An operations summary of Leg 196 Site 808 is presented in Table T1. Previous coring operations at Site 808 conducted during Leg 131 are reported in Taira, Hill, Firth, et al. (1991).
After the 1-hr, 8-nmi
transit from Site 1173, we arrived at Site 808 at 1800 hr on 18 May. During the
transit the drill crew slipped and cut the drill line. The seafloor positioning
beacon was deployed at 1937 hr. The precision depth recorder indicated a water
depth of 4674.5 meters below sea level (mbsl), or 4685.4 meters below rig floor
(mbrf). The vessel was offset 1 km west (upcurrent) to allow assembly of the
casing while the ship drifted with the current. The reentry cone was positioned
on the moonpool door. The drill floor was rigged up to run 20-in casing, and the
drill crew started running the 20-in casing at 2215 hr on 18 May. The casing
string included a shoe joint (10.63 m), 12 joints of casing, and a short (pup)
joint of casing attached to the casing hanger. The casing running tool was
attached to the casing hanger. A compound (Baker-Loc) was applied to each
threaded connection to ensure that it did not come apart, and all casing
connections were tack-welded. The casing was lowered and latched into the
reentry cone by 0515 hr on 19 May. We then assembled the drilling bottom-hole
assembly (BHA) that included an 18
-in
drill bit, a 22-in underreamer (Smith Service 15000 DTU), a
positive-displacement mud motor (Drilex Model D825MSHF), 16 drill collars, and
the casing running tool. The drilling BHA was lowered into the reentry cone and
casing and was latched into the casing hanger. The casing, reentry cone, and
drilling BHA were deployed through the moonpool at 0900 hr on 19 May. The
drilling BHA (160.14 m) extended 3.54 m beyond the casing assembly (156.60 m).
The casing, reentry cone, and drilling BHA had been lowered to 3606.9 mbrf by 1500 hr on 18 May. The vibration-isolated television frame (VIT) camera was deployed, and we continued lowering the pipe to 4646.1 mbrf, where we spaced out the drill pipe in preparation to spud. Hole 808H was spudded at 1805 hr on 18 May. The casing was drilled into the seafloor from 0 to 126.7 mbsf (4685.0-4811.7 mbrf). While drilling ahead, we circulated sepiolite mud sweeps at 4710, 4735, 4780, 4792, 4802, and 4811 mbrf.
At 0230 hr on 20 May the downhole positive-displacement motor (mud motor) that provides rotational torque to the underreamer and drill bit appeared to stop working. At 0430 hr, we decided to pull the casing and reentry cone back up to the ship and replace the mud motor. The bit cleared the seafloor at 0615 hr on 20 May. The pipe trip continued until 0645 hr, when we began to recover the VIT camera; the pipe trip resumed at 0815 hr. The casing and reentry cone were set on the moonpool doors at 1625 hr. After unlatching the running tool from the casing, the BHA was taken apart until the mud motor was reached.
The mud motor was tested from 1800 to 1830 hr. The mud motor would not rotate when we pumped seawater through it at 165 gallons per minute (gpm) at 1025 psi. Testing prior to drilling operations showed that it freely rotated at 275 gpm with 125 psi. The mud motor was laid down, and the drill bit cleared the rig floor at 1910 hr on 20 May, ending Hole 808H.
After its installation at 1915 hr on 20 May, the backup mud motor was successfully tested with circulation rates of 25 strokes per minute (spm) at 50 psi, 50 spm at 225 psi, and 70 spm at 450 psi. The BHA and casing running tool were attached to it, and the entire assembly was lowered and latched into the casing and reentry cone; it was deployed through the moonpool at 2205 hr.
When the bit was at 3809.2 mbrf, the VIT camera was deployed. We continued tripping down to 4646.1 mbrf, where we spaced out the drill string in preparation to spud. Hole 808I was spudded at 0720 hr; the seafloor depth was 4686 mbrf. We successfully drilled the casing down to 160.1 mbsf (4845.1 mbrf). The average drilling parameters were a weight on bit (WOB) of 5-15 klb and a flow rate of 400-550 gpm (80-110 spm) with 900-1150 psi.
The casing running tool was released at 2110 hr on 21 May, and the bit cleared the seafloor at 2245 hr on 21 May. When the drill string had reached 4588.6 mbrf, the VIT camera was retrieved. With the bit at 124.1 mbrf, the casing running tool was removed, the BHA was disassembled, and the mud motor and underreamer were flushed with freshwater.
On 22 May at 1245 hr, the
LWD/MWD (measurement while drilling) BHA was made up with a new 9
-in bit. The MWD mud pulse communications were tested with a flow rate of 305
gpm (61 spm). The trip down to the seafloor started at 1630 hr; the VIT camera
was launched at 0030 hr when the bit had reached 4309.0 mbrf. While the VIT
camera was being lowered, the drill crew slipped and cut the drill line. The
drill string was tripped to 4654.1 mbrf, and we started searching for the
reentry cone at 0315 hr on 23 May. The cone was successfully reentered at 0345
hr. The LWD/MWD tools were lowered from mudline (4686 mbrf) to 141.4 mbsf
(4827.4 mbrf). After the VIT camera was retrieved at 0630 hr, the drill string
was lowered to 160.1 mbsf (4846.1 mbrf). We began LWD/MWD logging/drilling at
0745 hr on 23 May.
The goal was to log/drill at a controlled rate of 50 m/hr. With connection times, the average rate of penetration (ROP) from 4846.1 mbrf (160.1 mbsf) to 5214.5 mbrf (528.5 mbsf) was 22.9 m/hr. Sepiolite mud sweeps (15 bbl) were pumped at 329, 413, and 471 mbsf to help remove cuttings and improve hole conditions. The drilling parameters used were a WOB of 5-20 klb, torque of 175-225 A, and a flow rate of 67 spm (327 gpm) with 1250-1350 psi standpipe pressure.
On 24 May, the logging/drilling continued from 5214.5 mbrf (528.5 mbsf) to 5435.7 mbrf (749.7 mbsf), and the average ROP dropped to 21.0 m/hr. Twenty-barrel sepiolite mud sweeps were circulated at 557 and 634 mbsf, with a 30-bbl sweep at 749 mbsf.
Before continuing deeper into the hole, a wiper trip was made to determine hole conditions from 5435.7 mbrf (749.7 mbsf) to the casing shoe at 4846.1 mbrf (160.1 mbsf). The drill string was pulled with the top drive to 5230.7 mbrf (544.7 mbsf) with a maximum overpull of 20 klb and torque of 400 A. The top drive was then removed and the string was pulled to 4827.4 mbrf (141.4 mbsf). The drill string was tripped back to 5256.5 mbrf (570.5 mbsf), where we picked up the top drive and reamed down to 5435.7 mbrf (749.7 mbsf). One tight spot was encountered at 5413 mbrf (727 mbsf).
Logging/drilling then continued from 5435.7 mbrf (749.7 mbsf) to 5474.3 mbrf (788.3 mbsf). The average ROP dropped to 15.7 m/hr as a result of the wiper trip and the time needed to make pipe connections. One 30-bbl sepiolite mud sweep was circulated at 759 mbsf. The drilling parameters were a WOB of 15-20 klb at 60 revolutions per minute (rpm), torque of 200-300 A, and a flow rate of 67 spm (327 gpm) with 1500 psi standpipe pressure.
On 25 May, LWD/MWD logging/drilling continued from 5474.3 mbrf (788.3 mbsf) to 5676.3 mbrf (990.3 mbsf). Sepiolite mud sweeps (20 bbl) were circulated at 846, 923, and 968 mbsf.
The décollement zone was encountered at 5654 mbrf (968 mbsf). We experienced drilling problems (high torque and high pump pressures) from 1115 to 2245 hr. Efforts to improve hole conditions included circulation, mud sweeps, and reaming up and down with the drill string. A short wiper trip was made from 5676.3 mbrf (990.3 mbsf) to 5615.4 mbrf (929.4 mbsf). During the wiper trip, the maximum overpull was 25 klb, maximum torque was 550 A, and the maximum pump pressure was 2200 psi.
Logging/drilling operations continued from 5676.3 mbrf (990.3 mbsf) to 5695.5 mbrf (1009.5 mbsf). The average ROP dropped to 13.3 m/hr as a result of connection times, hole conditioning, and the wiper trip. The LWD/MWD logging/drilling parameters were a WOB of 15-20 klb at 60 rpm, torque of 275-350 A, and a flow rate of 67 spm (327 gpm) with 1550 psi standpipe pressure. Sepiolite mud sweeps (20 bbl) were pumped at 958, 961, and 990 mbsf. Another wiper trip was made from 5695.5 mbrf (1009.5 mbsf) to 5615.4 mbrf (929.4 mbsf).
Most of 26 May was spent fighting the poor hole conditions, making wiper trips, and troubleshooting an overheating top drive. LWD/MWD logging/drilling extended from 5695.5 mbrf (1009.5 mbsf) to 5743.6 mbrf (1057.6 mbsf). The LWD/MWD logging/drilling parameters were a WOB of 15-20 klb at 60 rpm and torque of 275-350 A with a maximum of 600 A. The pump pressure was 60 spm (300 gpm) with 1550 psi standpipe pressure. At 2330 hr the drill string became stuck with the bit at 5702.1 mbrf (1016.1 mbsf). We were unable to rotate the drill string or move it up or down. Rotation and movement were successfully regained at 0045 hr on 27 May. We decided not to continue operations in this hole and began to trip out using the top drive for rotation and circulation up to 5432.7 mbrf (746.7 mbsf). The top drive was removed, and the bit cleared the seafloor at 0615 hr. The drill collars were racked back in the derrick, and the LWD/MWD logging tools were laid down. The drill bit cleared the rig floor at 1710 hr on 27 May, ending Hole 808I. The thrusters and hydrophones were raised, and the seafloor positioning beacon was turned off. We began the transit to Kochi at 1715 hr on 27 May.
At Site 808 we used the MWD tool to measure downhole drilling parameters, including downhole WOB, torque, and bit bounce. Given the strong MWD pressure signals obtained at the surface during Site 1173 MWD operations, we increased the MWD data transmission rates at Site 808 to 6 bits/s. Surface data from the RIS, including surface rotations per minute, torque, WOB, ship heave, pitch, and roll, were recorded synchronously. Ship heave was between 0.5 and 1.5 m during drilling at this site.
Eight experiments were conducted while drilling ahead with the AHC off (4917.2-4946.0, 5013.2-5032.1, 5118.6-5137.9, 5214.5-5233.7, 5320.2-5349.1, 5416.5-5435.7, 5522.3-5541.6, and 5724.4-5734.0 mbrf). One of these intervals (5522.3-5541.5 mbrf) was partially drilled with the AHC preloaded at a set surface weight to evaluate this practice in comparison to similar tests conducted at Site 1173. The comparison of downhole MWD parameters with the surface information will be analyzed postcruise to evaluate the shipboard heave compensation system and to compare drilling conditions with those encountered at Site 1173.
The 96-nmi transit to Kochi, Japan, began at 1718 hr on 27 May and ended at 1118 hr on 28 May. Because the local port authorities would not allow the vessel into the harbor before 1130 hr, the harbor pilot was aboard at 1126 hr, and the first line ashore was at 1230 hr. After customs and immigration formalities were completed at 1345 hr, 20 scientific staff members departed and 9 boarded the ship. The afternoon was filled with public tours for local television stations, newspapers, government officials, Kochi University officials, and university students. Nearly 100 people toured the ship.
At 1345 hr, we began
offloading the LWD/MWD tools, 10 joints of 20-in casing, 30 joints of 10
-in
casing, a broken mud motor, a 22-in underreamer, a broken air conditioner
compressor, and three-dimensional seismic computer work stations. Equipment
loaded included food, scientific equipment for the ACORK and thermistor
experiments, 64 joints of 4-in
casing, nine external casing packers, 11 joints of 10-in
screened casing, two ROV platforms, one replacement mud motor (Drilex 7
in), inner core barrels, 10-in
casing centralizers, 4-in
casing centralizers, one 30-ft knobby drill pipe, one 20-ft knobby drill pipe,
one tapered drill collar, one pup joint of 16-in casing, one 16-in casing
hanger, and a few steel I-beams.
Loading/offloading operations were completed at 0330 hr on 29 May, and the
equipment and casing were stored and secured for departure. After the port call,
we returned to Site 1173 for ACORK operations.
After finishing ACORK operations in Hole 1173B, we returned to Hole 808I. Despite the short distance between Sites 1173 and 808, we were unable to transit in dynamic positioning (DP) mode while tripping the pipe because of the current. We arrived back at Hole 808I at 0140 hr on 14 June after a 1.5-hr transit. The seafloor positioning beacon was turned on at 0240 hr. Because of our drill pipe loss at the end of Hole 1173B, our operations began with retrieving 75 additional joints of 5-in drill pipe from the riser hold, making them up into stands, and putting them in the drill pipe racker.
During LWD/MWD drilling
earlier in the leg, Hole 808I had been drilled with a 9
-in bit to 5743 mbrf (1057 mbsf). Our next step in preparing this hole for an
ACORK was to open the existing 9
-in hole to 17 in. A
drilling BHA was assembled with a 9
-in pilot bit and a 17-in
hole opener. The aluminum pipe protectors were removed from six joints of 5-in
drill pipe so that these joints could be used as transition joints in the BHA.
Once this was assembled, the bit was lowered to 4380 mbrf, and the VIT camera
was deployed at 2115 hr on 14 June; the drill line was slipped and cut while the
camera was being lowered. At 2315 hr we resumed lowering the drill string until
it reached 4669 mbrf, where it was spaced out for reentry.
We began the search for the reentry cone at 0000 hr on 15 June. Hole 808I was reentered at 0210 hr. Once we were in the hole, the drill pipe was lowered to 4812 mbrf (126 mbsf), and the top drive was picked up at 0345 hr. The VIT camera was retrieved; at 0615 hr we started drilling ahead at 4846 mbrf (160 mbsf). Twenty-barrel mud (sepiolite) sweeps were pumped every 40-50 m. The hole opening continued until we reached a total depth of 975.3 mbsf at 2025 hr on 17 June. The net penetration rate for the last 24 hr of drilling was ~10 m/hr. The driller noted tight spots from 5632.0 mbrf (946.0 mbsf) to 5660.0 mbrf (974.0 mbsf). Drilling parameters were a WOB of 10-20 klb, 60-70 top drive rpm, 200-350 A torque, and 130 spm at 1600-1700 psi.
We circulated a 40-bbl mud sweep and then started pulling out of the hole. The top drive was racked back at 0430 hr on 18 June when the bit reached 5129 mbrf (443 mbsf). As the pipe trip continued, tight spots were encountered from 4985 mbrf (299 mbsf) to 4971 mbrf (285 mbsf) and around 4956 mbrf (270 mbsf), which required up to 30 klb overpull. No additional problems were encountered and the bit cleared the seafloor at 0730 hr on 18 June. The bit cleared the rotary table at 1555 hr.
After the drilling tools
were secured, we began moving the ship ~70 nmi upcurrent (west-northwest;
~250°) to begin assembling the ACORK. The transit began at 1600 hr on 18 June.
During the transit, the drill floor crew prepared to run the 10-in
ACORK casing. As soon as the transit ended at 0000 hr on 19 June, we began
assembling the ACORK.
The first screen joint was
moved to the rig floor; a casing collar was welded to the bottom pin connection
to act as a guide shoe. The lowermost packer was made up as the second joint in
the string, and a single 
-in
stainless steel tube was run from the top of the screen to the bottom of the
packer. The hydraulic umbilical was started at the top of the lowermost packer.
As the casing, screens, and uppermost packer were assembled, hydraulic
connections were made at the top and bottom of each screen and packer. Three
centralizers were added to each joint, and the umbilical was strapped to the
casing near each centralizer with 
-in metal banding.
During the remainder of 19
June, 68 additional joints of 10-in
casing and five more screen joints were added to the ACORK assembly. From 0000
to 0500 hr on 20 June, eight more casing joints, one packer, and the landing
joint were added to the string. At 0500 hr the casing running tool, ACORK head,
and ported sub assembly were moved to the rig floor and attached to the top of
the landing joint. The total length of the ACORK assembly was 964.3 m. The
completed ACORK assembly was lowered below the rig floor and hung off the
moonpool doors by 0700 hr on 20 June.
The next step was to
assemble the drilling BHA that would pass through the ACORK assembly, allowing
the bit and underreamer to extend below the guide shoe on the bottom of the
ACORK. First, we had to remove the crossover subs that came with the mud motor,
as they had 7
-in instead of 6
-in connections; an ODP crossover sub connected the mud motor to the 8-in
drill collar above. We then discovered that the crossovers for the underreamer
were incorrectly listed in inventory as 4-in
regular connections and therefore could not be securely made up to those on the
underreamer. Because of the previous BHA losses on this leg, no other
appropriate crossovers were available, so we welded one of the existing
crossover subs directly to the underreamer. The other crossover required the
fabrication of a spacer ring to fill a gap between the mating shoulders before
it could be welded to the underreamer. A total of 9 hr was required for these
modifications.
The remainder of the
drilling BHA was assembled without additional problems. The complete drilling
BHA contained the following components: a 9
-in bit, a bit sub with float valve, three crossover subs, a 17-in underreamer,
a crossover sub, a mud motor, a crossover sub, nine 8-in
drill collars, a crossover sub, 91 joints of 5-in
drill pipe, one 5-ft 5-in
drill pipe pup joint, a crossover sub, a landing saver sub, one 8-in
drill collar pup joint, a jet sub, a motor-driven core barrel (MDCB) latch
sleeve sub, the casing running tool, two 8-in
drill collars, one tapered drill collar, six joints 5-in
drill pipe, and a crossover sub. The total length of BHA from the running tool
to the bit was 975.43 m.
When the drilling BHA was assembled and the casing running tool latched to the ACORK head, the complete assembly was lowered so that the final umbilical connections could be made in the moonpool area. After the ACORK assembly was dipped into the water for purging air from the hydraulic lines, the ACORK assembly and drilling BHA were lowered until the bit was at 1294 mbrf. At 0400 hr the VIT camera was deployed so that we could verify the space out of the underreamer with respect to the bottom of the casing as well as the functioning of the mud motor and underreamer. Once these tests were finished, the VIT camera was recovered at 0545 hr on 21 June. We then lowered the pipe to 3462.2 mbrf, where it was kept during the transit back to the site in DP positioning mode.
During the entire assembly/testing of the ACORK and the drilling BHA, the ship was in a controlled drift back toward Hole 808I. The initial offset from the site was ~70 nmi at 0000 hr on 19 June. We arrived back at Hole 808I at 0330 hr on 22 June.
On 21 June (0620-0645 hr), the Aso Maru arrived to deliver two engineers to assist with the thermistor string deployment later in the leg. A number of small packages, fresh fruit, and vegetables were also delivered. Two small pieces of failed drill pipe were offloaded for forensic analysis. Also, during the DP transit back to the site, 113 aluminum pipe protectors were removed from the drill pipe, as they would no longer be used.
We resumed lowering the drill pipe once we were back on site and ran the pipe down to 3798 mbrf, where we stopped to deploy the VIT camera at 0430 hr on 22 June. The pipe trip then continued at 0500 hr. Once the drill string reached 4662 mbrf, it was spaced out in preparation for reentering Hole 808I. After only 20 min, Hole 808I was reentered at 0805 hr on 22 June.
The drill bit/mud motor and casing shoe encountered some resistance passing through the reentry cone hanger at 4686 mbrf. After lowering the drill string to 4842 mbrf (156 mbsf), we began washing and drilling in the ACORK, reaching 5269 mbrf (583 mbsf) at 2400 hr on 22 June. Twenty-barrel mud (sepiolite) sweeps were circulated at 295, 430, 497, 555, and 574 mbsf. The drilling parameters were a WOB of 0-20 klb with a circulation rate of 100-120 spm and a pressure of 1450-2200 psi.
On 23 June, we continued drilling in the ACORK from 5269 mbrf (583 mbsf) to 5452 mbrf (766 mbsf). The VIT camera had to be retrieved at 2030 hr. Twenty-barrel mud (sepiolite) sweeps were circulated at 660, 670, 747, and 764 mbsf. The penetration rate for 22 June was 7.6 m/hr. The drilling parameters were a WOB of 10-20 klb and a circulation of 120 spm with a pressure of 2200 psi.
From 24 to 26 June, the ACORK was advanced from 5452 mbrf (766 mbsf) to 5611 mbrf (925 mbsf) with a penetration rate of only ~2 m/hr. Twenty-barrel mud (sepiolite) sweeps were circulated every 10-20 m. The drilling parameters were a WOB ranging up to ~180 klb and a circulation of 70- 120 spm with a pressure up to 2200 psi.
As the penetration rate was so low and any increase in WOB would cause concern for the safety of the casing and drill string, we decided to deploy the VIT camera at 1400 hr on 26 June and perform some WOB tests. Once the camera was on the bottom at 1615 hr, we noted that the drill string, ACORK head, casing, and reentry cone all appeared to be intact. We then attempted to increase the WOB to the maximum deemed safe and saw no significant increase in penetration or bending of the drill string or ACORK. We also tried to raise the drill string to see if recovering the entire assembly was an option; it could not be raised. During these tests, circulation was observed coming out of the reentry cone, which indicated that, although a flow path existed, the casing was being held by skin friction. The VIT was retrieved at 1950 hr on 26 June.
Because it was clear that we could not pull out of the hole, we continued to try to advance the ACORK in the hope that we could advance enough to either latch it in the reentry cone or get it low enough so that it could remain upright on its own. On 27 June, we advanced from 5611 mbrf (925 mbsf) to 5620 mbrf (934 mbsf) at ~0.4 m/hr. Two 100-bbl mud sweeps were pumped at 927 and 929 mbsf, as were two 20-bbl sweeps at 931 and 933 mbsf. The drilling parameters were a WOB of 180 klb and a pump rate of 70-120 spm with a pressure of 1000-2200 psi.
From 0000 to 0430 hr on 28 June, we continued to work the drill string, but it would not advance below 5620 mbrf (934 mbsf). In a last-ditch effort, we pumped 500 bbl of heavy (11.3 lb/gal) mud into the hole at 0430 hr and continued to work the pipe until 0730 hr with no success. At this point, we decided to inflate the packers. The circulation sub was shifted open with the wireline-shifting tool. The go-devil was dropped; once it landed, a slow pump rate was used to inflate the two packers and close the spool valves. This was completed at 0900 hr.
We then released the casing running tool at 1100 hr and raised the drill string from 5620 mbrf (934 mbsf) to 5574 mbrf (888 mbsf). Because it seemed unlikely that conditions would allow a bridge plug to be installed, the science party requested heavy mud be placed in the hole, so 100 bbl (10.5 lb/ gal) of mud was pumped into the hole. Tripping operations resumed at 1215 hr, with the top drive being racked back and the 20-ft knobby laid out.
The VIT camera was deployed and run in the hole from 1245 to 1315 hr on 28 June to observe the withdrawal of the drilling BHA from the ACORK head. At 1533 hr the drilling BHA came out of the reentry cone, and the ACORK head slowly bent toward the seafloor and went out of sight (Fig. F31). The drill string and VIT camera were lowered for a closer inspection and the ACORK casing was observed to be bent over the edge of the reentry cone (Fig. F32). With the camera, we followed the casing away from the reentry cone. The compass on the camera indicated the casing fell to the north. No damage was seen to the ACORK umbilical, casing, or head. The ACORK head was lying flat on the seafloor, and ~60% of the ACORK head appeared above the seafloor. Most important, the underwater-mateable connector was sticking straight up so that a submarine or ROV can still connect to download the data.
We started to recover the VIT camera at 1645 hr on 28 June. The drill string was pulled to 4491 mbrf, and knobbies were installed so that the drill string could be hung off during the DP transit back to Hole 1173B. The seafloor positioning beacon was released at 1815 hr. Once the VIT camera was back on board at 1845 hr, the hydrophones were raised and the ship began the DP transit back to Hole 1173B.
After completing the camera inspection at Site 1173 (see "Operations" in the "Site 1173" chapter), the drilling assembly used to drill in the Hole 808I ACORK was recovered. The underreamer arms were missing. This likely contributed to our inability to fully lower the ACORK into the hole.
