Boundary Conditions

Contents

Boundary Conditions

Description

Several native Abaqus and modified keywords can be used to define boundary conditions. These comprise:

1*BOUNDARY for defining prescribed nodal values. This is a native Abaqus command but subject to the limitation that the nodal values must be defined either

ausing an ELSET or NSET (in which case the nodes will all be assigned the same value)

busing an *INCLUDE file (in which case the nodes may have specific values that will be applied conversion to a ParaGeo spatial grid)

2*BOUNDARY, TYPE=NORMAL STRESS (which is used to assign normal surface tractions to an NSET or ELSET)

3*TOP SURFACE to assign conditions to the surface of a geomechanical model; e.g. surface stress, pore presure and temperature

4*WATER LEVEL for assigning the overlying fluid.

Boundary conditions

Usage

*BOUNDARY, NAME=Name_ID, AMPLITUDE=Curve_assignment, TYPE=Load_type

Description

Defines input of boundary conditions for the model at geometry sets. The data should be provided in a tabular form with values separated by commas as follows:

Optional Parameters
NAME	Defines a name ID for the boundary conditions set so it can be referenced.
AMPLITUDE	Assigns an amplitude curve to define the rate for the boundary conditions applied. If not defined it will be assumed a lineal ramp over the current simulation step / stage.
TYPE	- NODAL / NODE - Prescribed nodal values (displacement, pore pressure, temperature) (Default) - NORMAL STRESS - Normal stress (surface traction)

TYPE=NODAL

The values expected are:

Value 1	Value 2	Value 3	Value 4
Geometry set name	Boundary condition number	Boundary condition number	Value for the boundary condition

The boundary condition numbers are as follows:

Displacement in X	1
Displacement in Y	2
Displacement in Z	3
Pore Pressure	8
Temperature	11

TYPE=NORMAL STRESS

The values expected are:

Value 1	Value 2
Geometry set name	Normal Stress

Examples

*BOUNDARY

East, 1, 1, 0.0

West 1, 1, 0.0

North, 2, 2, 0.0

South, 2, 2, 0.0

Base, 3, 3, 0.0

1In this example fixity in perpendicular direction to East, West, North, South, and Base boundaries is prescribed.

*BOUNDARY

East, 1, 1, 0.0

East, 3, 3, 0.0

West 1, 1, 0.0

North, 2, 2, 0.0

South, 2, 2, 0.0

Base, 3, 3, 0.0

1In this example East boundary will have fixity in X and Z directions but not on Y direction. Fixities for east should be repeated twice.

*BOUNDARY

East, 1, 3, 0.0

West 1, 1, 0.0

North, 2, 2, 0.0

South, 2, 2, 0.0

Base, 3, 3, 0.0

1In this example East boundary will have fixity in X, Y and Z directions.

*BOUNDARY

East, 1, 1, 0.0

West 1, 1, 0.0

North, 2, 2, 0.0

South, 2, 2, 0.0

Base, 3, 3, 0.0

Top, 12, 12, 10.0

1In this example fixity in perpendicular direction to East, West, North, South, and Base boundaries is prescribed.

2A prescribed stress of 10 MPa (problem units) is defined on the top surface.

*BOUNDARY, TYPE=NORMAL STRESS

Top, 5.0

1In this example a normal stress of 5 MPa is defined on the top surface.

*BOUNDARY

*INCLUDE, INPUT=Pore_pressure_txt

*INCLUDE, INPUT=Displacements.spat

1A .txt file containing pore pressure values and an .spat file containing spatial grids with displacements are included as boundary conditions. This will generate Spatial_boundary and Spatial_grid data structures to apply the prescribed boundary conditions.

Notes

•Note that the displacement boundary conditions are defined in terms of displacement increment whereas normal stress, pore pressure and temperature are defined in terms of absolute values.

•Note that the stress defined in *BOUNDARY is defined in a local coordinate system and therefore is positive for compression (direction within the element) and negative for extension (direction outside from the element).

Stratigraphy surface boundary conditions

Usage

*TOP SURFACE, STRESS=Value, PORE FIXITY=Yes_or_No, PORE PRESSURE=Value, TEMPERATURE FIXITY=Yes_or_No, TEMPERATURE=Value, AMPLITUDE=Load_curve_name

Description

Defines boundary conditions for the top surface of the problem's domain. It is used to create Stratigraphy_surface_load data structure in ParaGeo. The NSET name corresponding to the top stratigraphy unit should be provided in a new line after last *TOP SURFACE parameter.

Compulsory Parameters
STRESS	Defines a stress load value to be applied to the top surface.
PORE FIXITY	Defines whether the pore pressure is constrained at the top surface. Valid values are: - YES - NO
PORE PRESSURE	Defines the pore pressure value to be applied as a boundary condition at the top surface in case the PORE FIXITY=YES. If PORE PRESSURE is not specified but PORE FIXITY=YES, then the pore pressure at model top surface will be assumed zero.
TEMPERATURE FIXITY	Defines whether the temperature is constrained at the top surface. Valid values are: -YES -NO
TEMPERATURE	Defines the temperature value to be applied as a boundary condition at the top surface in case the TEMPERATURE FIXITY=YES. If TEMPERATURE is not specified but TEMPERATURE FIXITY=YES, then the temperature at model top surface will be assumed zero.
AMPLITUDE	Assigns a defined time curve (by name) for the application of the top surface boundary conditions. If AMPLITUDE=RAMP is specified the stress and/or pore pressure at top surface will be ramped up during the current simulation Stage/Step.

Examples

*TOP SURFACE, STRESS=0.2, PORE FIXITY=YES, TEMPERATURE FIXITY=YES, TEMPERATURE=5, AMPLITUDE=Ramp

Top_horizon

1At the top surface of the model (Top_horizon) a compressive stress of 0.2 MPa will be applied, zero pore pressure is prescribed, a temperature of 5 degrees is prescribed and such conditions will be ramped up during the current simulation stage.

Notes

Definition of a water level table

Usage

*WATER LEVEL, ELEVATION=Value, DENSITY=Value, TYPE=Relative_or_Absolute

Description

Defines Water Level data to create ParaGeo Water_level_data structure which will be used to prescribe pore pressure boundary conditions. The level may be fixed during the entire simulation or can change according to an Amplitude curve.

Compulsory Parameters
ELEVATION	Defines the elevation for the water level. If TYPE is set to ABSOLUTE the value provided in ELEVATION will be the absolute water table elevation. If TYPE is set to RELATIVE the value provided in ELEVATION will be relative to the top surface elevation at reference coordinates (which should be provided using sub-keyword REFERENCE COORDINATES). If the water level is transient (varies with time) it should be provided in a tabular form via the sub-keyword VALUES
DENSITY	Defines the fluid specific gravity that will be used to compute the pore pressure as a function of depth.
TYPE	Defines whether the water level is specified as: - ABSOLUTE - RELATIVE

Sub-Keywords

Reference coordinates for RELATIVE water level

Time dependent values for water level

Examples

*WATER LEVEL, ELEVATION=200, DENSITY=1050, TYPE=Absolute

1The water level is defined to be at Y=200 m (2D problems) / Z=200 m (3D problems)

2The water density for calculation of the hydrostatic pore pressure at model top surface is 1050 kg/m3.

*WATER LEVEL, ELEVATION=500, DENSITY=1050, TYPE=Relative

*REFERENCE COORDINATES

450, 0

1The water level is defined to have an elevation of 500 m relative to the model top surface at the specified reference location of X=0 m, Y=0 m.

2The water density for calculation of the hydrostatic pore pressure at model top surface is 1050 kg/m3.

*WATER LEVEL, ELEVATION=10, DENSITY=1050, TYPE=Absolute

*VALUES

0, 10

1, 50

2, 200

1A transient water level is defined.

2The absolute water elevation is defined to be 10m at t=0, 50 m at t=1 and 200 m at t=2.

3The water density for calculation of the hydrostatic pore pressure at model top surface is 1050 kg/m3.