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Hydrodynamic database files viewer

The HDB Viewer is a graphical module which allows to display any data contained in the hydrodynamic database (.HDB) files, such as RAO amplitude and phases, excitation and drifts forces.

This module is invoked by clicking the icon from the toolbar or from the floater properties window Motion tab. A click on the button named HDB Viewer will launch the module with the HDB file associated with your floater.

The panel contains the following controls :

  • An edit and browse control File which contains the entire path to the HDB file. A button Reload allows you to re-load the graphics if you change the HDB file in the browser.

  • A list-box named Category with different types of variables listed below.

  • Series of tabs associated with different wave headings.

When selecting a given type of variable, the GUI will display the corresponding values for all the headings and periods found in the HDB file.

The X-axis corresponds to the period and the Y-axis corresponds to the values of the variable. Three curves are plotted on the same graphic (X, Y and Z values or RX, RY and RZ values according to the type of the category) :

  • Excitation force Module (X, Y, Z)

  • Excitation Moment Module (RX, RY, RZ)

  • Excitation force Phase (X, Y, Z)

  • Excitation Moment Phase (RX, RY, RZ)

  • RAO Module (X, Y, Z)

  • RAO Module (RX, RY, RZ)

  • RAO Phase (X, Y, Z)

  • RAO Phase (RX, RY, RZ)

  • Interpolated RAO Module (X, Y, Z)

  • Interpolated RAO Phase (X, Y, Z)

  • Drift Force Module (X, Y, Z)

  • Drift Moment Module (RX, RY, RZ)

Hydrodynamic database files creation

On the second tab of the same screen, a tool enables the creation of hydrodynamic databases for Generic floater objects using the potential flow solver Diodore. The Generic floater's mesh is imported from an external data file in the floater object (2 formats are possible, DeepLines native format, and Diodore native format).

An already created generic floater has to be selected. Once the floater is selected, it will update the HDB creation path accordingly, this can be further modified. The following commands are available:

  • Create Files : create the .inp and .dat files as input files for Diodore that will create the HDB file.
  • Edit .inp : open the .inp file in an embedded editor to edit it manually.
  • Edit .dat : open the .dat file in an embedded editor to edit it manually.
  • Calc HDB : if not already created, it creates .inp and .dat files from the content of the mesh of the floater, then run Diodore process to create the .hdb file. Note that second order loads (drift forces and moments) can be obtained at this stage by clicking on the option "Generate second order loads on fixed floater"
  • Gen batch file : creates .cmd files that contains commands which do the same as set on screen, for use outside DeepLines (by clicking on the .cmd file) or by loading in the batch processor
  • Stop proc : stop any running creation process.
  • Update HDB : if the HDB with first order load has been already created, this enables to update the HDB file with first order motions and also second order loads based on first order motion. Note that is possible only if a frequency domain analysis has been defined and previously run.

The steps are:
1) Generate the first order loads based on the floater's mesh (as defined above)
2) In the floater motion, the option "Coupled first and second order wave motion" must be selected in the "General panel", then in the "Calculated Motion" panel, select the HDB file crated in the first step together with the floater name
3) Define a frequency domain analysis in the "Default" analysis folder that includes the floater and a regular wave. All other components (mooring, loadings, turbine, ...) may be added. Then run the frequency domain analysis. If successful, go back to the HDB file creation panel and select the frequency domain analysis. It is now possible to update the HDB file

Note

Note that at least 3 periods and 1 incidence should be filled to be able to launch the HDB creation process.

The second order loads computed are the drift forces and moments. These loads are function of floater's response to the waves (first-order motion). Two options are available: - Calculation of the drift loads with floater fixed - Calculation of the drift including first order motion. This option requires as an input the first-order motion. This is obtained from frequency domain analysis.

Also, it is possible to derive the loads from the potential flow simulation with "Mixed" or "Pressure" method. With the "Pressure" method, drift forces are calculated by integration of pressures. With the "Mixed" method, the vertical directions are still computed with the "Pressure" method while the horizontal forces are obtained from the theorem of Lagally reformulated by Molin (A. Ledoux, B. Molin, G. Delhommeau, F. Remy. "A Lagally formulation of the wave drift force," 21st International Workshop Water Waves and Floating Bodies, Apr 2006, Loughborough, United Kingdom. pp.27.)

Note

Note that the accuracy of the hydrodynamic database and specifically of the second order loads is dependent on the quality of the mesh that is provided to DeepLines.

Note

The creation process can take a relatively long time if the mesh is complex. The "HDB" windows can be minimized and the user interface can still be used during the creation process. A log is displayed on the right side of the incidence definition control. It will keep you informed of the creation progress and steps. Do not close DeepLines' interface. Information can be found in the .MES files for the first order loads generation and some error message in GenRAO.day file for first order motion generation and drift.day for drift load generation.

Note

Some other files are also copied next to the .HDB files. These files are created while generating the .HDB files that are used for post-treatment of the external pressure on the mesh.

Non-linear first order calculations can be performed with this .hdb file. When using this functionality, the hydrodynamic stiffness and first order froude-Krylov loads are automatically computed at the center of motion of the floater (depending on the selected option as defined in the "NL Loads" panel when "Use non-linear loads" is selected in the floater's motion). This point is defined in the floater motion type. The user must be careful if changing the floater motion type of a floater, or changing the property of the motion type, or using a different motion type for the floater in an analysis. The .HDBfile that is generated is linked to the floater motion type and if it is modified, the .HDB file must be regenerated.

Also, the weight and inertia matrix are considered null in the .HDB file generated. A specific fairlead should then be defined in the floater's fairleads panel. A lump mass is created on this fairlead (for a diagonal inertia matrix) or a negative Z constant force for the weight together with a complete inertia matrix are loaded on this fairlead.

Note

This function has been created to overcome problems often faced about the introduction of HDB file on a floater:
o Confusion between the Centre of gravity and Centre of motion defined in Deeplines model and the [CENTER OF GRAVITY] defined in the HDB
o Hydrostatic instability when the lever between the [CENTER OF GRAVITY] and the [CENTER OF BUOYANCY] is large.
o These problems have been highlighted when dealing with wind turbines since the global COG of the whole system is quite different from COG of the floating platform alone.
o Note that this can also happen for an installation barge loading with a heavy load…

The leading idea is that the HDB creation is there to compute the diffraction/radiation loads on a ‘generic floater’ associated with a mesh.
The position of the mesh in Deeplines model fixes the draft for which the diffraction/radiation loads are computed.

An important point is that “diffraction/radiation loads” mean pressure loads coming from the fluid, the floater mass is not included at that stage, the hydrostatic stiffness in the HDB only refers to the restoring force coming from the fluid
The floater mass is defined in Deeplines model :
o For a simple floater, a lump mass and an inertia matrix may be ok,
o For more complex system, different elements connected to the floater will be part of this global mass (a crane, a wind turbine etc…)

In Deeplines, the option “non-linear hydrostatic loads” compute the hydrostatic stiffness, once the equilibrium is found and depending on the selected option it can be kept constant or updated during the time domain simulation as well as non linear Froud Krylov loads.

SubStructures

If the mesh used to generate the HDB files contains substructures, Diodore can generate information on each substructure for first order wave loads, added mass and radiation damping. Contrary to hydrodynamic coupling, all substructures move as a single body and not independently. This can be used for instance to compute hydrodynamic loads on different parts of a floater, to extract loads in the connections between those elements. An example is shown in Figure "Structure and substructures". The floater is represented as a rigid structure in the left figure and an .HDB file of the whole floater is generated. If the objective is to define the floater as presented on the right with beam elements between the three outer columns to extract loads and stresses in the braces, then the external columns are defined as substructure during the hydrodynamic calculations.


Structure and substructures

To run the dynamic simulation in DeepLines, three floaters are defined pointing on the same HDB file but different substructure. To work properly:

  • Mass/Inertia is defined for each substructure in DeepLines at substructures CoG,
  • The mesh of each substructure independently is entered in DeepLines in order to recompute the hydrodynamic stiffness matrix,
  • The reference node for the loads stored in the HDB is the same for all substructures as defined in the HDB files.
  • The heading is the same for all substructures.