OPERATIONS SYNOPSIS1
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 (GPS) 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.
Hole 801D
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).
Hole 801C
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.
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., 1-2 m/hr) and
lowest (i.e., 3.3 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.
Diamond Core Barrel System Test
Two additional days had been allocated to Leg 185 for testing
the DCB system. A 7 1/4-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 over 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.
Although the DCB cut 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 3/4-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.
1The Operations and Engineering personnel aboard the JOIDES Resolution during Leg 185 were ODP Operations Manager Glenn Foss and Schlumberger Engineer Steve Kittredge.