OrcaFlex Manual - Orcina

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Figure: Stress joint profile at End B, correctly modelled
6.8.18 Modelling Bend Restrictors
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System Modelling: Data and Results, Lines
We begin by introducing some terminology. A bend restrictor is any device that controls, restricts or limits bending
on a line. A bend limiter is a bend restrictor that has no effect until a certain curvature is reached, and then
curvature is prevented from going above that value. A bend stiffener is a bend restrictor that provides increased
bend stiffness in order to distribute more widely the bending.
Modelling Bend Limiters
Non-linear bend stiffness can be used to model a bend limiter. The approach is to specify a relationship between
curvature and bend moment that has:
� Low stiffness for curvature values lower than the lock-out curvature.
� High stiffness for curvature values greater than the lock-out curvature.
Typically the low stiffness value will be close to zero and the high stiffness value will be one or two orders of
magnitude greater than the stiffness of the protected line. Try to avoid using too large a value since doing so can
result in numerical instability. It may also help to smooth the transition from low to high stiffness.
The most common modelling approach for bend limiters uses a single equivalent line type object to represent both
the protected line and the limiter. The bend stiffness for this equivalent line type must account for both the
protected line and the limiter. You may choose also to account for mass, displacement and hydrodynamic properties
but often these properties are of lesser importance.
An alternative to the equivalent line approach is to model the limiter as a separate object using the bend stiffener
attachment (see below). The main difference from an elastomeric stiffener is that a general category line type with
non-linear bend stiffness must be used for the attachment line type. The main advantage of this approach is that it
becomes easier to check that the data is specified correctly because you can keep the data for the protected line
separate from the data for the limiter.
Modelling Bend Stiffeners
Bend stiffeners are modelled in OrcaFlex using two separate lines to represent the stiffener and the line which it
protects, which we refer to as the protected line. The region of the protected line which is covered by the stiffener
is called the protected region.
The two line approach enables reporting of separate results for the protected line and stiffener. In particular this
makes fatigue analysis of the protected line quite simple since the reported loads and stresses for the protected line
do not include the contributions of the stiffener.
The protected line can have linear, non-linear elastic or hysteretic bending properties. The stiffener is modelled as a
profiled homogeneous pipe. The stiffener can have linear or non-linear elastic material properties.