The influence of high harmonic force on fatigue life and its prediction via coupled inline-crossflow VIV modeling
Massachusetts Institute of Technology. Department of Mechanical Engineering.
Michael S. Triantafyllou.
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Vortex Induced Vibrations (VIV) of a flexibly mounted rigid cylinder placed in a flow is a canonical problem of fluid-structure interactions and the study of VIV and the resulting material fatigue is particularly important in design for ultra-deep water oil exploration and development. Long cylindrical structures such as risers and mooring lines can be outfitted with VIV-cancellation systems or subjected to aggressive replacement schedules. However, the cost of these measures can be high. Effective VIV and fatigue prediction programs offer the potential for design and replacement schedule optimization. For a long time, researchers believed that these oscillations were mainly in the crossflow direction (a direction perpendicular to the flow direction) and were purely harmonic ones with only one major frequency close to the Strouhal frequency (a dimensionless representative of the frequency of vortex shedding). The study of inline motion of long flexible cylinders caused by VIV has been long neglected due to its small amplitude compared to the cross-flow response amplitude. However, it is shown that the inline motion has a major impact on fatigue life due to its higher frequency (second harmonic). More importantly, it triggers a third harmonic stress component in the crossflow direction along with a chaotic frequency stress component. In this thesis, first the impact of the higher harmonic components and the chaotic response on the resulting fatigue damage of the flexible cylinders was first systematically assessed. Statistical methods were employed to generate a stress time series based on the power spectrum density (PSD) plots similar to those of the experimental signals. It is shown that the high harmonic signal has a significant influence on the fatigue life, while the wide-band chaotic components have limited effects. Thus, prediction of high harmonic forces through coupled inline-crossflow VIV modeling is the key to the problem. Thereafter, the semi-empirical crossflow VIV prediction model was revisited. Prediction results from VIVA - a VIV response prediction program widely used in the offshore industry - were improved through the systematical adjustment of hydrodynamics coefficients based on field and laboratory experiments. The results were benchmarked with controlled laboratory experiments, including the Norwegian Deepwater Programme (NDP) Riser High Mode VIV test, the Chaplin experiment, the ExxonMobil 10m long riser model datasets, and field experiments including the Miami experiment (MIT-DeepStar datasets) and the BP bare full-scale riser datasets in the Gulf of Mexico. Afterwards, pure inline VIV was studied using both experiments and a semiempirical model. A pure inline VIV hydrodynamic force coefficients database was constructed from forced motion experiments in the towing tank. A spring-danper rigid cylinder VIV model was created and the pure inline hydrodynamic database was employed to calculate the fluid forces to the system. The motion predicted by the model was compared with free vibration experiments. Additionally, an inline response prediction module was introduced to VIVA to predict the inline response of flexible cylinders. The results were benchmarked with the Norwegian Deepwater Programme (NDP) Riser High Mode VIV test data. Finally, the coupled Inline-crossflow VIV problem was systematically addressed. Extensive forced inline-crossflow experiments were designed and carried out in the MIT towing tank, which provided hydrodynamic coefficient databases for VIV prediction models. The constructed 2D VIV hydrodynamics coefficients database 'ZDMiT' was thoroughly studied and is expected to be useful for semi-empirical programs predicting coupled inline-crossflow VIV in the field. Additionally, a rigid cylinder VIV prediction model was developed to estimate the coupled inline-crossflow VIV response and the high harmonic VIV forces.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 383-389).
DepartmentMassachusetts Institute of Technology. Department of Mechanical Engineering.
Massachusetts Institute of Technology