Qualitative Risk Analysis of Emergency BOP Control Systems to Ensure Availability

Abstract

Abstract Many of the latest generation of ultra deepwater capable rigs included emergency BOP (Blow Out Preventer) control capabilities, sometimes referred to as secondary intervention systems. Such systems represent the last line of defense in containing a well. Should it be necessary and unavailable, the result could be environmentally and humanly catastrophic. Building on installations that have been in service for many years, these capabilities range in functionality and purpose, from providing an alternate means to operate BOP functions in the event of total loss of the primary control system to assisting personnel during incidents of imminent equipment failure or well control problems. They can be actuated automatically or manually, and utilize components of the primary BOP control system or be totally independent. With as many permutations as there were rigs built, an understanding of the capabilities and limitations that exist on a particular rig is of critical importance in assessing the risks associated with a drilling program. While there currently are no standard terms in use to describe the essential attributes of systems, this paper recommends definitions and terms for a common understanding. The defined terminology is then utilized to compare and contrast system parameters, identifying various system strengths and weaknesses for use in risk analyses. Possible enhancements to existing emergency control systems will then be discussed, as well as their benefits and anticipated costs. Finally, the paper will recommend best practices for moored rig operations and those for operations utilizing DP (Dynamic Positioning). Introduction Systematic analysis of emergency control systems can only begin with an agreement upon critical parameters based on common terminology. From this starting point, best systems and practices can be identified as well as recommended opportunities that could enhance these systems' effectiveness. Critical performance issues depend on two issues - type of control system (hydraulic or multiplex) and method of station keeping (anchored or DP). Consequently, four industry best systems and practices are suggested based on the possible permutations of these issues. The most important elements of a well designed secondary intervention system were defined as follows:Fast responseSufficient capacityIndependence from primary system (commonality)Environmentally independentAutomatic activation by loss of hydraulic and electrical power to subsea stackWorks in presence of mud plume or noiseContains well if LMRP (Lower Marine Riser Package) accidentally disconnected and well kicksManually secures non flowing well Accordingly, system capabilities were evaluated against these criteria. Executive Summary For rigs with a multiplex BOP control system operating in DP mode, the recommended system is what is defined herein as a deadman system, with suggested enhancements noted in the Recommended Enhancements Section below to supplement it. For this type of control system operating in anchored mode, the EDS (Emergency Disconnect System) and auto disconnect systems can be eliminated or bypassed. In both cases, an ROV (Remote Operated Vehicle) would be required to manually secure a non flowing well. For rigs with hydraulic control systems, addition of an auto shear circuit is recommended to provide the automatic closure of the well in the event the LMRP is accidentally unlatched. Again, an ROV would be required to secure a non flowing well.