3. Site 7351
Shipboard Scientific Party2
OPERATIONS SUMMARY
Overview and Significant
Accomplishments
We began Leg 176 with the first line
ashore in Cape Town, South Africa, at 1000 hr on 8 October 1997. Time as reported in this
summary is local ship time (Universal Time Coordinated [UTC] + 2 hr). Leg 176 drilling
operations were quite different from those of most Ocean Drilling Program (ODP) legs. We
conducted all drilling operations at a single site in a single hole. Hole 735B was first
established during Leg 118. During that leg, a hard-rock base (HRB; Fig. F1) was set in 731 m of water and a 9
-in hole was cored to a
depth of 500 meters below seafloor (mbsf). Another 0.7 m of penetration was made on that
leg using the 3
-in Navidrill Core Barrel, establishing a "cored" total
depth of 500.7 mbsf. During Leg 176 we set out to reoccupy Hole 735B and deepen it to a
depth of at least 1500 mbsf. We were overwhelmingly successful in meeting this objective,
deepening Hole 735B from 1235.8 meters below rig floor (mbrf), or 504.8 mbsf, to 2239.0
mbrf, or 1508.0 mbsf. We have not established why our Leg 176 beginning sub-bottom bottom
depth was 4.8 m deeper than the total depth documented at the end of Leg 118.
We used the rotary core barrel (RCB)
wireline coring system exclusively during the leg. Ten RCB C-7 core bits yielded the
recovery of 122 cores. A total of 1003.2 m of new hole was cored, from which 865.99 m of
core was recovered, or 86.3% of the section (Table T1).
Penetration rates varied from 1.3 to 6.1 m/hr in the gabbro and olivine gabbro. We
achieved faster rates in coarser grained olivine gabbros and in fractured rock. Drilling
rates were slower in more massive, finer grained, or strongly foliated rocks. Conversely,
recovery rates were high, close to 100%, in both massive and fine-grained gabbro, but were
much lower in highly fractured rock. The average rate of penetration (ROP) for the leg was
2.7 m/hr. We took drift measurements at eight stations between 500 and 1400 mbsf. Hole
deviation based on the Tensor electronic multishot data was moderate, varying between
4.3º and 4.8º.
Our progress in the hole ended when
a 5-in drill pipe connection failed, leaving 131 joints of 5-in drill pipe and a coring
bottom-hole assembly (BHA) in the hole, a total of 1403 m of drill string. The failure
occurred when the pipe was 97 m off bottom during a routine wiper trip to replace
wear-knotted drill pipe with standard 5
-in drill pipe. At the
time of the failure, as during most of our operations during this leg, we were
experiencing heave of the rig floor in excess of 2 m. On more than one occasion, including
this episode, the pipe caught on a ledge in the borehole and lifted out of the elevator at
the rig floor. We attempted to fish the severed pipe using a 9-in overshot dressed with a 6-in basket grapple and mill control. We
successfully engaged the pipe and as it was being lifted to its total weight of 130,000 lb
the drill string parted again. The new failure point coincided with a point at which the
pipe had apparently buckled upon impact with the bottom of the hole. We ultimately
recovered a total of 497 m of 5-in drill pipe. The fish remaining in the hole (906 m)
consists of 26 stands of 5-in drill pipe (734 m) plus the coring BHA (172 m). We made
seven unsuccessful fishing attempts over a period of 7.4 days before abandoning attempts
to clear the hole.
Besides the drill string, several
historical records were also broken during Leg 176. More hard rock (866 m) was recovered
during Leg 176 than during any other leg in the history of ODP or the predecessor Deep Sea
Drilling Project (DSDP), nearly doubling the previous record, also made in Hole 735B
during Leg 118, of 434 m. We also cored more than twice the hard rock (1003 m) of any
other single leg, shattering the previous Leg 118 record of 501 m. Hole 735B is the fifth
deepest hole in ODP history. In addition, Leg 176 coring in Hole 735B now qualifies as the
third longest single-hole continuously cored interval (mud or rock) in ODP history.
We conducted logging operations in
two phases. The first phase (two logging runs to 492 mbsf) was done after the initial
reentry into the hole and before commencing coring operations. It included the
Triple-Combo (natural gamma sonde [NGS], accelerator porosity sonde [APS], hostile
environment lithodensity sonde [HLDS], and the dual laterolog [DLL]) plus temperature log.
The second string included the NGS, the dipole shear sonic imager (DSI), and the Formation
MicroScanner (FMS). We ended this logging run early, however, because of apparent data
acquisition difficulties.
We conducted the second phase of
logging and vertical seismic profile (VSP) experiments near the end of the leg, after
several attempts to recover the lost drill string. The first tool string consisted of the
HLDS, caliper, APS, and the hostile environment spectral natural gamma sonde (HNGS). The
second logging run consisted of the natural gamma tool (NGT), DSI, General Purpose
Inclinometry Tool (GPIT), and the FMS probes. The third tool string was composed of the
NGT, GPIT, and DLL probes.
During the first logging run on
phase II, we obtained good density, porosity, and gamma-ray measurements from 49 to 595
mbsf, or 11 m above the hole obstruction (606 mbsf). The second logging run (DSI-FMS)
resulted in good data recorded by the DSI. Cross-dipole and P- and S-wave
mode data were recorded during the first pass, and cross-dipole, upper dipole, and
Stoneley modes during the second pass. The FMS, however, did not fare as well. After
spending some time testing different gain options to improve the data quality from the
FMS, we opted to recover the tool and repeat the logging run with a replacement FMS.
Unfortunately, the results of the second tool run were similar to those of the first.
Shore processing later showed that the tools could not respond quickly enough to the
extreme resistivity contrasts between lithology types. The deployment of the third tool
string resulted in very good resistivity data with the DLL.
The fourth and final logging run
consisted of the Schlumberger three-component VSP tool. The objectives of the VSP were to
acquire data over the section of hole not covered by the Leg 118 VSP and to acquire better
VSP data using the new tool. We hoped that new data would resolve sub-bottom reflectors
below 1500 mbsf and give better observations of the seismic attenuation at the site. The
VSP operation took about 16 hr rig floor to rig floor. We shot both air gun and water gun
sources with the sonde clamped at 23 depths in the hole. The tool did not give appreciably
better data than were acquired on Leg 118, but we did fill in the coverage between 500 and
600 mbsf.
After concluding the wireline
logging and VSP runs we round-tripped the drill string one last time for a final fishing
attempt. The fish was contacted ~2 m deeper than before at 1339 mbrf, or 608 mbsf. During
subsequent working of the pipe, we contacted the fish at the shallower (606 mbsf) depth;
however, all attempts at engagement proved futile. We abandoned our final fishing attempt
at 1400 hr on 1 December 1997 and prepared to get under way for Cape Town, South Africa.
We departed Hole 735B at 1900 hr on 1 December 1997.
Transit to Site 735
At 1300 hr on 15 October 1997, we
passed the last line ashore and headed out of Table Bay, Cape Town harbor. At 1346 hr we
discharged the pilot and got under way at full speed for Site 735. Rough seas rounding the
Cape of Good Hope, strong headwinds, and the effects of the Agulhas Current slowed our
transit; however, we ultimately accomplished the 2003-nmi transit to Site 735 in 9.0 days
at an average speed of 9.3 kt. Propulsion motor P-17A was taken off-line during the
transit due to armature damage when a field coil failed. The engineers rebuilt the
assembly while on site, and it was placed back on line before our departure for Cape Town.
We were also forced to reduce speed for ~1 hr using manual steering and magnetic compass
for heading when an emergency generator test caused a malfunction of the automatic station
keeping (ASK) system. This system is linked to the ship's gyrocompass. The problem was
ultimately traced to a dead cell in the ASK back-up battery bank. Subsequent system
testing was successful once the bad battery was replaced. Besides routine
preventive-maintenance system (PMS) tasks, the installation of a rig-floor
waste-collection system was completed during the transit and efforts continued on the
installation/testing of a new waste-burning incinerator system for future Antarctic
expeditions. We arrived on location 24 October 1997, and at 1345 hr the first positioning
beacon was dropped using the 10-yr-old Satellite Navigation coordinates (32º43.395'S,
57º15.959'E) for Hole 735B.
Site 735
Locating Hole 735B
We immediately went to work making
up an outer core barrel assembly and four stands of 8
-in drill collars. Three
stands were temporarily stored in the derrick because our first pipe trip used only a
single stand of drill collars in the BHA for logging the existing 500-m-deep hole. After
tripping the drill string to the seafloor, we deployed the vibration-isolated television
camera and began searching for Hole 735B. Because this hole was being reoccupied, and the
seafloor depth was known to be 731.0 m, we did not require a precision depth recorder
reading. In >1 hr, using an expanding-box search pattern, we located the HRB deployed
during Leg 118. Final GPS location coordinates for Hole 735B were ultimately determined to
be 32º43.3928'S and 57º15.9606'E, or ~5 m away from the original coordinates of 10 yr
prior.
Initial Logging of Hole 735B
At 2000 hr on 24 October 97, we
reentered Hole 735B; however, resistance was encountered almost immediately at a depth of
736 m (5 mbsf). We pulled clear of the seafloor, picked up the top drive, and using slow
rotation reentered for the second time at 2130 hr. This time the end of the drill string
was placed at a logging depth of 780 m (49 mbsf) without incident.
Logging operations began with the
Triple-Combo consisting of natural gamma, porosity, lithodensity, and temperature tools.
We also obtained temperature data using the Lamont-Doherty Earth Observatory temperature
tool. The first logging run advanced to 492 mbsf, or just 8 m short of the total depth of
500.7 mbsf. Our second logging run included the NGS, the DSI, and the FMS. We had to end
this run early because of data acquisition difficulties. We completed the first phase of
logging operations by 1700 hr on 25 October 1997.
Following a pipe trip to change over
to a coring BHA, we again reentered Hole 735B and ran to bottom. The top drive was used at
several points to rotate past obstructions; however, minimal resistance was encountered on
the trip. Because the last core recovered during Leg 118 was taken with the Motor-Driven
Core Barrel (MDCB), we expected the lowermost 0.7 m of the hole would be under gauge for
RCB operations. The bit tagged bottom at 504.8 mbsf, or 4.8 m below the depth that RCB
coring ended on Leg 118, and 4.1 m below the depth of MDCB drilling. The discrepancy
between the depth to the bottom of the hole recorded at the end of Leg 118 and the depth
at which Leg 176 coring operations began remains unexplained.
Core Bit 1
We began continuous coring
operations with Core 176-735B-89R at a total depth of 1235.8 m, or 504.8 mbsf. For BHA
configurations, bit type, drill-string configurations, and bit-specific recovery for all
bit trips, the interested reader is referred to Table T2.
Partial cores (ranging in length
from 3.7 to 8.0 m) were taken beginning with Core 176-735B-92R because of the extremely
slow ROP. Core jamming inside the acetate butyrate core liners occurred on three cores in
a row, so, beginning with Core 176-735B-96R, we deployed the core barrels without liners.
We used bentonite gel mud exclusively during Leg 176, circulating the mud in 20-bbl sweeps
every other core to enhance hole cleaning. Upon recovering Core 176-735B-96R, we found
that the core catchers and cored material were missing. The threaded connection between
the core catcher sub and the 11
-in inner barrel sub unscrewed, leaving the
core barrel components and core material inside the pipe just above the bit. We decided
that attempting to fish the items out of the pipe would be futile; therefore, the bit run
was ended at a depth of 556.3 mbsf. The bit cleared the rig floor at 2145 hr, and Core
176-735B-96R was recovered from the pipe. We found the core bit to be in good condition
except for the inner (nose) row of carbide buttons.
Core Bit 2
Using the same BHA configuration we
deployed the subsea television camera and reentered the HRB in less than 15 min of ship
maneuvering. These quick (15 min) reentries proved to be the rule for the remainder of the
leg. In this case, the driller again could not induce the bit into the hole, and another
reentry had to be made with the top drive picked up. On the second attempt we reentered
the HRB at 0300 hr on 28 October 1997 and, using slow top-drive rotation, walked the bit
into the off-center hole (Fig. F1) without incident. Continuous
coring began with Core 176-735B-97R from 556.3 mbsf. Coring on this bit run was affected
by large long-period swells, which translated into rig floor heaves of 2.0 to 3.0 m,
making it difficult for the driller to maintain optimum weight on bit (WOB). The seas
moderated on the third day, but by then it was time to pull the bit for replacement. Our
original goal was to obtain a minimum of 40 rotating hours on this bit, but because of the
large load fluctuations, we decided to be conservative and terminate coring operations
slightly earlier than planned. In addition, the driller noted elevated pump pressure
during the final three cores, and we suspected that two of the bit jets may have become
plugged. We suspended coring after cutting Core 176-735B-111R to a depth of 642.7 mbsf.
The bit cleared the rig floor at 0930 hr on 30 October 1997. The core bit was in good
condition except for the inner (nose) row of carbide buttons. The bit did have two jets
plugged with a combination of cuttings and Teflon bit-seal material. In addition, a
5-in-long crack extended longitudinally outward from the base of the pocket in the
drilling-jar body.
Core Bit 3
After reentry with this bit run, we
encountered 8 m of loose fill at the bottom of the hole. We initiated continuous coring
with Core 176-735B-112R at 642.7 mbsf. Our goal on this run was to replace the bit after
completing Core 176-735B-124R; however, observations of core tapering and some suspicious
markings in Core 176-735B-123R persuaded us to halt operations with only 3.4 m of advance
on Core 176-735B-124R. The bit cleared the rig floor at 0530 hr on 2 November 1997.
Surprisingly, we again found the bit to be generally in good condition with effective
bearing seals and no sign of imminent catastrophic failure. The internal cutter wear was
more advanced than on the two earlier bits with the inner (nose) row of carbide buttons
completely worn back into the matrix material. One of the two opposite cutters with three
carbide inserts was also worn back to the matrix material. In addition, there was some
wear at the tip of the core guide, indicating that ROP would have soon begun to suffer.
Core Bit 4
As a precaution, because of the
earlier cracking incident, we again laid out the drilling jars for nondestructive testing
(NDT) inspection and pressure testing prior to running them back in the hole with the
fourth core bit. During the NDT inspection, we identified small cracks initiating from the
two corners at the base of each pocket in the jar body. We set these jars aside, and
because the remaining set of drilling jars had not yet passed pressure testing, this BHA
was made up without jars. At 0845 hr on 2 November 1997, we reentered the hole and ran to
742.0 mbsf, or ~10 m off bottom. Using the Tensor electronic multishot tool we conducted a
wireline drift survey from 742.0 to 500.0 mbsf. Six measurements made at 50-m increments
indicated a hole deviation of 4.5º ± 0.3º. Upon completing the survey, we began
continuous coring with Core 176-735B-125R from 752.1 mbsf. While cutting Core
176-735B-136R, circulating pressure dropped by 100 psi, and we assumed that the RCB bit
seal had failed. Although the bit continued to recover gauge core and advance at a
respectable rate, we decided again to be conservative and pulled the bit after 46.8 hr. We
halted coring after recovering Core 176-735B-140R to 889.3 mbsf. The bit arrived at the
rig floor by 0500 hr on 5 November 1997. Once again, the bit was in good condition with no
sign of imminent catastrophic failure. As expected, the internal cutter wear was slightly
more advanced than on previous bits. The inner (nose) row of carbide buttons was
completely worn back into the matrix material, and both of the two opposite cutters with
three carbide inserts each were also worn back to the matrix material. There was slight
wear at the tip of the core guide. The bearing seals were still effective, and the bearing
condition was actually slightly better (tighter) than that of the previous bit.
Core Bit 5-First Deployment
During the previous bit run we
rebuilt the final remaining set of drilling jars and added packing to the seal chamber.
The jars passed a pressure test of 1500 psi for 10 min and were judged acceptable for use.
A new C-7 core bit was made up, and after 15 min we made yet another routine reentry at
0800 hr on 5 November 1997. We were tripping the bit to bottom when it encountered a hard
bridge at a depth of 123 mbsf. After picking up the top drive we again encountered an
obstruction, this time at 117.0 mbsf. We spent close to 1 hr attempting to clear
the bridge with the C-7 core bit to no avail. The top drive repeatedly stalled out with
little or no WOB each time it contacted the obstruction. After deliberation, we decided
that a better strategy would be to use a more robust tricone drill bit to clear the hole.
The drill string was tripped back to the rig floor by 1545 hr that same day.
Tricone Drill Bit
Leaving the BHA configuration
unchanged, we exchanged the Rock Bit Industries C-7-style core bit for a Smith F57 tricone
drill bit. We reentered the hole at 1915 hr on 5 November 1997 (15-min reentry), and the
pipe tagged the obstruction at the same depth as before (117.0 mbsf). Top-drive stalling
continued to be a problem during our attempts to clear the bridge. This was aggravated by
significant heave resulting from long-period swells generated by a significant
low-pressure cell some 50 miles to the south of the drilling location. Ultimately, by
using high rotation speeds (100-130 rpm) coupled with low WOB and some skillful drilling
supervised by the Overseas Drilling Limited (ODL) drilling superintendent, we cleared the
bridge in the hole. There were, however, top-drive stalling torques of 750 A and overpulls
of up to 60,000 lb during the episode. We theorized that an angular piece of material must
have sloughed off of the side of the hole, leaving a high angle bridge that caused both
core and drill bits to wedge immediately upon contact. We subsequently washed/reamed the
hole to bottom to ensure that any other bridges or hard fill on bottom would be broken up
by the more rugged tricone drill bit before resuming coring operations. No other bridges
were identified, however, and only 2.0 m of soft fill was encountered on bottom. We
circulated this out in short order and pumped a 50-bbl bentonite gel mud sweep. We then
tripped the pipe back to return the BHA to a coring configuration. The tricone drill bit
cleared the rig floor at 0630 hr on 6 November 1997.
Core Bit 5-Second Deployment
As the bit used in our fifth
deployment showed no signs of wear, we ran the same bit back into the hole following
hole-cleaning operations. The drilling jars failed a routine pressure test. Because they
were the only remaining set of jars, we delayed reentry until the packing on the jars
could be replaced and pressure tested. We then made up the jars with the remaining BHA and
tripped the drill string to bottom. The hole was reentered at 1230 hr on 6 November 97
after another routine 15-min ship maneuver. We encountered no ledges, and the pipe went
straight to bottom without incident. No fill was detected at total depth, and we began
continuous coring with Core 176-735B-141R from 889.3 mbsf. While we were cutting the
second core of this bit run, the drilling torque escalated to 600 A and we experienced
60,000 lb of overpull picking up off bottom. High torque, top-drive stalling, and overpull
of 40,000-60,000 lb continued through Core 176-735B-145R, but we encountered no problems
after Core 176-735B-146R. We halted coring while cutting Core 176-735B-151R at a depth of
987.5 mbsf when drilling torque escalated to 40-50 or more amps from the normal and ROP
dropped off to 1.6 m/hr. The bit cleared the seafloor at 0250 hr and was at the rig floor
by 0415 hr on 9 November 1997. As before, we found the bit to be in good condition with no
sign of imminent catastrophic failure. The internal cutter was again severely worn, with
the inner (nose) row of carbide buttons completely worn back into the matrix material.
Both of the two opposite cutters with three carbide inserts were also worn back to the
matrix material. Bearing seals were effective, and the bearing condition was good.
Core Bit 6
We made up yet another new C-7 core
bit with the BHA. However, drilling jars were not included on this run because the final
remaining set also showed evidence of cracking during NDT inspection. After reentry at
0700 hr on 9 November 1997, the drill string was run to the bottom without incident. We
initiated continuous RCB coring on Core 176-735B-152R at a depth of 987.5 mbsf and
continued coring through Core 176-735B-163R to a depth of 1099.4 mbsf. We terminated
coring early while cutting Core 176-735B-163R because the drilling torque abruptly
increased 40-50 or more amps and the ROP dropped to 2.5 m/hr from earlier, higher rates.
The bit reached the rig floor by 2330 hr on 11 November 1997 and again was in good
condition with no sign of imminent catastrophic failure. The internal cutter wear was
severe, however, and the inner (nose) row of carbide buttons was completely worn back into
the matrix material, as were both of the two opposite cutters with three carbide inserts.
One insert had fallen out of the core-gauge row and another had broken off. Bearing seals
were effective, and bearing condition was as good as, or better than, on any of the
previous bits.
Core Bit 7
We made up the seventh new C-7 core
bit and welded closed a crack in the latch sleeve before running in the hole. The BHA was
run in the hole as before without drilling jars. We reentered at 0245 hr on 12 November 97
and ran the drill string to bottom without incident. We initiated continuous RCB coring
with Core 176-735B-164R at a depth of 1099.4 mbsf and continued to a depth of 1191.1 mbsf.
No fill was identified after any of the connections. However, while cutting Core
176-735B-174R, the drilling torque increased dramatically from the normal 360-400 A to
500-600 A. The torque dropped back to normal when we picked the bit up off bottom. We had
only 33.3 rotating hours on the bit; nevertheless, we could not rule out the possibility
of a pending bit problem. We therefore decided to err on the side of conservatism and
brought back the final core after advancing only 3.0 m. Upon clearing the rig floor at
0430 hr on 14 November 1997, we found the bit to be in excellent condition; similar to the
bit used during the second core-bit run of 36.0 hr. There was, instead, a high degree of
wear and burnishing on the crossover sub between the 5-in and 5½-in drill pipe during the
pipe trip. A check of the pipe tally placed the crossover at or near seafloor in the
vicinity of the HRB when the bit was on bottom. This may have been the source of the
excessive drilling torque.
Core Bit 8
We ran the eighth new C-7 core bit
in the hole and replaced the crossover sub between the 5-in and 5-in drill pipe during the pipe trip. In addition, we added 10 more
stands of 5-in drill pipe to the string to remove the crossover from the area of the HRB.
The bit went to the bottom of the hole without incident after another routine 15-min
reentry (0700 hr on 14 November 97). We began continuous RCB coring with Core
176-735B-175R at a depth of 1191.1 mbsf and continued to a depth of 1360.6 mbsf. Coring
was terminated after advancing 5.6 m on Core 176-735B-193R when the drilling torque
increased by 50-100 A. The bit had 53.8 rotating hours at that time so we conservatively
pulled the bit. The bit reached the rig floor at 1530 hr on 17 November 1997 and was in
good condition. One cover plate from the grease reservoir was missing, however, and this
may have contributed to the increased torque downhole.
Core Bit 9
On our ninth bit run, we used
another new C-7 core bit with an additional five stands of 5-in drill pipe to keep the
5-in × 5-in crossover sub well above the HRB. After slipping and cutting the
drill line for the first time this leg, we reentered the hole at 2045 hr on 17 November
1997 and ran to a depth of 1295.0 mbsf, where the bit met an obstruction and was unable to
pass. We picked up the top drive and were able to ream the remaining 65.6 m without
difficulty. We found no indication of any fill on bottom, and the drilling torque was
normal. Continuous RCB coring was initiated with Core 176-735B-194R at a depth of 1360.6
mbsf and continued to a depth of 1386.4 mbsf. We stopped coring 3.1 m into Core
176-735B-197R because of worsening weather. In addition to the heave compensator beginning
to exceed operating limits, we experienced five 3% and two 5% positioning alarms. We
stopped coring after only 8.8 rotating hours on the bit and began to pull out of the hole.
After pulling the pipe to a depth of 739.0 m (8.0 mbsf), the drill string was hung off on
the elevators while the driller laid out the previously pulled stand of drill pipe. At
that time the bit contacted a ledge in the hole, causing the drill string to jump upward
in the landing elevators. Using the stand in the string above the rotary, the string was
lifted ~2.0 m off the elevators until the bit was free from the ledge. Two attempts were
then required before the drill string could be lowered past the ledge to the next tool
joint. The bit cleared the seafloor at 2115 hr on 18 November 1997 with marginal weather
and sea conditions. Winds were ranging from 33 to 41 kt gusting to 51 kt, and swells were
running 8-18 ft at 8- to 9-s periods. The vessel was heaving 10-18 ft. Because of
extremely rough seas at the time, we decided to hang off the BHA/transition pipe below the
keel and wait for the weather to abate. At 2215 hr on 18 November 1997, the string was
hung off on the elevators, and we commenced "waiting on weather." By 0245 hr the
following morning, conditions moderated sufficiently to recover and conduct a NDT of the
BHA. We found no cracks during the "magnaflux" inspection; however, two drill
collars (2 and 3) were bent. This was obviously the result of the incident just described.
The recovered bit was in excellent condition although (nose) cutter wear was much higher
(50% of the carbide inserts worn away) than expected of a bit with less than 9 rotating
hours.
Core Bit 10
By 1500 hr on 19 November 1997 our
sea conditions had moderated, and the captain felt confident that the storm system was
both weakening and moving away. We made up another new C-7 core bit, ran in the hole, and
reentered at 1945 hr on 19 November 1997. When the bit reached a depth of 1295.0 mbsf it
was again unable to pass, just as in the eighth bit run. As before, we picked up the top
drive and reamed to bottom without incident. We proceeded to rotary core with Core
176-735B-198R at a depth of 1386.4 mbsf and continued to a depth of 1508.0 mbsf. We took
drift measurements at 1100 and 1400 mbsf and determined hole deviation at those points to
be 4.6º and 4.8º, respectively.
We interrupted coring after Core
176-735B-210R to short trip five stands of wear-knotted drill pipe out of the hole and
replace it with standard 5-in drill pipe. We could have continued coring
using knobby drilling joints, but the cores would have been shorter (9.2 m vs. 9.6/9.7 m)
and the knobbies would have been more time-consuming to handle, whereas the wear-knotted
pipe could be handled using the automated pipe racker. After pulling all but four joints
of the wear-knotted pipe from the hole, we suffered a drill-string failure. The pipe was
landed in the elevators while the driller placed a double joint of drill pipe into the
mousehole with the top drive. During this time the drill string twice came into contact
with a ledge in the hole when the vessel heaved down, causing the string to rise ~0.5 m
above the landing elevators. The driller moved as quickly as possible to reinstall the top
drive, and the string was lifted off of the landing elevators. However, by that time the
damage had been done; the weight indicator showed a loss of 130,000 lb of string weight.
Our calculations indicated that the
string parted in the 5-in drill pipe at or near the seafloor. We sent down the subsea
camera to inspect the drill string/HRB, confirming that the failed pipe was not above the
seafloor, and then recovered the drill string. At 0600 hr on 22 November 1997, we
identified the point of failure as the last-engaged thread of a 5-in drill-pipe pin
connection located at 739 m (8 mbsf). The fish we left in the hole was 1403 m in length
and consisted of a 172-m-long BHA plus 43
stands of 5-in drill pipe (1231
m).
Fishing Attempts
We made a total of eight attempts to
fish the drill string from the hole. Interspersed with the fishing attempts were several
pipe trips with milling tools and one for logging operations. During the first fishing
attempt, we were able to latch onto the severed drill string. We were lifting it to its
total weight of 130,000 lb when it parted once again, leaving only 35,000 lb of weight
suspended below the overshot tool. We pulled the fishing string to the surface and
recovered a total of 497 m of 5-in drill pipe by 1930 hr on 22 November 1997. The drill
string had parted in the 5-in drill pipe tube 2 ft below a box tool joint. This was at a
point where the tube had buckled when the string impacted the bottom of the hole after the
initial failure. The portion of drill string remaining in the hole (906 m) consisted of 26
stands (734 m) of 5-in drill pipe plus the coring BHA of 172 m.
During subsequent fishing/milling
trips, we repeatedly encountered difficulties with ledges near the top of the hole.
Although we tried several different fishing and milling tools, many fabricated on site by
the ODL crew, none of these was successful in engaging the severed drill string (for a
complete list of fishing and milling drill string configurations, see Table T2). A common result of milling operations was advancement of the
drill pipe beyond the depth where the top of the severed drill string was initially
encountered. Subsequent fishing attempts, however, routinely stopped at the original depth
of the top of the severed pipe, indicating that the mills were side-tracking along the
drill string rather than grinding away the top of it.
On our third milling attempt, we
inserted a drill string stabilizer into the BHA to help keep the mill straight and
potentially reduce side-tracking. Unfortunately, unacceptably high torque immediately
after reentry indicated that the upper part of the hole was too narrow for such a
configuration, and the stabilizer was removed from the milling BHA. Our fourth and final
milling attempt went much better than previous attempts, with constant torque and a very
slow ROP. On recovery of the milling tool, we noted a concentric wear pattern with a 5-in
diameter in the center of the mill face. We were encouraged enough by this evidence to
plan a final fishing trip, after allowing time for our logging program.
Wireline Logging and VSP Experiment
We used a shortened BHA for logging
and installed a landing saver sub with a "special" landing ring two drill
collars up from the bit to provide a landing point for the Kinley Crimper/Cutter assembly,
should it be required. The landing ring inside diameter had to be opened up to 3.918 in to
accommodate the oversized Schlumberger VSP tool. We reentered the hole at 0745 hr on 29
November 1997, and the pipe was placed at 780 mbrf, or 49 mbsf, with a 30-ft drilling
knobby through the guide horn. At 0915 hr we began to rig up the Schlumberger logging
sheaves and wireline compensator.
The first tool string consisted of
the high-temperature lithodensity tool (HLDT), caliper, APS, and HNGS probes. The second
logging run consisted of NGT, DSI, GPIT, and FMS probes. The third tool string was
composed of NGT, GPIT, and DLL probes. The fourth and final run consisted of the
Schlumberger BGKT three-component VSP tool. During the experiment, a hydrophone that had
been hung off the rear port side crane to monitor noise in the water column failed. To
avoid tangling the cable in the main screws, we brought the hydrophone back on deck for
the remainder of the experiment. We rigged down the VSP tools and logging sheaves by 0415
hr on 1 December 1997 and proceeded to pull the drill string, clearing the seafloor at
0430 hr and reaching the rig floor by 0530 hr.
Final Fishing Attempt
While the logging/VSP efforts were
under way, we fabricated another modified fishing assembly. An overshot assembly was
shortened as much as possible, bringing the mill control and basket grapple closer to the
open end of the overshot. In addition, "Cutrite" hard facing was applied to the
lip guide, creating a "milling-capable" overshot assembly. We hoped that the
combination of the shorter fishing assembly and the limited milling ability would allow
the basket grapple to engage the drill pipe. We encountered the top of the severed drill
string ~2 m deeper than before at 1339 mbrf, or 608 mbsf. During subsequent attempts to
grasp the pipe, however, we made contact at the shallower (607 mbsf) depth. To our dismay,
even after several hours and a herculean effort by the drilling crew, we were not able to
extract the drill string from Hole 735B. We abandoned our final fishing attempt at 1400 hr
on 1 December 1997 and prepared to get under way for Cape Town, South Africa. When the
fishing tool was recovered, it showed obvious indications that something had at some time
been inside the overshot as far as the mill control assembly.
Transit to Cape Town, South Africa
We began the return voyage to Cape
Town, at 1900 hr on 1 December 1997. During the transit, the rig crews worked on the usual
cleanup, painting, report writing, and PMS activities. In addition we held a meeting with
rig personnel and the Leg 176 co-chief scientists/staff scientist to discuss approaches
and operational options for a possible future deep hole (2500+ mbsf) in the Hole 735B
Southwest Indian Ridge operating area.
We arrived at the pilot station at
1030 hr 9 December 1997 after a 1998-nmi voyage that averaged 10.9 kt. The first line was
ashore at 1130 hr on 9 December 1997, officially ending Leg 176.
1Examples of how to
reference the whole or part of this volume can be found under "Citations" in the
preliminary pages of the volume.
2Shipboard
Scientific Party addresses can be found under "Leg 177 Participants" in the
preliminary pages of the volume.
Ms 176IR-103
