OPERATIONS SYNOPSISTransit to Site 1185
The plan at Site 1185 was to drill two holes. The first was to be a pilot hole drilled to basement and the second a reentry hole with a reentry cone and short section of 16-in casing. We spudded Hole 1185A with the RCB at 2100 hr on 10 October and began a jet-in test. The purpose of the jet in test was to determine the amount of 16-in casing to affix to the reentry cone on the second hole. The crew observed the bit with the VIT system to tag the seafloor at a depth of 3898.9 mbsl. The jet-in test was concluded at 2300 hr after the bit had advanced 38 m. The results of the test indicated that a 30-m string of 16-in casing would be sufficient to anchor a reentry cone. Following the jet-in test, we drilled ahead with a wash barrel in place to a depth of 250.6 mbsf, where we initiated coring. The average rate of penetration through the washed interval was 53 m/hr.
We rotary cored the sediment portion of Hole 1185A from 250.6 to 308.5 mbsf (57.9 m interval) with an average recovery of 23.7%. Headspace gas analysis of the sediment cores yielded <2 ppm of methane and no heavier hydrocarbons. We contacted basement at 308.5 mbsf. Rotary coring continued into basement to a depth of 328.7 mbsf. The average recovery for basement was 56.1% (20.2 m), and average penetration rate was 2.3 m/hr. After determining the depth to basaltic basement, we realized the objective of the pilot hole, and we ceased coring in Hole 1185A. We retrieved the drill string, replaced the bit, and the vessel was offset 20 m west of Hole 1185A in preparation for the next hole.
The objective of Site 1185 was to penetrate at least 100 m into basaltic basement. Because of the difficulty of locating FFFs that had sunk into the sediment at previous sites, we decided to set a reentry cone with 28 m of 16-in casing in Hole 1185B to ensure that we would be able to make multiple round trips of the drill string in order to change bits.
A reentry cone that was fabricated for Hole 1183A but unused was positioned in the moonpool and centered under the rotary table. After removing and securing the upper guide horn, the 16-in casing hanger was connected to the Dril-Quip cam-actuated-drill-ahead (CADA) tool and secured in the derrick. Two joints of 16-in casing and a shoe joint were then assembled and affixed to the casing hanger. Once the casing was assembled, a stand of 5.5-in drill pipe with the 16-in CADA tool was connected to the casing hanger. The hanger and casing assembly were then lowered through the rotary table just above the throat of the reentry cone. However, the throat of the reentry cone was too narrow and prevented the assembly from landing properly. After the welder modified the internal throat clearance, the hanger and casing were successfully latched into the reentry cone. The CADA running tool was then removed.
The crew assembled a jetting-in BHA with a 14.75-in tricone drill bit and a stand of 8.25-in drill collars. This was followed by connecting the CADA tool to a stand of two 8.25-in drill collars and one tapered drill collar. We then lowered the BHA and CADA tool through the rotary table and connected them to the casing hanger. We lowered the complete assembly the seafloor and deployed the VIT during the pipe trip. We spudded Hole 1185B at 0930 hr on 13 October. It required 2 hr to jet in the 16-in casing. After successfully uncoupling the CADA tool from the casing hanger, we recovered the drill string and prepared a RCB BHA with a new C-7 bit.
The crew lowered the BHA to the seafloor and deployed the VIT. We positioned the bit over the reentry cone and reentered Hole 1185B at 0833 hr on 14 October after an 8-min search. We deployed the BHA to 31 mbsf and recovered the VIT. At 1000 hr, we began to drill ahead with a wash barrel in place and advanced to 308.0 mbsf. The wash barrel was retrieved and a fresh core barrel dropped. We began rotary coring at 1900 hr and contacted basaltic basement at 309.5 mbsf.
We continued coring in basement and advanced to a depth of 434.6 mbsf (125.1 m into basement) by the afternoon of 17 October. After we added a new stand of pipe and dropped a fresh core barrel, the bit nozzles clogged and the pump pressure reached a maximum of 3000 psi, indicating that all circulation was lost. The driller was able to work the drill string and partially clear some of the blockage. Because the bit had accumulated 52 rotating hr, we decided to conduct a round trip of the drill string to replace the bit. Average recovery in the cored basement section was 40.1%, at an average penetration rate of 2.7 m/hr.
After the drill string was recovered, inspection of the bit showed that it was evenly worn, had two plugged nozzles, and had a slightly undergage body. The drilling jars could not be mechanically operated and were replaced. The crew affixed a new C-7 RCB bit to a mechanical bit release and deployed it. At 1330 hr on 18 October, we successfully reentered the hole after a brief search and resumed rotary coring at 1700 hr. Pump pressure was initially high (1500 psi at 75 strokes/min), indicating that a few of the bit nozzles were clogged. After advancing the bit several meters, the pressure gradually stabilized to a nominal value of 650 psi at 75 strokes/min. After heavy use of mud flushes to clean out the hole, we resumed rotary coring in basement. Coring advanced from 434.6 to 502.1 mbsf with an average rate of penetration of 1.7 m/hr and average recovery of 58.5%.
When Cores 25R and 26R were retrieved with no recovery, we deployed a bit deplugger twice in an attempt to clear a possible obstruction in the throat of the bit. We then cored a 1-m interval but had no recovery, so the deplugger was deployed a third time. After this, we dropped another core barrel, advanced 1 m, and recovered 0.98 m of basalt and a small segment of a bit seal O-ring. The latter suggested that the blockage of the bit was caused by partial disintegration of the bit seal. We dropped a new core barrel (Core 192-1185B-29R), but, after coring 7.6 m, the top-drive motor stalled at 600 A. The torque was slowly released, and the driller observed a loss of 500 psi standpipe pressure. The crew immediately turned off the active heave compensator, so they could use the Martin Decker weight indicator to determine downhole conditions. The driller noted a loss of ~40,000 lb of drill-string weight, indicating a failure somewhere in the BHA.
The crew circulated a 50-bbl sepiolite sweep to ensure a clean hole for fishing operations and
pumped a 20-bbl slug of heavy mud downhole. They set back the top drive and pulled the drill
string out of the hole. After considering the time that would be required to attempt fishing of the
BHA vs. the time that would be lost for scientific operations, we decided to abandon the site. Upon
recovery of the BHA, we found that the outer body of the drilling jars had broken apart. We
inspected the remaining components of the BHA above the jars but found no deficiencies.
During operations at Hole 1185B, we cored 216.6 m of basaltic basement with 42% average recovery. At 0515 hr on 22 October, the vessel began the transit to Site 1186.
Site 1186 | Table of Contents