OFFSHORE OIL RIGS CHAT
The petroleum platforms are in the Gulf of Mexico.
By Philip Smith and Mike Kimball
Oil production is a fiercely competitive business.
Oil companies searching for oil and gas reservoirs on the continental shelf of the U.S. coastline typically lease a 2¼-square-mile area from the U.S. government to drill for oil.
Because oil deposits are finite resources, the leased area becomes quickly and densely populated with drilling platforms and production, including pumping and transporting operations.
Platforms connect: Platform complex with existing ore process (Siri), unmanned satellite platforms (Nini and Cecilie), and offloading facilities to an oil tanker
Competing oil companies will attempt to tap any oil deposits found, making it essential for efficient production by the first company that discovers the deposit.
To maximize productivity of each oil platform, all aspects of production involving monitoring and control of repetitive functions need to automate, thus allowing the platform to operate around and from a central control point.
The logical extension is to use one platform as a central control point for operating multiple unmanned platforms. At this point, a decision must take place concerning the method employed to connect (network) the manned platform to the unmanned platforms.
Here is the rationale for leveraging a wireless network to network oil platforms.
DIFFICULTIES IN ENVIRONMENT
Operating an oil and gas extraction facility offshore presents a list of difficulties.
The marine environment is highly corrosive, meaning any platform erected will require intensive maintenance and upkeep. Platforms are typically located in 150 to 350 feet of water, up to 10 miles offshore.
The platforms usually must drill several thousand feet below the ocean floor to reach oil and gas deposits. Weather is a major factor in platform operation. When high seas and heavy winds or hurricanes approach a platform, the crew must evacuate to prevent injury or loss of life.
These challenging conditions require platforms to operate continuously to maximize profitability. When a corporation erects platforms in a given area, competing oil companies will lease adjacent areas and tap the same oil deposit located below.
Don’t leak in my backyard
Pennzoil’s Bonito pipeline extends 72 miles into the Gulf of Mexico off the coast of Louisiana and gathers crude oil from 18 separate offshore platforms.
Pipeline diameters vary from 4 to 14 inches, and the pipeline transports up to 85,000 barrels per day. Bayard Engineering installed a PC-based SCADA system, flow computers for each platform, and a leak detection system.
The leak detection system is an application software product that collates the instrument readings from each wellhead and processes them using Barton flow computers.
This data then passes to the Wonderware In-Touch system, where it becomes available to the operator through a graphical user interface. The leak detection system, LeakWarn, accesses the data at this point.
The software computes the corrected volume—line pack—of each individual pipe leg at the end of each SCADA cycle. The model accounts for changes in line pack caused by all operating transients, temperature changes, and product changes.
The change in line pack from one cycle to the next, as well as incremental volumes entering and leaving the pipe legs, go through mathematical integration over various time periods from a few minutes to several hours.
This allows detection of a full range of leak sizes while keeping the instance of false alarms to a negligible level.
Source: Simulutions (www.simulutions.com)
Competitors continue extracting oil and gas from the same reservoir, and that reservoir’s capacity is finite.
AUTOMATIC SEQUENTIAL SHUTDOWN
Oil and gas production must continue on an uninterrupted basis to ensure profitability. To further this end, labor-intensive, repetitive tasks such as opening and closing valves, monitoring levels, and performing transfer functions at given intervals must be automated.
The obvious benefit is to reduce labor costs and ensure timely completion of tasks. The programmable logic controller (PLC) serves extensively to continuously monitor and handle routine operations.
The PLC allows valve states, fluid levels, and pipe pressures to be monitored; opens or closes valves; sounds alarms; or releases pressures, depending on the actions programmed into the PLC.
The PLC is part of the supervisory control and data acquisition (SCADA) system. The SCADA system allows the PLCs to operate autonomously while reporting to a host computer, which provides an operator interface to the system as a whole.
Other components of the SCADA system include a PC, which acts as the LAN controller, and man-machine interface (MMI) software. The MMI software allows a graphical representation of the system and is more intuitive than a display comprised of text.
The operator may take control of the unmanned platform as necessary to change alarm set points, monitor and control tank levels, bring wells online and offline, and shut down or start up the platform.
Using a SCADA system enables platform operations to take place smoothly with results reporting in a timely manner to a central monitoring point, where an operator may fine-tune system parameters as necessary.
In case of a communications failure, the PLC will continue to operate the platform autonomously unless conditions indicate the well needs to shut down to prevent damage to the platform.
The use of PLCs also allows a platform to start up or shut down in a controlled manner if an emergency arises. A sequential shutdown, called shutting in, of the well is desirable to avoid equipment damage and allow the well to be restarted efficiently.
Indeed, a PLC can also operate the platform should the crew need to leave the rig and shut in the well automatically should safety systems indicate a problem.
HARD WIRE PCS AND PLCS. . . ?
The obvious advantage of networking multiple platforms to a central control point is reduced staffing at each platform. By operating a specific platform as the control point, the other networked platforms can function effectively without direct intervention.
This reduces personnel requirements to only the number required for maintenance and repair. Additionally, control may shift to an alternate platform, should heavy weather at the primary control site force its evacuation.
Connectivity among platforms runs through a PC-to-PLC interface. One possible method for connection is through the use of satellite or cellular telephones and modems. This approach is difficult due to the scarcity and cost of available phone service.
An alternate approach is to hard wire the PCs and PLCs together, but the cost of installation and upkeep of underwater cabling makes this approach economically unfeasible.
The best alternative is the use of a private radio network. By installing a licensed, narrowband data radio modem, one can avoid these high costs and scarcity issues. Narrowband radio modems provide a private, secure medium for data transfer.
A case in point is Pennzoil’s facility that started with two well sites. The oil platforms are located in the Gulf of Mexico approximately 20 miles apart and operate 24 hours a day, seven days a week.
Standing wave ratio The ratio of the maximum to the minimum amplitudes of corresponding components of a field, voltage, or current along a transmission line or waveguide in the direction of propagation.
PLC A programmable logic controller is an industrial control device with multiple inputs and outputs that contains an alterable computer program.
SCADA Supervisory control and data acquisition is a generic name for a computerized system that is capable of gathering and processing data and applying operational controls over long distances.
These platforms are subject to the corrosive salt air of the marine environment and the full force of hurricanes. They operate in several hundred feet of water and drill several thousand feet beneath the seabed to tap their oil and gas deposits.
The oil platform serving as the control, or host, has the computer and MMI software in addition to the GE Fanuc Series 90/30 PLCs operating the platform.
The MMI software graphically displays the host platform operation and also the unmanned or remote platform status on the host computer and allows the operator to refine system parameters to optimize production on the platforms.
The remote platform also uses a PLC to operate the platform and will require a means to communicate with the host platform.
An ESTeem UHF radio modem links the platforms together. In conjunction with the PLC and MMI, a SCADA system allows the manned platform to control and monitor operation of the unmanned rig results.
The PLC will operate the platform and report status to the manned platform when polled. The plan is to bring additional platforms into the network and operate them from this central control point.
PROGRAM THE RADIO MODEMS
Requirements for using a radio frequency (RF) network solution are uncomplicated. The suggested way to begin installing the system is to conduct an on-site radio site survey.
First, a spectrum analysis takes place in order to find a clear frequency of operation. The radio system temporarily installs to record signal strength measurements at the host and remote sites.
Then one must check the locations for the permanently mounted antennas to make sure there are no problems with respect to clearance to nearby antennas. This is to avoid cochannel interference.
Other states of art
The state of the art in modern wireless technology revolves around the advancements in wireless Ethernet. These new products leverage the IEEE 802.11b wireless LAN standard and withstand the rigors of industrial applications. They can provide data rates of 11 mega bits per second for ranges of 7 or more miles.
This type of communication link will be the standard in industrial communication in a very short time.
Once the system is mocked up, reliability of the RF link is tested. Installation, the final step for the radio modems, is simple to perform. However, attention to detail is the key to system reliability.
When installing the system, one must securely mount and correctly position the antenna with respect to polarization: vertical or horizontal. The feed line between the radio modem and the antenna must lay in physically undamaged and not exceed the minimum cable bend radius.
In high-lightning environments, lightning arrestors are required to prevent equipment damage during electrical storms. Connectors install properly, securely fastened and weatherproofed, to ensure optimal system operation.
An uninterruptable power supply should be used to provide clean, stable power to the radio modem. After the antenna, feed line, and lightning arrestor have been installed, the reflected power should be measured to ensure the standing wave ratio (SWR) is within specification: less than 1.3.
Scheduled maintenance includes measuring the output power of the radio modem and the SWR and weatherproofing the feed line connectors. IT
Behind the byline
Philip Smith was a technician at Electronic Systems Technology (www.esteem.com) when he worked on this project. Mike Kimball is an electrical engineer at Bayard Engineering.