Oil and gas subsea equipment gets a network application profile

CAN networks on the ocean floor: To observe and protect the drilling process in sub-sea clusters of sensors, meters, and valves are used. But this is insufficient for reliability and interoperability requirements. SIIS (sub-sea instrument interface standardization) group worked with CiA (CAN in Automation) to network the equipment on the ocean floor, creating a new application profile for sub-sea equipment, which will help in standard formation.

11/21/2014


Figure 1: Combined pressure/temperature sensor compliant with the CiA 443 profile. Courtesy: Teledyne DalsaAlthough electric-vehicle mobility is in the headlines, combustion engines power most cars and trucks and it will remain this way for a long time, requiring the oil and gas industry to find new fields from which to extract hydrocarbons. The ocean floor is one of them.

An industrial network application profile has been developed to withstand ocean floor oil and gas industry applications, with help from more than 30 industrial companies. However, it is challenging to go into deeper waters for oil while protecting the environment and lowering the risk of another Gulf of Mexico oil spill such as the one in 2010.

Christmas trees and CiA at the bottom of the sea

The subsea drilling process consists of placing Christmas trees over wellheads. The trees comprise simple sensors, complex meters, and valves, which are used on producing wells to control flow rates or inject water or chemicals into mature wells to maintain steady production levels. In the past, these transducers were connected by EIA 485 serial links, but this is insufficient for reliability and interoperability requirements. Therefore, the subsea instrument interface standardization (SIIS) group and the CiA (CAN in Automation) association jointly developed a CAN-based solution to network the equipment on the sea floor.

CANopen as the base of the network

The CAN network uses fault-tolerant or high-speed physical layers compliant with ISO 11898-3 and ISO 11898-2, respectively. Of course, both types of physical layers are used with data rates of only 50 kbit/s due to the long distances between the devices. As an application layer, CANopen was selected due to its maturity and flexibility. To improve interoperability between the devices from different manufacturers, the organizations developed the CiA 443 CANopen application profile for subsea equipment, which include SIIS level-2 devices. Level-3 devices are connected by Ethernet, while level-1 devices use EIA 485 communication.

Protocol in transmission and communication

The CiA 443 profile specifies the process data (measured values and commands), the configuration parameters, and diagnostic information. All values are represented in the devices' object dictionaries and are accessible via the CAN network. The transmission of process data is performed through process data objects (PDO) protocol. Only some PDOs are predefined, for example, pressure and temperature sensors. For more complex devices, there are no predefined PDOs. The system designer is responsible to configure them consistently in the transmitting and the receiving nodes. Normally, the PDOs are event-triggered transmissions (change-of-state). The PDOs use the inhibit-time mechanism to avoid high network traffic caused by one PDO.

There are specifications for pressure and temperature transducers as well as for encoders and inclinometers. The profile also covers more complex meter implementations such as multi-phase flow, wet-gas, water-cut, and sand meters as well as oil and gas, vibration, and corrosion/erosion monitors. There are also specifications for leakage detectors and chemical injection valves.

Figure 2: Single-phase flow-meter supporting CiA 443 communication. Courtesy: FMC TechnologiesThe CANopen network manager (NMT) master resides in the subsea host controller. In most applications, there are two NMTs in the network. One operates, while the other is in hot standby mode. They comply with CiA 302 specifications, where the flying NMT master concept is standardized. The subsea system also uses the boat-loader approach of CANopen. This means that all devices start in the boat-loader mode. After being double-checked by the subsea host controller and downloading the appropriate application software, the devices are set in operation. All additional functions guarantee reliable communication. In addition, the redundancy improves the availability of the Christmas tree.

To detect failures in the specification as early as possible, the participating parties organized a plug-fest at which the device prototypes were tested. The detected misinterpretations and misunderstandings were fixed in the next revision of the CiA 443 specification. The tree system designer still has to conduct many tests before the system goes live on the sea floor. 

Subsea host controller connection to the offshore platform

The subsea host controller can be connected to the offshore platform by means of a transmission control protocol (TCP) capable network, which enables remote communication to the CiA 443 network devices. The communication is based on the CiA 309 specification, which supports remote access to the subsea host controller. This allows the configuration of devices and downloading of software as well as a remote diagnostic of the devices on the sea floor.

- Holger Zeltwanger is managing director, CAN in Automation; edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering eeissler@cfemedia.com 

Key concepts 

  • The CAN network uses fault-tolerant or high-speed physical layers compliant with ISO 11898-3 and ISO 11898-2, respectively.
  • Normally, the PDOs are event-triggered transmissions (change-of-state). The PDOs use the inhibit-time mechanism to avoid high network traffic caused by one PDO.
  • Christmas trees comprise simple sensors, complex meters, and valves, which are used on producing wells to control flow rates or inject water or chemicals into mature wells to maintain steady production levels.

Consider this

What standards might you use and apply to increase reliability and quality?

ONLINE extra

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