Significant riser oscillations were observed during deep water drilling operations. The riser frequency was above and below the rotation speed of the drill pipe. Laboratory tests and of a riser section has been conducted for these riser drilling-induced vibrations. The first test consists of first measuring the fluid forces between a rotating drill pipe and a riser. In the second test, an elastically supported riser self excites about a rotating drill pipe. An analytical model is developed for predicting these drilling induced riser oscillations. The results show that fluid coupled riser vibrations can result from the fluid forces generated in the annular fluid between the rotating drill pipe and the riser.
A deep water riser assembly in 6000 ft (2000 m) of water is shown in Figure 1. During drilling, fluid flows downward inside the drill pipe and upward in the annulus between the drill pipe and the riser. The drill string rotates at 30 to 120 rpm, (0.5 to 2 cycles per second). The drill pipe has 5 inch (12.5 cm) outside diameter and riser has an outside diameter of 13 (33 cm). A submerged buoy supports riser weight. Riser tension is set to about 40% higher than the wet weight of the riser so the riser is always in tension. The drill pipe is also in tension along the riser section; it goes into compression below the mudline where the riser is cemented into the earth.
ROV field measurements made of the riser motion during drilling with sea water as the drill fluid show that the riser oscillates approximately 1 ft (0.3 m) amplitude. The riser vibrates between the mudline and the buoy at an unsteady range of frequencies between 40 to 60 cycle per minute, 0.6 to 1 Hz. The vibration only occurs during drilling. The currents were nominal on the third and final day of drilling, when the vibrations caused fatigue failure of the riser at the buoy. l.
