When use PWHT for offshore oil and gas pipelines? 1

[PaoF]

Hi everyone,
Apologies for the basic question. It would be great if someone could help

I am trying to understand when/how much PWHT is used in offshore pipelines. I had different indications from different people (never used/used often on the weld/not used on the weld up to WT of 1.5" but maybe used for thicker walls)

Many thanks!

 

[LittleInch]

PWHT is only used if you absolutely have to as part of the welding procedure. Usually reserved for thick wall pipes over 25 or 40mm or some such thickness and as required by the welding procedure. Each weld takes a min of 1-2 hrs and sometimes longer to do it properly.

Offshore it would add a massive time delay when time really is money.

People may be confused by pre-heat which is pretty common to heat up the pipe and prevent cracking and or heating the pipe end up to spray on FBE and other coatings.

Many people don't really understand what it is they are looking at and only hear the word "Heat"...

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[1503-44]

I have not looked for that ref for awhile now, but I think you will find in ASME B 31.4 & 8 that it is a requirement when WT is 0.75" (19mm) +

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[LittleInch]

434.8.9 - 32mm (1 1/4")

Think B 31.8 is the same.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[PaoF]

Many thanks for your help. So it is fair to say it is often (always in the US) used for LSAW pipes with WT above 32mm? Would a double-seam allow to do without?

 

[1503-44]

So its 1" + 1/4, not 1" - 1/4". Thanks for actually looking. .
It seems that I'll have to get used not to relying on memory so much ... :-(

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[LittleInch]

Don't know.

You want to avoid PWHT like the plague. So you need to talk to your welding engineers on what fancy things you can do to avoid it. Offshore the cost in time is enormous. it would pay for much more expensive materials which don't need PWHT.

The codes allow different things and sometimes you can avoid it by testing and different welding techniques.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[PaoF]

I see. But would the time cost be an issue if the PWHT was handled by the pipe mill directly? I am referring to the longitudinal well here

Many thanks!

 

[1503-44]

It would be a rather large cost, even if the mill did it. And you will still have to weld the ends. Pipelines usually have so many welds, that PWHT MUST BE AVOIDED, NOT ONLY FOR OFFSHORE, FOR ONSHORE TOO. As far as most of us pipeliners are concerned, it is a speciality, for in-plant use only.... "BY OTHERS".

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[LittleInch]

I don't think anyone PWHTs pipe manufacturing welds or if they do its a pipe mill thing which I can't comment on.

As Mr 44 says, if you need to PWHT the seam weld do you need to do the same to the girth weld? Don't know - each pipe is different.

The only time I've seen PWHT in the field is when someone screwed up and didn't realise he needed to to it until after the pipe was on order and delivered. As he was the client he didn't get fired, but we had to fix his mess.

The only other time is in sour pipe welds when they want to reduce hardness in the weld and HAZ zone. Then you're in for a doubling of your construction time.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[PaoF]

All clear now, many thanks!

 

[blacksmith37]

ERW seams are are required to be PWHT , but that is local and very short. It is intended to basically be a normalize. Otherwise I never saw PWHT of line pipe in any mill around the world.

 

[SJones]

DNV sets the threshold thickness at 50 mm, and states that PWHT can be avoided by means of fracture toughness testing (presumably leading on to an engineering critical assessment). In my experience, the latter option is the one usually adopted.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.