Investigation On Dynamic Characteristics For Deep-Water Risers In Internal Wave, Current And Water Wave

The ocean environment where the deep-water risers stay will become much worse with the depth increasing of producing offshore oil. When the ocean current flows the riser, vortex-induced vibration occurs once shedding vortices exert periodical forces on the riser. Such a VIV phenomenon can damage the riser. If there exist surface waves or internal waves, the responses of riser will become large suddenly, which will reduce the riser's life.On the basis of reviewing recent progress of such subjects, the works in the thesis are presented as follow:(1)Based on the mKdV theory on the internal solitary wave, the Airy wave theory on the surface wave, and the VIV theory, a method was presented for computing lift and drag forces on the riser by modifying the Morison equation due to internal wave, current and water wave.(2) Based on the dynamic equation of the elastic beam where the effect of the internal flowing fluid is taken into account, a coupling nonlinear dynamic model for analyzing longitudinal and transverse deflections of the cantilever pipe conveying fluid considering the nonlinear fluid loads combined wave-current is presented, where a numerical method based on the Galerkin method is presented for solving the problem.(3) The dynamic responses including the primary resonance induced by current and the composite resonance under combing wave-current for a cantilever riser were considered, and dynamic characteristics of the riser were obtained due to such primary and composite resonances. (4) The dynamic responses in the transverse and longitudinal directions for a top tension riser under combined internal solitary wave and non-uniform current were considered. Results show that the multi-modally response will be excited due to such a non-uniform current, and bending moment and stress will become large due to internal solitary wave.Results show that the sudden impact loads induced by the internal solitary wave or surface wave will lead to not only the remarkable increase of the transversely composite resonance amplitude, but also much complicated deflections in longitudinal directions during the multi-modal vortex-induced vibration of the riser.