The Vibration Response Analysis Of Deepwater Drilling Riser

Now days, the exploration and exploitation of worldwide oil and gas resources is moving to deepwater and the drilling riser is the key element for drilling in deepwater. The global dynamic response and vortex induced vibration (VIV) have a significant effect on work and operation of drilling riser, and become of increasing concern in the field of offshore engineering.The response amplitude operators (RAO) of Semi-submersible platform are obtained by building and analyzing of hydrodynamic model. The time history of floating platform motion is stimulated. The nonlinear dynamic equation of deepwater drilling riser subjected to waves, ocean currents and vessel motion has been developed. The effects of top tension, mud density, wave and current on the global response of drilling riser are studied by solving the fourth-order nonlinear partial differential equations. The vessel motions effects on the global dynamic response of deepwater riser are investigated, including mean offsets, wave frequency motions, low frequency motions, period and amplitude of the motion. The results show that vessel motion has an important effect on the dynamic response of deepwater riser, and lead to significant cyclic stress in the upper part of the riser, and result in important fatigue damage.The Time-Frequency model for the predication of VIV responses of riser are proposed, using the fluid force model based on the date from Gopalkrishnan's forced oscillation tests of rigid cylindrical bodies. The real response frequency and the distribution of added mass are obtained by considering the relationship between added mass and non-dimensional frequency. Using the frequency method of VIVANA, the model identifies the response frequency of the riser, and establishes the lift force model and damping force model using identified response frequency, and then obtains the VIV responses of the riser by solving the partial differential equation in time domain. The proposed Time-Frequency model is validated with VIV experiments on the riser in stepped current and shear current. Compared to the frequency model, the predicted responses of the present model agreed well with the experimental ones.