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doi:10.2204/iodp.proc.347.104.2015

Operations

Transit to Hole M0060A

At 0835 h on 22 September 2013, the Greatship Manisha sailed from Site M0059 to the second coring site (M0060; proposed Site BSB-1) southeast of Anholt, Kattegat. The vessel arrived at 0100 h on 23 September.

Hole M0060A

Operations in Hole M0060A commenced at 0145 h on 23 September 2013 with a downpipe camera survey of the three proposed holes at this site (M0060A–M0060C) to assess the seabed, in accordance with the permissions required at this site.

Coring operations were then established (Table T1), with a longer setup period than expected due to installation of a new sample wire sealer for the piston corer system (PCS). However, on the second run, the sealer failed, and operations returned to the previous system of using an overshot release. Other technical issues slowed operations throughout 23 September, including breakage of the hydraulic hose to the seabed template, failure of the sample wire requiring the drill string to be retrieved, and cracking of the slips bowl.

Thirteen cores were recovered during the morning of 24 September, with operations running smoothly. Following this, a series of technical issues arose. The sample winch rope snapped, which led to tripping of the pipe following an unsuccessful fishing attempt. Once the overshot was recovered, the rope was tested to assess the cause of the breakage. Coring operations were reestablished but halted again because of damage to the manual roughneck. By 2240 h, the hole had been cleaned and washed down to 75.8 meters below seafloor (mbsf), and coring operations continued throughout the night.

At 0600 h on 25 September, a decision to switch to the hammer sampler (HS) was made because of difficulties recovering harder lithologies, and a sample of compacted clay was recovered. At 97 mbsf, a lithologic change was noticed, and this change coincided with an overpressurized mudline. Coring continued, with the sampling methodology determined by the lithology encountered. PCS, HS, push coring assembly, and extended coring system coring were all undertaken in an attempt to optimize recovery in unpredictable lithologies. At 1545 h, the overshot was unable to pick up the piston corer because of sand infill, and it was necessary to repeatedly flush the hole before recovery could be achieved. PCS coring continued until 0220 h on 26 September, when the seabed template was recovered to deck because of failure of the clamps. Repairs were then carried out, and coring recommenced.

As drilling progressed, recovery rates varied, primarily because of the presence of sand lithologies. At 1335 h on 26 September, following use of the rotating core barrel, the barrel was recovered with no sample. However, the catchers were inverted and disturbed material was brought up. In order to avoid damage to the core, the coring strategy was changed and a flapper catcher with one basket was used. From 1725 h on 26 September until 0315 h on 27 September, coring continued with good recovery rates. During this period, deformation of the liner due to suction pressure was noted.

At 0315 h on 27 September, the overshot rope snapped, and it was necessary to trip pipe to recover the barrel (despite fishing attempts). Issues arose as a result of overtight joints and stuck drill collars. By 1400 h, it was possible to recommence running pipe back into the hole. The hole was then washed down with the noncoring assembly (NCA) in place to the level of the last sample taken before piston coring restarted (188.60 mbsf).

Coring continued throughout 28 September using a combination of open-hole and push core sampling. At 1715 h, it was noted that the string had become difficult to rotate. One further core was recovered before a sand blockage caused the string to become stuck.

This sand blockage resulted in the termination of Hole M0060A, and efforts were made throughout the evening of 28 September and the early morning of 29 September to free the string. Various options were investigated, which included use of the HS and running high pressure through the mudline. At 1020 h, it was decided that the best option was to back off the string and recover the remaining pipe. This plan was executed, but unfortunately a stick up of 14 m remained above the seabed (assessed with the remotely operated vehicle [ROV]). This was hammered in as far as possible and then flattened using the seabed template so that ~9 m of pipe was left lying horizontally on the seabed. ROV footage showed 4 m of visible pipe, with the remainder buried in sand at a shallow angle.

A total of 101 coring attempts were made in Hole M0060A to a maximum depth of 232.50 mbsf. Eighteen of these involved use of the NCA, covering 42.49 mbsf. Hole recovery was 83.06% when the open-hole sections were discounted.

Hole M0060B

Operations in Hole M0060B commenced at 1600 h on 29 September 2013. Because of the drilling issues associated with sand in Hole M0060A, it was agreed that the target depth of the hole would be 100 mbsf, with termination determined by the first presence of the sand lithology.

This hole was a designated microbiology hole, and intensive sampling was conducted in accordance with the developed scheme. PCS coring ran smoothly at this hole, with GS550 mud used for stabilization of the loose sand encountered at the seabed. The concentration was reduced as the hole progressed into clayey material to 30 mbsf; seawater was used as the drilling fluid for Cores 10H through 27H. There was excellent recovery throughout the hole until an aquifer was encountered at 82 mbsf (Core 28H). After difficulties in core recovery and a tight drill string, it was decided to terminate the hole slightly shallower than anticipated at 85.70 mbsf. By midnight, coring operations were complete and the drill floor was being prepared for downhole logging.

Downhole logging was completed in Hole M0060B by 0735 h on 1 October, with three successful tool runs to a maximum depth of 67 mbsf. The imaging tool did not appear to be operating correctly, but later investigation showed that despite a black screen data had been recorded by this instrument. The remaining drill string was tripped and the bottom-hole assembly bit cleaned and checked. Once the seabed template was on deck, the drill floor and all containers were prepared for transit.

A total of 28 coring attempts were made in Hole M0060B to a maximum depth of 85.7 mbsf. Hole recovery was 98.32%.