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Prescribed displacement

Prescribed displacements may be used to model displacements that apply to one node in your model. This node must be either a primary point - also referred to as connection point - or any point that belongs to a vessel, rigid body, line... Each displacement may consist in several sub-displacements, whose properties are detailed below. In addition to defining each sub-displacement, you must define a number of general data for your displacement, as follows :

  • Name : Enter the name of the Imposed displacement component.

  • Colour : Select the colour that will be used for this displacement in your model.

Note it is important to select these the next three items for each sub-loading before clicking 'create...' as they cannot be edited later.

  • Object : First the object which the displacement is imposed to has to be defined: It can be a floater or a line or any object defined in your model.

  • Location : When selecting a floater the displacement can be imposed to the floater COG or floater fairleads. When selecting a line, displacement will be applied to one of the line ends (END_1, END_2).

  • Type : Four types of displacement can be selected :Incremental displacement, Sinusoidal displacement, Uniform acceleration displacement and Time dependant displacement

Each sub-displacement is created by selecting the Create new displacement button or a chosen displacement can be erased by selecting the Remove selected displacement button. After clicking on 'create new sub-displacement', select the line in the table, and then edit the data in the editing fields in the lower part of the properties window.

Sub-displacement properties

Several sub-displacements (different types and locations) can be defined in the same main displacement. Name, Object, Location and Type of each Sub- displacement is displayed in the upper part of the displacement pane. For each type of sub-displacement, additional data is required, as noted in the following pop-up paragraphs:

Incremental displacement:

  • Sub-Displacement Name: Enter the name that will be used to for this sub-displacement.

  • Direction: Select the sub-displacement direction in the specified reference frame. A translation (X,Y,Z) or a rotation (RX,RY,RZ) can be defined. Horizontal displacements may also be defined through the amplitude and heading wrt X-axis.

  • Amplitude: Enter the sub-displacement amplitude.

  • Quasi-static evolution: Quasi-static evolution can be linear or user defined. If user defined is selected Step and Ratio displacement have to be chosen. Enter for each quasi-static step the displacement ratio defined between 0 and 1. For example, 20% of the displacement can be applied at step 10 of a quasi-static analysis selecting step = 10 and Ratio = 0.2. An additional step can be entered selecting the Insert step button. A step can be removed selecting the Remove selected step button.

Sinusoidal displacement:

  • Sub-Displacement Name: Enter the name that will be used to for this sub-displacement.

  • Direction : Select the sub-displacement direction in the specified reference frame. A translation (X,Y,Z) or a rotation (RX,RY,RZ) can be defined. Horizontal displacements may also be defined through the amplitude and heading wrt X-axis.

  • Amplitude: Enter the sub-displacement sinusoidal signal amplitude.

  • Period: Enter the sub-displacement sinusoidal signal period (s).

  • Phase: Enter the sub-displacement sinusoidal signal phase (deg).

Uniform acceleration displacement:

  • Sub-Displacement Name: Enter the name that will be used to for this sub-displacement.

  • Direction: Select the sub-displacement direction in the specified reference frame. A translation (X,Y,Z) or a rotation (RX,RY,RZ) can be defined. Horizontal displacements may also be defined through the amplitude and heading wrt X-axis.

  • Starting time: Enter the starting time of the acceleration.

  • Duration: Enter the duration of the acceleration.

  • Initial velocity: Enter the initial velocity.

  • Initial velocity: Enter the Final velocity.

Time dependent displacement:

  • Sub-Displacement Name: Enter the name that will be used to for this sub-displacement.

  • File: The time dependent displacement is defined through an ASCII file. The ASCII file structure is as follows:

T0 DX DY DZ DTX DTY

T1 DX DY DZ DTX DTY

T2 DX DY DZ DTX DTY

T3 DX DY DZ DTX DTY

TN DX DY DZ DTX DTY

DX, DY, DZ are the displacements from the static positions expressed in Meters.

DTX, DTY, DTZ are the rotations from the static positions expressed in Radians.

Connection / Disconnection:

  • Sub-Displacement Name: Enter the name that will be used to for this sub-displacement.

  • Static tab: This tab is used to specify changes in boundary conditions that shall occur at a specified quasi-static step of the analysis.

1. Connection/Disconnection step: enter the step of the disconnection

2. New Connexion Type: unchanged, free, pin, clamp and user-defined

Note

if user-defined is selected for the New Connexion type, the user must specify for all degrees of freedom if they are free, constrained or clamped and if the considered frame is the global or local frame.

Example: Global 000111 means that the considered frame is the Global frame. X, Y and Z are free whereas RX, RY and RZ are constrained.

  • Dynamic tab: This tab is used to specify changes in boundary conditions that shall occur at a specified time of the time-domain dynamic analysis.

  • Connection/Disconnection Time: enter the time of the disconnection in the dynamic analysis

  • New Connection Type: unchanged, free, pin, clamp and user-defined

  • Mass: enter the mass of the QCDC

  • Horizontal drag global coefficient: enter the horizontal drag global coefficient of the QCDC

  • Horizontal inertia global coefficient: enter the horizontal inertia global coefficient of the QCDC

  • Vertical inertia global coefficient: enter the vertical inertia global coefficient of the QCDC

  • Lift global coefficient: enter the lift global coefficient of the QCDC

  • Reference tab: This tab is used to specify a change in the master model component the slave model component connects to. You may for instance have your model component originally connected with a model component A and then specify that your model component must connect with another model component B. Data in this tab are used to define the node to which your model component shall be connected once the change in boundary condition has occurred.

    • Use reference: Activate this option in case you wish to specify that your model component that was originally connected with a certain model component shall be connected to another model component once the change in boundary condition has occurred.

    • Object: Specify the name of the model component to which your model component shall be connected once the change in boundary condition has occurred.

    • Location: Specify the connection point that belongs to the model component selected above.

Note

Specifying a new "master" model component does not necessarily mean that the coordinates of both the "slave" node and the "master" node shall be identical. The "master" node will solely serve as a reference to prescribe its motion to the "slave" node and this will account for any lever arm between these two nodes that would arise from non matching coordinates of the "slave" and "master"nodes at the step/time of the change in boundary condition.

Control displacement:

  • Sub-Displacement Name: Enter the name that will be used to for this sub-displacement.

  • Type: the type of control displacement which can be one of the following, Z-rotation, Static displacement via dll, Dynamic displacement via dll, Dynamic displacement via dll with reference and Turbine start up.

  • Z-rotation:

    • Amplitude: the amplitude in deg.

    • From step: Beginning step.

    • To step: Ending step.

  • Static displacement via dll:

    • Load dll file: the dll path and name.
    • Procedure's name: the dll's procedure name used to manage the displacement.
  • Dynamic displacement via dll:
    • Load dll file: the dll path and name.
    • Procedure's name: the dll's procedure name used to manage the displacement.
  • Dynamic displacement via dll with reference:
    • Load dll file: the dll path and name.
    • Procedure's name: the dll's procedure name used to manage the displacement.
    • Reference point / Object: The object which will serve as the reference.
    • Reference point / Location: The object part which will serve as the reference point.
  • Turbine start up:
    • Proportional term: the proportional term value.
    • Derivative term: the derivative term value.
    • Integral term: the integral term value.
    • Corner frequency: the corner frequency value.
    • Maximum torque: the maximum torque value.
    • Maximum torque rate: the maximum torque rate value.
    • End of start up: the delay, in seconds, at which the start up ends.
    • Prescribed speed: the reference to a variation table which defines the prescribed speed..

Note

the direction must be given in terms of a specific axis direction. This means that multiple sub-loads must be used if you need to have a load whose resultant is not along a primary (or local) axis. The use of local coordinates depends on the object and connection point.