OrcaFlex Manual - Orcina

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System Modelling: Data and Results, Vessels
the WAMIT global origin relative to the WAMIT vessel origin (i.e. in "body-fixed axes"), using the values of XBODY,
YBODY, ZBODY and PHIBODY in the WAMIT output file, and (ii) translating this value to be relative to the OrcaFlex
vessel origin, using the difference between the centres of gravity in the two systems.
You will therefore need to ensure that the centre of gravity is set appropriately in OrcaFlex before importing the
WAMIT data.
Note: In some cases, WAMIT does not output the horizontal position of the centre of gravity. If so, then
OrcaFlex will warn you of this, and you will need to set these reference origins yourself.
Added mass and damping
WAMIT outputs added mass and damping as a list of (i,j) components. OrcaFlex imports these values, but uses only
the upper right triangle of each matrix. You will receive a warning if any of these matrices, in the imported file, are
significantly non-symmetric, since OrcaFlex is unable to handle this. Components for which an (i,j) value is not given
are assumed to be zero. The non-dimensional WAMIT values for added mass (a) and damping (b) are scaled to
dimensional (A,B respectively) values according to the formulae
where
RAOs
Aij = ρL k aij
Bij = ρL k ωbij
k = 3 for i,j=1,2,3;
k = 4 for i=1,2,3, j=4,5,6 or i=4,5,6, j=1,2,3;
k = 5 for i,j=4,5,6 and
ω is the frequency in rad/s.
Displacement RAOs are imported in a straightforward way. They are re-dimensionalised by multiplying by the
factor L k , k = 0 for i = 1,2,3; k = 1 for i = 4,5,6. WAMIT may calculate load RAOs in one of two ways, and either or both
methods (labelled "Haskind" and "Diffraction") may be present in the file. If both are given, OrcaFlex will prompt
you to choose one method and will import the data for that method only. The re-dimensionalising factor for load
RAOs is ρ g L k , k = 2 for i=1,2,3; k = 3 for i=4,5,6. The RAO directions in OrcaFlex are relative to the vessel, and are
determined by subtracting the value of PHIBODY in the WAMIT input file (the vessel heading at rest) from each of
the values of wave heading, which are given in WAMIT with respect to global axes.
QTFs
Wave drift QTFs are re-dimensionalised with the factor ρ g L k , k = 1 for i=1,2,3; k = 2 for i=4,5,6. Directions are
adjusted in the same way as for RAOs.
WAMIT may calculate Newman QTFs in a number of different ways: Momentum Conservation, Pressure Integration,
and Control Surface. OrcaFlex is presently unable to import data resulting from the Momentum Conservation
calculation, so this method is disregarded. As with load RAOs, if data from more than one of the remaining methods
are present, then you will be prompted to choose just one of them.
Similarly, full QTFs (if included) are calculated by WAMIT in two different ways: a Direct and an Indirect method. If
data from both of these methods are present in the .out file, then you will be prompted to choose one of them; if both
difference and sum frequency QTFs are present, you will have to make this choice for each.
Notes: WAMIT outputs Newman QTFs for pairs of wave headings. OrcaFlex allows only unidirectional
Newman QTFs, so imports only the data for which the two headings coincide. This restriction does
not apply to full QTFs.
Stiffness and equilibrium position
The heave, roll and pitch components of the hydrostatic stiffness matrix are imported. The equilibrium position is
not specified by WAMIT, so you should edit this value yourself.
Mass and inertia
Mass and moments of inertia are not imported from WAMIT files. You should enter these data yourself.