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Line data: Seabed |
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Line data: Seabed |
The data on this page can be used to control the interaction (normal and tangential resistance) between the line and the seabed, and to designate any parts of the line which may be buried. The normal reaction is the component of the seabed reaction force normal to the seabed tangent plane (i.e. the plane perpendicular to the seabed normal) at each point of contact; it resists penetration of the line into the seabed. The tangential reaction is the component that lies within the seabed tangent plane. The archetypal example of a tangential reaction force is the friction which resists the sliding of the line across 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).
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.
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. |
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.
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. |
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. |
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).
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.
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.
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.
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. |
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 resistance to the magnitude of the normal resistance. 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, $\lambda$. This can be set to ~, N/A, or any non-negative number. A value of ~ specifies that the initial slope of the profile, taken at zero displacement, will be used; a value of N/A specifies that the resistance unloads elastically upon reversal of motion (i.e. the resistance tracks back down the original profile whenever the displacement decreases).
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. In this case, the resultant tangential resistance forces are summed. | |
| In a restart analysis, the internal history of the tangential resistance calculation (the analogue of the target position for Coulomb friction) will be persisted from the parent model, except in certain circumstances. |
This feature can be used to model buried lines. Each row in the table specifies an interval of the line that has been buried. The material under which the line is buried is known as cover; it will tend to resist any upwards motion of the line, thereby protecting the line against upheaval buckling (UHB).
| Note: | The cover on top of a line is specified on a per arc length basis. This means that the cover above each line node moves with that node when it translates. The cover resistance always acts vertically and is a one dimensional function of the vertical separation between the line and the seabed. Motions of the node that are tangent to the seabed do not change the vertical separation and therefore do not change the value of the cover resistance. |
Defines the arc length range, $[L_-, L_+)$, within which the nodes of the line are considered to be buried. 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. |
The penetration into the seabed (of the nodes in the covered section) immediately after the line was buried. This specifies the vertical position of the underside of the line, relative to the seabed. The underside of the line is assumed to lie one outer contact radius (i.e. half the outer contact diameter) below each node.
The penetration can be either a constant value or a variable data function of covered arc length (the arc length measured from the start of the covered section). It defines an origin for the cover resistance model that is analogous to the as laid position used to compute meaningful friction forces in statics.
Covered sections are primarily intended to be used in restart analyses. With this in mind, a special value of ~ indicates that the line is considered to have been buried in whatever position it was in at the start of the analysis. For a restart, this will be its position at the end of the parent analysis.
| Warning: | For a standard analysis, ~ should be used with care because the position at the start of the analysis will generally be its reset position. It is therefore advisable to set an actual value for the as-buried seabed penetration (i.e. not ~); or, even better, run a static analysis without cover first, then add the cover in a restart. |
The seabed penetration can be positive or negative. Negative values mean that the underside of the line was above the seabed surface when it was buried. In this situation, the cover is considered to completely fill the gap between the line and the seabed. The associated theory topic explains this in more detail.
The cover type embodies the properties of the material under which this covered section is buried.
The height of cover, measured from the uppermost point of the line's cross-section (at each node) to the surface of the cover, immediately after burial (i.e. with the line at its as-buried seabed penetration). The cover height can be either a constant value, a variable data function of covered arc length (the arc length measured from the start of the covered section), or can be provided by a cover height model. In general, cover resistance forces depend upon both the type and height of cover above the line.
Sometimes, you might just want to apply a cover resistance load profile that is independent of cover height, or for which the notion of cover height has no physical meaning. You can indicate this by setting the cover height to N/A, which informs OrcaFlex that this property is not relevant; in this case, an error will be raised if any cover height dependent data has been associated with this covered section. You might use this approach when modelling the weight of concrete mattresses laid over a line.
| Notes: | The definitions of cover height and as-buried seabed penetration are illustrated diagrammatically in the associated theory topic. |
| It is possible for a single node to belong to multiple covered sections. In this case, the interactions between the cover types are ignored and the resultant resistance forces are summed. This approach has been adopted for maximum flexibility, such as when laying additional concrete mattresses over an already buried line. | |
| In a restart analysis, the internal history of the cover resistance calculation will be persisted from the parent model, except in certain special circumstances. |
Specifies whether a warning should be reported if any part of the line's outer contact surface emerges from the cover. This choice is for the line as a whole, not for each covered section individually.
| Note: | Covered sections with a cover height of N/A are always considered to cover the line. |
A cover height model allows you to specify the cover height in terms of an exact mathematical formula. It is a Python variable data source with the following outputs and inputs:
More details and an example of a cover height model can be found in the associated theory topic.