Fang, Jie; (1992) On structural damping and vortex induced vibrations of marine cables and risers. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes a theoretical model developed to predict vortex induced vibrations of marine cables and risers with an emphasis on catenary flexible risers. Experimental tests were carried out to improve our understanding of structural damping of cable-like structures, and vortex cell properties, which are two important aspects of vortex induced vibration of risers. After an introduction to the background of vortex induced vibration problems and the progress of offshore oil production systems into deeper water, a review is made of research work in areas of vortex induced vibration, structural damping, and flow visualisation techniques. A wide ranging parametric study on structural damping of model scale pipes is presented. Tests were conducted on free-hanging, tensioned and catenary pipes and chains to identify the effects of parameters such as temperature, frequency, static preload, dynamic stress level and choice of materials on structural damping. An analytical solution of structural damping which originated from material damping is derived using a hysteretic damping model. It reveals the relationship between the basic material damping and the structural damping, and explains well the test results measured. Further investigation is needed on other forms of damping such as end losses and friction between the components of a structure as indicated by the test results. A separate series of full scale tests using a 10 meter long, 2 inch bore flexible riser section, provides typical damping values of such structures. This information is used as input for vortex induced vibration analysis as well as in general dynamic analysis of risers. Work on flow visualisations is reported. A number of methods such as hydrogen bubbles, dye, and pressure sensors were used in order to detect vortex cell shed from model circular cylinders undergoing vortex induced vibration in steady and unsteady flows. These methods did not provide as much information as desired. The vortex induced vibration model is developed for circular cylindrical bodies in steady flows. It is extended to oscillatory or wave flows and then extended further to catenary flexible risers. Predictions have been made for rigid riser models and two realistic catenary flexible risers in a range of flow conditions. The prediction results show that two factors are of particular importance in determining the amplitude of the vortex induced response. One is how closely the natural frequencies of a structure are spaced and the other is the degree of shear of the relative normal velocity profile along the riser length. The combined effect of these two factors determines the extent of lock-on regions for each mode excited, this in turn determines the level of fluid damping and excitation and finally the response amplitude. The prediction results also show that there is no significant response of the two catenary flexible risers studied under realistic working environments, mainly because their natural frequencies are too low and too closely spaced.

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