The nonlinear properties of a mooring line can be defined by a set of load extension characteristics.
To define these properties, a mooring line local Cartesian coordinate system Mxyz is introduced, where the origin is the lower attachment point of the mooring line, the z-axis points vertically upwards, and the mooring line is in the local Mxz plane, as shown in Figure 9.4: 2-D Load Extension Database.
To build a 2-D extension database a number
of z-levels are chosen; in other words, 
. On
each z-level a number of points are selected as 
. The mooring tension components 
along
the local x- and z-axes are then given at those points. The stiffness
matrix that can be formed from the 2-D load/extension database is
evaluated as follows:
 | (9–24) |
It should be considered that the values
of 
and 
for a completely elastic mooring line must be equal. You must to
input values of 
at 
that obey this condition. Failure to do so will
result in an unphysical mooring system that generates or absorbs energy.