6D buoys: Spar buoy and towed fish properties

A spar buoy is defined relative to the buoy local frame of reference $\Bxyz$, and is built from a series of coaxial cylinders mounted end-to-end along the local $z$-axis. The geometry of a towed fish is identical except that the symmetry axis is aligned with the local $x$-axis. In either case we call this series of cylinders the stack.

The cylinders are numbered backwards along their axis. This may seem counter-intuitive but it means that tables on the buoy data form, arranged in cylinder order, then have the same ordering as a vertically-oriented spar buoy with the cylinder at the base of the stack (lowest $z$) being at the bottom of the table.

If you are modelling a CALM or SPAR buoy in particular, there are extensive guidelines for setting up your model under the topic modelling a surface-piercing buoy.

Geometry

The shape of a spar buoy or towed fish is determined by its geometry data.

Stack base centre

The centre of the base of the stack, relative to buoy axes.

Cylinders: outer diameter, inner diameter and length

The diameters and length of each cylinder.

The stack built of these cylinders represents the buoy geometry from which buoyancy forces and moments are determined. When the buoy pierces the water surface, OrcaFlex allows for the angle of intersection between the sea surface and the buoy axis when calculating the immersed volume and centre of immersed volume, and includes the appropriate contributions to static stability.

If the inner diameter is greater than zero then the cylinder is a hollow cylindrical pipe. In this case:

• If the buoy added mass and damping is given by values for each cylinder, then the internal region of the cylinder is treated as being free-flooding with sea water up to the instantaneous sea surface level. When the cylinder is at least partially submerged, the mass of this contained sea water is added to the cylinder's inertia in the directions normal to the cylinder axis, but it is not added to the cylinder's inertia in the axial direction – the sea water is treated as being free to flow in the axial direction relative to the buoy.
• If the buoy added mass and damping is given by RAOs & matrices, then the inertia contribution from any contained sea water is assumed instead to be included in the user-specified added mass matrices.

Hydrodynamic loads

Hydrodynamic parameters are, in general, given for each individual cylinder (with RAOs & matrices option being an exception). Loads are calculated separately for each cylinder, taking surface-piercing into account on a per-cylinder basis, and summed to obtain the total load on the buoy. See the added mass and damping and drag and slam topics for more details.