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5.2 Relation to other analysis tools HAZOP may be used in conjunction with other dependability analysis methods such as
Failure mode and effects analysis (see IEC 60812) and Fault tree analysis (see IEC 61025).
Such combinations may be utilized in situations when:
· t he HAZOP analysis clearly indicates that the performance of a particular item of
equipment is critical and needs to be examined in considerable depth; the HAZOP may
then be usefully complemented by an FMEA of that item of equipment; · h aving examined single element/single characteristic deviations by HAZOP, it is decided
to assess the effect of multiple deviations using FTA, or to quantify the likelihood of the
failures, again using FTA. HAZOP is essentially a system-centred approach, as opposed to FMEA which is componentcentred. FMEA starts with a possible component failure and then proceeds to investigate the
consequences of this failure on the system as a whole. Thus the investigation is
unidirectional, from cause to consequence. This is different in concept from a HAZOP study
which is concerned with identifying possible deviations from the design intent and then
proceeds in two directions, one to find the potential causes of the deviation and the other to
deduce its consequences.
5.3 HAZOP limitations Whilst HAZOP studies have proved to be extremely useful in a variety of different industries,
the technique has limitations that should be taken into account when considering a potential
· H AZOP is a hazard identification technique which considers system parts individually and
methodically examines the effects of deviations on each part. Sometimes a serious hazard
will involve the interaction between a number of parts of the system. In these cases the
hazard may need to be studied in more detail using techniques such as event tree and
fault tree analyses. · A s with any technique for the identification of hazards or operability problems, there can
be no guarantee that all hazards or operability problems will be identified in a HAZOP
study. The study of a complex system should not, therefore, depend entirely upon
HAZOP. It should be used in conjunction with other suitable techniques. It is essential that
other relevant studies are co-ordinated within an effective overall safety management
system. © BSI ISB ©
1002−8028ISB © 2001 13
31 BS IEC 61882:2001
6 1882 Ó I EC:2001 – 15 – M any systems are highly inter-linked, and a deviation at one of them may have a cause
elsewhere. Adequate local mitigating action may not address the real cause and still result
in a subsequent accident. Many accidents have occurred because small local
modifications had unforeseen knock-on effects elsewhere. Whilst this problem can be
overcome by carrying forward the implications of deviations from one part to another, in
practice this is frequently not done. · Licensed Copy: Puan Ms. Norhayati, Petroliam Nasional Berhad 4397000, 01 October 2003, Uncontrolled Copy, (c) BSI · T he success of a HAZOP study depends greatly on the ability and experience of the
study leader and the knowledge, experience and interaction between team members. · H AZOP only considers parts that appear on the design representation. Activities and
operations which do not appear on the representation are not considered. 5.4 Hazard identification studies during different system life cycle phases HAZOP studies are one of the structured hazard analysis tools most suitable in the later
stages of detailed design for examining operating facilities, and when changes to existing
facilities are made. Application of HAZOP and other methods of analysis during the various
lifecycle phases of a system is described in more detail below.
5.4.1 Concept and definition phase In this phase of a system’s life cycle, the design concept and major system parts are decided
but the detailed design and documentation required to conduct the HAZOP do not exist.
However, it is necessary to identify major hazards at this time, to allow them to be considered
in the design process and to facilitate future HAZOP studies. To carry out these studies, other
basic methods should be used. (For descriptions of these methods, see IEC 60300-3-9.)
5.4.2 Design and development phase During this phase of a life cycle, detailed design is developed, methods of operation are
decided upon and documentation is prepared. The design reaches maturity and is frozen. The
best time to carry out a HAZOP study is just before the design is frozen. At this stage the
design is sufficiently detailed to allow the questioning mechanism of a HAZOP to obtain
meaningful answers. It is important to have a system that will assess the implications of any
changes made after the HAZOP has been carried out. This system should be maintained
throughout the life of the system.
5.4.3 Manufacturing and installation phase It is advisable to carry out a study before the system is started up, if commissioning and
operation of the syste...
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