Page 39 - Pipelines and Risers

Introduction 13

The hydrodynamic forces are derived using traditional fluid mechanics with suitable
coefficient of drag, lift and diameter, roughness and local current velocities and accelerations.

The effective flow to be used in the analysis consists of two components. These are:

The steady, current which is calculated at the position of the pipeline using boundary layer
theory;
The wave induced flow, which is calculated at the seabed using a suitable wave theory.

The selection of the flow depends on the local wave characteristics and the water depth.

The wave and current data must be related to extreme conditions. For example, the wave with
a probability of occurring only once in 100 years is often used for the operational lifetime of a
pipeline. A less severe wave, say 1 year or 5 years, is applied for the installation case where
the pipeline is placed on the seabed in an empty condition with less submerged weight.

Friction, which depends on the seabed soils and the submerged weight of the line provide
equilibrium of the pipeline. It must be remembered that this weight is reduced by the fluid lift
force. The coefficient of lateral friction can vary from 0.1 to 1.0 depending on the surface of
the pipeline and on the soil. Soft clays and silts provide the least friction whereas coarse sands
offer greater resistance to movement.

For the pipeline to be stable on the seabed the following relationship must exist:
Y(FD -4)s P(Kb - FL) (1.3)

where:
y = factor of safety, normally not to be taken as less than 1.1
FD = hydrodynamic drag force per unit length (vector)
F, = hydrodynamic inertia force per unit length (vector)
p = lateral soil friction coefficient
=
Wsh submerged pipe weight per unit length (vector)
FL = hydrodynamic lift force per unit length (vector)

It can be seen that stability design is a complex procedure that relies heavily on empirical
factors such as force coefficient and soil friction factors. The appropriate selection of values is
strongly dependent on the experience of the engineer and the specific design conditions.