Vessel types

Each vessel has a vessel type: the vessel type determines much of the vessel data, and defines them on the vessel types form. You can define a number of different vessel types, each type having a different name and one of which is then chosen on the vessel data form to specify the type of that particular vessel.

Different vessels can have the same type. Consider, for example, a pipe being towed by two identical tugs. This may be modelled by creating a vessel type called 'Tug' and then creating two vessels, each of type 'Tug'. The RAOs, for example, are data of the 'Tug' vessel type, since they apply to both tugs. On the other hand the two tugs differ in their positions and any prescribed motion, so these are properties of the individual vessel objects. By sharing the data common to both vessels in this way, we avoid a great deal of data duplication.

You do not have to use all, or even any, of the vessel types you define. For example you can set up a data file that defines a number of vessel types but contains no actual vessels. Such a file can then act as a library of vessel types which can be imported into other OrcaFlex data files.

Vessel type data

Draught

For each vessel type you can enter data for multiple draughts, each draught having a user-specified name. Each vessel in the model must specify (on the vessel data form) which draught to use. It is not possible to use different draughts at different times during the same simulation.

Some of the vessel type data apply to all draughts, but a lot of the data are draught-dependent and so separate data are defined for each defined draught.

Properties data

The vessel type data are then specified on the following separate pages of the data form:

Structure define the length of the vessel type (same for all draughts) and its mass, inertia and centre of mass (draught-specific).
Conventions define the meaning of the RAO and QTF data, and the sense in which directions are positive. The conventions apply to all draughts.
Displacement and load RAOs. Separate RAOs for each different draught.
Wave drift and sum frequency QTFs. Separate QTFs for each different draught.
Sea state RAOs define the modification of the sea around the vessel as a result of the diffraction and radiation of waves by the vessel body.
Stiffness, added mass and damping. Separate values for each different draught. Wave radiation damping is specified here.
Other damping. For modelling other sources of damping, e.g. viscous roll damping. Separate values for each different draught.
Current and wind loads. Separate values for each different draught.
Drawing and shaded drawing. A single set of drawing data applies to all draughts.

Default vessel type data

When you create a new vessel type, it is given initial default data that correspond to a particular 103m long tanker. You should replace this with accurate data for the vessel you are modelling.

If your vessel is similar to this default tanker, then you might find the default data useful – but only if you have no better data available. OrcaFlex automatically Froude scales vessel type data to the vessel length you give, so if your vessel is a tanker of a different length to this but otherwise similar, and you have no better data, then these default data might still be useful. Be aware, however, that the default tanker has a significant heave resonance in beam seas at 7s period.

If your vessel is not similar to this default tanker, then we recommend that you do not use the default data, regardless of Froude scaling.

The default data were obtained as follows.

The default structure data, wave load RAOs, wave drift QTFs, stiffness, added mass and damping, and other damping data all come from an NMIWave diffraction analysis of a 103m long tanker in 400m water depth. The tanker used in this analysis had the following properties: Length between perpendiculars 103 m, breadth 15.95 m, draught 6.66 m, transverse GM 1.84 m, longitudinal GM = 114 m, block coefficient 0.804. The diffraction analysis used 8% extra damping in roll about the centre of mass, which is modelled in OrcaFlex using non-zero linear other damping.
The default displacement RAOs have been calculated from the first order wave load RAOs and the stiffness, added mass and damping matrices.
The default hydrodynamic and wind drag coefficients are based on graphs given in the standard OCIMF book. Note that the OCIMF book gives different coefficients for different vessel types and draughts. The default vessel type data are approximate averaged deep water coefficients, and they have been rounded to only 1 or 2 significant figures.
The hydrodynamic and wind drag areas and area moments are appropriate for this 103m tanker. The wind drag areas are based on an assumed average upperworks height of 12m for surge and 9m for sway.
The vessel origin for these default data is on the centreline at midships and at the mean water level.

Note:

Some default data values have been taken directly from NMIWave output, and so have only the few significant figures that are included in the NMIWave output file. But other default data have been derived from NMIWave output values by calculation (for example non-dimensional NMIWave output values were dimensionalised for use in OrcaFlex, and the default displacement RAOs are derived from the load RAOs, stiffness, added mass and damping matrices). The derived default data values therefore have many significant figures, but they are really no more accurate than those values with only a few significant figures.