Essential Tank Plate Types in API 650 Storage Tanks

Oil and LNG storage tanks (API 650) are built from four primary plate types: Shell, Bottom (floor), Annular, and Roof plates. Each serves a distinct structural role. Shell plates form the cylindrical wall and resist hoop and axial stresses; bottom plates form the tank floor and support the liquid load; annular plates are the ring-shaped plates at the shell–floor junction that transition loads into the shell; and roof plates cover the tank with a fixed conical/dome roof. Selection and design of each plate must account for load demands, welding methods, corrosion allowance, and material availability.

Shell Plates

Shell plates form the vertical walls of the tank. They are cut and rolled into courses – horizontal bands that stack to the full height. Thickness is calculated from hoop stress due to liquid head, plus corrosion allowance. Under API 650, shell plates are limited to a maximum of 45 mm (1.75 in) thick. If design stresses or material strength demand more than 45 mm, a higher-strength material (Group IV–VI steel) must be used. Common materials include ASTM A36 or EN S235JR (yield ~250 MPa) for lower-height tanks, and A516 Gr 70, A537 CL2, or EN S355 (355 MPa) for taller or more demanding tanks. Shell plates must be steel-killed and fine-grained for weldability.

Challenges & Solutions: Thick shell plates are heavy and difficult to roll and weld without distortion. Manufacturers often prebend plates and use sequence welding with controlled interpass temperature to manage deformation. All vertical weld seams must be radiographed per API 650 Sec. 8.3; horizontal (circumferential) welds and annular-plate welds also require radiography. Grade A36 or similar steels lack impact toughness at low temperatures, so in cold climates, designers switch to low-temperature–toughened steel (e.g., ASTM A553) or ensure impact testing. Finally, the shell is anchored to the tank foundation via anchor chairs welded to the annular plate or shell base.

Bottom (Floor) Plates

Bottom plates form the tank floor and must support the hydrostatic load and vacuum events. Multiple steel plates (6–12 mm thick plus corrosion allowance) typically cover the entire tank bottom. Standard layouts include overlapping “floor plates” and a heavier annular plate around the edge. The plates rest on a concrete foundation ring or pilings. Bottom plates are welded together in a grid; square or bevel-groove butt welds are used for full penetration, as required by API 650 (Sec. 5.1.5.5). Tack-welded backing strips (≥3 mm thick) may be used to maintain root openings. The nominal width of both rectangular and sketch plates should be ≥1800mm unless otherwise agreed by the purchaser. The required thickness of the bottom plates is the corroded thickness plus corrosion allowance.

Design considerations: Bottom plates must be flat and level to avoid ponding. They are fitted with seal welds to the shell or annular plate. By API 650, bottom plate butt welds are often laid out parallel to the shell for easier anchor pouring. Sloped “herringbone” patterns or radial layouts can also be used. Tanks may include a sump pocket in the center for drainage.

Challenges & Solutions: Bottom plating must resist positive pressure (hydrostatic head) and negative pressure (vacuum). A failing vacuum can cause collapse, so designers include vacuum relief valves and consider reinforcement (e.g., compensating plates). Welding distortion is mitigated by restraining the plates and welding symmetrically. Quality control is crucial: while roof and bottom welds are not typically radiographed, all shell-bottom connections and floor joints receive 100% magnetic-particle or dye-penetrant inspection to ensure leak-tightness. Delivery lead times can be long for large floor plates (especially thick annular rings), so early procurement is advised.

Annular Plates

Annular plates are the ring of plates immediately inside the tank shell at the bottom course. They transfer shell loads into the floor, providing an attachment point for shell base angles and anchor chairs. Per API 650 Sec. 5.5.2, annular plates must be at least 600 mm (24 in) wide (measured radially) from the shell to any lap joint when the tank diameter≥30m (100 ft) or when the bottom shell course is designed using the allowable stress for the materials in the Group IV, IVA, V or VI. In practice, designers often make annular plates significantly thicker than inner floor plates (e.g., 12–16 mm instead of 6–8 mm) to handle the high circumferential forces.

Welding and joints: Annular-plate radial joints must be full-penetration butt welds. A continuous backing strip (3 mm min..) is allowed beneath these welds, but the weld must be unflawed. For tanks >30 m in diameter or using high-strength shell steel (Group IV–VI), API 650 mandates butt-welded annular plates. Smaller tanks or low-stress cases may allow lap-welded “sketch” plates, but inspectors often prefer the butt-welded ring for safety. The inner edge of the annular ring may be cut straight or polygonal; by API definition, the inside circumference can form a regular polygon with as many sides as plates.

Challenges & Solutions: Because annular plates are large and thick, they are heavy and difficult to transport. On-site alignment with the shell is critical. Fabricators often butt-weld them to the shell in the shop or early in field erection—careful fit-up and welding (preheating if needed), control heat input. The annular ring is a hot spot for leak risk if undersized or welded poorly, so many engineers add an extra corrosion allowance and thorough NDE (radiography or PAUT) on these joints.

Roof Plates

Fixed roofs (cones or domes) cover aboveground tanks. Roof plates are metal panels welded together and attached to a top curb angle on the shell. API 650 divides roof design into three load cases: internal pressure (Annex F tension formula), external loads (Annex F buckling), and general loads (Sec. 5.10). In practice, roof plate thickness is often governed by buckling under roof weight or wind, not by internal pressure. API 650 requires a nominal roof sheet thickness ≥ 5 mm (3/16 in) plus corrosion allowance. Shallow cone roofs may use 6–10 mm steel; dome roofs often use 8–12 mm.

Construction: Roof plates are cut in a “pie-slice” pattern (with a polygon equal to the number of plates) or in concentric rings. Plates are welded together by lap fillet welds or bevel butt welds, with continuous fillet welds on the top side only. Plates must be fully supported at the perimeter. For supported-cone roofs, API 650 Sec. 5.10 requires that plates not be welded to the rafters (they rest on them instead), to allow slight movement. All roof panels attach to the curb angle with continuous top-side fillet welds.

Challenges & Solutions: Roof plates are thinner and often deformed by welding, so builders fabricate the roof on the ground in sections or use lifting frames. Dimensional control is critical to avoid gaps. Since roof welds generally have lower stress, API does not require radiography on roof plate welds, but 100% visual/MPI inspection is standard. Roofing steel is often A36 or similar; high-strength is rarely needed unless huge roof spans require higher buckling strength.

Plate Materials and Specifications

API 650 groups plate steels by allowable stress and application. Commonly specified materials for tank plates include:

ASTM Standards

ASTM A36 (26 ksi yield, ~250 MPa) – Widely used for shell and bottom in moderate conditions. It is inexpensive and widely available, though unsuitable for cold environments unless impact tested.
ASTM A283 Gr. C (also ~205–290 MPa) – A general structural steel sometimes used for low-height tanks.
ASTM A285 Gr. C (Plate for Pressure Vessels, 195–260 MPa) – Approved by API 650 but limited to thinner sections. More ductile, often a lower-cost alternative.
ASTM A516 Gr. 70 (Plate for Moderate-/Lower-Temp Vessels, 485 MPa tensile) – Common for higher-strength shells/bottoms. Has better toughness than A36.
ASTM A537 CL.2 (pressure vessel plate, ~450 MPa yield) – Higher strength and toughness for large tanks.
ASTM A553 (Type 1 & 2) – Low-temperature carbon-manganese plate (nickel-alloyed) for cryogenic service. A553 Type 1 (≈9% Ni) is specified in API 620 Appendix Q for LNG tanks.

EN Standard

EN 10025 S235JR / S355JR – European structural steels roughly equivalent to A36 (S235JR) and higher-strength A572/A656 (S355JR). Note that API 650 requires J0 or J2 impact-tested grades (tested at 0 °C or -20 °C) for S275/S355; ordinary “JR” grade (tested only at 20 °C) is not allowed for thicker plates.

JIS Standard

JIS G3101 SS400 / SS490 – Japanese equivalent structural steels (Y.S. 205–245 MPa and 245–295 MPa). SS400 is weaker than A36, so some designers avoid direct substitution unless the thickness is increased.

Other National Standards

API 650 permits “recognized national standards” if mechanical properties and chemical limits meet its Groups I–VI criteria. For example, CSA G40.21 (Canada) grades 300W/350W, or ISO 630 S275/S355, are often accepted.

For all plates, API 650 Section 4 requires the steel to be killed (fully deoxidized) and fine-grain practice, with careful control of C, Mn, P, S, etc. Higher-grade materials (Groups IV–VI) often need specific impact testing at 0 °C or -20 °C, even for service at ambient, to avoid brittle fracture in upset conditions. When selecting foreign steel, verify through mill test certificates that composition and impact quality meet API 650 requirements. (For example, Chinese SS400 may have lower impact energy than A36.)

LNG vs. Crude Oil Tanks

LNG storage tanks operate at –162 °C and impose far stricter material demands. Conventional API 650 plates (A36, A516, etc.) become brittle at cryogenic temperatures. Instead, inner tanks or baskets for LNG often use 9% Ni steel (ASTM A553 Type 1 or ASTM A553M) for excellent toughness. Recently, 7% Ni steels have been developed as cost-saving alternatives. These steels meet Charpy impact criteria (e.g.,≥34 J longitudinal at –196 °C for A553T1) per API 620 Appendix Q. Outer storage tanks (or roof and foundations) may use regular carbon steel at ambient temperatures.

Design differences include double-walled tanks with insulation and stricter leak-tight requirements. API 620 (not 650) is usually the governing code for aboveground cryogenic tanks, incorporating Appendix Q for materials. In summary, for LNG service, always use cryogenic-grade steels (A553, A553M, or higher-nickel alloys) for the wetted plates; standard API 650 steels are only for the insulated outer shell or above-ambient secondary containment.

Compliance with API 650 (2020)

Ensuring API 650 compliance involves following the code’s material, design, and fabrication rules:
Plate thickness and material limits: Adhere to Sec. 4.2.1.4: maximum 45 mm shell thickness. Use Section 4.2.2’s thickness limits per grade (for example, A537 may go thicker than A516). Specify plate classes that meet the required impact tests for the expected service temperature.
NDE and welding: Perform 100% radiography for shell-to-shell and annular joints. Roof and floor welds need 100% MPI/dye penetrant. Follow API 650 Sec. 8 for welder qualification (ASME IX), joint preparation, and testing.
Design rules: Use Sec. 5 and Appendices (e.g., Annex F/V) to compute thickness for shells and roofs. Ensure annular plate width ≥600 mm. Size bottom plates to meet deflection and buckling limits. Dimension weld lap/edge distances per Sec. 5.1.5 and 5.5.
Documentation: The tank nameplate and documentation must cite “API 650 – Twelfth Edition” (the 2020 edition is the 13th). Keep mill test reports for all plates (chemical, mechanical, impact) and welding records. Obtain third-party inspection as needed, especially for critical joints.
Corrosion allowance: Always add appropriate CA (often 2–5 mm) to plate thickness in calculations to allow for corrosion and potential mill surface defects.

Challenges and Best Practices

Weld quality and distortion: Thick plates (>10 mm) require preheat and controlled interpass temperature. Use sequence welding or shrinkage control to minimize warpage. Full-penetration butt welds must be achieved without defects. Inspect all completed welds (especially at shell and annular joints) with NDT.
Corrosion protection: Choose plated materials compatible with the stored product or apply coatings (epoxy or zinc-rich primer). The bottom plates often see water or solids, so a higher corrosion allowance or abrasion-resistant liners can be used.
Material availability and lead time: Large-diameter or extra-thick plates are specialized. Plan procurement months in advance. If importing, verify quality standards (e.g., don’t assume SS400 equals A36). Work with suppliers to ensure certifications meet API requirements.
Construction sequencing: Install the annular ring early, using a strong back or temporary braces to hold shell alignment. Use a wind girder (scaffold-like ring) during shell erection to keep the circular shape. When possible, pre-assemble roof panels on the ground, then lift them onto the completed shell.
Field adjustments: On-site deviations (e.g., foundation settlement or slight misalignment) should be accommodated by shim plates, slotted anchor bolts, or flange cuts, not by re-rolling plates. Verify the flatness of bottom plates before final welding to ensure watertightness.

Conclusion

By understanding the role of each plate type and following API 650 rules, EPC/EPCM teams can design and build safe, compliant tanks. Proper material selection (from A36 to A553), diligent welding practice, and attention to code details (plate width and weld quality) are essential for durable crude oil and LNG tanks. If you have steel plate RFQs for marine tank projects, don’t hesitate to contact us at [email protected] for a competitive and professional quote!