Abstract

This paper summarizes a quantitative risk assessment (QRA) performed on a new generation blowout preventer (BOP) and multiplexed control system to be used in water depths of up to 10,000'. Prevention of severe environmental damage and minimizing danger to personnel are the critical functions of a BOP. In order to assure high reliability in the systems, a risk assessment was performed on the design before final assembly. The reliability was examined in two manners, by development and quantification of a fault tree model of the systems and the development of a failure modes and effects analysis (FMEA). Both techniques yielded basically the same results, however the FMEA benefited from the insights gained during the more rigorous interdependency evaluation from the fault tree analysis. Application of QRA techniques can be used to improve the reliability of critical systems. The use of these techniques during the design phase allows reliability concerns to be addressed prior to initial system operation. The techniques are applicable to all types of automatic and manually operated mechanical, electrical, and control systems. The reliability of the systems examined is high due to the large amount of redundancy and continuous status monitoring of the system components. Component specific failure data was limited due to the uniqueness of the design, however representative data was developed based on generic sources, discussions with designers, and engineering judgment. The failure combinations, called cutsets, were reviewed with the designers to ensure that the model accurately reflected the systems. Importance measures were used to assess the relative importance of the various design features and the sensitivity of the results to the assigned failure data. Although the reliability of the BOP and control system was sound to be high, several recommendations resulted from the analysis results to improve or maintain the high level of reliability.

Introduction

Hydril Company has developed and built a blowout preventer (BOP) and control system for installation and operation onboard the Transocean Offshore Inc. drillship Discoverer Seven Seas. The BOP is designed for deepwater drilling operations up to 10,000 feet below sea level and incorporates the latest technology advances in micro-processors to increase system flexibility, system diagnostics, reduction in the electronic component count and the physical size required to house the components. In order to assess the overall reliability of the system, EQE International has developed and quantified a fault tree model as part of a quantitative risk assessment (QRA). The QRA process is composed of the following subtasks which are discussed in more detail following the backgrounddiscussion:

  • Success Criteria Determination,

  • System Boundary Defination

  • Fault Tree Development

  • Fault Tree Quantification, and

  • Results Interpretation

Background

BOP control system changes have been brought on by advances in technology and by the demand for increased capabilities associated with the control system:

  • The new deep water stacks have more functions and monitoring capabilities which exceeds the capacity of the older control systems.

  • The new multiplexed (MUX) control systems are software driven, computer based, networked systems. The operation, maintenance, and trouble shooting of the new MUX control system is significantly different from the older control systems.

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