OPERATIONS

The beacon at Site 1254 was released at 1106 hr and was on deck at 1221 hr on 26 October 2002 (all times reported are local ship time, which is Universal Time Coordinated - 5 hr). The ship moved the ~1.1 nmi to Site 1255 in dynamic-positioning mode while the CORK-II wellhead was being recovered. We deployed a seafloor position beacon at Site 1255 at 2116 hr on 26 October.

A reentry cone was then moved onto the moonpool doors and two joints (20 m) of 16-in casing were assembled to a 16-in casing hanger and latched into the reentry cone. A jetting bottom-hole assembly (BHA) was assembled and latched into the 16-in casing hanger. At 2000 hr on 26 October, the reentry cone was lowered to the seafloor. Because of the complex seafloor topography in the vicinity of the drill site, the 3.5-kHz precision depth recorder returned six different echoes. The most conservative shallowest depth indicated (4234 meters below rig floor [mbrf]) was used. The casing was lowered to 4325 mbrf before the seafloor was detected by a reduction in drill string weight. Hole 1255A was started at 0550 hr on 27 October when jetting-in of the 16-in casing began.

The 16-in casing was jetted in to 4343.49 mbrf (20.49 mbsf) in 5 hr. The casing running tool was released, pulled out of the hole, and was back on the rig floor at 2045 hr on 27 October. The next step was to prepare the hole for installing 10-in casing. A BHA with a 14-in drilling bit was assembled and lowered to the seafloor, and Hole 1255A was reentered at 0430 hr on 28 October. We drilled from 4343.49 to 4446 mbrf (20.49 to 123 mbsf). The hole was swept with 50 barrels of sepiolite, and then the complete borehole volume was circulated with seawater. We then conducted a wiper trip by raising the bit to the 16-in casing shoe at 4343.49 mbrf (20.49 mbsf) and then lowering it back down to 4429 mbrf (106 mbsf), where slight weight was taken. The hole was washed and reamed from 4429 mbrf (106 mbsf) back to total depth (4446 mbrf; 123 mbsf). The hole was again swept with 50 barrels of sepiolite, twice the hole volume was circulated with seawater, and 85 barrels of sepiolite was pumped into the hole in preparation for deploying 10-in casing. The bit was then pulled out of the hole and was back on the rig floor at 0526 hr on 30 October.

Ten joints of 10-in casing (115.5 m) were made up with a 10-in Dril-Quip casing hanger. A casing cementing plug was then made up to the Dril-Quip casing running tool that, in turn, was latched into the casing hanger. The 10-in casing string was then lowered to the seafloor. Hole 1255A was reentered with the casing at 1629 hr on 29 October. The casing was lowered to 4425 mbrf (102 mbsf) when slight resistance was encountered. The top drive was picked up and the casing was washed in to a total depth of 117.42 mbsf. After the casing hanger was confirmed to be latched into the reentry cone (10,000 lb overpull), the casing was cemented in place with 20 barrels of 15.8-lb/gal cement.

The casing running tool was released at 1940 hr on 29 October and pulled clear of the reentry cone. Before the drill string was tripped back to the ship, the drill line had to be slipped and cut. The casing running tool was back on deck at 0405 hr on 30 October.

A rotary core barrel (RCB) BHA was assembled and lowered to the seafloor, and Hole 1255A was reentered at 1207 hr on 30 October. It took 4.5 hr to drill out the 10-in casing shoe with the RCB bit before coring began.

We then cut four RCB cores (Cores 205-1255A-1R through 4R) (Table T1) from 4446.0 to 4480.0 mbrf (132.7 to 157.0 mbsf) and recovered 7.22 m (21%). Once the base of the décollement was identified (based on a substantial change in penetration rate), we began preparing the hole for installing the CORK-II and OsmoSampler.

We swept the hole with 50 barrels of sepiolite, and the borehole volume was circulated with seawater. The bit was raised up to the 10-in casing shoe and then lowered back down to 4467 mbrf (144 mbsf), where the bit took weight; the hole was reamed and washed from there to 4476.0 mbrf (153.0 mbsf). Once again, we swept the hole clean with 50 barrels of sepiolite and then circulated the borehole volume with seawater. The bit was again raised up to the 10-in casing shoe and then lowered back down to total depth with only slight drag near the bottom of the hole. To keep the hole open without exposing the décollement to barite-weighted mud, 20 barrels of sepiolite was placed in the bottom of the hole. The bit was pulled up to 4416 mbrf (93 mbsf) inside the 10-in casing, and the hole was displaced with 32 barrels of heavy mud. The bit was pulled out of the hole and was back on the rig floor at 2015 hr on 31 October.

A CORK-II consisting of a screen, packer, 123.4 m of 4-in casing, and a CORK-II wellhead was assembled. The CORK-II assembly was lowered to the seafloor to just above the reentry cone. The top drive was picked up so we could pump seawater through the end of the 4-in casing as it was lowered in the hole to displace the heavy mud and keep the screen clean. Hole 1255A was reentered at 1000 hr on 1 November, and the 4-in casing was lowered to 4465 mbrf (142 mbsf) while seawater was circulated.

The OsmoSampler seat was attached to the coring wireline and lowered down the drill string. The seat landed and was jarred on to latch the lock mandrel in place. When the wireline was recovered, the seat was still attached and the running tool was partially sheared and the seat seals were missing. The running tool was redressed, new seals were installed on the seat, and the seat was lowered back down the drill string. The seat landed in what appeared to be the upper latch nipple at top of the packer. The mud pump was engaged, the drill string was pressured to 300 psi, and the seat passed through the upper latch nipple. The seat was then lowered until it landed in the middle latch nipple at the top of the screen. Once again the pump was engaged, and the seat passed through the middle latch nipple. The seat was then lowered until it landed in the latch nipple inside the screen. The pump was engaged to fully latch the seat. The seat was jarred down on to latch the lock mandrel in place. An overpull of 3000 lb was applied and held, indicating that the lock mandrel was latched in place. The running tool was then jarred up on and released. The running tool was then pulled out of the hole and redressed.

When the OsmoSampler was attached to the running tool on the rig floor, the screws holding the OsmoSampler top connection in place were sheared. The running tool was removed, and the broken OsmoSampler was replaced with another one. The OsmoSampler fill port pipe plug (inside the top connection of the OsmoSampler) was protruding above the bottom of the blind box, preventing the double pin crossover sub to be properly attached. Bakerlok was applied to the double pin sub, and the sub was made up to the OsmoSampler top connection. Bakerlok was also applied to the collet housing, and the housing was made up to the OsmoSampler probe tip.

The OsmoSampler assembly picked up with a tugger and landed in the top of the drill pipe. The coring wireline was attached to the assembly, and it was lowered down the drill string in stages to allow pressure equalization of the osmotic pumps. Because the OsmoSampler lock mandrel had no seals, it easily passed through the upper latch nipple and was landed on top of the seat.

With the OsmoSampler probe tip stung through the seat, all of the OsmoSampler sampling ports were sealed off from the 4-in casing above. The pump was engaged to fully seat the OsmoSampler and help hold the assembly down during jarring. The running tool was jarred in order to latch the lock mandrel in place. When the wireline was raised, no overpull was observed and the weight of the OsmoSampler was lost, indicating that the running tool had released.

The OsmoSampler seat allows the pressure below and above the ported seal bore and plug to equalize. Thus, when the OsmoSampler probe tip is stung into the seat, the OsmoSampler is pressure balanced, so confirmation of latch-in (requiring pulling the OsmoSampler and deploying it again) was not critical to the installation. Once we verified that the OsmoSampler was at the correct depth (by tagging it with the wireline), we pulled the running tool out of the hole.

The modified packer setting go-devil was then dropped down the drill string and chased with the coring wireline without latch. The go-devil was tagged inside the CORK-II running tool, and the wireline was pulled out of the hole.

The CORK-II wellhead was then landed and latched into the reentry cone at 2000 hr on 1 November. The drill string was pressurized to 1200 psi for 30 min to inflate the packer. The drill string pressure was then increased to 1800 psi for 10 min to shift the spool valves. While the packer was set, the vibration-isolated television (VIT) camera system was pulled out of the hole in preparation for deploying the ROV platform.

The ROV platform was assembled around the drill string in the moonpool. The deployment bridle and the logging line were then attached. The platform was lowered down the drill string on the logging line and landed on the reentry cone.

At 0155 hr on 2 November, the acoustic releases were activated. When the logging line was picked up, overpull was observed, suggesting that the ROV platform was not completely free. The logging line was lowered and the acoustic releases were activated again. When the logging line was picked up, overpull was observed again. The overpull was increased above the weight of the platform, indicating the deployment bridle was hung up on something. The logging line was worked for 30 min without freeing the bridle when a straight pull was applied. The bridle came free with ~4500 lb overpull (9100 lb total wire load) and was pulled out of the hole.

When the bridle was recovered, one of the nylon strap termination eyes was found to have failed. The acoustic release yoke may have hung up in the nylon strap eye and had been pulled down to the platform strap bail. When overpull was applied, the platform was pulled up on one side only until the strap failed. If this scenario is correct, there should be no damage to the wellhead. After recovering the platform deployment bridle, the camera was lowered to inspect the installation and no problems with the installation were observed. The CORK-II running tool was released at 0615 hr on 2 November. The running tool and VIT camera system were then recovered; the running tool was back on the rig floor at 1600 hr on 2 November.

Once the seafloor positioning beacon was released (0759 hr) and back on deck (0929 hr on 2 November), we moved back to Hole 1254B in dynamic-positioning mode to attempt to fish the screen, packer, and 4-in casing that was left in the hole.

The Site 1255 CORK-II was visited ~1 month following installation. After fully closing one of the sampling valves, the CORK-II was functioning properly. For details of the Alvin dive and video for the CORK-II installation see "Postcruise Alvin Submersible Visit to Site 1253 and 1255 CORK-IIs" in the "Leg 205 Summary" chapter.