OPERATIONS SUMMARYLeg 193 was logistically different than most ODP expeditions in that all sites occupied were less than 1 km apart on the crest of Pual Ridge. Events in this operations summary are presented in chronological order (see Table 1).
Guam Port Call
Leg 193 began with the first line passed ashore to the fueling dock at 1700 hr on 7 November 2000. Port call activities including loading the equipment required in support of the arsenal of tools we brought along to ensure the success of this expedition. These tools included, above and beyond all of our routine paraphernalia, two developmental systems, the advanced diamond core barrel (ADCB) and the hard-rock reentry system (HRRS). Engineering observers, scientific observers representing Papua New Guinea (PNG), and an observer representing the mining company holding a lease on our operational area sailed as part of the ship's complement. At 0800 hr on 14 November the last line was cast off and we were under way at full speed for our first drilling target. During the entire transit to Manus Basin, we enjoyed fair weather.
Upon arrival at Site 1188 (Snowcap Knoll diffuse venting area) on the evening of 18 November, arrangements for customs and immigration clearance to PNG had not been finalized. We could not commence operations until clearance had been received, which resulted in delaying our start until 0845 hr on 19 November, when a helicopter carrying customs and immigration officials arrived on the JOIDES Resolution from Rabaul, Papua New Guinea. Our overall strategy was to drill a series of pilot holes in each of the three high-priority areas and to assess from those operations how best to pursue our scientific objectives. A short survey with the subsea camera identified our first target, and Hole 1188A was initiated at 1230 hr on 19
November with a jet-in test to determine the thickness of sediment cover and/or soft formation, which might dictate our operational approach. Coring continued until 0900 hr on 21 November, when the pipe stuck in the borehole while we recovered Core 193-1188A-23R (to 211.6 m below seafloor [mbsf]) (Table 2). This was the last of several episodes of stuck pipe, which along with poor recovery (slightly >10%) indicated poor hole conditions. Despite several hours of attempts to free the drill string, including releasing the bit, we eventually were forced to sever just below the seafloor, ending operations at this hole at 0830 hr on 22 November.
After assembling a new bottom-hole assembly (BHA), a second seafloor camera survey located a drilling location at our next high-priority site (Roman Ruins high-temperature area). A 3 m jet-in test at 1615 hr on 22 November was followed by continuous coring through Core 193 1189A-13R (to 125.8 mbsf). Once again, several incidents of stuck pipe and poor recovery (< 7%) plagued operations, resulting in a second pipe-severing episode. The severed pipe cleared the rotary table at 2045 hr on 23 November, concluding operations in Hole 1189A.
Our third high-priority target was an area that was not known to host surficial evidence of extensive hydrothermal alteration. This area was far enough from known active venting that we hoped to use it a reference site. After a jet-in test failed to penetrate the surface, three holes (Holes 1190A, 1190B, and 1190C) were attempted between 0300 hr on 24 November and 0520 hr on 25 November. None of these holes penetrated >17 m, nor did they return more than three short intervals of core before hole conditions became the basis for abandonment.
First Return to Site 1188
With the arrival (via a second helicopter) of batteries for our logging-while-drilling/resistivity at-bit (LWD/RAB) operations, we elected to attempt a RAB experiment at Site 1188. Our plan was to drill to ~75 mbsf to protect our BHA by keeping the decrease in drill string diameter (a potential spot for material to pinch off the drill string if the hole collapsed) above the level of the seafloor and establish a reentry site by deploying a free-fall-funnel (FFF). We were to follow this with our first deployment of the ADCB, to determine if this method of coring provided improved hole conditions or recovery.
The RAB experiment went smoothly, starting at 1615 hr on 25 November, but the rate of penetration (ROP) decreased markedly at ~68 mbsf and we terminated penetration at 72 mbsf. We deployed a standard FFF, ran the vibration-isolated television (VIT) frame to bottom, and observed as the RAB tool was extracted from Hole 1188B without disturbing the FFF. The ADCB followed into Hole 1188B, but as there were several meters of fill in the bottom of the hole, we could make no new penetration. Attempts to deepen Hole 1188B were suspended at 1400 hr on 27 November. Despite not deepening Hole 1188B, the success of the RAB experiment in a hard-rock application was our first operational highlight.
Having attempted rotary core barrel (RCB) coring at our primary sites, and recognizing that realizing our ultimate objectives would require significantly deeper penetration than we had been able to achieve with bare-rock spudding, we decided to attempt setting a casing string to prevent upper borehole wall collapse while we drilled deeper. Inasmuch as we were able to achieve >200 m of penetration at Hole 1188, and only just over 100 m in Hole 1189A, we decided our attempt at deep penetration should be at Site 1188, where we could set casing significantly deeper without missing a significant thickness of the section. Our engineers advised us that preparation for this deployment would take as much as a day. Because we had not brought any core on deck during operations at Hole 1188B, we chose to utilize the time required for assembling a reentry system by attempting another bare-rock spud at Satanic Mills, an alternate high-temperature venting site.
Hole 1191A was spudded at 2045 hr on 27 November, and RCB coring continued through to a depth of 20.1 mbsf before the pipe stuck fast. The driller was eventually able to free the pipe and recover the core barrel, but on deployment of the subsequent barrel there were indications that something was amiss. Low-circulation pressures indicated the core barrel was not seating properly, but indicators on the core barrel latching assembly appeared normal. In the meantime, the borehole started to collapse (indicated by >3 m of fill), and even through we were eventually able to attain 20 mbsf again, we could not advance the hole. By this time we were ready to begin our deep penetration attempt, so we abandoned operations at this site.
Second Return to Site 1188
Considering the challenges we had faced during operations in our pilot holes, designing a reentry and deep penetration assembly introduced numerous complications to our standard deployment. Variability in local topography and in the depth of soft material covering hard volcanic rocks and the tendency of the borehole walls to be unstable were factored into our design. We foresaw the requirement for a minimum of at least 5060 m of large diameter casing in our initial deployment to offer the best chance of success of eventually casing to the approximate depth of Hole 1188A (211 mbsf).
Based on these considerations, we decided to deploy the conventional reentry cone/casing hanger in an unconventional manner. Because we needed to select a drilling target based on the assurance we could jet in at least 3 m of casing below the seafloor away from any obstructions, we could not emplace the reentry cone by running it on the end of the pipe and releasing it. In our standard configuration, it is impossible to see around the cone with our VIT camera and pick a specific target on the seafloor. Additionally, because we were limited in the number of attempts we could perform by the amount of casing hanger hardware in stock, if we deployed the cone only to discover later that we could not drill a hole, we would potentially lose our opportunity for deep penetration. Our innovative strategy was to drill a large-diameter (14.75 in) borehole to an intermediate depth for our initial casing deployment, drop the standard reentry cone in free-fall mode, and round trip the pipe to install our first casing string.
Three attempts at drilling a large diameter borehole (Holes 1188C, 1188D, and 1188E) were not successful at achieving our desired depth (confirming the merit of our strategy), but our fourth attempt (Hole 1188F) yielded the best hole conditions and drilling parameters we had enjoyed to date. By 0745 hr on 2 December, we had drilled to a depth of 104.0 mbsf (as deep as we could drill without placing the top of the BHA in jeopardy below the seafloor) with no significant indications of hole instability. Pulling the pipe to just below seafloor, the reentry cone was dispatched through the moonpool and made for the most rapid conventional reentry cone installation in ODP history (under 9 min at 187 m/min). After tripping the pipe, we decided that our best chance at installing a second casing string would rely on having a hole with the largest diameter possible to our target depth, so another 14.75-in bit was assembled, this time with a much longer BHA. By 0230 hr on 4 December, the hole had reached a total depth of 195.0 mbsf, although we experienced several intervals of minor hole blockage. Because the stability of the borehole was suspect, we adapted the fluid hammer HRRS to precede the large-diameter casing into the hole as a hole-cleaning system. This deployment was not without complications, requiring a couple of iterations and additional hole-cleaning operations, but by late in the evening of 8 December we had installed the casing to a depth of ~59 mbsf, with ~2.5 m of casing protruding above the throat of our standard reentry cone.
Although in other circumstances our inability to fully insert the casing string into the hanger might have been disastrous, our engineers immediately realized that the geometry of our casing assembly (out of the throat of our reentry cone and sticking up ~2.5 m above the casing hanger) was identical in concept to the design of our HRRS. A quick adaptation to the FFF and centralizer guide was all that was required to create a nested funnel system that was installed without complication. We commenced hole cleaning beneath the large-diameter casing string, and after several intervals of stuck pipe, we were prepared to deploy our second casing string. Our second casing run went without incident and by 1205 hr on 12 December, we had a hole cased to 190 mbsf, cemented at the bottom for stabilization, and cleaned to a total depth of 218 mbsf in preparation for continuous coring.
At 1645 hr on 12 December we initiated ADCB coring in Hole 1188F with slow circulation rates while the drillers and engineers monitored our drilling progress. Slow coring rates were maintained at first to learn operational parameters, and recovery was intermittent, although on occasion excellent. Inasmuch as our goal was to achieve maximum penetration, we did not spend much time attempting to optimize our coring process for recovery. This resulted in poorer-than expected average recovery, exacerbated by the fragmented formation. Continuous ADCB coring proceeded until reaching a depth of 327.2 mbsf. A failure in a core barrel landing ring necessitated a slightly premature bit trip, but as it turned out, the bit was worn and performance deterioration would have required a change in any case.
A second ADCB run (temporarily interrupted by another core barrel landing failure) continued through Core 193-1188F-47Z to a depth of 386.7 mbsf. At that point a steadily decreasing ROP (< 1.0 m/hr) and no recovery for the last three cores prompted us to terminate ADCB coring. ADCB coring in Hole 1188F averaged nearly 20%, despite intensely fragmented core and core catchers not optimized for recovery of such material. ROP was as high as 4.5 m/hr and averaged ~3 m/hr. One of the primary lessons we took from this operation was how difficult it can be to optimize bit and core catcher design as well as coring parameters in variable lithologies. Further details of the ADCB deployment in Hole 1188F can be found in the Operation Manager's report for Leg 193 (available from ODP); however, the success of this endeavor should not be underestimated. The casing installation was accomplished with innovation and adaptation, and our earlier attempts at coring with an RCB system indicated to us that we would not have achieved our science objectives at this site without the development tools we employed. Coring operations at Hole 1188F were augmented by a series of wireline logging runs, following which we moved to Hole 1188B to attempt to measure borehole temperature and collect a water sample. Alas, neither the temperature tool nor the water sampler would pass an obstruction a few meters below the seafloor.
Return to Site 1189
Yet another technical highlight of operations during Leg 193 was the first deployment of the complete HRRS. At 0015 hr on 23 December, we began a brief camera survey and initiated Hole 1189B in the middle of a chimney field. This hole was spudded with >30 m of casing using our developmental hammer-in-casing system. Although penetration rates were variable (as we expected from our previous coring operations in fresh and altered rocks), the casing was installed much faster than we anticipated (33 hr from start to finish), and resulted in a reentry hole, cased to 31 mbsf, in an area of extremely rugged local topography where no conventional reentry system would have had much chance for success. RCB coring through the HRRS commenced at 0815 hr on 24 December. Although recovery was poor (~8% for Cores 193-1189B-1R through 18R), coring continued without incident to a depth of 206.0 mbsf. At that point hole conditions began to deteriorate, and because we were within 2 m of the maximum depth we could achieve without placing the top of the drill collars below the bottom of the casing, we decided to terminate operations in this hole. Wireline logging commenced at 2015 hr on 25 December and were complete by early afternoon on 26 December, ending operations in this hole.
Third Return to Site 1188
With the logging assembly near the seafloor, we planned to collect a second temperature profile with the ultra-high temperature multisensor memory (UHT-MSM) tool and a water sample with the water-sampling temperature probe (WSTP). The temperature run indicated the borehole had heated up significantly since our logging operation (from ~100° to >300°C in 5 days), but we were not able to rapidly estimate a temperature gradient, resulting in a water sample from a much lower temperature interval than we had hoped.
Second Return to Site 1189
Building on our successes, we decided to perform another LWD/RAB penetration, this time within the confines of the Roman Ruins chimney field. We had seen no evidence of high temperature fluids in our earlier operations and were confident that our drilling fluid circulation would keep the borehole cool. The RAB experiment was designed to penetrate beneath a distinct lithology change and concomitant depth marker in wireline logging data from Hole 1189B (±25 m away). After reaching 166 mbsf in Hole 1189C, we decided to multiply our achievement by dropping a FFF and adding wireline logging in this hole to our operations strategy. This experiment provides not only the deepest LWD/RAB penetration within hard rock in ODP history, but it also represents the first time we will have the opportunity to directly compare wireline logging and RAB data in a hole drilled in fresh and altered volcanic rock on the seafloor.
Final Return to Site 1188
With only a few days left for operations, we decided to attempt temperature profiles and water sampling runs in Holes 1188B and 1189B, followed by another attempt to core at Site 1190. Although the first of these objectives was accomplished, a compassionate leave emergency required us to suspend operations and transit to the port of Rabaul. Upon arrival in Rabaul, PNG authorities opted to clear the vessel from their waters 2 days early (being the millennium change weekend) and required our immediate departure without further subsea operations.
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