OrcaFlex Manual - Orcina

System Modelling: Data and Results, Lines
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For many cases, e.g. when modelling a simple homogeneous pipe that carries all the loads, these load factors should
be set to 1, the default value.
In some cases, values less than 1 may be suitable. For example, consider a case where the line models a composite
structure that consists of a main carrier pipe and an external piggyback pipe. You might estimate that the main pipe
takes all of the tensile and torsional loads, but only carries 70% of the bending loads, the other 30% being taken by
the piggyback pipe. Then to obtain stress estimates for the main pipe you could set the Stress Outer and Inner
Diameters to '~' and set the bending and shear stress loading factors to 0.7.
Note: The Stress Loading Factors only affect the wall tension results, stress results and fatigue analyses.
These results are derived after the simulation has run, and because of this OrcaFlex allows these
data items to be modified after a simulation has been run.
Friction Data
Seabed Friction Coefficients
OrcaFlex applies Coulomb friction between the line and the seabed. The friction force applied never exceeds μR
where R is the seabed reaction force and μ is the friction coefficient.
Lines lying on the seabed often move axially more readily than they move laterally. To enable this effect to be
modelled, you can specify different friction coefficients μ for motion normal (i.e. lateral) and axial to the line. For
intermediate directions of motion OrcaFlex interpolates between these two values to obtain the friction coefficient μ
to use. If the axial friction coefficient is set to '~' then the normal friction coefficient is used for μ for all directions of
motion. This provides a convenient way of using the same friction coefficient for all directions of motion.
See Friction Theory for further details of the friction model used.
Note: The friction coefficient for contact with elastic solids is specified on the Solid Friction Coefficients
data form.
Typical values
Published data are sparse. Some information is given in Puech (1984) and Taylor and Valent(1984). Both references
distinguish between sliding friction and starting friction: starting friction is greater to represent the "breakout"
force. OrcaFlex does not draw this distinction. In most cases, the sliding friction coefficient should be used; this will
usually be conservative. Both references are written in the context of the contribution of chains and cables to anchor
holding power, so we assume the friction values given are axial. Transverse values will be greater, perhaps by 50%
to 100%.
The values given below are recommendations from Taylor and Valent.
Line type Seabed Type Starting Friction
Coefficient
Chain Sand 0.98 0.74
Mud with sand 0.92 0.69
Mud/clay 0.90 0.56
Wire rope Sand 0.98 0.25
Mud with sand 0.69 0.23
Mud/clay 0.45 0.18
Structural Damping Data
Rayleigh Damping Coefficients
Sliding Friction
Coefficient
A named Rayleigh Damping Coefficient data set. This data item can be set to "(no damping)", in which case no
Rayleigh damping will be applied for this Line Type.
This data is only available when using the implicit integration scheme.
Equivalent Line Data
The properties of an equivalent line type are calculated from properties of other line types. For example, consider a
pipe-in-pipe system. These are often modelled by combining the properties of both external and internal lines into a