ODP Technical Note 10


C. RCB Orientation

As with APC cores, orientation of RCB cores includes running a multishot tool inside a nonmagnetic drill collar to record continuously the orientation of the core barrel. RCB cores tend to be cut in short pieces, however, with centimeter- to meter-long pieces of core breaking off from formation when the torque transferred from the cutting surface to the core exceeds the rotational strength of the rock. Thus two additional tools are needed to determine the orientation of each core piece while it is being cut, before the piece can brake off and rotate independently within the core barrel. These two tools are the sonic core monitor (SCM) and the scribing core-catcher. The entire hard-rock (RCB) core-orientation system is illustrated in Figure V-1.

The SCM is a battery-powered electronic tool that monitors and records core height relative to the core catcher. The SCM is programmed via an IBM-compatible PC, and mounted above the core receptacle inside the top of a corebarrel, which is then lowered or dropped down the pipe to the BHA. The SCM monitors the height of a core as it is cut by transmitting ultrasonic waves down the barrel at a target, which rides atop incoming core. As more core enters the barrel, the time for the reflected ultrasonic wave to be returned to the SCM receiver is reduced. At present, the rate of bit penetration is recorded at the rig floor with the same computer used to program and operate the SCM. This is done by having the driller record drilling and pipe progress manually, as sections of pipe pass through the Kelly bushing at the rig floor. When the core barrel and SCM are returned, bit depth and core height are merged to indicate the depth of origin (relative to the rig floor) of each part of the core.

The scribingcCore-catcher (Fig. V-2) uses three asymmetrically arranged carbide blades to mark the orientation of hard rock as it enters the core barrel. Three knives are used to prevent pieces of core from becoming eccentrically loaded and jamming in the core catcher; they are arranged asymmetrically to allow the "primary" scribe mark to be determined. The primary scribing knife is aligned mechanically with a magnetic reference on the electronic multishot, thus allowing relative core orientation to be tied to the absolute orientation of the barrel. Individual pieces of core tend to rotate relative to each other once the pieces enter the core barrel; each of these pieces must be individually reoriented after the core is recovered on deck.

In order to orient a piece of hard-rock core, the interested scientist must merge four distinct data sets: bit depth, core height, core-barrel orientation (relative to magnetic north), and core-piece orientation (relative to the core barrel). Bit depth and core height are presently merged within the same program used to operate the SCM, but this prototype system, like the rest of the hard-rock core-orientation system, is still in development. Example records from preliminary SCM deployments are shown in Figure V-3. The hard-rock orientation system is compatible only with the rotary core barrel. There are no plans at present to adapt hard-rock orientation to XCB cores.

To CORK, Section A.
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