Maximum bending of a cold formed bend

[KVdA]

Does someone has an idea what the maximum is that i can bend a cold formed bend, bearing in mind that there is no opportunity to check for wrinkles or minimum wall thickness.
Or is the answer simply no additional bending stress/strain allowed.

Any reference to known acknowledged codes might help also.

Thanks in advance

 

[KevinNZ]

The B31 series of codes cover cold forming bends.

for example B31.1 clause 129.1

129.1 Bending
Pipe may be bent by any hot or cold method and to any
radius that will result in a bend surface free of cracks. Such
bends shall meet the design requirements of para. 102.4.5
with regard to minimum wall thickness. Where limits on
flattening and buckling are not specified by design, as delineated
in para. 104.2.1, manufacturing limits of PFI ES-24
shall be met. When defaulting to PFI ES-24, mutual agreement
between purchaser and fabricator beyond the stated
manufacturing limits shall not be allowed without the
approval of the designer.
The use of bends designed as creased or corrugated is
not prohibited.

 

[KVdA]

Thanks KevinNZ but my question actualy was

- Does a cold formed bend react on the same way as a straight pipe?
- What is the bending limit on a pipeline containing cold formed bends?
- Any code references?

 

[1503-44]

ASME B31.4 & .8 both have cold bending limits for pipeline pipes. They are easy to find, well that is if you know which code you should be reading. It seems like you do not know. Maybe that is the more important question.

 

[LittleInch]

Are you asking " Can I further bend an existing cold bend??"

What bend radius are we talking about. Please be more specific.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[KVdA]

Are you asking " Can I further bend an existing cold bend??"
The answer is yes.

Pipes (diameter 24") are cold bend to a certain radius (lets say 260m), welded together and subjected to a load which will induce a bending moment (in this particular case during installation).

For simplicity sake, is it correct to assume that the maximum bending stress is the same as for a straight pipe section?
My feeling is that this assumption is incorrect and might underestimate possible problems (inducing wrinkles, ovality...).

In this case my design code is ASMEB31.8 but I'm wondering if any code has a more detailed/complete verification.
ASMEB31.3 has flexibility and stress intensification factors for bends but not for cold bends and not for these relative large bending radiuses.

 

[chicopee]

"For simplicity sake, is it correct to assume that the maximum bending stress is the same as for a straight pipe section?" No but there are formulae to determine bending stresses of curved sections.

 

[KVdA]

"No but there are formulae to determine bending stresses of curved sections. "
And those are/could be found in code .... paragraph....?

 

[LittleInch]

Can you check your numbers? A radius of 260m for a 24" pipe is 430D. That won't be a cold bend.

I'm still struggling to understand what you want to do? Any chance of a drawing or a better explanation?

Are you trying to get more of a bend angle or decrease the bend radius?

What sort of machine or operation are you thinking of?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

See the bendinng limits as stated in B31.8

 

[acedude1]

It is essential to define the intended service of the pipe (i.e. power piping, process piping, oil pipeline, gas pipeline....etc.)This would then identify the appropriate design code that will govern the design. Associated The ASME codes generally have a code clause that addresses criteria for "cold bends", "Pre-formed bends", Field bends, ....etc. For example if the pipe is intended to be for cross country oil pipeline service, the governing ASME code is B.31.4. Please refer to the following clauses in this code:
404.2
404.2.6
434.7
451.6.2.8

The table in Clause 4.2.2 provides some insight to your question. This table provides the allowable minimum cold bend radius for the range in pipeline diameters. In order to convert this information to answer your question for a crude oil pipeline, some calculations need to be performed. Refer to the results in the attached table.

As important is the equipment to then be used for the actual cold bending process in the field to comply with these code requirements. In general prior to any production cold bending operations, the selected cold bending equipment must be validated via a cold bending test using the actual pipe to be used for the pipeline project. This is referred to as the field cold bending verification test performed by the construction contractor and his cold bending engineer. A few random length pipe joints (aprox 40 ft length each) are cold bent and then inspected to verify conformance with the code requirements and to assess any damage or adverse wrinkling or cracking.

Diameter Achievable Cold Bend (Degrees)
(in) Case 1 Case 2 Case 3
12.75 100 47 24
14 78 43 21
16 60 38 19
18 47 33 17
20 38 30 15
24 32 25 13
30 27 20 10
36 26 17 8
48 21 13 6

Notes:
1) Cold bend angles are total angle per 40 ft double random pipe length
2) Case 1 - Theoretical based on table in B31.4, clause 404.2.2
3) Case 2 - Achievable in practice with API 5L X60, schedule 40 line pipe using CRC
or equivalent bending machine with internal mandrel and experienced bending engineer.
4) Case 3 - Achievable in practice with API 5L X60 or higher, Thicker wall pipe using CRC
or equivalent bending machine, no internal mandrel and experienced bending engineer.

 

[acedude1]

Would appreciate if someone could advise a better way to insert a table from excel into the "Submit Post" script so that appears in an understandable manner...thanks (refer to my table in the post above)....if someone can help I will send the table in another post

 

[acedude1]

 

[acedude1]

My response above is an answer to your original question in relation to cold bending of line pipe in reference to design codes (i.e. what is the minimum allowable cold bend angle per code?). Please note that the bend angles stated in my first reply are also applicable to your B31.8 pipeline design. As I re-read and further more carefully assessed the responses,it is concluded that your question is not that, but is something quite different. But firstly a few comments regarding information in the the other responses and your replies to these. You state that the line pipe is 24-inch diameter and the cold bend radius may be 260 meters. LittleInch is correct in that this is equivalent to a 430D bend radius.... but further, this is equivalent to a 2.7 degree total bend angle in a 12 meter long pipe joint. This is a relatively small cold bend angle for this length of 24" pipe joint as this diameter pipe welded together and laid in the trench should be able to elastically flex to reasonably conform to this trench bottom grade transition without the pipe becoming over stressed during operating conditions, unless if it is very heavy wall thickness and/or there is no elastic bending reserve in the wall thickness design.

By piecing together the various comments and responses it is suspected your real question is something like, "... given the that the there are 430D cold bends in the pipe segments, will they be over stressed during the contractors' lowering-in operation or afterwards during operational phase?". If the contractor has mobilized sufficient side boom tractors to properly support the pipe string during the lowering-in operation, and the lowering in crew/operators are sufficiently experienced then any concerns with over stressing the cold bent segments will be mitigated. Also, to answer your question, "...is it correct to assume that the maximum bending stress is the same as for a straight pipe section?", the answer is "yes" for all practical purposes (considering minor effects of bend thinning of the cold bent segments). In fact it is believed that the localized cold bent locations have been strain hardened due to the cold bending process. In addition please note that during installation, the pipeline is unpressurized and therefore can accommodate signficantly greater bending stress without yielding than for the pressurized condition for which the pipe wall thickness was designed.

As a side note, please be advised that pipeline construction contractors do in many instances "cut corners" in their bids and during construction...especially the low bidders. During detail design phase, the construction aspects should have been addressed, including the size and number of sideboom tractors required for the lowering in operation so as to prevent pipeline overstressing concerns. This is presented in a "Method Statement" that should include a drawing of the lowering-In operation with sidboom tractor capacities. Alternatively, if this was not provided during the detail design phase, the Client should require the contractor to provide such Method Statement to be submitted to them for their review and approval prior to the contractor performing any lowering-in operations for the project during the construction phase. This type of analysis generally requires the use of pipe stress analysis software (such as CAESAR II) to properly model the lowering-in operation. From this modeling, the loads on each of the sideboom tractor lift points can be identified, and then the appropriate side boom equipment capacity (with safety factor)to do the job can be identified using vendor equipment data sheets obtained on the internet.....the contractor will need to validate that he has mobilzed this capacity sideboom tractors or equivalent for the job.