The sea density can be constant at all positions in the sea or it can vary with depth and/or horizontal position.

By default the sea density is constant. This is the most common value to use, since in most models the effects of density variation are not significant. For some systems, however, density variation is important because it causes buoyancy variation.

Note: | Density variation only affects the buoyancy of objects. OrcaFlex does not allow for density variation when calculating hydrodynamic effects such as drag, added mass, etc. For these effects, and for object properties reports (where the object position is not known), a nominal sea density value is used, which is taken to be the density value at the sea density origin. |

Vertical density variation governs whether, and how, the water density varies with depth. The vertical density variation may be set to one of the following:

**Constant**allows you to specify a single density value which applies at all depths.**Bulk modulus**indicates that the density varies with depth purely due to the compressibility of the water. You must specify the water's surface density and bulk modulus. The bulk modulus quantifies the degree to which a given mass of water shrinks under pressure, using the same volume formula as for buoys and line types; OrcaFlex derives the density variation with depth assuming that the water column has the given bulk modulus and is at uniform temperature and salinity.**Interpolated**allows you to specify a density profile as a table of density against depth. Linear interpolation is used to obtain the density at intermediate levels, and the data are truncated at both ends of the table.

It is possible to model a dry land system in OrcaFlex by using constant density with value zero.

Modelling density variation with horizontal position requires you to define a horizontal variation factor variable data source, which specifies the variation along a given axis. The horizontal variation factor is assumed to be constant in the direction normal to the axis. The variation is presented as a dimensionless multiplicative factor. Where variation of density with depth has been defined, the factor will be applied at all depths.

You may also give a constant numeric value here, which will be applied to the density throughout as a uniform scaling factor. This allows you to, for example, scale a vertical density profile given non-dimensionally. The default value of ~ indicates that no horizontal variation or scaling is performed.

As explained above, for all hydrodynamic calculations other than buoyancy OrcaFlex uses a single sea density value, that at the sea density origin. We define the sea density origin as follows:

If there is no horizontal density variation, then the sea density origin is at $(0, 0, \text{Sea Surface Z})$. The $X$ and $Y$ coordinates are arbitrary; the sea density value used is that at the still water level, which is the same anywhere in the horizontal plane.

If horizontal density *does* vary, then the sea density origin is at $(X, Y, \text{Sea Surface Z})$, where $X$ and $Y$ are the origin coordinates for the horizontal variation axis.