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Fatigue Analysis of Flexible Risers: A comparative study of different models for metal fatigue in flexible risers
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Marine Technology.
2013 (English)MasteroppgaveStudent thesis
Abstract [en]

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 model. 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 ii was conducted. 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 been calculated. The M3 model showed a pronounced hotspot damage in the root, iii 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.

Place, publisher, year, edition, pages
Institutt for marin teknikk , 2013. , 143 p.
URN: urn:nbn:no:ntnu:diva-22034Local ID: ntnudaim:9451OAI: diva2:646772
Available from: 2013-09-09 Created: 2013-09-09 Last updated: 2013-09-09Bibliographically approved

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