﻿ Line data: Seabed

# Line data: Seabed

The data on this page determine the form of the tangential reaction between the line and the seabed. The tangential reaction is the component of the seabed reaction force that lies in the seabed tangent plane at each point of contact (i.e. the plane perpendicular to the seabed normal). The archetypal example of a tangential reaction force is the friction which resists the sliding of the line across the seabed (as opposed to the normal reaction force, which resists penetration of the line into the seabed). On this page you can modify the form of the applied friction force and define more general reaction forces to model phenomena such as soil resistance, lateral breakout (perpendicular to the axis of the line) and axial slip (parallel to the axis of the line).

### Decouple lateral and axial friction

When this option is selected, the decoupled friction model will be applied to this line whenever it makes contact with the seabed. If the option is not selected, then the coupled friction model will be used.

### Apply contact forces at centreline

Determines whether contact resistance forces (including friction and general tangential resistance profiles) generate a moment about the line centreline. Applying the contact force at the contact point is appropriate in an idealised situation between two impenetrable solid surfaces. However, there may be other effects that are not being modelled, such as embedment of the line into soft soil, which make it more physically appropriate to ignore any twisting forces.

 Note: This option is only available for lines that include torsion; if the line does not include torsion, then the friction force is always applied at the centreline.

## Seabed sections

This table allows seabed contact properties to be controlled on a per arc length basis and permits general tangential resistance profiles to be defined. Each row in the table specifies a particular arc length range within which to apply bespoke seabed contact behaviour.

### Arc length from/to

Defines the arc length range, $[L_-, L_+)$, within which the bespoke contact data should be applied. Nodes of the line whose reference arc length, $L$, satisfies $L_- \leq L \lt L_+$, will be affected by the remaining data associated with this row. Setting $L_-$ (arc length from) equal to ~ means beyond and including end A of the line; setting $L_+$ (arc length to) equal to ~ means up to and including end B of the line.

 Warning: It is inadvisable to choose $L_-$ or $L_+$ to lie exactly on a node. If this happens, numerical precision issues may make it hard to predict which side of the boundary the node in question will fall.

### Normal stiffness

The seabed normal stiffness. Either a constant value or variable data, overriding the seabed normal stiffness value of the environment. A value of N/A means that the environment value will continue to be used.

 Note: The nonlinear soil model cannot be specified on a per arc length basis; only elastic stiffnesses are permitted. If an elastic stiffness has been defined in the seabed sections table, then the nonlinear soil model will be not be used for the nodes within that seabed section and the elastic stiffness will be applied instead. Otherwise, the choice of whether to use the nonlinear soil model will be determined by the environment data.

### Lateral stiffness

The seabed shear stiffness used to compute frictional forces in the lateral direction. A value of N/A means that the original seabed shear stiffness determined by the environment data will be used; a value of ~ indicates that the normal stiffness value is to be used (in the case that the normal stiffness is nonlinear, then the value corresponding to zero penetration is used).

### Axial stiffness

The seabed shear stiffness used to compute frictional forces in the axial direction. A value of N/A means that the original seabed shear stiffness determined by the environment data will be used; a value of ~ means that the axial stiffness is the same as the lateral stiffness.

### Damping

The constant of proportionality of the damping force as a percentage of critical damping. A value of N/A means that the original seabed damping determined by the environment data will be used. Seabed damping is always zero when using the implicit integration scheme.

### Lateral friction coefficient

The friction coefficient for lateral motion of the line. A value of N/A means to use the lateral friction coefficient of the underlying line type.

### Axial friction coefficient

The friction coefficient for axial motion of the line. A value of N/A means to use the axial friction coefficient of the underlying line type; a value of ~ means that the axial friction coefficient is the same as the lateral friction coefficient.

 Warnings: Each node of the line can be associated with at most one bespoke value (i.e. values that are not N/A) for each of the above properties (normal, lateral and axial stiffnesses; damping; lateral and axial friction coefficients). Multiple N/A values are permitted and will effectively be ignored. Bespoke friction coefficients are not applied in analytic catenary calculations. For technical reasons, the friction coefficients of the underlying line type are always used by the analytic catenary model.

### Lateral/axial resistance profiles

The lateral and axial resistance profiles that apply within the specified arc length range. These are specified by variable data tables, with displacement as the independent value and normalised resistance as the dependent one. Normalised resistance is the ratio of the magnitude of the tangential reaction to the the magnitude of the normal reaction. It is also known as effective friction coefficient because of the similarity with the standard definition of a friction coefficient, $\mu$.

For each profile table you must also define an unloading stiffness, $\lambda$. A value of ~ can be used to specify that the initial slope of the profile, taken at zero displacement, will be used.

The variable data table must define zero normalised resistance at zero displacement. This is to avoid a discontinuous force being applied following a change in the direction of motion. Failure to satisfy this condition will lead to an error message being displayed.

 Notes: Resistance profiles are designed to be used in conjunction with Coulomb friction. Coulomb friction can be disabled, if desired, by simply setting the seabed friction coefficients to zero. The seabed friction policy also applies to lateral and axial seabed resistance profiles. It is possible for a single node to have multiple resistance profiles associated with it. If this is the case, then the resultant tangential resistance forces are summed.