Tomakomai oil tanks in which the floating roofs sunk during the 2003 Tokachi-oki earthquakeWhen oil tanks are shaken due to long-period (several seconds to more than ten seconds) strong ground motions, the surface of oil inside tanks may swing dramatically. This is known as sloshing. Some oil tanks have "floating roofs" in which the top cover or roof floats on the oil within. When sloshing occurs, the floating roof shakes as well. During the 2003 Tokachi-oki earthquake, the "floating roof" of oil tanks in the city of Tomakomai shook dramatically (maximum sloshing wave height of 3 m). This resulted in the damaging of the floating roofs, which ended up sinking into the tanks. This was an extremely dangerous situation because the oil was exposed to the atmosphere. The naphtha tank that suffered such damage burned fiercely for some 44 hours straight.

What sort of measures should be taken to prevent the damage and sinking of "floating roofs"? How does the degree of shaking correlate to the amount of damage to "floating roofs?" Solving such problems requires an in-depth understanding of the shaking of "floating roofs" during earthquakes. To that end, we are conducting shaking experiments for "floating roofs" using small model tanks and actual oil tanks. As a result, we are gaining data useful for improving the ability to accurately predict the sloshing wave height and assessing the effects of floating roofs on sloshing wave heights.

Experiment to assess the behavior of a floating roof in liquid sloshing in an actual oil tank

Experiment results on the shaking of a floating roof on a model tank: Relation between the intensity of the shaking applied to the tank and the sloshing wave height

Oil surface shape in a sloshing tank (computer simulation)