Time domain versus frequency domain VIV modelling with respect to fatigue of a deep water riser
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- Institutt for marin teknikk 
In this thesis the new time domain model developed by PhD. Mats J. Thorsen is compared with the current practice, with respect to fatigue of a deep water riser. Current practice is to superposition the fatigue damage due to Vortex Induced Vibrations (VIV) obtained from the frequency domain and the first order wave dynamics from time domain analysis. Thorsen's model combines the Morison and VIV forces in one equation solved in the time domain. Thorsen's Model is implemented in RIFLEX which is compared with a superposition of RIFLEX for the time domain analysis and VIVANA for the frequency domain analysis for the current practice. A riser installed at a water depth of 1200 meters with 2 meter elements is investigated. The cross section is constant for the whole riser with a diameter of 0.3 meters. The riser is subjected to sea stated based on data from the Norwegian Sea. The fatigue analysis is carried out in MATLAB based on the result from RIFLEX, both for the new time domain model and for the dynamic analysis based on current practice. The stress standard deviation and maximum accumulated damage are found directly in VIVANA for the frequency domain model. For the first cases investigated in this thesis, a mean current with no deviated waves, the two methods showed a good agreement for stress standard deviation and maximum accumulated damage. With applied waves, a good agreement for stress standard deviation were still obtained while the maximum accumulated damage showed some deviations. The maximum accumulated damage from the current practice was higher than for the new time domain method. This is due to the way fatigue is calculated, the accumulated damage is a function of the stress to the power of four. When the stress standard deviation is equal, one would expect equal damage as well. Since the current practice uses superposition is the total stress standard deviation from two different contributions while the new time domain method calculates it as one. The two contributions from the current practice raised to the fourth power and then added yields a smaller value compared to the total stress raised to the fourth power. The second case, a one year current with and without waves, showed huge differences between the new time domain model and current practice. One reason for these huge differences might be that VIVANA uses frequency domain and the option simultaneously acting frequencies to calculate the response. The excitation zones where the dominating frequencies are acting, according to VIVANA, are at the bottom of the riser while the new time domain model showed highest response at the upper half of the riser. For the third case was response amplitude operators (RAOs) added to the model. The new time domain model was more affected by the RAOs than the current practice, but no huge differences in the fatigue damage because of the added RAOs. The most important difference between the methods is the way fatigue damage is calculated. The current practice underestimates the maximum accumulated damage by use of superposition. Based on this is the new time domain model calculating a more accurate fatigue damage than the current practice.