Technical Note 25


One of the important conclusions of the workshop was that the drilling equipment used on Legs 147 and 153, while adequate for coring shallow single-bit holes, was not up to the task of establishing deep re-entry holes. Furthermore, the need of the scientific party for core to study militated against operational time being spent on overcoming the difficulties associated with setting guide bases and casing. But although the inadequacies of some existing equipment were brutally exposed on these two legs, the consensus of drilling engineering opinion at the meeting was that 500+ m holes could be achieved, using drilling technologies that have yet to be tested in these harsh seafloor environments and strengthened versions of existing equipment. A clear case can therefore be made for an engineering leg to test this hardware in an offset drilling location before another science leg is scheduled with the aim of achieving a deep hole.

The overall objectives of an engineering leg would be to concentrate on establishing a re-entry structure with casing down to 100-200 m. Work would be restricted to one or two sites and only a skeletal scientific party (maximum 10 scientists) carried. Some coring would be conducted mainly to confirm rock type or to test particular coring systems, but maximizing core recovery would not be an objective of the leg. Indeed, the opportunity to core in the top 100-200 m would be sacrificed if that improves the chances of setting casing.

The location of an offset drilling engineering leg would be based on the following criteria:

To Realistic Strategies for Offset Drilling

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