girders
Key Takeaways Explain This Topic AI
Tank shell plates are graded in thickness from Course 1 (bottom) to Course N (top), reflecting decreasing hydrostatic head pressure while vertical weld seams carry twice the hoop stress of horizontal seams.
Shell dimensional tolerances under API 653 Section 10.5 dictate maximum out-of-plumbness of 1/200 of tank height, with peaking at vertical welds and banding at horizontal welds strictly limited to 1/2 in. (13 mm).
Top and intermediate wind girders protect open-top and tall tanks against wind buckling; all stiffening rings require functional drainage holes to prevent ponding water and catastrophic crevice corrosion.
The critical shell-to-bottom corner weld is the highest stress concentration zone in the tank, requiring rigorous visual, magnetic particle, or vacuum box examination to detect toe cracking and fatigue tears.
Last updated: September 2026
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 N N ).
Hydrostatic Head Mechanics: Hydrostatic pressure exerted by the liquid is a direct function of liquid depth: P = ρ g H = γ H P = \rho g H = \gamma H P = ρ g H = γ H where ρ \rho ρ is product density, g g g is gravitational acceleration, and H H H is liquid height above the point of interest. The circumferential membrane tensile stress (hoop stress) in a thin-walled cylinder is: σ h = P ⋅ D 2 t \sigma_h = \frac{P \cdot D}{2t} σ h = 2 t P ⋅ D where D D D is tank diameter and t t t is shell thickness.
Plate Thickness Grading: Because P P 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:
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.
Flush on Centerline: Centerlines of plate thicknesses align, producing small internal and external offsets at course transitions.
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 ( σ h \sigma_h σ h ). In a pressurized cylinder, hoop stress is twice the longitudinal axial stress ( σ h = 2 σ L \sigma_h = 2\sigma_L σ h = 2 σ 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 E E ): Because vertical seams resist the primary containment stress, weld quality directly impacts shell minimum thickness calculations ( t min t_{\min} t m i n ). 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 E = 1.00 E = 1.00
- Spot Radiography per API 650: E = 0.85 E = 0.85 E = 0.85
Un-radiographed / Unknown historical joints (e.g., early API 12C): E = 0.70 E = 0.70 E = 0.70 or 0.35 0.35 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 H/200 H /200 ). For example, in a 40-ft high tank, the maximum permissible tilt is: Plumbness Limit = 40 × 12 in. 200 = 2.4 inches (61 mm) \text{Plumbness Limit} = \frac{40 \times 12\text{ in.}}{200} = 2.4\text{ inches (61 mm)} Plumbness Limit = 200 40 × 12 in. = 2.4 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:
Stairways, Platforms, and Ladders: Spiral stairways, straight ladders, and gauging platforms must be inspected for corroded structural stringers, loose bolting, and cracked attachment welds.
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 .
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.
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
Feature / Distortion Governing API Standard Maximum Allowable Tolerance Field Measurement Tool Corrective Action
Shell Out-of-Plumbness API 653 §10.5.2 / API 650 §7.5.2 1/200 of tank height ( H / 200 H/200 H /200 ) Total station, theodolite, or suspended plumb bob Structural re-evaluation, foundation re-leveling, or shell jacking
Peaking at Vertical Welds API 653 §10.5.4 / API 650 §7.5.4 1/2 in. (13 mm) 36-in. horizontal radius sweep board Engineering fitness-for-service (API 579), strong-back jacking, or plate replacement
Banding at Horizontal Welds API 653 §10.5.5 / API 650 §7.5.5 1/2 in. (13 mm) 36-in. vertical straight edge Engineering assessment, stiffener ring addition, or seam repair
Wind Girder Web Ponding API 650 §5.9 Zero ponding allowed (Drain holes required) Visual inspection, feeler probe for clog Clear clogged holes; drill minimum 3/4-in. drain holes at all low points
Corner Fillet Weld Defects API 653 §12.1.6 Zero cracks or rejectable linear indications MT, PT, 90° corner vacuum box, shear-wave UT Weld gouging, grind repair, qualification per ASME IX, and 100% re-NDE