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15:47 Jan-28-2009
Reliability of pipeline inspection using guided waves

Hi! I have observed that many operators doing pipeline inspection using Guided waves first go for a visual inspection of the entire pipeline, all supports and try to look for the possible corrosion. If found then they try to look for that in their GW signals and try to correlate by confirming distances. Actually it should be other way round. If GW system works this way how relaible this technique is ? And what these operators do in case of buried or insulated lines?


16:25 Jan-28-2009

James Gauthier

NDT Inspector, Operations Manager
GE Inspection Services,
Joined Nov 2007
Re: Reliability of pipeline inspection using guided waves In Reply to Mark at 15:47 Jan-28-2009 (Opening).

Hi Mark,

You are certainly correct that a lot of operators do perform a visual exam of the area prior to taking the guided wave shot. I believe that it is imperative to "walk" down the line in both directions and take note of the types of supports, bends, valves, drains, and the general condition of the pipe before you take your data shot. Then you can just put your all your "known" data points in and then look at the actual signal characteristics. You should be able to determine what the signal is without doing that step but it certainly makes it easier. Be very careful to not ignore signal characteristics just because you have a support or some other item on the pipe. Very serious defects can be within an area that you might determine to be a perfectly normal bend, etc. A good operator should concentrate on his/her ability to grade a pipe with just the data set but why not also record all your visual comparators? If you don't walk down the line, how do you determine that your shot placement is the best possible position? There are many different approaches to inspection and my opinion is to collect what I can see and then look very closely at the data to determine if any obvious defects "jump" out at me, then I look at all my "knowns" and see if there is any indications possibly being masked by typical pipe configuration signals.

Good Luck in your endevours.


17:29 Jan-28-2009
Christoph Schnitger
Re: Reliability of pipeline inspection using guided waves In Reply to Mark at 15:47 Jan-28-2009 (Opening).

Hello Mark,

some comments from Germany. We are involved in the LRUT buisiness since 2006, working with 2 of the big 3 manufacturers together to open the market and promote the technique to the end-client. We checked buried & insulated lines and found out that there is a big gap between the technique on paperwork and reality. For un-insulated pipework or insulated pipework where you have directly visual access to this technique works fine; - but please do it like jamie explain !!!!!!! In this case you will have correct & and sometimes surprising datas, you will find nice corrosion ( or something else ) let me say => system works fine !


If ypu need more information, pre-shots, manufacturer links etc....drop me a line !

17:47 Jan-28-2009

Thomas Vogt

R & D, - -
Guided Ultrasonics Ltd,
United Kingdom,
Joined Apr 2007
Re: Reliability of pipeline inspection using guided waves In Reply to Mark at 15:47 Jan-28-2009 (Opening).

Hi Mark

this is a good question, which in fact touches on a principal question probably not unique to guided wave (GW) testing.

An experienced operator would in fact most likely do as you suggest it should be done - they do the test and then confirm using visual and UT (so e.g. the insulated pipeline, he would tell you where to take the insultation off). Only he must have a look around beforehand to see what is a good test position.

But you seem to imply that maybe guided wave is no good because people do a visual in addition to the guided wave test? If this is so, then in my opinion you are neglecting a very important issue.
First, no single testing method can find you everything, and guided waves is no different. Would you want a defect found doing the visual but not found using guided waves excluded from the GW report? I don't think so.
Second, a GW indication can sometimes have several explanations, and the interpretation of an indication can be difficult. I believe most inspection scenarios do to some extent require a mix of different testing methods so you can draw the right conclusions from the results. To make the most of an inspection and give the best quality to the client, you need to use every tool that is available to you. I can't help but welcome any effort an operator undertakes in order to raise the confidence in the interpretation of his GW result. If this means walking the line beforehand, great (chances are though he will have to again afterwards anyway...).

Which is preferable? An operator that relies entirely on a single method to deliver a report about the state of a pipeline, or one who can say: "I used three methods: visual, GW and UT, and based on all this I have come to the following conclusion..." My choice is obvious.

If you have a buried pipeline, then you don't have the luxury of being able to prove up with visual and UT, and in this context don't forget that GW is a screening tool which does not give you an exact wall loss reading. This is exactly why buried GW inspection should be considered an advanced application only to be attempted by operators who have been specially trained in an appropriate training course. Same goes for road crossings.

Hope this helps.

21:42 Jan-28-2009

Sang Kim

Consultant, NDT Trainer
Guided Wave Analysis LLC,
Joined Feb 2008
Re: Reliability of pipeline inspection using guided waves In Reply to Thomas Vogt at 17:47 Jan-28-2009 .

Hello Mark!
I agree with what Tom wrote.
If operator doesn’t have isometric drawing or other sketch of the pipeline beforehand, he/she has to check the geometric features in order to determine the best probe installation position. For finding the installation position, the operator only needs to know the locations of elbow, flange, valve, T-joint, soil interface location that significantly affect the guided-wave propagation. After acquiring data, the data need to be checked with drawing if it is available. If the drawing is not available, the visual checking of geometric features is needed for making an inspection report. The drawing and checking the geometric features of pipeline is one of main procedure in guided wave inspection because the inspection report is made with comparing data and drawing information. Therefore, the guided-wave inspection cost per location should be reduced by about 10 to 20 percent if the same location is tested again. For guided-wave inspection of pipeline using MsS System, the inspection cost per location will be reduced by about 30 percent if the inspection is performed at the same location with the following reasons:
1) The sketch of the pipeline is already done with the previous inspection.
2) The MsS probe is already installed at the previous inspection.
3) Making inspection report is relatively easy by comparing the acquired data with those acquired in previous inspection.
4) The inspection speed is much faster up to 20 locations depending on the accessibility.
5) It takes less time to explain the results to client because the client already knows the condition of pipe with the previous inspection.

If the probe is installed for monitoring, the monitoring cost per location will be reduced by about 40 percent because the inspectors do not need to access to the installed probe. The guided-wave monitoring can be done up to 30 locations per day by only accessing the monitoring terminal.

If you need more information, please contact me at skim@gwanalysis.com.


09:08 Jan-29-2009
Re: Reliability of pipeline inspection using guided waves In Reply to Mark at 15:47 Jan-28-2009 (Opening).

Hi! Thanks everyone for the valuable comments. I am not against doing visual inspection while doing GW inspection. Infact visual inspection is the first step in every failure analysis process. I agree that it is always better to use multiple technique approach to get the best possible picture of the components under test. But when we select these techniques each method should provide some complementary information which other methods are not capable of. That’s the whole idea behind using multiple techniques. Lets start with bare pipelines. What complementary information GW testing can provide which Visual inspection is not capable of finding? One such vital information of interest is internal corrosion. How reliable GW systems are in locating areas of internal corrosion? And next obvious question is how to cross verify the results of GW inspection?

10:04 Jan-29-2009
Godfrey Hands
Re: Reliability of pipeline inspection using guided waves In Reply to Mark at 09:08 Jan-29-2009 .

Your comment "And next obvious question is how to cross verify the results of GW inspection?" desseres a reply.
GW is a tool that identifies areas of the component under inspection where there is an anomaly. This anomaly then needs local investigation, be it erection of scaffold and manual intervention with UT and visual, or excavation followed by UT and visual. On simple straight pipe lines, only UT and visual are normally required.
In any case some supplementary tests are necessary to verify the origin of your target signals

11:05 Jan-29-2009

Thomas Vogt

R & D, - -
Guided Ultrasonics Ltd,
United Kingdom,
Joined Apr 2007
Re: Reliability of pipeline inspection using guided waves In Reply to Mark at 09:08 Jan-29-2009 .

Hi Mark

I now see the purpose of your initial post, we have the same approach. Use each NDT method where it is good and within its limits, and if that is not enough try to cover the rest using other methods.

So what can GW do that visual testing (VT) cannot? You named the obvious examples: anything that you possibly cannot visually test for like internal corrosion, defects in welds, under supports when you don't want to lift them etc.
If that is not enough, lets assume that all the defects are visible so you should be able to find them all using VT? Recent research by one of our clients as part of a dissertation has shown that GW is in fact at least as good if not superior to VT even in this restrictive case biased to VT.
Now put the pipe high up into a pipe rack, let it jump over a road etc. In principle you can to VT, but access costs a lot of money. The main draw of GWUT as a screening method is of course its cost and time effectiveness.

Also, I sometimes get the remark that GW is not as good a manual UT because of the minimum defect size you can find. So, they conclude, the POD is lower for GWUT. This is wrong as they are not comparing like for like. Of course manual UT can find very small defects, but only if you know where to look! In a pipeline you don't stand a chance to find defects with manual UT, which are as small as you can find with GW, unless you have a grid that is so small that it will take you a very long time but takes 10 min for the GW (unless someone can suggest a way to be consistently lucky). Using GW and then proving up with UT and visual is in fact the way to greatly improve POD, at the same time as reducing cost and time.

We can go into an awful lot of detail regarding what you can and cannot find with GW because it depends on a few parameters e.g defect type and size, but if it helps, within its limitations you can find internal corrosion just as well as you can find external corrosion.

Cross-verifying is sometimes very difficult! You may have to throw everything you have at it to find out what it is: VT, UT, RT... what one method can find, the other isn't necessarily able to.


02:51 Feb-02-2009


Director, - -
Metalcare Group Inc.,
Joined Oct 2008
Re: Reliability of pipeline inspection using guided waves In Reply to Thomas Vogt at 11:05 Jan-29-2009 .

I agree with Tom.
Good response.
1) In general, GWUT is more of corrosion screening tool rather pin pointing each and every small discontinuities with sizing. Offcourse, if we identified the problem areas, other techniques/test methods (like UT thickness gauging) may required for further investigation like any other NDT method.
2) Visual inspection is the primary most important tool that must be performed as much as possible prior to apply any NDT method (this is even applicable to basic surface NDT methods like PT and MT). Also, the NDT technician must have as much as information on the product/line he is going test such as material specification, service history, inspection history, P&IDs and configurations etc., in order to his interpretations/evaluations to be most effective. This is the reason, mostly you will find these checks are preliminary mandatory requirement on any NDT procedure.


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