HAMMER DRILLING SYSTEM (HDS) ENGINEERING
Drilling the igneous foundation of the ocean crust has always been a challenging undertaking. Since the inception of the Deep Sea Drilling Project (DSDP) and its successor, ODP, one of the principal objectives of the science community has been to penetrate an entire section of the ocean crust and to reach the boundary between the Earth’s crust and mantle. Attempts at accomplishing this objective have been limited by drilling technology originally designed for recovering sediments by the oil industry but adapted by ODP for sampling igneous basement. There have been spectacular successes in this endeavor, witness Holes 504B and 735B, but challenges confronted in establishing, maintaining, and reentering boreholes in fractured, hard rock have been more common.
In response to these challenges, the Ocean Drilling Program has embarked on the development of a new technology, the hammer drill-in casing system, which will allow us to initiate a hole, then simultaneously deepen that hole and stabilize its walls with casing. This system is an adaptation of pneumatically driven drilling systems that have successfully drilled in environments not unlike those that present our greatest challenge. Owing to the water depths in which we operate, however, pneumatic power is not an option. This innovative design employs a hydraulically actuated hammer, which drives a drill bit into the ocean floor. Following the bit is a string of casing to stabilize the borehole walls and improve our ability to clean drill cuttings from the hole. The bit design allows it to be withdrawn through the casing system, after deployment of a reentry funnel, such that most of the documented problems associated with drilling hard rock in the ocean’s basin are alleviated. This new technology not only solves the technological problems, but it also reduces our dependency on site specific surveys, because the hammer can initiate a hole without regard to local topographic variability, thin sediment cover, debris, or rubble lying on the surface.
HDS testing was undertaken adjacent to the Atlantis II Fracture Zone along the Southwest Indian Ridge on an uplifted platform where two other ODP legs had successfully cored in hard rock using conventional drilling technology (see Robinson, Von Herzen, et al., 1989, Dick, Natland, Miller, et al., in press). We hoped that the shallow but variable water depth and locally flat but regionally rugged topography would both adequately emulate other environments where the HDS might be employed and test the limits of the system. By choosing this location, we also had a proven record of our best performance in hard rock penetration rate and recovery for comparison.