Page 148 - Pipelines and Risers

Vortex-induced Vibrations (VIV) and Fatigue 121

In Grytten and Reid (1999) typical values of soil damping ratios for various types of soil and
span lengthlpipe diameter (L / D) ratios, are given. The damping values, as used in VIVA, can
be interpolated for the correct span length. For continuous spans, taking the largest span
length will give the most conservative value for soil damping.

It should be emphasised that the determination of pipeline soil interaction effects is
encumbered with relatively large uncertainties stemming from the basic soil parameters and
physical models. It is thus important that a sensitivity study is performed to investigate the
effect of the above mentioned uncertainties.

Structural Damping
Structural damping ratio is the damping due to internal friction in the pipe steel material. A
value of 0.005 (0.5 %) to be used if no other information is available, which is considered to
be very conservative.

Effective Mass
The effective mass is defined as:
m, = m(s) = m,$,, m, i m, + mcon (9.3)
i
where:
me structural mass (including coating),
m= mass of content,
m, added mass, thus
n
m, =-D2.p.C, (9.4)
4
where:
C, is the added mass coefficient

If it is assumed that the entire span is oscillating and vortex shedding occurs over the entire
length, the effective mass can be defined by Equation 9.4. This assumption will contribute to
a somewhat lower natural frequency and is considered to be conservative.

The Eigen period will increase as the added mass increases. The Eigen period calculation is
computed during the Eigen value analysis. Secondly, Ks, the stability parameter will increase
as the added mass increases. Thereby the effect of the damping will increase.

9.2.3 Soil Stiffness Analysis
Soil data is needed for setting up the structural model and for calculation of soil damping.
ASTM Unified Soil Classification Systcm (USCS) is very convenient system for soil
description in connection with pipeline projects.