May 1, 2001 12:00 PM

One method of analysis used in the petroleum industry is simply to require the pontoon ring to make up 30% of the area of the floating roof. The 30% method is based purely on area and would be insufficient for floating roofs that had corrosion allowances that would affect the weight of the roof. When compared to more precise analysis techniques, this method tends to provide non-conservative calculations for small tanks and very conservative results for large tanks.

Another simple approach is to assume the floating roof is rigid and all deflections are vertical (no tilting or local deflections). This method typically uses a 1.25 flexibility factor to account for additional deflection from the roof tilting and from local deformations. It provides a fairly consistent design, but when the floating roof has corrosion allowance applied the 1.25 flexibility factor is no longer adequate.

A third method assumes rigid planar tilting of the floating roof. This method typically assumes that local deflections are not significant. It appears to be over-conservative on smaller floating roofs and non-conservative on larger floating roofs, according to Erdmann.

A popular analysis technique is finite element analysis (FEA). It models a floating-roof structure as a mesh of connected elements. STAAD III is the software used for FEA. “This method of analysis produced a deflected shape that was very close to the measured deflected shape of the full scale punctured pontoon roof that was tested,” Erdmann says. “However, the maximum deflection of the outer rim appeared to be slightly conservative.”

Another method of evaluating punctured pontoon roofs is to model the pontoon ring as a beam on an elastic foundation. This analysis was performed, and the results very closely matched the measurements of the tested pontoon roof. Though the deflected shape of the tested floating roof was closely matched, the maximum deflection was conservative.

“One primary consideration is how accurately does each method calculate the deflection of the pontoon ring in the punctured area,” he says. “The curved beam and the STAAD III FEA analysis methods were both slightly conservative at the point of maximum deflection. The curved beam provided the closest approximation of the deflected shape.

“The FEA and curved beam methods provide sufficient results. In the future, additional full-scale punctured tests should be performed to validate the results of these two methods of analysis over a wide range of tank diameters.”

Since the first floating-roof storage tanks were developed in the 1920s, it has been the most widely used system for storage of volatile petroleum products. Many design changes and improvements have been made since then.

The first floating-roof storage tank was demonstrated by Chicago Bridge & Iron Company in 1923 as a means to reduce product evaporation, and that is still the reason floating-roof tanks are in use. Another obvious reason is safety. Crude and refined petroleum products are volatile and will readily evaporate at normal storage and handling conditions, producing vapors that are combustible over a range of concentrations with air.

“The floating roof originally was developed for use in open top tanks,” Gallagher says. “As product, weather, and environmental concerns became more of an issue, floating-roof designs were adapted for use in a tank with a fixed cover. The most advanced type of installation is used in applications that require absolute control of all emissions from the storage tank.”

New equipment developments have reduced overall environmental impact from a floating-roof tank. Thus, recent developments are important from a regulatory compliance standpoint.

A floating-roof design can range from a simple pan roof to the complicated structure of a double-deck floating roof. Design selection depends on many of the same factors used previously to select the tank configuration — product characteristics, site weather conditions, system operating requirements, storage capacity, and required through-put.

“The first floating-roof designs were stiffened pans,” Gallagher says. “In 1923 when the first floating roof was tested, the only application was as an external floating roof. The roof deck was sloped to the center for water drainage and to permit vapors to pass from under the deck to the perimeter rim space.”

The need for improved rim seals, roof drains, manual and automatic bleeder vents, rolling ladders, and other tank equipment has never stopped.

Floating roof systems are complex structures when it comes to design, analysis, and construction, Gallagher says. Many of the products currently in use were developed before the computer. Design work now may be completed using computer analysis and verified data obtained from the original field test programs. However, designs of a floating-roof structure still require a rigorous analysis.

“Design conditions for an external floating roof differ significantly from those that impact an internal floating roof,” Gallagher says. “A major difference is the added requirement that the external floating roof design must accommodate loads due to varying weather conditions. Other differences include floating roof access and the design of product emission control hardware. However, many of the basic design principles remain unchanged.”