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Coastal Engineering: Theory and Practice
7.7.3 Scour around pipelines due to current action
Pipelines which are laid on the seabed will in general, be exposed to the
lower part of the current profile. The pipeline thus protrudes a certain
height into this profile and détermines the flow pattern around it, resulting
in the érosion and sédimentation of bottom material.
The sudden increase in transport capacity of the water passing through
the pipeline can be a reason for scour under a pipeline. Sédimentation/
settling of sédiments occurs when there is a decrease in transport capacity.
These gradients in transport capacities exist only temporarily until the bottom has changed into its equilibrium configuration according to the instantaneous flow conditions. The transport capacity will be denoted by “S” and
subscripts “o” and “p” will be generally used to signify locations beyond the
influence of the pipe and underneath the pipe, respectively. Although bed
transport and suspended transport certainly should be treated separately,
unless stated explicitly total transport will be used here for simplicity. When
an equilibrium situation has been established, both the depth of the scour
hole, ys, and the gap, y, between the pipe and the bottom of the scour hole
remain constant. The transport capacities satisfy the equality.
S« = S„
(1)
When a scour hole is developing underneath a pipe, there is no balance
of transport capacities:
Sp > So
(2)
A more detailed description of the érosion phase distinguishes two cases;
(1) no transport upstream of the pipe (Sou — 0), and transport only underneath and possibly just downstream of the pipe (Sp, Sod # 0); (2) transport
at ail locations (Sp > So > 0). In the second case there is a supply of sédiment to the developing scour hole, with a possibility of exchange of sédiment
between the suspended and the bottom layer. Prédiction of the rate of érosion and the ultimate scour depth on the basis of a sédiment balance now
seems to be possible, given a particular-flow pattern around the pipe.
Leeuwestein and Bijker [1985] identifled three basic forms of érosion
around a submarine pipeline:
1. Luff érosion, which occurs at the upstream side of the pipe and is caused
by an eddy formation upstream of the pipe.
2. Lee érosion, which occurs at downstream side of the pipe and is caused by
reemergence of the main flow over and the turbulent wake downstream
of the pipe.
Coastal Engineering: Theory and Practice
7.7.3 Scour around pipelines due to current action
Pipelines which are laid on the seabed will in general, be exposed to the
lower part of the current profile. The pipeline thus protrudes a certain
height into this profile and détermines the flow pattern around it, resulting
in the érosion and sédimentation of bottom material.
The sudden increase in transport capacity of the water passing through
the pipeline can be a reason for scour under a pipeline. Sédimentation/
settling of sédiments occurs when there is a decrease in transport capacity.
These gradients in transport capacities exist only temporarily until the bottom has changed into its equilibrium configuration according to the instantaneous flow conditions. The transport capacity will be denoted by “S” and
subscripts “o” and “p” will be generally used to signify locations beyond the
influence of the pipe and underneath the pipe, respectively. Although bed
transport and suspended transport certainly should be treated separately,
unless stated explicitly total transport will be used here for simplicity. When
an equilibrium situation has been established, both the depth of the scour
hole, ys, and the gap, y, between the pipe and the bottom of the scour hole
remain constant. The transport capacities satisfy the equality.
S« = S„
(1)
When a scour hole is developing underneath a pipe, there is no balance
of transport capacities:
Sp > So
(2)
A more detailed description of the érosion phase distinguishes two cases;
(1) no transport upstream of the pipe (Sou — 0), and transport only underneath and possibly just downstream of the pipe (Sp, Sod # 0); (2) transport
at ail locations (Sp > So > 0). In the second case there is a supply of sédiment to the developing scour hole, with a possibility of exchange of sédiment
between the suspended and the bottom layer. Prédiction of the rate of érosion and the ultimate scour depth on the basis of a sédiment balance now
seems to be possible, given a particular-flow pattern around the pipe.
Leeuwestein and Bijker [1985] identifled three basic forms of érosion
around a submarine pipeline:
1. Luff érosion, which occurs at the upstream side of the pipe and is caused
by an eddy formation upstream of the pipe.
2. Lee érosion, which occurs at downstream side of the pipe and is caused by
reemergence of the main flow over and the turbulent wake downstream
of the pipe.