Hazard and Operability study aka "what if" is a technique applied by a team on a project design, or part of it, to identify and assess hazards and problems that may arise in operation; lately done in a systematic way to reduce the chance that something is missed. Risks tend to concentrate on start-up, shut-down and abnormal or transient operating conditions. The purpose is, of course, that the consequences of those risks be prevented already in the design stage.
If you can get access to Trevor Kletz's HAZOP and HAZAN "Identifying and Assessing Process Industry Hazards" published by the Institution of Chemical Engineers (UK) you'll get a better idea with worked out examples.
There is a European standard for HAZOP. Quite good as both a practical guide and introduction to the method.
A couple of years ago, we had in Australia an interesting court case. A gas plant had exploded and the company was prosecuted for breaches of the OHS Act. The first charge was that they had not conducted any hazard identification study. The judge required the prosecution to nominate the method that should have been used, and they selected HAZOP.
The jury had to decide if HAZOP would have identified the particular hazard that lead to the explosion. A tricky task for engineers, let alone twelve lay persons. They decided guilty. Scary result because HAZOP is not a precise method with guaranteed, repeatable outcomes. It's not like some analytical laboratory test
