ENGINEERING RESULTS AND ACCOMPLISHMENTS FOR LEG 179
Although the complete HDS test plan could not be conducted because of the premature failure of the underreaming bits, a great deal of data was collected and a much better understanding of the HDS as deployed at sea was gained. Of primary concern was the performance of the hammer. Considering the sea state during most of the test, the hammer performed quite well. It must be noted that the hammer used for the tests was designed for drilling a 12-1/4-in borehole and that during the testing it was used to drill a 14-3/4-in borehole, an ~45% larger hole, thus reducing the efficiency of the hammer. The water hammer was also designed to work at maximum efficiency with a 2250-psi pressure drop across the piston. Because of the excessive vibrations in the pumps and stand pipe, the maximum continuous pressure drop across the hammer piston that could be maintained was ~1750 psi, which further reduced the hammer’s efficiency. However, in spite of the low hammer efficiency, a rate of penetration of 4.8 m/hr was achieved in Hole 1106E in massive gabbro, using a 12-1/4-in standard hammer drill drilling bit.
Further analysis of the underreaming bits is required. However, first impressions are that the BHA was leaning over during spudding during the early stages of drilling. Being composed of 9-1/2-in drill collars, the BHA was very stiff and so, as the BHA leaned over from being placed in compression, it caused the underreaming bits to be rotated about the horizontal axis (perpendicular to the drill string axis). This rocking of the underreaming bit in the hole during drilling probably caused extremely high loads to be placed on the low side of the bit underreaming arms. The high loads resulted in shearing off the TCIs and severely abrading the underreaming arms themselves. Once the TCIs were broken off of the underreaming arms, the arms acted much like a bearing, preventing further penetration by the bit.
It is encouraging to note that the pilot portion of all the underreaming bits came out of the hole in good condition, further indicating that the hammer drill can penetrate subsea hard rock formations and that the premature failure of the underreamer arms is what prevented deep penetration by the bit. The standard hammer drill drilling bit used in Holes 1106C and 1106D showed no signs of wear when retrieved plus the excellent penetration rate observed in Hole 1106E further indicate that the overloading of the underreaming arms was the cause of lack of penetration with the underreaming bits.
There were three primary problems associated with the hammer drill during the tests. Twice during the testing, the hammer internal control valve cracked, thus allowing pressure to escape past the valve, preventing the piston from cycling. Failure of the valve appears to have been from two causes. First, the valve has some ports in a highly stressed area, causing a definite stress riser in the valve body. Second, the hammer was constantly opened and closed because of heave. When this occurred, the pump could not be stopped in time to prevent the pressure drop across the valve from dropping to near zero and then almost instantaneously increasing to ~1750 psi. The combination of the stress riser and the pressure cycling probably caused the cracking to occur in the valves.
The second problem that occurred with the hammer drill during the tests was that on two separate occasions the piston began galling to the lower bushing. A hard coating had been applied to the piston where it passes through the lower bushing. A close tolerance fit is used in place of a dynamic seal between the piston and lower bushing. It appears that the hard coating may have been spalling as a result of cavitation erosion. The small flakes of the hard coating appeared to be wedging between the piston and the lower bushing, causing the galling to occur. This theory will have to be studied further in a metallurgical laboratory. In any case, the galling resulted in sluggish and erratic operation of the hammer; and thus, the hammer had to be retrieved and the piston and lower bushing had to be replaced each time the galling occurred.
The third problem with the hammer drill during testing was associated with the stroke length, 40 mm, to open the bypass, thereby stopping the hammer from cycling. This stroke may be too short for deployment of the hammer drill from a floating vessel. Adding to the problem is a piston effect on the bit caused by the pressure drop across the hammer acting on the bit shank cross section area. This piston effect causes the bit to be pumped downward, thus opening the bypass, as the hammer drill indrills off in or is raised off bottom. Once the bypass opens, the power fluid is diverted around the piston, preventing the hammer from cycling. Thus whenever a large heave occurs that the heave compensator can not completely adjust for in the drill string motion, the hammer bypass is opened and the hammer stops cycling. None of the seafloor hardware or casing running tools was deployed during the testing, so no data on the performance of this equipment was obtained. However, this equipment was assembled and fit tested without problem.