Seismic Analysis of Double Deck Floating Roofs of Siraf Storage Tanks with Condensate, Light and Heavy Crude Oils

Seismic vibration of the double deck floating roof of Siraf storage tanks located in southern Iran has been studied. Condensate of Nar and Kagan gas field in the south of Iran as a very light hydrocarbon, Lavan as light and Soroosh as heavy crude oil content have been chosen. In addition to fluid-structure interaction, intermediate stiffeners, foam seal with nonlinear radial compression behavior and contact friction between the seal and the inner side of the wall are also considered. Under the above conditions, modal and time history analysis have been performed. For time history analysis, Sarpol-e Zahab and Shonbeh earthquakes in Zagros seismotectonic province of Iran and Sakaria as an earthquake near Iran were selected. Dominant natural frequencies, mode shapes of the roof parts and damping ratios of the first and second natural frequencies in addition to overall and spectral behavior of the roof in each liquid case were obtained and discussed. Changing condensate to Soroosh oil made about 17% hydroelastic natural frequency decrement and about 10% damping ratio decrease for the first natural frequency. The results showed that dominant natural frequencies and the relevant damping ratios decrease with moving from light to heavy liquid. The vibration of the roof fundamentally depends on the frequency content of earthquakes to such natural frequencies. Also, a floating roof in heavier liquid is more vulnerable to vibration according to the scaling method and steady-state amplitude.