Measurement length clarified

A post a couple of weeks ago on measurement length and “No Rupture” resulted in unexpected comment on how the measurement length is interpreted for determination of location classes.  The problem arises because AS 2885.1-2007 somehow omits a clear definition of the area to be considered when you are deciding on location classification.  I guess the intention was so clear to the committee that we failed to realise the oversight.  Most people have managed to read the committee’s mind correctly but the recent comments revealed that others came to a different conclusion.  After double checking with some other committee members and also the issue paper written early in the process of preparing the 2007 revision I can set the record straight …

The location class shall be determined by the land use within one measurement length either side of the pipeline.

(A secondary requirement is that a higher location class shall extend into a lower location class for one measurement length beyond the land use boundary.  That is clearly spelled out in Clause 4.3.3(a) and was also subject to some debate, but I don’t want to dilute the point above by discussing it further here.)

The first thing I do when starting location classification is use either Google Earth or the project GIS to create a zone defined by lines parallel to the pipeline and one measurement length either side of it.  Anything outside that zone is of not much interest, everything inside it is important.

Having said that, land uses close to the borderline can’t necessarily be ignored, depending on the circumstances.  If the measurement length is 1000 m, a feature at 1001 m is not “safe”.  On the other hand, a small cluster of houses at the very edge of the zone in an area that is otherwise R1 might not change the location class to R2 or higher, whereas the same cluster close to the pipeline might.

But I don’t want to create confusion with subtleties – the basic principle is to look at land use within one measurement length either side of the pipeline.

Advertisements
This entry was posted in Pipeline design, Risk assessment, Standards. Bookmark the permalink.

4 Responses to Measurement length clarified

  1. tom Prezwanski says:

    Just to add to the confusion. How would you measure the measurement length in the area where the pipeline is at the bottom of a steep slope and the housed are built at the top of the hill? If measured horizontally, the houses are within the zone; if along the slope they would be outside of the zone.

    • petertuft says:

      Tom,

      I suspect you are trying to be overly precise. Unless the slope is extraordinarily steep (near-vertical) the difference between slope length and plan length is insignificant. Even for a 50% slope (about 27º) there is only about 10% difference in the lengths. If someone is at serious risk of death or dreadful burns at a distance of say 100 m from the pipeline, moving to 110 m isn’t going to make a huge difference.

      The whole calculation of measurement length is fraught with uncertainty. No-one really knows the pressure in the pipeline at the time of failure, or the configuration of the crater and wind speed and direction which affect the flame shape and direction. And the measurement lengths in Appendix Y of AS 2885.1 are based on assumptions about the length of pipeline (volume of linepack), gas heating value, radiation fraction of the energy release, etc, which may or may not apply to your particular case. All of those things could add up to much more than 10% variation in the radiation distance.

      The point is that the measurement length is a guide to be used with discretion, not a hard number.

      Having said all that, a more direct answer to your question is that since the issue is thermal radiation from a flame that will probably be very tall, the horizontal distance is probably what counts.

      On the other hand, if there are just a few houses right on the edge of the zone I probably would not change the location class for that reason alone.

      I’m afraid that in matters of risk there are few definitive answers. Hope this is at least a bit of help.

    • Chris Hughes says:

      The measurement length calculations are generally conservative in that they assume almost perfect radiation conditions i.e. line-of-sight between source and target. If there is a hill between the rupture point and the target the radiation intensity will obviously be reduced considerably, but the standard as written does not make any allowance for this. Humidity also affects radiation intensity, so calculations for Darwin should be different to those for Adelaide, but again this is not really covered.

      As Peter said, all these bits of the standard are open to interpretation (Appendix Y is Informative, not Normative) and at the end of the day the SMS is the document we stake our reputations on.

  2. Chris Hughes says:

    Peter, as the person who started the discussion in the first place I will say that I agree totally with what you say: I do exactly the same with regard to the parallel lines. Then in order to determine the length of pipeline which requires special treatment I draw a circle of measurement length radius from the extremities of the house/ school/prison/township and the section of pipeline which forms a chord of the circle(s) is the extent of the line referring to that location class.

    I still disagree with you regarding your “secondary consideration” but we’ll leave that for now.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s