Line with floats

You can model floats or buoyancy modules attached to a line with discrete buoyant clumps attached at the relevant points. However when a number of floats are supporting a length of line it is often easier to model the buoyancy as if it were smeared, i.e. spread out evenly, along that part of the line. This allows the length and segmentation of the buoyed section to be varied easily without having to add and remove individual floats.

This smeared properties approach involves the following steps:

1. Create a new line type.
2. Set the new line type's properties to be equivalent to those of the original line+floats combined. This is done by spreading each float's buoyancy, drag, etc. uniformly over the length of line from $s_\mathrm{f}/2$ before the float centre to $s_\mathrm{f}/2$ after the float centre, where $s_\mathrm{f}$ is the float pitch, i.e. the spacing between float centres (see diagram below). The result is an isotropic circular cross section line which will experience the same forces per unit length as the original line plus floats.
3. Set up a line section to model the length of line supported by the floats. The section's line type should be set to the derived line type and its length should be $n\,s_\mathrm{f}$, where $n$ is the number of floats. Note that this length is a little more than the length between the start of the first float and the end of the last one, since each float is effectively being smeared equally both ways from its centre; see the diagram below for the case $n{=}3$.

Step 2 is performed by the line type wizard. It will automatically derive the data for the new line type, based on those of the original line and the floats.

Note that this approach is also suitable for modelling a regularly weighted section of line: 'floats' are not required by OrcaFlex to have positive buoyancy.

In subsequent topics, we define the base line type notation, that of the underlying line onto which the floats are to be attached, and the notation for the floats themselves.

The following properties are derived by the line type wizard: click on the link for details of the calculation of each of these values.

• mass per unit length
• outer, inner and contact diameters
• normal drag coefficient
• axial drag coefficient
• added mass coefficients
• stress diameters and allowable stress

These properties are considered to be unaffected by the addition of floats to the base line, so the wizard sets them to have the same values as those of the base line type:

• axial and bending stiffness
• compression is limited
• allowable tension
• minimum bend radius
• torsional stiffness