RATIONALE FOR ADCB ENGINEERING COMPONENT
The advanced diamond core barrel (ADCB) is a development technology undertaken by ODP and IODP staff to overcome challenges routinely encountered in hard rock coring. The ADCB is a mining-style, narrow-kerf, diamond-impregnated bit that employs high rotation speed abrasion cutting as opposed to heavy impact (jack hammer) roller cone technology. Development of ADCB technology is crucial to improving our ability to core in young ocean crust and achieving many of the lithosphere objectives outlined in the IODP Long-Range Plan.
In our development configuration, the ADCB is significantly different than other IODP coring systems. The bottom-hole assembly (BHA) employs 6.75 inch drill collars (as opposed to the more robust 8.25 inch collars used in rotary coring). This BHA features a continuous, smooth outer surface (conventional BHAs have expanded thickness at the tool joints). Whereas this slickwall BHA likely contributes to more efficient operation (material falling into the hole can lodge against the upsets at tool joints in a conventional BHA, potentially leading to a pipe stuck in the hole), it is significantly weaker than a conventional BHA and cannot be used to initiate a hole. The inherent weakness of the tool joints requires that the ADCB be deployed in a predrilled hole deep enough to support the BHA. The bottom of the hole must be free of debris to ensure a flush contact of the bit with the formation.
The ADCB cuts a core with 44% greater volume than conventional coring, so our standard protocol is to cut 4.5 m long cores (to reduce the weight of the recovered core and facilitate core handling). This requires twice as many wireline runs as conventional rotary coring, thus potentially increasing overall operations time. In addition, maximizing recovery with the ADCB requires retrieving the core barrel when there are indications that the throat of the bit is clogged (restricted circulating seawater flow rates), potentially necessitating many more wireline trips and subsequent reduction of overall penetration rates.
The ADCB was deployed during two recent ODP expeditions, Legs 193 and 194. Although both of these applications were operations of opportunity and yielded design and deployment improvements, neither directly addressed tool development specifications. During Leg 193, the material cored with the ADCB was fractured, hydrothermally altered dacite. Whereas the ADCB outperformed conventional rotary drilling in terms of quantity and quality of recovery, the scientific objectives of the mission obviated optimizing circulating fluid flow rates and rotation speeds in favor of rapid penetration. During Leg 194, operations were in shallow water (more complex operations in terms of weight-on-bit fluctuations) and the drilling target was carbonate. In this environment, ADCB recovery was significantly better than extended core barrel (XCB) coring and somewhat better than rotary core barrel (RCB) coring.
For the continued development of this system, several parameters require testing and monitoring. Operations during Leg 193 suggested that recovery and/or rate of penetration may be improved by optimizing circulating fluid flow rates and bit rotation speeds. Testing this requires recovery of several cores while adjusting and monitoring these parameters. In addition, we need to determine if the active heave compensation system limits weight-on-bit variations to within operational tolerances in open ocean deepwater environments. Bit designs need to be tested in basalt, gabbro, and peridotite, the lithologies of interest at mid-ocean-ridge and deep ocean crust exposures.
The ADCB development work will take priority for as long as 3 days during Expedition 304. If results exceed those typically obtained with the RCB, additional use of the ADCB may be requested.
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