Technical Note 20/5
PART I: HAMMER DRILLING ENGINEERING LEG
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, new hardware and techniques have been developed to establish a borehole in these environments to meet the scientific objectives of hard-rock legs. Establishing a borehole refers to actual borehole spudding, emplacement of casing to stabilize the borehole, and establishing reentry capability. Establishing a borehole 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 ODP's 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 179 will be dedicated solely to testing a hammer drill-in casing system in a fractured hard-rock environment. This part of the leg will be carried out on the Southwest Indian Ridge in the vicinity of Site 735 (Fig. 1).
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 as it is being drilled. This rubble, along with the drill cuttings and material dislodged from the borehole wall, must be continuously removed. However, the size and density of this in-fill material make it difficult to remove it from the borehole. Maximum penetration is dependent on borehole stabilization. Stabilizing boreholes in fractured hard rock requires emplacement of casing and some form of reentry structure to support the casing.
The hammer drill-in casing system (Fig. 2) is composed of a hydraulically actuated percussion hammer drill, a casing string or multiple casing strings, free fall deployable reentry funnel, and a casing hammer. Reentry capability is established by a free-fall reentry funnel specially designed for this purpose. Once the casing string has been drilled into place and the reentry funnel installed, the drilling assembly is unlatched from the casing string and removed. The borehole is left with casing and a reentry funnel in place. If required, the casing string may be cemented in place and multiple casing strings may be installed in the same borehole.
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, the Icelandic system is pneumatically driven and, thus, not suited for use in deep water. However, a hydraulically actuated hammer drill suitable for use by ODP is currently under development in Australia. ODP is assisting in the development of this hammer drill and will incorporate it into the hammer drill-in casing system.
A viable hammer drill-in casing system would:
- 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 onboard operational characteristics of the hammer drill by deploying it independently of the drill-in casing system. The hammer drill will be thoroughly land tested before it is deployed at sea; however, it is difficult to simulate the shipboard deployment environment. Therefore, the hammer drill will be deployed by itself for evaluation prior to using the entire hammer drill-in casing system.
- 2. Determine the viability of the hammer drill-in casing system. Once the shipboard operational characteristics of the hammer drill are established, the complete hammer drill-in casing system will be deployed for evaluation. Three boreholes of increasing difficulty are planned to completely test the equipment.
- 3. Determine the maximum slope that can be spudded with the hammer drill. Once the hammer drill-in casing system is fully evaluated, the maximum slope at which the hammer drill can spud will be determined. Multiple shallow (1-3 m) boreholes will be spudded on increasing slopes to determine maximum slope spudding capability.
The test site location is the same shallow-water platform on the east rim of the Atlantis II Transform on which Hole 735B is located. This location provides a range of water depths from 700 m to over 6 km. This site also provides a variety of spudding surfaces ranging from relatively level massive outcroppings with clean surfaces to severely sloped talus covered surfaces.
The proposed drilling plan addresses the minimum requirements to evaluate the potential of a hammer drill-in casing system. No coring is specifically planned; however, should time allow, a core or two may be recovered through one of the established boreholes. Several reenterable boreholes will be established that may be used for future scientific exploration.
The drilling plan will proceed as follows:
- 1. Initially the hammer drill only will be deployed on top of the platform in 700 m water depth. Several shallow holes 1-3 m deep will be drilled to establish the shipboard operational parameters.
- 2. Once the hammer drill shipboard operational parameters are determined, the entire hammer drill system will be assembled and deployed. One specific location is on the north flank of the platform in 1.5-2.5 km water depth. A short casing string, 20-40 m, will be drilled as a first step in evaluating the system.
- 3. In order to maximize our knowledge of the system, a second hammer drill system emplacement will be attempted at the same location. A longer casing string, 40-80 m, will be drilled in.
- 4. A talus covered shelf with severe slope will be located on the western flank of the platform for the third hammer drill system emplacement. This will be the most severe test of the system and will most closely simulate conditions found at MARK and Hess Deep.
- 5. Time permitting, the following tests will also be carried out. The tests are listed in order of priority.
- A. Several shallow boreholes 1-3 m deep will be drilled using the hammer drill on shelves with increasing slope. This test will determine the slope spudding capability of the hammer drill when suspended from the drill ship.
- B. One of the established boreholes will be reentered and the casing cemented in place. Slightly different tools and techniques from those typically used by ODP will have to be employed. This test will verify the proposed procedure.
To Leg 179 Proposed Site Information
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