|dc.description.abstract||Fatigue analysis of marine risers due to vortex-induced vibrations (VIV) is normally based on semi-empirical frequency domain methods which only allow for linear structural models, excluding non-linear effects (e.g., tension variations and riser-soil interaction).
In order to alleviate this limitation, a time domain VIV model based on the synchronization concept was firstly proposed by Thorsen. Then, Thorsen and Ulveseter contributed to updating and validating the model with respect to various marine riser VIV tests. The latest version of the model by Ulveseter enabled the analysis of combined cross-flow (CF) and in-line (IL) vibrations. On this basis, the main purpose of the present work is to further improve the VIV load model and to validate the model against deepwater riser VIV tests by also including relevant non-linear structural effects, which were not considered in previous works.
Four relevant studies are included in the thesis. The first two focus on the highmode VIV response representative for deepwater riser VIV. Basic validation of the time domain VIV model was performed by applying a linearized test riser model. In the following validation study, focusing on the high-mode VIV response, a non-linear riser model, and an updated time-domain VIV model were considered for validation. Simulations of the non-linear behavior caused by VIV were also included. The last topic of the study addressed the VIV response under three-dimensional currents which is another feature of the deepwater riser VIV problem. The time domain model prediction performances for different types of three-dimensional current profiles were investigated. In addition, fatigue analysis results were discussed through a case study focusing on different current profiles and riser model assumptions.
Overall, the results of the studies showed that the time-domain VIV model can reasonably describe the characteristics of deepwater riser VIV responses. In addition, it was shown that the non-linear phenomena related to the VIV response can be predicted. Therefore, it is expected that the results of this thesis can contribute to improving the current design practices, especially for cases where non-linear structure phenomena will contribute to the responses.||en_US