OPERATIONS SYNOPSIS
Site 1149 (see Fig. 2), the second drill site of Leg 185 was located just east of the Bonin Trench, ~1040 nmi northwest of Site 801 and 300 nmi south-southeast of Japan's Boso Peninsula. Site 1149 was tentatively located on an east-west reference profile (Conrad 2005, Line 39) but a site survey was required to determine the best location for the site. JOIDES Resolution approached the Conrad reference seismic line from the southeast, slowed to 6 kt, and streamed seismic gear ~12 nmi before crossing the reference line. The survey continued until a location with favorable basement characteristics was found. After a triangular seismic survey pattern was completed, the profiling gear was recovered and the ship returned to the GPS coordinates of the site. A positioning beacon was launched at 1500 hr on 23 May. The transit from Site 801 to Site 1149, including the 7-hr seismic survey was accomplished in 4.05 days, at an average speed of 11.4 kt.
Hole 1149A
Hole 1149A was spudded with a "mudline" core at 0200 hr on 24
May. The core barrel contained 4.2 m of core, which was
interpreted as the seafloor interface, fixing the seafloor depth
at 5829.3 m from driller's datum. Continuous advanced piston cores
(APC) cores were taken with >100% recovery to 164 mbsf, when an
abrupt increase in the stiffness of the sediment resulted in an
incomplete APC stroke on Core 18H.
APC cores were oriented in azimuth beginning with Core 4H.
Temperature recording shoes were also run on Cores 4H, 6H, and 8H,
and the Lamont-Doherty Earth Observatory (LDEO) drill string
acceleration tool (DSA) was run on Cores 2H and 10H. Tracer
experiments, to determine potential core contamination for
microbiology studies, were carried out by fixing a bag of
fluorescent microbeads into the core catchers of Cores 3H, 6H, 9H,
and 12H, and by pumping perfluorocarbon tracer into the drilling
fluid on Cores 6H, 9H, and 11H.
The APC system was replaced by the extended core barrel (XCB),
which cored 24.3 m (i.e., Cores 19X-21X) with an average recovery
of 21% in stiff clay with interbedded porcellanite. After 2.2 m
penetration when cutting Core 22X, the XCB encountered a hard
streak that halted penetration.
Further progress with the XCB system was highly unlikely because
of the inability to penetrate the chert or porcellanite horizon.
The situation had been anticipated, and the plan called for using
the motor-driven core barrel (MDCB) system to attempt to core the
chert layers. Although conditions were not favorable for the MDCB
because of the presence of steel and tungsten carbide junk in the
hole from the XCB shoe, it was nevertheless decided to attempt a
MDCB core as a "last chance" to core the chert with an alternative
system before tripping for the RCB system. When the MDCB was
recovered, the corehead was noted to have all the diamonds worn
off the crown and to show signs of junk damage. The core barrel
also showed grooving from junk. When the core catcher was removed,
only 30 cm of chert and clay debris from the hole was found in the
split and collapsed plastic liner. There was no indication that
any core had been cut or that any new hole had been made by the
corer.
Coring attempts in Hole 1149A were abandoned at that point,
because there appeared to be no remaining alternatives to tripping
for an RCB coring assembly. The MDCB system was rigged down, the
top drive was racked, and the pipe trip began. Hole 1149A ended at
0725 hr on 26 May, when the bit arrived on deck.
Hole 1149B
Hole 1149B was 30 m northwest of Hole 1149A. No seafloor core
was planned, so the water depth was assumed to be the same as at
Hole 1149A. The new hole was spudded at 2000 hr on 26 May and
drilled ahead to core point at 161 mbsf in 3.5 hr. The "wash
barrel" then was retrieved, which contained ~3.5 m of cored
material and drilling rubble.
Continuous RCB coring then commenced. At ~190 mbsf, interbedded
chert caused core recovery to drop sharply, and drilling
conditions deteriorated. Up to 4 m of fill accumulated between
cores, causing high torque, and the bit hammered on the chert
ledges when the motion compensator was locked out. Only chert
"rollers" were recovered in the core barrels, with traces of soft
brown clay, probably representing a large portion of the
unrecovered material. At ~261 mbsf, the ROP slowed considerably
and hole conditions improved markedly with depth. Recovered cores
contained representative quantities of calcareous interbedded
material with interbedded chert. Only a small amount of fill was
present by the time basaltic basement was encountered at 410 mbsf
while drilling Core 29R. Average core recovery in the cherty
interval from 190 to 410 mbsf was only ~6%. Although it had
penetrated >200 m of cherty sediments, the core bit had accrued
only 21 rotating hr when it reached basement. The condition of the
cutting structure was unknown, but the plan for the hole called
for running the bit to "destruction." Penetration of the highly
altered basalt continued at >2 m/hr through Core 32R with a
recovery rate of ~20%. At the time the core barrel for Core 33R
had been dropped, however, without warning the drill string became
partially stuck. Indications were that chert or basalt fragments
were wedging the BHA, either at the top or at the bit. The string
could be moved up or down in irregular intervals before it would
torque up and bind vertically. A mud pill was circulated while the
string was worked free, and the inner core barrel landed during
the process. Eventually three "knobby" joints were removed from
the string as the bit was pulled upward. When the bit passed the
approximate depth of the sediment/basalt contact, the resistance
ceased. A joint of drill pipe then was added to the drill string,
but circulation was nearly plugged off when the pump was
restarted. Because the inner barrel had held the float valve open
while the pipe was being freed, cuttings had been able to flow
back into the jets of the bit. It was then necessary to make a
wireline trip to retrieve the inner barrel and regain normal
circulation. A total of 4 1/2 hr was spent on the consecutive hole
problems.
Although additional basement penetration was planned, the
highest remaining scientific priority of Leg 185 was a good set of
logs of the sediment section at Site 1149. The onset of
difficulties with the hole was a reminder that coring to bit
destruction could result in loss of the hole and the opportunity
for logs. Because there was ~35 m of "rathole" for the logs to
record the base of the sediment section, plans were changed to
terminate coring in Hole 1149B and proceed with logging
operations. When logging operations were completed at 0900 hr on 2
June, the drill string was tripped, and Hole 1149B was completed
when the mechanical bit release top connector arrived on deck at
2000 hr on 2 June.
Hole 1149C
Hole 1149C was spudded at 0820 hr on 3 June and was drilled
without coring to 283.6 mbsf. Two "wash" core barrels were pulled
at 237 and 283.6 mbsf as a precaution because of the accumulation
of loose chert fragments in Hole 1149B. After the wash core barrel
at 283.6 mbsf was retrieved, preparations began for retrieving
four spot cores requested by the science party. A short trip back
to 188 mbsf to replace the "knobby" drilling joints was slowed by
the torquing and sticking of the drill string. Although a
considerable amount of soft fill was found in the hole, the string
was free as it returned to total depth, and coring began.
Mud was circulated while the first core (3R) was being cut, but
torquing and sticking tendencies returned during the wireline trip
to retrieve it. It was necessary to "work" the pipe for 3 1/2 hr and
to pull back to 245 mbsf before Core 4R could be cut to 303 mbsf
and retrieved. Almost immediately after the barrel for Core 5R was
dropped, the drill string began to torque and stick again. While
the string was being worked up the hole with restricted
circulation, the inner barrel landed, opening the float valve.
When the next joint of pipe was removed from the string, backflow
plugged the pipe. Rotation and vertical movement were possible, so
the bit was pulled to 235 mbsf before a wireline trip was made to
retrieve the inner barrel and re-establish normal circulation.
The cause of the hole cleaning problems was believed to be the
interval of dark brown clay with lower porosity and shear strength
between 140 and 155 mbsf that had stopped logging tools in Hole
1149B. Therefore, a "wiper trip" was made to 130 mbsf to ream out
any clay restriction. No resistance was noted in either direction
until the bit returned to ~235 mbsf, where a solid ledge was
encountered. Several other ledges were noted as the top drive was
used to clean the hole back to total depth, and ~4 m of fill were
found on bottom. After the inner barrel was pumped into place,
Cores 5R and 6R were cut and retrieved without incident, but the
combined recovery was only 78 cm of chert and chalk core pieces.
With a wash barrel in place, the hole was drilled ahead to 388
mbsf, the next requested core point to sample the
sediment/basement interface. Because of the history of hole
problems, the knobby joints were laid out and a precautionary
short trip was made back up to 130 mbsf, above the unstable clay
zone. No resistance registered on the weight indicator, but the
top drive was picked up to ream through the clay zone and to clean
the hole back to total depth with circulation and rotation. The
hole was clean, with only 3 m of soft fill, but an additional 50
bbl of mud were pumped to sweep out any debris. Core 8R then was
cut with all parameters normal, and 1 m of chert/chalk core was
recovered.
Before the inner barrel was dropped before cutting of Core 9R,
the pipe began to torque and stick. Additional working of the pipe
resulted in more sticking. For 4 hr, various circulation rates,
amounts of overpull, and stuck-pipe techniques were tried. Success
was slowly achieved by "drilling up" or backreaming with tension
just short of the amount that would produce stalling of the top
drive. Progress up the hole of about 1 m/hr was achieved with that
technique. Circulation had been held to moderate rates to avoid
excessive hole erosion, but a desperation move involving use of
both mud pumps at 90 strokes per minute (spm) produced enough
improvement in the backreaming progress that a joint of pipe could
be removed. The pipe was set on the elevators with 80 kips
overpull to break out the joint. When it was reconnected to the
top drive and lifted off the elevators, it was free. The string
then was pulled to ~60 m off total depth with no resistance and
run back to bottom, where ~4 m of fill were found. The cause of
the sticking was believed to be gravel-sized chert fragments that
had settled around the BHA.
Hard basement drilling was encountered 2 m into drilling Core 9R
(at 401 mbsf), allowing a higher coring circulation rate to be
used for the remainder of the core. The pattern of stuck pipe
following core retrieval continued after both Cores 9R and 10R,
with average core recovery of only ~15%. It became apparent that
chronic sticking problems were leading to excessively slow
recovery of material and that continuing to operate in Hole 1149C
eventually would result in loss of the hole and the BHA. While
Core 11R was being cut, a decision to relocate had been made, and
the trip out of the hole began as soon as the core had been
recovered. The bit arrived on deck at 0215 hr on 7 June. The
vessel got under way at 0300 hr, ending Hole 1149C.
Hole 1149D
The 3.5-kHz echosounder was used to refine the desired offset
position on a nearby basement high. A new beacon was dropped at
0515 hr on 7 June ~3.1 nmi east-southeast of Hole 1149A, and Hole
1149D was spudded at 1530 the same day. Hole 1149D was drilled to
basement without coring and without pulling the "wash barrel."
Drilling parameters indicated chert stringers beginning at ~155
mbsf. No hole problems were encountered except for an incident of
sticking during the rereaming of the interval to 263 mbsf. At 272
mbsf, a "wash" core barrel was pulled to initiate continuous
coring in anticipation of encountering basement. Three consecutive
cores then produced a combined total of 1.2 m of chert fragments.
Basement was encountered at 307 mbsf while coring Core 5R. During
coring of Hole 1149D, some problems were encountered with sticking
and torquing of the pipe, which caused some delays. However,
coring continued until through Core 19R, when the time allotted
for coring had expired. Hole 1149D was deepened to a total depth
of 6319.4 m (440.4 mbsf). Departure from Site 1149 was at 1615 hr
on 13 June.