6D buoys

6D buoys are rigid bodies having all six degrees of freedom – three translational $(X, Y, Z)$ and three rotational $(\text{Rotation } 1, 2, 3)$. Broadly speaking, buoys are intended for use in the drag/inertia regime in which Morison's equation applies, requiring that their diameter be much smaller than the wavelength they experience; if this is not the case, then diffraction effects tend to dominate and an OrcaFlex vessel object may be more suitable (though spar buoys do have some limited diffraction capability).

Buoys have both mass and moments of inertia, and forces and moments from many different effects can be modelled, including:

Lines connected to a 6D buoy can experience both moment effects and translations as the buoy rotates. Lines can be connected to an offset position on a buoy – this allows the direct study of line clashing, including the separation introduced by spaced connection points.

Three types of 6D buoy are available: lumped buoys, spar buoys and towed fish. These share some common data, but they differ in the ways in which the buoy geometry is defined and the fluid loads and surface-piercing effects are calculated. These differences are summarised below.

Lumped buoys

These are the simplest type, and have an abstract geometry: their shape is not defined. This necessarily restricts the accuracy with which interactions with the water surface are modelled. When a lumped buoy pierces the surface it is treated for buoyancy purposes as a simple vertical stick with a length equal to its specified height. Its buoyancy therefore changes linearly with vertical position, without regard to orientation, neglecting the rotational stiffness that would be experienced by most surface-piercing buoys. See lumped buoy properties for more details of this buoy type.

Spar buoys

These are intended for modelling axisymmetric buoys whose axis is normally vertical and where surface-piercing effects are important (such as for a CALM buoy).

Spar buoys are modelled as a series of co-axial cylinders placed end to end along the local $z$-axis (see spar buoy and towed fish properties). This allows you to define their shape by building up a number of cylinders of appriopriate lengths and diameters. A conical or spherical shape can be approximated as a series of short cylinders of gradually increasing or diminishing diameter.

Spar buoys model surface-piercing effects in a more sophisticated way than lumped buoys. Effects such as heave stiffness and righting moments in pitch and roll are calculated based on the intersection of the water surface with each of the cylinders making up the buoy, allowing for the instantaneous position and orientation of each individual cylinder in the wave. Slam forces are also calculated and applied separately for each individual cylinder.

Hydrodynamic loads on spar buoys are calculated from Morison's equation. Added mass and drag forces are applied only to those parts of the buoy which are instantaneously in the water. For partly-immersed cylinders, added mass and drag are scaled according to the proportion of the individual cylinder volume that is submerged.

Towed fish

These are intended for modelling bodies whose principal axis is normally horizontal. Towed fish are identical to spar buoys except that, for convenience, the stack of cylinders representing the buoy is laid out along the $x$-axis of the buoy, rather than along the $z$-axis.

For further details see spar buoy and towed fish properties.