Sea Riser & Under-reamer for CRP-2/2A.

The two new flotation systems of the refurbished sea riser worked well this season. The syntactic "foam" rigid floats provided reliable service this season. Cracks were observed in a few floats immediately after arrival in Antarctica at WINFLY, when they were subjected to rapid temperature change from the aircraft to lower than minus 40 degrees Celsius, but these did not appear to cause any failure during deployment. The floats were assembled in 4 unit modules and 7 modules deployed on the sea riser at the CRP-2 site. On one or two modules the clamps appear to have slipped during operations but this did not adversely affect the riser operation.

The new inflatable flotation also performed well and enabled precise control of the tension in the riser even when the riser was not anchored into the sea floor. Both flotation systems will require minor modification for operations in 1999.

The riser was initially embedded into the "hard" sea floor to a depth just over 6 m bsf by slow drilling ahead with a 4 inch roller bit and 100% loss of drill fluid to the sea floor. The sediments appeared to be a clast-supported diamict with a soft dark grey muddy matrix. At this point two attempts were made to cement the riser, but the cement did not set up properly on either attempt, and we continued to core ahead to 57 m with minimal top tension on the riser. There was 100% loss of drill fluid thoughout most of this period.

The initial purpose of coring was to provide a better understanding of the strata we were drilling so that the two armed under-reamer could be deployed, but this also provided useful core from the youngest sediments for the science community. Coring was slow, but good recovery was achieved (70%). It should be noted that with this slim hole drilling system that down-hole progress is generally as fast by coring as drilling with a roller bit even with the drill collars that were hired specifically for the roller bit drilling.

The riser was bumped down with the main winch to where we thought we had clast-poor sediments that were suitable for deploying the under-reamer. The process of bumping down the riser is very slow as you are using the hardened riser shoe to cut clasts only millimetres at a time and also flush out the cuttings to the sea floor. The under-reamer was deployed but became disconnected from the drill string even though the standard preparation and torquing up of the tool was carried out. It is still not clear how disconnection occurred but probably resulted from the combination of the following.

• clasts in the sediment causing intermittent cutting,
• the two arm design which could experience high torque down hole,
• the API thread which only requires 3 1/4 turns to make up and store rotary energy ("twisting") in the HQ drill string.

The under-reamer was however recovered after some very careful work by the drillers.

In the process of embedding the riser by the bumping down procedure we found that the scanned images of the core obtained from drilling ahead to be an extremely useful tool to help plan the embedment. We now know that we can drill by rotation in the riser for a time with minimal top tension while coring ahead. Coring ahead is vital to plan the embedment process, and bumping the riser into the sea floor appears to be the only reliable way to penetrate clast-rich sediments near the sea floor without redesigning the lower part of the sea riser and using down hole hammer techniques.