At present, one of the keys to develop deepwater natural gas hydrate resources is to control the downhole pressure during drilling of hydrate reservoirs. This paper analyzes the effects of slow drift motion of mobile offshore drilling equipment on the heave motion of the drill string and downhole pressure in a riserless dual gradient drilling (DGD) system. A coupling model of platform motion-drill string vibrationdownhole pressure in a hydrate reservoir was established. The effects of drilling platform positioning, environmental factors, drilling fluid density and mud pump parameters on wellbore pressure fluctuations were considered.
The riserless drilling method is a type of dual-gradient drilling system. Compared with the conventional riser drilling system, it directly exposes the drill string to the seawater and separates the annulus of the wellbore from the seawater by a subsea suction device. When drilling without a riser, the drilling fluid is pumped into the drill pipe by the sea surface pump, and then reaches the bottom of the well from the drill pipe, and returns to the mud line from the annulus, and is diverted into the subsea lifting pump unit through the subsea suction device. The small-diameter up return pipeline is returned to the drilling platform. After the returned drilling mud is processed, it enters the mud circulation system again. The aim of controlling the wellbore pressure during the drilling of Marine hydrate reservoir is achieved by controlling the speed and flow of the submarine lift pump and controlling the mud slurry height in the rotary bop.
Deepwater riserless drilling fluid recovery drilling technology (RMR) was first proposed and developed by Norwegian AGR Subsea Company, which is mainly used to solve various problems caused by conventional riser systems and is currently used in industrial applications was discovered (Alford and Asko and Campbell, 2005; Scott and Ledbetter and Chester, 2006; Stave and Farestveit and Yland and Rolland, 2005). Xu Qun (Qun and Guoming and Guodong and Changjing and Zhiming, 2011) summarized the development status of riserless offshore drilling technology and analyzed the application characteristics of RMR technology. Gao Deli (Deli and Hui, 2012) established a model of lateral and longitudinal vibration generated by external current load under the condition of riserless drilling and provide a new guiding method for the process design and strength check of the pipe string feeding process of deep water riser drilling system.However, the influence of the platform motion on the string vibration was not considered. Su Kanhua (Kanhua and Wei and Jilin and Chengwei, 2013) established a mechanical calculation model of the drill string by analyzing the motion boundary conditions of the upper part of the drill string in a deepwater without riser environment, and comprehensively analyzing the motion of the platform and the effect of external wave loads. However, the influence of the positioning mode of the platform on the heave motion of the drill string was not considered.