Faster, cheaper to install
William Furlow
Editor-in-Chief
Cameron recently launched its all-electric subsea production system. Central to this concept is the all-electric control system, which replaces the conventional multiplex electro-hydraulic (MUX E/H) technology and actuates tree valves and chokes without hydraulic fluid. The introduction of all-electric technology addresses issues that have become increasingly challenging as the industry has moved into deeper and more remote waters.
Subsea advantage
Subsea systems are less expensive to install and can be brought on stream faster. They offer flexibility in the location and size of the host facility and can reach into water depths where conventional development of all but the largest fields would be prohibitively expensive. The drawback is that if something goes wrong with a subsea well, intervention can be costly.
The key to a successful subsea system is reliability. Cameron has found that when such a system does fail, the culprit is typically the control system. As water depths and step-out distances from the host facility to the subsea system increase, more complex conventional MUX E/H systems are required to overcome hydrostatic pressure and response time challenges. The industry has traditionally tackled this problem by building redundancy into the production system. Such redundancy obviously added to the cost of the system, but it was necessary to increase system availability or uptime. The trade-off here is increased system complexity as more and more components are added. The inherent reliability of an all-electric control system means reduced dependence on redundancy, fewer components, and increased system simplicity.
Another key concern in deepwater, long step-out applications is communication speed and response time. As requirements for downhole data increase in complexity, the modest communication speeds of current MUX/EH systems are challenged. Use of the all-electric control system virtually eliminates this problem as communication and power transmission are accomplished via high-speed coaxial cables.
Response time is also critical for actuation speed and accurate positioning of critical subsea components such as chokes. Traditionally, subsea chokes are actuated by a series of hydraulic pulses – or steps – to move the choke orifice from open to closed in controlling the flow of oil and gas from the well. Over long distances, the use of an electrically actuated choke rather than a hydraulically actuated one means that choke operation becomes a smooth continuous movement from open to closed, reducing response time and ensuring accurate control of well fluids at all times.
Central to the design of Cameron's all-electric subsea production system, Cameron DC, is a patented all-electric control system, which replaces conventional multiplex electro-hydraulic technology.
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Reliability drivers
Assuming reliability is equivalent, subsea systems offer many advantages over dry tree technology. Subsea systems are less expensive than their floating dry-system counterparts and can be brought on stream faster. In addition, they offer flexibility in well placement not always afforded by a dry host facility and can facilitate producing in water depths that would be otherwise uneconomical.
Cameron's all-electric subsea system begins with the surface power and communication system installed on the host facility to provide power and interface to the equipment on the seabed. Both power and communication are transmitted through the system's coax umbilical cable. The system provides bi-directional communication, giving the operator real-time feedback on the conditions of the equipment subsea. The umbilical terminates into a voltage regulation assembly mounted on the tree and into a control module also mounted on the tree. The control module (pod) provides communication to and control of all the functions mounted on the tree. The tree valves and chokes are opened and closed by electric motors instead of hydraulic pressure. This will be accomplished without subsea accumulators or even subsea batteries.
Hydraulics eliminated
Using electricity in lieu of transmitting hydraulic fluid through an extended umbilical makes communication with the equipment almost instantaneous. No hydraulics also means there is no hydraulic fluid vented to the environment, either through normal operation of an "open loop" system or a leak.
Topsides installation and intervention equipment is also simplified with the elimination of hydraulics as the primary workhorse. Simpler equipment also means easier maintenance and storage of such equipment when not being used.
Field trials
Cameron's all-electric system has gone through three stages of development, starting in 1999 with sub-assembly and qualification testing. In 2002, work began to develop the topsides and power distribution and control equipment. This led to the manufacture and testing of an integrated system in 2003. Dock trials were then conducted in Stavanger, Norway, in 2003.
In March of this year, field trials of the system began off a platform in the North Sea in 180 m of water. These trials will extend through the majority of the 2004.
Subsea system failure can most often be traced back to problems with complex hydraulic systems. The elimination of hydraulic actuation simplifies subsea production systems and reduces the number of seals – potential leak paths and wears components -- ultimately increasing availability. A simplified system is easier and less expensive to install, commission, and maintain. Operational expense is also reduced because there is no need for consumable hydraulic fluids, which are also an environmental hazard.
A recent cost benefit analysis performed by a British university on behalf of a major subsea operator indicated that the benefits of the all-electric system vary depending on the size of the field. Estimating the cost savings (or added profit) of a hypothetical 24-well subsea development, the study indicated that an all electric design could add as much as $130 million in value over the life of the field. Ironically, this number is about what today's subsea equipment (trees, chokes, manifolds, and controls) costs for a 24-well development.
