Pipe Ovality Question

[kander]

Guys,

Consider a pipe of 100mm OD and lets say this is subject to a maximum ovality of, say, 10%.

Taking a section through the pipe, does this mean that:

1) The pipe can be 110mm in the x-plane with 90mm in the y-plane?

2) The pipe can be 105mm x-plane / 95mm y-plane?

3) The pipe can be 110mm x-plane / 100mm y-plane?

4) Something other than this?

 

[narenr]

Pipe Ovality = [Dmax - Dmin] / D


Narendranath R
Pipeline engineering is made easy with state of the art computer software, visit

http://www.narendranath.itgo.com.

 

[zoomi]

Interestingly enough BS 8010 Part 3 1993 has a different formula. Looking at section C.1.2 External pressure
it states

fo = [Dmax - Dmin] / [Dmax + Dmin]

Where fo is the initial ovalization of the pipe cross section (not to be taken as less than 0.025.

The same as the above is for flexible pipes with a maximum permanent ovalisation of 0.2% considered acceptable.


But

For reeling calcs the formula is the same as narendranath's. eg DNV OS-F101. Section 6 Clause E1200 for measurments of tolerances.

 

[BigInch]

zoomi, Interesting point.

They probably spell ovalization with an "s" too.

http://virtualpipeline.spaces.msn.com

 

[zoomi]

My Trevor Jee notes avoid the "z" vs "s" problem by talking about "ovality".

The notes talk about "a runaway flattening of the pipe". Ulp!

Trevor Jee's ovality is the same as narendranath's but they state that BS8010 has a different inherent factor of safety and that the ovality in BS8010 is defined differently, having a less conservative lower limit.

 

[BigInch]

:-?

Runaway flattening at a pressure lower than critical pressure can happen once colapse has been initiated at a local buckle. The pipeline needs to have stiffener joints to stop it.

http://virtualpipeline.spaces.msn.com

 

[zoomi]

Yes - I can see an analogy of a column subjected to buckling loads. Axial load producing more lateral deflection which produces more LD and so on.


Those stiffener joints. Can you recommend a code for these stiffeners?

 

[BigInch]

No pipeline code I know of covers the design of a pipeline stiffener joint. You might find an applicable method used in structural design, but I really don't know.

http://virtualpipeline.spaces.msn.com

 

[rconner]

There is some information at least relating to the design of steel water pipe "stiffening rings" and "ring girders" in the Manual of Water Supply Practices, "AWWA M11, Steel Pipe -- A Guide for Design and Installation".

 

[narenr]

Large diameter water pipelines are designed more like vessels than pipelines hence the requirement for stiffners. Normal pipelines are not designed with stiffners for column (upheaval) buckling instead accessed and mitigated by routing to avoid buckle initiating outcrops.

Pipe ovality is an input data for local buckling and buckle initiation/propogation in offshore pipelines which are prone for collapse (due to external pressure)is mitigated with buckle arrestors. These are higher wall thickness pups designed for buckle resistance and spaced considering the water depth for laying.

I think the original question is more looking in geomtrical sense as pipelines with 10% ovality cannot be used at all.


Narendranath R
Pipeline engineering is made easy with state of the art computer software, visit

http://www.narendranath.itgo.com.