The economics of marginal field development in the UKCS can be made much more attractive by reducing capital investment costs in drilling equipment, platform equipment, and jacket structure sizing and construction. This can be achieved through the introduction of semi-submersible Tender Assisted Drilling (TAD) techniques. The classical approach to TAD requires the use of a standard flat floating barge in support of a small jacket mounted drilling package. This technique cannot be used in the severe environmental conditions of the North Sea, but the viable alternative of using a modified semi-submersible drilling unit has been identified and more recently put into place.

An international drilling company, under contract to a major North Sea operator, has modified a Sedco 700 series design semi-submersible drilling unit (Sedco 704) to perform drilling operations in both TAD and conventional stand alone drilling mode for a UKCS development project. The planning, design, modifications, and performance tests were completed within the contract schedule and budget cost and the unit was delivered ready for platform hook-up and drilling operations in December 1992.

This paper discusses the philosophy behind the selection of the unit, the studies associated with the complicated mooring requirements, and the modifications made to allow for operations in TAD or stand alone drilling mode. Solutions to interfacing problems with the transfer of power, drilling fluids, and personnel between the unit and the jacket are also described and discussed.

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