Free Span Analysis (Clarification #1)

[subsealinepipe]

Introduction:
To carry out free span analysis (DNV-RP-F105 Feb 2006)for subsea pipeline, at 1st bottom roughness analysis is done to identify the span length and gap, and also moments and effective forces (or stresses/ strains) along the pipeline for various phases (as laid, flooding, hydro test, operation), and I do the calculation by ANSYS and the soil stiffness is calculated based on STATIC vertical soil stiffness.
Thereafter, for each identified span (bottom roughness analysis result) and using DYNAMIC vertical and horizontal stiffness, and using ABAQUS each span (or multi span in case of interaction between spans) CF and IL frequencies and unit stresses are extracted.

Question:
1st alternative (for a single span) to calculate frequency and unit diameter stress based on FEA(ABAQUS), the concrete effect is considered only on the mass, added mass, drag, inertia forces effects. The bending stiffness of concrete (as per RP-F105 Sec 6.2.3) is ignored. In the FEA (ABAQUS), the Pipe21 (or Pipe31) element is used and the equivalent density and diameter is considered for the contribution of concrete on the weight and hydrodynamic effects.
2nd alternative (for a single span) to calculate frequency and unit diameter stress based on RP-F105 Sec. 6.7, in these calculation the effect of stress concentration due to concrete stiffness in the field joints is considered. In this case, I have assumed that in case effective force is larger than half critical buckling load, the effective force should be adjusted equal to the critical buckling load. In addition, since critical buckling force depends on the soil stiffness (static or dynamic), for calculation of IL/CF frequency critical buckling load is calculated based on soil dynamic lateral/vertical stiffness, but for calculation of static moment (or stress) critical buckling load is calculated based on soil static stiffness.

However, there is difference on the above results. Can somebody advise on the likely points that I have missed (for instance shall I consider concrete bending stiffness contribution in FEA, or etc)
I hope the above question is clear, otherwise kindly advise till I will explain further, and thank you in advance for your time.


Regards,
Amir

 

[APipeliner]

RP-F105 does not consider the effect of stress concentration for natural frequency calculation, it considers the effect of additional stiffness due to concrete coating. So, if you considered this effect in you analytical calculation you should consider it in your numerical model.
I would like to recommend a step by step validation of your FEM model. It means, first remove everything (concrete, soil stiffness, added mass, static deflection, ...) and just model a fixed-fixed beam, then, add other parameters step by step to find which parameter affect your results.

 

[subsealinepipe]

Thanks APipeliner, I will run a FEM considering concrete stiffness effect. (I had run a simple beam without concrete and the FEM and manual results were nearly the same)
In F105 Sec. 6.2.3 it is quoted “the positive effect on the stiffness and strength is normally disregarded”, of course the concrete weight and diameter effect (as effective mass, inertia / drag loads) are considered. But as per F105 in manual calculation of frequency and unit diameter stress the CSF (stiffness of concrete to pipe)(Sec. 6.7) (the effect of concrete bending stiffness) is considered. These are not very in line.
(Ref is made to DNV-RP-F105 (Feb 2006))

Regards,
Amir

 

[APipeliner]

Understand. I meant if you considered CSF you should consider it in FEM as well.
Another question, don't forget to apply static deflection and axial force for on your model prior to natural frequency extraction.