The largest structures of Freeport LNG’s facility on Quintana Island are the two LNG storage tanks. As part of the plant design features to accommodate expected export volumes, a third tank is being constructed. This tank was designed and is being built by CB&I. It will be capable of holding 160,000 cubic meters of LNG, approximately the amount of liquefied gas that one large LNG tanker can carry.
Over the last year, work has progressed on the walls of Tank No. 3, specifically the carbon steel liner and exterior concrete wall. On January 12th, the roof was raised to cap the tank. With the tank enclosed, the interior construction of the nine-percent nickel-steel tank will commence.
Roof raising for a structure of this size represents an impressive event. The roof itself is constructed inside the tank. Upon completion, it weighs approximately 1,600,000 pounds and is raised 123 feet from the ground to its final anchoring place. This herculean task is accomplished with nothing else but compressed air.
High-volume fans are attached to the ground-level door sheets to provide the four to five pounds of air pressure necessary to lift the roof. The roof’s edges that meet the walls are sealed with sturdy plastic sheets acting as a "gasket" to maintain the air pressure.
Twenty-eight cables crisscrossing the domed roof structure, top and bottom, act as a guide system and help keep it aligned with the tank body. The speed of the lift can be controlled with a series of nozzles on the roof's top to let air out and slow the progress. There is a redundant set of fans and generators to power the lift and backup the primary fans in case of a mechanical failure.
The process of raising the roof can take an average of two hours, but was somewhat shorter on January 12th.
This herculean task is accomplished with nothing else but compressed air
Once the roof arrives at it anchoring point, a crew of 20 welders and 20 fitters are arrayed around the tank's circumference to attach it permanently to the tank's compression ring. They use a series of H-braces with wedges and some U-straps that they weld to the top of the tank. When it is securely wedged, they begin welding the roof to the compression ring. This welding process can take several hours before the fans can be safely turned off to make sure the roof will stay put.
The new roof arrives at its welding point with a network of Nelson metal studs attached. These aid in constructing the rebar mesh needed for the concrete layer that will be poured two-foot thick across the entire roof. The outer concrete completes a double-containment system that is able to withstand external forces that might impact the outside of the tank and that protects the vapor barrier and nickel-steel tub.
"The most challenging part is to get all the different people involved in it to do the things they need to do at one time," remarks Nick Tabler, Freeport LNG Tank 3 Supervisor. "CB&I has a lot of experience raising roofs of this nature, and we at Freeport LNG also have some experience doing it too... There's probably none better in this immediate area for safety right now. And we intend to keep it that way."