| Applies To | |
| Product(s): | AutoPIPE |
| Version(s): | ALL; |
| Area: | Calculations |
| Date Logged & Current Version | April. 2014 09.06.00.15 |
Problem:
By calculating the Von Mises stress in a separate way, the result was different with respect to the automatically calculated ASD. Why?
Solution:
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 * (Do4 – Di4) / (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.