OrcaFlex Manual - Orcina

System Modelling: Data and Results, Lines
Data
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Although the stiffener is modelled as a separate line you do not need to create this line manually – OrcaFlex creates
it automatically as an attachment. The procedure for setting up a bend stiffener is as follows:
1. Create a Line Type which defines the material, structural and hydrodynamic properties of the stiffener. Usually
this will be a profiled homogeneous pipe.
2. Create a Stiffener Type which uses this Line Type.
3. Create a line attachment based on this Stiffener Type.
4. Set the line attachment position and the Stiffener Type connection arc length so that the stiffener is attached at
the desired location on the protected line.
If you have multiple protected lines which all use identical bend stiffeners then you can create a single Stiffener
Type which can be re-used on each protected line.
The stiffener profile uses the convention that profile arc length increases from End A towards End B of the stiffener.
If you have a bend stiffener connected at End B of a line then you will need to define the profile so that the arc length
0 refers to the tip of the stiffener. The Modelling Stress Joints topic illustrates this issue in some more detail.
Although the discussion there centres on stress joints many of the points covered are equally applicable to bend
stiffeners.
We strongly recommend that you use the Profile Graph available from the Line Data form to check that the stiffener
is connected at the correct location on the line with the profile defined as you intended.
Segmentation
The stiffener line that OrcaFlex creates is modelled with uniform segment length – that is every segment in the
stiffener has the same length. The segment length is determined by the segment length of the protected line in the
protected region.
The stiffener modelling (see below) requires that each node on the stiffener line is associated with a node on the
protected line. Each stiffener node is effectively clamped to its associated protected node.
These constraints have the following implications for the segmentation of the protected line:
1. The protected region must have uniform segment length.
2. The stiffener length must be an exact multiple of the segment length.
One simple way to satisfy these requirements is to model the protected region as a single section with length equal
to the stiffener length.
Note that it is not essential for the protected region to be a single section. The protected region could comprise
multiple sections each using different line types, so long as you satisfy the two rules above.
Drawing and Results
The stiffener line is drawn using the drawing data of the protected line to which it is attached. Note that the stiffener
is not drawn when the program is in reset state; it is only drawn after the static or dynamic analysis has started.
Results are available for the stiffener line exactly as they are for any other OrcaFlex line.
OrcaFlex reports results separately for protected line and stiffener line and this does need some explanation. For
example, consider bend moment at a particular location in the protected line and at the corresponding location in
the stiffener line. Suppose that the bending stiffnesses are EIp and EIs for protected line and stiffener respectively
(we are assuming linear bend stiffness for simplicity). The bend moment carried by the protected line and stiffener
ensemble is given by BMtotal = C(EIp + EIs) where C is the curvature at this location. For the protected line OrcaFlex
reports the local protected line bend moment BMp = C.EIp and likewise for the stiffener line OrcaFlex reports BMs =
C.EIs. It is straightforward to see that BMtotal = BMp + BMs.
The total load is also split into separate protected line and stiffener loads for effective tension, wall tension, shear
force, torque and stress results. However, the method for doing this varies for axial components as explained in the
next section.
Modelling details
As mentioned above the stiffener is modelled as a separate OrcaFlex line which is created automatically by OrcaFlex
as an attachment. The stiffener line inherits a number of properties from its protected line, namely: