FALSE. Whilst HazID and HazOp studies cover similar ground, HazID uses a “top-down” approach, whilst HazOp uses a “bottom-up” approach, and these two methods often identify different issues in a design. The big advantage of HazID is that it can be carried out earlier in the project lifecycle, before full Piping & Instrumentation Diagrams (P&IDs) are available and can therefore identify safety and operability issues earlier in the project, thus saving design time, and expensive re-work later in the project.
If HazID isn’t carried out then it’s true that the HazOp may find the same issues, but the design may be too far progressed by that stage to make fundamental changes (for example, to the design basis, or to allow “inherently safer design” choices). Even if the HazID does not identify fundamental design changes, we find that the discipline of carrying out a HazID leads to a more efficient HazOp – as a rule of thumb, we find that every day spent on HazID can save up to two days of HazOp.