OPERATIONS

Leg 185 began at 0835 hr on 12 April 1999 with the port call in Hong Kong. At 1005 hr on 18 April the port call ended when JOIDES Resolution departed the Yiu Lian terminal. The planned transit of 2400 nmi to Site 801 was accomplished in near-perfect weather and calm seas. The vessel navigated to the Global Positioning System coordinates of Hole 801C and launched a positioning beacon at 1000 hr on 28 April. The transit had been accomplished in 235 hr (9.79 days) at an average speed of 10.5 kt.

Immediately after the beacon launch, assembly of the bottom-hole assembly (BHA) began. Once the BHA and the vibration-isolated television (VIT) frame arrived at reentry depth, a quick search was made to locate the reentry cone of Hole 801C. When a strong sonar target had been determined to be the cone, the ship was positioned 100 m east of the cone in preparation for Hole 801D. A seafloor punch core had been requested for microbial studies; therefore, the top drive was picked up and water was pumped through the drill string at a high rate to clean it. A rotary core barrel (RCB) system inner core barrel then was pumped into place, and the drill string was lowered. Under TV observation, the core bit tagged the seafloor at a depth of 5685 m below driller's datum at 0710 hr on 29 April. To increase chances of recovering an adequate amount of the soft sediment, the bit was pushed as far as possible (19.3 m) into the seafloor. The bit then was pulled clear of the seafloor, and the inner core barrel was retrieved by wireline. About 9.4 m of soft mud was recovered when the core barrel was laid out, ending Hole 801D at 0830 hr. Because an interval greater than the length of a core barrel had been penetrated, the recovery was designated a "wash core" (1W) (Tables T1, T2).

As soon as the top drive had been rigged down, the bit was repositioned for reentry. About 45 min of maneuvering were required to coax the drill string over the reentry cone. Hole 801C was reentered at 1015 hr on 29 April.

The bit then was lowered into the open hole to 468 mbsf. A water sample from the undisturbed borehole had been requested for microbial studies, and the top drive was again picked up in preparation for a sampling run with the water sampling temperature probe (WSTP). The bit was lowered without circulation to 490 mbsf, the WSTP was landed at the bit, and time was allowed for the preset timer to open the sampling valve. Upon recovery of the WSTP, however, no sample had been collected. A second WSTP attempt then was made with the bit lowered to 540 mbsf, just below a known interval of unstable hole. A good water sample was obtained, and the pipe trip continued with the top drive deployed and a RCB inner core barrel in place.

An obstruction, or "bridge," that had stopped logging tools in this hole during Leg 144 was encountered by the drill string at 546 mbsf. The obstruction was cleared easily with rotation and circulation, but the effect of "pushing something down the hole" was noted. Rubbly fill was encountered ~3 m off the recorded total depth. The fill was washed to total depth at 594.3 mbsf, where torque and rough running indicated the presence of loose rocks on the bottom of the hole. After a few minutes, the rocks were broken up and the coring of Core 13R began (Tables T1, T2).

Continuous RCB coring then proceeded under ideal weather and motion conditions for 3 days. The first two cores were cut at a rate of penetration (ROP) of ~2.0 m/hr. The ROP increased slightly on each core because of increasingly fractured rocks with an accompanying downtrend in recovery from excellent to low (i.e., from 89% in Core 15R to 32% in Core 20R). Anomalous drilling parameters on Core 21R (i.e., an abrupt decrease in ROP accompanied by increased drill string torque and slightly low circulating pressure readings), and over 36 rotating hr on the bit were considered sufficient reason to trip the drill string for a bit change. Average recovery for the 79.3 m cored with Bit No. 1 was 52%. At 2230 hr on 2 May, the core bit arrived on deck and was found to be in excellent condition, with bearing seals still effective and only a few broken tungsten carbide inserts.

Reentry scanning began at noon on 3 May after pipe tripping with Bit No. 2. Maneuvering for reentry consumed a frustrating 5 hr, primarily because of the subdued response of the long drill string to changes in the ship's position. Reentry was made at 1700 hr, and the bit was run into the hole without incident. Coring of Core 22R began at 2200 hr on 3 May. Core recovery and ROP were quite variable, but there was a trend toward more massive rock units with depth with resultant higher recovery and lower penetration rates. Weather and hole conditions remained good, and 83 m of new hole was made before the bit was tripped on the basis of its 47.7 rotating hr. Core recovery for the interval was 44%.

A third bit was installed, and the drill string was tripped back toward the seafloor. After 7 hr lost to repairs of the cable head of the VIT and subsequent problems with flooding of the short oil-filled cable that connects the coaxial cable head to the telemetry pod of the VIT, maneuvering for reentry began at 0200 hr on 8 May. Again, the inhibited response of the long drill string to ship movement slowed the operation. At 0538 hr, a successful reentry stab was made. The trip into the hole was uneventful except that 14 m of soft fill was found. It was removed without difficulty, and coring recommenced at 1030 hr, 8 May. Core recovery and ROP were quite variable, with both the highest (i.e., 3.3 m/hr) and lowest (i.e., 1-2 m/hr) penetration rates to date encountered within the interval. Coring continued without incident through Core 42R at 869.1 mbsf. At that depth the bit had accrued 50 rotating hr, and the trip began for the fourth RCB bit. Bit No. 3 cored 112.5 m at an average ROP of 2.2 m/hr and with average core recovery of 53.5%.

A fourth RCB bit was tripped to the seafloor, and the swinging of the pipe was less troublesome with reentry accomplished in only 24 min of maneuvering time. Coring resumed at 869.1 mbsf at 0100 hr on 13 May. The first four cores were in altered pillow basalt units and produced relatively low recovery at fairly high penetration rates. On the next two cores, penetration slowed in fractured flow units, but recovery did not improve correspondingly. The bit run cored 57.1 m with an average ROP of 2.1 m/hr and an average recovery rate of 34.9%. To divide the remaining site operating time between logging operations and coring with the diamond core barrel (DCB) system, coring was stopped with only 27.3 rotating hr on Bit No. 4.

Two additional days had been allocated to Leg 185 for testing the DCB system. A 7-in diamond core bit with "carbonado" stones was selected for coring basalts. The drill string and VIT were run to reentry depth, and another quick reentry of ~10 min was achieved. Because more than 90 m of hole fill had been encountered by the logging operation, considerable caution was used on the return to total depth. The hole then was cleaned out to firm resistance, which was felt at 928.3 mbsf, 2.1 m deeper than the bottom of the previous core. The discrepancy was interpreted to be because of a difference in pipe stretch caused by the 40,000 lb difference in BHA weight between the two strings.

Coring with the diamond core bit deepened the hole by 7.4 m in 20.5 hr, and core recovery averaged 42% in hard, fractured basalt (Tables T1, T2). Although the DCB cut very slowly through the basalt, the recovered core appeared to contain more delicate features, such as complete interpillow hyaloclastites and abundant veins, than is typical for RCB core.

After the test was concluded, the drill string and the original acoustic beacon were recovered without incident. The 6-in drill collars were returned to the drill collar racks, the rig was secured for sea, and JOIDES Resolution departed Site 801 at 1330 hr on 19 May.