B31.8 Hydrotest- Allowable Hoop Stress?

[JT100]

Can anyone guide me to an allowable hoop stress value for the hydrotest case for subsea piping to ASME B31.8 (Offshore Code?).

I have read through the code and can see no clear definitions. In offshore code (VIII) A847.2 makes reference to at least 1.25 times the maximum allowable operating pressure. Does this mean 1.25 x 0.72 x SMYS is the allowable hoop stress? As 0.72 is the design factor for operating case (hoop).

I've also read through 841.33-841.353 and Tables 841.322(f) and 841.33 but again there doesnt seem to be a definitive answer. Thanks.

 

[BigInch]

1.25 x MAOP is the minimum test pressure at the highest point of the pipeline that is mentioned in B31.8 s847.2

It is possible that the MAOP is not equal to the hoop stress allowable of 0.72 x SMYS, but yes, most of the time MAOP is also targeted to that same value, HOWEVER we also know that can only be true, if all the combined loading cases that include an internal pressure condition and the corresponding combined stresses are also less than their allowables too.

For example, you would not be allowed any significant bending stress, or perhaps axial stress, temperature stress, wave load or sea current load bending etc. to happen simultaneously with hoop stress as calculated from MAOP, and even less, if you had concurrent combined stresses from, bending, axial stress, temperature stresses, residual installation, or others. See 833.2 and 833.4 (frictional contact with the sea bottom may provide restraint)

B31.8 2007 warns about one of these possible combined conditions that includes external pressure in that same section 847.2 with this quotation, "CAUTION: When an external pressure, Pe, greater than zero is used in the hoop stress formula in para. A842.221, there is a possible combination of conditions where the yield strength of the pipe could be exceeded during the hydrostatic test. Therefore, the hoop stress shall be checked to see that it is within allowable limits considering both the internal and external pressures when determining the maximum hydrostatic test pressure."

One of the reasons, for example, that you verify that there is no significant spanning and that resultant bending stress are limited to acceptable values using divers or ROVs as the pipeline is being layed, because those bending stresses might affect the combined stress during later hydrotesting or operation. See s841.11 for combined stress limitations and the other sections referenced therein.

But yes, the short answer is 1.25 x MAOP.



**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[BigInch]

BTW there is a pipeline forum applicable to questions about both onshore and offshore pipelines, "Pipelines, Piping and Fluid mechanics".

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[JT100]

Cheers BigInch.

Thanks for clarifying, I was a little confused by a statement in Appendix N- N-5(c):

"If the hoop stresses during the test exceeds 100% of SMYS then ..."

Will post future questions to the pipeline forum.

 

[BigInch]

That's basically for if you want to test higher than what is typically required and get to yield, or over at lower elevations. Sometimes its hard to make hydrotest sections correspond exactly with both minimum test and maximum pressure based on SMYS, if the pipeline elevation profile isn't cooperating. One company I worked for at one time back in the olden days routinely tested offshore pipelines to 110% SMYS as a "burn-in" procedure, just because they had always done that for onshore lines. They had only done a couple of offshore lines and had IMO been very lucky not having any bursts up to that time. I convinced them that it wasn't necessary and, in fact very unnecessarily, increased the construction risks considerably, something not very desirable anywhere, but especially offshore. They dropped that procedure.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[JT100]

So for hoop stress during hydrotest the allowable stress is 1.25 x MAOP = 1.25 X 0.72 X SMYS = 0.9 SMYS?

What is then the allowable longitudinal or combined stress during hydrotest, is that also 0.9xSMYS?

 

[BigInch]

In as much as hydrotests have max allowable stresses, yes, but by s833.5 You can go over yield, if strains are limited to 2%.

s833.3 SL for restrained pipe is limited to
0.9 * SMYS * T, temperature derating factor.
b]s833.6 For unrestrained pipe[/b]
SL <= 0.75*SMYS*T

s833.4 gives combined stress allowable for restrained pipe at 0.9 for long term loads, or 1.0 for short duration loads. Note that this is for operating cases, so if hydro is included in that, debateable, use the short term, so 1 * SMYS * temperature derating factor.


**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[JT100]

Thanks again BigInch.