ABSTRACT

Successful Surface BOP drilling operations from moored semisubmersible drilling rigs of Transocean Sedco Forex Inc. have been progressively developed in deepwater regions offshore Indonesia since 1996. These techniques developed specifically for environmentally benign areas have illustrated significant commercial advantages over traditional methods using subsea BOP and low-pressure riser systems. As a result of these proven potential cost savings, significant effort is being focused on developing of similar Surface BOP and high-pressure riser systems suitable for offshore floating drilling applications in more moderate operating environments typical of the Gulf of Mexico, West Africa, Brazil and the Mediterranean.

Current industry design guidelines and standards applicable to MODU's are specific to the traditional subsea BOP and low-pressure riser drilling systems. Efforts to develop alternative deepwater Surface BOP technology suitable for moderate environment MODU application have resulted in a requirement for a rational design basis to be established. Development of this technology within the industry is currently fragmented between several groups and an industry-coordinated effort to standardize the design basis and guidelines has yet to evolve.

The guidelines presented in this paper evolved from the procedures, equipment, and operational experiences obtained during the successful deepwater Surface BOP drilling operations for the past five years. This experience, combined with various applicable industry deepwater procedures and guidelines is used to develop a methodology to move this technology into moderate operating environments. Further refinements have resulted from HAZOPs, risk analysis and detailed system design along with sound engineering and operational judgement. Implementing the resulting design basis may produce a higher initial capital and operational cost than the benign environment configuration, however, this method achieves or exceeds safety and operational risk factors benchmarked by conventional deepwater drilling operations in moderate environments. The following moderate environment guidelines herein detailed will continue to evolve as the technology advances, but, the established basis of safety to personnel and the environment should remain consistent.

INTRODUCTION

Simply defined, Surface BOP Operation is the practice of utilizing a floating drilling unit fitted with a BOP that is suspended above the waterline in the moonpool area. The BOP, usually a land/jackup type BOP, is connected to a high-pressure riser serving as a conduit to the sea floor. Typically, the high-pressure riser has been 13-3/8" casing deployed in one continuous length from the casing shoe to the surface wellhead, Figure 1. The equipment configuration is similar to a jackup utilizing added top tension, with the exception the vessel is floating and the water depth may be thousands of feet deep.

This concept goes back to the early days of offshore floating drilling. A long-term example of Surface BOP is the Sedco 135A that drilled in the Far East offshore Borneo for over 20 years in water depths as deep as approximately 250 ft. This program began in the 1960's. The Sedco 135 series rigs were triangle shaped submersibles capable of being used in semisubmersible floating drilling applications.

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