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doi:10.2204/iodp.proc.310.105.2007 Maraa western transect: Sites M0005–M00071Expedition 310 Scientists2OperationsHoles M0005A and M0005BThe DP Hunter arrived at Site M0005 at 2330 h on 6 October 2005 and immediately conducted dynamic positioning (DP) calibration tests ~250 m south of the first hole, Hole M0005A. By 0250 h on 7 October, the vessel was in position to start drilling. After overcoming problems with a hydraulic ram that caused the moonpool doors to fail to open, Seacore’s drilling and reentry template (DART) was deployed on an American Petroleum Institute (API) drill string (used as conductor pipe for this expedition) to just above the seabed. From 1335 h, a seabed survey around the position of the first site was conducted using the downpipe underwater camera. A suitable site was located ~2 m from the planned position, and the DART was placed on the seabed with 8 T of weight. After removing the camera, the HQ (piggyback) drill string was run through the API conductor to the seabed and coring operations in Hole M0005A commenced. The first core arrived on deck at 2000 h. Thereafter, coring continued with difficult barrel latching, and short core runs were used to avoid drilling rather than coring until the best coring parameters were established. The first cores displayed large cavities, and recovery was poor. Core barrel jams and problems with blocked bit–related mislatching led to slow progress in this hole. When reentering the hole after one particular drill string trip to remove a core blockage from the bit, coring appeared to recommence at 10.4 meters below seafloor (mbsf). Good core was obtained, and it appeared that the borehole had been restarted in a deviation or had avoided a previous deviation. This was taken as the start of a new hole, Hole M0005B, at 1840 h on 8 October. At 0140 h on 9 October, the core barrel failed to latch in at 20 mbsf. A replacement barrel also failed to latch. The hole was verified as clear by the chisel tool. It was suspected that the HQ pipe was bent, and the string was tripped. On deck, the third and second pipe stands (four joints) of the HQ drill string were bent. The decision was made by all to terminate Hole M0005B, not to log, and to make modifications to the DART before continuing with a new hole. The downpipe camera was prepared, and an environmental impact inspection of the coring area was carried out by raising the DART from the seabed and lowering the camera through the DART. There was difficulty in identifying the drilling area because of the minimal impact of the DART on the seabed. The camera was recovered, and the inspection was completed by 0615 h on 9 October. Starting at 0615 h on 9 October, the API string was tripped and the DART was brought on deck and secured above the moonpool. The vessel departed Site M0005 and headed for a nearby sheltered bay to carry out work on the DART. The DART feet were removed, and a stinger was installed. The vessel departed the sheltered bay and was back at Site M0005 by 1330 h. Hole M0005CThe DP Hunter took position above Hole M0005C, ~10 m east-southeast along slope from Holes M0005A and M0005B, and was ready for operations by 1510 h on 9 October 2005. When lowering the DART on the API string, a burst hydraulic union on the rig hydraulics caused an oil spill on the deck. There was an operational shutdown until the spill was contained and repairs were made. At 1835 h, the downpipe underwater camera was deployed for a predrilling site check, and the stinger was engaged with the seabed under camera observation. The DART was drilled 1.3 m into the seabed and stabilized, ready for the coring string. The HQ coring string was run at 2000 h, and coring operations commenced in Hole M0005C. At 0545 h on 10 October, sand in the drill string caused the core barrel to jam, and the string had to be tripped to free it. At 1110 h, a heading change of the vessel was requested, which was carried out with the HQ string off bottom but still in the hole. After the heading change, the core barrel was unable to be run into the HQ string and it was observed that the API pipe was not central in the moonpool. The HQ string was tripped, and nine stands were removed with bends. A problem with the DP was analyzed and resolved before resetting the DP to average the previous drilling position. At that position, the downpipe camera was run at 1515 h to see if the HQ hole was still in the stinger. No HQ hole was apparent, but the decision was made to trip the HQ string with an insert bit and drill down to the previous depth. Reentry into the earlier borehole was not confirmed even though the template had not moved, so a new hole (Hole M0005D) was started. Hole M0005DCoring commenced in Hole M0005D at 2330 h on 10 October 2005. Coring was steady and recovery moderate to good throughout the day on 11 October. At 0510 h on 12 October, the decision was made to stop coring in Hole M0005D after reaching 102 mbsf and without reaching the basaltic basement. The HQ string was pulled and the logging tools were prepared. At 0820 h on 12 October, the gamma logging tool was lowered, but it got stuck just below the seabed. It was suspected that the top of the hole was unstable, within a zone of low recovery noted while drilling. The top 5 m of the hole was cased with the HQ pipe. The gamma logging tool was lowered but was still unable to enter the hole from the pipe. The HQ pipe was run all the way to the base of the hole, a gamma log was run inside the HQ pipe, and then, after ensuring that the pipe was free from obstruction, the HQ pipe was pulled back in 25 m increments and the logging was conducted in the open hole below. The logging operation in Hole M0005D continued with excellent logs being obtained until the hole finally collapsed after pulling back the HQ pipe to 17 mbsf. The logging tools were in danger of becoming stuck in the top-hole section and were retrieved. After a heading change, the remaining HQ pipe was pulled, the DART was lifted, and operations in Hole M0005D ceased at 1310 h on 13 October. A postdrilling seabed survey using the downpipe camera was conducted before leaving the hole. Hole M0005EBefore departing Site M0005, four hammer samples were taken in Hole M0005E (~5 m along slope from Hole M0005D; 61.34 m water depth). Four samples were taken to a depth of 2 mbsf with typically 80% recovery. After curation, these samples were handed to the microbiologists to sample at will before the remainder was passed to the sedimentologists and coral specialists for description. Operations in Hole M0005E were concluded by 1635 h on 13 October. Before progressing to the next site, the DP Hunter conducted a 360° maneuver to verify that positioning offsets at the drill string were zero. Hole M0006AA move to deeper water (~80 m) was undertaken, and the DP Hunter was repositioned above Hole M0006A in 81.58 m water depth. Another four hammer samples were collected primarily for microbiology before the drill string was lowered onto what appeared to be a very steep slope. Both the ship’s transponder and tautwire indicated that they were sliding downslope and suggested that bathymetric data were insufficient to allow a reasonable picture of the seabed. Any slope instability may have led to the DART sliding away downslope, and the decision was made to avoid the outer reef edge until either more seabed information was obtained or modifications were made to the DART to allow greater stability on steep slopes. Site M0006 was departed at 2300 h on 13 October 2005. Hole M0007AHole M0007A was located further back from the reef edge than Sites M0005 and M0006, in 44.45 m water depth. The seabed was surveyed using the downpipe camera, and at 0330 h on 14 October 2005, four hammer samples were taken. After hammer sampling, the HQ string was run and rotary coring commenced in the same hole. Coring continued with good recovery for the rest of the day. Total depth (TD; 44.4 mbsf) was reached at 0120 h on 15 October. Prior to tripping the HQ pipe, the gamma tool was run inside the pipe. Logging results from Hole M0005D indicated that the shape of the gamma trace through the pipe, when scaled accordingly, did not differ significantly when compared to the open-hole trace, although statistically the results are less useful. After gamma logging, the HQ pipe was tripped, the core barrel was removed, and the string was rerun with a casing shoe to 7.5 mbsf. Open-hole logging commenced from TD to the casing shoe. After running the resistivity and hydrochemical tools in the open-hole section, the acoustic imaging tool would not progress much beyond the casing shoe, despite being run with a sinker bar. Logging was terminated in Hole M0007A at 0700 h on 15 October. Hole M0007BAfter a postdrilling seabed survey using the downpipe camera, the DP Hunter moved 50 m along slope to Hole M0007B. Two hammer samples were collected before rotary coring commenced in the same hole. Core recovery was poor in the top part of the hole, and the coring parameters were checked for any changes that may have been responsible. Zero bit weight and high rates of penetration confirmed that an open structure was being drilled. The formation became more compact with depth, and recovery improved accordingly. Coring continued until 0630 h on 16 October 2005. The HQ pipe was tripped and a casing shoe fitted before the pipe was rerun to the base of the hole for in-pipe gamma logging. After gamma logging, the HQ pipe was pulled to 24.6 mbsf and the resistivity, hydrochemical, acoustic imaging, and optical imaging tools were run. It was not possible to log the entire bottom-hole section because of hole blockage (below 30 mbsf). Logging in Hole M0007B was completed by 1410 h on 16 October. Hole M0007CBefore returning to Site M0007, Sites M0008–M0014 at Tiarei were cored (see the “Tiarei outer ridge” and “Tiarei marginal sites” chapters). Site M0007 was reached at 0200 h on 26 October 2005. Prior to coring in Hole M0007C, a seabed camera survey was conducted, which revealed a surprisingly barren sloping seabed in 43.35 m water depth. Coring operations began at 0315 h and continued until 2215 h. The hole progressed quickly, with core recovery generally poor and little formation resistance being evident on the bit. At ~30 mbsf, the hole started to become sticky and then collapsed at a wireline trip, filling the outer core barrel with cuttings. The string was tripped, the barrel was cleaned out (2 m of coral cuttings), and the HQ string was rerun with an insert bit as the inner core barrel. The base of the hole was reached with difficulty, where rotation also became difficult. The string stuck a few times, and the hole was abandoned at a TD of 32.25 mbsf. After Hole M0007C was complete, the vessel moved to Site M0015 on the Maraa eastern transect (see the “Maraa eastern transect” chapter). |
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