TRANSIT CAPE TOWN TO SITE 1104
During the transit from Cape Town to Site 1104, the SDS water hammer drill was picked up and deck tested on 28 April. The test assembly consisted of an SDS concentric underreaming bit, SDS 12-1/4-in water hammer, SDS jet sub, with three blank nozzles installed, and the required crossover subs to the top drive. After making up the assembly, the bit breaker was placed on top of some dunnage and rubber matting on the rig floor. The bit was then lowered into the bit breaker and 5000 lb applied to the bit via the top drive. The mud pump was engaged, and the flow rate slowly increased to 75 gpm at 700 psi, when the hammer first began to cycle. As the flow rate was increased the hammer cycled intermittently and erratically. The flow rate was increased to 375 gpm at 1770 psi, and the hammer began to cycle more evenly. The flow rate was increased to 400 gpm at 1900 psi and the hammer cycled smoothly. The hammer was cycled for several minutes before the flow rate was reduced to 240 gpm at 900 psi. The hammer was then cycled for a few minutes more before shutting down the pump.
It was theorized that the initial erratic behavior of the hammer was caused by air in the pumping system and excess grease left in the hammer from assembly. To test the theory the mud pump was once again engaged and the flow rate slowly increased to 75 gpm at 270 psi, and the hammer began cycling very smoothly. To create a base line pressure vs. flow rate curve, the flow rate was increased in 50-gpm steps and the corresponding pressure recorded as follows:
|
Flow rate (gpm) |
Pressure (psi) |
Comments |
|
75 |
270 |
Hammer begins cycling smoothly |
|
150 |
360 |
Hammer cycling smoothly at higher frequency |
|
200 |
560 |
Hammer cycling smoothly at higher frequency |
|
250 |
820 |
Hammer cycling smoothly at higher frequency |
|
300 |
1125 |
Hammer cycling smoothly at higher frequency |
Large vibrations were noted in the stand pipe and derrick, presumably from pressure pulse reflections from the hammer traveling back to the pump.
Also during the transit, a frequency analyzer was assembled on board to monitor the hammer-induced pulsation frequency in the stand pipe as an aid in determining when the hammer was cycling. Although the initial frequency spectrum recorded was not a clear indication of when the hammer was operating, the voltage output spectrum from the pressure transducer installed in the stand pipe gave a good indication of when the hammer was cycling.
It is interesting to note that later in the hammer drill tests, additional filtering that made the frequency spectrum more prominent was added to the frequency analyzer. The frequency spectrum indicated a notable peak at ~30 Hz, the known operating frequency of the hammer. However, there was an even more prominent peak at ~60 Hz that was believed to be an indication of the power stoke of the hammer at 30 Hz, plus the return pulse as the hammer piston moved upward at 30 Hz and 180° out of phase, essentially doubling the frequency to 60 Hz. The increased amplitude of the 60 Hz signal may indicate that more energy is transmitted up the stand pipe by the hammer piston up stroke than is reflected by the power stroke when the piston moves downward. This further supported the theory that the stand pipe vibrations were indeed caused by hammer-induced pressure pulse reflections traveling back to the pump.