Collapsible fabric tanks can provide cost-effective liquid storage solutions for GOM contractors
Since World War II, collapsible pillow or bladder tanks have been employed by both the armed forces and the resource industry for temporary and long-term liquid storage, including fuel, oil, kerosene, water and other chemicals. As the resource sector explores increasingly harsh terrain, tank designs and materials must keep pace with the industry.
The biggest change in North American pillow tank construction over the decades has been the material used and the manufacturing methods. While vulcanized rubber is still used for collapsible storage in Europe, the North American industry has moved to polyurethane-coated fabric.
SEI Industries Ltd. with headquarters in Delta, British Columbia is one of a number of manufacturers of pillow tanks. It has designed, manufactured and commissioned structural engineered fabric products and related systems for remote site logistics since 1983. The GOM sector accounts for about two-thirds of the company’s sales, with military contracts making up much of the rest. Aviation, environmental and firefighting customers round out the client roster.
The difference between military and GOM clients?
“The civilian customer is looking for a large containment system that’s transported easily,” says Paul Reichard, division manager with SEI. “They’re looking at how this product will save them money versus a steel tank and drums to provide storage capacity where none exists. They demand a turnkey operation with everything required for immediate deployment. The military doesn’t care about price. They want instantaneously usable storage that’s tough as nails.”
Product life depends on a number of factors, including the tank design, the client’s needs, and how often the tank is moved. Tanks built to U.S. military spec, for example, are designed to last 18 to 36 months and/or three moves. Tanks designed to Canadian military spec must last six to 10 years with no limits to the number of moves.
Introducing a liquid initiates the product life cycle. At the end of that service life, tanks are rolled up, shredded and used either as insulation or boiler fuel.
Thermal or RF construction
The tanks are primarily constructed using one of two methods. In the first, the tanks are constructed thermally with seams heated by either hot air or a hot metal wedge that liquefies the surface coating of the tank material. The material is then squeezed between contact rollers and cooled as the seam is completed.
“This method is fast and economical, but the alternate heating, rolling and cooling cycles can allow gaps to form if it’s not done properly,” says Reichard.
SEI uses radio frequency (RF) welding, a process that liquefies the entire thickness of the fabric coating in a consistent fashion by agitating the molecules at the optimum frequency under a hydraulic press. “The big challenge with RF is the slower speed of welding,” says Reichard. “For a two-foot seam it can take 30 to 45 seconds per weld.”
The company now uses a much larger purpose-built RF welder that achieves the economics of hot air or hot bar welding. But no matter how carefully constructed, pillow tank seams are subject to diffusion, the migration of liquid molecules from a high-pressure area to a lower-pressure area between the panels. “These are not fuel leaks, but represent the migration of some of the volatile components in the fuel, which have a lower flash point,” says Reichard.
Use determines tank design
The three prime considerations for GOM sector tank design: how much the tank will hold, the type of liquid contents, and the climate where the tank will be employed.
The maximum fuel capacity of a collapsible tank is determined by safety regulations in each country. In the U.S., regulations allow fuel tanks to hold a maximum of 50,000 gallons, while in Canada they’re limited to 30,000 gallons. In Chile, by contrast, the tanks are limited to 10,000 gallons. The smallest effective size for a stationary pillow tank is approximately 500 gallons. “Any smaller than that, and you’d find it more economical to use a drum,” says Reichard.
Physics determines the maximum tank height. “We build them to inflate to a maximum of about 60 inches when full,” says Reichard. “Liquids just naturally tend to spread out beyond that height.”
The tanks can be configured in any shape, from wide-open and flat, to long and narrow, or from ellipses and donuts to octagons.
Cloth weight indicates the strength and puncture-resistance of the enclosure. A light-duty water tank is built using 18-ounce fabric, while a fuel tank may be constructed of 38- or 45-ounce fabric.
“We construct the tanks using different coating materials, depending on the contents,” says Reichard. “A water tank can only be used with water and a fuel tank can only be used with fuel. If you put fuel inside a water tank, for example, it would degrade the coating.”
Even apparently similar materials may require specific tank design. Gray water, for example – wastewater generated by such activities as washing laundry at a remote site – must be stored inside containers made from different fabrics and coatings than those used to store either potable water or sewage.
Climate likewise alters tank design. In a moderate climate, flexible gasoline tanks generally require an ester-based coating, while diesel tanks require an ether-based coating. However, diesel would require an ether-based coating in the jungle, but an ester-based coating in the arctic and desert.
Arctic, desert, and jungle
SEI differentiates its product lines with an identifiable color scheme. Camo-green represents jungles, brown represents deserts, and white is designated for the Arctic.
“That’s as much camouflage as it is marketing,” says Reichard. “We want them to be easily identified by the user, and easily differentiated.”
The essential differences between climate-specific models:
Arctic tanks require significant UV resistance and are designed to be unfolded, deployed and re-folded at temperatures falling as low as -58 degrees F where they must remain flexible and strong. The vent system is designed to accommodate deep snowfalls. Reichard says the most common question asked about Arctic tanks is whether polar bears like to snack on them. “To date, there has not been one documented case of polar bears eating collapsible fuel tanks,” he says.
Desert tanks require larger venting capacity to release pressure during significant changes between cold night temperatures and broiling daytime ground temperatures as high as 176 degrees F. The tanks can be shaded by the sun using a tent accessory, or covered with a new-style liner, essentially a pillowcase.
Jungle tanks require extra protection from a combination of ultraviolet light and hydrolysis, a chemical reaction in which water molecules act to break down polymers. “Hydrolysis could chew up a tank and spit it out,” says Reichard. “We now use a hydrolysis-resistant urethane bonded to a polymer cured substrate.” While the other models are suitable for fuels with aromatic content to 60 percent, the jungle models are best used with fuels containing fewer than 40 percent aromatic content.
Preparing a site for pillow tanks isn’t rocket science. First locate level terrain away from buildings or watercourses, and remove rocks, sticks and debris. The pillow tank is never placed directly on the ground but must be placed inside a secondary containment system, or berm.
“The berm structure depends on the regulations of each country,” says Reichard. “It could be an in-ground geotextile liner placed inside an excavated depression, or a frame surrounded by a liner.”
Once in place, the contractor will install appropriate accessories, including pumps, vents and hoses. Pillow tanks can also be interconnected to form a tank farm.
“You dig the site holes, lay out all the tanks and connect them through a manifold system,” says Reichard. “You can install and fill a 10-million-liter-tank farm inside three weeks.”
The most common pillow tank user error: overfilling. The tanks have a design capacity for some overfill, but adding too much liquid puts a strain on tank material and creates inaccurate content readings. Second most common user error: leaving a valve open.
SEI recommends that tanks be inspected monthly, including connections, hoses and fittings.
Contractors should also keep a record of what’s going into the tanks and what’s coming out, says Reichard. “If it’s fuel, we recommend limiting access to the site to authorized personnel only, because theft is a major problem, even on a remote site.”
A berm supporting a pillow tank full of fuel must remain empty of precipitation to perform its functions as a containment system.
“What we’re concerned about is that a berm might fill with rain or melting snow,” says Reichard. “If there was a fuel spill, we don’t want fuel breeching the secondary containment system.”
Likewise, pressure vents on the tanks must also remain free of fluid or fallen snow. The systems are fitted with gravity drains that remove water, but will shut down if hydrocarbons make contact with a special cartridge inside the drain.
SEI is testing a remote monitoring service that will allow the company to inform contractors of the condition and contents of pillow tanks when tanks are untended, often for two to three months.
“One of the misconceptions about something called a ‘pillow’ tank is that it isn’t tough,” he says. “You can’t simply walk up to a pillow tank and stab it with a knife—it takes a lot more than that to destroy one. Even under a shock wave from a missile strike 30 meters away, the force of the shock wave tends to move over the surface of our pillow tank, not through it, so it doesn’t rupture.”
Field repair techniques
Pillow tanks can be repaired in the field, in the same way a patch is applied to an inflatable swimming pool. In military settings, the tanks have been repaired after taking a bullet. “I have never heard of a bullet igniting fuel like you see in the movies,” says Reichard.
To repair small holes, the company supplies temporary plugs that can be screwed into the tank. For small tears, the area of the fabric with the hole in it is clamped to affect a temporary repair, and then the hole is repaired beyond the clamp.
While mining currently represents the company’s most significant GOM business, Reichard says he expects oil and gas to take the lead with the introduction of collapsible frac tanks. “The average frac tank will be moved 12 times per summer and 12 times per winter,” he says. “It has to be rapidly deployable, hooked to a Cat and unrolled quickly. The frac market required us to develop a 50-ounce fabric with a thicker and denser coating that’s high in abrasion resistance and puncture-resistant to -58 degrees F.”
The company has also designed and launched the Bulk Aviation Transport Tank (BATT), a collapsible, double-walled, baffled fabric tank designed to transport fuel to remote sites via aircraft.
The new design is part of a five-year rollout. The BATT must be designed specifically to fit in each type of aircraft and approved for use in each country. The design has been approved in Colombia and the company expects approval in Canada this year, with Alaska next on the list. SEI is also applying to the International Civil Aviation Organization for worldwide approval.
“Using it in the GOM field, you don’t have to take back empty drums on the return air trip,” says Reichard. “Roll up the tank and you can return with personnel, cargo or core samples instead.”