3 October 2005
Forces/Moments. In this case the design wind speed is 45 knots (23
m/s). Currents, waves, and tidal effects are neglected for these `fair weather' moorings.
The bow-on ship wind drag coefficient is taken as the value given for normal ships of
0.7, plus 0.1 is added for a clutter deck to give a drag coefficient of 0.8. Methods in
Section 4 are used to compute the forces and moments on the ship. The computed
bow-on wind force is 68.6 kips (3.0 E5 newtons) for 45-knot (23-m/s) winds, as shown in
Figure 8-2 .
Quasi-Static Design. Quasi-static design procedures place the ship
parallel to the wind for this example, because in this position the forces and moments
on the ship are balanced out. Two mooring hawsers were specified for this design.
Extra factor of safety was specified for the two 12-inch nylon mooring hawsers, which
had a new wet breaking strength of 406 kips (1.8 E6 newtons), to account for poor load
sharing between the two hawsers.
Mooring Hawser Break. The ships were moored and faced into 15-knot
winds. The weather was unsettled, due to two nearby typhoons, so the ships had their
engines in idle. A wind gust front struck very quickly with a wind speed increase from
15 to 50 knots. As the wind speed increased, the wind direction changed 90 degrees,
so the higher wind speed hit the ships broadside. The predicted peak dynamic tension
on the mooring hawsers was 1140 kips (5.07 E6 newtons), (Seelig and Headland,
1998). Figure 8-3 is a simulation predicting the dynamic behavior of the moored ship
and hawser tension. In this case, the mooring hawsers broke and the predicted factor
of safety dropped to less than 1. In this event, the peak dynamic tension on the
mooring hawser is predicted to be 13.5 times the bow-on wind force for 50-knot (25.7-
This example shows that single point moorings can be susceptible to
dynamics effects, such as those caused by wind gust fronts or other effects.