External floating roofs (EFRs) are commonly used for petroleum products like gasoline, crude oil, petrochemicals and more. They are a cost-effective way of reducing losses of product to vapor and preventing the risk of fires. Unlike fixed roofs, EFRs move up and down with the liquid in storage. They operate by floating on top of the liquid's surface, allowing little to no space for air above the liquid. However, as a heavy moving part partially exposed to the elements, their inclusion in tank systems can introduce new modes of failure.
For example, Hurricane Harvey recently resulted in numerous oil spills throughout the South. This has happened with previous tropical storms such as Hurricane Katrina. One mechanism of failure to consider in high-precipitation situations -- like in a hurricane -- is buckling or sinking of a tank's floating roof. The floating roof can accumulate too much water and buckle under its weight. Normally, gravity draws precipitation from floating roofs through a drain line and valve located on the tank shell near ground level. However, in hurricane conditions, where rainfall intensity can be in excess of 10 inches in 24 hours -- i.e., the API-required design criteria for floating roofs -- secondary containment can flood and floating roofs may not be able to drain adequately.
Consider a 150-foot-diameter tank with an EFR filled to 40 feet. It has a 6-inch drainage hose for the roof, snaking down to an outlet nozzle 2 feet above the ground. From the physical laws of hydraulics, the driving force for drainage is the 38 feet of vertical distance between the drain's inlet and outlet. Assuming no blockages, it would drain at about 1,100 gallons per minute (gpm). Rainfall of 8 inches per hour over the 150-foot-diameter tank, which is possible during the high-intensity periods of a hurricane or storm, would result in 1,450 gpm of water landing on the EFR. In this situation, water would accumulate on the EFR at 350 gpm. Given API 650 C requires EFRs to maintain buoyant under 10-inch rainfall -- or 110,000 gallons for the 150-foot tank -- it would take about 5.5 hours for the tank to reach the 10-inch threshold. Perhaps this would be enough time for the high-intensity period to end and the roof to drain.
Now consider a different scenario. Consider the same tank and rainfall but instead, the tank is drained; the EFR is landed, resting against the ground, and the drain inlet is now only 8 feet from ground level. The driving force for drainage is only 6 feet of vertical distance, and the hose only drains at 450 gpm. This is ignoring any changes on the outlet side. During Hurricane Harvey, the secondary containment of many tank farms filled up with water as high as 6 feet. This means there would be only 2 feet of vertical distance to drive drainage. With secondary containment flooded, the EFR only drains at 250 gpm. In this case, drainage is practically stalled, and water would accumulate to 10 inches in only 1.5 hours, which is far too quickly for safety. In a short time, the EFR could buckle just as floating roofs did when Hurricane Harvey hit.
Under adverse rain conditions and ground flooding in hurricanes and tropical storms like Harvey, draining EFRs can slow to unacceptable levels or even stall, leading to their possible mechanical failure and millions of dollars' worth of oil spills. It is difficult to determine the actual flow requirements of an EFR's drainage system, as rainfall intensity is a probabilistic function depending on the kind of storm encountered. Rainfall intensity is transient to the minute with changing meteorological conditions. Additionally, drainage flow can even vary by about two or three times, depending on the height of the stored liquid and the EFR.
Drainage flow can even vary by about two or three times, depending on the height of the stored liquid and the EFR.
Before the next storm sinks any of your floating roofs, we recommend calculations for drainage be performed on all of them using a probabilistic rainfall criteria that is reasonable for your area. Armed with this information, you can make decisions such as whether you will fill the tank with product to ensure proper drainage
For more information, email Andy Wong at andy.wong@berkeley.edu.
