This question was posted elsewhere…
The short answers:
- PHA/HAZOP (qualitative methods) with a good team of experts can judge the risk of 950 to 980 out of 1000 hazardous scenarios
- LOPA can help reach a risk-based decision on the rest
- QRA is almost never needed (though many times others will require you to use such methods); we have found QRA is only needed for about 1 in 10,000 scenarios
RE Software: Do not buy any frequency assessment tool. But, you will likely benefit from use of Excel for LOPA. Download ALOHA and use it for FREE to do a few consequence models to calibrate the site staff on how far something can reach (concentration or overpressure). Then, after calibration to the right order of magnitude you will not need it again.
This may be a contrarian view to QRA, but note that we have done thousands of LOPA and a great many QRA; and more than 10,000 PHAs/HAZOPs in the past 20+ years, so there is a lot of data behind these conclusions. But, some of my closest friends in the industry think QRA are necessary more than I have stated above… Our data indicates that aside from a few exceptions, it is not needed much in the chemical-related industries.