Seismic Risk of an Oil-Gas Storage Plant

The wide range of induced effects of earthquakes, from direct damage due to seismic shaking to indirect damage caused by secondary effects (e.g. liquefaction, soil densification and landslides) makes the seismic risk one of the most common cause of structural failures among natural hazards. The degree of vulnerability and the level of exposure of the threatened elements may further amplify such effects. In this sense, the seismic risk induced by an oil-gas storage plant located close to an important commercial harbour in Southern Italy is analyzed. The plant is situated in one of the areas with the highest levels of seismic hazard in Italy, hit in the past by earthquakes as large as 7 in magnitude. Moreover, the plant lies near to the shoreline and the facing seafloor is characterized by the presence of a deep submarine canyon filled by loose, unconsolidated soils coming from the excavation of the harbour channel. Given these conditions the following phenomena have been investigated: local site amplification, liquefaction, submarine landslides and seawaves run-up. The stability analyses considered both the plant’s structure itself and the site. A vulnerability analysis provided the response to the ground motions of the steel tanks forming the structure, while dynamic analyses gave the response of the soils to the wide range of possible ground failures. Joining all the possible effects that could destabilize the plant, an overall probability that the safety of the plant may be affected was computed. The total risk was then assessed considering the effects, in terms of human life losses, produced by the failure of the plant. This risk was then compared with those deriving from other human activities to provide a reasonable basis for risk the acceptability assessment.