Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 199

3.15 VORTEX-INDUCED VIBRATIONS
In fluid dynamics, vortex-induced vibrations (VIVs) are motions induced on
bodies interacting with an external fluid flow and are produced by periodical
irregularities in the flow.
A classic example is the VIVs of an underwater cylinder. If you put a pipe
into the water and move it through the water in a direction perpendicular to its
axis, you can see this vortex. Since real fluids always present some viscosity, the
flow around the cylinder will be slowed down while in contact with its surface,
forming the so-called boundary layer. At some point, however, the boundary
layer can separate from the body because of its excessive curvature. Vortices
are then formed, changing the pressure distribution along the surface. When
the vortices are not formed symmetrically around the body, different lift forces
develop on each side of the body, thus leading to motion transverse to the
flow. This motion changes the nature of the vortex formation in a way that
leads to a limited motion amplitude.
The tubular members of the flare/vent booms should be checked for VIVs. If
the members and booms are found to be dynamically sensitive, they should be
checked for fatigue during detailed design.
VIVs are an important source of fatigue damage to offshore platforms, espe-
cially for oil exploration and production risers. These slender structures experi-
ence both current flow and top-end vessel motions, which give rise to the flow-
structure relative motion and cause VIVs. The top-end vessel motion causes the
riser to oscillate and the corresponding flow profile appears unsteady.
The possibility of VIVs due to the design current velocity profiles should be
considered for all appurtenances, including risers, sump pipes, caissons and any
individual members considered potentially susceptible.
One of the classical open-flow problems in fluid mechanics concerns the
flow around a circular cylinder, or, more generally, a bluff body. At very low
Reynolds numbers, according to the diameter of the circular member, the
streamlines of the resulting flow are perfectly symmetrical, as is expected
from potential theory.
The Strouhal number, named after

Č ě

k (Vincent) Strouhal, a Czech scien-
tist, relates the frequency of shedding to the velocity of the flow and a charac-
teristic dimension of the body (diameter, in the case of a cylinder). The Strouhal
number is defined as St = f st D/U, where f st is the vortex shedding frequency (or
the Strouhal frequency) of a body at rest, D is the diameter of the circular cylin-
der and U is the velocity of the ambient flow. The Strouhal number for a cylin-
der is 0.2 over a wide range of flow velocities. When the vortex shedding
frequency comes close to the natural frequency of vibration of the structure,
large and damaging vibrations can occur.
In general, wind, current or any fluid flow past a structural component may
cause unsteady flow patterns due to vortex shedding. This may lead to oscilla-
tions of slender elements normal to their longitudinal axis. Such vortex-induced
oscillations (VIOs) should be investigated.

en