Analytical calculation of lightning strike probability for floating roof tanks

Fire hazard studies have shown that lightning is a major threat to petroleum storage facilities, especially floating roof tanks (FRT) due to the risk of lightning-induced fires which may ultimately result in tank boil over, and an increased risk of the fire spreading to adjoining facilities. Lightning is a natural occurrence with scientific attributes that have been studied for decades towards mitigating lightning-related hazards. The probability of a direct lightning strike to various points on a FRT can be estimated by applying the dynamic electro-geometrical model (DEGM) using numerical techniques, and this provides information on the high-risk lightning-strike points on the FRT. This numerical approach involves surface discretization resulting in millions of meshed points in some cases, both on the structure and the surrounding space. This requires a lot of computer resources, and the simulation is slow to compute. In this study, simple and easy to compute equations were developed and hereby proposed as alternatives to numerical DEGM by applying analytical equations directly from first principles. The result confirms the susceptibility of the shell’s rim region to direct lightning strikes with 90.61% probability of a direct strike for a 20 m high tank and 88.58% for a 15 m high tank with 60 m in diameter. The results show close approximations to the numerical model and also reveals the effects of the discretization size on the accuracy of the numerical approach, while also reducing the computation time from several hours to a few minutes.


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