|
Importing hydrodynamic data: AQWA |
|
|
Importing hydrodynamic data: AQWA |
AQWA results are output in dimensional form, to a text file with a .lis extension.
| Warning: | The OrcaFlex import of AQWA output data has been developed without assistance from the developers of AQWA. As a result there may be deficiencies in the OrcaFlex import code and you must carefully review the imported hydrodynamic data to confirm the correctness of the import procedure. |
| One known issue is that the AQWA output file contains no reliable indication of any yaw rotation between the AQWA fixed reference axes and the analysed structures. In the absence of this information OrcaFlex interprets motions imported along and about the AQWA output x-direction as surge and roll, motions imported along and about the AQWA output y-direction as sway and pitch, etc. This may be significant for users making use of the AQWA "move structure" data record. There may also be further difficulties of similar nature that we have not yet seen. |
OrcaFlex first attempts to determine the units in use in the file, from the values it contains for acceleration due to gravity and water density and the knowledge that the unit of time is the second. If this is successful, then OrcaFlex will scale the data (if necessary) from the units in use in the file to the units prevailing in the OrcaFlex model. If the units cannot be determined, or if they are a mixture of SI and US, OrcaFlex will be unable to convert the data in this way: in this case, the raw data will be imported from the file unscaled and a warning will be issued.
| Figure: | Position vectors used for reference during import of AQWA data |
You must ensure that the vessel type centre of mass coordinates are set appropriately in OrcaFlex before importing AQWA data.
Almost all of the AQWA data have their reference origin at the vessel centre of mass, so OrcaFlex sets the RAO origins, QTF origin, and reference origin for stiffness, added mass and damping to the value of the centre of mass given on the vessel types structure page. RAO phase origins, likewise, are at the vessel centre of mass, so are set appropriately by OrcaFlex.
It is also possible to define a hull model in AQWA which does not correspond to a horizontal vessel condition, but instead represents that vessel at non-zero heel and trim. The AQWA reference axes remain horizontal, but the vessel properties provided to AQWA can still be set appropriately for a rotated vessel condition. In this case, as for centre of mass, those angles should be entered as the datum rotations for the OrcaFlex vessel type, before importing the AQWA data.
The only vessel reference point imported from AQWA which is not placed at the centre of mass is the centre of buoyancy. As with the centre of mass itself, the AQWA value is referred to the AQWA global origin. OrcaFlex deduces the position of the AQWA global origin relative to the OrcaFlex vessel origin from the difference between the centres of mass in the two systems, following which the position of the centre of buoyancy can be expressed, as required, relative to the OrcaFlex vessel origin. Finally, the imported position is rotated through the vessel type datum heel and trim angles.
Displacement RAOs are imported in a straightforward way. Load RAOs are imported including the Froude-Krylov contribution. Since AQWA and OrcaFlex use the same conventions for vessel headings, RAO directions are imported directly and require no adjustment.
QTFs are imported and, as with RAOs, no adjustment is necessary for directions. AQWA writes full QTFs to a separate text file with the same name as the .lis file but a .qtf extension: if this .qtf file is present in the same folder as the .lis file, OrcaFlex will automatically import the QTF data it contains. Newman QTFs are imported directly from the .lis file.
AQWA outputs the whole $6{\times}6$ matrices for added mass and damping but, on theoretical grounds (at least for zero forward speed), this matrix should be symmetric. OrcaFlex enforces this symmetry, so imports only the upper right triangle of each matrix and sets the lower left triangle values to match. You will receive a warning if any of these matrices, in the imported file, are significantly non-symmetric.
The $Z$-component of the reference origin datum position is imported. AQWA does not output the heel and trim values at the reference origin datum position, so these values may be required as input to OrcaFlex before the AQWA data are imported. The datum rotations are therefore left unchanged by import of AQWA data.
The value of displaced volume is taken from the figure given by AQWA for "mesh-based displacement". Centre of buoyancy is read in if given in the AQWA file (it isn't always), and translated from AQWA body origin to OrcaFlex vessel origin as described above.
The heave, roll and pitch components of the hydrostatic stiffness matrix are imported. Again, only the upper right triangle is used, and a warning is issued if the AQWA data are non-symmetric.
| Note: | AQWA may output the stiffness matrix twice, under the hydrostatic and hydrodynamic headings. If so, and if the two matrices are not the same, a warning will be given and the 'hydrostatic' form will be imported. |
Later versions of AQWA output what is labelled "additional damping (frequency independent)" as a $6{\times}6$ matrix. OrcaFlex interprets this as other damping having a linear damping effect on the wave frequency motion of the vessel, and assumes that the same reference origin and direction conventions as for frequency-dependent damping apply to these data. Upon import of this data, the other damping frame of reference is set to matching calculation mode and other damping calculated from primary is set to wave frequency motion.
Only the diagonal values of the matrix are imported, since those are the only values OrcaFlex allows for. In general these data are likely to be zero, with the possible exception of the roll-roll term.
Mass and moments of inertia are imported if requested. Since they are referred to the centre of mass in both AQWA and OrcaFlex, no reference-origin transformation is necessary for these data. The full inertia tensor will, however, be rotated from the diffraction frame to vessel axes if the datum rotations are non-zero.
We have no information on the form of AQWA sea state RAOs, so they cannot be imported from AQWA files.
If the file contains data for more than one structure, OrcaFlex will treat all the structures as being members of a multibody group and import the multibody data accordingly.