A Study of Influence of Locally Reduced Thickness on Stress of Bottom Annular Plate of Oil Storage Tank During Uplifting by Seismic Loading

In 1978, Off-Miyagi Prefecture Earthquake in Japan caused damage to large oil storage tanks. Their annular plates had been greatly reduced in thickness due to local corrosion, and they were broken by uplifting due to the earthquake. In order to examine the stress tendency of annular plates with local metal loss during uplift by earthquakes, nonlinear static uplift analyses by three-dimensional shell FEM were carried out on a 110,000kl oil tank. The sizes of locally reduced parts, such as the circumferential and radial dimensions, were taken as the analytical parameters. Many useful results were obtained regarding the relationship between the range of local metal loss and the stress in annular plate. The “stress increase ratio” is defined in this study as the ratio of radial surface stress in an annular plate with metal loss to that without metal loss. The following results were obtained. As the radial width of metal loss part decreases, the annular plate radial stress increases. The stress increase ratio is 1.47 when the thickness of radial metal loss part of radial width 23mm is reduced to 18.4mm from an original thickness of 21mm. For a circumferential length of metal loss part of less than 2.1m that is calculated by multiplying the radius of tank with the tank central angle of three degrees, the stress increases proportionally to the length of metal loss part. On the other hand, when the length is more than 2.1m, the maximum stress has a tendency to saturate.