FREE
S P A N
THREATS
There has been a strong drive to extend the operating lives of subsea
pipelines. Pipeline free spans are a potential source of fatigue damage, as
Graham Wilson, Jee Ltd, UK
, explains while discussing numerous free
span assessment projects.
P
ipeline free spans are a common threat to
pipeline integrity in certain parts of the world,
and can occur at any time during the pipeline’s
life, particularly when the seabed is mobile or the
local water particle velocities are high. As the length
of a free span increases, so does the susceptibility
of the pipeline to buckling (under the application of
operational and environmental loading) and accelerated
fatigue damage due to direct wave loading and vortex-
induced vibrations (VIV). These vibrations are caused by
the shedding of vortices in the wake behind the pipe
as the water flows past, resulting in the pipe oscillating
at the natural frequency of the spanning section when
the combination of current and wave-induced water
particle velocities is appropriate.
The challenge for the industry is to determine
whether such free spans could lead to pipeline failure
during the original or extended operating life of the
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