BOUNDARY

Q: How is the nub-valley contact accounted for? I can’t se the contact pressures in XPOST when opening the xx_bflex2010.raf file. Shall it be activated in the VISRES or similar card?

A: You will see the contact pressures directly by looking at the Normal stress (sigma-zz) result type for the boundary group

Q: In BFLEX, which friction coefficient is used for the nub-groove contact in BOUNDARY?

A: The friction coefficient will follow the value provided in the FLEXCROSS input for the corresponding layer.

Q: Nub-Valley friction induced flexlok section transverse stresses (boundary), can they be visualized?

A: Look at Transverse stress (sigma-yy) and Shear stress (sigma-xy) result type for the boundary group

Q: INOD, would there be results differences depending on which INOD is selced?

A: The user can select the position of the boundary model. This is to be able to choose a critical position along for example a bend-stiffener. So yes, changing INOD may mean different curvature histories and hence difference in the boundary and pflex stresses.

Q: The Axial stress results in the -boundary model looks strange?

A: The Axial stress, Normal curvature stress and Transverse curvature stress results are populated by PFLEX, even though the results can be seen in the -boundary model part in Xpost. You should hence perform the PFLEX analysis before the inspecting these results. Note that for the Transverse curvature stress results, the -boundary model results only contain the dynamic, bending induced part of the stresses, excluding the axi-symmetric part due to tilting of the profile when gaps are closed.

Q: During tension and pressure loading my boundary model of the zeta profile moves a lot in the transverse direction?

A: Check your transverse curvature parameter trcurp in the *.boi file. If this is zero or very small, an increased value will provide a small transverse stiffness.

Q: I observe a longitudinal stress pattern which are present from the beginning of the analysis, with horizontal stripes of stress variation?

A: For a non-linear material in a pressure spiral, the initial stress-free configuration is straight. (This differs from elastic material, where the initial helical shape is assumed to be stress-free.) Before the analysis load steps are applied, the production-induced stresses are accounted for. Consequently, the pressure spiral is pre-curved to a level that yields a net-zero bending moment for the model’s helix radius. This implies a spring-back from a larger initial curvature, resulting in the stress pattern observed.

Q: The Axial stress and Normal curvature stress are observed to be identical for the -boundary and -pflex model part in Xpost. However the Transverse curvature stress differs slightly. Why is this?

A: The Axial stress, Normal curvature stress and Transverse curvature stress are populated by PFLEX both for the -boundary and -pflex part of the model. For Transverse curvature results the -boundary model results only contain the dynamic bending induced part of the stresses, excluding the axi-symmetric part due to the tilting of the profile when gaps are closed.

Q: The Total longitudinal stress differs from the -boundary to the -pflex model part in Xpost. Why is this?

A: The BOUNDARY and PFLEX procedures are different analysis models with different underlying assumptions. The obtained stress components may hence differ. PFLEX is responsible for populating the Axial stress sigma_xx, Normal curvature stress sigma_nc and Transverse curvature stress sigma_tc result types both for the -boundary and -pflex part of the model. For the -pflex model results sigma_xa + sigma_nc + sigma_tc = sigma_xx, Total longitudinal stress. However BOUNDARY also calculates its own axial and transverse stresses in the analysis step before PFLEX has been executed. The Total longitudinal stress reported in the -boundary part of the model is based on these results, and will normally differ from the one reported in the -pflex part of the model. As PFLEX has populated the separate stress components, sigma_xa + sigma_nc + sigma_tc is not equal sigma_xx for the -boundary results.

Q: Which stress components are relevant to include in the fatigue assessment?

A: For fatigue assessment the -pflex model results stress results can be applied for transverse cracking of the wires.

For longitudinal cracking, a combination of the results from BOUNDARY and PFLEX must be applied. As the BOUNDARY model does not include the Normal curvature stresses due to ovalization and the dynamic part of the Transverse curvature stresses, these stress components must be added to the Total longitudinal stress from the -boundary model. The Normal, Transverse and Shear stress from the -boundary model must also be included in the von Mieses stress calculation.

To obtain consistent results with LIFETIME the following approach must be taken: