Page 449 - Pipelines and Risers

416 Chaprer 22

calculated assuming the drift motions are Rayleigh distributed. The total fatigue damage from
each window is then calculated assuming the same drift motions apply to each seastate in the
window. For each scatter diagram window, the mean and RMS drift offset are conservatively
selected based on the extreme values of any of the seastates in the window.

22.2.3 VIV Induced Fatigue
Vortex-induced vibration (VIV) is probably the single most important design issue for
metallic catenary risers, particularly for high current locations. High frequency vibration of
the riser pipe due to vortex shedding leads to high frequency cyclic stresses, which can result
in high rates of fatigue damage.
Vortex-induced vibration occurs anytime when a sufficiently bluff body is exposed to a fluid
flow that produces vortex shedding at, or near, a structural natural frequency of the body (see
Figure 22.1 & 22.2). Deepwater risers are especially susceptible to VIV because:
1. currents are typically higher in deepwater areas than in shallower areas;
2. the increased length of the riser lowers its natural frequency thereby lowering the
magnitude of current required to excite VIV; and
3. deepwater platforms are usually floating platforms so that there are no structures adjacent
to the riser to which it could be clamped.

IN-LINE
DIRECTION

U 3 3 3

SYMMETRIC VORTEX SHEDDING,
INDUCES IN-LINE VIBRATIONS
mow
VELOCITY

I CROSSFLOW

333

0 3333 DIRECTION

NON-SYMMETRIC VORTEX SHEDDING,
INDUCES CROSSFTOW VIBRATIONS
FLOW
VELOCITY

Figure 22.2 Typical flow behind a cylinder.