Applies To Product(s): AutoPIPE, Version(s): 2004, XM, & V8i Environment: N/A Area: Calculations Subarea: Original Author: Bentley Technical Support Group Comments, Questions, and Answers about DNV piping code calculations in AutoPIPE Item #1, Question: The problem is: by calculating the Von Mises stress in a separate way, the result it is different respect to the automatically calculated ASD. Why? Answer: The ASD check uses the literal equations presented in DNV-OS-F101. For each point, the combined bending moment M (comprising of in-plane and out-plan moments), and combined pipe wall force N (combined axial force at the location) are used for calculation of longitudinal stress. The hoop stress is also calculated as outlined in DNV-OS-F101. The above results are then combined to calculate the effective stress as per the equation provided. The results for the ASD check will be different from those of Von Mises tress calculation, which is calculated differently. Item #2, Question: In the AutoPIPE on line manual (help/contents/code/DNV 2012/ASD) there are references to formulas numbered as per DNV 2010, not as per 2012 edition. At the section 5-F200 of DNV 2012 a new definition of bending is presented, that is not reported in AutoPIPE manual. Answer: Online help shall be updated to reference the correct equation numbers. Item #3, Question: The AutoPIPE (automatic) calculation of the ASD is lower than the hoop stress and that is not possible! At bend flexural deformation occurs due to thermal/pressure elongation: the bending determines tensile and compressive stress at the pipe section. Together with the hoop stress, Von Mises calculation shall also consider the compressed fiber of the pipe section (as required in the of bending moment definition in section 5, F200, of DNV 2012). Answer: Bentley has identified the following issues with DNV 2012 Automatic ASD calculations: A. AutoPIPE ASD calculations currently does NOT calculate the maximum bending stress (compression or tension) - a fix is currently in progress and will be issued shortly B. Our interpretation of DNV 2012 requirements is that the bending moments are not intensified for ASD combinations i.e. SIF values are not used for calculating bending moment stresses for all components. After the fix for item a) above, there may still be differences between Von Mises, General Stress, and Automatic ASD calculations owing to SIF values not being used for ASD bending stress calculations. C. Bentley is investigating the interpretation and use of effective wall force vs. true wall force for calculation of axial stress for ASD calculations. We would welcome any comments or suggestions on its application for ASD. D. Please refer to workaround under Item #4, "Difference in values of general stress results and user Von mises results". Item #4, Question: Difference in values of general stress results and user Von mises results Answer: Von Mises category for DNV 2012 code is a legacy category based on requirements from DNV 1981. Note: There is no explicit mention of Von Mises stress calculation in DNV 2012 other than under ASD criteria. Results Output report AutoPIPE General Stress Report Typical General stress report shown below: Following are the differences between General Stress calculations and DNV 2012 Von Mises code combination: Section modulus The code combination available Von Mises for DNV 2012 is based on requirements of DNV 1981. The section modulus calculation formula required by DNV 1981 is as under: Z = π * (Do-t) 2 * t Where: Do = Outside diameter t = wall thickness General stress calculations use the following section modulus formula instead: Z = π * (Do 4 – Di 4 ) / (32*Do) Where: Do = Outside diameter Di = Inside diameter Wall thickness Von Mises code combination always use nominal wall thickness for calculation of cross sectional area and section modulus as per requirements of DNV 1981 General stress calculations use wall thickness based on Result Model Option “Use nominal thickness”. If the check-box Use nominal thickness is unchecked, wall thickness shall include corrosion allowance. Wall thickness will effect cross sectional area, section modulus and pressure area for longitudinal pressure stress. Longitudinal Pressure formula Von Mises code combination always uses PD/4t for calculating longitudinal pressure stress as per DNV 1981 General stress calculation use pressure area formula P*Ap/As for calculating longitudinal pressure stress Summary: Recommendation As a current workaround, General stress calculations will be a better match to ASD calculations for DNV 2012, as it uses the same section modulus, wall thickness value used can be controlled, and longitudinal pressure formula P*Ap/As uses actual pressure area. Note: Bentley's interpretation of DNV 2012 requirements is that the bending moments are not intensified for ASD combinations i.e. SIF values are not used for calculating bending moment stresses for all components. This can be achieved by specifying a user SIF value of 1.00 for both In-plane and Out-plane SIF on the Joint Type & User SIF dialog (Insert Xtra Data Joint Type & User SIF). This will set SIF values to 1.0 for both General Stress Report as well as Von Mises code combination. Item #5, Question: The results for DNV code combinations Collapse and Prop Buck are 0.00 or very small, why? Answer: Typical reason for 0.00 or low values: 1. Be sure to check that the node points are actually located below the Water surface. View Show options "Water Surface & Sea Level" is checked on, press OK button and confirm node point is with int he blue are of all the waves specified in the model. Only those points below the water surface, will have calculated values 0.00. 2. The model units may a limiting factor, with the "DNV-OS-F101-INITIALMODEL.DAT" provided in the Example folder (C:\Bentley\AutoPIPE V8i SELECTseries\Examples), change the Tools Model options General units from SI to English and re-run the analysis report: If the values remain 0.00 there is a problem with one of the settings in the model, see next suggestion for possible reason.. 3. Please see the following AutoPIPE help section: Help Contents Contents Tab Reference Information Code Compliance Calculations DNV 2012 Collapse Limit (DNV - 2012) Equation 5.14, Code stress value is calculate based on Pext - Pmin, The external pressure at any point less minimum pressure which is always is assumed to always be 0.00 psi. Confirm the following dialog settings are correct: Load Wave Tools Model Options DNV 2012 Result Options Load Buoyancy Note: 1. Check to be sure the water level / ground elevation on the Wave, Wind, and Buoyancy dialog screens all agree. They could have different elevation values that would cause the respective load to be applied to the model incorrectly. To keep things simple, may consider using the Vertical axis location = 0.00 as the water / ground elevation and locate the piping arrangement accordingly. Thus making it easy to understand that anything above the Vertical axis (default Y) = 0.00 will be exposed to wave or wind loads accordingly. 2. On the Buoyancy dialog screen, specific gravity must be greater than 0.00. 3. O n the Buoyancy dialog screen, check ON " Apply buoyancy to all segments" otherwise, use segment input grid for selecting segments to which buoyancy will be applied in the analysis. Item #6, Comment: Tools Model Options DNV 2012 Results Options dialog explained Item #7, Question: Can you tell me if a version of AutoPIPE had DNV F-101 Year 2000 as a code as the current version has 1981 and 2012? Answer: I apologize for the inconvenience, however that particular code is not available in any version of AutoPIPE (v.09.06.00.xx and lower). I have logged the following requested enhancement, TFS-E102639: Add DNV 2000 piping code. Please log a Service Request referencing this TFS number if you would also like to see this enhancement added to the program. Item #8, Question: Regarding the LCC (Load Controlled condition) check according to DNV 2012 (section 5 – equation 5.19). In the AutoPIPE output, for Load Controlled Buckling check (see figure below) the “stress value” and the “allowable value” are reported: please could you explain what are the meaning of these values ( values like 0.001 N/mm2 and 0.007 N/mm2 respectively) ? how these values are calculated ? Answer: Notice that some values are displayed in front of Stress and Allowable rows under "Maximum load controlled buckling stress ratio". There are no stress or allowable values for this category, only an interaction ratio should be displayed. This has been logged under TFS-D119449, and will be added to AutoPIPE V8i 09.06.01.xx and higher. Item #9, Question: An AutoPIPE client use DNV-OS-F101 standard. When they analyze with section 5 "DESIGN – LIMIT STATE CRITERIA", D 600 Local buckling - combined loading criteria, in that standard, they are wondering about which category should be chosen in Combination setting dialog. Answer: Section 5 D600 is combined loading local buckling (AutoPIPE only considers Load Controlled Condition only) which calculates interaction ratio as below: The results for this calculation are listed under code calculation category = Buck. Comb. as seen below on the Code Comb tab of the Load combinations dialog screen. This can also be seen in the output report where the equation number (5.19) used for calculation are also printed: See Also Bentley AutoPIPE External Links Bentley Technical Support KnowledgeBase Bentley LEARN Server Comments or Corrections? Bentley's Technical Support Group requests that you please submit any comments you have on this Wiki article to the "Comments" area below. THANK YOU!
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