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Prescribed loadings

Prescribed loadings may be used to model local external forces or moments that apply to one node in your model. This node must be either a primary point of a line - also referred to as connection point - or any point that belongs to a floater, rigid body, buoy...

Each load may consist in several sub-loadings, whose properties are detailed below. In addition to defining each sub-loading, you must define a number of general data to your loading, as follows :

Name: Enter the name of the Imposed loadings component.
Colour: Select the colour that will be used for this loading in your model. This applies to ALL the sub-loadings.

Note

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

Object: First the object the loading is imposed on has to be selected: It can be a floater or a line or any object defined in your model.
Location/Position on line: When selecting a floater the loading can be imposed at the floater COG or any fairlead. When selecting a line, the loading shall be applied to one of the line ends (END_1, END_2), or at a defined abscissa location.
Type: The following types of loads can be selected : Constant loading, Incremental loading, Sinusoidal loading and Time dependant, concentrated buoyancy module type loading, inertia matrix, and lumped masses.

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

Sub-loading properties

Several sub-loadings of different types and at different locations can be created under the same main loading. Name, Object, Location and Type of each Sub-loading is listed in the data table for each sub-loading. Additional data must be entered as noted in the following pop-up paragraphs:

Constant loading :

Sub-Loading Name: Enter the name that will be used to for this sub-loading.
Reference: Select the sub-loading reference frame :Global frame, local frame of user defined frame (U1,U2,U3).
Direction: Select the sub-loading direction in the specified reference frame. A force (X,Y,Z) or a moment (RX,RY,RZ) can be defined.
Amplitude: Enter the sub-loading amplitude.

Incremental loading :

Sub-Loading Name: Enter the name that will be used to for this sub-loading.
Reference: Select the sub-loading reference frame :Global frame, local frame of user defined frame (U1,U2,U3).
Direction: Select the sub-loading direction in the specified reference frame. A force (X,Y,Z) or a moment (RX,RY,RZ) can be defined.
Amplitude: Enter the sub-loading amplitude.
Step/Ratio amplitude: Enter for each quasi-static step the sub-loading amplitude ratio defined between 0 and 1. For example, 20% of the loading 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 loading :

Sub-Loading Name: Enter the name that will be used to for this sub-loading.
Reference: Select the sub-loading reference frame :Global frame, local frame of user defined frame (U1,U2,U3).
Direction: Select the sub-loading direction in the specified reference frame. A force (X,Y,Z) or a moment (RX,RY,RZ) can be defined.
Amplitude: Enter the sub-loading sinusoidal signal amplitude.
Period: Enter the sub-loading sinusoidal signal period (s).
Phase: Enter the sub-loading sinusoidal signal phase (deg).

Time dependent loading:

Sub-Loading Name: Enter the name that will be used to for this sub-loading.
Reference: Select the sub-loading reference frame :Global frame, local frame of user defined frame (U1,U2,U3).
Direction: Select the sub-loading direction in the specified reference frame. A force (X,Y,Z) or a moment (RX,RY,RZ) can e defined.
Amplitude: Enter the sub-loading time-dependent amplitude.
Time Step: Enter a time step which be used as multiplicative coefficient in the time dependent loading definition.
Step / %Amplitude: Enter a step and amplitude percentage (%). Each step corresponds to a constant loading with the following duration : Step x Time step (defined previously) = T end (s). The amplitude for each step is adjusted through the amplitude percentage. A Linear evolution can be chosen between each step. An additional step can be entered selecting the Insert step button. A step can be removed selecting the Remove selected step button.

Time dependent from file:

Sub-Loading Name: Enter the name that will be used to for this sub-loading.
Reference: Select the sub-loading reference frame :Global frame, local frame of user defined frame (U1,U2,U3).
Load file: Select the name of the text file that contains the time-evolution of the load components. This file must include 7 column of data as listed below:
- Column #1: The time in seconds, from the start of the dynamic analysis (i.e. 0) to the end of the dynamic analysis.
- Column #2: The time-evolution of the X-load force component expressed in N, based either on the global or the local coordinates system.
- Column #3: The time-evolution of the Y-load force component expressed in N, based either on the global or the local coordinates system.
- Column #4: The time-evolution of the Z-load force component expressed in N, based either on the global or the local coordinates system.
- Column #5: The time-evolution of the X-load force component expressed in N.m, based either on the global or the local coordinates system.
- Column #6: The time-evolution of the Y-load force component expressed in N.m, based either on the global or the local coordinates system.
- Column #7: The time-evolution of the Z-load force component expressed in N.m, based either on the global or the local coordinates system.

Time dependent via DLL:

External Dynamic Link Libraries (DLLs) may be invoked tto model controls applied to any nodes in the model. These dynamic control routines define loads that applies to a node as a function of the node's motion. Typical applications of this feature include modelling of any PID control systems, controlled thrusters loads, dynamic positioning systems, lift on pitch- controlled wings.

Sub-Loading Name: Enter the name that will be used to for this sub-loading.
Load type: The nodal force type only is available for now.
Load DLL file: Enter the path and name of the DLL file containing the routine used to calculate the dynamic loads applied to the node as a function of its motion.
Procedure's name: Enter the name of the routine included in the DLL file

Input and outputs parameters of the routine must be specified as follows: RoutineName (for, pos, vit, acce, time, dt)

with :

Parameter	Data type	Format	Description
for	Output	A vector of 6 real values	Force and load components calculated by the routine
pos	Input	A vector of 6 real values	Position components provided by the FE engine to the routine
vit	Input	A vector of 6 real values	Veocity components provided by the FE engine to the routine
acce	Input	A vector of 6 real values	Acceleration components provided by the FE engine to the routine
time	Input	A single real value	Time provided by the FE engine to the routine
dt	Input	A single real value	Time-step provided by the FE engine to the routine

Concentrated Buoyancy type loadings:

Sub-Loading Name: Enter the name that will be used to for this sub-loading.
Property: Select the name of the buoyancy module type to be used. A buoyancy modules loads type must have been previously created in the Types folder of the Model Browser.

Warning

Drag and inertia coefficients setup within Concentrated Buoyancy Modules properties are defined with respect to the global XYZ axes and therefore produce pure horizontal and vertical drag and inertia loads. These coefficients are not defined with respect to the local line axes (which are usually referred to as normal and axial directions). Using these loadings is therefore not recommended to model distributed drag and inertia along a line in case the orientation of the line is expected to vary over the simulation or along the line.

Note

Concentrated buoyancy loads can be distributed along the line by specifying sets of curvilinear abscissa instead of a single value. This allows to define distributed concentrated masses and inertia, added masses, drag coefficients, lift coefficients.. from the same sub-loading component.

Note

Use the following syntax to define sets of multiple locations along the lines and define distributed buoyancy type loads :

(S_min>S_max):Delta_S

For instance (0>100):10 will define several points between 0 and 100m curvilinear abscissa separated by 10m intervals.

Inertia Matrix:

This type of imposed loadings features a symmetric 6x6 inertia matrix which affects the inertia of the node the imposed loading is applied to. The inertia matrix has not impact on the weight. Unit for the 3 main translational terms I11, I22 and I33 is [kg]. Unit for the 3 main rotational terms I44, I55 and I66 is [kg.m2/rad].

Lumped masses:

lumped mass combines mass, rotaional inertia, and weigth components. It requires the definition of:

Mass (in kg): used to derive translational inertia effects and to define the weight in air.
Buoyancy (in N): used to define a buoyancy force. The submerged weight is then automatically derived as being the weight in air (calculated from the mass term and the gravity) minus the buoyancy force.
Moments of Inertia in 3DOF (Ixx, Iyy and Izz). These are expressed in kg.m2/rad.

Note

Moment of inertia components Ixx, Iyy and Izz are expressed in the local coordinates system of the node to which the prescribed lumped mass loading is applied.

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.