Deepwater Bop Control Systems - A Look At Reliability Issues
Authors: Shanks Earl , Dykes Andrew , Quilici Marc , Pruitt John 
Published: October 2008

Abstract
Historically, drilling contractors have accepted without many questions the reliability of the Blowout Preventer (BOP) components and overall control system. A statistical reliability approach to qualifying, purchasing, and maintaining deepwater BOP control systems should provide a high level of confidence of being able to have long periods of time between planned maintenance of these systems with very few, if any, failures.
A study of deepwater BOP control systems has been performed to look at reliability issues and a means to qualify systems and components for a determined period between maintenance. Of special attention are the regulators and how they are typically arranged and used in the system. This paper will describe a statistical process to determine the reliability and failure rate necessary to accomplish the maintenance goal. In addition, the qualification process will be described and a discussion of the pressure control regulator issues discovered in the study will be provided.

Introduction
Transocean, like many other offshore drilling contractors, recently went through an extensive rig newbuild and upgrade program, which required purchasing a significant amount of customer-furnished equipment for the various shipyards. As with most "boom" cycles, the industry activity before the building cycle had developed ideas for new rig technology, but lacked R&D resources to make them available to be manufactured as already proven systems. Therefore, this building cycle, similar to all the rest, resulted in R&D efforts in parallel with the manufacturing of new equipment to be installed on new rigs. And, as before, this resulted in design and related problems while in service that drove significant downtime, in many instances.
At times, it appears the industry attitude is that we cannot afford R&D in advance of a defined need. However, the industry seems to be able to afford to fix the problems associated with downtime due to an incomplete design.
Many of these problems are directly related to not having a detailed set of design and functional specifications to give to the equipment manufacturer. Plus, the purchaser usually does not understand the duty cycle requirements, or demands, of the particular equipment for an interval that is acceptable to perform maintenance on the equipment without sustaining downtime.
For offshore floating drilling operations, especially in deepwater, one of the most expensive downtime events is associated with having to pull the marine riser and subsea BOP because of a problem. Any problem or failure that requires the riser and BOP to be round tripped will result in a cost of approximately ﹩1.00 MM per event. And whether the contractor or the operator absorbs this cost, it is expensive.
One of the more common causes for pulling the marine riser and subsea BOP is associated with the BOP control system. The deepwater BOP control system associated with dynamically positioned (DP) rigs is typically a Multiplexed Electro- Hydraulic (MUX) Control System. This is schematically shown in Figure 1. The demand on the subsea control system is initiated at the surface. The demand signal is multiplexed down the control umbilical to the subsea control system. There, the signal is decoded, confirmed, and performed.