Abstract
In this research, in order to investigate the safety risk of drilling riser during deep-water Christmas tree lowering process and determine the basic parameters for the safety calculation of the upper part of the Christmas tree, wave-current force calculation method is explained based on modified Morison equation. According to differential element method, the dynamic equations of vertical and horizontal bending of riser under wave and current conditions are derived and equation solution method and calculation stability conditions are analyzed. By combining the upper and lower boundaries of riser during Christmas tree connection and environmental load factors, dynamic responses of riser under different wave, top tension, platform drift, operating water depth and other parameters are investigated. The obtained results show that maximum lateral displacement, corresponding water depth and bending moment of riser as well as the amplitude and period of displacement fluctuation at riser bottom are increased with the increase of wave velocity and water depth. It is also found that increase of top tension reduced the maximum lateral displacement of riser and increased corresponding water depth, but excessive tension increased the fluctuations of displacement and bending moment at riser bottom. Increase of platform drift increased maximum lateral displacement fluctuation range, bottom string displacement and bending moment of the riser and decreased corresponding maximum displacement water depth. Drift is found to increase the fluctuations of dynamic parameters at riser bottom. Increase of operating water depth increased riser maximum lateral displacement and decreased the period and amplitude of bottom bending moment fluctuations; also, maximum water depth position approached to "1/3" position on sea surface.
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