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Technical Discussions
krish
krish
16:04 Dec-07-2009
Comparison between RT & AUT

In one of the critical sub-sea pipeline project ,the erection contractor/PMC is recommending to perform AUT(Combination of PAUT + TOFD)instead of RT claiming that the propability of Detection is more in AUT compared to RT.I really want to know for very critical sub-sea flow line which one is better to detect typical defect encountered in the automated PGAW process. Further i understand that the combination of AUT with ECA(Engineering critical analysis using DNV software or others)will prevent to carry out unwarranted repair or otherwise carried out using RT methods.The only inherent disadvantage with RT is defect height cannot be measured otherwise RT can capture all type of defects whether it is relevant or irrelevant.I need the experts opinion who used AUT technique for critical pipeline(like sub-sea)erection in lieu of RT.
with advance thanks,

    
 
 
Roger Duwe
NDT Inspector, API-510, 570, 653
MISTRAS, USA, Joined Jan 2009, 148

Roger Duwe

NDT Inspector, API-510, 570, 653
MISTRAS,
USA,
Joined Jan 2009
148
17:58 Dec-07-2009
Re: Comparison between RT & AUT
In Reply to krish at 16:04 Dec-07-2009 (Opening).

Very generally, TOFD finds everything and dispositions [accept/reject] very little. Typically, Phased Array is used to disposition indications found by TOFD. Both work in 3-dimensions. Thus, the AUT combination of PA and TOFD is synergesic and superb.

RT is the interpretations of shadows onto a 2-dimensional piece of film. It has a low probability-of-detection for sidewall lack-of-fusion, a critical defect. If you RT a book, it looks like a solid piece of wood. Both PA and TOFD find fine, tight Lack-of-fusion very easily, with a VERY high probability of detection.

RT is traditional. Period. Traditional methods hang on in spite of better and faster [cheaper] alternatives. Either TOFD or PA alond are better at finding fusion defects. Togather, the synergy gives the best detection and sizing in the industry.

    
 
 
krish
krish
05:43 Dec-08-2009
Re: Comparison between RT & AUT
In Reply to Roger Duwe at 17:58 Dec-07-2009 .

Thanks roger for the quick reply.I fully agree with you that AUT has better POD compared to RT.But the AUT is heavily depends on the equipment calibration and operator skill.Another biggest problem with AUT is intrepretation of signals and there is no consistency in this regard.Even the signal capturing and detection are varying between one brand to another one.Further dead zone effect in AUT is the biggest handicap in detecting the defect close to the surface as well as root?Is it true that defect lying in 2-3 mm at the surface cannot be detected by AUT?

    
 
 
Jawahar B. Goradia
Jawahar B. Goradia
06:34 Dec-08-2009
Re: Comparison between RT & AUT
In Reply to krish at 05:43 Dec-08-2009 .

Comparison between RT and UT/AUT is a longtime ongoing unconcluded process. With all pros and cons of imperfection (flaw-discontinuity) POD, sizing, and acceptance criteria both have no direct correlation to fracture mechanics which is most important for true acceptance. State-of-art scenario today is still in favor of RT whenever possible to radiograph.

    
 
 
KRISH
KRISH
06:42 Dec-08-2009
Re: Comparison between RT & AUT
In Reply to Jawahar B. Goradia at 06:34 Dec-08-2009 .

For fracture mechanics point of view AUT is the best tool by providing the defect height and if you have base data by AUT you can monitor the defect in future.

    
 
 
Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
07:20 Dec-08-2009
Re: Comparison between RT & AUT
In Reply to KRISH at 06:42 Dec-08-2009 .

Hello Krish

Your contractor is following a well-established trend in pipeline construction, especially offshore, to favour AUT for girth weld acceptance. An ECA programme for the linepipe will radically change the API 1104 or DNV-OS-F101 'workmanship-based" acceptance criteria. Long lengths of inter-run lack of fusion of no measurable through wall height will become acceptable and very short "stacked" defects must be repaired.

Offshore AUT in particular performs as a form of process control, especially for automated welding such as PGAW. Often a particular welding station's parameters will drift with time from the established procedure. If this goes unchecked eventually an out of code defect will occur requiring repair. The AUT operator will, at the earliest evidence of such a drift, inform the welding foreman as to which weld station is responsible and whether "up or downstream" so that appropriate parameter adjustment can be made thus saving repair.

    
 
 
Abbas Bombaywala
NDT Inspector
Free Lance, India, Joined Jul 2007, 91

Abbas Bombaywala

NDT Inspector
Free Lance,
India,
Joined Jul 2007
91
08:54 Dec-08-2009
Re: Comparison between RT & AUT
In Reply to Jawahar B. Goradia at 06:34 Dec-08-2009 .

Pipelines are now using Fitness-For-Service (FFS) for accept/reject of weld defects. FFS requires accurate measurement of defect height for Fracture Mechanics assessments. The standard pipeline weld inspection technique of radiography is incapable of such measurements. However, the newer technique of Pipe Line AUT can not only measure the defect height accurately it can also discriminate between types of defects very well.
Defects invariably occur from welding, even with the most stringent procedures. In practice it is not practical to remove all defects by repair, so some acceptance criteria must be used to determine which defects should be removed and which left in place. This situation has become even more important with the advent of high strength steels, where grinding and rewelding typically destroy the controlled microstructure; repair may create more damage than leaving the defect in.

From many years industry has switched from the “Workmanship” criteria, where defects were accepted or rejected primarily on what the inspection system could detect, to “Fitness-For-Service” (FFS) criteria, which are based on Fracture Mechanics (also called Engineering Critical Assessment, or ECA). FFS/ECA uses the material toughness, crack growth data and the component duty cycle to estimate the service life, and hence acceptable initial defect size.

AUT is becoming the inspection method of choice due to FFS. The use of AUT and FFS in the pipeline industry has significantly lowered reject rates (though this is partially due to the ability of AUT to perform process control).

In Pipeline AUT the weld is divided into the zones and the focusing is done on different zone with the suitable angles to detect the defect in that zone, root cracks are also easily detected with AUT,

Regarding surface limitation (Near Dead Zone), this dead zone is a major concern when performing ToFD and not in Phassed Array, Pipeline AUT is combination of TOFD and Phased array. Surface Transverse cracks can also be detected with AUT. And u can even use creep wave probes for surface breaking defects.

One more benefit with Pipeline AUT is u can collect all this data in a single scan and it is faster then radiography.

Many Oil & Gas Industries prefer radiography cause its cheaper then AUT and they trust radiographs more then AUT, obviously we will trust things for which we are sure with our knowledge and skills.

    
 
 
Jawahar B. Goradia
Jawahar B. Goradia
10:48 Dec-08-2009
Re: Comparison between RT & AUT
In Reply to Abbas Bombaywala at 08:54 Dec-08-2009 .

RT/UT cost ratio will be different for different locations. Objective of discussion is to evaluate technique from safety point of view irrespective of cost. FFS and BCA are also based on certain data assumptions. Best approach is to follow proven case studies or Code cases which are trustworthy.

    
 
 
S.R.G.PRABHU
Consultant, AUT specialist
FREELANCE, India, Joined Aug 2008, 63

S.R.G.PRABHU

Consultant, AUT specialist
FREELANCE,
India,
Joined Aug 2008
63
13:58 Dec-08-2009
Re: Comparison between RT & AUT
In Reply to Jawahar B. Goradia at 10:48 Dec-08-2009 .

Apart from working in more than eight offshore pipelay projects both S lay and J lay deepwater all over the world, and worked with ECA and workmanship and also company acceptance criteria , I found AUT superior than RT in many aspects , especially the radiation hazard by taking RT in a barge.Apart from inability to find many defects such as lack of side wall fusion, the time factor is also important. When a 42 inch dia pipe can be scanned and iterpreted in 3 to 5 minutes, RT is taking more time for sure. And the repair locations and causes for repeated similar defects are advised to the welding stations for immediate repair/rectification of the welding system parameters.In Iran offshore last Dec/Jan , we did a 40'' pipelay along with a 4'' piggy back, all AUT , and finished a 110 kms line in 30 days( with repairs in both dia lines)!!! Also the AUT contarctor in CHINA were qualified on the basis of a girthweld on two pup pieces welded together with a number of defects induced intentionally. We took RT, manual UT, and AUT was done on that weld. And the results favoured AUT to a remarkable extent. The defects are interpreted with precise height,location and depth, with hundreds of test welds scanned and also macro tested , and compared...!!
Also I found experienced and old people in the inspection field have a aversion towards AUT , could be because lack of understanding or long time familiarity with RT...

    
 
 
Abbas Bombaywala
NDT Inspector
Free Lance, India, Joined Jul 2007, 91

Abbas Bombaywala

NDT Inspector
Free Lance,
India,
Joined Jul 2007
91
14:24 Dec-08-2009
Re: Comparison between RT & AUT
In Reply to Jawahar B. Goradia at 10:48 Dec-08-2009 .

Dear Goradia,

There are lots of case studies which has proven that POD of AUT is better then RT,
there are some litreatures which proves the POD of AUT is better
one I will mention
RT VS AUT paper 2005 NPE Conf. Banff by I J Roux in this case study they have mentioned a project where RT & AUT were conducted on the same weld for the comparison of POD and below mentioned was the result

Total welds RT and AUT = 136 welds
Number of welds – no defects = 17 welds (Confirmed by both RT and AUT)
Number of welds with defects identified by RT only = 22 welds
Number of welds with defects identified by AUT only = 67 welds
Number of welds with defects identified by RT & AUT = 47 welds
So the POD of RT was 51% where as AUT was 84%.

There are many more examples in which it has proved that POD of AUT is more then that of Radiography.

As u mentioned that this discussion is to evaluate the techniques in safety point of view i dont get ur concern over here, because UT is the safest technique as compare to Radiography.
And yes best approach is to use the guidelines from proven case studies and code cases which are trustworthy. so yes it has been proven in many case studies that AUT (ToFD + Pulse Echo) is having the higher POD.

Regards

    
 
 
Abbas Bombaywala
NDT Inspector
Free Lance, India, Joined Jul 2007, 91

Abbas Bombaywala

NDT Inspector
Free Lance,
India,
Joined Jul 2007
91
14:33 Dec-08-2009
Re: Comparison between RT & AUT
In Reply to S.R.G.PRABHU at 13:58 Dec-08-2009 .

I agree with Mr. Prabhu, some of the people they just want to stick to Radiography because they have in depth knowledge about the technique and they trust it more as compare to a technique which they are not familier with.

Regards

    
 
 
JB Goradia
JB Goradia
19:46 Dec-08-2009
Re: Comparison between RT & AUT
In Reply to Abbas Bombaywala at 14:24 Dec-08-2009 .

Dear Bombaywala,
Your data is definitely educative.
Do you have data on POD of transverse crack by RT and AUT?
My present understanding on POD of transverse crack is that RT is far superior to AUT.
Your input will help me to experiment further.
Thanks and regards.

    
 
 
John Bomb
John Bomb
01:07 Dec-09-2009
Re: Comparison between RT & AUT
In Reply to JB Goradia at 19:46 Dec-08-2009 .

AUT will detect transverse cracks with a set of ID and OD transverse probes.

    
 
 
JB Goradia
JB Goradia
04:31 Dec-09-2009
Re: Comparison between RT & AUT
In Reply to John Bomb at 01:07 Dec-09-2009 .

Please eloborate furhter on "AUT will detect transverse cracks with a set of ID and OD transverse probes." as not understood clearly.
Thanks.

    
 
 
Abbas Bombaywala
NDT Inspector
Free Lance, India, Joined Jul 2007, 91

Abbas Bombaywala

NDT Inspector
Free Lance,
India,
Joined Jul 2007
91
07:33 Dec-09-2009
Re: Comparison between RT & AUT
In Reply to JB Goradia at 19:46 Dec-08-2009 .

zoom image


POD of Defect VS NDT Technique

Dear Mr. Goradia.

Definately I have a data for POD of Different Types of deffect with different NDT method please see the table attached.

Sir we also have to keep in mind that what are the chance of occurence of the defects, In my carreer I have come through only one time with transverse crack found in the weld.

In the Above table the POD of Transverse Crack for Radiography and Pulse Echo + TOFD is same.

As Mr. John Bomb said in AUT there are four sets of probe which are used for Transverse defects, the wedges used for this probes are not having paralel beam exit along the weld but are at 45 deg angles and the transducers used are tofd transducers and pitch catch method is applied, two probes are focusing at the ID of the weld and two probes are set to focus at the OD of the probe. A 0.2 mm Edm notches are used as a reflector from transverse deffect, and are easily detected by Trasnverse Channel both ID & OD.
I Think this might help u.

Regards.
    
 
 
Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
08:39 Dec-09-2009
Re: Comparison between RT & AUT
In Reply to JB Goradia at 04:31 Dec-09-2009 .

Hello Jawahar

Where required, AUT companies should use a pair of probe mounts orientated at 45 degrees to the girth weld axis. Which probes they use is job specific. In my experience it is not often a requirement.

The following link is to a technical paper which touches on transverse crack detection with AUT. A further search of this site may reveal others.

http://www.ndt.net/article/ecndt02/morgan/morgan.htm

    
 
 
Abbas Bombaywala
NDT Inspector
Free Lance, India, Joined Jul 2007, 91

Abbas Bombaywala

NDT Inspector
Free Lance,
India,
Joined Jul 2007
91
11:06 Dec-09-2009
Re: Comparison between RT & AUT
In Reply to Nigel Armstrong at 08:39 Dec-09-2009 .

I Hope now peoples might be convinced that AUT is better then Radiography. The only main thing which is very very important while performing AUT is the operators skills and knowledge.

Regards.
Abbas Bombaywala

    
 
 
Tyler
Consultant, Pipeline
NDT Consulting, Germany, Joined Feb 2009, 5

Tyler

Consultant, Pipeline
NDT Consulting,
Germany,
Joined Feb 2009
5
11:24 Dec-09-2009
Re: Comparison between RT & AUT
In Reply to Nigel Armstrong at 08:39 Dec-09-2009 .

100% POD is a myth.
Thank you Nigel for the link to this very informative article, which shows that the only 100% that we find, is that 100% of the transverse defects were missed by at least 1 company in these blind trials. RT and UT or AUT are complementary, they often see different defects and the comparisons done between RT and AUT are often unfair. For example AUT being done with no time pressure and compared to RT done with IR192 on D7 film. Not that I'm against AUT but we should try to be as realistic as possible. AUT and RT have there distinct advantages and the method which is best suited should be used. If we make claims in the industry that we have a system which has a POD of 100% we are only going to cause problems for ourselves.

And POD is not everything, in the Lewis Morgan article you can read that craze cracks found and interpreted easily by RT were detected by all AUT systems but generally characterized as porosity. Here RT is defiantly the stronger method.

Going back to the original question:
“I really want to know for very critical sub-sea flow line which one is better to detect typical defect encountered in the automated PGAW process.”

If it really is “very critical” then do 100% AUT and 100% RT.

Other than that I would generally say, thicker welds go to AUT, thin wall stick to RT.

    
 
 
JB Goradia
JB Goradia
13:40 Dec-09-2009
Re: Comparison between RT & AUT
In Reply to Abbas Bombaywala at 07:33 Dec-09-2009 .

Dear Abbas,
You have provided a very valuable data.
In late sixties, seventies, and eighties I had come across n-number of transverse cracks then after frequency has reduced drastically.
Adoption of AUT requires rigorous vigilance which is also a point of concern.
Thanks and regards.

    
 
 
JB Goradia
JB Goradia
13:42 Dec-09-2009
Re: Comparison between RT & AUT
In Reply to Nigel Armstrong at 08:39 Dec-09-2009 .

Nigel,
Yes, link is very educative like fundamentals which each person in the field should be aware of.
Does any Code specify the use a pair of probe mounts orientated at 45 degrees to the girth weld axis? I ask this only as information.
Thanks and regards.

    
 
 
Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
14:09 Dec-09-2009
Re: Comparison between RT & AUT
In Reply to JB Goradia at 13:42 Dec-09-2009 .

Hello JB

Before Code matters, apologies from me for a tardy welcome to this forum.

The author of the linked article was previously a British Gas (BG) employee, and BG were the only company I recall who specified obliquely mounted probes for transverse flaw detection.

Codes do not usually stipulate the detailed technique to be utilised - this is subject to agreement between the end-user and the AUT contractor. Actual set-up details will be written into a job-specific instruction approved by the end-user.

    
 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1252

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1252
15:54 Dec-09-2009
Re: Comparison between RT & AUT
In Reply to Nigel Armstrong at 14:09 Dec-09-2009 .

This thread has become rather long in a relatively short time! It would seem the topic is one of some interest. The initial posting asked about which is better (AUT or RT) "to detect typical defect encountered in the automated PGAW process".
Somehow the thread moved off to a discussion on transverse crack detections in welds in general. First and foremost, as it relates to the initial posting, transverse cracking is not one of the "typical defects" in the PGAW process. The small gap weld uses a small angle bevel for the fill region but it is not zero degrees so AUT with a tandem path approach has proven to be the best option for the most typical flaws. For 4 years we ran parallel 100% AUT and 100% X-ray with class 1 film on thousands of pipeline production welds and concluded that AUT provided the best option for reliability of the "typical flaws" formed in the process.

The concerns for transverse crack detection raised seem to be a different topic. DNV OS F101 has a clause in Appendix E (B 106 in the 2007 edition) that states " The ultrasonic system shall incorporate facilities for detection of transverse defects, when it is clearly identified that the weld process, parent material, application and environmental condition may increase the risk for transversal type flaws".
If there is a clearly defined need for the detection of transverse defects then the NDT process best suited for the task and considered to provide the required probability of detection should be developed. 45° skewed pitch-catch arrangements for ID and OD initiated transverse flaws may be OK for thin wall pipe welds. This technique may not be adequate in heavy wall sections where the initiation point is midwall. Other NDT methods would then be considered. But part of this consideration is the preparation required. Most fabricators will balk at the requirement to then remove the weld reinforements that interfere with both UT and RT options that would be used to achieve the required detections.

    
 
 
John O'Brien
Consultant, -
Chevron ETC , USA, Joined Jan 2000, 278

John O'Brien

Consultant, -
Chevron ETC ,
USA,
Joined Jan 2000
278
13:24 Dec-24-2009
Re: Comparison between RT & AUT
In Reply to Ed Ginzel at 15:54 Dec-09-2009 .

The transverse inspection requirement was originally driven in many specs due to hydrogen cracking primarily experienced in pressure vessels with SAW welding. the flaw was also observed in some SAW linepipe. With good flux hydrogen control the flaw has generally disappeared but the requirement still exists in many specifications. I get many requests to waive this requiremnet these days and we commonly do if it can be demonstrated that the correct hydrogen controls are in place. If not AUT is employed both at pipe mills and during girth weld examinations. Most specifications state the requirement not how it must be achieved.
The original question was about subsea equipment and why you would go for a more stringent examination regime. the answer is the cost of getting it wrong. Deep water intervention costs to lift flawed equipment are millions of dollars just to start thinking about - get it right first time is the key so do the best examinations possible.

    
 
 
Toni Hakkarainen
Director, Technical manager of industrial services
Bureau Veritas Branch Office Finland, Finland, Joined Jun 2006, 4

Toni Hakkarainen

Director, Technical manager of industrial services
Bureau Veritas Branch Office Finland,
Finland,
Joined Jun 2006
4
14:52 Dec-26-2009
Re: Comparison between RT & AUT
In Reply to krish at 16:04 Dec-07-2009 (Opening).

First! STOP comparing methods, none of the available techniques is above others. In NDT we are in service business and we need to serve our clients. They define the required performance, not your opinion on performance of different techniques.
Example 1. Gas container 100 cubic meters on volume under dynamic load, wall thickness of 32 mm, X-bevel. Service time of 44 years. 100 % X-ray performed in manufacturing stage. During inservice inspection a LOF with length of 1,5 meters and height max 8 mm was detected (UT). It is a fact that X-ray may not detect LOF like this. It is also fact that this imperfection has not influenced the life time of this vessel !!
Example 2. Gas container, 100 cubic meters on volume under dynamic load, wall thickness of 17 mm, X-bevel. Realized life time of 13 years. 100 % X-ray performed in manufacturing stage. Vessel exploded due to small miss weld. Length 3 mm, depth 1,5 mm. Was not detected with MT.
These example shows that in some cases the smallest detectable imperfection size is too small and in some cases the NDT is not even needed. Due to these facts we do as procedures required because we do not have the knowledge about planning requirements and material requirements. as the designers have.
In case you are forced to provide a NDT solution, please keep in mind what our clients really want. Do not compete with methods and techniques.

    
 
 

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