Geostatic Initialization (Multi Stage)

Geostatic Initialization (Multi Stage)

 

Description

 

Here are described the Keywords and their corresponding parameters for defining a multi stage Geostatic initialization of a problem. This is done in ParaGeo via the Geostatic_control_data structure. Geostatic control facilities:

    •Multi-step geostatic initialization where in each step specific assumptions can be applied, particularly related to the material and contact models.

    •Specification of additional initialization options; e.g. re-initialisation of displacement and state variables after establishing geostatic equilibrium.

 

Geostatic control may be used for cases where the initial geostatic state cannot be fully defined a-priori or when additional initialization options are required. For example, when discrete contact surfaces are present two situations can arise:

    •The displacement associated with establishing contact stress results in horizontal stresses that differ significantly from the target values, in which case an additional initialization step can be used to improve these values

    •Undesirable plastic deformation is observed adjacent to the contact surfaces during initialization. This can be ameliorated by performing the initial geostatic step using an elastic material assumption then performing an additional step where there is a gradual transition to nonlinear response.

    •Undesirable slip or de-bonding occurs on the contact surfaces. In this case setting the contact model as elastic in the first step and then performing a subsequent step with a gradual transition to slip assumptions may be beneficial.

 

 

GEOSTATIC CONTROL    Definition of geostatic control options

Usage

*GEOSTATIC CONTROL,  NAME=Name_ID,  DISPLACEMENT=Yes_or_No,  POROSITY=Yes_or_No,  POROELASTIC=Yes_or_No,  STATE=Yes_or_No,  AMPLITUDE=Ramp

Description

It is used to create Parageo Geostatic_control_data structure. This data structure facilitates:

•Multi-step geostatic initialization where in each step specific assumptions can be applied, particularly related to the material and contact models.

•Specification of additional initialization options; e.g. re-initialisation of displacement and state variables after establishing geostatic equilibrium.

 

 

Optional Parameters

NAME

Defines a name with which the Geostatic Control can be refereed.

DISPLACEMENT

Defines whether a displacement reinitialization should be performed during the current geostatic stage or not. Valid options are:

- YES

- NO (Default)

POROSITY

Defines whether porosity should be initialized at the start of the geostatic step. Valid options are:

- YES (Default)

- NO

POROELASTIC

Defines whether poroelastic constants should be initialized using approximate end-of-step stresses. Valid options are:

- YES (Default)

- NO (Initialize poroelastic constant using actual stresses)

STATE

Defines whether state variables should be reinitialized after geostatic. Valid options are:

- YES

- NO (Default)

AMPLITUDE

Assigns a defined time curve (by name) for the application of the displacement reinitialization during the current geostatic stage. If AMPLITUDE=RAMP the displacement reinitialization will be performed throughout the entire current geostatic stage.

 

 

Sub-Keywords
hmtoggle_plus1*GEOSTATIC CONTACT         Control of contact model during geostatic initialization

Usage

*GEOSTATIC CONTACT, MODEL=Contact_model,  TYPE=Initialization_type

Description

This keyword is used to define which contact model is going to be used during a given geostatic initialization stage and the algorithm for contact stress initialization. It is equivalent to keywords Contact_constitutive_model and Contact_initialization_type in ParaGeo Geostatic_control_data structure.

 

Parameters

MODEL

Defines the contact model to be used during the present geostatic stage. Valid options are:

- ELASTIC:  Elastic contact (No slip nor separation, sometimes denoted as "tied contact").

- NO SLIP:  No tangential slip is allowed but separation may occur.

- STANDARD:  Used the contact model defined in the data file (Default).

- ADHESIVE SLIP:  No separation is allowed by slip in tangential direction may occur.

TYPE

Defines the type of contact initialization. Valid options are:

- STANDARD:  Standard contact with geostatic stress ramped up from zero stress (Default)

- RELEASE:  If the contact model was previously set to elastic ( MODEL=ELASTIC) then a gradual transition from an elastic state to a nonlinear state will be performed in this geostatic stage.

 
hmtoggle_plus1*GEOSTATIC STRESS         Control of constitutive model during geostatic initalization

Usage

*GEOSTATIC STRESS, MODEL=Constitutive_model,  TYPE=Initialization_type

Description

This data is used to control the constitutive model to be adopted during a given geostatic initialization stage and the algorithm for stress initialization. It is equivalent to keywords Stress_constitutive_model and Stress_initialization_type in ParaGeo Geostatic_control_data structure

 

Parameters

MODEL

Defines the constitutive model to be used during the present geostatic stage. Valid options are:

- ELASTIC:  Elastic model using elastic properties defined in the material file)

- POROELASTIC:  Poroelastic model using poroelastic properties defined in the material file

- STANDARD:  Constitutive model as defined in the material file

TYPE

Defines the type of stress initialization. Valid options are:

- STANDARD:  Starting from zero stress apply stress using data defined in the geostatic data structures (Default)

- NONE:  Ignore the data for stress initialisation in the geostatic data structures

- RELEASE CURRENT:  If the material was previously set to elastic ( MODEL=ELASTIC) then a gradual transition from an elastic state to a nonlinear state will be performed in this geostatic stage by:

    1 - Saving the internal forces due to the current elastic stress

    2 - setting the material as nonlinear,

    3 - performing a nonlinear stress update based on the current stress state,

    4 - gradually relaxing the internal forces from the elastic stress state to the nonlinear stress state

Consequently, with this method the stresses are gradually ramped up from zero.

 

 

 

Examples

 

*GEOSTATIC CONTROL, NAME=Gravity

 *GEOSTATIC CONTACT, MODEL=ELASTIC, TYPE=STANDARD

 *GEOSTATIC STRESS, MODEL=ELASTIC, TYPE=STANDARD

 

(data for geostatic step 1)

 

 

*GEOSTATIC CONTROL, NAME=Contact_release

 *GEOSTATIC CONTACT, MODEL=STANDARD, TYPE=RELEASE

 *GEOSTATIC STRESS, MODEL=ELASTIC, TYPE=NONE

 

(data for geostatic step 2)

 

 

*GEOSTATIC CONTROL, NAME=Constitutive_release

 *GEOSTATIC STRESS, MODEL=STANDARD, TYPE=RELEASE CURRENT

 

(data for geostatic step 3)

 

 

 

*GEOSTATIC CONTROL, NAME=Disp_reinit, DISPLACEMENT=YES

 

(data for geostatic step 4)

 

 

1In this example a multi staged geostatic initialization is presented. Geostatic stages are described as follows:

a.Stage 1: Gravity is applied while considering elastic contact and an elastic constitutive model

b.Stage 2: Contact is released changing gradually from elastic to standard contact model defined in the data.

c.Stage 3: Constitutive model is gradually released from elastic to the standard model defined in the material data (e.g. SR4 Elasto-plastic model)

d.Stage 4: Displacement are reinitialized during this stage to recover the initial geometry (while preserving the stress and balance computed during previous geostatic stages).