PE coating of aboveground gas pipeline in desert 1

[cedric]

Hi guys,
I am looking for an advice on the type of external coating to consider when the design temperature of the line reaches 75oC. Do PE coatings going up to 75oC exist or shall I use another type of linepipe coating (& field joint coating). Taking into account ageing due to UV, what type coating shall be specified.
Cheers.
Cedric.

 

[stanier]

My experience is that PE coatings will not last in such an environment and you would be better off with a fusion bonded expoxy or an epoxy paint system. Sharing knowledge is a way to immortality

 

[PVRV]

Please excuse me; I may not understand your question properlly. But why do you need to apply any external coating system above ground ?? Now considering a service temperature of 75 C PE should not be a problem.

Cheers

 

[cedric]

Hi guys,
Thanks for your reply. the pipeline will be buried; but it's a very long length and will be exposed to sun for quite sometime. my concern was to understand how UV could affect the material properties, and what protection could be required.
Epoxy will not be considered here (thks Stanier). the first idea was 3layer-PE, but it seems that max temp is 65-70oC. so it has been decided to go for 3layer-PP.
This is definitely a buried pipeline, large diameter, without corrosion allowance...
any advice or comments.
Cheers,
Cedric.

 

[PVRV]

cedric,

We have been using 3-Layer HDPE, extensively for the past 4 years. Most recently a 48" pipeline (roughly 120 km) to be buried underground. As for your concern regarding the degradation of the external coating system in view of prolonged exposure to direct sunlight; we have not experienced any problems in this regard.

stanier, 3-layer HDPE consists of:

a) A thermosetting powder epoxy primer based on epoxy
resin. Which is a fancy way of saying FBE.

b) An adhesive layer (PE copolymer) which should not
contain property degrading extenders.

c) Prime, virgin pigmented polyethylene coating.

d) The final polyethylene coating should be black in color
containing carbon black to resist UV degradation. At
times, it is stipulated to be suitable for 2 year
storage in direct sunlight without any signs of
degradation.

Your concern was evident with other coating systems; coal tar enamel, Polyolephic & Bituminous Rubber PVC Tape. All of which have degraded due to UV.

Hight density does cater for high temperature resistance versus LD or MD. However, it would be prudent to provide protection (covers) prior to lowering and backfilling; it does coincide with sound engineering practices.

As for the joints; you have to be careful with the type of heat shrink sleeve.


If it appears that I am advocating the use of 3-Layer HD/MD/LD PE am not. At several instances I was of the opinion that other coating systems would be as effective and towards value engineering practices.

Cheers,

 

[cedric]

thanks for ur complete answer. what would say about 3-layer PP?
another subject... I am worried that no corrosion allowance has been considered on my project (48",X70, 270km, buried, gas). corrosion monitoring has been specified, but I am not convinced that it is sufficient.

 

[PVRV]

cedric,

1. PP is very expensive. I can recall its use on off-shore
pipelines since PE has adhension problems (slippage) with
concrete encasement; of course this does not apply to your
case. Besides PE is sufficient to address EXTERNAL corrosion.

2. I agree with you, it is grave misjudgement not to have
provision for corrosion allowance. I can understand why,

a) Design Pressure is perhaps around 800 psi; adding corrosion
allowance may require each joint to be PWHT'ed in view of
thickness exceeding 32mm

b) What pushed the designer towards X70 is to reduce
the wall thickness; bearing in mind that it is
diffucult to weld (and I would caution you not
to increase Mg content beyond 1.6 to attain the strenght
required)

c) There is a huge cost impact on any type of wall thickness
reduction.

3. General

- If the design factor is 0.6 or lower and thickness under
tolerances are considered in calculating the required
thickness.
- If the gas is adquately dehydrated and corrosion inhibitors
are injected.
- If sour service is not encountered during the service life
of the pipeline.
- If thickness required considers maximum corroded conditions
towards design factor and maximum under-tolerance.
- If there is a periodic intelligent pigging program

All the above or a comibination of, may justify nil CA; but I
would not allow it and yes I would be very conservative; which
may not be correct.

It is worth while to consider optimizing the pipeline diameter as much as possible. Yes the wall thickness would increase with larger diameter 48" --> 60" by about 1.16 but the flow rate under the same operating pressure would increase by a much higher factor around 4; the above requires simulation to verify; but there is a potential to lower the operating pressure substantially. I believe a larger diameter pipe with lower wall thickness is much more cheaper. Besides, you can easily add corrosion allowance


Cheers,

 

[stanier]

I must admit the original question had me fooled as the description didnt infer that the PE would be a multi part wrapped system. In fact it didnt even mention that the pipe was buried.

Some PE coatings that are available such as Shaw's Yellow Jacket and Sintakote do have problems with UV degradation. Also bonding is problem if one considers that the thermal expansion coefficient of PE is 15-20 times that of steel.

The carbon black in the outer coating of the PE wrap is designed to prevent UV degradation. It works provided that the carbon is evenly dispersed in the polymer. Virgin PE is not resistant to UV.


Good information from PVRV, hence the Star.. Sharing knowledge is a way to immortality

 

[cedric]

PVRV, Stanier thank you very much for you answers, they are extremely useful for me.
My playground is the middle east (UAE, Oman, Qatar, Bahrain), if you need any info feel free (cedric6@gmx.ch).
Cheers,
Cedric

 

[PVRV]

Cedric,

It would be worthwhile to consider a larger diameter pipeline. I believe a 60" pipeline has the potential to:

a) Lower the material grade
b) Reduce the required design pressure
c) Substantially decrease material tonnage
(it would very much cheaper)
d) Add some corrosion allowance
e) Lower the power consumption on your compressors
f) Reduce compression stages

The above requires simulation.

Regards,