AutoPIPE [FAQ]
| Applies To | |||
| Product(s): | AutoPIPE | ||
| Version(s): | All | ||
| Environment: | N/A | ||
| Area: | N/A | ||
| Subarea: | N/A | ||
| Original Author: | Bentley Technical Support Group | ||
Index
- Can I add a new code material to my existing ASME B31.1 library so the program will update the hot allowable stresses for different temperatures?
- How do I interpret the elevation of ground relative to the piping model for wind loading?
- What does [??%] mean in the code compliance report?
- What is an appropriate way to model a offshore riser?
- When do I use the Xtra hydrodynamic data?
- What is the significance of Cm to buoyancy?
- How do I model Seabed piping with concrete mattresses?
- Why is my buried piping system is showing large Displacements in the gravity case?
- A consistency check shows near-zero elements reported in the model, will this affect my model?
- A consistency check finds many all of the following warning messages: * * * W A R N I N G - MODEL * * * W726-7: Pipe diameter change without a reducer at point B02. Why?
- My piping model with gaps and friction does not converge after five iterations, can I continue?
- When will the thermal expansion rates be updated for my code materials defined in the piping model?
- When are my material code allowable stresses updated?
- My ANSI flange check shows several flanges have increased from class 600 to 900, what does this mean?
- How do I model code compliance cases in accordance with the DNV code?
- Some of my segments have been deleted from the model and using RE-NUMBER/ALL in AutoPIPE 6.0 some of the the point names do not match the segment names e.g. a AB01, AB02 may be on segment V, how can I correct this?
- Why is my non-linear analysis giving unexpected results i.e large movements in the pressure case across my bellows when my tielink should restrain the pressure movement?
- Why is displacement showing 10mm in the E1 case when my guide gaps are 5mm?
- When I perform a static analysis it stops giving the error message "E801-1 Fatal error unstable system" also if I perform a modal analysis "E801-16 Fatal error zero energy modes".
- What is the warning message W90-24 "current model does not match the analysis"?
- What are modeling techniques for special valves and loads like ice/snow loading?
- When I see the Lift-off warning message after the static analysis how can I find out where the lift-off is occurring?
- For the pipe with the lowest point is at elevation -2 000 mm in the vertical coordinate but actually it is located 10 000 mm above ground, is it correct to specify the "Ground Elevation for Wind" at -12 000 mm?
- In the wind profile, if I specify "Global X" , does it mean +X or -X or the highest value of the two cases?
- How I can determine if a flange in the piping system is under tension or compression with AutoPipe?
- How do I model socket weld fittings?
- Are there any changes I need to make after I rotate my model about the vertical axis?
- Do I need to change the weld factors defined in the pipe properties screen?
- How do I model the offshore code compliance to B31.4, B31.8, BS8010 or CSAZ662 section 11?
- Why are spring or rigid hangers on vertical legs showing large reaction loads in the Hydrotest (Hy) case compared to the Gravity (Gr) case?
- I am performing a steam hammer analysis but when should I use ZPA correction method under Time History analysis?
- How I can be sure I have correctly modeled my fluid transient?
- Is marine growth thickness included for buoyancy loads?
- It is optional to include corroded wall thickness, axial force and torsion in the DNV stress calculations, what should I use?
- Can Autopipe open an Caesar input file?
- How do I calculate the DNV 2000 tension terms?
- How do I capture marine growth weight?
- I am carrying out a modal analysis on my offshore riser and what value of Cm should I use on the buoyancy screen?
- How do I model a rigid anchor in the local pipe direction?
- How do I model a large sweep bend with radius = 50ft?
- Do I need to run a static analysis after my hanger selection?
- What are the +/- points displayed in code compliance and forces/moments reports?
- Does Autopipe include axial stress for ASME B31.1 and B31.3?
- Can I apply local displacements to my guides?
- Which side of the pipe at a run point are the forces and moments are reported?
- How does Autopipe calculate the Pressure Extension & Pressure Thrust forces?
- How can I simulate soil settlement on part of my piping system?
- Can I model a Sway Brace?
- Can I model my pipe shoe supported on a spring can with friction?
- Why are all my dynamic results positive?
- Can I create a isometric model to use in AutoCAD or Autoplant?
- Why does the legend for the interactive code scan show a maximum value = 1.0 for stresses?
- My B31.3 code stress is showing an overstress in the hoop stress (Max P) case?
- I open my archived model and it appears blank. Why?
- Sometimes my Autoplant valves imported from the PXF neutral file have zero weight. Why?
- How can I change my Y-vertical axis model to Z-vertical?
- Why does my bend look disconnected from the pipe on the screen?
- How can I define a branch SIF using B31.1 Fig D-1 sketch d)?
- Does my expansion joint include contents weight?
- Why do I I keep getting a warning message "W831-1, Combinations include cases not analyzed"?
- How do I make a mirror copy of a section of piping?
- How do I define the coefficient of lift for wave loading?
- Can you provide any references for buried pipe design?
- How do I model the discharge piping accurately relative to my suction Piping?
- How do I model different parts of the piping system inside and outside a building for wind loads?
- What does LOC mean in the General Stress Report?
- How can I determine axial (tension or compression) and bending stress in the pipe?
- How do I model a ball joint with high inertial moment stiffness?
- Please confirm the recommended value of the added mass coefficient, Cm for a circular cylinder according to DNV Rules for Submarine Pipeline Systems - 1981 - Fig. A.7.
- How do I define one constant Hydrotest pressure over the whole model?
- What are the Participation Factors and the Captured Modal Mass in the frequency report?
- When should I perform a pressure analysis?
- The Autopipe help states "When the rise time several times larger than the 2L/a time, the calculated pressure rise in AutoPIPE might be conservative. For this special case, the use of a fluid simulation software is recommended if P2 case is critical." What does this mean?
- My seabed piping shows a large unexpected displacement. Can you please explain?
- How do I enter cold spring into my main steam model?
- Given that point A11 is the TIP of a long-radius bend, how are the near and far points related to A11 N-, A11 N+, A11 F-, and A11 F+?
- When I insert a midpoint there is a big difference in the natural frequencies, can you please explain?
- When performing seismic analysis, can AutoPIPE do multi support excitation?
- How can I print an echo of the input response spectra (load)?
- Where do I input stress range reduction factor?
- How can I look up the units for a flexible anchor for translational and rotational stiffness?
- I have been working with AutoPIPE 8.05.01.11. How do you print in version 6.3 when there are no grids?
- Can AutoPIPE run a flange check and if so, how?
- I am looking at a tutorial for water hammer called apham1. I have noticed that between A06 and C00 there is, essentially, a split between the supply and discharge lines of the imaginary pump (each is anchored and flanged). I didn't know this could be done. Could you help me to get this same type of arrangement set up for a similar project I am working on?
- When coding a tie/link for an expansion joint what is considered gap forward and what is considered gap backward?
- How do you close a loop?
- I am working on a system which a two phase liquid. For the stress analysis, which specific gravity should I use? Should I consider the true representative by getting the average or just use the higher specific gravity?
- Does AutoPIPE assume the contents of the pipes are full of water? If so, is there a setting available to modify the specific gravity (or other item) to model a different fluid?
- Is there a way to easily modify the allowable stress for the custom piping I have put into the model? I am using Code B-7159.
- Please explain the $T1 and $T2 load case data?
- Seismic load needs to be run with friction. How do I turn it on?
- How do I change a temperature over a range?
- In the Hanger Selection Analysis dialog, it is displaying V-Stop as supports whereas the user has spring hangers. What criteria does it use to determine the supports?
- If the modal displacements are a unitless number, what is their value based on?
- How do I hide node numbers?
- How do I print to AutoCAD DXF?
- How to increment point numbers by 5 instead of the default increment of 1?
- I am looking for a quick way of changing the piping orientation and I do not want to start to input the model again. I would like to change the orientation of the horizontal leg to start from the X and Z plane at 45 degrees.
- What is the method for stress analysis of pipe settlement when loading traffic?
- Is there an easy way to change temperatures over a range using the input grids?
- I am running 12 thermal loadcases (Code B31.1). I noticed in the code combinations that the range from ambient to T1....T12 are considered, however, the total range for all twelve cases are not shown. For example only "T1 to T2"; "T1 to T3" and "T2 to T3" are shown.
- Where can I find a sample for a pressure balanced expansion joint?
- How does AutoPIPE decide the allowable stresses on pipes?
- How do you know if the axial forces on a flange are in compression, simply by looking at the forces and moments report?
- For Hanger Selection what is considered for spring sizing, cold setting or hot setting?
- What is Weightless or As-built when considering gapped supports?
- In the output report why does the Results Summary reflect two different Occasional Cases?
- Why does the code compliance report show an SIF = 1 for the Sustained Case and 1.3 for all other cases?
- If a user hasn't saved his model in a while (old bak file) and lost his model by accident, can he use his U0x file and change it to a dat?
- Does AutoPIPE display "Von Mises" as well as "Tresca" stresses on the output report?
- How do you set up WinNOZL to work with AutoPIPE?
- On the support output report, the GR's load is not equal to the spring Preload. Why?
- How do you model Cryogenic piping?
- My AutoPIPE application runs slowly on my computer, what can I do?
Question 1:
Can I add a new code material to my existing ASME B31.1 library so the program will update the hot allowable stresses for different temperatures?
Answer: Yes, a document has been prepared to describe this process. Download it now by clicking HERE .
Question 2:
How do I interpret the elevation of ground relative to the piping model for wind loading?
Answer: For pre Autopipe 6.2X , both wind design codes ASCE-7 and UBC ground is always taken as the global vertical axis coordinate = 0. For the ASCE-7 wind code, when elevations in the piping model are < 15 feet, the program defaults to an elevation = 15 feet to determine the automatic code factors. For the UBC wind code, if elevations in the piping model are negative, the automatic code factors may become very small resulting in a small calculated wind Pressure. In this case, the piping system should be moved vertically to correctly position the ground elevations relative to the global vertical axis coordinate = 0. Use Modify/coordinate and enter the appropriate vertical movement. Note: For UBC code elevations > 400ft are considered with no wind.
User wind profile: The first height field entered references the ground level of the piping/framing system, located at the point in the model with the lowest global vertical axis coordinate value. Specific height values are the global vertical axis coordinates of points in the model, which means that negative heights (or 0) are possible. Highest = point in the model with the highest global vertical axis coordinate value.
In Autopipe 6.20 or later the ground elevation field should be used to define the location (positive or negative value) where ground is located relative to the global vertical origin e.g Y = 0 if vertical axis is Y.
Note: UBC:1997 and ASCE-7:1998 codes were updated in V6.2 and now UBC Table 16-G is interpolated for intermediate values and uses values at 400 ft for higher elevations.
Question 3:
What does [??%] mean in the code compliance report?
Answer: Intermediate stress points. A [%??}in code compliance report refers to % location of a point of the higher stress found than the adjacent run points e.g. AutoPIPE 5.03 default = 19, i.e. 19 intermediate stress points evaluated between any 2 adjacent run points or around a bend from the near to the far points.
Note: In AutoPIPE 6.0 the default for Intermediate stress points = 0 i.e. turned off.
Intermediate stress points can be important to capture points of highest stress e.g. a simply-supported Pipe with defined supports at either end has a maximum moment/stress at the mid-span but typically only the support points are defined in the pipe stress model and hence only the stresses at these points are calculated and reported.
Question 4:
What is an appropriate way to model a offshore riser?
Answer: It is important to add many points along the riser section of pipe e.g. at approximately every 8 to 10 feet to provide adequate mass discretization so the program can capture the distributed wave loading accurately across the riser pipe. Riser pipes are typically sloped at 10 to 15 deg and guide supports on the riser will be normal to the pipe axis and the reaction loads normal to the pipe can be seen in the a support forces report which shows Local and global displacement's and reactions. Note: Platform wave displacements should be applied at the platform anchor and riser guides.
Question 5:
When do I use the Xtra hydrodynamic data?
Answer: When the pipeline does not experience the wave or current effects then under xtra/hydro data set Cm=0, Cd =0 and CL = 0 across the range of pipe selected e.g the pipe is in a J-tube, seabed pipe is buried or when concrete mattresses are applied to the seabed piping. These Hydrodynamic coefficients will over-ride the ones defined under Load/Wave.
Question 6:
What is the significance of Cm to buoyancy?
Answer: Cm under buoyancy is only used to compute added mass effects during a modal analysis.
Question 7:
How do I model Seabed piping with concrete mattresses?
Answer: Either i) calculate "equivalent" soil properties for the concrete mattresses then insert these soil properties over this range ii) Model vstops over this section of seabed piping and use high value of friction e.g 1.5 to 2.0 plus additional distributed weight loading from the concrete mass.
Question 8:
Why is my buried piping system is showing large Displacements in the gravity case?
Answer: Soil supports are specified for most models with a final stiffness K2 = 0, although AutoPIPE can still solve this problem the results may be invalid if the nonlinear system goes a large deformation.
The soil should still have some stiffness after yield to restrain the pipe, this is accomplished by specifying a nonzero K2 value (e.g. K2= 0.01) or to include more restraints in the system or reduce the soil span in the soil identifier. Note that may also be possible that the K1 values for the soil are not providing enough stiffness to the system.
Question 9:
A consistency check shows near-zero elements reported in the model, will this affect my model?
Answer: Try to avoid near-zero elements in a model, particularly with defined soil properties. Generally near-zero elements can be deleted without affecting the model for example if a run point is defined at the same coordinates as the near or far point of a bend then delete the run point.
Question 10:
A consistency check finds many all of the following warning messages: * * * W A R N I N G - MODEL * * * W726-7: Pipe diameter change without a reducer at point B02. Why?
Answer: These warning messages can generally be ignored and will be displayed also where two pipe identifiers are connected with the same pipe nominal diameter but with different wall thicknesses.
Question 11:
My piping model with gaps and friction does not converge after five iterations, can I continue?
Answer: Yes, generally the default number of nonlinear iterations = 5 (in version 6.0 or earlier) is adequate for most models but for models with large gaps and friction it is not uncommon for convergence to occur after 20-30 iterations particularly with the thermal, wind and earthquake cases being analyzed in one model.
Generally if the model doesn't converge after 50 iterations, a solution cannot be found and reviewing the Model.log file will show bearing force and Displacement convergence errors at a particular support which may be the reason for the non-convergence.
Note: Default number of nonlinear iterations has been increased to 30 in version 6.10
Question 12:
When will the thermal expansion rates be updated for my code materials defined in the piping model?
Answer: Thermal expansion rates will be updated under the following operations:
- Export to a batch file (*.NTL) then importing the same batch file.
- Modifying the ambient temperature in the Tools/Model Options/General
- Modifying the Pressure temperature across the range or complete model.
- Change the number of temperature Pressure cases under edit/system (5.03) or tools/model options/General
Question 13:
When are my material code allowable stresses updated?
Answer: Material code allowable stresses (temperature dependent) are updated under the following operations:
- Changing the code material under a pipe identifier.
- Modifying the Pressure and temperature across a range.
Question 14:
My ANSI flange check shows several flanges have increased from class 600 to 900, what does this mean?
Answer: The ANSI flange check calculations are recommending increasing the Pressure rating from class 600 to 900 based on the combination of the design Pressure, axial force and bending moment across the flange, these calculations are shown in appendix O as based on ASME B31.7 nuclear piping code Para 1 704.5 (a). These flange checks are known to be conservative, and using the calculated effective Pressure in ASME VIII Div I appendix 2 for flange design, a more accurate design check can be completed.
Question 15:
How do I model code compliance cases in accordance with the DNV code?
Answer: The only relevant code compliance categories in the DNV code are functional and environmental as defined in Autopipe 6.0 as follows:
| Max Equiv | Represents "load case a - Functional loads". (Von-Mises code category) |
| Equiv.+U1 | Represents "load case b - Design environmental loads and simultaneously acting functional loads". (combined code category) |
| Hoop | Represents hoop pressure stress only (hoop code category) |
Notice the default code combinations under sustained, occasional and expansion have brackets around these cases such that they are not required by the DNV code and have been de-selected to not print out by default in Autopipe 6.0. Also refer to code compliance calculations in the Autopipe Reference Information for DNV'81 code calculations.
RECOMMENDATIONS FOR CODE COMPLIANCE CASES FOR DNV CODE
Step1
Start with the default DNV code compliance cases.
Tools/code combinations/reset all
Step 2
If you wish to create user code combinations
a) Functional user code case e.g. Gr + T1+P1
See image below
b) Functional + environmental code case. e.g. Gr + T1+U1+E1
Note: the forces and moments for all load cases are combined at the moment level then an effective stress calculated and the allowable stress includes a factor =1.33.
Question 16:
Some of my segments have been deleted from the model and using RE-NUMBER/ALL in AutoPIPE 6.0 some of the the point names do not match the segment names e.g. a AB01, AB02 may be on segment V, how can I correct this?
Answer: Currently in Autopipe 6.0 or 5.0x , the only workaround is or perform an Edit/Cut then Edit/Paste operation.
Select the complete model using Ctrl A or Select/All points such that all points are highlighted red, then select edit/Cut (ctrl X), clicked on or type the name of a base point for typically the first anchor in the system e.g. A00 then click OK. Click yes to delete all points and the model will disappear from the screen. Select Edit/Paste (ctrl V) from menu, uncheck the box "connect two selected points", then click OK. The complete model will reappear on the screen.
Question 17:
Why is my non-linear analysis giving unexpected results i.e large movements in the pressure case across my bellows when my tielink should restrain the pressure movement?
Answer: The tie-link has a large forward gap which under the default load sequence i.e default is "Use default sequence" is checked . Or to change the intial states i.e different load sequence then uncheck this option.
The following Initial states dialog shows the default load sequence.
i.e GR then thermal then Pressure case.
Since P1 follows the thermal case T1, the bellows moves across the gap under T1 providing "lift-off" or the gap is open then the pressure load causes the bellows to expand and moves back to take up the tie-link gap.
Further discussion on load sequencing is found in 5.0 appendices pg 190 or autopipe 6.0 Autopipe reference information under analysis considerations\static analysis\non-linear analysis\solution sequence
Note: The liftoff can be verified by checking the restraint loads for the tielink reveals Gr= -64500 and T1= 64500 confirming after the T1 case (gr+t1) no reaction occurs on the tielink hence tielink gap is open.s
A more expected result i.e pressure occuring before the thermal expansion thus the load sequence of Pressure after gravity as follows:
Giving very small movements in the vertical direction = 0.002mm across the tielink (i.e A02-A01) is probably due to any rigid anchor or support does have a very large finite stiffness in the finite element analysis within autopipe.
Question 18:
Why is displacement showing 10mm in the E1 case when my guide gaps are 5mm?
Answer: Your model is run as a non-linear analysis with the default load sequence ie. Gr then T1 then P1 as seen below in the initial states dialog.
Under a non-linear analysis the loads reported in the support and restraint reports are incremental not absolute total loads. To observe the resultant loads on supports it is important to create load combinations.
Note: There is a new option under tools/model options/result "Add Def non-code comb" when checked will automatically create many non-code combinations eg Gr+T1+P1.
eg at C23 Gr FY = -436 T1 FY= 210 i.e pipe is tending to lift off under the thermal case. but Gr+t1= -226 which means after T1 the pipe is still sitting on the support and hence 0 upward movement. If Gr Fy equal and opposite of T1(FY) then the total vertical reaction is 0 and expect to see +ve DY movement in the T1 case.
Extracts from online help autopipe 6.0
Total vs. Incremental Loading
A load case (e.g. gravity, thermal, wind, etc.) represents an increment of load, not a total load (except for gravity). Hence, pipe forces, displacements, support forces, etc. calculated for a load case represent the increments produced by that case regardless of the type of static analysis performed (linear, or nonlinear). In particular, the results for a thermal case define the changes in the forces and displacements due to thermal expansion, not the total effects due to combined gravity and thermal. In order to obtain total load effects, combinations must be defined which include the load cases that have been used to hold the specific loads of interest. "Superposition" of load cases is a commonly accepted principle for a static linear analysis. However, it is not so straight forward for a nonlinear analysis.
Nonlinear Solution Load Case Sequence
For a linear analysis, the results for each load case are obtained all at once. However, for a nonlinear analysis the results are obtained sequentially. There are two reasons for this. First, the analysis of a nonlinear system requires iteration (successive trials), and different load cases will usually require different numbers of iterations. Second, (and more important) the result for any AutoPIPE load case will generally depend on the initial state for that case. For example, the result for a thermal expansion load will generally depend on the state of the system after gravity load is applied (e.g. which gaps are open and which are closed). The solution for the gravity case must thus be obtained first, and used as the initial state for the thermal expansion case. For a nonlinear analysis, the user has the option of selecting a default load sequence or of specifying a user defined sequence. It is important to note that in an AutoPIPE analysis, each load case is an increment of load, not a total load. To illustrate the difference, consider two alternative procedures for obtaining thermal expansion effects. If analyses are performed for total loads, the steps are:
Preferred load sequence for correct loads on restraints
- You may wish to run a load sequence as follows which considers the earthquake cases after the operating i.e case of GR+P1+T1 and set T1 as the initial state of the E1 case. This equally applies to wind cases also.
- If more than one thermal case exists then the user may wish to set T1 as being the initial state for E2, E3 instead of T2 which may not be a operating case.
Regarding your Dx= 10mm in the E1 case since E1 follows Gr case in the load sequence and Dx = -5mm in Gr case the resultant position Gr+E1 is Dx= +5mm i.e hitting the gap hence the reaction in E1 case Fx = 179.
Question 19:
When I perform a static analysis it stops giving the error message "E801-1 Fatal error unstable system" also if I perform a modal analysis "E801-16 Fatal error zero energy modes".
Answer: Possible Solutions are:
- A segment is not connected to the rest of the model (Check the consistency check warning messages)
- Tee is not connected correctly e.g Notice the point names are overlapped (point name is not clearly seen on the screen) at 2 points A and B. and/or the tee arrowhead is floating in space. Place cursor on point A and select modify/point , change point name A to B and click ok Modify convert point to run, then Modify convert point to tee, view/redraw.
- A beam is not connected to the rest of the piping model with no anchor.
- A beam is too 'soft' e.g shear area is too small
- Flexible joint has some stiffness values = 0
- Nozzle element has very low stiffness.
- No anchors (rigid or flexible) in the piping system.
If frames are only used to model dummy leg supports. Then one useful tip in the future to find disconnected frame points is to batch the model i.e. file save as /NTL (batch) and scroll to the bottom of the NTL file. There is a section called BEAM NODES as seen below. This section should be empty if dummy legs are modeled correctly i.e beam from bend midpoint to dummy pipe segment.
A model has to be statically determinate which means an anchor can be traced back on every segment of pipe. In some cases the anchor does not have to be rigid but can only have 3 non-zero translational stiffnesses to prevent the pipe "floating off into space". You can also use rigid beam elements to connect pipes together which will also maintain a stable piping system.
Question 20:
What is the warning message W90-24 "current model does not match the analysis"?
Answer: This message is displayed when the model has been changed but either one or both of the static and dynamic analysis has not been re-run. If not wishing to re-run the dynamic analysis then suggest you delete the LIN & EIG files and re-run the static analysis only. This message does not affect the results.
Note that AutoPIPE 6.2 and later will prompt you for deleting these files.
Question 21:
What are modeling techniques for special valves and loads like ice/snow loading?
Answer:
Angle Valve
Refer to online Help topic /Autopipe Workbook/AutoPIPE Modeling Approaches/Valves/Relief and Angle Valves
3 way Valve
Insert a tee (set tee component = tee, tee type = Other and SIF =1.0), insert very small run pipe then a valve from the each leg of the tee. Select the tee and run points up to each valve but not including the valve (highlighed red) and Insert/Rigid options over range (include weight = OFF, include thermal expansion = ON)
4 way Valve
Insert a tee (set tee component = cross, tee type = Other and SIF =1.0), insert very small run pipe the a valve from the each of 4 legs of the cross. Select the tee and run points up to each valve but not including the valve (highlighed red) and Insert/Rigid options over range (include weight = OFF, include thermal expansion = ON)
Prestressed springs
Define the cold preload and spring rate for the spring hanger when Insert /support/spring instead of "Undesigned".
Snow and/or ice loading
Apply either :
- Select the range of pipe and apply distributed load (Insert/Distributed load) in the vertical down direction equivalent to snow or ice loading in the U1 case This is a uniform distributed load therefore enter as below:
- If all the model experiences the snow loading and same pipe size then can use a user-profile wind case in the -vertical direction if calculate the equivalent snow pressure to be applied i.e snow loading in lb/in divided by the projected pipe diameter (including insulation). Note: Wind pressure default units = lb/sq.ft.
Question 22:
When I see the Lift-off warning message after the static analysis how can I find out where the lift-off is occurring?
Answer: To verify lift-off has occurred first examine the Restraint report as below:
Where the thermal or occasional (e.g. wind, seismic or user) vertical load is equal and opposite to the Gravity (GR) load (-ve vertical load) then lift-off has occurred i.e. GR+T1 vertical load = 0. This can ALSO be confirmed in the displacement or support forces report by a +ve vertical displacement in the thermal or occasional (e.g wind, seismic or user) load case as seen below :
LIFT -OFF procedure to find all the supports which are lifting off.
- Run non-linear analysis and lift-off warning msg is displayed.
- Clear the selection set i.e Ctrl Q or Select/clear.
- Select Result/filter criteria /support.
- Select the logical method = "OR"
- Check the supports which could be lifting off i.e v-stops and guides. Note: springs can always move up or down.
- Check Dy = on , abs = off, and "greater than" , "0.001" Note: no quotes on the values entered.
Note: This assumes Y = vertical axis change to Dz if Z= vertical axis
All the support points that match this criteria will be highlighted red on the screen or can be printed in a support forces report provided the batch report option "apply filter criteria" = yes
Note: This procedure also highlights guides on vertical lines which can be ignored Also supports with gaps in the downward direction.
Modeling Options if Lift-off occurs
- Remove the support and re-run the analysis
- Use "gap above pipe" and accept pipe will lift off in hot condition
- Restrain the pipe from moving up e.g 0 gap above the pipe.
- Replace the support with a spring.
- Move the support to prevent lift-off.
- Re-arrange the pipe route to prevent lift-off.
Question 23:
For the pipe with the lowest point is at elevation -2 000 mm in the vertical coordinate but actually it is located 10 000 mm above ground, is it correct to specify the "Ground Elevation for Wind" at -12 000 mm?
Answer: Yes.
Question 24:
In the wind profile, if I specify "Global X" , does it mean +X or -X or the highest value of the two cases?
Answer: Global X = +X only.
Question 25:
How I can determine if a flange in the piping system is under tension or compression with AutoPipe?
Answer: If you check the box for ANSI flange check then print the Flanges report it will only show tensile axial load. If the ANSI flange report shows axial load = 0 then flange under compression.
Question 26:
How do I model socket weld fittings?
Answer: It is recommended based on the intent on many ASME codes to use conservatively an SIF value = 2.1.
ASME Codes state the following regarding socket welds:
B31.1 use 1.3 for socket welds with no undercut, and 2.1 for all others.
B31.3 states "2.1 Max or 2.1 T/Cx but not less than 1.3. Cx is the fillet weld leg length".
B31.8 states "2.1 Max or 2.1 T/Cx but not less than 1.3."
To apply this SIF to all bends and Tees perform the following:
Select/Bends to highlight all bends in the model, then Select/Tees to highlight all bends in the model, then Insert/xtra data/User SIF, Inplane SIF =2.1, Outplane SIF =2.1, Override all other SIFs at this point = unchecked (i.e maximum SIF used if automatic SIF > 2.1), Flexibility Factor = 1.0 (no flexibility).
Question 27:
Are there any changes I need to make after I rotate my model about the vertical axis?
Answer: Yes update the following to reflect the new model orientation.
- All imposed support and anchor displacements.
- Forces and moments are applied in a global direction and may also need updating.
- Supports other than springs, constants, guides and v-stops e.g inclined may need to be modified so the restraint direction relative to the pipe is maintained.
Question 28:
Do I need to change the weld factors defined in the pipe properties screen?
Answer: These are only used for the TDK and SPC piping codes and should be left as the default value = 1.0 for all other codes. Refer to the Autopipe help "Weld Factors" for more in-depth discussion.
Question 29:
How do I model the offshore code compliance to B31.4, B31.8, BS8010 or CSAZ662 section 11?
Answer: A document has been prepared to describe this process. Download it now by clicking HERE .
Note: In AutoPIPE 2004 edition (v8.50) offhsore codes B31.4 chapter IX, B31.8 Chapter VIII and CSA Z662 section 11 codes were added.
Answer: The vertical leg in the piping effectively becomes a restrained piping system between the hanger and an anchor or another vertical support above or below the current hanger support. Therefore the large reaction on the vertical leg support in the Hy case is due to the combined effects of pressure extension, any hydrotest thermal effects(usually taken at ambient) and deadweight under hydrotest case and fluid+pipe weight loads under the GR case. The pipe wants to expand due to pressure but is restrained by the locked spring hanger therefore generating a large pressure reaction.
Currently, the hydrotest load case in AutoPIPE is only a static linear solution. In other words, support gaps are considered closed and support friction is ignored. In the hydrotest load case, AutoPIPE automatically converts all designed and undesigned spring and constant hangers to a rigid hanger (V-stop) to simulate locked hangers. Gravity, pressure, and thermal loads (if any) are combined to simulate hydrotest loads.
As mentioned the Hydrotest includes rigorous pressure analysis which for high pressure systems causes significant pressure extension in the pipe e.g if remove the v-stop at A03N and run the hydrotest shows upward displacement of 0.005". i.e with the system pumped up with 4950 psi pressure.
With the v-stop at A03N it is assumed locked and generates this upward load = 27482lb trying to resist the pressure extension.
Note: If it is considered that the hanger rod assembly is not rigid in compression (i.e. some slack which is not unreasonable) then it would better to use a separate model with GR, P1 (Pressure Analysis = Checked) and T1 as hydrotest loads, and enter upward gaps for the v-stops then run the analysis as non-linear. This will allow the pipe to move up and not be restrained.
Note: To include the axial stress due to the pressure analysis cases (P1 - P3) in the calculation of longitudinal pressure stress (may be significant for high pressure systems), under tools/code combinations/longitudinal pressure, option "Include rigorous pressure stress" = checked. By default this option is OFF.
Question 31:
I am performing a steam hammer analysis but when should I use ZPA correction method under Time History analysis?
Answer: We recommend to perform two analyses, one with ZPA and one without. For flexible legs (legs with flexible or no axial supports) use no ZPA correction. If the system has pipe legs with rigid axial supports, use ZPA correction to determine realistic loads on these axial supports. Note: ZPA can be very conservative for flexible legs.
Question 32:
How I can be sure I have correctly modeled my fluid transient?
Answer: Some key points to check modeling a fluid transient :
- Define the flow rate with correct sign. Flow rate is positive for negative pressure rise. Note: When a pump is shutdown, there are two shock waves generated. A positive pressure wave on the suction end and a negative pressure wave on the discharge end are generated. The maximum possible negative pressure wave is equal in magnitude to the pump discharge pressure(Ps) less the liquid vapor pressure (Pv). The pressure wave amplitude is calculated in AutoPIPE using the Joukowski formula. DP = Fluid density*Fluid velocity*speed_of_sound
This pressure wave = dP should be less than Ps-Pv to avoid cavitation. This condition should be avoided since the Autopipe results will be invalid. Similarly the pressure rise will be positive upstream of a closed valve and negative downstream of an open valve.
- Typically use default SINE rise function
- Define time history duration as 1st period (1/first modal frequency, hz) + transient duration (as shown in the THL file).
- When click ok to the fluid transient check the red highlighted sections of piping are correct.
- Run the modal analysis with cut-off frequency at least 100 to 150hz. Recommended to perform modal followed by time history analysis at both cut-off frequencies to confirm the solution has converge i.e. time history results are similar.
- Run time history with and without ZPA correction. See Q31.
Note: Recommend to set under tools/model options/Edit "Mass points per span" = A to allow the program to automatically perform mass discretization on your model for improved accuracy for the dynamic analysis.
Support solution
Flexible is better. The restraint should only be stiff enough to sufficiently attenuate the low frequency gross deformation.
Question 33:
Is marine growth thickness included for buoyancy loads?
Answer: No Autopipe does not consider marine growth thickness (under load/wave) in the calculation of buoyancy loads but it does consider insulation around the pipe in the buoyancy load which can be used to simulate marine growth over a section of pipe and also capture additional weight of the marine growth.
Question 34:
It is optional to include corroded wall thickness, axial force and torsion in the DNV stress calculations, what should I use?
Answer: Yes for the DNV code, the program defaults to Use nominal thickness = checked, since this code explicitly uses nominal thickness for stress calculations. However other codes default to use corroded thickness for stress calculations which is more conservative. i.e for longitudinal pressure stress and corroded section modulus used in bending stress calculations.
Refer to the following help topic for code calculations.
Autopipe 6.X "Online help topic Autopipe Reference information / Code Compliance Calculations/ DNV" By default axial force and torsion effects are NOT included for DNV but it is recommended to include them for offshore risers especially when the line pressure is high e.g > 500psi.
To include axial force and torsion check the following options.
- Pressure Analysis = checked (under static/analyze)
- nclude rigorous pressure stress =checked (under Tools/code combinations /longitudinal pressure) to include axial pressure stress in the longitudinal pressure stress
- Under Tools/model options/results:
- Include axial force = checked
- Include Torsion = checked
Question 35:
Can Autopipe open an Caesar input file?
Answer: Autopipe can only open a Caesar neutral file (*.cii), In Caesar convert the caesar model file to a neutral file from the main menu , Tools/external interfaces/Caesar II neutral file.
Question 36:
How do I calculate the DNV 2000 tension terms?
Answer: In accordance with DNV 2000, AutoPIPE currently can output the following Local Forces and Moments results:
Note: Local forces convention
-ve = tension
+ve = compression
With buoyancy defined under Load/buoyancy the hydrostatic forces are calculated and automatically included in the GR case.
- GR = N + PeAe
- P1 = internal pressure forces in pipe wall not including PiAi (capped pressure term).
- GR + P1- PiAi = S = N + PeAe - PiAi
Since the sign conventions for S (Effective axial force) , N (True axial force in the pipe wall) , PeAe is consistent with respect to tension or compression i.e signs are automatically calculated by the program and included in the GR and P1 load cases.
Question 37:
How do I capture marine growth weight?
Answer: Marine growth thickness usually varies with depth therefore it is recommended to add a distributed load down the riser which can be triangular profile to simulate the varying thickness vs depth.
Note: There is no marine growth above mean water level, i.e., marine growth is assumed zero above water level for drag and inertia wave calculations.
Question 38:
I am carrying out a modal analysis on my offshore riser and what value of Cm should I use on the buoyancy screen?
Answer: Referring to the On-line Help a "value of Cm (coefficient of inertia) for cylindrical bodies in a incompressible, frictionless fluid is 2.0".
Also refer to DNV 1981 A.3.2 and fig A.7 which shows added mass coefficient as a function of M/D where M is distance from a fixed boundary. If no influence from a fixed boundary then use Cm = 1.0 otherwise Cm = 2.29 to 1.0. Most of offshore users use default value = 2.0.
Question 39:
How do I model a rigid anchor in the local pipe direction?
Answer: Enter 3 translational and 3 rotational stiffnesses in local direction for a horizontal pipe, add 1 rotational support about global Y direction (i.e local Y = Global Y), 2 inclined rotational supports (calculate the cosine restraint direction with respect to the pipe direction), linestop (forward and backward gaps = 0) and guide (left, right, up and down gaps = 0).
Question 40:
How do I model a large sweep bend with radius = 50ft?
Answer: Assuming the bend has supports along it then model the bend as series of straight pipe run sections with offsets calculated as a segmented miter bend. Alternatively enter the bend with radius = 50x12 = 600 inches (for ENGLISH units, by overriding the word Long or Short in the radius field. Radius units is displayed in the lower right of the main AutoPIPE window) and insert soil over the large bend with large value of downward soil stiffness, vertical up and lateral stiffness = 0, longitudinal stiffness = 0 (or some nominal value to include some frictional resistance).
Question 41:
Do I need to run a static analysis after my hanger selection?
Answer: Yes since the hanger selection only does a free thermal loading whereas the static analysis considers the spring hanger preload and stiffness.
Question 42:
What are the +/- points displayed in code compliance and forces/moments reports?
Answer: +/- points are displayed in the code compliance report:
- pressure or temperature change
- Pipe identifier change
+/- points displayed in forces and moments report:
- Flange points, supports, additional weight, concentrated force, tee point and uniform distributed load.
Question 43:
Does Autopipe include axial stress for ASME B31.1 and B31.3?
Answer: Both ASME B31.1 and B31.3 codes do not give explicit stress calculations for axial stress but for some cases e.g buried pipe, low modulus systems like FRP and high pressure systems it is recommended to consider axial stress. To include axial stress for all loadcases except pressure then use the option Tools model options /Results "include axial force" = checked. The sign is ignored when adding axial stress to the code stress calculation since this stress calculation is a gross longitudinal stress in the pipe. This is consistent with other pipe stress programs.
Note: To include the axial stress due to the pressure analysis cases (P1 - P3) in the calculation of longitudinal pressure stress (may be significant for high pressure systems), under tools/code combinations/longitudinal pressure, option "Include rigorous pressure stress" = checked. By default this option is OFF.
Question 44:
Can I apply local displacements to my guides?
Answer: Only the component of the global displacements in the local restraint direction i.e normal to the pipe for a guide is applied to the restraint. Hence for a skewed pipe, a local displacement can be resolved into global components and applied to the guide.
Question 45:
Which side of the pipe at a run point are the forces and moments are reported?
Answer: Global Forces and moments convention.
Refer to online help topic Figure H-3 on Autopipe 6.0 reference information/ results interpretation/ Pipe Forces and Moments: Global Option
Check the forward direction of the pipe and imagine all the pipe upstream of the reported run point is removed then local or global forces and moments reported (-ve point) will be those acting on the downstream pipe to keep it in equilibrium.
For bend and tees and support points and - and + points are reported by the program i.e Loads before /after the point
For example: To resolve the actual forces and moments at a tee defined with 3 segments. The branch segment direction is into the Tee and both the header segments are created away from the Tee. Then the global forces and moments at the tee can be read directly for the branch point but the sign must be reversed for the global forces and moments reported on the two header tee points.
Note: For skewed piping connecting equipment, you will generally consider the local forces and moments acting on the run point if the run direction is not aligned to the global axes. Enter option Tools model options /Results "Force (Global/Local)" = L for local forces and moments.
Question 46:
How does Autopipe calculate the Pressure Extension & Pressure Thrust forces?
Answer: Autopipe calculates the pressure thrust based on cap pressure force - poissons effect i.e using inside diameter for the poissons term. This term is at the analysis level acting on the inside diameter and not a code compliance hoop stress term which uses outside diameter.
- Pressure thrust reaction at anchor. Rpressure = p.Ai - v.p.Di/2t x As where Ai is the internal pipe area and As is the X-area of the pipe, t is the wall thickness.
- Pressure extension Ext = (p.Ai/As/Elong - v.p.Ri/t/Ehoop)*tangent length
Note: Autopipe does not report the cap internal pressure load at an anchor unless it is unbalance thrust across a flexible joint with the pressure analysis option checked when performing a static analysis.
Note: The program calculates displacements then back-calculates forces and moments from the stiffness matrix equations which many times can be difficult to manually re-produce.
Question 47:
How can I simulate soil settlement on part of my piping system?
Answer: If the piping system is above ground then at the v-stop and guide supports we recommend to apply settlement displacement in a user case e.g U1. This will allow the soil settlement to be observed as a unique loading. Then edit the sustained code case combination SUS + Gr to include U1 i.e. the settlement is added to the sustained stresses.
If the piping system is buried then the imposed displacement at defined node points (with soil) would give the soil settlement profile. In other words, the base of each soil spring is subject to a displacement interpolated from adjacent nodes with imposed displacements. Hence select the range of points and Insert/xtra data / imposed support displacement = settlement value in the U1 case.
To consider settlement for hanger selection, the following is recommended:
- Using model A, add the settlement in the T1 case and run the hanger selection to size the spring.
- Copy model A to Model B.
- Using model B, change the settlement from case T1 to case U1 and create user non-code combinations using U1 and add U1 into the sustained stress case GR+MaxP as above.
- Using a non-code combination e.g. GR+T1+U1 the combined movement and load on the supports including any spring hangers can be evaluated.
Question 48:
Can I model a Sway Brace?
Answer: You could use an inclined support in the direction of restraint and enter the stiffness corresponding to the sway brace spring stiffness. Add xtra data/conc force (i.e sway brace preload ) in the direction of restraint in the GR case such that non-code combination Gr + E1 will still have the preload.
Question 49:
Can I model my pipe shoe supported on a spring can with friction?
Answer: Yes insert a spring at the pipe location and run a hanger selection then change spring to inclined support with the calculate hanger spring rate, 0 gaps and add friction e.g 0.3. Insert concentrated force in GR case = hanger preload at this same point.
Question 50:
Why are all my dynamic results positive?
Answer: All results from any dynamic analysis are always signless That is positive displacements, forces and moments, support loads etc i.e. although dynamic loads are signed during the analysis phase of the solution, the results are without sign. The reason is that the results are maximum values over time and represent an envelop of maximum values. Hence in many piping configurations applied loads - whether +ve or -ve in direction will give an identical result.
Although all dynamic results are always reported as positive including time history, support loads should always be considered +/- for design but be aware that the restraint load will always be in the direction of the restraint and therefore has a vector direction (therefore signed) not shown in the reported restraint report for dynamic loads (always positive). Hence it is also not recommended to create user non-code combinations of static + dynamic loads. All stress programs have this limitation.
Also refer to online help - Autopipe reference information/Results Interpretation click on Dynamic Support Forces Results. Note: The Dynamic load analyses including time history are always linear i.e non-linear features such as friction, gaps, soil yielding are ignored.
Question 51:
Can I create a isometric model to use in AutoCAD or Autoplant?
Answer: Yes there are 3 possible DXF formats which can be exported from Autopipe.
DXF file export in either 3D or 2D format and import to Autocad R14 or later.
- TRUE 3D DXF model (File save as DXF/ Tools/Settings/DXF Drawing Border + include 3D model = yes )
- 3D LINE ISOMETRIC DXF model (File save as DXF/ Tools/Settings/DXF Drawing Border + include 3D model = no)
- 2D DXF model (Set the view mode = line then select Print graphics As Autocad DXF)
In the 3D formats a) & b) when the DXF file is first opened in Autocad the view is a plan view by default with the drawing sheet orientated to this plan view which is consistent with 3D CAD drawings. We recommend use the 2D DXF format for a 'flat' 2 1/2D Isometric view - unfortunately no drawing border but can be copy/pasted from standard CAD drawing. OR Create a drawing border in paper space to frame all the views you want on the 3D model.
Dimensions and node numbers can be sized under tools/settings/DXF export drawing border e.g change text height = 4 inches. Autopipe does not support dimensioning lines only length offsets shown on the pipe centerline.
Question 52:
Why does the legend for the interactive code scan show a maximum value = 1.0 for stresses?
Answer: When the stress criteria (not ratio) is selected in the Result/code scan, the color coding is based on the ratio of the calculated stress divided by the largest stress value in the model. The element is colored based on the highest stress ratio at either end of the element.
Question 53:
My B31.3 code stress is showing an overstress in the hoop stress (Max P) case?
Answer: Suggest check the mill tolerance, corrosion allowance and pressure at the points of overstress.
Note: Miter bends have a different hoop stress calculation to straight pipe. Autopipe is based on the more conservative B31.3 equation 3a.
Note: Autopipe always defaults to Y factor = 0.4 (i.e carbon steel <=900F), for different materials and/or higher temperatures change the Y-factor under Tools model options /Results "Y Factor". Default weld efficiency factor = 1.0.
Autopipe 6.X Online help topic Autopipe Reference information / Code Compliance Calculations/ASME B31.1 hoop stress calculation.
Question 54:
I open my archived model and it appears blank. Why?
Answer: When files are archived to read only media like CD-R they are assigned a file attribute = R (read only) but Autopipe requires the model file (*.DAT) to have a file attribute = A (archive).
Question 55:
Sometimes my Autoplant valves imported from the PXF neutral file have zero weight. Why?
Answer: Valve weights are read from the Autoplant PXF fil. If no weights are available in the AutoPLANT model then Autopipe will attempt to set the weight based on its own valve library using the valve size, valve type e.g. gate, globe etc and pressure rating e.g 150, 300 etc. If any of these three criteria do not exist e.g ball valves are not in the default valve library then the weight will be set = 0.
Question 56:
How can I change my Y-vertical axis model to Z-vertical?
Answer: Autopipe was not designed to change the vertical axis for an existing model since coordinates and supports (except springs, v-stops and guides) may be messed up.
However the model can be saved as a Autopipe neutral NTL file (File/save as batch NTL). Open this NTL file in Notepad.exe then change Y-vertical model to Z-vertical model by changing the Y to Z in the CTL line below Save the NTL file and in Autopipe select File open/ batch NTL.
When the model is imported back into Autopipe it will have to be rotated (possibly other transformations) to orientate the model as it was before and then check all the supports are correctly orientated.
Question 57:
Why does my bend look disconnected from the pipe on the screen?
Answer: Bends may appear disconnected but this is only a plotting limitation and to check bend connectivity with the piping system, use the keyboard left/right arrow keys to move the cursor along the pipe and around the bend.
Question 58:
How can I define a branch SIF using B31.1 Fig D-1 sketch d)?
Answer: Autopipe currently does not calculate to Fig D-1 B31.1 but the SIF can manually be calculated then applied using insert/xtra data/user SIF on the branch side of the Tee only. The B31.1 branch effective section modulus is calculated by Autopipe = pi rm^2 te since rm is same as rb on Fig D-1 B31.1 (d) then suggest insert a small run with different pipe identifier having a thickness when used in the section modulus calculation will be equivalent to tb. Where te = smaller of tn or i.tb hence equivalent branch thk = actual tb/ SIF calculated from Fig D-1 B31.1 assuming i.tb is smaller than tn.
Question 59:
Does my expansion joint include contents weight?
Answer: Autopipe does not automatically include the contents weight across a flexible joint therefore this weight must be included in the total component weight. Also valves and reducers do include contents weight but nozzle elements do not.
Question 60:
Why do I I keep getting a warning message "W831-1, Combinations include cases not analyzed"?
Answer: This warning message "W831-1" means either a user non-code or code combination has been created but a load case defined in that combination has not been analyzed. Note: When a user combination is defined both the non-code and code combination list is fixed i.e. if additional loads cases are analyzed then default code and non-code combinations are not created. To have the program create the new default combinations then select Tools/code combinations/reset all or Tools/non-code combinations/reset all, this will remove any user combinations.
Question 61:
How do I make a mirror copy of a section of piping?
Answer: To make a mirror copy :
- Select the pipe section to mirror copy.
- Edit / copy and select appropriate base point.
- Edit /paste to and paste the copied section to the new location.
- Select the copied section
- Edit/scale Enter -1 to mirror those offsets e.g of mirror about the Z-axis then enter -1,1,1 i.e reverses all the -ve X offsets
Question 62:
How do I define the coefficient of lift for wave loading?
Answer: The lift coefficient (CL) is typically applied only to the seabed piping and is only defined under Insert/xtra data/hydrodynamic data. By default CL = 0.
Question 63:
Can you provide any references for buried pipe design?
Answer: Some references are listed below:
Buried Pipe Design by A.P. Moser Publisher: McGraw-Hill Reference: Guidelines for the Seismic Design of Oil and Gas Pipeline Systems - ASCE Committee on Gas and Liquid Fuel Lifelines -1984
Soil Properties : Testing,Measurement and evaluation by Cheng Liu and Jack Evett
ISBN 0-13-0200069-7
Publisher : Prentice Hall
Soil Mechanics in Engineering Practice by Karl Terzaghi
Publisher : John Wiley & Sons
Essence of soil Soil Mechanics and Foundations : Basic Geotechnics by David F Mc Carthy.
Publisher : Prentice Hall.
Guidelines for the Design of Buried Steel Pipe - July 2001 by ASCE Click the following link.
http://www.americanlifelinesalliance.org/pdf/buried_pipe.pdf
Question 64:
How do I model the discharge piping accurately relative to my suction Piping?
Answer: To model the discharge piping accurately follow the steps below:
- Place the cursor on the suction point
- Insert / Segment (see screen bitmap below)
- Enter New first point name e.g AY00
- Enter suction point in the "Offset from which point" e.g AW07
- Enter offsets DX, DY, DZ
- Enter or select pipe identifier for the discharge piping
- Click Ok
- Then continue building the new discharge segment
Question 65:
How do I model different parts of the piping system inside and outside a building for wind loads?
Answer: You will need to model the section of pipe inside the building as a different segment to the outside piping.
On the main wind dialog, uncheck the option "All segments exposed to wind" and after all the wind loads have been defined, a segment dialog will appear which you can select or de-select the segments which will have wind loading applied to them.
Question 66:
What does LOC mean in the General Stress Report?
Answer: General Stress location Total stress is calculated every 15 deg and the zero degree axis for the total stress location angle in a clockwise direction around the pipe.
The 0 degree axis is the local axis of the pipe cross-section, i.e. perpendicular to the pipe axis, corresponding to the resultant in-plane bending moment. For example, if the local bending moments are my and mz, then arctan (my/mz) is the location of the 0 degree reference axis for the general stress location relative to the local y axis.
Note: For straight pipe in-plane bending is arbitrary since no plane of bending like an elbow.
Axial stress: fx/A
in-plane bending stress: my/Z
out-plane bending stress: mz/Z
where,
fx = local axial force
my = local in-plane bending moment
mz = local out-plane bending moment
Question 67:
How can I determine axial (tension or compression) and bending stress in the pipe?
Answer: Refer to the general stress report .
Axial stress = 1/2 x(sum of the longitudinal Max and min stresses)
Stress convention
+ve = tension
-ve = compression
Note: Local forces convention
-ve = tension
+ve = compression
Note: P1 force does not include the capped internal pressure force. However this pressure force PiAi can be manually subtracted from the Local-X P1 force to give a true P1 axial force i.e Total P1 = P1 - PiAi
Resultant Bending stress = 1/2 x(longitudinal Max - longitudinal min stresses) (always +ve stress since using resultant bending moment)
Question 68:
How do I model a ball joint with high inertial moment stiffness?
Answer: The conservative approach would be to add a constant moment at one end of the flexible joint.
Question 69:
Please confirm the recommended value of the added mass coefficient, Cm for a circular cylinder according to DNV Rules for Submarine Pipeline Systems - 1981 - Fig. A.7.
Answer: Sorry for the confusion Cm and Ci are inter-changeable in Autopipe i.e mass (inertia) coefficient in Buoyancy, Wave Load Hydrodynamic Data dialogs.
So Autopipe (Cm)is inertia coefficient but DNV 81 (Cm) is the added mass coefficient.
Where Autopipe Mass coefficient (Cm) is the inertia coefficient(i.e. 1 + added mass coeff) [where added mass coeff = Range 2.29 to 1.0 (no fixed boundary) as per DNV'81 Figure A.7), hence Autopipe Cm(Ci) = 3.29 to 2.0]
We will be updating the program and help in v7.0 to clarify the definition of these coefficients.
The only Cm (inertia) used in the modal analysis is the Cm value in the buoyancy loading dialog.
Question 70:
How do I define one constant Hydrotest pressure over the whole model?
Answer: The hydrotest pressure defined under Load/hydrotest dialog is taken as the pressure (usually case 1) multiplied by the factor e.g. 1.5 at each point hence for case 1 if different pressures in the system e.g branch line then the hydrotest pressure will be different for the branch.
A workaround if on hydrotest pressure for all points then define a dummy temperature and pressure case 2 which has one constant pressure for the whole model and no temperature loading i.e ambient and define the hydrotest pressure based on case 2.
Question 71:
What are the Participation Factors and the Captured Modal Mass in the frequency report?
Answer: The participation factor is a measure of the importance of the mode in earthquake type load. The captured modal mass is another way of quantifying the importance of the mode and the two are related. The captured modal mass percentage tells how much of the response is atributed to a particular mode and also tells on the mode orientation (X,Y or Z). The sum of modal masses should be 100% if all modes are counted. But since many modes are not counted, the sum is less than 100 and hence the importance of the ZPA and missing mass options for dynamic analysis. Please refer to the topic "Missing Mass and ZPA Correction" in the online help for more information.
The participation factors are calculated from the product of the mode shape, the mass matrix and a vector of ones. For mode i the participation factor is calculated as:
Participation_Factor_i= Transpose(ModeShape_i) * MassMatrix * {1}
The mode shape is mass normalized. The above equation is used three times for X, Y and Z directions.
The mass participation report illustrates how sensitive each of the piping system's modes are to the dynamic loading. High modal participation factors indicate that the mode is easily excited by the applied dynamic forces. If subsequent displacement reports indicate high dynamic responses then the modes having high participation factors must be dampened or eliminated. Once a particular mode is targeted as being a problem, it may be viewed in the mode shape report, or graphically via the animated mode shape plots.
Question 72:
When should I perform a pressure analysis?
Answer: Some guidelines are a pressure analysis should be performed when some significant strain due to pressure can result for example:
- Systems with expansion joints.
- Low modulus piping e.g plastic or FRP
- Large pressure on steel piping systems e.g >500psi
- Large diameter e.g >=30" diameter with moderate pressure.
Question 73:
The Autopipe help states "When the rise time several times larger than the 2L/a time, the calculated pressure rise in AutoPIPE might be conservative. For this special case, the use of a fluid simulation software is recommended if P2 case is critical." What does this mean?
Answer: Check for maximum surge pressure (static (362)+ rise (228) =590 psi). This should be added as a second pressure case (P2). Use Tools/Model Options/General and set number of operating cases to 2. Use Select/All Points and follow by Modify/Pressure & Temperature and set design pressure for P2 to 590 psi. When the rise time several times larger than the 2L/a time, the calculated pressure rise in AutoPIPE might be conservative. For this special case, the use of a fluid simulation software is recommended if P2 case is critical and causes an overstress condition in the pipework.
If P2=590 psi governs the design, that is critical, the use of fluid simulation software is recommended since AutoPIPE value would be too conservative.
Question 74:
My seabed piping shows a large unexpected displacement. Can you please explain?
Answer: When the 2nd soil stiffness (K2) is set to zero, the pipe displacement can be large or can cause instability as the soil yields. Most soils especially sand have a parabolic force-displacement shape and a larger K2 value is justified but it is conservative to assume a small K2 value. It is advisable with a model so sensitive to changing the K2 value, to examine the soil displacements and forces for more detailed evaluation of yielding e.g. as seen in the soil forces report the horizontal soil force is about 176 kg/m at A78 compared to the P1 value is only 145.8 kg/m hence the soil does not have enough lateral resistance to support this pipe.
Question 75:
How do I enter cold spring into my main steam model?
Answer: Cold Spring or Cut short in pipe stress programs can only be applied as a fixed displacement value not a percentage of thermal expansion since the amount of cold spring it is unknown until the completion of the static analysis. This cut short value is equivalent to removing this amount of pipe from the overall length. Therefore a previous static analysis run should have been performed
The maximum thermal expansion in all 3 global directions is found from a static analysis between 2 anchored points then the cutshort is calculated in the 3 global directions and applied generally at one location. To apply cutshort in 3 directions it is necessary to insert a small pipe in the direction of these 3 global cut short values e.g. Dx = 3", Dy = 4" , DZ = 5" then apply the cutshort as a resultant value e.g. 7.07" at the end of this short pipe run.
Important Note: Piping codes do NOT permit reducing sustained or thermal stress range using cold spring. Therefore it is recommended to apply the cut short in Autopipe using the User load case = U1.
Create user Non-code combinations = Gr+P1+T1+U1 and GR+U1 to examine the resultant loads on equipment and supports.
Question 76:
Given that point A11 is the TIP of a long-radius bend, how are the near and far points related to A11 N-, A11 N+, A11 F-, and A11 F+?
Answer:
A11 N- = on the straight pipe adjacent to near point
A11 N+ = on the bend side adjacent to near point
A11 F- = on the bend side adjacent to far point
A11 F+ = on the straight pipe adjacent to far point
Question 77:
When I insert a midpoint there is a big difference in the natural frequencies, can you please explain?
Answer: It is important you perform all modal and dynamic analyses with automatic mass discretization to capture the dynamic mass in the system.
Recommend you set under tools/model options/Edit "Mass points per span" = A to allow the program to automatically perform mass discretization on your model for improved accuracy for the dynamic analysis.
Question 78:
When performing seismic analysis, can AutoPIPE do multi support excitation?
Answer: No but it is common to envelope spectrums to provide a maximum response. This feature is available in AutoPIPE 2004 (v8.6).
Question 79:
How can I print an echo of the input response spectra (load)?
Answer: AutoPIPE cannot at this time print an echo of the input response spectra. You can, however, manually send the spectrum files (.spc) to the printer.
Question 80:
Where do I input stress range reduction factor?
Answer: Tools/Model Options/Results...right hand column, second line down.
Question 81:
How can I look up the units for a flexible anchor for translational and rotational stiffness?
Answer: Select unit in question and read units designation in lower right hand corner of screen.
Question 82:
I have been working with AutoPIPE 8.05.01.11. How do you print in version 6.3 when there are no grids?
Answer: Go to Results/Output Report and select which items you want to print. Then in the Output Report go to File/Print.
Question 83:
Can AutoPIPE run a flange check and if so, how?
Answer: Yes, AutoPIPE can run a flange check. In the flange dialog box check "ANSI Check", select the flange class and material and select the gasket material. AutoPIPE automatically runs the flange check.
Question 84:
I am looking at a tutorial for water hammer called apham1. I have noticed that between A06 and C00 there is, essentially, a split between the supply and discharge lines of the imaginary pump (each is anchored and flanged). I didn't know this could be done. Could you help me to get this same type of arrangement set up for a similar project I am working on?
Answer: Select the suction nozzle where you want to add a discharge nozzle. Press F3 and note the coordinates. Using your pump drawing calculate the coordinates of your discharge nozzle. Go to Insert/Segment and set the first point number (i.e if new segment is C then first point would be named C00), and input the discharge nozzle coordinates. Insert you anchor and proceed.
Note: If the pump is changed or if you need to adjust the position of the discharge then Select the discharge segment and use the Edit/Move command.
Question 85:
When coding a tie/link for an expansion joint what is considered gap forward and what is considered gap backward?
Answer: The gap forward will allow the near end of the joint to move forward (compress the joint). The gap backward will allow extension of the joint.
Question 86:
How do you close a loop?
Answer: Go to Insert/Run. The dialog box will show you the existing point (from) and the next point (to). Change the "to" point to the one you want to close with. Enter "OK" and the loop will close.
Note: A segment cannot be connected back on itself e.g. to form Ring manifold a 2nd small segment is required to close the 'Ring' using Insert /Segment.
Question 87:
I am working on a system which a two phase liquid. For the stress analysis, which specific gravity should I use? Should I consider the true representative by getting the average or just use the higher specific gravity?
Answer: For slugging flow induced by the two phase flow, the SG of the liquid should be used.
Question 88:
Does AutoPIPE assume the contents of the pipes are full of water? If so, is there a setting available to modify the specific gravity (or other item) to model a different fluid?
Answer: Go to Modify/Properties of a Pipe Identifier to enter the fluid Specific Gravity.
Question 89:
Is there a way to easily modify the allowable stress for the custom piping I have put into the model? I am using Code B-7159.
Answer: For Code BS7159 the Allowable Stress is not entered. The Design Strain is. The Design Strain value is used for the calculation of the allowable design stress corresponding to the operating load condition. The default value is set to the cold design strain as defined in the Pipe dialog. If a load combination does not include any operating loads, then the smallest design strain of all the operating loads will be used. Go to Modify/Pressure & Temperature for the operating design strain and Modify/Properties of a Pipe Identifier for the cold design strain.
Question 90:
Please explain the $T1 and $T2 load case data?
Answer: Loadcases T1 and T2 already existed under a linear analysis and AutoPIPE could not create duplicate loadcases with the same name. So, it renamed the cases as $T1 and $T2. Please note that when you ran a non-linear analysis and established user cases T1 and T2 the other T1 and T2 cases did not exist. But when you ran a linear analysis, and T1 and T2 were created. AutoPIPE renamed your existing user T1 and T2 cases to avoid a programming conflict.
Question 91:
Seismic load needs to be run with friction. How do I turn it on?
Answer: Go to Analyze/Static and turn on Gaps/Friction/Soil. Select OK. Uncheck the Ignore Friction for E1 to E10 box and select OK again.
Question 92:
How do I change a temperature over a range?
Answer: Select the starting point of your range and, while holding down the Shift key, select the end point of your range. Go to Modify/Pressure and Temperature and type in the new temperatures. To view go to View/Show/Temperature.
Note: Modify/Operating Load by value allows any temperature or pressure in the complete model to be changed to a different value.
Question 93:
In the Hanger Selection Analysis dialog, it is displaying V-Stop as supports whereas the user has spring hangers. What criteria does it use to determine the supports?
Answer: If the movement at the location where you want the spring is less than the value in the Rigid Hanger Criterion box, the program will enter a V-stop. The default value is 0.1 inch. To always have a spring selected then change this value to 0.
Question 94:
If the modal displacements are a unitless number, what is their value based on?
Answer: The modal displacements are mass normalized which means they divided by the the following matrix transformation.
Question 95:
How do I hide node numbers?
Answer: There are three ways to Hide/Show node points.
- Hold down Ctrl and depress N.
- Click the toolbar icon with E-15 on it.
- Go to View/Show/Point Names.
Question 96:
How do I print to AutoCAD DXF?
Answer: Go to File/Save As/DXF
Note: Tools/Settings/DXF Export Drawing border provides many options to change the drawing sheet size, units, text size, line thickness and drawing margins.
Question 97:
How to increment point numbers by 5 instead of the default increment of 1?
Answer: Tools/Model Options/Edit and change "Default point name offset" to 5.
Question 98:
I am looking for a quick way of changing the piping orientation and I do not want to start to input the model again. I would like to change the orientation of the horizontal leg to start from the X and Z plane at 45 degrees.
Answer: Highlight the area in question by selecting the first point and holding the shift button down while selecting the second point. Go to Edit/Rotate and input the base point (the point that you want to rotate about) and enter OK. Input the number of degrees rotation desired and the axis of rotation and enter OK.
Question 99:
What is the method for stress analysis of pipe settlement when loading traffic?
Answer: After modeling soil points, use support displacements at piping points which have soil defined over them to simulate the soil settlement and then apply a uniform load to simulate the traffic loading.
Note: If no Soil is defined then insert V-stops with applied support displacement = soil settlement.
Question 100:
Is there an easy way to change temperatures over a range using the input grids?
Answer: See steps below.
- Using the Pres/Temp/Pipe ID tab of the Input Grid enter the temperature at the first point that you want to change.
- Hold down the Shift key, select the last point in the temperature range. This will highlight the range of cells.
- Hold down the CTRL key, press Enter, and the temperatures over that range will be updated.
Note: Using the CTRL key can do multiple unconnected selection ranges like EXCEL. The new temperature can be entered after the range is selected also.
Question 101:
I am running 12 thermal loadcases (Code B31.1). I noticed in the code combinations that the range from ambient to T1....T12 are considered, however, the total range for all twelve cases are not shown. For example only "T1 to T2"; "T1 to T3" and "T2 to T3" are shown.
Answer: Go to Tools/Model Options/Results and enter 2 to 20 in the No. of thermal ranges option. Note: v8.5 has a new Include Max Range comb option which automatically shows only the maximum thermal stress and the corresponding combination it is occuring at every point including both ambient to all hot cases and all the thermal ranges. This is very useful to quickly find the maximum thermal stress and not have review and print all the ambient to hot and thermal range cases.
Question 102:
Where can I find a sample for a pressure balanced expansion joint?
Answer: Go to Help/Contents/Modeling Approaches/Modeling Approaches/Flexible Joints/Pressure Balanced Expansion Joints.
Question 103:
How does AutoPIPE decide the allowable stresses on pipes?
Answer: Material allowable stresses are per code. The default values for weld efficiency factors are 1, but these values can be adjusted by going to Insert/Xtra Data/User Weld Efficiency Factors and inputting a lower value when warranted.
Question 104:
How do you know if the axial forces on a flange are in compression, simply by looking at the forces and moments report?
Answer: Go to Tools/Model Options/Results and set Force (Global/Local) to L for local. That will set X as axial to the pipe regardless of its global orientation. Now if you examine your Forces_Moments and Flange Reports, you will see that a negative value will show a tensile force. Also see questions 67.
Question 105:
For Hanger Selection what is considered for spring sizing, cold setting or hot setting?
Answer: The current default hanger selection method uses hot setting or hot load design. There is no automatic option at this time to use cold setting or cold load design.
To perform a cold load design:
Method A
- Tools/Model Options/General , add one more temperature case for the model and define at ambient temperature.
- Set rigid displacement critera = 0 (so springs are automatically selected even though no thermal displacement)
- Run the hanger selection with variation = 0.25 at the new ambient temperature case only.
- Springs are all designed now at cold load, change number of temperature cases in Tools/Model Options/General to the original number before step 1 or ignore analyzing this thermal case.
Method B
- Set rigid displacement critera = 100"
- Run hanger selection with variation = 0.25
- Save hanger report
- Re-run hanger selection with rigid displacement critera = 0.1"
- Edit all hangers and replace the cold load value by the V-stop reaction found in step 3.
Method A is simpler to execute but all springs selected are the stiffest lowest range hence if design load variation is low e.g. 10% then the spring rates for many springs will need to be updated.
Method B requires all cold loads to be updated manually on all springs. The load variation on all springs also needs to be checked and some spring rates may need to be changed.
- Now run a static analysis and confirm vertical displacement = 0 under GR at each spring hanger point.
- Also recalculate the load variation ratio as [(GR+T1)-GR]/GR = T1/GR
- Confirm that both hot and cold reactions can be satisfied by the selected hanger
Advantages of cold load design:
- Hanger stops are easier to remove
- GR displacement small
- Spring loads can be adjusted before system is brought up to temperature
- Some consider that cold load approach yields a more dependable design
- In some system configurations, operating loads on connected equipment are lower. A typical configuration resulting in this load reduction is one where a hot vertical riser, anchored at the bottom, turns horizontally into a nozzle connection.
Disadvantages of cold load design:
- In some systems, in the hot condition the loads on rotating equipment may be increased by a value proportional to spring rate times the travel.
- Most installations are done on a hot load design basis since the piping system is operating in the hot condition for most of its project lifecycle.
Question 106:
What is Weightless or As-built when considering gapped supports?
Answer: Gaps may be specified to be set either As- built (weighted) or weightless. If a gap is As- built, it is assumed to be open (i.e. no support) for the gravity load case, then it is set to the specified gap AFTER the gravity load is applied. The stop will then engage only if the pipe displacement due to thermal expansion or any other loads exceeds the gap. In effect, a support with an As- built gap moves with the pipe for gravity load, then remains stationary for thermal and other loads. On the other hand, if the gap is specified in the weightless condition, the gap can change when the gravity load is applied, and the stop may engage. There is one important exception. If a value of zero is specified for any gap, it is always assumed that this zero gap is of the weightless type (i.e., the corresponding stop may engage under gravity load). This is done because we have found that users will generally assume that a zero-gap support acts in this way. If a gap is required to be zero and to be set in the As- built condition (i.e. just closed under gravity load, with zero support load) specify a small, nonzero value (e.g., 0.002 inches).
Question 107:
In the output report why does the Results Summary reflect two different Occasional Cases?
Answer: The first summary lists the Maximum Occasional Stress and the second lists the Maximum Occasional Stress Ratio. These are very often the same but not always Max Ratio=Stress/Allowable Stress.
Question 108:
Why does the code compliance report show an SIF = 1 for the Sustained Case and 1.3 for all other cases?
Answer: See tools/Model Options/ Results/Set Sustained SIF to 1. This field is only active for the B31.1-1967, B31.3, B31.4, B31.8, DNV, SNCT, CAN-Z662, and KHK level1 codes (it is closed for all others). These codes define stress intensification factors (SIF's) for the expansion stress category only. This option has been provided since no guidelines are presented for the sustained and occasional stress categories. If disabled, AutoPIPE will apply the expansion category SIF's to all other stress categories. When it is enabled, an SIF of 1.00 is used for the sustained category at all points except bends, and the expansion category SIF's are applied to the occasional category. By default this field is disabled for all applicable codes except B31.1-1967.
Question 109:
If a user hasn't saved his model in a while (old bak file) and lost his model by accident, can he use his U0x file and change it to a dat?
Answer: U01, U02 etc up to U99 are actually DAT files. Rename any of the U0x files to DAT and you can retrieve the latest model.
Question 110:
Does AutoPIPE display "Von Mises" as well as "Tresca" stresses on the output report?
Answer: Yes, select Tools>Model Options>Results>Total Stresses (Oct/Max). 'O' represents the maximum octahedral shear (Von Mises) stress, and M represents Max Shear Stress (where Max Shear Stress = Tresca stress/2).
Create 2 output reports in order to display the stresses in the General Stress Calculation section or uncheck the "Overwrite output file" option on the Tools>Model Options>Results dialog.
Also, these stresses could be displayed as a Code Combination, select Help>Contents>Command Reference>User-Defined Code Combinations Category>For all code options …
Question 111:
How do you set up WinNOZL to work with AutoPIPE?
Answer: Install WinNOZL to the default installation directory. Move the entire WinNOZL folder/directory to become a subfolder/directory of AutoPIPE, i.e. C:\Program Files\Bentley\AutoPIPE\WinNOZL. Update any desktop icons and Start>Program folder icons as necessary.
Question 112
On the support output report, the GR's load is not equal to the spring Preload. Why?
Answer: The output report GR reaction is calculated and may not be the same as the design Preload. The Preload (cold load) is extracted from the Hanger analysis report. Using the calculated hot load and the travel distance from the design operating case, the cold load is extracted from the spring manufacturers tables.
Question 113:
How do you model Cryogenic piping?
Companies like Air Products and York Refrigerant use B31.3 for all their refrigerant or cryogenic piping using AutoPIPE. It is not necessary to use B31.5
Please follow the suggested steps below:
B31.3 code compliance case to figure 323.2.2B to avoid Impact Testing for carbon steels.
Combined longitudinal stress due to pressure, dead weight, and displacement strain (stress intensification factors are not included in this calculation) divided by S, at the design minimum temperature. In calculating longitudinal stress, the forces and moments in the piping system shall be calculated using nominal dimensions and the stresses calculated using section properties based on the nominal dimensions less corrosion, erosion and mechanical allowances.
S = nominal design stress at low temperature
AutoPIPE Modeling:
Create a model with one(1) T1, P1 case, with low temp and pressure, so the sustained case (SUS) and (LONG) are based on correct pressure.
Analyze with GR, T1, pressure analysis = ON
Create a user defined code combination as follows:
- method = abs sum
- category = occasional/li>
- combination = (LONG) + GR + cold to T1(s) + P1
- The automatic Allowable = K. SH (i.e. S at low temp)
- Set the K factor = 1.0 under tools/ code combinations/ occasional load factor.
- This combination does add GR, and P1 at the moment level (abs sum) before calculating its stress, which is reasonably conservative, i.e. no moment cancellation in summing the load cases. Less conservative method is to use SUM combination method. Also adding thermal stress separately is conservative.
- Select all Tees and Bends and Insert>Xtra Data>User SIF = 1.0 (both in-plane and out-plane).
- Tools>Model Options>Results, "Use Nominal Thickness" = unchecked
The calculated stress ratio can be used to evaluate the temperature reduction below the minimum design metal temperature for carbon steel to clause 323.2.2d(1) and Fig 323.2.2B.
Question 114:
My AutoPIPE application runs slowly on my computer, what can I do?
One or more of the following suggestions should help a slow computer using AutoPIPE:
- Does the computer meet the minimum requirements? See Help>Contents>Bentley AutoPIPE>Getting Started>Installation and Authorization>Installation>System Requirements.
- Change hardware acceleration inside AutoPIPE, View>Settings, uncheck Enable hardware acceleration.
- In AutoPIPE, View>Level of Detail, check the option to switch to line mode (and may be also try 3 other options).
- Change computer display properties: Control Panel>Disply>Settings tab>Advanced button>Troubleshoot tab, select a lower hardware acceleration. This may be an iterative process.
- Control Panel>Display>Appearance?>Effects, the 'Use transition effects for menus and tool tips' option should be unchecked.
- Update computer's OPENGL display drivers.
- Check to verify that model size has a limitation of < 10,000 points: Tools>Model Input Listing>Extended Components>Number of Points in the System.
- Also, you may want to consider defragmenting your hard drive. This can be done (in XP) by going to Programs/Accessories/System Tools/Disk Defragmenter. If your hard drive is very fragmented, the program has to write data in a lot of different locations, and it slows down considerably.
- If that does not work, suggest updating the video card, increasing the systems memory, and/or updating the CPU.
- Check out a license from the Select Server and unplug the network/internet connection. Reboot the system and start AutoPIPE.
If none of these suggestions have helped, please submit a help desk request to support@bentley.com. Please include the computer's system information. Start>Programs>Accessories>System Tools>System Information. After the program has been opened, select File>Export, then save the file and send a copy for inspection.
See Also
Structural Product TechNotes And FAQs
External Links
Bentley Technical Support KnowledgeBase
Comments or Corrections?
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Where the bend near point is the start of the bend and the Far point is the end of the bend.
Question 30: Why are spring or rigid hangers on vertical legs showing large reaction loads in the Hydrotest (Hy) case compared to the Gravity (Gr) case?
- analyze for gravity;
- analyze for gravity plus thermal; then
- subtract Step 1 from Step 2 to get thermal.
If analyses are performed for load increments, the steps are:
- analyze for gravity; then
- analyze for thermal, specifying gravity as the initial state.
AutoPIPE uses the second of these procedures. Thus, to obtain the results for gravity plus thermal, a load combination must be defined in using the commands in the Result menu.