Line types: Friction data

Seabed friction coefficients

OrcaFlex applies Coulomb friction between the line and the seabed. The friction force applied never exceeds $\mu R$, where $\mu$ is the friction coefficient and $R$ the seabed reaction force.

Lines lying on the seabed often move axially more readily than they move laterally. This is represented by having different friction coefficients for motion in the lateral and axial directions. For intermediate directions, OrcaFlex interpolates between these two values. The axial friction coefficient may be set to '~', meaning that the lateral friction coefficient is used for all directions of motion.

Note: These friction coefficients apply to seabed contact only. The friction coefficient for contact with elastic solids is specified on the friction coefficients data form.

Typical values

Published data are sparse. Some information is given in Puech (1984) and Taylor and Valent(1984), both of which distinguish between starting friction and sliding friction: starting friction is greater, to represent the "breakout" force. OrcaFlex does not, however, support this distinction. Instead, in most cases, the sliding friction coefficient should be used; this will usually be conservative. The context of both of these references is the contribution of chains and cables to anchor holding power, so we assume the friction values given are axial. Transverse values will be greater, perhaps by 50% to 100%.

The values given below are recommendations from Taylor and Valent.

Line type Seabed type Starting friction
Sliding friction
Chain Sand 0.98 0.74
Mud with sand 0.92 0.69
Mud/clay 0.90 0.56
Wire rope Sand 0.98 0.25
Mud with sand 0.69 0.23
Mud/clay 0.45 0.18