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Current

The most common categories of currents are:

1)                  Tidal currents (associated with astronomical tides)

2)                  Circulation currents (associated with oceanic-scale circulation patterns

3)                  Strom-generated currents

4)                  Loop and eddy currents

The vector sum of these four currents is the total current, and the speed and direction of the current at specified elevations is the current profile. The total current profile associated with the sea state producing the extreme waves should be specified for floating platform design. The frequency of occurrence of total current speed and direction at different depths for each month and/or season may be useful for planning operations. In certain geographic areas, current force can be the governing design load. Consequently, selection of appropriate current profiles requires careful consideration.

Current force

Model tests

Model test data may be used to predict current loads for mooring system design provided that a representative underwater model for the unit is tested, and that the contribution to current load made by thrusters, anchor bolsters, bilge keels, and other appendages be accounted for. Care should be taken to assure that the character of the flow in the model test is the same as the character of the flow for the full-scale unit.

Current force calculation

If current forces on semi-submersible hulls are to be calculated, the following equation should be used:

 

Fcs = Css(CdAc + CdAf)V2c

 

where: 

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    Fcs        =          current force [N]

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    Css        =          current force coefficient for semi-sub hulls (515,62) [Ns2/m4]

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    Cd          =          drag coefficient (0,5 for circular shapes. See figure 2.1)

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    Ac          =          summation of total projected areas of all cylindrical members below the waterline [m2]

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    Af           =          summation of projected areas of all members having flat surfaces below the waterline [m2]

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    Vc          =          design current speed [m/s]

 

                                    Figure 2.1: Drag coefficients

       

                                                                     

 

 

 

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Last modified: 10/16/06