There are a number of ocean currents found around the Earth. Ocean currents are rivers of hot or cold water within the ocean. The currents are generated from the forces acting upon the water like the planet rotation, the wind, the temperature and salinity differences. The depth contours, shorelines and other currents influence the current direction and strength.Maximum current speeds are usually chosen for survival design purposes. One could select a speed corresponding to a maximum spring tide current. In some cases a velocity profile is given the current as a function of depth.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

If there isn’t any velocity profile available, it is conservative to assume the maximum currents acts over the entire depth. In general, these physical phenomena are independent and it will have its own direction. This is most obvious for the relation between the tidal current direction and the wave direction, these have seldom much correlation. On the other hand, wind and wave direction in a major storm are usually correlated. The conservative choice is to assume these three phenomena are collinear and they all come from the same direction. But be sure to keep the direction bookkeeping correct. The wind direction is specified as the direction which it comes, while a current direction is most commonly stated as the direction to which it goes. For example, a northwest wind and southeast current goes in the same direction.

 

Current Load Calculation

If current forces are to be calculated, the equations here below should be used.

 

 

Force due to bow or stern current on ship-shaped hulls

 

 

 

where

 

 

 

 

 

 

 

 

Force due to beam current on ship-shaped hulls

 

 

 

where

 

 

 

 

 

Force due to current on semi-submersible hulls

 

 

 

where

Fcx  =   current force [N]

Ccx  =   current force coefficient for semi-submersible hulls = 515.62 [Ns2/m4]

Cd   =   drag coefficient (dimensionless) = 0.50 for circular members, see figure here below for members having flat surfaces

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

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

 

 

 

Figure: Semisubmersible current drag coefficient for members having flat surfaces