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All three tools correctly measured the correct absolute MTF. The only difference between this test and every other test performed was that the tools were tested without their pressure barrel. This suggested that the source of error was related to the pressure barrel. Further inspection of the pressure barrels revealed that the pressure barrel snubber used for Icefield 2043 during this expedition was out of alignment by 180 degrees (Figure 2). This misalignment would account for the incorrect MTF angles in cores 1 through 4 of Hole U1554B. The snubber was realigned. It is conceivable that the mis-aligned snubber and/or pressure barrel could have been used with different tools in the past which would account for instances of other tools recording incorrect MTF angles which are 180° off.
Figure 2 Snubber Alignment (Middle snubber is 180 degrees out of alignment)
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The orientation tool connects to the top of a sinker bar assembly via the keyed ‘T-slot’ fitting (Figure 3). This fitting The orientation tool can only be connected fit into this fitting one way. The T-slot fitting on top of the sinker bar is threaded onto the sinker bar and the alignment is set using shims to limit the distance the T-slot fitting can be threaded on. These shims (Figure 3), which are essentially washers of varying thickness, are adjusted so that when the T-slot fitting has been threaded on and tightened, the orientation point of the T-slot (the center of the T opening) is in alignment with the orientation point on the sinker bar, which corresponds to the apex of the curve in the D-pin receiver (discussed later). Getting the shim spacing right could be a difficult process and care must be taken to ensure the alignment is correct. However, once the spacing has been set and the alignment verified to be correct, no further adjustments should be needed until the T-slot fitting needs to be changed. Technicians should double check the orientation of the T-slot prior to coring to ensure that it has been set correctly. The T-slot fittings can take a lot of abuse and may wear out over time. These fittings should be inspected periodically to ensure they are in good shape and replaced if necessary. Due to the difficulty in getting the shim spacing correct, there may be a reluctance on the rig floor to change the T-slot fitting.
Figure 3 T-Slot Fitting and Alignment Shims on Sinker Bar
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