By noon, a north-to-south seismic line was concluded and the seismic equipment retrieved. The vessel came about and slowly approached the location. A beacon was dropped on the prospectus Global Positioning System (GPS) coordinates at 1215 hr. After the hydrophones and thrusters were extended and the vessel settled on the location, the corrected PDR depth referenced to the dual elevator stool (DES) was 5066.4 m. A nine collar bottom-hole assembly (BHA) was made up, comprising a C-4 four cone rotary bit, a mechanical bit release, a head sub, an outer core barrel, a top sub, a head sub, seven 8 1/4-in drill collars, one tapered drill collar, six 5 1/2-in drill pipes, and one crossover sub. Although no logging was anticipated, the mechanical bit release was affixed as a means of freeing the BHA should the drilling assembly become stuck at the bit.
Hole 1152A was spudded with the rotary core barrel (RCB) at 0830 on 27 November. The bit tagged the seafloor at 5066.4 m. Instead of coring into a sediment pond, the bit appeared to contact hard rock immediately. After RCB coring 11 m with slow penetration and high and erratic torque, we decided to again attempt to contact sediment by offsetting the vessel upcurrent of the present location. The vessel was repositioned in dynamic positioning mode ~100 m north-northeast of Hole 1152A.
Hole 1152B was spudded at 1330 hr and was washed ahead to 22.6 meters below seafloor (mbsf) where we encountered a hard contact. Rotary coring was initiated in basalt at this sub-bottom depth and advanced to 40.6 mbsf with low recovery. At this depth, the drill string stuck and stalled the top drive. The driller worked for 1.25 hr to free the drill pipe with overpulls as large as 100,000 lb to regain control of the situation. After freeing the drill string, the interval from 40.6 to 45.3 mbsf was cored with improving recovery (28%). The bit was only advanced 1 m to 46.3 mbsf when the drill string stuck again. This time the drill pipe was freed with 40,000 lb of overpull. At this juncture the hole was too unstable to deepen, so coring operations were abandoned. The bit cleared the seafloor at 1500 hr and cleared the rotary table at 2345 hr on 28 November. Concurrent with the retrieval of the drill string, the beacon was successfully recovered. After the thrusters and hydrophones were retracted and the drilling equipment secured, the vessel was under way to the next site by 0000 hr on 29 November.
Although the average recovery of the site was 12%, this provided adequate material to attain our scientific objectives. During the operations at this site, the environment was very mild for this region with vessel heave not exceeding 2 m.
Site 1153 | Table of Contents