Received a good question the other day about the measurement length applicable to pipelines carrying oil, LPG or even CO2. I’ve previously written a bit about measurement length here and here, but the emphasis was on gas pipelines.
Radiation contours from pipelines carrying oil or HVPL (high vapour pressure liquid) are difficult. The note following Clause 4.3.2 in AS 2885.1-2012 says as much. However I’ve done a few SMSs for oil lines and the difficulty has not been a serious practical problem (yet). I think the key thing to remember is that the measurement length is a guideline to help you make judgements about the severity of consequences. It doesn’t need to be precisely calculated in order to fulfil that guideline function.
The other day I saw the measurement length for a gas line given as 199 m. That implied level of precision is ridiculous, given that no-one has any idea about the operating pressure in the pipeline at the time of failure, the wind direction, the type of crater and its effect on the type and direction of the fire, etc, etc. Also the measurement lengths graphed in Appendix Y of AS 2885.1 are based on a bunch of assumptions about the length of pipeline (and hence the linepack available) and gas properties. Further, the gas pipeline radiation intensities apply at a moment 30 s after rupture. Because the gas release is a highly transient phenomenon the initial radiation intensity is a lot higher and it decays very rapidly. Hence although it appears that the measurement length for gas lines can be calculated with great precision that’s a complete illusion. For the particular pipeline I mentioned, to me it seems that the measurement length is very roughly 200 m, and anyone within about 300 m had better run hard and anyone within 100 m is in very serious trouble indeed.
(If your recognise your pipeline as the one with the 199 m measurement length, don’t feel singled out – I’ve seen many other documents with similar unjustifiably precise values for measurement length.)
For oil lines various approaches are possible. If you have access to specialist process risk software then pool fire calculations can be done, but of course they depend enormously on assumptions made about the rate of spill from the pipeline, size of pool, spread of pool, topographic effects on drainage, etc. So again, although the results might look precise they are built on a very shaky foundation.
Sometimes in the past I have just used the gas pipeline measurement lengths as an indicative approximation. At other times, for particular pipelines in well-developed areas, we haven’t bothered at all because it is just so obvious that the radiation distance will encompass a large number of houses and hence the location class is T1 (or T2 or S depending on what is there).
HVPLs such as LPG or ethane have the potential to spread into low-lying areas. I have not yet had to deal with this in practice so don’t have any experience-based advice to give. I suspect that in this case it might be more necessary to have some dispersion modelling done so that there is a reasonable understand of how the vapour spreads. That becomes complex partly because of the range of atmospheric conditions and partly also because of the possibly large number of leak locations that have different topography.
For CO2 the problem has been recognised by Appendix BB of AS 2885.1-2012 and the Energy Pipelines Cooperative Research Centre. There are some research projects just getting under way to look at this. CO2 is even more complicated because of the potentially 3-phase nature of the stuff. Its behaviour when released to atmospheric pressure from supercritical conditions is not well understood, to say the least. Hence the need for research on how it behaves both within a suddenly ruptured pipeline and then in the surrounding environment. The other aspect of CO2 measurement length is that is unresolved is the CO2 concentration corresponding to the 4.7 kW/m2 radiation level. CO2 pipelines are not here yet but I suspect they are coming soon and this will a subject for ongoing discussion.