Page 155 - Pipelines and Risers

128 Chapter 9

9.4.2 Cross-flow VIV in Combined Wave and Current
This section applies to all cross-flow loads in all types of regions (a4.5 & 0.5ca4.8 wave
dominant and d.8 current dominant).

Onset of Cross-flow Lock-on
For steady current dominated flow situations on-set of cross-flow VIV of significant
amplitude occurs typically at a value of reduced velocity, VR, between 3.0 and 5.0, whereas
maximum amplitude vibrations occur at a value between 5.0 and 7.0. For wave dominated
flow situations, cross flow vibrations may be initiated for VR as low as between 2 and 3 and
are in this region apparently linked to the in-line motions. For high values of VR the motion
are again de-coupled.

(9.9)

(pJp) is the specific mass to be taken as:

(9.10)

Thus the onset of cross-flow motion will not occur if the reduced velocity is below VR,,,,~~~.

Stress Range
If it is established that cross-flow VIV may occur, the span will have to be checked for fatigue
damage. An important parameter is the stress range, S(Un), associated with the response
amplitude. The stress range may be approximately estimated as:
S=2.S, .R. f,(V,,Kc,a) (9.1 1)

where:
Sm= stress amplitude (stress due to unit diameter mode shape deflection) to which is to
be estimated by a dedicated FE analysis package.
R= amplitude reduction factor accounting for damping and gap ratio.

The characteristic (maximum) amplitude response fy(VR,KC, a ) in combined current and
wave flow may be taken from Figure 9.4 (DNV, 1998).