Specifications

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5.8.6 Floats
The purpose of floats is to keep the mooring free from the seabed. Datawell provides two types
of floats, a 3 Kg and a 10 Kg float. The 0.2 m diameter, 3 Kg buoyancy float can be tied with a
8 mm nylon rope to the polypropylene rope by two times a clove hitch, see Figure 5.8.4. This
way of mounting is called in-line float and it leaves the main rope intact. In this way the
strength of the polypropylene rope is not decreased. Its main function is to lift the lower end of
the mooring line from the seabed. The working depth of the float is 1.4 Km.
Figure 5.8.4. In-line float (0.2 m diameter, 3 Kg buoyancy) and mounting instruction.
Another float supplied by Datawell has a 0.3 m diameter and provides 10 Kg buoyancy.
The float is attached to line terminals in the mooring line with a shackle and split pin or directly
linked by a PP terminal. The larger buoyancy is needed to lift the heavier rubber cord terminals.
The working depth is 1.4 Km too.
5.8.7 Rubber cords
As mentioned, the rubber cords are essential for high quality wave measurements. They allow
the buoy to follow the wave motion, thus guaranteeing that the measured motion of the buoy is
indeed the same as the motion of the water due to waves.
With a single point vertical mooring using 15 m of rubber cord the buoy has sufficient
horizontal freedom to guarantee good wave following properties even for small motions at low
frequencies. The high flexibility of 30 m of rubber cord allows the Directional Waverider to
follow wave direction up to the specified frequency. Current velocities up to 3 m/s (6 knots) can
be accepted.
Natural rubber is chosen for its high tear strength and low tear propagation specifications.
This reduces chances on mooring failure in operation due to, initially, minor cuts in the rubber
cord.
The rubber cords are checked for hardness degree, tear strength and tensile strength. The
rubber cord is rejected when the strength falls below 50 Kg/cm
2
(Delft method). After the
rubber cord terminals have been fitted, the cord is stretched to about 3.5 times its original length
and kept under tension for 24 hours. Any holes or cuts, if present, will thus be enlarged and
found at visual inspection afterwards.
In case of strong breaking waves of the plunging type, the buoy has to be pulled through a
wall of water running at considerable velocity. In that case the induced forces may incidentally
exceed the breaking strength of the rubber cord. In this situation, the use of a polypropylene
safety line parallel to the rubber cord is advised: limiting the maximum elongation of the rubber
cord to 4 times the unstretched length, it helps the mooring to survive the peaks of the water
forces.
Standard rubber cords are 30 m or 15 m in length, but can be supplied in any length up to
30 m. Their diameter is either 27 mm for 0.7 m diameter buoys or 35 mm for 0.9 m diameter
buoys.