TRANSIT TO SITE 1116 (ACE-16A)
While under way to Site 1116 (ACE-16A), the drill crew prepared the RCB coring and BHA with a new C-4 core bit and MBR. At 0330 hr on 23 July, the seafloor positioning beacon was deployed, beginning Hole 1116A.
After lowering the BHA to 1819.0 mbsl and picking up the top drive in preparation for spudding, the driller began the normal process of slowly circulating seawater ("slow circulating rates," SCRs) to establish baseline rig floor pressures for various flow rates before spudding. The pump pressure immediately rose to 600 psi (at 40 spm). Because this was abnormally high, the sinker bars were run in the hole to recover the core barrel. There was no sign of any sediment nor were any mechanical problems noted with the core barrel or its retrieval. Another core barrel was pumped to bottom, and the driller started circulating the seawater. This time the pump pressure rose immediately to 1400 psi and continued to climb until the driller shut down the pumps. It appeared that all of the bit nozzles were completely plugged. We decided not to make any more attempts to clear the blockage because in this relatively shallow water it would not take too long to retrieve the BHA to physically investigate the problem.
The bit reached the rig floor after a 4.5-hr pipe trip and a thorough inspection of all the BHA components failed to identify any mechanical problem or blockage. The bit, bit jets, support bearing, float valve, MBR, and core-barrel latch sleeve were all found to be unobstructed and in normal condition. Furthermore, there were no indications that any of these components had been below the seafloor, nor had the drillers seen any reduction in drill string weight to indicate that the bit had contacted the seafloor during the pipe trip.
While the drill crew was reassembling the original core barrel components for a rig floor flow test, the coring technician inspected another bit release assembly that was in the process of being rebuilt in the core technicians shop. He then noticed that when the lower support bearing was installed in the pocket of the bit disconnect, there appeared to be no flow path remaining for the circulating fluid. Upon checking the machine drawing of the bit disconnect it became apparent that this component had a manufacturing flaw. A 60% taper at the base of the pocket was missing, which prevented nearly all of the flow around the bearing. The only flow area remaining was that left between the inside diameter of the support bearing and the outside diameter of the core barrel landing sub. The difference in pump pressure between the first and second core barrel deployed may have resulted from different amounts of wear on the respective landing subs, resulting in a more or less internal flow area.
The rig crew reassembled the original outer core barrel components and when the circulating pumps were activated the pump pressure was significantly higher than normal (600 psi at 15 spm). This confirmed that there was a mechanical blockage somewhere in the coring assembly, and upon removing the suspect MBR for the second time, it too was missing the critical taper. After installing a new MBR with the correct taper, a rig floor flow test indicated normal pump pressures. The rest of the BHA was prepared and lowered to the seafloor.
Hole 1116A was spudded at 1915 hr on 23 July and, based on a reduction in drill string weight, seafloor depth was determined to be 1851.3 mbsl. Because of the indurated nature of the seafloor sediments, the first few cores progressed slowly. After a few drill collars were buried beneath the seafloor, the drillers were able to increase the WOB and the top drive rpm. The first 48 mbsf cut at an average rate of 2.5 m/hr with 500010,000 lb WOB and 40 to 50 rpm. RCB Cores 1R through 18R were taken from 0 to 158.9 mbsf. Core recovery in Hole 1116A was generally poor, ranging from 2% to 33% (average 21%), and the overall penetration rate was 3.8 m/hr.
When the drill string was being raised after cutting Core 18 (0215 hr on 26 July), the drill string started experiencing torque. The driller continued to raise the drill string up to 153.0 mbsf, where rotation was lost and the pipe became stuck. For the next 1.75 hr, the driller attempted to free the drill string. He was able to maintain circulation in the hole, but could not rotate or raise/lower the pipe. Initial attempts to actuate the drilling jars were unsuccessful, leading the drill crew to believe that the drill string may have been stuck above the jars. Overpulls of up to 200,000 lb were unable to free the drill string.
After recovering Core 18R, we resumed efforts to free the drill string. At 0600 hr, after working the pipe for another 1.25 hr, the drilling jars were successfully actuated with ~40,000 lb of overpull. Drill string rotation and vertical movement was immediately established; however, pump pressures remained slightly higher than normal.
A short wiper trip of the hole was made up to 91.0 mbsf and all drilling parameters returned to normal. At 0630 hr on 26 July, a core barrel was deployed and the driller prepared to run it back to bottom to resume coring. When pump pressures did not rise as normally occurs when the core barrel has landed, the driller began to suspect that the bit may have released during the efforts to free the drill string. Another explanation may have been that a drill string component had been bent and was keeping the core barrel from landing properly. The sinker bars were run in the hole to determine if the core barrel had landed properly, but the sinker bars passed below the depth where the top of the core barrel should have been and then passed even slightly below where the bit should have been. This confirmed that the bit had indeed released; therefore, we retrieved the drill string and abandoned the hole. The positioning beacon was recovered at 1030 hr, and the pipe trip was completed at 1100 hr, ending Hole 1116A.
At the rig floor the MBR was inspected. It was determined that the release sleeve had not shifted and the latch segments ("dogs") remained in place. Brinelling (deformation of metal) evidenced on the upper surface of the segments indicated that the bit disconnect portion of the MBR had failed and been stripped over the latch segments. It is not clear whether the bit disconnect mechanically failed because of the jarring action or the overpull used in freeing the drill string. It is speculated that the bit was stuck in the hole and that the overpull had weakened or cracked the bit disconnect above the latch dog windows. Subsequent jarring with the mechanical drilling jars likely caused the disconnect to fail completely releasing the bit and freeing the drill string.
To 180 Site 1117
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