Bottom Plate and Annular Ring Design Under API 650: When Is an Annular Plate Required?

Bottom Plate Layout: Sketch Plates and Annular Ring

The bottom of an API 650 tank is not a single uniform plate — it is an assembly of two distinct zones. The central field is made up of sketch plates: rectangular or trapezoidal plates that are lap-welded together to form a continuous membrane. The perimeter zone, directly beneath the shell, is the annular plate (also called the annular ring or annular bottom plate): a continuous ring of thicker plate that connects the shell to the bottom and resists the uplift and bending forces at the shell-to-bottom junction.

When an annular plate is not required, the bottom is composed entirely of sketch plates, and the shell sits directly on the outermost sketch plates. When an annular plate is required, it replaces the outermost sketch plates for the full perimeter, and the interior sketch plates lap-weld onto the inner edge of the annular ring. The distinction matters because the annular plate is subject to both a minimum thickness requirement (driven by Table 5-1a) and a minimum width requirement (driven by the shell-to-bottom weld development length).

The annular plate must be a single continuous ring — it cannot be fabricated from isolated segments. It is butt-welded at its radial joints (the joints running from inner to outer diameter). The lap welds connecting the interior sketch plates to the inner edge of the annular plate are continuous fillet welds, typically on the top side only for the inner joint.

When Does API 650 Require an Annular Plate?

API 650 Table 5-1a provides the trigger condition. The annular plate is required when the product of the maximum liquid height H (in metres) and the design specific gravity SG exceeds a threshold that depends on the first shell course thickness. For most practical tank geometries, the condition reduces to: if H × SG is large enough that the first shell course thickness exceeds approximately 19 mm, an annular plate is mandatory.

In plain terms: any tank storing a heavy product (SG approaching 1.0 or greater) at a height above roughly 10–12 m is almost certain to trigger the annular plate requirement. This is where many designers make an error — they classify a tank as "small" based on diameter and skip the check. Diameter does not appear in the Table 5-1a trigger; only H × SG (through its effect on first-course shell thickness) governs.

Common examples where the check is incorrectly skipped:

• A 12 m diameter, 18 m tall crude oil tank (SG ≈ 0.85): H × SG ≈ 15.3 — likely triggers annular plate.
• A 15 m diameter, 14 m tall diesel tank (SG ≈ 0.84): H × SG ≈ 11.8 — check carefully against first-course thickness.
• A 10 m diameter, 20 m tall produced water tank (SG ≈ 1.05): H × SG ≈ 21 — almost certainly triggers annular plate.

The correct procedure is: (1) calculate the first shell course design thickness using the 1-foot method or VDP method, (2) look up that thickness in Table 5-1a to determine whether an annular plate is required and what its minimum thickness must be.

Calculating Minimum Annular Plate Width

Per API 650 §5.4.1, the annular plate must satisfy two independent width requirements, and the larger governs:

1. 600 mm inside the shell: The annular plate must project at least 600 mm inside the inside face of the shell. This is an absolute minimum regardless of tank size or plate thickness.
2. Weld development length projection: The width must be sufficient to develop the shell-to-bottom fillet weld. The minimum projection L_min beyond the inside edge of the shell-to-bottom fillet weld is given by the formula in §5.4.1:

Note that t_b in the formula is the annular plate thickness (not the sketch plate thickness). Because the required annular plate thickness increases with shell course thickness (per Table 5-1a), the minimum width also increases for heavier-duty tanks. For a large crude storage tank with a 25 mm annular plate and a 60 m diameter, L_min can exceed 1.0 m, making the 600 mm absolute minimum irrelevant.

The total annular plate width (from the outer edge of the shell to the inner edge of the annular plate) must also account for the shell-to-bottom weld size and the plate extension beyond the outer face of the shell, which is typically 50 mm minimum for lap-welded details. The governing total width is the sum of the outer extension, the shell thickness, and the inside projection.

Minimum Plate Thickness Requirements

API 650 specifies separate minimum thickness requirements for annular plates and sketch plates.

Annular plate minimum thickness is governed by Table 5-1a, which maps first shell course design thickness to required annular plate minimum thickness. Representative values from the table are:

• First shell course design thickness ≤ 19 mm → annular plate t(min) = 6 mm
• 19 mm < t ≤ 25 mm → annular plate t(min) = 8 mm
• 25 mm < t ≤ 32 mm → annular plate t(min) = 10 mm
• 32 mm < t ≤ 45 mm → annular plate t(min) = 13 mm
• t > 45 mm → annular plate t(min) = 16 mm

The critical point: use the design thickness of the first shell course — not the ordered thickness — as the input to Table 5-1a. Using the ordered thickness (which is rounded up) may push you into a heavier annular plate requirement than the standard actually demands. This is one of the most common unnecessary over-designs seen in API 650 calculations submitted for third-party review.

Sketch plate minimum thickness: Per API 650 §5.4.1, all bottom sketch plates have a minimum thickness of 6 mm, regardless of tank diameter. There is no graduated minimum by diameter for the sketch plate (unlike the shell, which has a minimum linked to diameter). In practice, many engineers specify 8 mm minimum sketch plates to allow for settlement grinding — the process of grinding the bottom to restore flatness after initial hydrostatic testing, which consumes 1–2 mm of plate locally.

Corrosion allowance on the bottom: API 650 does not mandate a standard corrosion allowance on tank bottoms. Any CA on the bottom is owner-specified and must appear in the design data. However, API 653 (the in-service inspection standard) requires remaining-life calculations for tank bottoms, and owners of long-lived tanks typically specify a nominal bottom CA of 1.5–3 mm in the original design to extend inspection intervals. If the owner specifies no CA, the ordered bottom plate thickness equals the structural minimum — and the inspection programme must account for that.

The Shell-to-Bottom Weld and the 6 mm Rule

The shell-to-bottom weld is the most critically loaded weld in the entire tank structure. It must transfer the full hydrostatic shear from the shell into the bottom plate, and it must resist uplift loads from internal pressure, wind, and seismic events. API 650 §5.1.5.7 requires a continuous fillet weld on both the inside and the outside of the shell at the shell-to-bottom junction.

The minimum fillet weld size at the shell-to-bottom junction is governed by the 6 mm rule: the fillet weld size must be at least 6 mm, but need not exceed the annular plate thickness t_b if t_b is less than 6 mm (which can only occur when t_b = 6 mm minimum — in practice the weld is then specified as 6 mm to match). The common error: specifying a 5 mm fillet weld (perhaps to match a lighter weld specification used elsewhere) on a tank with a 10 mm annular plate. This is non-compliant and will be flagged in any competent third-party review.

A second common error: specifying an intermittent fillet weld at the shell-to-bottom junction. API 650 §5.1.5.7 is explicit — the weld must be continuous on both sides. No intermittent welds are permitted at this joint, regardless of product service or design pressure.

For tanks in corrosive service, owners frequently specify a weld leg size larger than the 6 mm minimum — 8 or 10 mm is common — to provide a corrosion reserve on the weld itself. This is good practice but must be noted on the drawing as an owner requirement, not a code minimum.

Practical Tips

Pulling together the above into actionable guidance for everyday design:

1. Always run Table 5-1a against the design thickness of the first shell course, not the ordered thickness. Calculate the design thickness (before rounding), look it up in Table 5-1a, and record the required annular plate thickness. Then order the annular plate at or above that minimum.
2. Check the trigger for every tank — regardless of diameter. Diameter does not appear in Table 5-1a. A 10 m diameter tall tank can require an annular plate while a 50 m diameter squat tank does not. Automate or checklist this step so it is never skipped.
3. For tanks on a ring-wall foundation, verify the annular plate extends to or beyond the outer edge of the ring wall. The ring wall provides bearing support, and the annular plate must span the full width of the bearing surface to avoid a cantilevered edge condition at the outer shell. This is particularly important when the ring wall is wider than the typical 300–400 mm.
4. When the annular plate is very wide (greater than approximately 1.8 m), consider butt-welding annular plate segments at the radial joints rather than lap-welding. Wide lap-welded annular plates can develop localised uplift stresses at the lap weld edges during hydrostatic testing and seismic events. Butt-welded joints flush-ground on top provide a flat bearing surface and eliminate the stress concentration at the lap edge.
5. Record the governing criterion for the annular plate width. In your calculation record, note whether the 600 mm minimum or the L_min formula governed the inside projection. If L_min governs, show the input values of D and t_b used. This makes the calculation transparent and easy to verify during design review or third-party audit.
6. Bottom plate CA is owner-specified — confirm it in writing. If the owner has not specified a CA for the tank bottom, document this explicitly in the design basis. A missing CA specification on the bottom is a common gap in design data packages, and it creates disputes during API 653 assessments years later.

The annular plate check is triggered by product height × SG — not tank diameter. A 12 m diameter, 18 m tall crude tank almost certainly needs an annular plate, while a 40 m diameter, 6 m tall water tank does not.

Related reading: Continue with Shell Course Thickness Design, Corrosion Allowance Strategy, and Foundation Loads for API 650 Tanks to keep the full API 650 design workflow connected.