This thesis used different methods to model and predict the fatigue
from wave loads on the tensile armor layers in a flexible riser in the
bending stiffener area. The additional aim of the thesis was to give
the reader an understanding of flexible pipes and risers, and present
the necessary theory to understand and complete a dynamic analysis
of riser with the finite element method, and how to do lifetime
predictions from irregular time series.
Different methodologies exist for fatigue calculations for risers. However,
the general approach is to use the wave response from a global
riser model, and to apply this to a local model which has a more
accurate representation of the pipe?s cross-section. The actual fatigue
calculations are done using a SN-curve and Miner sum. The
variations of the different methods lie in how the models are made,
what kind of wave loads the global model is subjected to, and what
loads are used and how these loads are transformed in to the local
The scenario was a new field development close to the Enchova field
located in the Campos Basin off the Coast of Rio De Janeiro. This
field will be developed with a turret moored floating unit with lazywave
riser configuration at a water depth of 125 meters. The system
was modeled with simple tubular elements given the pipe?s bending
stiffness in slip. An irregular wave analysis using the local wave data
The transformation of the global time series was done using the Rainflow
method. The range spectra were divided into 40 bins. Three
different local models were made in BFLEX. The first model, M1,
was a stub, just a representation of the pipe?s cross-section. This
is a known approach in the industry. The two last models, M2 and
M3, were full sized representations of the bending stiffener, which
were submitted to axial and lateral loads by decomposing the tension
with the riser?s hang-off angle. This approach is less used. M2
and M3 used different types of elements. Their difference is in the
way the tensile armor layer is represented.
M2 and M3 have shown to give a less conservative result than M1.
This can be understood from the fact that M2 and M3 give a more
realistic representation of the load history. They capture other effects
as well, like for instance the interaction between the pipe and
the bending stiffener (BS) at the BS-tip.
It was shown that the BS was too stiff in the local model, this resulted
in an abrupt curvature response at the BS-tip. M2 suffered
the most to this to this. The fatigue response in M3 was more jagged
along the riser than the response in M2. A comparison of M3 with a
more refined mesh was conducted. This did not give the desired improvement.
The jaggedness could be understood from nature of the
shear interaction model in ITCODE0, whereas ITCODE31 uses elementary
beam theory after the moment-curvature relationship has
The M3 model showed a pronounced hotspot damage in the root,
which was believed being caused by the forced displacement the BS
exerted on the pipe seeing that both the pipe and the BS were modeled
in the same nodal system. This was tried solved with a contact
model, M3X, where the BS and the pipe were modeled in independent
nodal systems with the contact element, CONT130. M3X
proved itself difficult to converge. In the end a separate load history
was applied to M3X and in addition to M3 to demonstrate this load
history also had the same tendencies as witnessed earlier. It was
confirmed that the element type ITCODE0 has the same issues for
a dependent and an independent nodal system, and it was therefore
concluded that this element model must be revised in the future if
it should be applied in similar fatigue assessments.
The use of hang-off angle and full representation of the BS paves the
way for a less conservative fatigue assessment than the stub model.
The biggest damage found along the length of the BS is 51% less
than what is predicted from the stub model. However, it must be
noted that the too stiff BS could have lead to less motion over its
length since the pipe will get a hinge-like transition over the BS-tip
for the upper threshold of wave responses. Even so, it is reasonable
to believe that result for a correctly modeled BS will still give a
lower estimated damage than what is found in the stub model. The
conclusion is that the hang-off angle approach together with the ITCODE31
element is an interesting path to pursue. In addition, the
stub model approach could always be conducted as a supporting
model because of its fast solution time. As such, the already established
method does not necessarily need to be abandoned.
Institutt for marin teknikk , 2013. , 143 p.