Page 222 - Pipelines and Risers

Installation Design 195

P@ = ASP, i- 4QrPwr + LP,, (12.16)
4
where:
A, = Cross section area of stee
A,, = Cross section area of coating [m]
A,,,, = Cross section area of concrete [m]
Di = Pipeline internal diameter [m]
ts = Pipeline steel wall thickness [m]
Corrosion coating thickness [m]
&on= Concrete coating thickness [m]
ps= Steel density [Kg/m3]
pm~ Corrosion coating density [Kglm3]
peon= Concrete coating density (with 4% water [Kg/m3]
finp= Density for input [Kg/m3]

This density together with the steel pipe outer diameter and wall thickness as input makes
ABAQUS able to calculate the dry weight of the pipe. The dry weight of the pipe is here
thought of as the weight of the pipe in air.

During the installation, a part of the pipeline will be above the sea-surface and rest of the
pipeline will be under the sea-surface. From a point at the stinger, the pipeline will be into the
water. The pipeline will then be exposed to a buoyancy force and hydrostatic pressure.
This is applied to the pipeline in ABAQUS by using a command named PB. This command
applies a distributed pressure load and a distributed buoyancy load to the submerged part of
the pipeline.

When computing the distributed buoyancy loads (load type PB) ABAQUS assumes closed-
end conditions. The pressure field varies with the vertical co-ordinate z.
For the hydrostatic pressure the dependence on the vertical co-ordinate is linear in z,
P'P dzo-z) (12.17)

Here zo is the vertical location of the free surface of the fluid, p is the density of the water and
g is acceleration due to gravity.

The calculation of the pressure load and buoyancy load on the pipeline is based on the outer
diameter of the pipeline. This is a problem since the outer diameter for computing the
buoyancy load has to be different from the outer diameter that is used for computing the right
pressure load on the pipeline. This is because the pipeline is covered with concrete and
coating which contribute to the buoyancy load but not to the pressure load. The outer diameter
of the steel pipe is defined to give the right pressure load. This means that the buoyancy load
is too small when the command PB is used to specify the buoyancy and pressure load on the