We used the RCB wireline coring system exclusively during this leg. Performance was excellent. Recovery for the leg averaged 86.3%, which is impressive for hard-rock coring. Bit seal failure was evident on most bit runs, and it was clear that the bit seals will not outlast bit life in this type of formation. We could identify nothing catastrophic as a result of the bit seal failures because the ~100 psi pump pressure increase had no negative impact on the coring process. On one occasion, however, Teflon material from the bit seal did become lodged in one of the bit jets. Given the experience gained on Leg 176, the RCB bit seal does need to be redesigned for normal coring operations where severe core erosion is often experienced while drilling deep holes in sedimentary formations where bit life is even longer.
About midway through the leg, we began to use the sleeveless core catchers designed for use with the extended core barrel (XCB) flow control system. The hard-rock coring was taking a toll on the standard core catchers. The core catcher dogs became so tightly pressed into the sleeves that we could not remove the core catchers from the core catcher subs without using an air chisel. This, of course, destroyed many of the core catcher components, primarily the outer sleeves. Also, early on in the coring process, we experienced a lot of core jamming inside the butyrate core liners. Beginning with Core 176-735B-99R, we deployed the core barrels without core liners and the problem was not repeated.

Two RCB coring system failures occurred during the leg. On the first occasion, a core catcher sub backed off from the inner barrel sub after 6+ hr of downhole rotating time. We had to make a pipe trip to get the cored material and coring components back. Subsequent to this, we coated all core barrel connections with liquid Teflon and made them up with a 48" wrench and a cheater pipe. The incident was not repeated after that. The second failure occurred when a pin thread failed between inner barrel connections during unloading of core at the rig floor. This was attributed to threads/barrels that had been in service for quite a while and could not stand up to the rigors of high-recovery hard-rock coring. The barrels were replaced and no further failures occurred. An Incident Report was sent in documenting the details surrounding each of the failures.

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