Next Section | Table of Contents


Drilling Strategy and Operations

After steaming to the site and lowering the pipe to the summit of the seamount, we planned to conduct a brief seafloor television survey of the conduit to locate the springs and mussel beds identified in Shinkai 6500 dives and to identify sites near the springs for rotary core barrel (RCB) coring and logging and relatively clast-free sites for advanced piston corer/extended core barrel (APC/XCB) coring, jet-in tests, and the establishment of a reentry hole. After conducting a jet-in test to establish the depth of the first casing string for the reentry hole, we planned to core and log an RCB pilot hole to 450 meters below seafloor (mbsf) to determine the nature of the formation and the ease of drilling on the seamount, a major concern because drilling during Leg 125 at Conical Seamount had been plagued with drilling problems.

Depending on the results of the RCB hole, we then planned to offset and jet in a reentry cone and 20-in casing to about 25 mbsf and then drill a hole in stages to 420 mbsf for the CORK installation. In anticipation of hole instability problems, an elaborate casing program was envisioned, with cemented 16-in casing to 200 mbsf followed by 10.75-in casing to 400 mbsf, including 23 m of screened casing and a casing shoe at the bottom to prevent the serpentine mud from slowly invading the installation from below. After the hole was drilled and cased, the instrumentation installed, and the remotely operated vehicle (ROV) platform emplaced, we planned to drill an APC/XCB hole to 420 mbsf to collect a continuous, undisturbed section for petrologic and pore water studies. By drilling the APC/XCB hole last, the time allocated for APC/XCB coring could be held in reserve as contingency time if it took longer than anticipated to drill the reentry hole and install the observatory. Not surprisingly, the actual operations at Site 1200 unfolded rather differently.

The JOIDES Resolution arrived at Site 1200 (proposed Site MAF-4B) at 2100 hr on 11 March 2001. Following a 4-hr camera survey, Hole 1200A was spudded adjacent to a vent mussel community at the top of South Chamorro Seamount at 2200 hr on 11 March with the RCB and cored to a depth of 147.2 mbsf (Fig. F6; Table T1). High torque and lost rotation at this depth resulted in a stuck drill string that ultimately forced us to abandon the hole. Recovery in Hole 1200A was poor (147.2 m cored; 7.9% recovered), with little recovery of the mud matrix material surrounding the hard ultramafic clasts. The drill string was pulled out of the hole, and the seafloor was cleared at 1535 hr, ending Hole 1200A.

The ship was then offset 25 m to the east, and Hole 1200B was spudded at 1640 hr on 13 March. Our intention was to wash to 147.2 mbsf and then start coring to the target depth of 450 m. By 1615 hr on 14 March, the hole had been advanced to a depth of 98.0 mbsf. Once again, high torque and overpull began to plague the hole. Despite consistent mud sweeps and multiple reaming attempts, the hole could not be stabilized. At 0645 hr on 14 March, we decided to abandon further drilling/coring efforts in the pilot hole and to start the reentry hole because it was obvious that deep penetration could not be achieved without the use of casing.

Once the drill string was pulled out of the hole with the top drive, the ship was offset and Hole 1200C was spudded at 0450 hr with the 20-in casing attached to the reentry cone. After a total of 18.25 hr of drilling with an 18.5-in bit, 22-in underreamer, and drilling motor, the reentry cone base reached the seabed, placing the 20-in casing shoe at 23.7 mbsf. Drilling of the 22-in hole for the 16-in intermediate casing string, without the motor this time, advanced smoothly and without incident. At 0130 hr on 18 March, the hole reached a depth of 140 mbsf. While we washed/lowered the casing string into the hole, however, the casing shoe encountered an obstruction 6 m off bottom that prevented the casing hanger from landing. After three futile hours, the drill string was recovered back on the ship and one joint of casing was removed from the string. The shortened string was run back into the hole and washed down without incident. With the 16-in casing shoe placed at a depth of 107.4 mbsf, the casing string was cemented in place. The drilling process was then reinitiated using a 14.75-in drill bit and 20-in underreamer, dressed with 20-in cutter arms, and advanced to a depth of 266.0 mbsf. The penetration rate deteriorated to zero at that point, and a subsequent wiper trip found 9 m of soft fill at the bottom of the hole that was easily removed by circulation. Upon reaching the rig floor, the underreamer was missing two out of three cutters. Because the hole had penetrated below the base of the summit knoll but could not be deepened further, the 10.75-in casing was deployed, reaching a depth of 224.0 mbsf by 0300 hr on 24 March without incident. However, progress beyond this point was not possible, and the string was pulled out of the hole to remove three joints of casing. The shortened casing string finally placed the shoe at 202.8 mbsf, with the screened interval extending from 202.3 to 148.8 mbsf (Fig. F7).

The CORK assembly, with two stands of 5.5-in drill pipe used as the stinger, was run in the hole, and the drill string was lowered to 53.0 mbsf. This left the CORK shy of landing out by ~8 m. At 1300 hr on 25 March, the thermistor/Osmo-Sampler assembly was slowly run in the hole, and at 1430 hr the data logger was landed in the CORK body. After 1 hr of pressure transducer calibration, the data logger was latched and the CORK body was then lowered the final few meters and latched in place as in Figure F8. The CORK installations at Hole 1200C ended with a successful free-fall deployment of the ROV platform. The pipe was then returned to the surface; not counting the pilot holes, nine round pipe trips totaling 52 pipe-km of tripping pipe were required to deploy the observatory.

After the successful installation of the geochemical observatory, the vessel was offset 40 m to the south and Hole 1200D was spudded with the APC to sample the serpentine muds for petrology, pore water, and microbiology studies at 1415 hr on 26 March. Coring continued to a depth of 44.4 mbsf using the APC advance-by-recovery method. Hard clasts were drilled with an XCB center bit assembly. After coring was halted by a hard clast at 44.4 mbsf, the XCB was used to deepen the hole. After advancing 9.0 m, however, the penetration rate fell to zero and the decision was made to abandon Hole 1200D.

The vessel was then offset back to the location of the earlier identified mussel beds, and Hole 1200E was spudded with the APC for further pore water studies. Coring with the APC/XCB continued, again using the advance-by-recovery method for the APC cores, to a depth of 50.4 mbsf, where the scientific objectives of the hole were met.

The ship was then offset 20 m to the north, and Hole 1200F was spudded at 1315 hr. Coring proceeded until the time allocated for operations at Site 1200 ran out. The hole depth reached 16.3 mbsf, with the recovery of APC Cores 195-1200F-1H through 3H. At 0200 hr on 29 March, the ship was under way to the Guam pilot station.

Next Section | Table of Contents