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doi:10.2204/iodp.proc.347.108.2015

Operations

Transit to Hole M0064A

The Greatship Manisha commenced transit to Site M0064 (proposed Site BSB-5) in Hanö Bay at 1215 h on 19 October 2013 and arrived on station by 1200 h on 20 October.

Hole M0064A

Operations in Hole M0064A commenced at 1200 h on 20 October 2013 with a remotely operated vehicle survey conducted to assess the seabed for the presence of WWII munitions. The survey covered Holes M0064A, M0064B, and M0064C. No evidence of any dangerous materials on the seabed was identified.

Following the survey, the vessel moved back to Hole M0064A and coring operations commenced (Table T1). The first four runs used the piston corer system (PCS) to recover clay. For Run 5, the nonrotating core barrel (NRCB) was used because of the presence of stiffer material indicative of the top of the diamicton unit. Run 6 used the push coring assembly (PCA) in an attempt to improve recovery. Following this, Runs 7–24 (16.05–34.5 meters below seafloor [mbsf]) used the NRCB, and Guar replaced seawater for pumping because of the increased sand content of the sediments. Run 25 was a PCA core, after a perceived failure to latch with the NRCB (no core recovered). Following the PCA, the string was flushed. The NRCB was lowered on the overshot, and latching was achieved. Coring with the NRCB continued for Runs 26–30. Runs 29 and 30 had no recovery, so a hammer sample (Run 31) was taken to prove the ground. The hammer sample recovered 0.08 m of gravel. The final NRCB (Run 32) core recovered 0.15 m of gravel with chalk beneath.

A total of 32 cores were recovered from Hole M0064A to 41.5 mbsf. Recovery for the hole was 66.82%.

Downhole logging operations started on 21 October at 1300 with drill pipe tripped to 1.5 m. The first tool string with the total gamma ray and induction tools reached only 5 mbsf. At 1515 h, the option of reaming the pipe to total depth, doing a wiper trip, and setting the pipe to 10.5 m was discussed, but as the ship required a heading change, the decision was made to abandon the downhole logging attempt at this hole. Logging operations were completed at 1520 h.

Hole M0064B

Following completion of operations in Hole M0064A, the vessel moved to Hole M0064B under dynamic positioning, and operations commenced at 1605 h on 21 October 2013. The second PCS run (3.3–6.6 mbsf) unexpectedly recovered coarse-grained sand. A third PCS run was attempted, with poor recovery. It was therefore decided to open hole in order to try and penetrate the sand feature. A final hammer sample taken at 10.1 mbsf confirmed that sand was still present at this depth, and because of potential damage to drilling equipment and the unanticipated lithology, the hole was terminated here.

A total of four cores were recovered from Hole M0064B to a maximum depth of 10.20 mbsf, with one open-hole section. Recovery for the hole was 96.94% when the open-hole section was discounted.

Hole M0064C

The vessel arrived over Hole M0064C at 1915 h on 21 October 2013, and drilling operations commenced. The first PCS core was recovered to deck at 1925 h. Three more PCS cores were recovered to deck before the top of the diamicton unit necessitated use of the NRCB.

Coring in Hole M0064C continued on 22 October, with 21 cores recovered by midday. This included one small interval of open holing to clean the hole and penetrate a possible obstruction that had resulted in zero recovery from the previous core run. Following a significant drop in recovery rates at 38.2 mbsf, a hammer sample was conducted to ascertain the exact lithology. Following this, five more NRCB cores were taken, with very limited recovery of rounded pebbles and gravel. A final hammer sample returned granite and chalk basement. The hole was terminated at target depth. Preparations were made for downhole logging. However, despite the hole being flushed and wiper trips being carried out to condition the hole, the hole collapsed from 9 mbsf. It was therefore not possible to conduct any logging operations.

A total of 35 core attempts were made in Hole M0064C, with one open-hole section of 2 m. The hole reached 45.10 mbsf, and hole recovery was 74.98% when the open-hole section was discounted.

Hole M0064D

Operations in Hole M0064D began at 1555 h on 22 October 2013. Based on the lithologies observed in the other holes at this site, it was decided that this hole should be cored at a new location, midway between Holes M0064A and M0064C, in an attempt to avoid the sand unit encountered in Hole M0064B and prevent the need for a second camera survey (as this hole lay on the A–C transect surveyed initially).

Operations began successfully in Hole M0064D with five piston cores being recovered. During recovery of the fifth core, a spline came off the overshot, but this was successfully retrieved using a magnet. As a soft diamicton was recovered in this run, the decision was made to change to the NRCB and continue operations.

NRCB Runs 6–13 recovered material from the diamicton layer to 23.4 mbsf. Run 14 was an open-hole run, using the insert bit to clear a blockage. Following this run, NRCB coring resumed to 33.90 mbsf. At this point, it was no longer possible to core, so an open-hole section was drilled, ending at 41.0 mbsf where a hammer sample was taken to prove the ground. The hammer sample recovered 0.2 m of gravel.

This marked the end of the hole (at 0755 h on 23 October), and the drill floor was prepared for downhole logging operations.

Downhole logging operations started in Hole M0064D on 23 October at 0715 h with rigging up the Weatherford logging setup while the drill pipe was tripped to 9 mbsf. The first tool string comprising the total gamma ray and induction tools reached 31 mbsf, and the second tool string comprising total gamma and spectral gamma ray tools reached 26 mbsf from where an uplog was started. Logging operations were finished at 1035 h.

A total of 21 cores were recovered from Hole M0064D. A further four recorded runs were open-hole sections with no recovery attempted. Hole recovery was 77.94% when open-hole sections were discounted.

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