Technical Note 25


At its April 1991 meeting, the JOIDES Planning Committee established the Offset Drilling Working Group (OD-WG), charging it with the tasks of prioritizing the scientific objectives and developing a strategy for a program of drilling offset sections of oceanic crust and upper mantle. In addition, the working group was asked to identify target areas, site survey and technological requirements for such a drilling program. The OD-WG report was presented to the Planning Committee in August 1992.

Concurrently with this study of the objectives and needs of offset drilling, the JOIDES structure was considering individual drilling proposals that formed a part of this larger strategy. At the December 1991 meeting of the Planning Committee, the proposal to drill lower crustal and mantle rocks in Hess Deep was scheduled as Leg 147. A year later, Leg 153 was earmarked for drilling in the MARK area of the Mid- Atlantic Ridge. Leg 147 took place between 25 November 1992 and 21 January 1993; Leg 153 was at sea from 27 November 1993 to 24 January 1994. The exigencies of ship scheduling rather than any expressed wish of the scientific participants led to both of these legs being at sea over the Christmas/New Year period.

Three drill sites were identified in the Scientific Prospectus for Leg 147. If operations went well, a single re-entry hole using a hard-rock guidebase (HRB) and multiple strings of casing would be established at the highest priority site (HD-3) and RCB coring would proceed to a depth of at least 500 m. In actuality, considerable difficulties were encountered in establishing guidebases and setting casing and, rather than a single cased deep hole, a dozen much shallower holes were cored at two sites. 123 m of core were recovered from a total interval cored of 487 m (Table 1). Substantial equipment losses were suffered, but the HRB, which toppled over at Hole 894C, was recovered at the end of the leg.

The objectives of Leg 153, as defined in its Scientific Prospectus, were similar to those of Leg 147. Two multiply-cased re-entry holes were to be established with HRBs, one at a gabbro site (MK-1) and the other at a serpentinized peridotite site (MK-2). It was hoped that each hole would be cored to between 200 and 400 m, and that both could be deepened on future legs to as much as 1000 mbsf. The reality of the leg was quite different. Use of the HRBs was quickly found to be impractical, and considerably more time was spent coring than on Leg 147 (Table 2). Fourteen shallow holes were cored rather than the two cased re-entry holes planned. A total of 261 m of core was recovered from a total interval cored of 798 m (Table 1).

In comparison with previous hard rock legs, Legs 147 and 153 were by no means unsuccessful. The rocks recovered on the two legs were remarkable in many ways and of great scientific interest. Core recovery of the gabbros and serpentinized peridotites encountered was substantially higher than that of the hard basalts during the deepening of Hole 504B on Leg 140 (Table 1). However, neither leg progressed anything like as planned and the maximum penetrations achieved (154 m at Hole 894G on Leg 147, 201 m at Hole 920D on Leg 153) fell short of what had been hoped. Both legs ended up drilling a large number of unsupported, single-bit holes, and operations with the hard-rock guidebases were repeatedly frustrated. Neither leg left behind a cased re-entry hole that could be deepened on a subsequent leg (such as Hole 735B). Furthermore, considerably greater expenditure was incurred by ODP in preparing for these two legs than for a "typical" ODP leg. As a rule of thumb, the drilling operations budget for a "typical" leg at this time was about $250K. The budget for 147 and 153 was about $500K for each leg.

The Offset Drilling Workshop, which took place on 13-14 September 1994, was convened by ODP-TAMU to discuss the proble ntered on Legs 147 and 153 and how the objectives of the Offset Drilling strategy might be better achieved in the future. The participants (Appendix 1) consisted predominantly of scientific and ODP-TAMU participants on the two legs together with a lesser number of representatives of relevant JOIDES panels and ODP-TAMU. Travel and subsistence costs for participants from outside College Station were paid with commingled funds from the ODP-TAMU budget. The costs of the workshop were minimized by holding it in College Station, since that limited the number of outside participants to 10. Furthermore, that also allowed other ODP-TAMU personnel to attend on an occasional basis.

Over the last few years the proportion of ODP legs that are technologically complex and operationally difficult has been increasing. The sequence of difficult legs over the past five years includes 132, 142, 147, 153, 156, 158. These legs can be regarded as high risk in the sense that the chance of failure to meet the objectives set out in the Scientific Prospectus is greater than for "typical" legs. The cost of preparing for such legs and the risk of losing equipment on them is also greater. It is likely, therefore, that similar gatherings of ODP-TAMU personnel and scientific participants will be needed in the future to review the progress of the increasingly high risk operations that ODP is being asked to undertake.

To Operation Superintendent's Perception of Leg 147

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