Discoveries and developments of oil and gas resources in increasingly challenging areas feature ultra-deepwaters, ultra-long step-outs, complex well technologies, and challenging weather conditions. This drives the subsea market to look into technologies and design solutions that reduce the overall cost for subsea installations.
The search for new discoveries in Arctic areas has a strong focus on reducing the environmental footprint and minimizing emissions. A common feature of the Arctic is the high cost of marine operations. Additionally, in the Arctic, the ability to perform intervention operations is often restricted by seasonal weather.
Over the last several years, more complex systems, traditionally associated with topsides, have been marinized and installed subsea. The terms "subsea factory" and "all subsea" are applied to these types of subsea installations. These systems include equipment such as separators, high power liquid and multi-phase pumps, and gas compression systems. These systems are increasingly being relocated to the seafloor to complement the traditional subsea systems associated with production and injection.
These system designs often involve a large number of remotely controlled processes that include controllers, instruments, and valves of various types and sizes. Some control loops require dynamic performance of valves normally not feasible using hydraulic systems. This relates both to the cost of a high-capacity umbilical system for long step-outs and also to the lack of control components suitable for a dynamic control loop.
A key design requirement for this new generation of equipment is the focus on reducing the cost of marine operations by introducing autonomous operations and light intervention work. This shift requires equipment with higher reliability and with built-in systems to monitor condition and performance for early detection of equipment faults.
To meet these emerging challenges, FMC Technologies has developed a variety of all-electrical technologies. The key benefits of electrical technology compared to conventional electro-hydraulic systems are as follows:
- Improved HSE. No pressurized equipment and reduced emissions
- Improved quality that enables better condition and performance monitoring, plus increased reliability and shorter repair time
- Reduced cost. Umbilicals are simplified without hydraulics, and the hydraulic infrastructure/consumption is eliminated both subsea and topside
- Technology benefits. The all-electric equipment is less sensitive to water depth and long distances, offers faster and more accurate choke/valve operation, has ROV retrievable actuators and distribution, and lower power consumption.
New eActuators contain the controls electronics to communicate with supervisory control systems and for internal control and monitoring of the eActuator. Electrical power is supplied from the battery to the built-in motion control system that generates power at variable voltage and frequency to the electrical high-speed motor. A speed reducer (gear) converts the high speed and low torque into the required torque at a lower speed.
These systems use electric energy and electric motors to generate torque subsea, and accumulate energy in subsea batteries (trickle charged).
This design allows for a low power requirement for the actuator/system, precise control of the valve position and torque, ROV installation, different gear boxes with a range of maximum torques and speeds, and implementation can be done without a new design of topside power or communication systems.
Statoil's Statfjord field got its first electric actuators from FMC Technologies in 2001. The actuators were used to control the ROV-operated choke valves on the manifold. This system is battery-based and consists of a subsea controls module (eSCM) and multiple electric actuators, communicating over CANbus. The eSCM includes the electronic devices and batteries (NiCad batteries).
For Statoil's Norne field a new generation of actuators was developed and deployed in 2005 using Li-ion batteries.
Benefits of electrical actuators in ultra-deepwater
Since no springs are required to move a valve to a fail-safe position, electric actuators can be smaller and lighter than hydraulic actuators. The required torque for electric actuators is not water depth dependent.
Statoil's Aasgard Subsea Gas Compression project has ordered 82 electrical actuators to operate process and manifold valves ranging in size from 2 in. to 14 in. (bore).
Key features include:
- Fully redundant electronics
- Dual motor winding
- Dual barrier on all interfaces
- Operates on 400 VAC
- IPC class 3 compliant
- TCP/IP and modbus interface to control system
- Built-in datalogger
- Built-in condition and performance monitoring system
- Weight in water 78 kg (172 lb)
- Length is 130 cm (51 in.) including ROV handle
- Actuator housings are Titanium
- API 17D interface toward valve.
The actuators will be produced in two versions, high torque with up to 2,700 Nm (1,991 lb/ft) and high speed with up to 250 Nm (184 lb/ft).
Control systems integration
The subsea parts of the control system consist of two main parts. The electrical Subsea Control Module (eSCM) provides the power and communication interface to topside equipment, actuator control, ESD/PSD control, power management for the subsea harness, including battery power switch, and housekeeping management. The subsea distribution system contains the electrical and communication connections.
The operator uses a logical name for the actuator in the control interface (Smart Tool). The topside processor unit (TPU) converts this logical name into a subsea device number and sends the message to the eSCM (Subsea). The eSCM (Subsea) converts the device number to a CANbus ID and transmits the messages on the CANbus. The addressed actuator receives the command and the reply is sent back to the eSCM, which sends the message back to the TPU.
The battery-based products feature low power consumption. The eSCM typically consumes less than 20 watts, the electric actuator consumes less than 2 watts, the electric actuator with local battery consumes less than 6 watts, an operated electric actuator consumes less than an additional 10 watts, and typical charging power is less than 30 watts. This means that it is possible to configure an eSCM with multiple electric actuators with a power requirement less than 100 watts. The system controls the charging of the batteries by turning the charge on and off when necessary.
The equipment uses the same topside infrastructure and communication systems as an existing hydraulic control system. With a topside communication system that has multi-drop capability, it is easy to add an electric control system on the same communication line as the hydraulic control system or another electric control system. A SCADA system on top of the TPU must ask for the newly defined tags to receive information from the electric equipment.
Electric actuators with a local battery can operate from a conventional hydraulic control module, since the actuator only requires CANbus communication and system power (24 volt DC). A new configuration file for the TPU must be generated and the new SCM definitions must be downloaded to the SCM before the electric actuator can operate.
The future of electrical subsea systems
Future subsea systems will require additional and new functions compared to today's electro-hydraulic systems. A key design priority in the development of subsea systems is to reduce the size and weight of equipment for very deepwater, and an all-electrical solution has the potential to replace the valve actuator spring packages at a reduce weight.
Another major application for this technology is in subsea processing. These typically include gravity-based separation and liquid scrubbers. Though these technologies are robust and proven, they are big and heavy. There are initiatives to reduce the size and weight of this equipment by moving to compact separator technology including coalescers. A more compact system design will require a dynamic and accurate control system that is feasible with all-electric technology.
The cost of equipment and intervention is high; a key way to minimize cost is to implement better condition monitoring and equipment protection. Initiatives are on-going to implement the Safety Integrity Level (SIL) into the equipment protection systems. The all-electrical design is optimal for these system requirements and will help meet the requirements related to SIL by using SIL building blocks.