DYNAMIC Soil Stiffness for Free Span Analysis

[subsealinepipe]

Introduction:
Ref. DNV-RP-F105(Feb 2006) for subsea pipeline free span analysis, I want to calculate the DYNAMIC lateral/vertical soil stiffness. Since the calculated specific mass ratio between the pipe mass and displaced water is not in the range of 1.2 and 2, the formula in sec 7.4.10 can not be used and I want to do calculation as per sec. 7.4.6 and sec. 7.4.7.
Questions:
1) How is span support length on one shoulder is calculated? (the previous version of F105 provided a table for this, but in new version there is not any table).
2) What is cyclic shear strain (table D-2 appendix D) and how it is considered?


Regards,
Amir

 

[Eyeoftheneedle]

subsealinepipe (Amir)

1) It's worth checking this one with DNV as normally they have a good reason for removing tables when they update codes and quite often you will find that previous tables and previous equations will yield the same results as the new equations. In any event, it would be worhtwhile doing some sensitivity (for your own piece of mind) on varying Lsh to see if it's significant as certainly the equiations for vertical and lateral stiffness have changed bith with regards to the Cv and Cl inputs and the fact that poisson's ratio is now included in the denominator of the equation.
2) the cyclic shear strain is a soil parameter defined through triaxial testing of the soil and relates to the plasticity index (and so pore pressure, etc.) of the soil. There are quite a few papers (google them) which will give you the basics...alternatively get hold of a geotechnical specialist who can give you a simple explanation }