For new developments, HAZOP studies are an integral part of the overall project activity and should be carried out at key stages in the project cycle. For existing facilities, a HAZOP study should be carried at regular stages to confirm that hazards have not been introduced unwittingly by changes to procedure, material quality, personnel responsibilities, minor plant modifications, etc.

This is a hazard identification technique using similar analysis and brainstorming techniques to HAZOP, but designed to be used at a much earlier stage in the project. HAZID concentrates on wider health, safety and environmental issues with less of a process focus. HAZID is complementary to Preliminary HAZOP and in certain circumstances the two study types can be combined.

Preliminary HAZOP
This study should be held at the end of the project specification phase (front end engineering phase) of a project. It uses the full HAZOP methodology but it is intended to be used early in a project's life where there is still the ability to change the design, operating and maintenance policy. The plant is divided into a small number of large nodes appropriate for an early stage in the design development.

Detailed HAZOP
This study is held at the detailed design stage when drawings are ‘Approved for Design'. Its objective is the assurance of the safety of the current design (‘will it work?') rather than making radical recommendations for change.

This study is held at the end of the detailed design stage when drawings are ‘Approved for Construction'. The technique is the same as the detailed HAZOP except that the study focuses on changes since the detailed HAZOP, rather than reviewing the agreed design.

This study, usually linked to a change control procedure, can be applied at any stage in the design development or subsequent operation of the facility to assess proposed changes or vendor package details. The methodology is similar to that of a HAZOP for an existing facility.

Pre-start HAZOP
This study is held during commissioning to check for correct / complete construction, operable safety systems, clear procedures, removal of temporary equipment, and any concerns arising from simultaneous activities occurring during start-up.