FBE Tail protruding from 3L-PP anticorrosion coating on pipes

[bafometto81]

Hi guys,

I'm working as coating engineer and I have a question regarding the FBE tail protruding from a 3L-PP or 3L-PE anticorrosion coating on pipes.
In my opinion any residual FBE at the cutback area should be avoided as it could be detrimental for field joint coating and AUT operations...

Nevertheless I hear many people asking for a minimum 10 mm tail for example.

What's your opinion on that?

Regards,

NB

 

[brimmer]

My opinion is it may depend on what type of field joint coating you are using, the whole subject of coating compatibility should come up, and then what you do to prepare the area for the specific coatings. There is no issue with a 10mm FBE tail, as long as you prepare this surface as well when you put the field joint coating over top. Putting the field joint coating over top with no preparation over the FBE will not work very well no matter what the coating, the FBE should be roughened as well, and adhesion of the field coating should be fine. I should think the tail would provide a slight advantage rather than cutting it back all the way to the PE, if your coating seam above fails, at least there will be some epoxy below in the area of this seam (if your contractor is not preparing the FBE as well but leaving it smooth)then you have a problem. I also would recommend an epoxy coating as the field joint coating, I have seen too many pipelines built with shrink sleeves etc. where contractors don't know how to apply coating properly and then you have a problem years later at many of the girth welds. Epoxies are also fail safe in terms of cathodic protection. Anyway, if there is a tail, roughen it...

 

[bafometto81]

I'm using a heat shrinkable sleeve polypropylene based (the factory applied coating is polypropylene).
I agree that the main point is to ensure a proper bonding between the FBE and the PP, but I don't see how the tail could help that. when we preheat the pipe with induction coils in order to apply the HSS we pass the Tg of the FBE so we destroy the tail which in fact will be just residual FBE, not a proper protective tail. Furthermore during the blasting of the field joint area this tail can be damaged and again we will have just a damaged coating, not at all protective.
So it seems to me that this tail will not serve as a protective coating but will result just in a residual FBE wich in my opinion could cause some problems during the AUT test being seen as a defect.

thanks,

Nicolò

 

[brimmer]

You would prepare FBE by blasting, but not as you normally would the steel. The idea is to roughen the surface of the FBE just enough to provide an anchor pattern for the new coating, not to remove it. So when blasting, they would hold the nozzle farther away and at a different angle, or reduce pressure so as not to take the FBE off... Your blasters may not realize this and probably have never done it.
In any event if you are damaging FBE from the induction coil and there is no chance of saving the tail, then you would want to remove all of it during blasting (as you probably are now) so there is no residual FBE left and the shrink sleeve can adhere properly. The main thing is proper adhesion of the coating, whether it be on steel or FBE. Don't know if you will have a seam where the shrink sleeve joins the 3PLE, or the sleeve extends over top, but you don't really want to leave that seam, this is where environment can get in and start the disbondment of the shrink sleeves.
As far as AUT giving a false signal by seeing the FBE, I think that this would not be possible, as providing there is no coating under the sensors, it is shooting into the weld, the FBE should be on the backside of the machine. If you mean FBE directly under the sensors, then yes, this would actually block the signal and need to be removed.