Weld defects - how much is too much?

[KiwiTanks]

I have a tank currently underconstruction, which has had about 35% of the Radiographed welds fail. These failures are only small in nature (but still large enough to be a failure obviously) so not huge gaping holes etc.

I have scoured API650 and some of ASME IX but can not find if there is anywhere that states how much failure is too much.

I 'feel' a third is too much, but I can't state that to the contractor...
Anyone got any suggestions?

 

[SJones]

You will not find a repair rate limitation in the standards that you are using. Generally, it is introduced as a separate, and generally woolly (excuse the pun - not related to your nickname!), contractual requirement by the owner; something along the lines of: 'if the owner is not happy with the repair rate then.......' If the repairs are down to welder skill issues: porosity, slag, etc, then one could argue that there is a motivational issue, a training issue, or both in the contractor's organisation. If the contractor has an ISO 9001 certificate, then they should be interested in quality improvement and reduction of rework; so, get together with the contractor and thrash it out - it's a win win situation.

As an aside, how are you calculating the weld repair rate? As a percentage of the number of welds, or as a percentage of the total weld length? I suspect the former. Try the latter and see what rate you get then - your 'feel' might be assuaged somewhat.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[KiwiTanks]

Thanks for the advice Steve,

The weld defect rate was simply calculated by a pass/fail rate on the x-ray, so not a percentage of total length. I will get someone to check out the % as a weld / x-ray length.

Most of the failures are inclusions, so will bring it up with the Contractor along the lines of quality improvement.

[small](Let's not get in to the whole woolly thing. It's NZ's West Island that has a Swagman jumping up with glee after a Jumbuck (in their 2nd national song)...)[/small]

Thanks
Dave

 

[JoeTank]

A repair rate of 35% is never a win-win situation. Your contractor is failing to meet any normally accepatble failure rate by competent tank companies. If the failure rate went above 10% of the RT shots taken, I'd be jumping up and down about it. In API-650, the Purchaser has the priviledge to perform additional radiography at his own expense. The expenses for RT, repair and re-RT pass the the Manufacturer if additional defects are disclosed. Does your 35% rate include the tracer shots for defects too?

Joe Tank

 

[JStephen]

That repair rate is higher than normal, and is also higher than what a manufacturer would expect for economy in construction. However, the rate isn't limited by the tank standards.

While the owner may have the option of doing additional radiography, this should be included in the contractural requirements if applicable. Increasing the degree of weld inspection is normally an extra-cost item, even if the actual inspection is done by others.

 

[JoeTank]

JStephen,
When the Purchaser refers to API-650 in the bid documents, that already includes the provision for extra RT by the Purchaser. It's been in the Standard for quite a while now. It's nothing new. As you already know, things have gotten pretty bad by the time the Purchaser looses confidence in the weld quality being provided.

Joe Tank

 

[SJones]

Joe,

What I meant by win -win was that both Owner & Contractor should have the mutual aim of maximising weld quality. If the Owner expresses dissatisfaction, a quality driven organisation should react to that situation in some way, perhaps by more preventive action. If the Contractor doesn't care, then the wrong Contractor was selected.

Repeated volumetric defects are indicative of two things:

1. Welder(s) with marginal skills
2. Poor welding inspection

So, the first thing I would be discussing with the Contractor is: which welders are producing the most defects and what is the Contractor going to do about getting them back on track or replacing them. Then I would ask why the welding inspector(s) is/are not picking up slag before more weld metal is deposited over it.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[JoeTank]

Steve Jones,
I see your point, but too often the RT is done late in the job and it's sometimes too late to fix the problem and get to the win-win situation. At that point the Purchaser sometimes has to grab the bull by the horns and invoke his priviledge to conduct further NDE on the poor workmanship he now suspoects in in-place.

Joe Tank

 

[JStephen]

Joe, what part of 650 are you referring to?

 

[KiwiTanks]

JStephen

Check out
[blue]7.3.2.3 Responsibility[/blue] for starters. We also write an allowance of further testing in to our contract docs to make it clear.
(In fact we are actually performing 10% radiography of vertical welds, and 5% of horizontals in line with our Client's global specifications)

At this point in time we have 2 horizontal welds completed in one tank out of 6 times 3 tanks - so now if the opportune time to iron this out with the Contractor.
Thanks for your input guys - much appreciated.
Dave

 

[JoeTank]

JStephen

The dreaded 7.3.2.3 is pasted below...

7.3.2.3 Responsibility
The Manufacturer shall be responsible for making radiographs and any necessary repairs; however, if the Purchaser’s inspector requires radiographs in excess of the number specified in Section 6, or requires chip-outs of fillet welds in excess of one per 30 m (100 ft) of weld and no defect is disclosed the additional inspections and associated work shall be the responsibility of the Purchaser.

Joe Tank

 

[JoeTank]

KiwiTanks,
Your use of 10% radiography of vert seams may not be sufficient to meet API-650 rules. For example, all thicknesses greater than 1" required full RT

Joe Tank

 

[JStephen]

Thanks, Joe and Kiwi- I was looking in the following section where it spells out number of tests, and it didn't show that option there.

 

[KiwiTanks]

Joe Tank

These tanks are only babies - 30m (100')diameter & 16mm thickest strake (11/16"), so no probs with the full RT.

Ta
Dave