OrcaFlex Manual - Orcina

System Modelling: Data and Results, Lines
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Each attachment attached to the Line is specified by giving the Attachment Type and the arc length, measured from
End A, at which it should be attached. The attachment is then attached to the nearest node to that arc length.
Attachment Types are similar to Line Types – they are simply named sets of attachment properties. The properties
themselves are then given separately, on the Attachment Types data form. This allows the same set of attachment
properties to be used for a number of different attachments.
The two ends of a Line are referred to as End A and End B and each end can be Free, Fixed, Anchored or else
connected to a Vessel or Buoy. The two ends of a line are treated in essentially the same way, but some aspects of
the line are dependent on which end is which. In particular the numbering of parts of a Line is always done starting
at End A.
6.8.1 Line Data
For every line in the system there is a data form defining its structure and interconnection. It is on these data forms
that the system is built up by connecting lines between the objects that have been defined.
Name
Used to refer to the Line.
Include Torsion
Torsional effects can be included or ignored, for each line in the model. If torsion is included then the line type
torsional properties must be specified. See Torsional Stiffness.
To see the line orientation visually on the 3D views, select Draw Node Axes on the View menu. OrcaFlex then draws
the node axes Nxyz at each node, and these axes allow you to see how the line is behaving torsionally.
Top End
Notes: The node axes are drawn using the node pen, specified on the line data form.
If torsion is included for a line, you must specify the torsional orientation at each end of the line.
This is done by setting the Gamma angle of the end connections on the line data form. The Gamma
angle determines the torsional position of the line end – for details see Line End Orientation. To
check visually that you have the orientation you expect, select Draw Node Axes on the View menu.
If torsion is included for a line, the static analysis should also include the effects of torsion –
otherwise the simulation will start from a position that is not in torsional equilibrium and an
unstable simulation may result. We recommend that the Full Statics option is selected because this
is the only statics option in OrcaFlex that includes the effects of torsion.
This data item is used to give OrcaFlex information about the sense of the Line. Various calculations performed by
the program need to know which end of the line (End A or End B) is at the top, and which end is at the bottom. You
specify which end is at the top, and the program assumes that the other end is at the bottom.
Suppose you have a line with the top end connected to a vessel, and the bottom end anchored to the seabed. If you
wish to measure arc length from the vessel then you should connect End A to the vessel, make End B anchored and
set Top End to End A. On the other hand, if you wish to measure arc length from the seabed then you should connect
End B to the vessel, make End A anchored and set Top End to End B.
The setting of the Top and Bottom Ends is used by the program as follows:
� The Lay Azimuth data defines a lay direction starting from the Bottom End and moving towards the Top End.
� The Touchdown results point is determined by starting at the Top End and then moving towards the Bottom
End until the first node in contact with the seabed is found.
� The Contents Pressure Reference Z level can be set to '~' (indeed this is the default value) which OrcaFlex
interprets as the Z level of the Top End in the reset state.
� The Line Setup Wizard uses the bottom end when calculating anchor positions and also for the layback
calculation.
If the Line is not in contact with the seabed then this data is somewhat arbitrary. You are free to make whatever
choice suits your model, but remember that the contents pressure will be referenced from the Top End. If the entire
Line is in contact with the seabed then again you are free to make whatever choice of Top and Bottom Ends suits
your model.