You may be aware of the phrase "...it is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail." which comes from a concept known as the law of the instrument or Maslow's hammer, as an over-reliance on a familiar tool.
In the Process (Safety) industry, our 'familiar tool' is HAZOP and it has served us well for over 40 years but it is not the multifunctional tool (or sonic screwdriver if you are of a certain demographic) it always appears to be.
Remember that, even as an adjustable spanner/wrench, it is not infinitely adjustable i.e. the jaws only open so far and HAZOP also has these limitations.
Trevor Kletz (in his book HAZOP and HAZAN) recognises it's limitations as;
"HAZOP cannot, of course, detect every weakness in design. In particular, it cannot draw attention to weaknesses in layout. It will also miss hazards due to leaks on lines that pass through or close to a unit but carry a material that is not used on that unit".
"HAZOP assumes that the design assumptions are followed during construction and operation. If, say, the wrong material is used or equipment is not tested as assumed, then problems may result."
The HSE also highlight the potential weaknesses, including three major assumptions underlying HAZOP;
A general level of competent management.
The plant will be operated and maintained in the manner assumed by the design team and in accordance with good management and engineering practice.
Protective systems will be tested regularly and repaired promptly when necessary.
They emphasise "Constant vigilance is required to ensure that these assumptions remain valid. A good proportion of major accidents have arisen from situations where these assumptions were invalidated, in particular through dereliction of management and failure to follow proper procedures."
HAZOP might best (or most easily) be described as a systematic challenge of the design of a process plant based on deviations from the intended operation.
Perhaps there is still too much emphasis on DEVIATION and not enough on the other potential failings due to;
DESIGN - Has the plant actually been built in accordance with the engineering concept.
DEGRADATION - Has the integrity of the plant been maintained/sustained i.e. is it still as strong as it was designed/constructed to be.
DEFEAT - Are the protection measures actually able to respond (both in function and time) in the event of a demand or have they been overridden.
DISCIPLINE - Are the procedures up-to-date and the personnel executing them suitably trained.
DAMAGE - What external threats are there such as dropped objects, vehicle collisions or fire/explosion on adjacent equipment.
I've previously posted on what I call the 5D's (now actually 6D's) above, maybe we need to think of how me address them as the 6C's of;
CONSTRUCTION to ensure the DESIGN is realised correctly.
CONTROL to ensure DEVIATIONS are constrained.
CONDITION to ensure that plant strength is not subject to DEGRADATION.
CHANGE management to ensure that protection is not DEFEATED either permanently or temporarily without due assessment.
COMPETENCE to ensure that personnel are properly trained (and refreshed) to execute current procedures and reduce lapses in DISCIPLINE.
COMMUNICATION to ensure equipment/vehicle movements or other nearby operations or activities do not DAMAGE the piping or equipment.
I'm sure the pages of industrial history are littered (or stained) with incidents where plants or processes were HAZOP'd and therefore assumed to be 'safe' - so we need to approach the challenge not just with a hammer or a wrench (which you will also recognise makes a good hammer when you are desperate) but a toolbox of appropriate tools - at least until we develop the equivalent of a sonic screwdriver.