Experience gained on Ocean Drilling Program (ODP) Legs 147 (Hess Deep) and 153 located at the Mid-Atlantic Ridge at Kane Transform (MARK) indicates the current hard-rock base design is not optimal for establishing boreholes in fractured hard rock environments with moderate slope. This is especially true on thinly sedimented slopes covered with debris or rubble. Therefore, the engineering department has worked on developing new hardware and techniques for establishing a borehole in these environments in order to meet the scientific objectives of hard rock legs. Establishing a borehole refers to actual borehole spudding, emplacement of conductor casing, and establishing reentry capability. This requires some form of seafloor structure, whether it be an independent structure such as a seafloor template, a hard-rock base, or some form of a hard-rock drill-in casing system.

The tool with the most promise of dramatically increasing our ability to establish a borehole in a hard-rock environment is the hammer drill-in casing system. Thorough testing of this tool prior to deployment, at sea in an actual hard-rock environment, will greatly increase the success of future hard-rock legs. Therefore, the engineering portion of Leg 174B will be dedicated solely to testing a hammer drill-in casing system (Fig.2) in a fractured hard-rock environment.

Drilling and coring operations in fractured hard rock must overcome many unique challenges. The boreholes must be spudded on hard, sometimes fractured rock, with little or no overlying sediment cover to help stabilize the bit. The dipping slope generally associated with these areas further compounds the problem. An additional challenge is keeping the borehole open long enough for the emplacement of casing. Rubble and debris from the seafloor continuously sift into the borehole. This rubble, along with the drill cuttings and material dislodged from the borehole wall, must be continuously removed, but the size and density of this in-fill material make it difficult to remove it from the borehole. Because maximum penetration of a borehole is dependent on borehole stabilization, and stabilizing boreholes in fractured hard rock requires emplacement of casing, some form of reentry structure must be installed that is capable of supporting casing.

The hammer drill-in casing system is composed of a hard-rock hammer drill, used to drill the borehole, a casing string with integral reentry funnel, and a casing hammer attached to the top of the casing string. Once the casing string has been drilled into place, the drilling assembly is unlatched and removed, leaving the casing string in place. Reentry capability would be established by means of the integral reentry funnel. The reentry funnel also provides a landing point for additional casing strings if required.

This type of drill-in casing system is currently being used in Iceland to drill-in large diameter casing (18-5/8 in) up to 100 m deep in fractured basalt. Unfortunately, this system is pneumatically driven and thus not suited for use in deep water depths. However, a hydraulically driven version of this hardware is currently under development in Australia; it may be modified for use by ODP.

A viable hammer drill-in casing system would have the following attributes:

1) Eliminate the need for any form of independent seafloor structure such as the hard-rock base or seafloor template.

2) Allow spudding boreholes on much steeper slopes than can be achieved using an independent seafloor structure.

3) Be less sensitive to thin sediment cover, debris, or rubble lying on the spudding surface.

4) Be less dependent on precise site surveys.

In general, there are three objectives, listed in order of priority, that must be explored to establish a borehole in a hard-rock environment.

1) Determine the viability of the hammer drill by deploying it independently of the drill-in casing system for evaluation. The hammer drill will be thoroughly land tested before it is deployed at sea; however, it is difficult to simulate deployment of such tools from the ship.

2) Determine the viability of the hammer drill-in casing system. Once the hammer drill viability is established, the complete hammer drill-in casing system will be deployed for evaluation. Two or more boreholes will be drilled using the hammer drill-in casing system.

3) Determine the maximum slope that can be spudded with the hammer drill. Once the hammer drill-in casing system viability is established, the maximum slope at which the system can spud will be determined. Multiple boreholes will be spudded on increasing slopes to determine maximum slope spudding capability.

Ideally the Offset Drilling Engineering "Quarter" Leg will be conducted in the Mid-Atlantic Ridge at Kane Transform (MARK) near ODP Site 395 (Fig.1). This would provide for a direct comparison between the performance of a hammer drill-in casing system and that of hardware and techniques already deployed in the same area. Also, pre-site survey requirements would be minimized.

The proposed drilling plan addresses the minimum requirements to evaluate the potential of a hammer drill-in casing system. No coring is planned during the half leg because the main focus is establishing a borehole using a hammer drill-in casing system. However, it is hoped that several reenterable boreholes will be established that can be used for future scientific exploration.

To Leg 175

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