Hole 1093A was spudded with a seafloor APC core at 2355 hr on 10 January. An 8.5-m core set the seafloor depth at 3635.0 mbrf. Continuous cores were taken with orientation beginning on Core 4H. APCT measurements started with Core 4H and continued through Core 16H. Core recovery was excellent, with an average of 103% achieved through APC refusal. Withdrawal of the corer required 50-60 kips beginning with Core 23H. Cores 25H and 26H failed to stroke completely in the stiffening sediments, and a severe plastic liner failure necessitated pumping Core 25H from the inner core barrel. Those failures were considered to signal effective refusal depth for the APC, and coring switched to the XCB mode.
The first three XCB cores recovered low-quality core with only 38% recovery. At the request of the scientific party, an additional APC core was attempted which had incomplete stroke and recovered 3.9 m of core of partly excellent and partly disturbed recovery and a badly broken liner. An additional four XCB attempts recovered only pebbles of ice-rafted debris in the core catchers. Though weather and motion conditions were deteriorating at the time, the inability to recover core was attributed to the properties of the diatomaceous sediment. Coring attempts were abandoned at 309.4 mbsf after Core 34X. The bit was pulled clear of the seafloor at 1245 hr on 12 January.
In an attempt to recover the seafloor interface, the bit was positioned 4 m higher for the new spud than it had been for Core 1093A-1H. The first core attempt produced no sediment, and only plastic fragments from a break at the top of the core liner were found in the core catcher. A second coring attempt from 9.5 m deeper recovered 7.1 m of sediment and was designated Core 1093B-1H. (Later hole-to-hole correlation revealed that several meters apparently had been penetrated and lost from the first core barrel.)
Vessel heave, which had continued to build after the earlier strong winds, died off and reached 11 ft around the time of spudding. It decreased as coring continued, however, and good results again were achieved in favorable operating conditions. Cores were oriented beginning with Core 4H. APC refusal was declared when Core 24H failed to achieve complete stroke, and the drill string was raised for the third hole.
To maintain a stratigraphic overlap, the mudline APC core was shot from 2 m deeper than the initial attempt at Hole 1093B. Again only a trace of the soft seafloor sediment was recovered, and the interface was missed. The second core recovered 8.0 m of sediment and was designated Core 1093C-1H. Core orientation began with Core 3H.
Routine inspection of the aft coring line revealed several broken wires just above the ropesocket. The line was removed from service for reheading. During the attempt to put the forward sinker bar into the drill string, the upper wraps of line on the winch drum became fouled because the spooling of the wire was excessively loose. Efforts to correct the situation resulted in a severe kink in the wire. It was then necessary to cut and rehead the aft wire and put it back into service. A loss of 1.5 hr of operating time resulted.
Winds were light, but a moderate swell persisted as continuous cores were taken over the interval covered by the preceding two holes. The swell built rapidly in the early morning hours of 14 January and was joined by a large cross swell. Vessel heave increased to as much as 12 ft and core recovery and quality dropped concurrently. Average recovery for Cores 13H-18H was 42%. A knobby drilling joint was picked up for Core 18H and the situation was evaluated as the operational roll limit was reached and pitch and heave limits were approached. At 169.5 mbsf, coring operations were terminated and the drill string was pulled above the seafloor to wait out the swell and weather conditions, which continued to deteriorate. Knobby drilling joints again were located at the top of the drill string to handle bending stresses at the guide horn, and the drill string was hung off with the bit suspended at 3527 mbrf.
Swell and weather conditions changed little during the afternoon and night hours, except that the cross swell gradually died out and the amount of vessel roll decreased. One set of swells of 25-30 ft persisted into the morning of 15 January. When the swell height had decreased slightly and roll and pitch were within operating limits, conditions were considered safe for rig floor personnel to handle pipe. Heave remained too high for successful coring, but operations resumed at 0500 hr, after 15.25 hr of downtime, in the hope that conditions would continue to improve. The drill string was run back to the seafloor and Hole 1093D was spudded at 0640. The hole was drilled to 136 mbsf, where continuous oriented APC coring began.
Vessel heave continued to have an adverse effect on core recovery into the afternoon of the first day. Recovery improved with environmental conditions, only to decrease as an incomplete stroke occurred with Core 12H at about 245 mbsf. Because of the inability to recover that material with the XCB system in Hole 1093A, coring continued with the APC and 9.5 m advance between cores through Core 18H (307 mbsf). Core recovery was mostly insignificant and severe liner failures occurred.
The coring mode then was switched to XCB and recovery was nil, as expected. Only pebbles of ice-rafted material were retained in the core catchers of the next three cores. As the seismic record and drilling parameters suggested that a different, softer sediment unit might have been reached, another APC core (Core 22H) was attempted. Recovery was 1.2 m of soft sediment from 329 mbsf, which was sufficient for age dating. XCB coring continued, without recovery, through Core 26X. During that period, recurrent high circulating pressures indicated that jets in the main bit were becoming plugged with the small pebbles that were plentiful at the bottom of the hole (as evidenced by recovery on the top of each core). Most of the jets apparently were cleared by pumping core barrel 27X into place at a very high rate (100 strokes per minute [spm]), and near-normal circulating pressure was regained. Core 27X recovered 6 m, but a total of only 4.3 m were recovered from the next five cores despite normal operating parameters.
The nature of the sediment then changed to a clay-rich material, and high recovery rates were enjoyed over the next 70 m to a depth of 492 mbsf. A return to diatomaceous ooze, however, brought the return of recovery problems, and clay-rich streaks were recovered selectively to 557 mbsf, with average recovery of only ~11%. At that depth, streaks of hard drilling, interbedded with softer material, were encountered, slowing the rate of penetration sharply with no significant increase in core recovery.
In addition to increasing signs of incomplete hole cleaning, bit plugging problems returned. Extremely high circulating pressure upon the landing of core barrel 49X indicated nearly complete blockage of the bit jet channels. Torque and weight indications also showed about 1.5 m of hard fill in the hole. Pressure was too high to attempt cutting a core, and the core barrel was retrieved. A mud pill was pumped, followed by a fresh core barrel at 100 spm. The effort was successful in reducing circulating pressure to an acceptable level, and another slow core was cut. The landing of core barrel 50X was a repeat of the previous attempt. The bit again was plugged when the barrel landed. As preparations were being made to retrieve the barrel, partial circulation was regained. Low rate of penetration (ROP) and core recovery had prompted the decision to terminate coring, but Core 50X was cut while a 50-bbl mud sweep was circulated to clean the hole for logging. The ROP was slightly higher, but only 50 cm of hard cherty mudstone was recovered.
A go-devil was pumped through the drill string to latch open the lockable float valve, and the drill string was pulled to logging depth. No drag was encountered on the up-trip, so a wiper trip was not considered necessary. Knobby joints were picked up, and the bit was placed at 3722 mbrf for the logging operation.
The triple-combination logging tool was the first deployed. The tool went to within 7.5 m of total depth after meeting minor resistance in the upper section of the hole. Weather and sea conditions were excellent, and a technically valid log was recorded. Its quality and usefulness were degraded by the large, washed-out hole diameter and extremely high porosity of the sediments.
The second logging run was made with the geological high-sensitivity magnetic tool (GHMT) string. The light-weight tool reached 581 mbsf, and a good magnetic susceptibility log was obtained over the entire hole interval. Because of the unfavorable hole conditions, the planned Formation MicroScanner log was not run. Logging operations were completed and the wireline sheaves rigged down by 2145 hr on 18 January.
The knobby joints were laid down, and the drill string was pulled clear of the seafloor for the final coring attempts at the site.
An additional attempt to obtain a seafloor interface core had been requested, as the uppermost sediments apparently had not yet been recovered at the site. The vessel was offset back to the coordinates of Hole 1093B, and the bit was positioned at 3629.5 mbrf. As vessel motion conditions were minimal, a single APC shearpin was used to reduce the impact of actuation. The APC appeared to fire at a relatively low pressure as expected, but was recovered with the shearpin intact. The APC assembly was sent down for a second attempt at the same depth. Again there was difficulty in actuating the APC, with anomalous pressure indications. The corer was recovered in the stroked out condition, but with a broken liner and no trace of sediment (a "water core") indicating a seafloor depth below 3639 mbrf and well below the 3635 mbrf indicated by Core 1093A-1H. A third attempt was made from 4 m deeper. The results were a broken core liner and traces of mud on the core catcher. Efforts to recover a mudline core were abandoned.
Hole 1093E was then officially spudded as the bit was "washed" to 4 m below the apparent seafloor depth of Hole 1093A and an APC core was taken to fill a gap in the composite section of the site. A split liner with 6.6 m of sediment was recovered. After an additional washdown to 33 mbsf, a second core was taken to fill another gap and to end the coring program at Site 1093. During recovery of the core, the coring line was coated for preservation, as the remaining site was in much shallower water. Core recovery was only 5.0 m, and the liner again had failed.
The drill string was pulled above the seafloor and preparations began for a scheduled slip and cut of the drilling line.
Review of the coring results by the scientific party resulted in concerns that a gap still existed in the section, but this could not be confirmed without results from the multisensor track core logging. The rig operation was halted for half an hour while it was confirmed that one additional core was needed.
Hole 1093F was spudded at 0640 hr on 19 January and was drilled to 34 mbsf (inferred from Hole 1093A). A final APC core was shot from that depth, and 6.2 m of sediment was recovered. The coring line was coated again on the retrieval trip.
When the bit had been pulled to about 75 m above the seafloor, the cut-and-slip operation was completed. During the ensuing pipe trip, both wind and swell had increased greatly, and most of the trip was made in steady rain with a wind chill factor of 8°F. The JOIDES Resolution departed the site at 1800 hr on 19 January.
To 177 Operations - Site 1094
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