OrcaFlex Manual - Orcina

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Theory, Object Connections
Directions relative to the global axes are defined in just the same way, simply replacing the local xyz directions
above with the global XYZ directions. A global declination of 0° therefore means vertically upwards, 90° means
horizontal and 180° means vertically downwards.
When a direction is being defined, the "sign" of the direction must also be defined. For example "vertical" does not
fully define a direction – it must be either "vertically up" or "vertically down" before the azimuth and declination
angles can be derived. The "sign" conventions used in OrcaFlex for directions are:
� For Lines, axial directions are always defined in the A to B sense, in other words from End A towards End B.
Thus a vertical line with End A at the top has declination 180°.
� For Winches and Links, axial directions are defined in the sense 'from first end towards second end'. Thus a link
with end 1 directly above end 2 has declination 180°.
5.3 OBJECT CONNECTIONS
Lines, links, winches and shapes are special objects that can be connected to other objects. First consider connecting
a line to another object. To enable connections to be made each line has two joints, one at each end, which are
drawn as small blobs on the ends of the line, when the model is in Reset state. To distinguish the two ends, the joint
at End A is drawn as a triangle and the joint at End B as a square.
Each of these line end joints can either be Free or else be connected to a Vessel, 3D Buoy, 6D Buoy, the Global Axes
or the seabed. Lines cannot be connected to themselves, to other lines, nor to a Link or Winch. When a line's joint is
Free, that end of the line is free to move and this is indicated by the joint being drawn in the same colour as the line.
When the joint is connected to another object, that end of the line becomes a slave and the object to which it is
connected becomes its master. The connection is then indicated by the joint being drawn in the colour of its master.
Links and Winches also have joints at each end (winches can also have extra intermediate joints) and these are
connected to other objects in the same way as with lines, but with the following exceptions.
� Link and winch joints cannot be Free – they must always be connected to some master.
� Link and winch joints can be connected to nodes on a line, as well as to Vessels, 3D Buoys, 6D Buoys, the Global
Axes or the seabed. This allows, for example, a winch to be attached to the end node of a line so that winching in
or out can be modelled.
Shapes have a single joint which can be connected to Vessels, 3D Buoys, 6D Buoys, the Global Axes or the seabed.
When a joint is connected to a master, the connection is made at a specified master-relative position and the master
object then determines the position of its slave – the slave is dragged around by its master as the master moves. In
response the slave applies forces and moments to its master – for a line these are simply the end force and moment
applied by the line.
Because neither the Global Axes nor the seabed move, a joint connected to either of them is simply fixed in one
position. The difference between them lies in how the connection point is specified. For a connection to the Global
Axes, the X, Y and Z coordinates of the connection point are specified relative to those axes and the joint is called
Fixed. For a connection to the seabed the X and Y coordinates are specified relative to the global axes, but the Z
coordinate is specified relative to the seabed Z level at that X,Y position; the joint is then referred to as being
Anchored. So for an Anchored joint, Z=0 means that the connection is exactly on the seabed and Z=1 means it is 1
unit above the seabed. By using anchored joints you can therefore avoid the need to calculate the seabed Z level at
the given X,Y position (not simple with sloping seabeds).
5.4 INTERPOLATION METHODS
OrcaFlex uses a number of different methods for interpolating data. These methods are described below:
� Linear. The data is assumed to follow a straight line between each (X,Y) pair. Linear interpolation is said to be
piecewise linear. Curves that are linearly interpolated are continuous but their first derivative is discontinuous
at each X data point.
� Cubic spline. Cubic spline interpolation fits a cubic polynomial over each interval in the data, so the fitted
interpolation curve is piecewise cubic. These cubics are chosen so that both the first and second derivatives are
continuous at each X data point. A consequence of this is that with cubic polynomial interpolation the (X,Y) data
specified at any given data point affects the interpolated curve over the whole range of X values, not just over
the intervals near that (X,Y) data point. In other words cubic spline gives a 'non-local' interpolation.