Analysis of the Seismic Risk of Major-Hazard Industrial Plants and Applicability of Innovative Seismic Protection Systems

Industrial plants are complex systems and it is such a complexity, due to numerous connections, equipment and components, together with the complexity of their operations that makes them particularly vulnerable (local vulnerability) to earthquakes. Activities carried out in process plants can also be arranged in series, which means that process activities are realized with specific sequence and boundary conditions. Consequently, the “failure” of a single element can get out of order the entire system. This is of fundamental importance for the seismic vulnerability of a plant (general vulnerability). Seismic action can cause serious accidents to industrial plants as shown in several occasions. The actual worldwide situation of major-hazard plants against earthquakes should be considered as critical. For instance, in Italy about 30% of industrial plants with major accident hazards are located in areas with a high seismic risk. In addition, in case of a seismic event, the earthquake can induce the simultaneous damage of different apparatus, whose effects can be amplified because of the failure of safety systems or the simultaneous generation of multiple accidental chains. A representative example is certainly the Izmit Earthquake in Turkey (Erdik and Durukal,2000), which induced severe damages to Tupras refinery (area of the plant, farm tanks, and landing place). An example of domino effect caused by a structural collapse was the breakdown of a concrete chimney that caused a big release of dangerous substances and damages to surrounding equipment. In a plant, an earthquake can cause many dead as consequence of components collapses, similarly to what happens to buildings; moreover, the consequences deriving from a seismic event, such as economic losses for interruption of the production, environmental damages due to releases of dangerous substances, damages to persons due to explosions, fires and release of toxic substances, have also to be taken into account. Therefore, the usual safety requirements applied to civil buildings for ultimate and serviceability limit states, together with the consequences of exceptional actions, are generally unsuitable for structures belonging to industrial plants. As a matter of fact, a critical damage for a process safety that can cause even a modest release of inflammable substances, such as a flange opening or a welding breaking, can result unessential under structural point of view, but, at the same time, might cause considerable accidental chains. Consequently, for process industry it is necessary to associate the indirect consequences caused by possible accidents (i.e. a seismic event) to the direct structural damages. During the last years, in order to increase safety against earthquakes, passive control techniques (PCT) have been developed, which are based on the concept of reducing the seismic action instead of increasing the strength (Housner et al., 1997). These techniques that for civil constructions are nowadays considered a consolidated alternative design tool, can also be used for seismic protection of industrial structures. Unfortunately a very limited number of applications to industrial plants components have been realized. For this reason, in the present chapter the applicability of such a technique is investigated, aiming at providing general applicability criteria. An example of base isolation of a steel storage tank is also presented, whose effectiveness is investigated by a wide numerical and experimental activity.