OPERATIONS

Transit to Proposed Site HE-4A (DSDP Site 432) and Underway Survey over Nintoku Seamount

The position of Site 1205, at the northwestern corner of the summit platform of Nintoku Seamount, was located nearly 600 nmi south-southeast of Site 1204. The area of Site 1205 was last visited during DSDP Leg 55 (Site 432) in early August 1977. Nintoku Seamount, one of the nine seamounts of the Emperor chain named by Robert Dietz in 1954, was named for the 16th emperor of Japan.

The 596-nmi transit to the first survey waypoint was accomplished at an average speed of 11.6 kt. During the 52-hr voyage, the skies were overcast, with occasional showers and fog hampering visibility. The sea and swell did not exceed 7 ft. The air temperature increased from 10 to 17C as the vessel entered the Kuroshiro Current during the early evening of 7 August 2001.

Prior to spudding Hole 1205A, a geophysical survey was conducted to acoustically image the stratigraphic and structural setting of the site. Crossing high-resolution seismic reflection profiles were obtained using a single 80-in3 water gun firing every 6 s. At 0030 hr on 4 August the vessel slowed to 6 kt to deploy the seismic equipment, and by 0600 hr the survey was completed. The 29.5-nmi survey was conducted at an average speed of 5.6 kt. To provide adequate sediment thickness to ensure lateral stability of the BHA while spudding, Site 1205 was positioned where the sedimentary blanket was estimated to be at least 60 m thick.

The thrusters were lowered as the vessel established location using the Global Positioning System (GPS) coordinates selected from the site survey, ~100 m southwest of proposed Site HE-4A (DSDP Site 432) in the Leg 197 Scientific Prospectus. After the vessel was on station at 0632 hr, the hydrophones were lowered, concurrent with the making up of a new BHA. A beacon was deployed at 0735 hr.

Hole 1205A

The corrected precision depth recorder (PDR) depth referenced to the dual elevator stool was obtained and indicated 1319.4 m. The BHA was made up of a 9.875-in rotary core barrel (RCB) hard formation C-7 bit, a mechanical bit release, a modified head sub, an outer core barrel, a modified top sub, a modified head sub, seven 8.25-in drill collars, a tapered drill collar, six 5.5-in drill pipe sections, and one crossover sub.

Hole 1205A was spudded with the RCB at 1230 hr on 4 August and tagged the depth of the seafloor at 1321.0 m. Coring advanced slowly through 42.7 m of sandstone and breccia rich in basalt fragments at an average rate of penetration (ROP) of 12.5 m/hr (recovery = 9.1%). Basement coring began with Core 197-1205A-6R (42.7-52.2 mbsf) and proceeded to a depth of 132.9 mbsf, when operations were suspended to change the bit, which had accumulated 52.1 hr of rotation.

Progress in the mostly fresh, fine-crystalline basaltic basement was the slowest thus far in the leg and was only uncommonly interrupted by sudden spurts of rapid penetration through thin soil horizons between lava flows. Until a depth of 59 mbsf was attained, slow penetration rate in basement was also attributed to the BHA not being sufficiently buried to allow the driller to apply more than 15,000 lb of weight on bit.

A total of 90.2 m of basaltic basement was cored (average recovery = 61.3%). The cored sedimentary section was 42.7 m (recovery = 9.1%). The ROP in basement varied from 4.8 m/hr (Core 197-1205A-6R [42.7-52.2 mbsf]) to 0.5 m/hr (56.9-58.9 mbsf). The average ROP in basement was 1.9 m/hr. The average total recovery for the 132.9 m of cored sedimentary and basement sequences was 49.0%.

Reentry

The bit cleared the free-fall funnel (FFF) at 1330 hr on 7 August and was captured by the vibration-isolated subsea television camera. The bit cleared the rotary table at 1630 hr and was quickly removed and replaced with another new C-7 hard formation rotary bit. The used bit was inspected and found to be in excellent condition with only 0.0625 in loss of outside gage. The cones were intact with no chipped or missing teeth. All nozzles were clear. Because of the excessive hours of rotation, most of which were accumulated in basement, the bit was retired from service in spite of the outward excellent appearance.

The used mechanical bit release was also replaced with a fresh unit. After spacing out the new assembly, the drill string was deployed and positioned over the FFF by 2115 hr. As the driller was making a near-perfect reentry, the bit lightly tapped the throat of the cone, which immediately toppled over on its side, rendering it useless for reentry purposes. Apparently, the sandy surface formation was too firm to allow the 2.5-m length of 13.375-in casing to sufficiently penetrate the hard seafloor during deployment. The FFF was perched on, rather than nestled in, the hard sandy bottom of the summit platform of the Nintoku Guyot. The bit tapping the FFF was enough to upset the unstable installation.

Considerable bottom mud was stirred up as a result of the FFF tipping over, which obscured the now-unadorned Hole 1205A. However, the driller and dynamic positioning personnel persevered, and the reentry of the open hole was achieved by 2209 hr.

Coring resumed in Hole 1205A and advanced to 215.8 mbsf, when flow-back at the drill floor connection indicated that the float valve was jammed open. It required seven runs with the depluggers and core barrel to clear the obstruction. This effort consumed 5 hr and involved two runs with the long deplugger, two runs with fresh core barrels, another run with the long deplugger, and two runs with the chisel deplugger.

Basement coring again resumed and penetrated to 274.6 mbsf, when once again flow-back at the rig floor required one run with the deplugger and one run with a fresh core barrel to clear the obstruction from the float valve. Time-scheduled coring in Hole 1205A was terminated at a final depth of 326.0 mbsf. The cored interval was 326.0 m with a recovery of 165.6 m of sediment and basalt (recovery = 50.8%) (Table T1). The total basement penetration was 283.3 m with 161.8 m recovered (recovery = 57.1%). The coring effort required two bits with a total rotating time of 110.8 hr. The average ROP in basement was 2.6 m/hr. The cored sediment was 42.7 m with 3.87 m recovered (recovery = 9.1%). At this juncture during Leg 197, 935.2 m of basement had been cored with 487.9 m recovered (recovery = 52.2%).

Logging Attempt

Following the retrieval of the last core, the hole was conditioned with a wiper trip up to 69 mbsf and down to 279 mbsf. The hole was washed and reamed from 279 to 326 mbsf with no apparent fill detected at the bottom. Following the displacement of the hole with 100 bbl of sepiolite mud, the bit was placed at the logging depth of 90 mbsf. The first planned logging run was with the triple combination tool string. Unfortunately, it was not possible for the loggers to deploy the tool deeper than 136 mbsf because the hole was blocked by a bridge and swelling formation. The logging effort was aborted, and the drilling equipment was retrieved.

The drilling equipment was secured, and the BHA was dismantled in preparation for the 400-nmi journey to proposed Site HE-6A, Koko Seamount. The beacon was recovered, and the thrusters and hydrophones were retracted. The moonpool was secured for the transit, as the vessel got under way at 0600 hr on 12 August to the last site of Leg 197.

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