Hurricane Harvey in 2017 was a grim reminder that understanding the risks and being prepared for storms before they take place is important. Indeed, the industry's performance was improved thanks to pre-planning and efforts on the part of many companies that implemented hurricane procedures and cooperatives. During Harvey, the total rainfall in some parts of the Texas coastline reached 50-60 inches over a few days. Some crude oil and gasoline tanks failed and spilled about 140,000 gallons.
Federal rules require companies to be prepared for spills but do not provide specific requirements to actually prevent these spills. Careful engineering and assessments can reduce and prevent the likelihood of storm-induced spills.
During hurricanes, we see wind, rain, lightning and storm surges. Each of these has effects that should be considered in a risk-screening exercise, which should accompany planning for these events. Wind has three concerns for the tank owner:
1.Overturning or tipping tanks over
2.Sliding or pushing tanks off foundations
3.Buckling
These events can initiate other risk events. Overturning will typically result in loss of tank contents. Sliding may result in loss of contents if connected piping is not sufficiently flexible to withstand the amount of movement. Buckling damages the tanks and may make the floating roof inoperable. Assessment of wind forces can be done using ASCE-7 or other standards, but the process is complex. Fortunately, determination of these forces will result in only a few wind pressure levels per site evaluated. Overturning, sliding and buckling forces can be determined in advance so the tank operator knows which tanks are the most vulnerable.
Another issue is, of course, buoyancy. To estimate the amount of ballast required to keep a tank from floating away, the weight of the tank must be estimated. It turns out it is easiest to do this by estimating the weight per square foot, although it varies from a 30-foot-diameter tank at 130 pounds per square foot (psf) to about 100 psf for larger tanks. Given this weight, the amount of liquid required to ballast the tank would range from 17-25 inches of water. But for light products with the specific gravity of 0.7, the height of product to ballast the tank ranges from 24-36 inches. An easy-to-remember rule of thumb is that 3 feet of product is needed just to keep the tank from floating away.
But recall that wind is also acting on the tank. This means there is potential for not only tipping the tank but also causing it to slide. Therefore, doubling the value to 6-8 feet of product would be a safe minimum level for product to stay in tanks under storm conditions.
Careful engineering and assessments can reduce and prevent the likelihood of storm-induced spills.
Another reason to fill the tank to high levels is buckling can occur from wind pressure. If the tank is filled, the buckling will not occur because the liquid product in the tank will act radially outward, countering the wind forces pushing inward. In practice, most tanks that have failed have had less than 6-8 feet of product in them. However, large tanks have more wind force on them, so filling the tanks with product is the best practice. This improves the drainage from the floating roof as well, since the head available to drain external floating-roof tanks is also increased by the enlarged floating-roof product level. I recommend that tanks should be filled to at least 50 percent of the tank shell height, as a general rule for these events.
Another good practice is to survey the tank bottoms externally before and after flooding events. When standing water infiltrates the foundation, accelerated settlement can occur. Many tanks may have new, highly stressed tank bottoms that could be subjected to failure from increased settlement.
For more information, contact Philip Myers at (925) 302-6707 or phil@pemyconsulting.com.
