Page 109 - Pipelines and Risers

82 Chapter 5

5.3 Modeling Friction and Breakout Forces

5.3.1 Anisotropic Friction
For pipelines not penetrating the seabed much, a pure Coulomb friction model can be
appropriate. But, as the pipeline penetrates the seabed, the forces required moving the pipeline
laterally become larger than the forces needed to move it in the longitudinal direction. This
effect is due to the passive lateral soil resistance that is produced when a wedge of soil resists
the pipe’s motion. An anisotropic friction model that defines different friction coefficients in
the lateral and longitudinal directions of the pipeline allows this effect to be investigated (Fig.
5.4).

Figure 5.4 Anisotropic Friction.

It may be mentioned that the torsional moment around the pipeline longitudinal axis,
produced by the lateral soil-resistance force is ignored. However, the impact of this on the
calculation of pipe response is believed to be negligible, unless pipeline twisting during
installation is to be simulated.

5.3.2 Breakout Force
The breakout force is the maximum force needed to move the pipe from its stable position on
the seabed. This force can be significantly higher than the force needed to maintain the
movement after breakout due to suction and extra force needed for the pipe to “climb” out of
its depression. An example curve is given in Figure 5.5.

The breakout forces, can be simulated in a finite element model, according to Brennodden
(1991), which gives the following equations for the maximum breakout force in the axial and
lateral direction:

Axial soil resistance (kN/m):
Fa,,, = 1.05 * A,,,,,, ’ S, (5.5)

Lateral soil resistance (kN/m):
4,- = 0.8*( 0.2. F, -I- 1.47. S, *A,,,,, / D ) (5.6)

where:
F, = vertical contact force (kN/m)
A,,,,,, = 2. R. ACOS( I - Z/ R) (m2)