John Bodine and Margaret Buckley
While electro-hydraulic multiplex prod- uction control system reliability and uptime availability have improved over the past 25 years, progress of drilling control systems has been relatively slower. With today's systems designed for ultra-deepwater applications to 10,000 ft and deeper, the drilling industry is demanding higher uptime availability and longer operating life. To reach reliability objectives, drilling control sys-tems could benefit from lessons learned in the production control systems environment.
Deepwater drilling control systems and production control systems serve similar functions. They ensure BOP operations and christmas tree functions. Hydraulic pressure provides the fluid power necessary to operate the subsea BOP and tree functions. On electro-hydraulic multiplexed systems, signals are sent from the surface to open and close subsea solenoid valves that perform the required functions. In addition, electrical sensors provide inform-ation on pressure, temperature, inclination, and other subsea system performance characteristics.
Drilling and production control systems share much of the same component technology. Both use one-atmosphere enclosures to protect sensitive electrical equipment from exposure to seawater and depth-induced pressure. Directional control valves are installed on subsea control pods and modules, and subsea distribution of electrical power and communications is typical. Electrical signals are transmitted from topside control stations through the subsea electronics to operate solenoid valves on the control pods/modules, which sends a pilot signal to operate the main stage of a control valve. The control valves enable fluid power to function the BOP or tree operators.
Moving production control reliability into drilling controls.
While the functionality of the systems is similar, one main difference is in the basic functions that the subsea control system operates. Prod-uction systems use fluid pressure to open tree-mounted actuators, which are typically spring- operated fail-safe close. Drilling systems use fluid pressure to open and close BOP functions. A secondary difference is in the volume and pressure of hydraulic power required for operating the two types of systems. Drilling systems require greater fluid volumes, while production systems require higher operating pressures.
Historically, due to the economics of the shallow-water drilling industry, contractors have not needed to accept systems employing new (to drilling systems) technology, increased redundancy, fault tolerance, and increased equipment cost that is necessary to deliver highly reliable, high uptime-availability systems. The shallow water industry is driven mainly by capital expenditure (capex) and schedule, not by investment in technologies and techniques that could improve system uptime. This is understandable in that drilling contractors typically receive a day rate for a contracted period of time, with little upside potential.
Over the past two to three years, however, a new trend has emerged that could change the conventional approach. The move tow-ard deepwater drilling creates a different focus from drilling in shallow water. High downtime costs associated with retrieving equipment from deepwater significantly impact contract economics. The costs are high enough now that additional upfront investment for drilling control systems warrants reconsideration.
The need for reliability
Typically, drilling controls suppliers have designed and provided systems that meet, but do not surpass or enhance, API 16D specification requirements. The cyclical nature of the industry causes suppliers to design systems that are "good enough" for the current business cycle. This is understandable in that higher price systems increase the risk of losing awards, even though a system could provide higher uptime availability and improved economics to the drilling contractor.
Exploration and production operators, while always considerate of capex, are also inclined to optimize field production rates for the long term. Capability to optimize production often requires additional investment in equipment and technology. Current trends in production systems are to invest in production control systems with statistically higher potential to successfully operate over (and at) a given period of time. Equipment and component reliability and uptime availability of control systems is of significant importance.
Incorporating new technology
To cost-effectively manage drilling control systems, the industry needs to incorporate new technology. One critical move would be to incorporate premium seawater tolerant materials. Corrosion resistant materials, especially for control valves and hydraulic circuitry, need to be developed to reduce the effects of seawater contaminated control fluid. Additional testing on fluid compatibility and tighter controls on the chemical composition of BOP fluids also needs to be explored. Value improvement could be expected – measured in days of higher uptime availability and lower spares expenditures.
Critical equipment/component modularity is also a goal. Standardization would improve manufacturability and schedule, improve product integrity and interchangeability, reduce equipment footprint size and weight, and reduce intervention and maintenance durations. Moving to modularity involves a change in thought from existing drilling systems where it has been normal practice for the drilling contractors to drive unique specifications and custom requirements. Improvement would take the form of better communications response between suppliers and contractors and in fast-track schedule and delivery capability.
A third technology improvement is ROV retrievability for critical equipment or components. Upfront planning should be incorporated to make intervention and maintenance easier and to eliminate BOP stack trips for controls-specific faults. Intervention plans should be based on failure mode effects and cause analysis (Fmeca) and system reliability availability maintainability (RAM) analyses. Improvement would take the form of greater uptime availability and significantly reduced intervention/retrieval trip times.
Dual barrier protection for critical components would also improve reliability. Precautions, such as dielectric fluid filled chambers to limit seawater exposure of electronics, electrical cables, control valves, and hydraulic circuits should be in place. Incorporating dual barrier protection could be expected to extend submersion days, reduce seawater/corrosion effects, and increased equipment design life.
Another vital issue is eliminating single-point failures. Again, by employing system analysis, such as an Fmeca and RAM, analyzed redundancy of electrical and hydraulic systems should be employed based on the criticality of the component. This needs to be offset with due consideration for cost and additional components. The result would be higher uptime availability and increased system design life.
Finally, supplying clean control fluid to NAS 1638 Class 8 or better would have a tangible impact on dependability. Use of ROV deployable/retrievable subsea filter modules and pre-mixed control fluid can reduce contamination induced control valve failures. Value improvement could be measured in days of higher uptime availability, reduced intervention/retrieval trips, lower spares investment, and increased equipment design life.
As the drilling industry moves to ultra-deepwater depths of 10,000 ft and more, system reliability, uptime availability, modularity, and ROV intervention flexibility will become increasingly more important factors for designing and supplying drilling control systems. By applying production control system technology to improve drilling control system reliability, drilling contractors could expect to see a significant reduction in non-productive time, as well as an increase in equipment and system mean time between failures. Expected results could include improved contractor economics and an equally important reduction in system failures that are always highly visible to those within and external to the energy industry.
John Bodine is product manager for Production Control Systems at Cameron Controls, with 15 years' experience in deepwater subsea production systems and subsea control systems. He holds an MBA in finance and strategy from Rice University, BS in mechanical engineering from Michigan State University and is a registered professional engineer (Texas).
Margaret Buckley is product manager for Drilling Control Systems at Cameron Controls, with 25 years' experience in hydraulic and mechanical control systems. She holds a BS from the University of Houston.