The all-electric subsea control technology on Statoil’s Norne field represents a second showcase reference for FMC Kongsberg Subsea. The company sees a bright future for electric subsea control systems, which offer cost and reliability benefits compared with conventional electro-hydraulic controls, especially as subsea step-outs get longer and water depths deeper.
FMC’s first delivery of an electric subsea control system consisted of electrically operated actuators for Statoil’s Statfjord Satellites in 2001/02. The actuators were installed on 16 chokes on the subsea production trees. The chokes, which had been operated using an ROV deployed from a supply vessel, now are controlled directly from the host platform. The electric controls have worked without a hitch, and, according to Statoil, the system paid for itself in about a year.
The Statfjord Satellites delivery constituted a pilot project, according to Vidar Sten-Halvorsen, R&D manager and project manager for FMC’s all-electric control technology. The Norne system features greater functionality and more flexibility in the control modules. It also incorporates a lithium-ion subsea battery which is more compact than the nickel-cadmium battery installed on the Statfjord Satellites.
The Norne delivery comprises 21 electric actuators covering both choke and manifold functions, plus rechargeable subsea batteries. Actuators will be fitted to all production chokes and two fail-safe valves. Each fail-safe actuator will have its own battery. The electric system will take over some of the hydraulic functions on the pig-loop valves, freeing lines in the umbilical which are occupied currently by hydraulic fluids to be used instead for chemical injection fluids.
Both the Statfjord Satellites and the Norne deliveries are retrofit systems, another benefit of all-electric controls, Sten-Halvorsen says. In both cases, they provided solutions to requirements arising in the course of production. Also, installation was effected without interrupting production.
For retrofit systems, batteries are a satisfactory alternative for actuator operation if the existing power supply is insufficient to meet the additional demand. For subsea systems designed from scratch with electric controls, power can be supplied through the umbilical, but if this makes the umbilical too expensive, batteries may prove more cost-effective. Batteries can be kept charged by a trickle charge sent down the umbilical when equipment is not operated. They are connected subsea with field-proven wet-mate connectors.
Electro-hydraulic systems use hydraulic fluids, thereby running the risk of fluid release into the sea. Such leaks are not uncommon and could be problematic in the face of ever tighter environmental regulation. By contrast, all-electric systems are clean - a factor which becomes more relevant as offshore operations head for environmentally sensitive areas such as Arctic seas.
In deep waters, electric systems provide other benefits over electro-hydraulic. Electric actuators and manifolds are more compact than the electro-hydraulic equivalents, leading to installation savings. As water depth increases, equipment such as an electro-hydraulic, spring-operated fail-safe actuator, which has to operate against the ambient pressure, can become extremely bulky, while the electric version stays the same size.
Electro-hydraulic systems also encounter problems as step-outs get longer, pushing umbilical costs ever higher, not to mention requiring greater amounts of hydraulic fluid. With an all-electric system a simpler umbilical is used, leading to big potential savings.
An electro-hydraulic control system would be impracticable for such an extreme step-out as Shtokman, if developed as a direct 550-km tieback to shore. But FMC has demonstrated that an electric control system is feasible. In such a case, subsea batteries would be used so the electric cable in the umbilical need not be sized to take a high intermittent load, which would increase the umbilical cost.
The fact that the control signal in the fiber-optic cable would have to be boosted by repeaters at intervals along the way provides a neat solution for charging up the batteries, which can be done by using the power feed to the repeaters, Sten-Halvorsen says.
Electric controls provide faster response time than electro-hydraulic, and also allow valves to be adjusted to any setting, whereas options available with electro-hydraulic controls are often limited to open or closed. The electric control technology also is suited for use with integrated operations, Sten-Halvorsen says.
FMC is developing the technology for use in subsea processing systems. It has qualified a flow control valve actuator for controlling the liquid level in a subsea separator. In connection with Norsk Hydro’s subsea compression pilot project for Ormen Lange it has qualified an electric anti-surge actuator. It also plans to use all-electric controls for the subsea compression technology it is developing with Siemens.
Although Statoil has been FMC’s leading customer for the electric subsea controls, other operators, including Petrobras, have accompanied the technology development closely.
