In recent years, Oklahoma has experienced hundreds of magnitude 3+ earthquakes annually — with each year since 2013 bringing a remarkable increase over the prior year. What is the cause of this continued increase in earthquakes? According to Oklahoma’s Office of the Secretary of Energy and Environment’s Earthquakes in Oklahoma: What We Know webpage, “The Oklahoma Geological Survey has determined that the majority of recent earthquakes in central and north-central Oklahoma are very likely triggered by the injection of produced water in disposal wells.” In other words, the practice of hydraulic fracturing, or fracking, and its byproducts is a real concern.
One area of concern involves the huge tank farms in Cushing, Oklahoma. Cushing bills itself as the “pipeline crossroads of the world.” At least 13 major oil pipelines are in place and actively delivering up to six million barrels of oil each day to the Cushing oil storage facility, which is the largest commercial crude oil storage hub in North America. How much crude oil is stored here? According to Reuters, earlier this year, Cushing’s steel tanks held more than 66 million barrels of crude oil.
The Cushing Hub, as the tank farm is often called, stores crude oil in above ground storage tanks spread out over several miles. With strong earthquakes occurring frequently in the area, the concern is that these steel storage tanks and the various pipelines running throughout Oklahoma will be at risk of damage or structural failure from the strong tremors.
According to a recent article in Tulsa World, engineers are looking at what possible API 650 design modifications need to be made to the Cushing tanks to ensure they can withstand more intense earthquakes. Earthquakes are not created equal because it depends on how the earth moves during the quake as to how it will impact the structures above ground. “Engineers say Richter Scale magnitude is not as important as other factors called ‘accelerations,’ which measure the ground’s horizontal and vertical movements.”
So what is needed to make sure the tanks can withstand the accelerations from the hundreds of annual earthquakes? It may be as simple as only filling the largest tanks to the limit of 90% capacity. According to many tests conducted on the above ground steel tanks, the biggest stress on their structure is the movement and weight shift of the oil inside of the tanks. If the amount of the product is decreased, the potential magnitude of the stress is also decreased.
These simple changes will help prevent a structural failure of an above ground storage tank in the hub as well as a possible disruption to the U.S. energy market. The Oklahoma Commission also does its part to control the impact fracking has on creating the earthquakes.
The Oklahoma Corporation Commission typically instructs well operators to temporarily halt or reduce wastewater injections after strong earthquakes. According to an NBC News report, 58 disposal wells have been shutdown by the commission.
While fracking remains controversial and earthquakes are indeed rattling nerves throughout Oklahoma, we can take comfort in the fact that all oil storage tanks are built to the API 650 construction standard. The standard applies to the construction of steel storage tanks intended for storing oil and petroleum products and is used as the international standard for building most large liquid storage tanks. The documentation for the API 650 standard is nearly 450 pages long — with a signifianct portion of the standard focused on the seismic design of storage tanks.
As a API 650 fabricator, we know what goes into the construction of each API 650 above ground storage tank. The API 650 standard for petroleum storage tanks are in place precisely because seismic activity and situations that cause stress to storage tanks. The API 650 standard creates a significant factor of safety to prevent structural tank failures.