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*Rigid Surface |
 
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*Rigid Surface
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*Rigid Surface |
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To specify the properties of a rigid seabed.
Refer to Seabed Interaction for further information on this feature.
Note also that the old *RIGID SURFACE keyword has effectively been superseded by the new seabed definition keywords which facilitate the modelling of arbitrary seabed profiles. Refer to *SEABED PROPERTIES, *SEABED PROFILE and *SEABED STIFFNESS for further information.
A single line with the coefficients of friction and maximum characteristic lengths may optionally be followed by a single line (to define a sloping seabed) or several lines (to define an arbitrary seabed profile).
Line defining the seabed properties:
Longitudinal Friction Coefficient, Transverse Friction Coefficient [, Maximum Longitudinal Characteristic Length]
[, Maximum Transverse Characteristic Length] [, Constant Friction Force Coefficient] [, Longitudinal Mobilisation
Length] [, Transverse Mobilisation Length]
Line defining uniform seabed slope:
Seabed Slope
Line defining a point of the arbitrary seabed:
Y Co-ordinate, Seabed Elevation
A model may not contain both rigid and elastic surfaces. The Seabed Slope is measured in degrees. A sloping seabed is assumed to pass through the global origin. A flat seabed is assumed if no slope is specified.
Both Maximum Characteristic Length inputs default to 10ft or 3.048m. Where a Mobilisation Length is explicitly specified, it takes precedence over the corresponding Maximum Characteristic Length (in this case the latter value is immaterial as it is unused). If a Mobilisation Length is omitted, then the mobilisation length is governed by the Maximum Characteristic Length and the finite element mesh discretisation. Refer to Seabed Friction for further details.
Input: |
Description |
Longitudinal Coefficient of Friction: |
The coefficient of friction in the longitudinal direction. This defaults to a value of 0. |
Transverse Coefficient of Friction: |
The coefficient of friction in the transverse direction. This defaults to a value of 0. |
Slope: |
The slope of a uniformly sloping seabed, in degrees. This defaults to a value of 0o, which gives a horizontal seabed. See Note (e). |
Maximum Longitudinal Characteristic Length: |
The maximum length used to simulate frictional restraint in the longitudinal direction on the seabed. This entry is optional. See Note (g). |
Maximum Transverse Characteristic Length: |
The maximum length used to simulate frictional restraint in the transverse direction on the seabed. This entry is optional. See Note (g). |
Constant Friction Force Coefficient: |
The constant friction force coefficient in the transverse direction to be used in addition to the classical limiting friction. This entry is optional and defaults to 0. See Note (h). |
Longitudinal Mobilisation Length: |
The mobilisation length used to simulate frictional restraint in the longitudinal direction on the seabed. This entry is optional. See Note (g). |
Transverse Mobilisation Length |
The mobilisation length used to simulate frictional restraint in the transverse direction on the seabed. This entry is optional. See Note (g). |
(a)The longitudinal friction coefficient refers to a local direction parallel to the pipe axis, and the transverse coefficient to a direction normal to the pipe axis.
(b)By default, no seabed is included in your analysis.
(c)The default friction coefficients give a smooth seabed, with no frictional resistance.
(d)This keyword and the *ELASTIC SURFACE keyword are mutually exclusive. If you have included an Elastic Surface in your analysis, then you may not include a Rigid Surface also.
(e)The seabed slope is input in degrees; a positive slope defines a seabed sloping upwards in the positive global Y direction, while a negative slope gives a seabed sloping in the opposite direction. A uniformly sloping seabed is assumed to pass through the global origin of co-ordinates. If a non-zero seabed slope is specified, then the definition of an arbitrary seabed bathymetry is not permitted.
(f)Note that, when determining the effect of hydrodynamic loading, Flexcom calculates water particle velocities and accelerations on the basis of the user-specified (constant) water depth, irrespective of whether an arbitrary profile rigid surface is present in the model or not. In other words, the presence of an arbitrary rigid surface is not considered to affect the hydrodynamic loading on a structure.
(g)The Maximum Characteristic Length and Mobilisation Length inputs relate to the operation of the Flexcom seabed friction model, and allow user control over the characteristics of non-linear springs used to model seabed frictional restraint. All these entries are optional. The Maximum Characteristic Length inputs default to 3.048m or 10ft. Where a Mobilisation Length is explicitly specified, it takes precedence over the corresponding Maximum Characteristic Length (in this case the latter value is immaterial as it is unused). If a Mobilisation Length is omitted, then the mobilisation length is governed by the Maximum Characteristic Length and the finite element mesh discretisation. Refer to Seabed Friction for further details.
(h)With the Constant Friction Force Coefficient input, the total limiting friction, in the transverse direction, is given by:
Ftotal = μN + CFF
where μN is the Coulombian limiting friction force, and CFF is the constant friction force. This equation is applicable only to transverse friction. The constant friction force is evaluated for each seabed contact element, and is given by the element length multiplied by the constant friction force coefficient.