﻿ 6D buoys: Common data

# 6D buoys: Common data

All types of 6D buoy use a local buoy axes coordinate system. The origin of the buoy axes $\Bxyz$ can be anywhere, but the axes directions should be coincident with the principal axes of structural inertia of the buoy.

### Name

Used to refer to the 6D buoy.

### Type

Three types of buoy are available: lumped buoys, spar buoys and towed fish. The following data items are common to all types.

### Wave calculation method

The wave calculation method to be used when computing the buoy's wave kinematics. The default value is specified by environment, meaning that the method specified on the environment form will be used; you can override that (for this buoy) by selecting a different method here if you wish.

### Disturbance vessel

Determines whether the 6D buoy will experience sea state disturbance generated by a particular vessel.

### Connection

A 6D buoy can be free, fixed, anchored or connected to another object.

### Initial position and attitude

$x$, $y$ and $z$ connection coordinates that determine the initial position of the buoy origin. The initial attitude is given by the three angles rotation 1, rotation 2, rotation 3, which are successive rotations that define the orientation of the buoy axes, $(B\urm{x}, B\urm{y}, B\urm{z})$, as follows:

1. align the buoy with the connection axes defined by the connection
2. apply rotation 1 about $B\urm{x}$
3. apply rotation 2 about the new $B\urm{y}$ direction
4. finally, apply rotation 3 about the new (and final) $B\urm{z}$ direction

### Degrees of freedom included in static analysis

Free buoys may have some or all of their degrees of freedom excluded from the static analysis. The degrees of freedom to be included may be:

• None, the buoy position and orientation are not calculated by the static analysis – they remain at the initial position and orientation as described above
• X,Y,Z, the buoy position is calculated by the static analysis, but its orientation remains at the initial orientation
• All, the buoy position and orientation are calculated by the static analysis, and the initial position and orientation supply the initial estimates for the static analysis

Normally, all should be used, so that the static analysis calculates the true equilibrium position and orientation of the buoy. It may, however, sometimes be useful to fix the buoy in position, for example if the static analysis is unable to find the equilibrium position or orientation.

Buoys which are fixed or anchored will, obviously, not move in either statics or dynamics. Buoys which are connected to a parent object have their static equilibrium position determined by that object.

### Mass

The mass of the buoy.

### Mass moments of inertia

The solid moments of inertia of the buoy, about the local $B\urm{x}$, $B\urm{y}$ and $B\urm{z}$ axes through the buoy centre of mass.

 Note: These moments of inertia are the diagonal terms in the structural inertia matrix about the specified centre of mass. The off-diagonal terms are taken to be zero, so the buoy axes should be chosen to be in the principal directions of inertia about the centre of mass.

### Damping relative to

You can choose whether the buoy velocity to which the damping data apply (see lumped buoy data or spar buoy or towed fish data) is relative to earth or relative to the fluid. To model wave radiation damping the velocity relative to earth should be used, whereas to model skin friction damping the velocity relative to the fluid is appropriate.

### Centre of mass

The centre of mass of the buoy, relative to the buoy origin. The weight force and moments of inertia are applied at this point.

### Bulk modulus

Specifies the compressibility of the buoy. If the buoy is not significantly compressible, use the value infinity to mean 'incompressible'.

### Characteristic scales

For some models it may be desirable to explicitly set a characteristic length and characteristic force for the 6D buoy. These characteristic scales directly affect the convergence criteria of the iterative solvers employed in the analysis. The data does not appear on the 6D buoy data form but can be found on the all objects data form.

## Contact

### Total contact area

This is used to determine contact forces when the buoy comes into contact with the seabed and with elastic solids. If the value is '~', OrcaFlex will calculate a default total contact area based on the buoy geometry.

A value of zero disables contact forces for the buoy.

### Seabed friction coefficient

OrcaFlex can model Coulomb friction between the buoy and the seabed and elastic solids. The friction force applied never exceeds $\mu R$, where $\mu$ is the friction coefficient and $R$ the contact reaction force.

 Note: The friction coefficient for contact with elastic solids is specified on the friction coefficients data form.