The short answer is you need to perform PHAs in phases, from conceptual stage, through preliminary and detailed design, and then complete these during the pre-commissioning stages (as mentioned by Bryan). The PHAs build upon each other and the completion of the final phase PHA becomes the Baseline PHA.
The tools used change from phase-to-phase, with What-if and some high level risk assessments being done early in the project, then HAZOP-based analysis as more details of designs become available (preliminary design, detailed design) and What-if and HAZOP of procedure steps as procedures become available., and then PHA sessions later in the project close actions from earlier phases and also cover gaps from earlier PHAs and more information is available; it is especially critical to perform a hazard review of skipping steps and performing steps of operating procedures wrong.
After thousands of PHAs during projects, we have not met any EPCs that knows when it is best to do PHAs, that multiple PHAs are required, and what makes a good baseline PHA. I would not let them make such decisions, if you are the owner of the process.
Read the free paper at:
This is the definitive paper on when to do PHAs during a major project. Pass this on to your EPC. Performing the PHAs progressively during the project phases as outlined in the paper will save millions in uncontrolled risk!