Taking the First Step to Safeguard LNG Storage Tanks from Earthquakes

Chris Hendrix, PE

Chris Hendrix explores the science of earthquakes, and how to ensure LNG storage tanks are seismically safe.

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Natural gas is a vital commodity and accounts for about a third of the United States’ total energy production. After natural gas is produced, it is liquified at extremely low temperatures (-260 °F) and shipped via tanker to U.S. terminals. Once into the U.S., this Liquefied Natural Gas (LNG) is distributed all over the country to local industrial facilities or gas plants where it is stored in tanks so that it may be vaporized and used whenever energy is needed.

Last year, a series of earthquakes in Ridgecrest, CA caused over $1 billion in damage. Highways cracked, chimneys fell, and storage tanks were damaged. While not often reported, earthquakes present a major hazard to LNG storage tanks. Shifting of the tank and connections due to an earthquake could cause gas leakage and insulation loss, which could lead to a major explosion and disaster.

No matter where LNG tanks are located within the U.S., it is important that they are seismically designed, which is required by federal regulation. In fact, all large tanks, even if they don’t contain hazardous materials, are considered “nonbuilding structures” by the building code and require seismic design.

The science of earthquakes

An earthquake occurs when two tectonic plates slip past one another, releasing an enormous amount of energy. In fact, a Magnitude 6.0 earthquake releases the energy equivalent of an atomic bomb. This enormous energy creates waves that travel within the earth and along its surface. As these waves move, they:

  1. Weaken (attenuate), similar to the rippling of waves caused by throwing a pebble in a pond
  2. Amplify as they reflect and refract off different soil layers

Because seismic waves can both weaken and amplify as they travel, determining the earthquake motions at a site is a complicated task. A seismic engineer needs two things to determine this motion:

  1. Geological location of the site (i.e. distance from known faults)
  2. Soil conditions underneath the site (usually determined by a geotechnical engineer)

The first step in seismic design is determining this motion. For a standard building, a structural engineer would use design maps and average soil properties to determine the potential earthquake motion at the site. This is a quick method that usually yields conservative results. However, for important items, such as LNG storage tanks, a more comprehensive analysis is required.

What is a Site-Specific Response Analysis?

A site-specific response analysis (SSRA) is a more comprehensive way to determine the earthquake motion at site's ground surface and requires a specialized skillset and software. Some examples of structures for which SSRAs are required include:

  • LNG storage tanks
  • Nuclear power plants
  • Structures on soft clay

Two steps are required for an SSRA:

  1. Determine the motion at the bedrock below the site. This is commonly referred to as the site’s seismic hazard and is determined based on probabilities and history of past nearby earthquakes.
  2. Calculate the motion at the ground level by modelling the waves reflecting and refracting through the soil layers. This is typically performed with special software.

Last year’s earthquakes at Ridgecrest demonstrate the importance of making our infrastructure seismically resilient. Our highly specialized structural/seismic safety team has the required skillset to ensure the design of LNG tanks meet the strict federal regulations and are seismically safe.

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