Longitudinal seam orientation in pipelines 1

[weld9]

Gentlemen - Virtually all client specifications call for the long seams in pipeline to be in the top quarters of the pipe, offset (not aligned) at girth welds. Bends have the seam at the neutral axis. What is the reason for not having the seams in the bottom quarters of the line?

 

[bimr]

Steve Jones recently addressed this question:

BS 4515-1:

"When a pipe with one longitudinal seam is used, this seam shall be located within the top half of the pipe circumference and the
longitudinal seams of adjacent full pipe lengths shall be offset by an angle of approximately 90º or by a circumferential distance of approximately 250 mm, whichever is the smaller."

The engineering and corrosion points of view are:

1. Fracture arrest of cracks initiated in the weld zone of the seam
2. The greatest accumulation of liquid water as a corrodent would be expected at the bottom of the pipeline and the seam, being a different material to the pipe body, might give rise to preferential corrosion

 

[rconner]

Am curious what did you mean by (and in what context)"neutral axis"?

 

[weld9]

The neutral axis of the bend, as opposed to the centerline axis of the bend. It is the line separating the region of compression from the region of elongation during the bending process, and lies just inboard of the centerline axis.

 

[zdas04]

By far most of the corrosion failures I've seen (and I've seen a lot as a consultant) have been MIC. The microbe waste products aggressively attack the HAZ of the longitudinal weld. At least half (maybe a bit more) of the MIC failures I've investigated have had the longitudinal weld seam in the bottom of the pipe in a low point with standing water.

If I have to choose between putting a weld on the neutral axis of a bend and putting it on the bottom of the pipe (i.e., in a situation where the neutral axis would be on the bottom of the pipe and you can't flip it) I will put the weld on one of the higher stress planes. This isn't often a problem with shop bends (because you can just flip the bend over), but for compound field bends that can get complex it can be a big deal.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Gator]

Is there a specific dimension/ratio for the offset of the neutral and centerline axes or am I misunderstanding the concept?

Piping Design Central

 

[weld9]

Gator - you have the right idea. It is explained in detail in Bend Tooling Encyclopedia, which you can access online. I don't think it can be simplified down to a fixed ratio, but the math is shown there I believe -

 

[Gator]

OK, I'll look it up. Thanks.

Piping Design Central

 

[SectionIX]

WRT longseam location, other reasons include:

1. Ease in visual insopection and NDE of long seam during integrity digs.

2. Seams in the bottom quadrants are more susceptible to damage caused by poorly prepared ditches (read: rocks) during lowering in and subsequent backfill. With other qualifiers, ASME B31.8 specifies "Dents that affect ductile girth or seam welds are injurious if they exceed a depth of 2% of the nominal pipe diameter..."


Each of the above especially relevant for A.O. Smith low frequency longseams.

 

[BigInch]

I agree with zdas. Keep longitudinal weld alignments alternating from 10 to 2 o'clock positions to avoid bottom of pipe corrosion from any number of sources. Long term corrosion of weld seams are far more dangerous than stretching a weld a little bit.