As has been pointed out, barriers are selected based on
scenarios, which are developed from so-called bowties.Combining
a fault tree and an event tree at their top events and
turning this on its side, results in a figure that resembles a
‘bow tie’. Scenarios then are formulated by describing escalation
paths through this bow tie, starting at initial events on
the fault tree side (‘threats’) and ending at unwanted ones on
the event tree side of the figure (‘consequences’). The use of
bow ties for this purpose has become quite common in the
last fewyears and, consequently, the development of scenario
based auditing techniques has become opportune.
To define scenarios and devise barrier solutions for these,
the company must have a risk identification system in place
and working. The outputs of this system are barrier solutions,
which initiate the barrier life cycle. Barriers have to
be designed or ordered according to particular specifications,
have to be built or delivered, installed and adjusted for use.
Importantly, when barrier solutions are deliberated and functions
are defined, certain trade-offs should be considered. For
instance, should the barrier be a passive hardware solution
like a wall or a layer of paint or should it be active like a
pressure relief valve or an interlock, and what should be the
desired or required involvement of people in its operation?
Hence, when the barrier life cycle starts, two other life cycles
start with it, namely a life cycle related to the development
of procedures and another one aimed at the competence of
people working with the barrier (see Ref. [5] for comparable
reasoning). Additionally, this competence can be defined at
the skill, rule and knowledge based level.