I am surprised too often by people who think that they can do an AS 2885 risk assessment by using some risk matrix other than that published in the Standard. AS 2885.1 makes quite clear that risk evaluation must be done using the risk matrix in Appendix F. The words in the Standard say that the severity class and frequency of occurrence shall be selected from Tables F2 and F3, and the risk rank shall be determined from Table F4. The only flexibility is some limited scope to adjust the severity scale to reflect the nature of the pipeline, which really applies only to the supply dimension of the consequences – interrupting the flow from a gathering line is clearly not of the same severity as interrupting gas supply to a major city.
Using the AS 2885 risk matrix is important partly because it ensures consistency across the industry, but more fundamentally because it has been calibrated and shown to produce results that are broadly consistent with the best alternative methods used internationally. I wrote about that here. (The calibration might not be perfect in absolute terms but at least it shows consistency with worldwide practice). I view this validation of the matrix as highly important.
Although the frequency scale in the AS 2885 matrix does not currently show numerical guidelines, its intent is clearly to span a range of many orders of magnitude. If the words don’t convey that then a bit of history will. When risk assessment was first introduced in the 1997 edition of the Standard it was thought that a handbook would help the industry come to grips with this new approach. Accordingly SAA HB105-1998 was published. It was superseded by the 2007 edition of the Standard which incorporated much of its content. However one thing that got lost was a table of numerical frequency guidelines. I won’t reproduce that table because the risk matrix was different then, but I presented an interpretation of it in this post.
Note that the lowest frequency (Hypothetical) implies a probability approaching the 1 in a million level. In contrast, I have seen corporate risk matrices in which the bottom of the frequency scale is a probability of 1%. Most company matrices do go lower than that but few seem to go as low as AS 2885 (the variability itself is remarkable). Notwithstanding their very wide use and acceptance, I really wonder if anyone has ever thought about calibrating their corporate risk matrix. Clearly, if you run the same analysis through a matrix where the bottom of the scale is 1% you are going to get very a different result from AS 2885. Quite apart from the fact that the AS 2885 matrix is mandatory, would you prefer to use some “approved” but arbitrary company matrix or the calibrated AS 2885 version?
Where a company insists on using their corporate matrix, I insist in turn that they do it in parallel or outside the AS 2885 SMS workshop, not as a substitute. There is no option – if you are doing an AS 2885 safety management study then the risk evaluations must be done using the AS 2885.1 risk matrix from Appendix F.