DRILLING STRATEGYOne of the most technologically challenging aspects of this leg will be drilling in material with an as-yet undetermined response to conventional drilling tools. Penetration rates and bit life are virtually unknown, as are downhole conditions. Our original operational strategy was based on an attempt to pinpoint locations for deployment of an HRGB. In bare-rock environments where ODP has experience drilling, our proposed penetration depths would require more than one bit run; hence, they would require reentry capability. Instability of borehole walls could also limit depth of penetration, so hole preparation options that include casing operations must be considered.
Based on detailed examination of seafloor video images, the operations team for Leg 193, in consultation with the co-chief scientists, have determined that few of the proposed sites are suitable for deployment of an HRGB without risk; however, given the bottom conditions and shallow water operations area and the expectation of good weather, calm waters, and undithered GPS navigation, we are confident it should be possible to conduct bare hole reentry (BHR) operations with assistance of the vibration-isolated television (VIT) underwater camera. Recent repeated success with similar BHRs bolsters our confidence. Possible innovations that might also be useful in this situation include the hammer drill (under development) or drill-in casing. An additional option includes drilling a large-diameter hole followed by deployment of a free-fall funnel. At least one target (proposed Site PCM-2A) could be suitable for jetting in a standard reentry cone, if hole instability in the pilot hole dictates a need for casing to reach our scientific objectives.
Proposed Site PCM-2A at Snowcap is first priority for both scientific and technical considerations. It lies at the center of a large zone of relatively soft but coherent altered dacite. Possible high temperatures and hole instability being the chief constraints, this site would be drilled to a maximum depth of 750 m. The drilling plan is to spud directly on bedrock and drill with the rotary core barrel (RCB) for ~50 hr rotation time or ~250 meters below seafloor (mbsf), whichever comes first. The time/depth allocated for this operation may be adjusted by the operations manager, inasmuch as one objective of this operational scenario is to provide a baseline for future operations once we determine rate of penetration (ROP) and bit life characteristics. The hole will then be reentered with the advanced diamond core barrel (ADCB; maximum depth penetration of 300 m without reaming) and drilled to target depth of 500-550 mbsf. If time and drilling conditions are especially favorable, and if particularly important intersections occur at this target depth, we may consider deepening this hole to as much as 750 m with a third bit (either the RCB or ADCB).
The next priority will be proposed Site PCM-3A at the site of focused high-temperature venting. To achieve the planned penetration of 350 m, at least two bits and a BHR may be required. The drilling plan will be similar to that at Site PCM-2A, but initial RCB penetration will be reduced to as little as 50 m if the first hole demonstrates superior performance of the ADCB, which would be used to continue to the target depth of 350 m. If justified by timing and exceptional results at this depth, the hole could be deepened to as much as 650 m with a third entry using the ADCB following reaming. It is unlikely that scenario will arise.
The need for and priority of a reference hole at proposed Site PCM-1A will depend on whether the previous two drilling targets provide an adequate assessment of volcanic architecture or whether sampling to allow direct comparisons between altered rocks and their fresh counterparts becomes vital (for instance, if the altered rocks appear to not have been derived from the well-characterized dacite lavas at PACMANUS). This site is targeted at 750 m depth with the aim of sampling the entire volcanic sequence of Pual Ridge as well as characterizing the basement below the ridge. Since at least three (and likely more) bits will be required, this target depth cannot be achieved during this leg unless predicted penetration rates can be significantly exceeded. The operational strategy for drilling at Site PCM-1A will be determined by the success of coring operations at previous sites.
Deletion of Site PCM-1A from the drilling plan or revision of the target depth might allow inclusion of the inflow target (proposed Site PCM-4A). Achievement of target depth and scientific objectives of drilling at Site PCM-4A, through the inferred fault zone and into basement, may require multiple reentries and potential coring with the ADCB but might be accommodated by a single bit RCB hole drilled to bit destruction.
If hole stability becomes a major problem during drilling at any site but we recognize the need to continue coring on scientific grounds, we may need to stabilize the borehole wall with casing. After extensive discussions of operational strategy, and given the bottom conditions as documented in video surveys and reports from submersible surveys and camera tows, we will hold in our arsenal of seafloor reentry platforms conventional reentry cones equipped with casing hangers and will carry sufficient casing for borehole stabilization. Because casing operations are time consuming (to the detriment of coring operations) we will deploy these only if we determine that deeper penetration is more important to achieving our overall scientific objectives than additional hole commencements.
Recognizing that drilling conditions may be challenging in this terrain, we have identified several alternate sites with the same scientific objectives and drilling strategies as outlined above. In the event we cannot meet our scientific objectives at Site PCM-1A, two contingency targets have been identified. Both locations are near the Pual Ridge crest but are further removed from areas of high temperature venting. We do not intend to occupy these sites in place of Site PCM-1A; they will be cored only if we cannot reach our scientific objectives at the primary site and there is enough time in the operations schedule to drill these targets. Proposed Site PCM-5A is ~600 m northeast of Site PCM-1A, whereas Site PCM-6A is ~300 m southeast of our primary target.
Two alternate sites for drilling below a focused high-temperature vent site (the scientific objective of drilling at Site PCM-3A) have also been identified (Fig. 7). Proposed Site PCM-7A is in the Satanic Mills chimney field, and proposed Site PCM-8A in the Tsukushi chimney field. We do not intend to investigare these locations in lieu of the primary target; however, one or both of these sites may be occupied if drilling at Site PCM-3A fails to provide adequate material for comparison with recovery at Site PCM-2A or if the alteration patterns in the high-temperature site prove to be so similar to those in the lower temperature site that the principal scientific imperative becomes to confirmation of this unexpected result by drilling additional sites.
Additional Operational Considerations | Table of Contents