Contact in the seafloor plane is modelled independently in the axial and lateral directions. Contact is initially modelled with linear springs.

Sliding will occur when an axial or lateral spring force reaches the friction force value. Springs will be reinstated if the line starts sliding in the opposite direction, or if the friction force increases and is greater than the spring force.

The spring stiffness is calculated as

where \(\mathrm {stiff}\) is the stiffness in axial or lateral direction and \(\mathrm {L_h}\) is the length of the element’s horizontal projection.

The seafloor friction forces are calculated as

and are directed against the axial or lateral displacements. \(\mathrm {fric}\) is the axial or lateral friction and \(\mathrm {F_{vert}}\) is the vertical contact force between the pipe and the bottom.