Numerical Simulation Of Dynamic Response And Fatigue Life Prediction Of Top Tensioned Risers

The aim of this dissertation is to investigate the dynamic characteristics and dynamic responses of top tensioned riser(TTR) under the excitation of waves, currents, internal flow, vortex-shedding and floating platform and vessel motions. It is the hope of this dissertation to provide theoretical foundation and guidance for the engineering.As one of the basic facilities in development of offshore oil-gas exploitation, top tensioned risers are the main connector between the platform and the mouth of a well at the seabed and they can be used for drilling, production and intervention. Marine risers are in very harsh environment in the deep ocean and they are subjected to many kinds of loads including waves, currents, floating platform motion at the upper end, earthquakes, ice, etc. While waves and currents generated by wind are the main loads on the riser. When waves and currents flow about the riser, due to the viscosity and inertia forces of the fluid, large static and dynamic displacements will be induced in the in-line direction of the flow while due to the presence of the riser there will also be flow separation at certain velocities resulting in shedding vortices and periodic wakes. Because of the periodic shedding of vortices, fluctuating forces in both the in-line and cross-flow directions are exerted on the riser and cause the riser to vibrate in the two directions. Also due to the action of the floating platform motion at the upper end and internal flowing fluids in the riser, more complicated coupled vibrations will be induced. All these phenomena will accelerate the fatigue failure of the riser, the failures of the riser will not only incure huge loss for the project itself but also lead to secondary haphazard. So further researches on the vortex-induced vibrations(VIV) and dynamic behavior of the riser under the excitation of waves and current are very necessary.At present, the VIV prediction software such as SHEAR7 and VIVARRAY is based on hydrodynamic coefficient database derived by large number of tests and also they are only used to model the cross-flow VIV. In consideration of the effect of the internal flowing fluid and the external marine environmental condition on the VIV of top tensioned riser, the in-line and cross-flow differential equations are derived based on work-energy principles and the riser near wake dynamics is modeled by a wake oscillator model. The corresponding Matlab numerical programs which solved the coupled equations directly in the time domain with iterative procedure are compiled, and the numerical simulation software of the vortex-induced vibration(named as NSVIV 1.0), which can be used to model the multi-mode and high order of VIV and fatigue life of the riser with arbitrary internal flow velocity and depth-dependent currents, is developed. The comparison of the predicted results with the recent experimental results, the prediction results of SHEAR7 and other's numerical results is performed and the results show the validity of the proposed method on the prediction of VIV of deep water risers.The moving boundary and effect of internal flowing fluid on the riser is most often not considered and the nonlinear drag force is linearized in the in-line dynamic behavior analysis of the riser. Also comprehensive analysis of the riser in consideration of various loads and boundary conditions is rarely seen. The dynamic behavior and fatigue life of the riser under combined excitations of ocean currents, random waves and vessel motion is calculated and parameters of various loads on the riser dynamic behavior and fatigue life are analyzed in detail.The following conclusions can be drawn. The internal flow has some influence on the fundamental frequencies, dynamic response amplitude and mode numbers and fatigue life of the riser, especially for risers with high flow velocities. Though the in-line VIV amplitude is just 10-30% of cross-flow amplitude, the in-line VIV may also have great contributions to the riser's fatigue damage for its vibration frequency is twice that of the cross-flow vibration. With respect to cross-flow vortex-induced vibrations, the in-line vortex-induced vibrations are more sensitive to top tensions and with the increase of top tensions the dominant vibration modes may shift from higher order modes to lower modes. Both the in-line and cross-flow vibration amplitudes of the riser decrease with the increase of top tensions when the vibration mode does not change. The riser mainly has two areas of extreme bending stress values under the excitations of random waves, currents and vessel motion which are located at lower and upper part of the riser, respectively. With the increase of the current velocity, the stresses at the lower part increase greatly while the stresses at the upper part decrease. The bending stresses of the riser increase with the increase of the significant wave height. The bending stresses of the riser decrease rapidly with the increase of the top tension while the stresses increase with the increase of the vessel mean offset. The low frequency motion amplitude mainly affects the bending stresses of the lower part of the riser while the low frequency motion period mainly affects the bending stresses of the upper part of the riser. The lower end of the riser is more prone to fatigue failure under the vortex-induced vibration, while both the lower and upper end of the riser are prone to fatigue failure at in-line vibrations under excitation of ocean currents, random waves and vessel motion.