Damage and influences to oil tanks caused by severe strong ground motion due to a large earthquake (Mw6.6) that occurred in the district of Iburi-tobu, Hokkaido, Japan on September 6, 2018 are reported in this paper. In the vicinity of the seismic source region, two large-scale crude-oil storage bases are located. The neighboring two bases had in total 86 large oil storage tanks with a capacity of 115,000 m3. The oil storage bases were hit by strong ground motion with peak ground accelerations of 590 to 1,570 cm/s2 and with peak ground velocities of 50 to 80 cm/s. Shell plates of a small bunker A tank with a capacity of 306 m3 suffered diamond buckling and elephant-foot buckling. No large oil storage tanks lost their function of oil storage despite of the severe strong ground motion. However, most of them splashed oil from the gap between the floating roof and the shell plate, and many of them had damage to their pontoons, gauge poles, guide poles, rolling ladders, liquid-level meters, and shoulders of foundation. One of the 115,000-m3-in-capacity tanks was equipped with a displacement gauge system to measure uplift of the bottom of the shell plate from the shoulder of tank foundation. The system recorded a maximum uplift of 4.4 cm. This is the world’s first record of uplift of a large tank caused by a natural earthquake.
In order to clarify the physical property of a heavy-duty coating, the impedance spectrum of a thick vinyl ester resin organic coating containing glass flakes as additives that had been used for 16.9 years was analyzed. The phase angles of the impedance of the coating deviated from the behavior of an ideal capacitor and had frequency dependence. These impedance characteristics could be explained by considering exponent distribution of the coating resistivity along its thickness direction, which is one of the interpretations for a constant-phase-element (CPE) behavior. Furthermore, this result suggests that the residual lifetime of the long-used heavy-duty coating can be predicted based on physical parameters.
For the development of a method to evaluate the residual lifetime of a heavy-duty coating on the surfaces of the internal bottom plates of oil storage tanks the impedance characteristics of the coating were investigated by the laboratory immersion test of the coated steel panel in a 3 wt. % NaCl solution for 85 days and the field study of an actual oil storage tank that had stored oil over 266 months. As a result the impedance spectra of the coating could be analyzed with the electrical equivalent circuit of one CPE Constant-phase element. It was found that the CPE behavior in the impedance spectra is caused by the electrolyte solution permeation in the coating. In addition it was suggested that the degradation with age of the coating can be evaluated quantitatively by using the fitted parameter values of the CPE.
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