Wireless control systems help prevent hardwired service loop failures for moving equipment
By Justin Rowden and Dustin Torkay, Seawell
Wireless control systems offer great potential for modern drilling rigs, but increased reliability and lower costs can only be achieved if changes to industry culture occur.
Moving equipment poses a challenge for reliable automation systems. For the types of tools that move along rails or a track, the control cables must trail behind the equipment as it moves along.
Often, the most vulnerable links in an automation system are the small conductor wires making up the communication cables and instrument cables in dynamic service loops. As the cables move, they are exposed to tensile forces, torsional stress and mechanical fatigue.
Most cables struggle to support their own weight if suspended vertically for many meters. As an example, Figure 1 shows top drive instrument cables being crushed by their own weight. This weight also causes the metallic conductors to stretch, resulting in Z-kinks in the bundle.
Even the specially designed service loop cables can only be expected to survive for about two years in a typical top drive installation. The material expense, along with any downtime incurred for a planned replacement of the service loop, is expensive to undertake every two years. This cost can soar if a failure of these cables causes an unplanned change-out.
Wireless technology is readily available and has been applied in other industries with great success. ArcelorMittal in Gent, Belgium, is an example. The company uses approximately 50 heavy lift cranes for handling large steel coils. To increase reliability of the systems, they selected wireless PROFINET to eliminate the complex routing of trailing communication cables between moving and stationary components. Eliminating the trailing cables saved approximately $45,000 per crane.
For drilling equipment, however, current industry culture poses a significant barrier to technology commercialization. By using wireless communication, it would be possible to operate equipment without relying on service loops or drag chains. Using this technology would require the cooperation of the original equipment manufacturers (OEMs) and drilling contractors. This technology has potential in many drilling-related applications, such as standalone condition-monitoring and measuring systems.
When a top drive service loop fails, the replacement value can be up to $100,000 and require up to 36 hours to install. Assuming the rig has a spare service loop on hand and is billed out at a rate of $450,000 per day, replacement of the service loop can cost $775,000 per failure, not including the operational costs or the cost of risk to the well. With a wireless system implemented, drilling contractors could recover the costs of installation by preventing just one hardwired service loop failure.
Before using wireless technology to move drilling tools, the following questions and concerns must be addressed:
• What frequency band should be used?
• How do we know wireless technology is safe?
• How reliable will wireless equipment be once installed on a drilling rig?
• The remote I/O devices must be relocated onto the drilling tool.
- Is there available space?
- Are the devices rugged?
• How will wireless transmission affect electric detonators?
• Is wireless transmission secure?
• Will wireless signals be affected by interference?
What frequency band?
Each country has its own government organization regulating which frequency bands are available for use license-free and which bands can be used only with special permission. The 2.4 GHz band is allowed worldwide license-free. Multiple wireless protocols have been developed using this band. The following are commonly used in automation:
• IEEE 802.15.1
• IEEE 802.15.4
• IEEE 802.11
- Wireless Ethernet/Profinet
Bluetooth: There are Bluetooth remote I/O devices on the market today that can be added to PROFIBUS networks. These devices act as a PROFIBUS slave to extend wireless to Bluetooth remote I/O modules. The transmission distance is typically between 10 m to 100 m.
WirelessHART: This is ideal for condition-monitoring sensors. The following types of wirelessHART-enabled process sensors are examples of ones readily available in the market now:
• Vibration sensors for monitoring electric motors.
• Pressure sensors.
• Temperature sensors.
• Flow meters.
• Control valve condition monitoring.
• Position sensors.
These devices can be installed in the field without the need for running any cabling. They can be battery-powered, solar-powered or field-powered. To conserve battery life, the transmission power can be adjusted according to the required transmission distance.
Additionally, the devices can be configured to use smart reporting so they don’t consume power by continually transmitting meaningless information.
This protocol allows for the ability to create a wireless mesh network. The devices pass information from one unit to another, and each device can act as a router to pass the information along. As a result, the devices do not need to have a direct transmission path to the network gateway.
Gateways and proxies are available to connect these devices to a main back-bone network, such as PROFIBUS or Profinet. In March 2010, the International Electrotechnical Commission (IEC) unanimously approved the wirelessHART protocol as a full international standard for wireless communication in process automation (IEC 62591Ed. 1.0).
Wireless Ethernet: Perhaps the most well-known protocol is wireless Ethernet according to IEEE 802.11. Industrial Ethernet is now more than 30 years old and is the international standard for area networking.
Ethernet is already widely used in automation systems at the SCADA level and for the human-machine interface (HMI).
To bridge the gap between Ethernet and fieldbus protocols, Profinet was developed, which supports direct connection of field-level devices to industrial Ethernet. By using Profinet, it is possible to ensure real-time deterministic control while also benefiting from the WLAN capabilities of IEEE 802.11 with raw transmission rates up to 54 Mbps.
Another major benefit of Ethernet devices is the addition of memory cards that store the configuration data. This allows for simple plug-and-play installation and eliminates the need for specially trained technicians when replacing a device in the field. The replacement can be completed in a matter of minutes.
Wireless Ethernet according to the Profinet protocol is fully compatible with the proven safety profile, PROFIsafe, which allows fail-safe communication over wired and wireless links. Continuous diagnostics are ensured, and the signal integrity is continuously monitored.
Redundant WLAN nodes can be used in a “rapid roaming” configuration to dynamically transfer from one radio field to another, preventing interruption of the communication. This is shown in Figure 3. Cyclic radio traffic with dedicated transfer rates can be defined to ensure real-time communication. Redundant radio channels can be implemented with a change-over time of a few milliseconds.
Traditionally, all I/O points on moving drilling tools are hardwired back to remote I/O installed at a fixed location. To eliminate the trailing service loop or drag chain cabling, it will be necessary to relocate the remote I/O onto the tool itself.
Finding the space to do this will be one of the biggest challenges. On tools where the service loop conductors are terminated in a junction box, the space allocated for this box will be gained and can be used to mount the remote I/O devices. If this doesn’t exist, brackets to hold the I/O component enclosures must be incorporated onto the tool. This is a fairly simple engineering problem that requires only some demand from the market to solve.
To locate the remote I/O onto the tool, the remote I/O must be resistant to shock and vibration. There are many rugged devices available in the market to meet this requirement. Additionally, the devices can be shock mounted.
Furthermore, drilling tools are often rated for operation in hazardous areas. In some cases, it will be necessary for the devices to be intrinsically safe or explosion-proof. There are wireless devices on the market that are rated to ATEX Zone 2, EEx and UL Class 1, Div. 2. For tools located in Zone 1 areas, WLAN product choices are limited. In these cases, the devices must be installed in a protected enclosure, or vendors must be pushed to develop more available options.
When it comes to wirelessHART condition-monitoring sensors, there are many products available rated for Zone 1 areas, as well as Zone 0.
Radio silence using electric detonators
Traditionally, it has been normal practice to enforce a strict rule of radio silence when using electric detonators during perforating operations. If planning to introduce wireless communication for use on drilling equipment, it is important to understand how it may or may not affect electric detonators.
Detonation by RF energy: The distance between the RF transmitter and the detonator wires is very important. Radio signals are attenuated as they pass through air. This attenuation is proportional to the square of the distance. This means that if the distance is doubled, then the received signal level will decrease to ¼ (-6dB).
The radio frequency also plays a major role. As radio frequency increases, the effective aperture of an antenna to receive a radio signal decreases. This relationship follows the same rule as with distance. The received signal decreases by a square relationship as the frequency increases. If the frequency doubles, the signal drops to ¼. To relate this to distance, the frequency and distance are inversely proportional. As frequency increases, reliable transmission distance decreases proportionally.
According to the Institute of Makers of Explosives (IME), the safe distance for a hand-held Citizen Band 2-way radio transmitting 27 MHz at 4 watts is 5 ft. By comparison, wireless Profinet is transmitted at 2.4 GHz at 40 mW using a Siemens Scalance W-786 wireless access point. This is an increase in frequency of nearly 9,000% and a decrease in power of 10,000%.
Based on the relationships described above regarding the effect this has on transmission distance, it would seem that the likelihood of triggering an electric detonator using wireless Profinet is practically nil. Nonetheless, the Profinet antennas can be strategically placed to maintain the 5-ft distance from the detonation areas. For example, placing an antenna on the roof of the driller’s cabin means that it would always be at least as far away as the height of the cabin (more than 5 ft).
Radio-safe detonators: Oftentimes, redundant, double-firing heads are used, such as mechanical percussion detonators, which are not affected by RF. Many offshore installations have adopted these SAFE detonators, but some rigs have not. These rigs often use a “rig voltage monitor” to measure the ambient rig voltage to determine if it is safe to have detonators at the drill floor in accordance with API RP 67.
The Profinet wireless access points on the market today offer WPA2 and AES encryptions for security and are in full compliance with IEEE 802.11i regulations. AES (Advanced Encryption Standard) is generally recognized as “unbreakable”; it is used by the US government to protect classified information.
The spectral analyses undertaken on drilling installations do not indicate significant background noise in the 2.4 GHz frequency band. Electromagnetic disturbances from arc welding or variable frequency drive systems occur in the kHz and MHz ranges. However, if several automation devices are introduced, they must share the same 2.4 GHz band reliably. To do this, collision avoidance techniques are implemented, such as CSMA, TDMA, iPCF and iQoS. These techniques allow several wireless devices to share the same band reliably while ensuring time-critical data for real-time applications. The detailed explanation of these methods is beyond the scope of this article.
Figure 4 shows a typical arrangement of the I/O devices associated with a moving drilling tool, followed by an arrangement where wireless communication has been implemented. The remote I/O devices have been relocated from a stationary junction box. Instead, they are shown to be installed onto the moving tool itself.
To implement the configuration of Figure 4 to an existing top drive, a wireless access point could be placed at a location approximately halfway up the derrick. The other wireless access point would be installed on the top drive itself.
This arrangement ensures direct line-of-sight between each access point and minimizes the required transmission distance to less than 30 m. Furthermore, the configuration can be made redundant by adding a second wireless access point as shown in Figure 5.
It is also possible to create a ring configuration in which one path is hardwired and the other is wireless.
Wireless technology has been advancing rapidly in recent years. It is being applied in countless new applications in many industries. When it comes to mobile equipment, wireless communication can offer an alternative or give redundancy to service loops, drag chains and slip rings. Eliminating dependency on these weak links in an automation system will increase uptime.
There are valid concerns when installing wireless systems. Reliable transmission distance, interference, transmission speed, security and governmental regulations all must be evaluated. With proper planning, a suitable arrangement using wireless devices can be implemented in most cases.
Wireless technology can increase the reliability of drilling rigs, and drilling contractors could recover the cost of installing a wireless system by preventing just one hardwired service loop failure.
This article is based on a presentation at the IADC Advanced Rig Technology Conference & Exhibition, 18 August 2010, Houston, Texas.