Page 157 - Pipelines and Risers

130 Chapter 9

The axial effective force, i.e. the sum of the external forces acting on the pipe is also
accounted for. It should be noted that the effective force will change considerably during
the various phases of the design life.

In order to achieve this, it is important to ensure that a realistic load history is modeled prior
to performing the modal analyses
9.5.2 Single Span Modal Analysis
Single span analyses are performed in order to assess the onset of in-line and cross-flow VIV
as well as to calculate fatigue damage if it is found that onset of VIV may occur.

The modal analyses of a single span with simple boundary conditions will be used to assess
the onset of in-line and cross flow VIV and associated fatigue damage. The justification for
this is as follows:

The multispan analysis can be carried out on the assumption that the pipeline along the
routing is an input to the FEM program and the actual span length is considered. In this
report the actual span lengths are unknown and the span criteria is the only interest. In
such cases the boundary condition will have to be assumed. The main reason for
performing a multispan analysis is to take into account the interaction between adjacent
spans, i.e. several spans may respond as a system. Although this may be important for the
vertical mode of vibration where the seabed between adjacent spans may form a fixed
point about which the pipeline may pivot during the vibration, this effect is less important
for the horizontal mode where the lateral seabed friction will oppose the movement and
where there is no fixed point to which to pivot about. The VIV analysis for the actual span
lengths will be considered in the inplace analysis.

0 The single span model is adequate for the VIV analysis based on that the actual span
lengths are unknown which is one condition to carried out the multispan analysis.

Both Fixed-Fixed and Pinned-Pinned boundary conditions have been analyzed together with a
range of axial effective forces.

9.53 Multiple Span Modal Analysis
Two dimensional multispan analyses are performed in order to assess the onset of Cross-flow
VIV as well as to calculate fatigue damage, if it is found that onset VIV may occur. The
multispan analyses will take account of the interaction between adjacent spans. The analysis
will not be performed in this report, but the analysis will be a part of the inplace analysis
report.

The multispan analysis can be carried out on the assumption that the pipeline along the
routing is an input to the FEM program and the actual span length is considered. In this report
the actual span lengths are unknown and the span criteria is the only interest. The VIV
analysis for the actual span lengths will be considered in the in place analysis report.