During pipeline routing
evaluation, consideration has to be given to the shortest pipeline length,
environment conservation, and smooth sea bottom to avoid excessive free
spanning of the pipeline. If the free span cannot be avoided due to rough sea
bottom topography, the excessive free span length must be corrected.
Free
spanning causes problems in both static and dynamic aspects. If the free span
length is too long, the pipe will be over-stressed by the weight of the pipe
plus its contents. The drag force due to near-bottom current also
contributes to the static load. To mitigate the static span problem,
mid-span supports, such as mechanical legs or sand-cement bags/mattresses,
can be used.
Free
spans are also subject to dynamic motions induced by current, which is referred
to as a vortex induced vibration (VIV). The vibration starts when the vortex
shedding frequency is close to the natural frequency of the pipe span. As the
pipe natural frequency is increased, by reducing the span length, the VIV will
be diminished and eliminated. Adding VIV suppression devices, such as strakes
or hydrofoils, can also prevent the pipe from vibrating under certain
conditions. The VIV is an issue even in the deepwater field since there exists
severe near-bottom loop currents.
There’s some pictures of
prevention methods from the table above.
Pipeline mattresses for
spanning (source: http://submar.com/)
Clamp
on support with telescoping legs (left) and clamp-on support with auger screw
legs (right)
Bouyancy
module or strakes
VIV Supression (Source: www.marktool.com)
Based on soil
conditions, water depth, and span height from the seabed, the appropriate
method should be selected. If the span off-bottom height is relatively low, say
less than 1 m (3 ft), sand-cement bags or mattresses are recommended. If the
span off-bottom height is greater than 1 m (3 ft), clamp-on supports with
telescoping legs or auger screw legs are more practical.
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