ABSTRACT

This paper is concerned with the design parameters, fabrication and sea trials of the first semi submersible equipped for dynamic stationing for drilling exploratory wells in rough, open, deepwater environments.

Design criteria are reviewed with regard to sea state, environment, propulsion requirements and drilling performance, including automatic control and sensing systems. A review of second generation sub sea BOP, control and re-entry equipment is presented.

Operating systems are summarized, and performance expectations are related to prospective utilization of the vessel in specific ocean areas. Preliminary conclusions are given with respect to vessel response to control and propulsion parameters.

INTRODUCTION

The trend in offshore exploration clearly indicates that the oil industry must extend its search for oil and gas into water depths beyond the continental shelf in the rough water areas of the world - the North Sea, Davis Strait, Gulf of Alaska, etc. The need for additional oil and gas reserves, and the belief that sizable reserves could indeed exist in greater water depths, have encouraged interest in deepwater exploration. This deepwater exploration has resulted in the need for new techniques and tools for water depths of 6,000 feet and greater and in rough sea conditions.

Dynamically stationed drill ships have been exploring deepwater out to 2,300' for the past four years. Ships perform satisfactorily in most areas of the world but motions greatly reduce their effectiveness in rough water.

IN ROUGH DEEPWATER OR ICE-INFESTED AREAS, A SEMISUBMERSIBLE UNIT WILL PROVIDE THE REDUCED MOTIONS NECESSARY TO MAKE THE OPERATION ECONOMICAL. The SEDCO 709 is a special, first-of-its-kind drilling unit with unprecedented capabilities. The new rig is the world's first dynamically stationed semi submersible (SSM) and the unit will be able to effectively operate in 36-ft. sign. seas and in water depths to 6,000 ft. The hulls have been strengthened for the ice-infested waters of Arctic regions and the onboard storage has been sized to make the unit self-sufficient for 60 days operation.

Dynamic stationing (DYNSTA) is not new to offshore oil operations. Drill ships have been using it successfully for about five years; however, the large water-plane area of the ship-shape hull with its attendant response to severe seas limits the drill season in some areas. This is the primary factor that led SEDCO to carry out research and development work on a DYNSTA SSM.

One of the SHELL companies saw a requirement for such a vessel and in March, 1974, entered into an agreement with SEDCO for construction of the 709. The SHELL co)any and SEDCO are equal partners in the rig which will be operated by MARINE DRILLING, S. A., a joint venture.

A special engineering task force undertook detailed final development of advanced forms of DYNSTA and underwater drilling equipment for this project " the first dynamically stationed semi-submersible".

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