Floating Roof Pontoon Sizing Under API 650 Appendix C: Buoyancy, Freeboard, and the Two-Compartment Rule

Single Deck vs Double Deck: Choosing the Right Type

External floating roofs (EFR) come in two primary configurations, and the choice between them is driven by buoyancy reserve, vapour emission requirements, and economics.

Single-deck EFR consists of a deck plate supported on a peripheral pontoon ring. The pontoon is the buoyant element — it is a closed annular compartment, divided radially into a series of individual watertight cells. The centre deck plate spans across the inner rim of the pontoon and sits close to the liquid surface; it is intentionally thin (typically 5–6 mm) and is not designed to be buoyant on its own. Single-deck roofs are the most common configuration for crude oil and clean product tanks where vapour emission regulations permit the small vapour space that exists between the centre deck and the liquid surface.

Double-deck EFR consists of two complete deck plates — an upper deck and a lower deck — with a sealed air space between them across the full diameter of the roof. This air space provides significantly more buoyancy per unit area than a pontoon-only design and eliminates the vapour space beneath the centre region of the roof entirely. Double-deck roofs are heavier and considerably more expensive to fabricate, but they are the preferred choice where strict vapour emission regulations apply, where a higher freeboard reserve is required, or where the product density is low enough that a single-deck roof cannot meet the two-compartment flooding requirement without an impractically large pontoon.

The choice directly affects the pontoon sizing calculation. A single-deck roof relies entirely on the pontoon volume for buoyancy, so every millimetre of pontoon depth and every centimetre of radial width matters. A double-deck roof has inherent distributed buoyancy and the sizing calculation is typically less sensitive to individual dimensions — but the weight is greater, and the structural adequacy of the deck plates under uniform load must still be verified.

The Buoyancy Calculation: Does the Roof Float?

The fundamental question is straightforward: does the upward buoyant force on the pontoon exceed the total downward weight of the roof assembly? API 650 Appendix C requires this to be checked under a defined set of load conditions, not just the nominal operating case.

The total roof weight to be supported includes:

• Dead load: centre deck plate, pontoon top and bottom plates, pontoon shell (inner and outer rim), radial bulkheads, structural rim members, all attached fittings (gauge poles, vents, legs, rolling ladder guides, seal hardware).
• Design rainfall (250 mm): API 650 Appendix C requires the buoyancy check to be performed with 250 mm of accumulated rainwater uniformly distributed over the centre deck area. This is not a conservative conservative — it is the mandatory load case. A roof that floats comfortably in dry weather may approach minimum freeboard with a full rain load if the pontoon is marginal.
• Two-compartment flooding weight: see below — this additional load is superimposed on the rain load for the governing check.

The upward buoyant force is:

Fₐ = ρₘₗₑₛ× g × Vₛₑₙ

where Vₛₑₙ is the volume of the pontoon (or, for a double-deck roof, the displaced volume of the full deck assembly) submerged below the liquid surface. For the roof to float at or above the minimum freeboard mark, this buoyant force must be at least equal to the total load.

In practice, the calculation proceeds by assuming the roof floats at a given draft (depth of submersion), computing the buoyant force at that draft, and checking that the freeboard (distance from the top of the outer rim down to the liquid surface) meets the minimum requirement. If it does not, the pontoon volume must be increased — either by adding depth, increasing the radial width, or both.

The Two-Compartment Failure Rule (Appendix C §C.3)

This is the provision that most often governs the pontoon sizing on single-deck roofs, and the one most often under-checked on early-stage designs.

API 650 Appendix C §C.3.3 requires that the floating roof remain afloat — with the minimum required freeboard — when any two adjacent pontoon compartments are simultaneously flooded with product. The word "any" is important: the designer must identify the most onerous pair of adjacent compartments (which is generally the largest pair, though for uniform designs all compartments are equal) and verify the roof still floats with those two fully flooded.

That check assumes only those two compartments are flooded; the remaining compartments are still intact and continue to provide buoyancy. The purpose of the rule is to verify that the roof has enough reserve buoyancy to tolerate that localized damage case without sinking.

The procedure is:

1. Identify the total volume of the two largest adjacent compartments.
2. Calculate the weight of product (at the design product SG) that fills those two compartments completely.
3. Add this flooded weight to the roof dead weight and the design rainfall load.
4. Compute the buoyant force available from the remaining unflooded pontoon compartments plus any distributed buoyancy (for double-deck roofs) at the expected draft.
5. Verify that the freeboard is still at least 150 mm under this combined loading.

The effect of this rule is that the pontoon must have substantially more total volume than the simple dead-weight buoyancy check would suggest. If there are twelve compartments and the roof just barely floats with all twelve intact, it will certainly sink when two are flooded. The standard is explicitly requiring a buoyancy reserve that can absorb the loss of two complete compartments without dropping below the minimum freeboard. This typically requires the pontoon to have a buoyancy reserve of at least 20–25% above the minimum needed for the dead-weight plus rain load case alone.

For new designs, the number of compartments should be chosen carefully. More, smaller compartments reduce the fraction of total buoyancy lost when two fail — but they also increase fabrication cost and complexity. The typical range is eight to twelve compartments for most tank diameters encountered in practice.

Minimum Freeboard Requirements

Freeboard is defined as the vertical distance between the top of the outer pontoon rim and the liquid surface when the roof is floating under the design load. It is the margin that prevents product from overflowing onto the centre deck, which would rapidly add weight and cause the roof to sink in a cascade failure.

API 650 Appendix C sets the minimum freeboard at 150 mm. This minimum must be met simultaneously with:

• The full design rainfall of 250 mm accumulated on the centre deck, and
• The two adjacent compartments flooded with product (§C.3.3).

These two conditions are not checked separately — they are checked together. The governing design load case is the rain load plus two-compartment flooding applied simultaneously, and the freeboard under that combined loading must be at least 150 mm.

This combined case is often significantly more demanding than either condition alone. A pontoon sized to float with 250 mm of rain and 200 mm of freeboard may drop to only 80 mm of freeboard when the two-compartment flooding is added — a clear failure of the combined requirement. The only remedies are to increase pontoon volume (deeper or wider pontoon), increase the number of compartments (reducing the volume lost when two fail), or switch to a double-deck configuration where distributed buoyancy reduces the sensitivity to compartment flooding.

For tanks that may operate at reduced product levels, there is also a low-liquid-level check. When the roof rests on its leg supports during very low product conditions, freeboard is not relevant — but the designer should confirm that the roof can re-float cleanly as product level rises without the rim catching on the tank shell or fittings.

How Product Density Governs the Design

This is the most commonly overlooked dimension of floating roof design, and it can lead to serious operational problems on tanks that receive variable-density products.

The buoyant force is directly proportional to the density of the liquid on which the roof floats. If the product SG decreases — for example, because a crude oil tank receives a delivery of lighter condensate, or a gasoline tank is cleaned and refilled with a different blend — the buoyant force per unit of displaced volume decreases. The roof sinks deeper into the product for the same weight, reducing the freeboard.

API 650 Appendix C requires the buoyancy and freeboard check to be performed at the minimum product SG expected in service, not the nominal design SG. This is a critical distinction. A floating roof designed for crude oil at SG 0.85 and verified to have 180 mm of freeboard under the full design load case will have substantially less freeboard — possibly below the 150 mm minimum — if the same tank later operates at SG 0.72 with a light condensate blend. The buoyant force drops by roughly 15%, and the draft increases accordingly.

In practice, this means:

• Always obtain the full range of expected product SG from the process engineer or operations team before beginning pontoon sizing.
• If the SG range is wide (e.g., SG 0.72 to SG 0.90), design the pontoon for the minimum SG. The roof may have excess freeboard at the higher SG — this is acceptable and conservative.
• If a tank is repurposed to a lighter product after original design, the floating roof must be re-verified. This is a real scenario that has caused floating roof failures in service.
• For tanks where the product SG may vary significantly batch to batch, consider specifying a minimum operating SG as an operational constraint and communicating it clearly to operating personnel.

Practical Tips and Inspection Notes

The following points come up repeatedly in design reviews and field inspections of floating roof tanks.

1. Always check both governing cases explicitly. The rain-load-plus-two-compartment case and the minimum-product-density case must each be evaluated independently, and whichever produces less freeboard governs. In most designs one case will be clearly controlling, but both must be documented. A design report that only shows the nominal-SG rain-load case is incomplete.

2. Number and label pontoon compartments on all drawings. This seems obvious, but it is frequently omitted on smaller projects. The two-compartment flooding check requires the reviewer and the construction inspector to be able to identify which compartments are adjacent and which pair is the governing case. Numbered compartments on plan drawings make the construction-phase weld inspection and the as-built verification straightforward.

3. Inspect and record pontoon drain plug condition annually. Each pontoon compartment should be equipped with a drain plug or access fitting that allows internal inspection and drainage. Blocked drain plugs — usually from corrosion or paint — allow water to accumulate inside the pontoon from condensation or from minor leaks at weld seams. Accumulated water inside a pontoon reduces the effective buoyant volume of that compartment. If this accumulation goes undetected across multiple compartments over several years, the freeboard can drop below minimum without any obvious external indication. Annual inspection of pontoon drain plugs and sounding for internal water accumulation is a simple and high-value inspection activity.

4. Verify the pontoon-to-shell clearance and seal condition. The secondary seal at the outer rim must maintain contact with the tank shell across the full range of operating levels. A damaged or detached secondary seal can allow product vapour to accumulate between the roof and the shell, creating a fire hazard. Seal condition should be checked at each opportunity when the tank is at low level.

5. Treat floating roof structural drawings as living documents. If the tank is re-rated, the product changes, or fittings are added to the roof, the original buoyancy calculation must be revisited. Additional weight from, for example, a heavier gauge pole assembly or an added sampling port can erode freeboard margin that was already close to the 150 mm minimum.

Design for the minimum product SG you will ever operate at — not the nominal. A floating roof that has adequate freeboard on crude oil (SG 0.85) may have insufficient freeboard when receiving a lighter condensate blend (SG 0.72). The difference is not academic — it is the difference between a compliant roof and one that is at risk of sinking under the design rain load.

For further context on the external pressure and stability checks that companion the floating roof design, see External Pressure Design and Wind Stability During Construction. For an overview of changes that affected floating roof provisions, see API 650 14th Edition Changes.

Related reading: Continue with External Pressure Design, Wind Stability During Construction, and API 650 14th Edition Changes to keep the full API 650 design workflow connected.