The operations and engineering personnel aboard the JOIDESResolution for Leg 179 were

Operations Manager and Chief Engineer: Tom Pettigrew
Development Engineer: Leon Holloway
SDS Consulting Engineer: Taras Olijnyk
SDS Consulting Engineer: Paul Speight

Drilling and coring operations in fractured hard rock must overcome many challenges not confronted in piston coring operations. These can be summarized as initiating the borehole, stabilizing the borehole, and establishing reentry capability. Until a drilling/coring bit can gain purchase, because it is not stabilized by sediment, it tends to chatter across the surface of a hard-rock outcrop. Difficulty initiating a hole is exacerbated if the drilling target is on a slope. Rubble from the seafloor, drill cuttings, and material dislodged from the borehole wall must continually be removed; however the size and density of this material complicates this task. Because of bit wear in hard rock, deep penetration (beyond a few tens of meters) absolutely requires the ability to perform multiple entries into a borehole. The ideal system for drilling in hard-rock environments would disregard local topographic variation, seafloor slope, and thickness of sediment cover or talus accumulation. Such a system should initiate a hole, then simultaneously deepen the hole and stabilize the upper part of the hole with casing. This requires the bit to cut a hole with a greater diameter than the casing, and then to be withdrawn through the casing string. The casing in turn would facilitate hole-cleaning operations by elevating the annular velocity of the drilling fluid and would ease reentry operations by eliminating the possibility of offsets in the borehole wall (ledges or bridges). Finally, this ideal system would leave behind a structure to simplify the required multiple reentries.

The hammer drill-in casing system is composed of a hydraulically actuated percussion hammer drill, a casing string or multiple casing strings, a free-fall deployable reentry funnel, and a casing hammer. 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 hammer drilling system (HDS) is currently being used in Iceland to install large diameter 18-5/8-in casing more than 100 m deep in fractured basalt. Unfortunately, the Icelandic system is pneumatically driven and, thus, is not suited for use in deep water. However, a hydraulically actuated hammer drill, suitable for use by the Ocean Drilling Program (ODP), is currently under development in Australia. ODP is assisting in the development of this hammer drill and has incorporated it into the HDS.

A viable HDS would (1) eliminate the need for any form of independent seafloor structure such as the hard-rock base, (2) allow spudding boreholes on much steeper slopes than can be achieved using an independent seafloor structure, (3) reduce sensitivity to thin sediment cover, debris, or rubble lying on the spudding surface, and (4) reduce dependency on precise site surveys.

ODP initiated its HDS project in 1994 with a worldwide industry survey of the available hammer-drill technology, techniques, and equipment. In July 1996, ODP was invited to visit an Iceland Drilling Company drill site where 11-3/4-in casing was being drilled into fractured basalt using a pneumatic hammer drill. It was determined that similar techniques could be employed by ODP. However, because of the water depths typically associated with ODP legs, the pneumatic hammer drill would have to be replaced with a hydraulic, or water, powered hammer drill. Further industry surveys resulted in locating SDS Digger Tools, Canning Vale, Western Australia, which had a 6-in prototype water-powered hammer drill that was ready for commercialization. Discussions with SDS Digger Tools resulted in an agreement between the company and ODP to work together to scale up the existing 6-in water hammer to a size suitable for drilling in 16-in casing.

To test the general concept, in August 1996, a field test of the existing SDS Digger Tools 6-in water hammer was carried out. The field test was successful in drilling 7-in casing into black granite in a quarry. Since SDS Digger Tools was not in the business of making underreaming hammer drill bits, in September 1996 the decision was made to use underreaming hammer drill bits manufactured by Holte Manufacturing, Eugene, Oregon, U.S.A. Holte Manufacturing has been in the business of drilling in casing into hard fractured rock for many years, in many locations around the world, using pneumatic hammer drills.

In October 1996, SDS Digger Tools presented the option to ODP of using an existing prototype 12-1/4-in water hammer. The 12-1/4-in water hammer could be used to drill 13-3/8-in casing and would cost less to complete development than developing an entirely new hammer capable of drilling in 16-in casing. Therefore, the decision was made to change the prototype HDS from 16-in casing to 13-3/8-in casing and to employ the SDS Digger Tools prototype 12-1/4-in water hammer. In January 1997, ODP engineers traveled to Perth, Australia, to witness bench testing of the prototype SDS 12-1/4-in water hammer. The bench test was successful, and the project was continued based on the 12-1/4-in water hammer drill.

The 12-1/4-in water hammer was field tested in black granite in April 1997. Although the field tests, from a drilling stand point, were successful, it was determined that the hammer closing forces were too high for safe operation from the drillship. A redesign of the 12-1/4-in water hammer was undertaken by SDS to lower the closing forces. A second round of field tests was carried out with a modified 12-1/4-in water hammer in September 1997 at Rogaland Research Center, Stavanger, Norway. The results of the second field test indicated that the closing forces were now in an acceptable range for use by ODP.

During development of the 12-1/4-in water hammer drill by SDS Digger Tools, ODP/TAMU (Texas A&M University) developed the supporting hardware required for the HDS system. This hardware included a hydraulically actuated casing running tool, a modified 13-3/8-in casing hanger, a bearing assembly between the modified casing hanger and casing string, and a free-fall reentry cone. The bearing assembly between the modified casing hanger and casing string was added to allow the drilling assembly to rotate independently of the casing string. The free-fall reentry cone was designed to be assembled around the drill pipe and dropped to the seafloor, coming to rest on the modified 13-3/8-in casing hanger. Besides making reentry easier, the free-fall reentry cone locks out the bearing between the casing hanger and casing string. Locking out the bearing is required for installation of other casing strings using conventional ODP casing running tools which must be rotated to latch and release. The HDS running tool, hanger bearing, and free fall-reentry cone were assembled and fit tested at ODP/TAMU in March 1998. All of the HDS equipment was shipped to Cape Town, South Africa, in April 1998, for testing at sea during Leg 179.

The HDS tested on Leg 179 was a concept assembly comprised of seven basic components: (1) underreaming bit, (2) water hammer, (3) jet sub, (4) running tool, (5) hanger bearing assembly, (6) reentry cone, and (7) the casing string (Fig. 14). The overall HDS is an adaptation of similar hammer drill systems used on land, in particular by the geothermal industry. However, some fundamental aspects of the HDS had to be changed, or added, for deployment at sea.

Hammer Drill Underreaming Bits
Hammer drill bits drill by crushing rock under extremely high point loads using hemispherical tungsten carbide inserts (TCIs) as the cutters. The cutters are driven into the rock with each impact of the hammer, thus chipping a small portion of the rock with each blow. The bits are rotated slowly, ~20 rpm, to index the cutters between impacts of the hammer. Underreaming bits are required to open the borehole large enough for the casing to follow behind the drill bit as the hole is being drilled. The underreaming bits are designed to collapse to a small enough overall outside diameter to be pulled up through the casing string once the casing has been emplaced.

There were two basic designs of underreaming bits used during Leg 179. Both types were direct adaptations of land-based hammer drill underreaming bits currently used in industry. The first type is called a concentric underreaming bit (CUB; Fig. 15) and is a relatively new development in the hammer drill industry. The CUB has a pilot bit, ~12-1/4 in in diameter, sized such that it will pass through a 13-3/8-in casing string. Immediately above the pilot bit are three underreaming arms that are retracted and expanded by rotating the drill string left or right, respectively. When retracted, the underreamer arms close to the same outside diameter as the pilot bit (12-1/4-in). When expanded, the underreaming arms open to an effective diameter of 14-3/4-in, thus creating a large enough borehole for 13-3/8-in casing to pass through. The advantage of the CUB is that the underreaming arms ream ~84% of the borehole circumference with each stroke of the hammer. Based on data collected from land-based operations, the CUB has proven to drill faster and last longer than conventional eccentric underreaming bits.

The second type of underreaming bit used during Leg 179 is called an eccentric underreaming bit (EUB, Fig. 15). In the EUB the underreamer and the pilot bit are one piece. The EUB is built such that when it is in the closed position, the pilot bit is off axis to the drill string and the overall effective diameter (12-1/4-in) of the bit is small enough to be pulled up through 13-3/8-in casing. When opened, the pilot bit moves on axis with the drill string and the eccentric is moved outward to perform the underreaming. Thus, when open, the EUB has an effective diameter of 15 in, capable of creating a borehole large enough for 133/8-in casing to pass through. As with the CUB, the EUB is retracted and expanded by rotating the drill string left or right, respectively. The EUB has been used for years in the land-based hammer drilling industry. The disadvantage to the EUB is that the eccentric only reams ~38% of the borehole circumference with each stoke of the hammer. Thus, the EUB drills slower and does not last as long as the CUB.

Water-Powered Hammer Drill
The heart of the HDS is the water-powered hammer drill (Fig. 16). As the name implies, the water-powered hammer drill is driven by pumping water through the hammer. The basic operating mechanism is an internal reciprocating piston. On the up stroke, the piston is slowed and stopped by compressing water. On the down stroke, high-pressure water drives the piston down until it impacts the top of the bit. The high-energy impact is transmitted through the bit body to the TCIs, thus creating extremely high, virtually point, impact loads on the rock.

Another feature of the hammer drill is a bypass mechanism that allows the driller to flush the borehole with high-viscosity mud without activating the hammer. When weight is applied to the hammer with the bit set on bottom, the bit shank moves upward closing the bypass and diverting all flow through the hammering mechanism. When the bit is pulled clear of bottom, the bit shank is allowed to move downward, opening the bypass and diverting all of the flow around the hammering mechanism so that the hammer stops operating when not in contact with bottom.

The water hammer used during the testing during Leg 179 is a proprietary product of SDS Digger Tools, Pty., Ltd., 49 Vulcan Road, Canning Vale, Western Australia 6155 (telephone (09) 455 4433; fax (09) 455 4399). Specific operational parameters of the water hammer can be obtained by contacting SDS Digger Tools.

Jet Sub
A special jet sub (Fig. 16) replaced the conventional water hammer top sub. The jet sub has receptacles for three nozzles capable of diverting part of the flow down the drill string and up the outside of the drill string at high velocities. When assembled in the complete HDS, the jet sub is placed ~2 m up inside the casing. While drilling in casing with the HDS, the cuttings are brought up the inside of the casing through the annular space formed by the casing inside diameter and the drilling assembly outside diameter. The jet sub is used to increase the velocity of the cuttings-laden water moving up the casing and out of the hole for more efficient hole cleaning.

Running Tool
When casing is conventionally drilled in with hammer drills on land, individual joints of casing are added to the overall casing string at the surface as the casing string is being drilled in. Unlike the conventional land-based hammer drill-in casing systems, the HDS, because it is to be deployed in deep water, requires that the entire casing string be made up as a single assembly with the HDS and lowered to the seafloor. Thus a special running tool, which becomes an integral part of the drilling assembly, is required to support the casing string as it is lowered to the seafloor and drilled into place. The running tool must also be able to unlatch from the casing string and be removed with the drilling assembly, thus leaving the drilled in casing fully open for reentry.

The HDS running tool employs a triangular cross section body (Fig. 17). The flats of the triangle provide flow paths for the cuttings to be circulated out of the hole while drilling. At each of the points of the triangle are latch dogs that, when extended, lock into mating grooves in the casing hanger. The latch dogs are held out, in the locked position, by a shifting sleeve inside the running tool body. To unlatch the running tool from the hanger bearing latch body, the shifting sleeve must be moved downward, out from underneath the latch dogs, thus allowing them to retract into the running tool body.

A special tool called a go-devil (Fig. 18) is used to move the running tool shifting sleeve. At the time when the running tool is unlatched from the casing hanger, the drill-string heave compensator is in operation and thus it is not safe to access to the drill-string bore to insert the go-devil. To get around this problem, there is a hydraulically actuated ball valve, on top of the top drive, which is normally opened and closed when retrieving core barrels during routine coring operations. When the HDS is deployed, the go-devil is placed on top of the ball valve, with the ball valve closed. To deploy the go-devil, the driller opens the ball valve, from the safety of the drillers shack, allowing the go-devil to fall into the drill string. Once the go-devil is inside the drill string, it is pumped down the drill string until it comes to rest on top of the running tool shifting sleeve. After the go-devil has landed on the shifting sleeve, the drill string pressure is increased to ~600 psi until the shifting sleeve overcomes a snap ring and moves downward, releasing the latch dogs. After confirming the running tool has been unlatched from the casing hanger, the driller increases the drill string pressure to ~1800 psi, shear releasing another sleeve inside the go-devil, which also moves downward and opens a circulation path to the hammer and borehole once again.

Hanger Bearing Assembly
A hanger bearing design was incorporated into the casing hanger to allow the drilling assembly and integral running tool to rotate relative to the casing string while supporting the weight of the casing string (Fig. 19). During the drilling in process, as the casing enters the borehole, the bearing assembly enables the casing to stop rotating, even though the drilling assembly is still being rotated. By doing so the total torque required to drill the casing in is reduced. Also, whatever torque is produced is a direct response from the bit, thus giving the driller direct feedback regarding torque on bit.

Casing String
As mentioned previously, unlike conventional land-based hammer drill in casing techniques where the individual joints of casing are added to the sting as it is being drilled in, the entire HDS casing string must be made up as part of the overall HDS assembly. The top of the HDS casing string must be compatible with other standard ODP casing tools and hangers. So, a standard ODP casing hanger is slightly modified by adding a hanger bearing assembly and shortening the casing pup joint for the HDS. For added protection at the bottom of the casing string, a hardened casing shoe is welded to the end, or shoe joint. The hardened casing shoe is more collapse- and abrasion-resistant than the casing itself.

Reentry Cone
A reentry cone was added to the HDS assembly to aid in reentering the borehole and to defeat the hanger bearing assembly (Fig. 20). The vibration-isolated television (VIT) camera is used to locate specific spud targets when spudding with the HDS. Because the HDS uses the drill string as a guide and must pass over the HDS assembly, the reentry cone can not be in place, on top of the HDS, while drilling in. Therefore, the HDS reentry cone was designed to be deployed after the casing has been drilled into place. The reentry cone is split into two halves and is attached around the drill string, while the drill string is still attached to the casing string, and free fall deployed. The falling reentry cone comes to rest on top of the HDS casing hanger. As the reentry cone is falling, a guide on top of the HDS running tool centers the reentry cone with respect to the drill-string axis so that the body of the reentry cone passes over the outside of the casing hanger and extends down to the hanger bearing assembly housing. Special lugs inside the reentry cone body, near the top, land on top of the casing hanger thus preventing the reentry cone from dropping below the casing hanger.

The standard ODP casing tools latch and unlatch by rotating the drill sting left and right respectively. Because the HDS hanger bearing allows the casing hanger to rotate relative to the casing string it must be locked out for standard ODP casing tools to be used during subsequent operations at an HDS installation. The HDS reentry cone also provides a mechanism for locking out the hanger bearing assembly. The special lugs that land on top of the casing hanger also lock into the bypass flow grooves in the body of the casing hanger, thus preventing rotation between the casing hanger and the reentry cone. There is another set of lugs inside the reentry cone, near the bottom, that engage lugs on the outside of the hanger bearing housing, thus preventing rotation of the reentry cone with respect to the hanger bearing housing. Therefore, rotation of the hanger relative to the casing string is prevented.

 To 179 Summary of Leg 179 Engineering and Drilling Operations

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