OrcaFlex Manual - Orcina

Theory, Static Analysis
Line Statics Step 2
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The second step, which is optional, is called Full Statics. If Full Statics is included, then it calculates the true
equilibrium position of the line. The calculation is iterative and hence it needs a starting configuration – for this it
uses the configuration found by step 1. If Full Statics is not included, then the line is simply left in the configuration
found by step 1, which (depending on the method chosen) may not be an equilibrium position.
Unstable equilibria
Sometimes Line Statics Step 2 finds an equilibrium configuration that is unstable. Everyday examples of unstable
equilibria include balancing a coin on its edge or balancing a pencil on its tip.
Very occasionally OrcaFlex statics converges on an unstable equilibrium. An unstable equilibrium can usually be
detected by the presence of large curvature spikes on a range graph. Typically a dynamic run will excite the line
enough to kick it out of the unstable equilibrium. However, statics and dynamics results from such unstable
equilibria are invalid.
Catenary
The Catenary method calculates the equilibrium position of the line, but it ignores the effects of bending and
torsional stiffness of the line or its end terminations.
The Catenary method also ignores contact forces between the line and any solid shapes in the model, but it does
include all other effects, including weight, buoyancy, axial elasticity, current drag and seabed touchdown (see
below) and friction.
Because bend stiffness (and other) effects are not included in the Catenary method, the position found is not, in
general, an equilibrium position. Therefore Full Statics should normally be included unless it is known that the
omitted effects are unimportant. Nevertheless, the Catenary position is often quite close to the true equilibrium
position, especially when bend stiffness is not a major influence.
The Catenary algorithm is robust and efficient for most realistic cases but it cannot handle cases where the line is in
compression. This is because, when bending stiffness is ignored, compression means the line is slack and there is no
unique solution.
The algorithm is an iterative process that converges on the solution. If necessary, you can control the maximum
number of iterations that are attempted, as well as other aspects of the convergence process – see Catenary
Convergence.
The Catenary solution has facilities for including seabed touchdown, with the following limitations:
� Seabed touchdown can only be included at End B of the line, not at End A. Therefore, if you want touchdown
then you should arrange it to be at End B.
� Touchdown is only included if End B is anchored exactly on, or else below, the seabed. If End B is above the
seabed, even if only by a small amount, then no touchdown will be modelled and the line may hang below the
seabed. This will be corrected when the simulation starts, since the nodes below the seabed will be pushed back
up by the seabed reaction forces. Alternatively, you should use the Full statics option, which can handle this
case.
� If End B is below the seabed, then the Catenary algorithm models touchdown by assuming that the line 'levels
out' at the level of End B. This will result in part of the line being below the seabed, but again this will be
corrected when the simulation starts, since the nodes below the seabed will be pushed back up by the seabed
reaction forces.
Spline
The Spline method gives the line an initial shape that is based on a user-defined smooth Bezier spline curve. It is
therefore not, in general, an equilibrium position, and so when it is used Full Statics should be included if you want
the equilibrium position to be found.
The Bezier curve is specified by the user giving a series of control points – it is a curve that tries to follow those
control points – and the Bezier curve and its control points (marked by +) can be seen on the 3D view when in Reset
state. The smoothness of the spline can be controlled using the spline order.
The Spline method puts the line into a position that, as far as possible, follows the Bezier curve. However the Bezier
curve may have the wrong length (depending on how accurately you have set up the control points), so the Spline