Two basic design plans are being considered by ODP Deep Sea Drilling (DSD). The favored is an eccentric or retractable bit design (see Overview below).
There are 5 basic components to a hammer-in casing system, a drill bit, a percussion-driven hammer at the bit, a casing string, a second hammer above the casing, and the drill string.
In conventional use there are two common configurations. In either case, the lower hammer-bit configuration is indexed such that it slowly rotates between each impact (20-30 rpm).
In one configuration, the center bit slides inside casing string, a compression fit oversized ring bit is pressed around the outside of the center bit, and the casing string rides on the ring bit. The upper casing hammer only activates if the casing gets hung and does not follow the ring bit into the hole. This technology requires 2 attachment points, at the bit and on top of the casing which translates to high stress on the casing.
A second bit design is an eccentric bit. As the bit rotates, it throws out and offcenter arm which cuts and overgauge hole for the casing to slide into. When rotation stops, the offcenter arm collapses in, allowing removal of the bit. The upper hammer has same function as above.
A new technology is in prototype testing which involves a retractable bit. As the bit rotates a mechanical cog extends four wings out from the bit, cutting an overgauge hole. The advantage to this system over the eccentric bit is it cuts ~80% of the hole per stroke (since it is cutting in all directions simultaneously, as opposed to ~15% for the offcenter arm version. With this set-up only one attachment point is required (at the top of the casing), the bit is free and the casing is not part of the bit, therefore less stress on the casing.
A tool or tools is required that is capable of setting all existing casing strings (20", 16", 13.375", 10.75"). ODP has contracted with an Australian firm (SDS Digger Tools) to do feasibility testing (Phase 1). This company has extensive experience with hammer development, but they have less experience with bit design and development. Basically, they use off-the-shelf bits, remanufactured to fit their patented hammer. The original plan was to contract hammer and bit design to this company, pending the results of the feasibility study. In the meantime, another manufacturer of percussion bits has been found, who is willing to work with SDS to develop the ODP system. The original specification was to develop and purchase two tools.
Leon Holloway has recently returned from Australia, where he was involved in the feasibility test. SDS currently has a 4.75" water hammer, which is designed to drive a 5.5" bit. In order to test the limits of its capability, the tests were conducted with a 7.5" bit, in granite, on sloped surfaces as high as 45º. Penetration tests were successful, casing tests less so, potentially because the bit design was the ring bit type, and adaptations were made to allow drilling fluid to escape around the casing string, rather than up through it. SDS is currently developing, for another client, a 12" hammer. This should be completed by the end of September.
The tests on land suggest that for up to~20m of penetration, the upper casing hammer is not essential, (the casing just trails behind). The design for Leg 174B does not include the upper casing hammer, because this involves significantly more engineering development that just the lower hammer and bit. Land tests make clear that for optimum efficiency, the hammer size should be close to the bit size. For example if only one size hammer is choosen, optimized for 13.375" casing, it will only operate at about 70% efficiency with a 16" casing, and 45% efficiency with a 20" casing. If two different size tools are choosen rather than two of the same, one can be optimized for 10.75" and 13.375" casing, and another optimized for 16" and 20" casing. The problem with this scenario is if one tool is lost, there is no back-up for that size casing.
[UPDATED August 20, 1996]