Impact assessment and risk matrices. In Section 4.4.1 we described the probability of leak events impacting the primary risk receptors that have been considered (basically the fault-tree analyses). In this section we address the final step of the MHR assessment that intends to answer the question: given the occurrence of a leak event in a risk receptor, what is the impact of such a leak? To answer this question, we calculate both the expected leak volume associated with that spill event, and the probability associated with the leak. The general problem of the impact assessment in this case can be solved using a simple event-tree implementation that, for all the considered cases, follows the structure depicted in Figure 20. Figure 20 - Basic template of the event-tree defined for assessing impacts on primary risk receptors for Phase 1: site construction and drilling – Redrawn form Ref. [12] In Figure 20, Node 1 refers to the estimation of the leak probability, which is determined from the output of the fault tree analysis (e.g., the top event, or some intermediate event of interest). Node 2 refers to the “size distribution” of the event (here for simplicity divided only in three categories defined as “small”, “medium” and “large”). These “size” events refer to the size of the typology of event associated with the leak at node 1. The combination of these two nodes provides the probability of each path of the event tree. The tree can be expanded further considering different directions for the leakage spatial distribution, as shown in Figure 21. Figure 21 - Template of the event-tree defined for assessing impacts on primary risk receptors considering spatial distribution of the leakage for Phase 1: site construction and drilling We have, thus, implemented the structure of the two event trees to assess the impacts on the primary receptors of choice: ground – Figure 21 – and groundwater – Figure 20, under the elicitaded assumption that 2% of the volume leaked reaches the groundwater. After calculating the expected leak volumes associated with each path of the event tree, impact assessment in this work is defined by considering the expected leak volumes for different incidental events along with the respective occurrence probability. This information can be summarized using a classical risk matrix – Figure 22. Figure 22 - Risk matrix showing results of the MHR assessment showing scenarios of ground (red) and groundwater (blu) pollution associated to oil leak in Phase 1 Moreover, considering the Eve...
