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doi:10.2204/iodp.proc.303306.114.2006

Operations

We arrived at Site U1315 (proposed Site 642) on the morning of 15 April 2005 after a 1190 nmi transit from Site U1314. By 1120 h, the positioning beacon was deployed and operations at the site officially began.

Hole U1315A

After we verified the seafloor depth with the television/sonar system (1283.0 meters below rig floor [mbrf]), the drill string was recovered back to the drillship and we prepared to assemble and deploy the “elevated” reentry cone assembly and 10¾ inch casing string. Following assembly, the drill string with the reentry cone structure, 10¾ inch casing, and the mud motor and underreamer drilling assembly were tripped to the seafloor, spudding Hole U1315A at 2130 h on 15 April. The base of the reentry cone reached the seafloor at 0515 h on 16 April, resulting in an average rate of penetration of 21.9 m/h. The drill string was released from the reentry cone/casing assembly at 0550 h and then recovered back to the ship.

The drill string was reassembled with a cementing bottom-hole assembly and reentry cleanout bit and tripped to the seafloor, and Hole U1315A was reentered at 2026 h on 17 April. A 5 bbl, 15.8 ppg cement plug was displaced to bottom. The pipe was pulled clear of the seafloor/reentry cone, and the vessel was offset 30 m south where the drill pipe was thoroughly circulated clean. After the cement set, we reentered the hole and lowered the pipe, tagging the top of the cement at ~164.2 mbsf or ~11.6 m above the casing shoe, which was within 1.6 m of our 10.0 m target height for the top of the cement column. After the casing string was displaced with bentonite mud, the drill string was retrieved in preparation for the CORK deployment in Hole U1315A.

CORK deployment

The CORK running tool was made up to a 2.0 m drill collar pup joint and was laid out on the rig floor. The antitorsion ring, a recent modification that is designed to prevent accidental release of the tool during deployment, was installed along with the seal stinger. The Hole U1315A CORK head was moved from the core tech shop roof to the starboard side of the pipe stabber using the number 2 crane. From there, the load was transferred to two tugger lines and the head was placed through the rotary table with the bushings removed. The head was supported using 13⅜ inch casing slips with a dog collar installed above. The running tool was picked up with the drawworks elevators and was easily installed on the CORK head.

Two stands of 8¼ inch drill collars were made up to the top of the running tool, and the CORK assembly was lowered to the seafloor. During the pipe trip, the subsea television/​vibration-isolated television (VIT) frame was being lowered when the video was suddenly lost. The VIT frame was recovered, and troubleshooting identified a cleared power circuit breaker resulting from water leakage. In addition, a broken strand of outer armor was discovered ~360 m above the cable head termination. The resulting snarl in the armor was cut off, and the ends were secured. Once the cable head was on deck, we discovered that water had entered the oil-filled section of the cable head. The cable head connection was repaired rather than spending costly time completely reheading the cable. The VIT camera was ready to deploy once again at 0345 h on 19 April. A total of 3.25 h was required to make the necessary repairs.

Hole U1315A was reentered for the third and final time at 0448 h on 19 April. Space out was tight given the short length of the CORK stinger (20.85 m). For thermistor string deployment, the CORK head was left 9.0 m shy of landing out in the 16 inch casing hanger. This left ~12.3 m of stinger in the hole and allowed the pipe to be hung off at the rotary table to break the drill pipe connection and deploy the thermistor string.

Thermistor string deployment

A total of 5.75 h was required to make up and deploy the thermistor string. This string was specially designed for long-term monitoring of the upper 150 m of the sediment column from the seafloor down. The thermistor string was premade with a ¼ inch diameter Spectra (Kevlar) rope attached to the thermistor cable with tie wraps and duct tape. The Spectra line (153.84 m) was designed to carry the load as a tension member and was terminated with a ~250 lb sinker bar (3.73 m). The thermistor cable was plugged into a battery pack/data logger assembly (0.97 m) at the top. This package was suspended from another short section of Spectra rope (2.22 m) with thimbles at each end and the XN latch assembly (0.31 m to landing shoulder). Cable “grips” were used to lift the thermistor string in ~50–60 ft lifts using a double sheave assembly and two tugger lines. This was the same process used on earlier thermistor deployments; however, this time, there were problems. For reasons yet to be fully understood, at the conclusion of each lift the thermistor cable tensed up and carried the load and the Spectra line would go slack. The weight probably transferred across the tie wraps. On most lifts, the load would eventually transfer back to the Spectra line as designed; however, on the last two lifts, the load never did transfer. Fortunately, the Hole U1315A thermistor string was relatively short, and the suspended load was fairly light. To rectify the problem, we cut loose the remaining tie wraps and duct tape. We then retied and retaped the cable and rope together as the remainder of the string was deployed. At the end, where the rope was to be attached to the bottom of the data logger/​battery assembly, we had a surplus of 9 ft of Spectra rope that had to be cut off, leaving the final rope length at 151.10 m. Once made up, the thermistor string assembly was deployed via wireline at 1030 h on 19 April. The XN latch assembly was landed without incident. The latch was jarred down for setting, and a 4000 lb overpull was taken to verify proper latch-in. The sinker bars were recovered via wireline, and by 1215 h we were ready to land and release the CORK assembly.

CORK landing and release

The drill string was lowered the remaining 9.0 m (~1 knobby joint), and the CORK head landed out at exactly the correct pipe depth. About 5000 lb were put down, and then ~8000 lb of overpull was taken to verify that the CORK head was latched. This weight was slacked off, and a small amount of right-hand torque was applied to the string. The CORK running tool was observed to rotate slightly, and when the string was picked up, the running tool lifted cleanly off the head. Installation of the Hole U1315A CORK was officially completed as of 1237 h on 19 April.

The top drive was racked back and the drill string was recovered, clearing the rig floor at 1605 h. During the pipe trip, positioning beacon SN 2039 was recovered by 1440 h. The upper guide horn or “piccolo” was then reinstalled, ending operations for Hole U1315A.

Hole 642E (ODP Site 642)

Search for Hole 642E reentry cone

During the trip out of the hole, the drillship moved back to the original Site 642 prospectus coordinates. From there, a search began for the Hole 642E reentry cone. A 100 m × 100 m box pattern search in 15 m swaths was initiated. The operations report for ODP Leg 104 indicated that the reentry cone base was left ~1.0 m below the seafloor, leaving ~1.5–2 m of cone above. Other entries in the report indicated that the last few reentries were difficult and the crew questioned whether future reentries would be possible because they felt that the cone may have been buried in drilled cuttings. It should be noted, however, that all reentries at that time were made using sonar and were not aided by any subsea television capability. The search for the reentry cone began at 2045 h on 19 April 2005 and extended through the night without success. The following morning, the ship returned to a target that was not close to where the reentry cone should have been; however, an obvious man-made object was visible on the seafloor and was detected on sonar as well. The object appeared to be an old-style reentry cone reflector. Four of these reflectors were mounted on the older-style reentry cones in the past. Convinced that the cone was buried, we spent several hours attempting to define where a cone might be relative to the single visible reflector. Ultimately a stab into the seabed was made with the drill string; however, this was to no avail as the driller noted drill string resistance after lowering the drill string only 9 m into the seabed. Upon reflection, we decided that the object was not the cone we were searching for; the sonar should have identified the remaining cone and reflectors even if submerged. Considering this and coupled with the fact that we were not even close to the area that the cone should have been, we decided to continue on with the search. In so doing, we revisited all that we knew about the location of the target cone and realized that we had started our search pattern at a longitude of 2°55.7′E, failing to recognize that Hole 642E had in fact been spudded at 2°55.8′E, a full tenth of a minute off. After entering new offsets into the dynamic positioning system, we finally located the reentry cone on sonar and then ultimately with the subsea television system as well. The cone was fully visible and was not submerged or covered in cuttings. The rim and all four reflectors could be clearly identified, and the cone rim appeared to be, as reported, 1.5 to 2.0 m above the seafloor. Using the new undithered Global Positioning System capabilities of the ship, the ultimate coordinates for Hole 642E are 067°13.1850′N, 002°55.7789′E. The cone was actually located 548 m south of Holes 642A and 642B on a bearing of 173°. According to the documentation, the cone should have been located 450 m to the southeast (a bearing of 135°). The drill string was spaced out, and Hole 642E was reentered at 1305 h on 20 April, a total of 16.5 h after the search was initiated.

Wireline logging

The primary goal of our return to Hole 642E was to obtain a high-resolution continuous temperature log of the hole that had been left undisturbed for nearly 20 y. This hole was drilled to a total depth of 1229.4 mbsf during Leg 104. It was left cased (with 11¾ inch 54 lb/ft casing) 62.5 m into basement, placing the casing shoe at 371.5 mbsf. To minimize disturbance in the hole, the end of the pipe was placed at only 15.3 mbsf. The LDEO Temperature/​Acceleration/​Pressure (TAP) tool was deployed with the standard triple combo (Dual Induction Tool model E [DIT-E]/​Hostile Environment Litho-Density Sonde [HLDS]/​Accelerator Porosity Sonde [APS]/​Hostile Environment Gamma Ray Sonde [HNGS]) tool string at 1710 h. A good temperature log was obtained on the first run downhole; however, ledges and/or chunks of basalt falling in from above the tools proved to be problematic. After the tools were “mouse trapped” between two such zones for nearly an hour, they were recovered, having only reached a depth of ~1888 mbrf (~599 mbsf) or ~228 m into the open hole below the casing shoe. The first tool suite was recovered at 2225 h on 20 April.

After rigging down the first tool suite, the second suite of tools consisting of the FMS-sonic tool string were made up and deployed. These tools were run in at 0135 h on 21 April and were recovered at 0700 h after reaching a depth of ~1878 mbrf (~589 mbsf) or ~218 m into the open hole.

The FMS-sonic tool string was rigged down, and a final run was made only to 1200 mbsf to test the Multi-Sensor Spectral Gamma Ray Tool (MGT) pressure case. This tool leaked during a logging run earlier in the expedition; however, nothing unusual was found to explain the leak. After cleaning the pressure case thoroughly, new O-rings were installed and the body was deployed without electronics to test its pressure integrity. The tools were run in at 0750 h and recovered by 0900 h. The limited test (relatively shallow depth) was successful, and the MGT was recovered without any signs of leakage.

All wireline tools and the Schlumberger logging sheaves were rigged down by 1000 h, and preparations began for drill string recovery. The bit cleared the seafloor/​reentry cone at 1005 h, and by 1310 h, all drill pipe had been recovered and all drill collars were laid out to the main deck tubular rack. During the pipe trip, positioning beacon SN 2199 was recovered on deck at 1100 h. The rig floor was secured, all thrusters and hydrophones were raised, and the drillship was under way for Dublin, Ireland, by 1315 h on 21 April 2005.

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