Page 158 - Pipelines and Risers

Vortex-induced Vibrations (Vlv) and Fafigue 131

The criteria used for Cross-flow VIV is to keep the natural frequency of the spans above the
VIV frequency corresponding to the onset of Cross-flow vibrations.

In order to ensure that the boundary conditions for each individual span are properly
accounted for, the cross-flow vortex shedding is subject to a more rigorous modal analysis in
which the longitudinal on-bottom configuration is assessed as part of the 2D static analysis.

The spans natural frequencies are checked against the corresponding frequency for onset of
cross-flow VIV for the various design conditions (installation, waterfilled and operation).
Those with a potential for experiencing significant cross-flow VIV are identified and
measures to prevent or limit fatigue of the pipe will be evaluated on a case by case basis.

9.6 Example Cases

9.6.1 General
To give an insight into design against free span fatigue, a 40" pipeline has been assessed
under operating conditions. For a complete analysis the damage during the empty and the
water-filled conditions should be assessed and included in the overall damage accumulation.
Each case represents a typical pipeline with characteristic dimensions and flow parameters as
provided below, see Table 9.4. (Reid et al. 2000).

Table 9.4 Pipeline Input Parameters.

Input parameters \ Pipe diameter 40
Outer pipe diameter (m) 1.016
Wall thickness (m) 0.030

-
Concrete coatinn thickness (m)
..
Concrete coating density (kg/rn3) I 2500
Residual lay tension (kN)
Content density operating (kg/m3)
Internal operating pressure (bar)
Operating Temperature ("C) 25
Internal hvdrotest uressure (bar) 200

A typical uneven seabed has been selected in order to obtain a wide range of span lengths
giving high fatigue damage. The soil is medium stiff clay.
The configuration and loads from the static analysis are used as the basis for the Eigen mode
analysis. The modal analysis is carried out for the horizontal (in-line) and the vertical (cross-