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Off-shore oil and gas drilling and production face
hazards caused by exposure of submerged rig components to underwater currents.
Critical among these components are the marine risers, consisting of a
series of steel pipes linking the surface platform to the sea bed. During
drilling, a marine riser contains the drill-string and carries the mud
(lubricant) and debris from the rock face, while during production it carries
the oil or gas. The integrity of the marine riser under a variety of conditions
is crucial to the entire operation. However, our current ability to predict
stress levels and fatigue rates in marine risers is inadequate, resulting in
design criteria that rely on anecdotal evidence and simplified models. This is
especially true for sheared flows, where the local fluid velocity of
the ocean current varies with depth. Pressure on development costs and
increasingly hostile field environments, including water depths over 1000m,
demand more refined design strategies.
The behaviour of a segmented elastic
cylinder (such as a marine riser) in the presence of fluid flow is a classical
problem in hydrodynamics. In certain conditions vortices develop which are shed
alternately from either side of the cylinder. The resultant lift and drag
forces excite forced oscillations of the cylinder known as vortex-induced
vibration. This is a complex interaction, dependent on both flow
parameters and mechanical properties of the cylinder. When the vortex-induced
vibration frequency nears one of the natural (normal mode) frequencies of the
structure, a resonance phenomenon known as lock-in takes place,
enhancing the vibration amplitude (and hence its destructive potential). |
