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*Elastic Surface |
 
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*Elastic Surface
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*Elastic Surface |
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To specify the properties of an elastic seabed.
Refer to Seabed Interaction for further information on this feature.
Note also that the old *ELASTIC SURFACE keyword has effectively been superseded by the new seabed definition keywords which facilitate the modelling of arbitrary seabed profiles.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.
Two lines, the second of which is optional, with the following format:
[Seabed Stiffness], Longitudinal Coefficient, Transverse
Coefficient [, Lateral Seabed Stiffness] [, Suction
Stiffness] [, Suction Zone Extent] [, 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. Omitting the Seabed Stiffness input means a non-linear seabed is being input, in which case the *EMBEDMENT keyword is required. 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 |
Seabed Stiffness: |
The elastic stiffness per unit length of the seabed. |
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 (f). |
Lateral Seabed Stiffness: |
This input allows you to specify a lateral resistance to horizontal motion for the elastic seabed. This defaults to a value of 0. See Note (g). |
Suction Stiffness: |
This input allows you to input a seabed stiffness for elements which have moved off an elastic seabed but which are still in a so called "suction zone" between the mudline and the elevation at which suction forces disappear. Suction Stiffness defaults to a value of 0. See Note (h). |
Suction Zone Extent: |
This is related to the previous input, Suction Stiffness. Suction Zone Extent represents the elevation or height above the mudline at which suction forces become zero. This input is meaningless unless the soil Suction Stiffness is nonzero, but is required if the Suction Stiffness is non-zero. |
Maximum Longitudinal Characteristic Length: |
The maximum characteristic length used to simulate frictional restraint in the longitudinal direction on the seabed. This entry is optional. See Note (i). |
Maximum Transverse Characteristic Length: |
The maximum characteristic length used to simulate frictional restraint in the transverse direction on the seabed. This entry is optional. See Note (i). |
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 (j). |
Longitudinal Mobilisation Length: |
The mobilisation length used to simulate frictional restraint in the longitudinal direction on the seabed. This entry is optional. See Note (i). |
Transverse Mobilisation Length |
The mobilisation length used to simulate frictional restraint in the transverse direction on the seabed. This entry is optional. See Note (i). |
Input: |
Description |
Y Co-ordinate |
Y Co-ordinate along seabed |
Seabed Elevation |
Elevation point along seabed |
(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)The seabed stiffness is input as a stiffness per unit length of riser or pipeline, in units of [Force]/[Distance]/[Distance] or [Force]/[Distance]2.
(e)This keyword and the *RIGID SURFACE keyword are mutually exclusive. If you have included a Rigid Surface in your analysis, then you may not include an Elastic Surface also.
(f)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.
(g)The Lateral Seabed Stiffness entry specifies a lateral resistance to horizontal motion on the elastic seabed. Refer to Lateral Resistance for further information on this feature.
(h)The suction or restraining force experienced by a riser element in a “suction zone” just above the mudline is modelled with a linear spring resistance similar to that provided against downward vertical motion by the elastic seabed itself. Refer to Suction Zone for further information on this feature.
(i)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.
(j)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.