Formular for calculating the elastic Bend Radius of an Offshore/subsea Pipeline

[Mathew95]

Good Evening All..

Please how do i calculate for the Elastic bend radius of an offshore pipeline ..

check a couple of codes & standards and i can't seem to find a definite formular for the calculating the elastic bend radius of an offshore pipeline ..
All i found was for Onshore ..

Thanks

 

[Snickster]

I believe you mean the allowable elastic bend radius which is after you subtract the allowable stress for pressure, etc. An elastic bend is an elastic bend whether onshore or offshore so for a given bend radius there is a given bending stress that must be within the elastic limits. However onshore and offshore piping are two different things altogether.

For onshore piping the pipe is routed fully supported along its length and typically buried. If the land is uneven it is cut out or filled in so that the pipe is laid with a gradually varying slope which is the allowable elastic bend radius calculated. The land is either cut or filled to provide constant support of the pipe.

For offshore pipe the pipe not routed in the cut and fill method (as this would not be possible) but purposely routed on level ground so there is no elastic bends and completely concrete coated. There are natural changes in elevations though on the seabed where the pipe routed over it does not make contact but is fully or partially unsupported for a certain length. Therefore the calculation/concern is not of an elastic bend but of an unsupported pipe length and how much unsupported span can exist, which is a very complex analysis since there are other forces involved such as wave forces.

There are many other calculations for underwater pipelines such as wave loads and stability of resting surfaces, and installation loads. Therefore, the design of underwater pipelines is very complex and specialized.

 

[BEMPE16524]

in addition to what Snickster has explained above, specific standards may not always provide a direct formula, the calculation is rooted in fundamental elasticity and bending mechanics. in DNV-ST-F101 (Submarine Pipeline Systems) you can find the detail guideline to achieve what you are looking for. focus on these sections:

• Section 5: Design Criteria and Safety Classes
• Section 6: Pipeline Design
• Appendix A: Material Properties

Note:
5.6.2.1 This standard does not provide any limit state criteria for pipeline bends.

 

[HTURKAK]

check a couple of codes & standards and i can't seem to find a definite formular for the calculating the elastic bend radius of an offshore pipeline ..

I do not have any idea how did you check the relevant codes and standards .
You may look ;
- API RP 1111 Recommended Practice - Design, Construction, Operation, and Maintenance of Offshore Hydrocarbon Pipelines,
- DNV -OS-F101-Submarine Pipeline Systems , ( i attached the doc.)
I will suggest you to look to the book ; Offshore Pipelines. Design, Installation, and Maintenance- (by Boyun Guo, Shanhong Song..)
You may also provide more data to get better responds.

 

[LittleInch]

I don't think for an offshore line this is directly calculatable.

The simple answer is calculate the maximum bending moment that doesn't exceed you allowable total stress (Von mises etc). Th problem with an offshore line is that unlike a onshore line it cannot be assume to be either unrestrained or fully restrained and hence other stresses such as thermal change and laying stresses lie somewhere between the two. Also the impact of e.g. thermal stress is that the pipeline can move sideways or indeed vertically, sometimes by a considerable margin. You have of course the hoop stress which is known.

The other aspect subsea is that whilst a vertical elastic bend is very feasible due to seabed undulations, horizontal bends require the installation tension forces on the pipe from the stinger to be more than matched by friction on the seabed, otherwise the line starts to straighten. In reality this max bending radius (2000D) is far larger than the pipe max elastic bending radius ( Usually 600-1000D).

Why do you want this and what are you trying to do with it?

 

[GD2]

For a layman, the elastic bend radius, R is given by:

R=(ED)/(2 Sb), where

E = Elastic Modulus of the pipe material
D= Pipe Dia
Sb = Bending stress

Of course the bending stress should not exceed the elastic limit. Bending stress will be created by the external bending moment, M to create the bend.

Sb= Mc/I. Max bending stress will be at the outer fibres of the pipe. i.e when c=pipe radius. I = pipe moment of Inertia.

Elastic bending means the pipe is bent without exceeding its yield strength.

 

[LittleInch]

Beware that bending radius ignores all the other stresses on the pipe....

 

[McDermott1711]

I found something in page 4 of the following document that may help.

Subsea Pipeline Installation

 

[LittleInch]

The formula is a bit vague about lay tension though.

Not a bad high level presentation though.