Under ice buoyancy for CRP-2/2A.

The two five tonne air bags were deployed under the sea ice below the Drill Rig with the assistance of Crary Lab Divers. About 9 tonnes of buoyancy was applied under the 17 tonne drill rig and this maintained the freeboard in the ice page break hole to 180 mm (210 mm when unloaded) by the end of drilling. No buoyancy was applied under the Mud Hut complex which is also a significant load especially when the mud tanks are full (about 9 tonnes) and at the end of drilling this freeboard was reduced to less than 100 mm. Both air bags have been recovered but will require an improved mooring system to prevent freeze-on under the sea ice and facilitate recovery at the end of drilling.

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.

Cementing.

The initial cementing operations were not successful. Previous cementing in CIROS- 1 and page break CRP-1 appeared to work satisfactorily, but none of this cement had been cored to confirm that cementing techniques and setting times were appropriate. Expert advice and cementing references had suggested that we would need to use high concentrations of Calcium Chloride accelerator in the cement for low temperatures. However, our situation was more complex than the "textbook examples" because we are also using sea water containing Sodium Chloride which is also an accelerator. We also knew that high concentrations of both accelerators can react as a retardant and or weaken the cement

At the time of the first cementing attempt for CRP-2 we therefore had not been able to confirm that the theoretical cementing techniques and previous cementing attempts were working properly at sea floor temperatures of −1.8 degrees Celsius. Placement of the cement was carried out with proprietary cementing plugs directly within the riser for the first two cementing attempts. We hoped these plugs would speed up the operation but the plugs were unsatisfactorily in the modified riser string. The third cementing of the sea riser at about 13 m bsf appeared to work, although we could not positively confirm this by recovering strong cement core.

The cementing of the HQ drill barrel at 199.31 m, where down-hole temperatures were theoretically +5 degrees Celsius worked well, with several metres of hard cement recovered. This confirmed that the cement/accelerator mix was appropriateand indicated maximum setting times. A long setting time was deliberately allowed for this primarily because it was vital that the HQ barrel landing ring and diamond coring bit were locked firmly in place to be successfully drilled through with the NQ drill string. Major delays could have occurred or even hole abandonment if drilling out of the barrel was not successful. However, even with this latest CRP-2 experience we still do not have positive confirmation of the setting times required for initial cementing at the sea floor at −1.8 degrees Celsius.

Down Hole Logging.

The down-hole loggers had two opportunities to log parts of the hole. They were not allowed access to the uppermost 60 m of hole because this part had already shown itself to be unstable, with some of the hole drilled twice. In the first logging session in the upper part of the hole (HQ-96 mm hole) the loggers were requested not to use their radioactive source tool in case this was lost. It is common practice in many countries not to allow a radioactive tool in an open hole. Loss of this tool could have meant the early abandonment and cementing of the hole because fishing operations could have been hazardous.

In the second logging session (NQ-75.7 mm hole) all tools were available for use at the loggers' discretion but unfortunately the hole became bridged around 440m after running some of the tools to the hole bottom then running the calliper tool. This was the first bore hole wall contact tool, and it's use presumably caused the bridge to form. The Drilling Manager and Science Manager agreed to attempt to try to clear the bridge with wire line tools. We were not prepared to re-run the NQ drill string and chase the bridge which could have meant re-drilling the lower 200m of hole because this could have taken several further drilling shifts. The Drilling Manager pointed out that even running the wire line tools in an open hole represented a risk because the tools would be operated below the casing. Subsequently the wire line tools and much wire line were lost down hole and two days were spent in clearing the hole to 130 mbsf. . We now have confirmed that the current wire line and winch used for the normal core inner tube recovery is inadequate for the operation of the ODP piston and Shelby soft sediment samplers. The remaining deep logging tool runs were now restricted to the middle third of the hole. Further bridging restricted the VSP study to the interval above 130 m bsf. The VSP logging was then treated as an experiment with 24 shots fired in marginal blowing snow conditions. The initial VSP results appeared to be good, although smaller charges will be required for data acquisition in the upper part of future holes.

At the end of the logging the HQ casing was cut and recovered, exposing about the upper 60 page break m of hole for logging that was obscured by casing in the first logging session. Most of this part of the hole immediately bridged, confirming the original drilling decision not to expose this part of the hole for logging prior to cementing the HQ casing. I believe that the drillers were impressed with the professionalism of the down-hole logging operations. Of minor concern was the time that some tools remained at the bottom of the hole where they were susceptible to being sedimented in. I think that the down hole logging was very successful considering the unstable nature of a major part of the CRP-2 hole. In retrospect, the attempt to clear the bridged hole was a mistake but the other decisions to allow access to only the stable parts of the hole have been proven correct.

CRP-2 Site completion.

The drilling plan called for coring to cease on November 23, allowing for 2 days of logging, 5.5 days for returning the drilling system to Cape Roberts, and 7 days for breaking down the camp and storing on Cape Roberts. This presumed a normal ice year in which heavy plant could move safely on the sea ice until the first week in December. In 1998 down-hole progress was much slower than expected early in the season. With the hole 200 m short of the target depth on November 21 and down-hole progress at 30 m/day, it was decided that drilling could be continued a little beyond the schedule date.

By the time the drill site was abandoned significant surface melt had occurred in the immediate vicinity of the Drill Rig and Mud Huts primarily due to the reflection of sunlight of the buildings. After the removal of the drill rig it also became apparent that the ice platform immediately under the drill rig was in very poor condition and had been strongly corroded by accumulated minor spills of salt rich drill fluid. We took extreme care in the operation of the heavy plant close to the drill site area during site abandonment. The surface prediction of the timing of surface melting is extremely difficult but can occur within a period of 2-3 days during warm temperatures and bright sunlight. We also hope to reduce the quantity of spilt drill fluid that accumulates around the drill rig in the future with better capture proceedures.. Sea ice offset was evident from the angle of the sea riser at the surface by 18 November, when a chain hoist was fitted to the top of the riser (Annular Diverter) to reduce the angle at the entry point of the NQ drill string. At this time horizontal ice offset from spud-in was 9 m. By the time coring finished on November 25 the offset was at least 11 m and some difficulty with rotation was evident during high current flow. This offset is about 6% of water depth and corresponds well with the latest engineering study and this can now be used to predict part of the system's operational constraints.

The transition route onto Cape Roberts had become flooded with sea water in the tide crack zone by 22 November. Other less suitable routes were available but these crossed more highly fractured ice in the 30-m-wide tide crack zone. The nature of the sea ice and ice foot transition at Cape Roberts varies from season to season which can influence the timing of flooding, melting and cracking and it's safe use.