OrcaFlex Manual - Orcina

System Modelling: Data and Results, Lines
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Ex and Ey are perpendicular to Ez and they are defined by specifying the Gamma angle, which is a rotation about Ez.
The Ex and Ey directions are used for reporting results (e.g. the 2 components of shear force). And if the line has
torsion included and the joint twisting stiffness is non-zero, then Ex and Ey also define the line end orientation at
which no torsional moment is applied by the joint.
The connection at a line end is modelled as a ball-joint with this orientation being the preferred "no-moment"
orientation, i.e. the orientation of the line end that gives rise to no moment from any rotational stiffness of the
connection.
If all of the end connection stiffness values are zero, e.g. to model a ball joint that is completely free to rotate, then
the end orientation angles have no effect on the line behaviour. The angles then only serve to define the local x, y
and z-directions that are used to define results (e.g. shear and bend moment components, stress components, etc.)
that depend on the local axes directions.
Bending and Twisting Stiffness
The connection at a line end is modelled as a joint with the specified rotational stiffness. The restoring moments
applied by the joint depend on the deflection angle, which is the difference between the end fitting orientation and
the orientation of the line. The end orientation is therefore the orientation of the line that corresponds to zero
moment being applied by the joint.
The connection stiffness is the slope of the curve of restoring moment against deflection angle.
The bending and twisting connection stiffnesses can be set to:
� Zero: free to rotate with no resistance.
� Non-zero, finite: can rotate but with resistance.
� Infinity: a rigid connection.
� Variable: non-linear (for bending connection stiffness only).
The x bending and y bending values specify the connection bending behaviour for rotation about the end Ex and Ey
directions, respectively. For an isotropic ball joint the two values must be equal; this can conveniently be specified
by setting the y-bending value to '~', meaning 'same as x-value'. A non-isotropic ball joint can be modelled by giving
different x and y bending values; in this case the line must include torsion.
The x bending and y bending behaviour can either be linear or non-linear, as follows:
� For a simple linear behaviour, specify the bending stiffness to be the constant slope of the curve of restoring
moment against deflection angle.
� For a non-linear behaviour, use variable data to specify a table of restoring moment against deflection angle.
OrcaFlex uses linear interpolation for angles between those specified in the table, and linear extrapolation for
angles beyond those specified in the table. The restoring bend moment must be zero at zero angle.
The Twisting Stiffness value is only relevant if torsion is included for the line. It specifies the rotational stiffness
about the end Ez direction. For the twisting stiffness this variation is always modelled as linear so the twisting
stiffness you specify should be the slope of the linear angle-moment curve.
A flex joint can be modelled by setting the stiffness values to be non-zero and finite.
Warning: Avoid specifying large connection stiffness values (except the special value Infinity) since they
require very short simulation time steps.
Release at Start of Stage
If desired each line end can be disconnected at the start of a given stage of the simulation. If no release is wanted
then set this item to "~", meaning "not applicable".
Structure
Each line can be made up of up a number of sections with different properties, the sections being defined in
sequence from End A to End B.
Line Type
This determines the properties of the section.