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Re: AUT simulator discussions
Posted by: Daniel H.Rice Consulting Services
(PID_1468), E-mail: danrice@uersw.com, on September 28, 2003 at 22:16
:
In Reply to: AUT simulator discussions posted by : Ed Ginzel 
, E-mail: eginzel@mri.on.ca, on September 26, 2003 at 16:16
:
I wanted to say something about a SIM but did not have the practical experince of performing the test.
Thankyou for your imput!!
In my experince with other SIMS, A SIM is a good place to start, a Level II inspector! Then we can let the inspector experince real world conditions, under direct supervision of a designated inspector or a level III adminisrator as your business case describes.
Follow the ASNT-TC-1A Training guidelines for "On the Job training and experince", ans you will recieve better inspectors.
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: AUT simulator discussions
: Due to the misreading of the forum item I posted by Mr. Rawlinson, I am posting another item where his concerns might be more appropriately addressed. Not having used the simulator indicated in his point, I cannot speak to the efficacy of the tool. If it is based on a raytracing algorithm it may provide a coarse explanation of amplitude deviation with respect to flaw orientation but to do justice to the REAL conditions EFIT modelling is required as described by Langenberg (see article online at http://www.ndt.net/article/ecndt02/195/195.htm). Unless the simulator can duplicate all the conditions of diffraction, mode conversion and echo-transmittance coefficient effects due to angle changes, it is not truly a decent simulation tool for indicating the REAL conditions of the AUT pipeline girth weld application.
: The other point Mr. Rawlinson makes is his perception that coupling monitor checks are “…the most critical factor in ACC/Rej criteria”
: He suggested that, “Until a company develops a system which automaticaly compensates amplitude levels to couplant loss we should be working at FBH+6DB (as the ACC/Rej level).”
: Perhaps Mr. Rawlinson does not understand the technique used by most AUT companies to monitor coupling and why the allowance for variation is incorporated. This is an ideal venue to explain the situation and dispel the notion that we need auto-compensation.
: In a girth weld that has been made where the weld axis is vertical (as in land-lay and S-lay projects) most AUT systems set up a pitch-catch configuration on the bottom of the pipe where, not only is the spring tension weakest but also, the crown of the weld is the highest and any extra losses due to the crown scattering portions of the beam can be compensated by adding gain to the receiver to ensure there is adequate signal received to indicate that there is couplant under BOTH the transmitting and receiving probes. This technique is configured that the zonal probe on one side transmit to the zonal probe on the opposite side of the weld. Verification that couplant is under the probe wedge is the sole reason for the couping check!
: But loss of coupling signal using this technique need not only indicate loss of fluid. It is often indicated by the hi-lo (mismatch) condition, i.e. the beam is redirected or scattered by a change in geometry (not a real loss of coupling). No matter how “wet” you make the surface under such conditions you are NOT going to get an indication in the coupling monitor gate.
: Coupling can also be lost when a flaw is present in the weld, i.e. the flaw interupts the through transmission. Providing this flaw is indicated as a Defect (i.e. the flaw is rejectable) this is not usually an issue because the flaw is removed in the repair. But if it is an acceptable flaw and still reduces the energy across the weld for coupling check it MIGHT cause some issues of concern for coupling.
: These shortcomings of the through transmission technique can be remedied by using a separate probe for coupling checks. This option was noted in Krautkramer’s text many years ago. These were zero degree probes built into the wedge and fired separately from the angle beam. Essentially the zero degree would monitor the first backwall for significant amplitude drops. The technique doubles as a thickness monitor as well as a coupling monitor.
: Another point Mr. Rawlinson has missed is that, in most AUT girth weld specifications, a requirement that EACH "probe" must have a monitoring channel. For a large multi-probe array using 20 or 30 separate wedges, that would technically be 20-30 coupling channels! Most systems have dodged the point and set up a single coupling channel for a zone and further reduce compliance by selecting only one pair of the tandems where tandem probes are used.
: There is a need to monitor for coupling but Mr. Rawlinson is not correct that over-sensitivity is the only solution or that auto-coupling correction be should eventually be made by adding gain by some sort of feedback loop to the receivers. One must consider that the loss of coupling “signal” does not always mean the same as loss of “coupling fluid”.
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