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Chapter 9

Vortex-induced Vibrations (VIV) and Fatigue

9.1 General

The objective of this Chapter is to present acceptance criteria with respect to Vortex Shedding
Induced Vibrations (VIV) of freespans and to outline the proposed methodology for the
detailed design of pipeline systems.

Traditionally, VIV of freespans is not allowed to occur at any time during the design life of a
pipeline system. In merit years a less stringent approach has become acceptable, in which
VIV has been allowed provided it is demonstrated that the allowable fatigue damage is not
exceeded.

Spans that are found to be critical with respect to VIV are usually corrected by placing rock
berms below the pipe in order to shorten the span lengths and thus increase the natural
frequency of the spans. In addition to the cost implication of placing a large number of rock
berms on the seabed, the main disadvantage of this approach is that feed in of expansion into
the spans will be restricted. It was demonstrated that allowing the pipeline to feed into the
spans reduces the effective force, which is the prime factor in the onset of pipeline buckling.
It is therefore advantageous with respect to minimizing buckling that the number of rock berm
freespan supports is kept to a minimum.

Based on the above, it is proposed that the VIV criteria are as follows:

Onset of in-line VIV is allowed during any phase of the design life provided it is
demonstrated that the allowable stress and allowable fatigue damage is not exceeded.
Onset of cross flow VIV is allowed during any phase of the design life provided it is
demonstrated that the allowable stress and allowable fatigue are not exceeded.

A flowchart listing the various analysis steps to be performed during the VIV assessment are
shown in Figure 9.1. (Grytten and Reid, 1999).