The number of earthquakes with magnitudes of three and larger continues to rise in the central and eastern regions of the United States. According to the US Geological Survey (USGS), these areas have experienced a 1000-fold increase in seismic activity since 2008. Most of these earthquakes cause little if any damage, but can be felt by humans.
On October 10, 2015, a 4.3 magnitude earthquake hit Cushing, Oklahoma. It was the largest earthquake recorded in the area to date. Cushing is considered to be the pipeline crossroads of the world, with a maze of pipelines and more than 85 million barrels of storage capacity.
This record-breaking quake caught the attention of terminal owner/operators and regulatory authorities. They wanted to understand how these smaller quakes were impacting aboveground tanks and if these tanks could withstand a larger, more powerful quake.
In 2016, the USGS released data that indicated a five to ten percent probability of a damage-causing earthquake for the area within the next year. This new data and the increase in seismic activity further intensified the need to assess the impact on storage infrastructure.
Aboveground Storage Tank Design & Seismic Loading
Aboveground storage tanks are built to withstand many environmental conditions. The American Petroleum Institute (API) and American Society of Civil Engineering (ASCE) have strict standards that guide tank design, fabrication, welding and construction. Seismic loading is one of the areas these standards address.
However, the standards related to seismic loading were based on data from USGS National Seismic Hazard maps, now outdated due to recent seismic activity, like the 2015 quake. A Cushing-based consortium engaged Matrix PDM Engineering to analyze how storage tanks responded to these earthquakes and if changes needed to be made. Matrix engineers selected six different open-top storage tanks in Cushing, Okla., designed to meet the minimum requirements of API 650. The tank diameters ranged from 84 to 295 feet, providing a good representation of the tanks in the area.
Peak Ground Acceleration vs. Magnitude
The team assessed the tanks’ response based on peak ground accelerations. Peak ground acceleration (PGA) is equal to the maximum ground acceleration that occurred during the earthquake at a given location. In other words, it measures how hard the earth shakes at a geographic point.
You may wonder why magnitude was not used in the calculations since earthquakes are typically defined by magnitude. Magnitude is measured on the Richter Magnitude Scale and measures the seismic energy released
by an earthquake. While this is helpful to describe the severity of the quake to the general public, engineers focus on seismic parameters in the design process. This is because they are evaluating structural response to the energy released and not magnitude.
How Aboveground Storage Tanks Respond to Seismic Events
An aboveground storage tank’s response to an earthquake can be broken down into two modes: impulsive mode and connective mode. The impulsive response is caused by the ground motion, which causes the tank to move and a portion of the tank’s contents to move as well. The connective response is caused by the sloshing of the product inside the tank.
Next, the tank’s size and the mass of the stored product are used to create pseudo forces that mimic impulsive modes and connective modes. A mathematical model of the tank takes the seismic parameters and the hypothetical forces from the quake to evaluate the tank’s response.
Evaluating Stress Caused by Earthquakes
The tank was evaluated in five different areas: lateral stability, dynamic hoop tensile stresses, overturning movement, shell buckling stress and sloshing. The Matrix team primarily focused on the tank structure, but it
should be noted that it is also important to consider secondary features of the tank. These would include piping attached to the tank, rolling ladders on the floating roof, floating roofs seals, etc. The team used PGAs recorded from two different locations near Cushing, Oklahoma, during the October 10, 2015 quake. The first location recorded a PGA of 13% g and this location was closest to the representative tank set. The second location was five miles away but recorded a maximum PGA of 60% g. These were the highest PGA data points recorded at area stations in the recent earthquakes.
The representative tank set performed well when subjected to 13% g PGA. The tank responses for lateral stability, hydrodynamic hoop tensile stresses, overturning movement, shell buckling stress and sloshing were all well within design norms for the tanks with a variety of diameters. As you may recall, all of the tanks were designed at minimum to meet API 650 standards. These results are supported by the fact that no damage was
reported following the earthquakes in Cushing.
The representative tank set was then tested against the 60% g PGA that was recorded at the second location. For some of the larger tanks, there was a slight variance over the design limit for hydrodynamic hoop stress.
Additionally, some of the smallest tanks, those less than 100 feet in diameter, indicated that they may need anchorage.
In both scenarios, the models were run assuming that the tanks were filled to capacity. Additional calculations were made to determine how a reduction in tank capacity would keep the tanks within API 650 design limits during a seismic event with a 60% g PGA. This could potentially be one way to mitigate risk without retrofitting tanks.
In conclusion, it is important that the industry looks ahead and proactively plans for increased seismic activity. This analysis relied on recorded PGAs to test the response of aboveground storage tanks. Ideally, tank design would be based on the maximum considered earthquake, expected PGAs and current seismic parameters for a given site. With this more accurate data, tanks and terminal operators can better prepare for the next earthquake. New tanks can be built to withstand greater seismic activity. Existing tanks and supporting equipment can be evaluated and, if necessary, be retrofitted to mitigate the increased risk for earthquakes we
are seeing in this area and across the central and eastern regions of the United States.