UT in lieu of RT question

[ralzzz]

API-650 4.2.1.2.3 allows a reduction of .01” (presumably for mill tolerance considerations) for newly purchased shell plate material. Appendix U states that ultrasonic testing can not be done in lieu of radiography for materials less than .375” in nominal thickness. Provided that the minimum required design thickness requirements are met, will that .01” tolerance value be applicable to the requirement for .375” nominal thickness value plates in Appendix U? The material was found to have thickness readings slightly less than .375”, but greater than .365”. In fact, the purchasing agent who bought the material specified .365" as the nominal due to this allowance. When the material was delivered, UT scans were made to determine overall thicknesses at the plate edges to be joined. The reason for this question is that this tank is being manufactured in Afghanistan where radiography is not allowed, but is still required to meet the design specifications in API-650.

 

[Bonswa]

Here's my logic on this...

The area of interest is the Weld itself and the adjoining side walls. That being said, if your base metal is 0.365" and you've got a small weld cap that would add to that thickness, I see no reason why you couldnt use the size of the weld to meet the minimum requirements. Get a fillet weld gauge and use it to figure out how high the weld cap is.

If you go to API-650 Appendix U, Table U-1b, you will see that the first column states "THICKNESS AT WELD(t)". Along with that, is Note "a".

If you read note "a" at the bottom, it clearly states the following:

a. "t" = thickness of the weld excluding any allowable reinforcement. For a butt weld joining members having different thickness at the weld, "t" is the thinner of the two.

Hope this helps!

 

[IFRs]

If the design thickness on the drawings is 0.375" then I think you can make a case for using Appendix U as it is written (greater than or equal to 0.375"). How the material was purchased could be thought of as a side issue as long as the received material meets code. The allowable underrun of 0.01" is in section 4.2.1.2.3, so this material meets the code.

To me the real question is: will the UT method be effective if used on material less than 0.375" thick and if not, then stop right here and re-think your position.

API specfis where the mearurements are to be taken:

U.3.1 The UT volume shall include the weld metal, plus the lesser of 25 mm (1 in.) or t of adjoining base metal on each side of the weld unless otherwise agreed upon by the Purchaser and the Manufacturer.

They also talk about the effectiveness of the method:

U.3.6 The examination methodology (including U.6.6) shall be demonstrated to be effective over the full weld volume. It is
recognized that Time of Flight Diffraction (TOFD) may have limitations in detection of flaws at the surface such that it may be necessary to supplement TOFD with pulse-echo techniques suitable for the detection of near-field and far-field flaws. The variety of surface and sub-surface category flaws in the test plate mandated by U.4.3a are intended to ensure that any such limitations are adequately addressed.

Good luck and please let us know what happens!

 

[ralzzz]

Thank you guys for the response. We have found a company in Dubai who can perform the UT on these tanks. They are reputable and have procedures specifically for performing UT on tanks. Because Afghani laws prohibit performing gamma ray radiography, we are somewhat limited in what we have to do for inspections.
In general, our plate thicknesses are greater than .375", around .379" or so, but there are some areas that did read at .369-.374", so we're only talking about a few thousandths of an inch. My problem is that I'm working on this government project and they don't realize any deviations than what is written in the code. They are looking at that .375" requirement and not wanting to apply the .01" tolerance value. But, the tanks are designed with a thickness of .375" on the drawings, so I think that point will suffice. If anyone has any input, I'd love to hear it.
Thanks again and have a great day.

 

[Ballbearing1]

ralzzz,
You mention gamma radiography is not allowed in Afghanistan.
Do they allow x-ray radiography ?
Regards,
BB

 

[Bonswa]

x-ray uses gamma radiation

 

[Ballbearing1]

Bonswa,
With all due respect you are wrong.
Gamma rays are generated by the natural degradation of a radioisotope. They are generated as a result of nuclear activity.

Xrays are generated by an x-ray tube containing a cathode that emits electrons which are accelerated towards the anode by a high voltage electric charge.

A gamma source is always radioactive whereas an xray source is like a light switch, you switch it on and off.

That is why you can quite often use an xray source in areas that a gamma source is not allowed.
Regards,
BB

 

[Bonswa]

My mistake. I wasn't considering X-ray tube at all.

'round this here part-a-town, all RT work is referred to as X-ray, but it's "technically" gamma radiation from a radioisotope. In our case, iridium192 (most common).

As a matter of fact, none of the facilities that i've worked for have utilized the X-ray tube. That being said, does it require "Radiation" labels all over it? I would imagine so, and if i'm right, i cant see the Afghani government allowing it.

ralzzz, Can you give me some specs on this tank?
Height, Diameter, Shell thickness, Shell Material, Joint Efficiency.

 

[ralzzz]

It is only gamma ray that is not allowed. The problem with x-ray tubes is that we can not find a company willing to do field work with their equipment. Cost is another factor, the one company in the US who would do it wanted more for the x-ray than the tanks cost.
The tanks are 90' diameter and 23'8" tall. All four shell courses are 3/8" A 36 carbon steel. The capacity is 25,000 barrels and the contents will be JP-8 fuel. We are using a specific gravity of 1.0 for the design, althought the actual SG is .82. Design temp is 200 F. All welds are specified to be double butt welds with complete penetration and no defects.
I'm going to go back to the Army Corps of Engineers and tell them that the tanks meet the .375" requirement for UT and they are going to have to accept it. If we had plates that were thinner than .365", I would see they had a point. Because API-650 allows the reduction of .01", I'm going to recommend they allow it. Somewhere I think there is a conflict between the Corps and the design company that I don't know about and I'm sort of mediating the issue. That's where my involvement is in this mess. Any other input would be greatly appreciated.

 

[Ballbearing1]

ralzzz,
We used x-ray exclusively on Caltex, Mobil and Shell Oil Tanks in New Zealand in the late 90's and it should be no more expensive than gamma.
Fabricate an aluminium frame that the tube sits in and is clamped into (height of legs will depend what FFD you are using) and puts wheels on the bottom of the legs.
Have a pulley system with wheels set up on the top of the tank and then hang the frame off the cable. Just push it around the tank and raise and lower as required.
Companies like CB&I use cages that hang over the top strake with wheels for there welders to propel themselves around in. These can be used for sitting the tube in but the advantage of a frame is it is rigid and all the shots are the same FFD so no changing exposure times.
Hope that helps,
Regards,
BB

 

[ralzzz]

Thanks BB, I know exactly what you're referring to as we used x-ray in a similar set up on projects all over the world. I think it's more of a stigma working in Afghanistan and trying to bring equipment into that country. I had to pay $4,000 in duties last year just to bring a UT scope into the country. As of now, we're still going to pursue the UT option and hope for the best.

 

[Bonswa]

what joint efficiency is required?

Could the tank possibly qualify for API-650 appendix A?

If so, you can wave rt or shearwave all together and use a joint efficiency of 0.70.

Just a thought.

 

[ralzzz]

That's a good point. The design does not stipulate a joint efficiency, but it does state that "tanks must have 100% full penetration" and that "all welds must be 100% free of slag, undercut or other defects". That almost sounds like they are specifying a JE of 1.0.

 

[Bonswa]

not necessarily. That just means full penetration butt-welds and all surface slag removed. I would look into it farther. See if the joint efficiency is specified or if it states a requirement for the extent of radiography.

Now, by lowering the joint efficiency, your tmins will be lower, but if the product isn't highly corrosive or if you're coating the internals, you won't need to really worry about tmin.

Let us know the outcome.

 

[ralzzz]

Hi again. Just to follow up, we finally got approval to do the UT on the tanks in Afghanistan as discussed. The job went without a hitch and everyone was happy. We are now looking at some other tanks that are much thinner in nominal thickness (6mm = .236"). We convinced our client that since our original UT project went so well on the thicker tanks that we would try it on the smaller tanks. We have shown that there may be a good reason why API does not allow UT on thinner materials as we are experiencing a high rate of defective welds. Thing is that when we cut the welds, we are not seeing any defects. The defects range from lack of fusion and incomplete penetration, but we know that these defects do not exist. Does anyone have a reason why these defects may be erroneously showing up? More importantly, does anyone have any ideas on another UT technique other than normal shear wave that may eliminate these false defects? I used to be a UT tech back in my early days so I do remember some theory, but that's been over 20 years ago and I'm not quite as up to speed as I once was. As always, any ideas would be greatly appreciated.

 

[Bonswa]

Although costly, I think Phased Array would be your closest option to RT. It's still looked at like UT, but can produce the results that you would expect from RT.

 

[ndeguy]

ralzzz

Acceptance testing of welds in 6mm thick plate is not he easiest of inspections. How experienced is your UT tech? It seems that it is a case of misinterpretation of signals from the root bead as LOF and IP. What angle probe was the tech using - a higher angle could be tried e.g. 80 degrees. PA may be useful in that the saved data file can be emailed to an expert for analysis and review, but it still will be a struggle dealing with 6mm thick plate.

You say Afghani law prohibits use of gamma isotopes - are you certain? But more importantly what about X-ray, you could get great images of 6mm thick welded plate with an X-ray tube. If isotopes it would need to be Selenium for a good image at that thickness.

Good luck

Nigel Armstrong
Lloyds Register
Independent Verification Body Surveyor