The operational plan for proposed site BJORN-1 called for 3 piston-cored holes to approximately 300 m below the sea bed, with one hole deepened to approximately 500 mbsf using the XCB coring system. Core orientation was desired on at least two of the holes along with the possibility of ADARA temperature measurements on one hole. All four logging strings were deployed on the deepest hole.
After a brief 57 nmi transit to Site 984 (BJORN-1), the vessel slowed to survey speed (6.0 kt) at 0636 hr on 24 July, and the seismic profiling gear was deployed for a short seismic site survey. By 1020 hr on 24 July 95 the seismic profiling gear was recovered, the vessel returned to the drilling location based on GPS coordinates, and the positioning beacon was deployed, initiating Hole 984A.
A standard APC/XCB BHA was used for all holes at Site 984, including a nonmagnetic drill collar. Subsequent holes (984B and C) were offset 15 m to the north. The mudline was established for each hole. The APC firing depth was offset by a few meters for subsequent holes in order to establish continuous sediment sections.
Routine piston coring at Hole 984A proceeded until the core barrel for Core 984-20H jammed inside the drill pipe while running in the hole. The barrel was stuck at approximately 1605 m depth. After several limited attempts to free the barrel the overshot shear pin was sheared, and the sinker bar string with the Tensor core orientation hardware was recovered. The orientation gear was removed and the sinker bar string was run in for another, more vigorous, but unsuccessful attempt at jarring the barrel free. The decision was made to abandon the hole, and the drill pipe cleared the seafloor at 0246 hr on 25 July 95, ending Hole 984A. Upon recovery the barrel was found to be jammed by a shear pin stub.
At Hole 984B, APC coring proceeded without incident until Core 984A-31H required 100,000 lb overpull and drillover to recover. Extended Core Barrel (XCB) coring was initiated with Core 984B-32X and continued with remarkable recovery (100%) until Core 984B-48X. Beginning at a depth of 446.1 mbsf, five straight zero-recovery cores were taken. All attempts to recover the formation proved futile. The final Core 984B-53X reached scientific target depth and recovered 6.6 m of core that appeared to be similar to, but perhaps slightly more friable than, the formation recovered earlier with good success. No satisfactory explanation of the recovery problem was found.
Upon completion of coring operations at Hole 984B, two annular-hole volumes of seawater were circulated, and a wiper trip with the drill string was made to 1759.0 m (100.0 mbsf). No overpull or drag was identified during the wiper trip and no fill identified on bottom. The go-devil for locking open the LFV was pumped downhole, and the pipe was pulled to a logging depth of 102.2 mbsf. The hole was considered to be in excellent condition, and therefore it was unnecessary to displace it with mud for logging. Four successful logging runs (Quad combo, FMS, GHMT, and geochemical tool) were made and all tool strings reached to within 3.7 m, or less, of bottom.
At Hole 984C, APC coring proceeded until the scientific target was reached. Severe liner collapse problems were encountered on the final two cores. The barrel for Core 984C-30H was actually removed from the pipe with several feet of liner (with core) dangling below the shoe. The liner collapsed and stretched resulting in a significantly reduced outer diameter.
Because Hole 984A was terminated prematurely, it was decided to once again offset the vessel 15 m north and spud Hole 984D. After establishing mudline with Core 984D-1H, the hole was deepened by drilling ahead an additional 157.8 m. Continuous APC coring then commenced at a depth of 166.2 mbsf and continued until the scientific target for the hole had been reached. Coring was terminated slightly higher than on Hole 984C to avoid a repeat of the severe liner collapse problems encountered on the final two cores of that hole.
ADARA temperature measurements were taken continuously on Cores 984B-3H through 13H and Cores 984B-15H, 17H, and 19H. The bottom-water temperature was measured with the ADARA tool before taking Core 984D-1H. The following cores were oriented with the Tensor tool: Cores 984A-3H through 19H and Cores 984B-3H through 19H.
The positioning beacon was released and subsequently recovered at 1420 hr on 29 July 95. The vessel was secured for transit and got underway for Site 907 (ICEP-1) at 1742 hr on 29 July 95.
To Operations Site 907
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