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Victoria University Antarctic Research Expedition Science and Logistics Reports 1998-99: VUWAE 43

1.1 Background - The CIROS-1 experience

1.1 Background - The CIROS-1 experience.

The drilling programme for CRP was based primarily on drilling experience and conditions found at CIROS-1 in 1986.

The CRP drilling programme adopted a sea riser and two coring drill string option in comparison to the three coring strings used at CIROS- 1. An increase in maximum operating water depth from 200 m at CIROS- 1 to 500 m for CRP sites was the main constraint in developing the current sea riser system. Cost and the belief that drilling could be successful with only 2 coring strings, HQ and NQ, led to the adoption of the present system. It was assumed from the sea floor conditions at CIROS-1 that 10-20 m of relatively soft Quaternary sediment could be washed through, and that the underlying older lithified strata would be suitable for embedding the sea riser to a depth of at least 20 m bsf with under-reaming techniques.

Drilling equipment purchases (drill rod etc) were based on the drilling strategy of four holes up to 500m deep in water depths of 150-500m as outlined in the CRP Workshop Report of December 1992. Mike Blong (Baroid, New Plymouth) analysed the CIROS--1 mud usage and developed a simple mud programme for CRP. Drill rod, sea riser and drill fluid products that were expected to be sufficient for all four holes were supplied in December 1994 for the January 1995 ship off-load at Cape Roberts.

Our Experience in 1997 while drilling CRP-1 identified inadequacies in the riser system designed and supplied by Austoil NZ Ltd. These were remedied for CRP-2 with a detailed engineering study of theoretical riser performance, improved deep-water rigid flotation and a sophisticated air bag tensioning system at the top of the riser. A new "off the shelf two arm under-reamer was also purchased to assist riser embedment. CIROS-1 found no significant down hole fluid over pressure problems although traces of methane were analysed and a page break bitumen residue found in sandstone.

The drilling of CRP-1 showed that the Quaternary sediment was more than twice as thick as expected from the available seismic survey data, and much more time was spent on seating the sea riser than had been planned. As a consequence it was decided that it was unrealistic to expect to have the time to drill two 500-m-deep holes in a season, and that we should instead set a single 700-m-deep hole as a goal. The drilling of CRP-2 has confirmed the wisdom of this view. A consequence of this decision has been that, following ODP advice, fluid control hardware has been procured for low over-pressure conditions (<1500 psi). For drilling below 500 mbsf this calls for the H casing string to be set in impermeable strata and cemented to withstand down hole pressures.