2007_6_Nr6_EEMJ

Methods and procedures for environmental risk assessment
o inside the contour of the events signs, their
description is inserted without directly
linking two connections (the events will be
described after each connection);
o the tree is elaborated on levels, downward
from the top;
o a level is constituted by an array of
connections situated at the same distance
by the top event and the prior events of the
respective connections;
o it is not allowed to go to the next level until
the current event is exhausted.
• solving of the free tree involves finding of
minimal sequences. A sequence represents an
array of events that results in an accident. The
minimal sequences are successions of this type
that contain a minimum number of events.
Before properly accomplishment of the tree,
the following steps should be done:
• Defining of the top event (e.g. high
temperature from the reactor).
• Defining of the determinant event: conditions
of occurrence.
• Defining of the not-allowed events: damages
at the system for power supply, faulting of the
switches etc.
• Defining of the physical conditions of the
process: the limits should not be taken into
account. For example, in elaboration of FT, the
units situated upstream and downstream of the
reactor will not be considered.
• Defining of the configuration of the
equipments from the system.
• Establishment of the detailing level.
After completion of these steps, one may
proceed to the properly elaboration of the fault tree.
First, the top event is drawn in the upper part of the
scheme. This will be labeled precisely for avoiding
the further confusions. Then, the major events that
contribute to achievement of the top events should be
identified. If these occur in parallel, will be linked
through an AND connection. If they occurs in series,
will be connected through OR.
3.3.3. Event Tree Analysis (ETA)
ETA is an inductive logic model that
identifies the possible results of a given initiating
event. An initiating event will commonly result in an
accident or an incident.
ETA considers the responses of the operators
and safety systems to an initiating event. This
technique is the most suitable for analysis of complex
processes that involve few safety systems or
emergency procedures.
The first stage in conceiving an event tree is
the defining of an initiating event that may lead to the
damage of the system: equipment or utilities faulting,
human error, natural disasters. The next step is the
identification of the intermediate actions for removal
or reduction of the initiating events effects.
The event tree contains two branches for
every intermediate event, one for a successful
exploitation and other for a faulty exploitation of the
system. The upper part represents the success, while
the bottom part represents the failure. Within a
simplified model, the initiating events become the
damage of P2. There are some response stages at the
initiating event that include the warning alarm for the
minimum flow rate, the response of the operator and
damage of P1.
The assessment using the event tree analysis
contains the following steps (an example):
1. equipment is damaged and becomes the
initiating events. Probability of this event
was defined as being 1.
2. The warning alarm for minimum flow at the
vessel may work or fail. If it works, the
upper branch is covered. If it doesn’t wok
the bottom branch is covered. The warning
has a success probability of 0.998.
3. The operator either respond or not to the
warning alarm. Probability of responding is
0.952.
4. The last response is the fact that the operator
put the equipment into operation. Probability
of this event is 0.995.
The event tree analysis is the best analysis
for the initiating event that may lead to the final effect
of the event. Each branch of the tree constitutes a
separate sequence of the relationships between the
safety functions of the initiating event. Considering
the same system and the same hypothesis concerning
the probabilities, identical results may results through
the both methods.
The fault tree is larger then the events tree
owing to the fact that the latter is based on a single
effect related to the damage. Many people are
tempted to think in a logic manner about the safety
systems using the events tree approaching. Risk
assessment throughout the tree event may be
summarized as follows (Gavrilescu, 2003):
• identification of the initiating events that may be
materialized in accidents;
• identification of the safety functions for
diminishing the initiating events;
• elaboration of the event tree;
• description of the results of an accident and its
probability.
3.4. Environmental risk assessment in accordance to
MAPPM Order no. 184/1997
The environmental risk assessment
(concordant to Ministerial Order no. 184/1997)
examines the probability and the severity of the main
components of an environmental impact. The
necessity of additional information regarding the risks
related to the identified pollution or to the pollutant
activities developed on a site may determine the
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