Dear all,

i had noticed that axial stress value will be different during perform the UG analysis by using the recommended Load case and customize load case.

- By recommended load case the Axial stress is a simple formula which is Sx= 0.3* Sh (similar formula of Sp, longitudinal stress due to internal pressure). Or it also using the same formula of Sx = R/A? which coincidentally can get the similar value by using Sp= 0.3*Sh?

- By Customized load case the Axial stress seem computer with the formula of Sx = R/A


I actually doing the UG analysis with customized load case below

1 W+P1+T1 (OPE) (Extreme hot operating condition; ignore stress)

2 W+P1+T2 (OPE) (Extreme cold operating condition; ignore stress)

3 W+P1 (SUS) (For sustained stress case; ignore forces and displacements)

4 L1-L2 (EXP) (Need Algebraic combination; for expansion stress check; ignore forces
and displacements)

5 L3+L4 (OPE) (Need Scalar combination method; for sustained plus expansion check;
ignore forces and displacements


May i know why? Please forgive me as this is my first time facing this problem as previously i always based on the Caesar II recommended load case

First formula (0.3 x Shoop) is for fully restrained pipeline, as defined by pipeline Codes (B31.4 & B31.8) and regards PRESSURE STRESS only. Second formula (Fax/A) is the accurate one is is valid for any load and/or Design Code.
If you performed a comprehensive CII analysis of the buried pipeline/piping system modelling the pipe-soil interaction by equivalent bi linear restraints (whose stiffnesses were established based upon actual Soil parameters), I strongly recommend you to use the accurate approach (Sax = Fax/A). This means to define ALL the pipe elements as UNRESTRAINED.

For stress analysis as per B31.4 Code, FAC parameter (from Allowable Stress input spreadsheet box) should be 0.001 for buried elements and 0.0 (default) value for aboveground/not-buried elements.For CII V. 9.00 & 10.00, this discrimination is done automatically (read User Guide manual).

For B31.8 Code, the stress calculation is done as required by the "Restrained/Unrestrained" radio button from from Allowable Stress input spreadsheet box.

Regards,

Few more details.

1) The ”Add F/A in Stresses” parameter from ”SIFs and Stresses” menu of
Caesar II Configuration File spreadsheet should initially be fixed to ”Yes” value, so that non-pressure (e.g. ”structural”) axial force/stress is included in Code Stresses calculation.

2) For buried pipe elements, the OPERATING stresses (OPE/L1, L2) assessment and qualification should be considered instead of L5 Case. The allowable limit is 0.90×T×Sy (T = Temperature De-Rating Factor as per ASME B31.8, Table 841.1.8-1, and Sy = Pipe Material’s Specified Minimum Yield Strength as per ASME B31.8, Appendix D), BOTH for SL (Longit. Code stress) and Se(Combined Biaxial Code Stresses - i.e. 3D Stress Intensity/Tresca or Octahedral Shearing Stress/Mises equivalent stress).

3) Displacement Stress Range qualification (L4) is correct.

4) Sustained (SUS/L3) stresses are qualified similarly to OPERATING stresses.

NOTE. Fully restrained pipe elements represent a simplified approach which was typically used in the past, in conjunction with simplified/hand calculations, in the absence of the comprehensive buried piping/pipeline stress model developed in accordance with the latest applicable standards/methods (such as American Lifelines Alliance / ALA).

Dear Dorin,

Many thanks for your advice. However can you show me how to get the Sax value by hand calculation via Sax= R/A? Because now my model is fully restraint after i input the soil parameter in American Lifelines Alliance / ALA.

Because i do not know whr the "R" which is the external force value come from as you know i only input the T1, T2,and P1 in the analysis. i need to show client how to compare the Sax value from the report by manual calculation. Please help me!!

Furthermore, to get the Summation of longitudinal stress (in restraint UG pipe) i just need to use this formula |SL|= Sp + Sax + Sb + St will do? All Sp, Sax, Sb, St just directly take from the output report and sum it will do? note that St is the longitudinal stress due to thermal expansion in restraint pipe.

Many thanks.

Aaron,

Have you checked Bourdon effect settings within ”Environment/Special Execution Parameters” sub-menu of the current Caesar II Input spreadsheet (I mean the latest buried stress model file)?

Normally, pressure elongation effect should be taken into account for unrestrained elements. Therefore, “Translation Only” option of ”Activate
Bourdon Effects (for this job)” parameter within ”Environment/Special Execution Parameters” sub-menu of the current Caesar II Input spreadsheet should be ENABLED.

Then, go to the first buried pipe element and change the element status from Restrained to Unrestrained. Check if this change was propagated to the subsequent buried elements. If not, using Input spreadsheet list, you can go to Allowable Stress page and change there the Restrained/Unrestrained status of the buried elements.

Re-run the analysis. I believe you'll get different results than you got for the previous analysis and, for the UG straight pipe/pipeline runs located far enough from AG/UG interfaces and/or from 90/45 deg UG elbows/curves (if applicable), the SL (longitudinal stress) values will not differ significantly between initial analysis (Restrained Elements) and latest analysis (Unrestrained) elements.

The point is that the Unrestrained elements method, using the settings/configuration as suggested above, provides more realistic results for the UG pipe/pipeline portions located nearby (or close enough to) AG/UG interfaces and/or 90/45 deg UG elbows/curves (if applicable), because such UG pipe/pipeline regions are actually NOT fully-restrained, pipe may experience significant movements there and the axial & bending stresses need to be assessed accordingly.


REGARDING STRESS VALUES CHECK for the "default" Fully Restrained Pipe Elements:

1) You should be aware that S_P = 0.3 * S_H formula is applicable ONLY for PRESSURE STRESSES on fully restrained piping/pipeline. As stated by B31.8 Para. 833.2(f),
<< The stress due to axial loading other than thermal expansion and pressure is: Sx = R/A. >>.

2) For Thermal Expansion AND Fully Restrained Buried Elements, the Axial/Longitudinal Stress is (see para 833.2(c)):
S_T = - E * alfa * (T2-T1), where E = Elasticity/Young Modulus (MPa), alfa = thermal expansion coefficient (1/C), T1 = Installation Temperature(C)and T2 = Calculation Temperature(C). For thermal heating case, this stress is always compression stress-type.
So, it appears that for Fully Restrained Piping/Pipeline, it is not specifically required to consider Tmax - Tmin full temperature range in order to qualify thermal expansion stresses.

3) In addition to Sx, S_P and S_T, the Bending Stress S_B shall be considered, where:
S_B = M/Z (for straight pipes) or
S_B = MR/Z (for fittings/elbows affected by Stress Intensification Factors - see para. 833.2(d)),
Z = Pipe Section Modulus;
M, MR = Resultant Bending Moment.

4) To conclude, for Fully Restrained Buried Elements, the RESULTANT Axial/Longitudinal Stress, as per your L5(OPE) case, will be:
S_L = S_P + S_T + Sx + S_B.

Therefore, you cannot compare S_P = 0.3 * S_H values with Sx = R/A values. Moreover, since you do not analyze the Pressure Load case separately, your S_L (resultant longitudinal stress) values cannot be normally equal to S_P values unless there is no thermal expansion load (e.g. T1=T2) and no other external forces applied (e.g. R = 0 / Sx = 0).
This is the specific case of L3(SUS) case, where for the Buried pipe elements, Weight stresses are negligible and therefore S_L is expected to be equal to S_P. You may check this in CII output stress report (e.g. the Axial Code stress for L3 Case).

5) If you'd like to check by hand calculation the S_L stress values for L5(OPE), you need to calculate all the S_P, S_T, S_B and Sx values in EACH NODE and to sum them accordingly.
If your external loads on Buried Piping/Pipeline System consist in Pressure, Weight and Thermal Expansion only, then, for the Buried Pipe Elements, the axial force R is expected to be negligible and therefore you may check for your L5(OPE) case: S_L = S_P + S_T + S_B.

Dear Dorin,

thanks for your advise, i had enable the Bourdon effect and re run analysis and result is more realistic. learned new stuff.

Besides that, from your advise regarding the longitudinal stress for full restraint pipe the S_L = S_P + S_T + Sx + S_B. So which mean if the CII output stress report show there is a Axial stress value there mean i shall include it into net SL formula right? Appologies for asking this kind of question as so far almost most of the analysis appear the Axial stress in the output report that i confuse with your statement below:

"If your external loads on Buried Piping/Pipeline System consist in Pressure, Weight and Thermal Expansion only, then, for the Buried Pipe Elements, the axial force R is expected to be negligible and therefore you may check for your L5(OPE) case: S_L = S_P + S_T + S_B."

Dear Dorin,

Furthermore, in my case currently is buried pipeline system and only consist of Pressure, weight and thermal expansion so it fulfill with your statement below right?

"If your external loads on Buried Piping/Pipeline System consist in Pressure, Weight and Thermal Expansion only, then, for the Buried Pipe Elements, the axial force R is expected to be negligible and therefore you may check for your L5(OPE) case: S_L = S_P + S_T + S_B."

Many thanks

<< Besides that, from your advise regarding the longitudinal stress for full restraint pipe the S_L = S_P + S_T + Sx + S_B. So which mean if the CII output stress report show there is a Axial stress value there mean i shall include it into net SL formula right? >>

NO. Axial Stress in CII Report means the longitudinal stress developed by the uniform tensile/compression force action.
For the ASME B31.8 "default" Fully Restrained Pipe Elements, this stress should normally include < S_P + Sax + S_T = 0.3 * S_H + R/ - E * alfa * (T2-T1)A > stress term.
S_L is the RESULTANT Axial/Longitudinal Code Stress, being the sum of the uniformly distributed axial stress (as given by CII Report) and the maximum tensile/compression bending stress (S_B, as given by the same CII Report). In fact, Caesar checks both stretched and compressed marginal fibers of the pipe cross section and prints the maximum resultant S_L value in absolute value.


<< Furthermore, in my case currently is buried pipeline system and only consist of Pressure, weight and thermal expansion so it fulfill with your statement below right?

"If your external loads on Buried Piping/Pipeline System consist in Pressure, Weight and Thermal Expansion only, then, for the Buried Pipe Elements, the axial force R is expected to be negligible and therefore you may check for your L5(OPE) case: S_L = S_P + S_T + S_B." >>

YES.


I suggest you to use CII EXTENDED STRESS Output Report , where all the stress components are displayed properly.
Moreover, for B31.3 SUSTAINED and OPERATING Load Cases, both Longitudinal (S_L) and Combined Biaxial Code Stresses (Se = 3D Stress Intensity/Tresca or Octahedral Shearing Stress/Mises equivalent stress, see para. 833.4) are to be checked against allowable limit (0.90×T×Sy). In this case, Caesar II displays/prints as CODE STRESS that stress that has the highest Stress Ratio against Code Allowable Limit.

Errata:

"Moreover, for B31.8 SUSTAINED and OPERATING Load Cases...."

Sorry for inadvertence

Dear Dorin,

understood. All this make more clear to me now. Many thanks. Happy new year.