Offshore– Foo-sure

Asian Moorings inc.

Wave Force Calculations

Three waves related phenomena affect mooring system design. They are:

1) steady state mean drift force,

2) surge, sway, and yaw response at or near the predominant period of the waves, and

3) oscillatory drift forces at or near the natural period of the spring/mass system of the moored vessel.

The steady state mean drift forces are typically much smaller than the wave forces that excite surge en sway response. However, drift forces may still contribute significantly to the total mean environmental force acting on the vessel. Therefore, wave drift forces should be accounted for in the mooring system design.

Mean wave drift forces may be predicted using model tests or using advanced hydrodynamic computer analysis. In the absence of available wave drift force predictions, the following procedure for estimating mean wave drift force may be used. This procedure uses design curves for typical drill-ships and semi-submersibles to facilitate calculation. These design curves were generated by and advanced vessel motions computer program which as been verified and calibrated by extensive model test data.


Mean drift force for ship-shaped hulls:

The mean wave drift force for ship-shaped hulls in bow, quartering and beam seas can be estimated by figures 1 through 12, according to the size of the vessel and the direction of the waves relative to the hull. Drift forces for stern and stern quartering seas are nearly equal to bow and bow quartering values, respectively.


Mean wave drift force for semi-submersible hulls:

The mean wave drift force for semi-submersible hulls may be evaluated by the curves in figure 13 through 15. The drift curve in each figure represents the upper bound of the mean wave drift forces generated by the advanced motions computers program for four semi-submersible designs including typical 2,6 and 8 circular column twin hull designs and the pentagon design.

The figures can be downloaded here