Support span for an underground pipe

[MPinson]

Hi,

I'm working on a gas project, for which I need to install an underground DN150 PE lined CS pipe.

I got a client standard imposing the use of concrete sleepers in the trench, around each fitting. Just before and after each elbow, just before and after each tee.
Nothing stating the use of concrete sleepers on the straight parts of the pipe.

One of my colleagues told me he used to install sand bags, at the right span.

Has someone already worked on this kind of topic?
How did you support the line on the straight lengths? Did you respect the support span of a "standard installation"?

Thx

Mathieu

 

[cvg]

search this forum for "thrust block" or "restrained joints".

you really don't need restraint for straight sections of pipe or for welded pipe.

 

[LittleInch]

Do you mean buried or just in some sort of open trench?

If you mean buried then most of that is a complete waste of money unless your trench bottom is made of liquid sand.

Do you have some sort of sketch or drawing for this so we can see what you can see??

you don't specifically support pipe when laid in a trench - that's what the bottom of the trench is there for.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MPinson]

The line is buried, that is why it is lined with PE (cathodic protection).

The soil is very unstable (along the sea) and is subsiding every year.

Point is the line is 500m long, with lots of direction changes, so we'll have to do some of the welds in the trench. And PE lining after that.
For at least this reason the line needs to be raised and cannot lay directly on the bottom of the trench.

 

[cvg]

several points

a) you need to provide erosion protection or ballast if this is affected by either wave action or high groundwater

b) setting it on blocks will not reduce the stress on the pipe due to earth loads or settlement. it might make it worse

c) sleepers will not prevent the ground from settling

d) plan on designing the pipeline to handle the stress or move it to a better alignment

the only time I have seen sandbags used was for temporary support while the pipeline was being encased in concrete slurry.

How is PE lining helping with the corrosion? shouldnt the exterior of the pipe be wrapped or coated instead?

 

[LittleInch]

There are several issues here which appear quite odd to me:
1) Lining steel lines with PE is not common for gas, mostly used for water or slurry, but once you put lots of bends or tees would become quite painful to undertake for 500m
2) Laying a gas pipeline in an area of subsidence and "unstable ground" sounds like a wholly bad thing to me
3) Pipelines are quite flexible, especially at 6 inch size, but not if you start to anchor them or provide hard points such as supports
4) I've never seen a sleeper big enough to allow someone to weld underneath it - at the bends lift the pipe up, weld the bend, lay it back down again - simple
5) This sounds like someone applying above ground piping layout design to a buried situation - not applicable and like cvg says, supports could easily make it worse.
6) The fact you say elbow instead of bend implies this is a piping type network and not a pipeline? Buried pipework is a big clash of cultures and experience between piping and pipelines so be aware the two don't usually meet - or get on very well.

A plan or some drawings would help.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MPinson]

The plant is a LNG terminal, close to the the sea. That is why there is settlement of the whole site. The buried line will never erode because of the waves

My mistake, I used the bad word, I meant PE coated ! This is a BOG line, DN150 carbon steel, 900lbs, with external PE coating.

I'm not talking about a pipeline here, but a piping type network.

Check the links below. You'll find the client standard for sleepers and the piping layout

https://www.dropbox.com/s/pgs4dtn7ddglr97/SUE-020.pdf

https://www.dropbox.com/s/dnhg5djhul4ksvm/Etude du tracé de la ligne enterrée.pdf

 

[LittleInch]

The pipe will not settle inside a plant to any degree otherwise the whole plant will collapse before a 6" bit of pipe will do. Someone is greatly exaggerating the stability of the soil here... However if there is any settlement of the thing your pipe is connected to you could easily shear it off as your pipe is effectively anchored independently of the connecting equipment.

As I said above, someone is trying to use what is clearly above ground piping sleeper designs for below ground pipework which doesn't need it as all the pipe is surrounded by soil / sand, so lifting one section off the bottom of the trench is totally irrelevant and worthless. However if they want to waste money it probably won't do any harm so long as someone packs soil or back fill UNDER the sections lifted off the ground otherwise you will induce large bending moments as the pipe tries to support 1m of soil and might even shear it or fatigue it...

My French isn't good enough to see if the notes said the sleeper designs were intended for inside a trench / buried pipe, but if so then it needs to be removed or apply for a dispensation

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way