PIPELINES: INTERACTION OF PIPELINES WITH SEA BED
Aug 06, 2015
INSTABILITY OF THE SEA BED
Traditional stability design method is to treat the seabed as stationary and immovable.
This assumption is not correct The stability of a pipeline cannot be considered in isolation from the
stability of the seabed on which its rests.. If the pipeline is unstable the stability of the seabed is likely to be
marginal at best. If the sea bed is unstable, the pipeline becomes unstable with it. In extreme wave and current conditions the seabed begin to move
in response to hydrodynamic forces, and the pipeline also moves due to active sediment transport.
Pipeline – partial buried
If the pipeline is initially partially buried and a top layer of the seabed
begins to move, then the following three things will occur:
1) The pipeline will be exposed to hydrodynamic forces over a greater
fraction of its diameter.
2) The fluid moving past the line will have a density greater than water
because of the presence of seabed particles.
3) A smaller fraction of pipeline will be embedded in stationary
material and the resistance to lateral movement will therefore be
much reduced.
Lay – barge method
Frequently used technique for marine pipeline construction
This method is versatile, flexible and self-contained.
Initially it is expensive to mobilize a lay – barge to remote location
but once the barge is in place, it can start work and operate as
efficiently as anywhere else.
It was originally developed in shallow water in the 1940s.
The first larger diameter installed by this method was about 100m.
The average lay rate was 6.9km/day and the peak lay rate was 7.8
km in 24 hours.
Two methods are S – Lay and J - lay