Page 454 - Pipelines and Risers

Fatigue of Risers 421

Calculating
- Support forces
- Pipe wall forces
- Velocities and accelerations from wave and vessel motion time series

- Statistical time series analysis, estimation of spectral densities

22.5 Vortex-induced Vibration Prediction
Accurate estimation of the fatigue life of a deepwater riser experiencing vortex-induced
vibration depends critically upon accurate estimation of the response amplitude and
frequencies (or mode numbers). Accurate estimations of the response amplitude and mode
number are, in turn, dependent upon several “basic” parameters that include:

- The current profile (both magnitude and shape variation with depth)
- The frequency and magnitude of the lift force imparted to the riser by the vortex shedding
- The excitation and correlation lengths of the lift forces and vortex shedding
- The hydrodynamic damping
- The structural properties of the riser including damping, mass, tension, bending stiffness,
and the cross sectional geometry (including surface roughness)

VIV is perhaps more sensitive to the current profile than to any other parameter. For short
riser spans the current magnitude determines whether or not VIV will occur, and determines
whether the response is in-line or transverse to the flow direction (or both). The cross-flow
response is more significant than the inline response, see Figure 22.3. For deepwater risers, a
very low current will, at least theoretically, produce some VIV due to the low natural
frequency of the riser in bending. The variation of the current along the riser span (i.e. with
depth) then determines which modes will be present in the response. Here it should be noted
that:

Current profiles that are conservative for platform offsets are not necessarily conservative
for deepwater riser VIV prediction (this is because VIV of deepwater risers is much more
dependent upon the shape of the current profile with depth)
The current profile should be varied during the analysis to determine the sensitivity of the
results to current profile shape
Currents change with time, so some kind of probabilistic description of the current
magnitudes and/or profile shapes is necessary for a sufficiently accurate VIV analysis
It is possible that even if numerous modes are potentially exited by a current profile
(typical of deepwater riser in a significant current), a single mode (or a small number of
modes) can dominate the response due to “lock-in” in which the vortex shedding tends to
adjust to the vibration frequency within certain limits (dependent upon mass ratio and
Reynolds number among other things)
Even in a highly sheared current it is possible for a single mode (or a small number of
modes) to dominate the response