Cellular Digital Packet Data Reduces SCADA Cost

By Ross Linnemann, Vinson Process Controls Co. and Robert Munda, Dobson Cellular Co.

Application Overview

Remote applications of RTU's for wellhead SCADA projects typically involve much more than electronic hardware for the measurement and processing of the field inputs. The communication systems of a typical RTU application are usually direct connect via RS485 or RS232, radio or standard telephone applications that are either land line or wireless cellular. These communications systems can, many times, be a large portion of the initial project cost and implementation, as well as long term operational expenses. Recent advances in digital cellular wireless communications have brought about a communication scheme known as CDPD, or Cellular Digital Packet Data which can greatly affect the entire scope of the project. Both system cost reduction and increased speed of commissioning can easily become significant benefits of utilizing this communication method. To fully understand the scope of the project, the following discussion will examine the various aspects of the project.

Application Description

In this project, the primary goal of the customer is to monitor, measure and control gas production, as well as the measurement and management of the produced oil and water. Remote monitoring and reporting will maximize the use of man power by allowing production managers to better direct personnel to the proper locations. This in turn will allow the number of wells per man to be increased. Additional benefits of improved production, reduced down time can easily be realized as well. To minimize start up costs, the customer chose the emerging technology of Cellular Digital Packet Data for their communications. This allowed for a quick start at minimum costs. The CDPD transceivers or modems were purchased from the local cellular provider and monthly costs per well site were set up to be charged on data volume as opposed to air time. The host application also utilized a CDPD modem and a flat monthly rate for that site was applied.

Most of the sites are located in Western Oklahoma and the host application is in Oklahoma City. The host computer is located in Oklahoma City operating in a modified telephone mode of communications. Since the CDPD emulates telephone type communications, configuration was somewhat similar to standard telephone applications.

The field Remote Terminal Unit for this project was the Fisher Controls' Remote Operations Controller (ROC) Floboss 407. This unit is a stand alone unit that utilizes the latest technology in digital sensors for measurement, local operator display and data acquisition via a keypad membrane. The unit is solar powered and can perform gas measurement per AGA standards, valve control, data archival and custom logic operations.

CDPD Communications Overview

The Cellular Digital Packet Data (CDPD) network was announced in 1992 by a consortium of cellular carriers that included: Ameritech Mobile Communications, Inc.; Bell Atlantic Mobile Systems Inc.; GTE Mobilnet Inc.; Contel Cellular, Inc.; McCaw Cellular Communications, Inc.; NYNEX Mobile Communications; PacTel Cellular; and Southwestern Bell Mobile Systems. The purpose of the consortium was to oversee the development of the CDPD Specification. CDPD is a fast and efficient digital system that overlays the existing analog cellular network. Currently, there are over 13,000 cell sites in the US. More and more carriers are taking the steps to implement this technology as markets emerge.

In order to implement the CDPD network, the cellular service providers must install additional (or, in some cases, upgraded) equipment in each cell site. Therefore, the CDPD network will grow in an evolutionary fashion.

Differences Between CDPD and Analog Data Transmission

Some of the major differences that exist between sending data over the Public Switched Telephone Network (PSTN) or the analog Cellular Network and CDPD are shown below :

Architecture Description

CDPD is an open architecture based upon current, proven data networking technology. This open architecture :

• Utilizes common, off-the-shelf networking technology;

• Is data oriented communications , rather than voice, oriented;

• Is secure by using sophisticated encryption techniques;

• Supports multiple, connection less protocols such as Internet; and

• Provides standard interfaces to user applications.

CDPD was designed to work with current data networks. This telephone emulation allows it to be used by many existing software applications that use dial up methods. This, in turn, provides seamless connections to most host computers. Also, the implementation of CDPD uses existing network hardware and proven network technology. Many of the components in the CDPD infrastructure are commercial off-the-shelf products.

CDPD was designed to minimize the impact on network software by not requiring any changes to the higher network protocols. This allows users to take advantage of the CDPD network with little or no change to their current applications. In addition, innovative frequency-hopping techniques minimize the impact on cellular voice traffic. Also, unlike current cellular voice technology, CDPD provides seamless service while roaming; there is no need to dial a roamer access number to gain access to the CDPD network.

CDPD in its most basic form can be viewed as a wireless extension of any existing TCP/IP network. It allows mobile workstations to communicate with host computers to retrieve sales data and communicate inventory levels. CDPD enables applications to monitor remote devices such as pumping stations, oil and gas leases and pipeline valves. CDPD can also send dispatch information to vehicle fleets. Of course, by allowing wireless access from anywhere to anywhere, CDPD helps to enable applications not otherwise possible.

Because the CDPD network is connection less, each and every unit of user data (a packet) is a self-contained entity. The network routes and handles each packet independently of every other packet in the network. In the event that the network loses a packet, the end-to-end connections are able to recover the packet just as they do in current wired networks. CDPD allows packets to be of variable size. The maximum packet size is 2000 characters. Since each CDPD packet contains "overhead characters" associated with ensuring the proper routing of the packet, it is important that applications designers make the most efficient possible use of CDPD packets.

Channel Hopping

The data packets sent by CDPD are meant to take advantage of the idle time between cellular voice calls. This methodology should provide for little to no impact on the existing voice network. The basic operation of the system allows for sending of data during idle moments, and, if while a session is occurring and a voice call is about to be initiated, CDPD will instruct the MES to acquire another channel to send the remainder of the message. This method of channel switching is often called channel hopping in the CDPD specification is accomplished faster than the voice call can be initiated. The potential for voice and data collisions are minute. In the cases where dedicated channels are allocated for CDPD, the potential does not exist.

Encryption

The CDPD Specification provides for the security of all data transmitted over the air between the MES and the MD-IS.

This security is built-in to the network and is transparent to any CDPD application. Security is controlled by off-the-shelf security software developed by, and licensed through, RSA Data Security. The CDPD Specification calls for the implementation of RC4 data security. An explanation of the exact nature of the CDPD security implementation is beyond the scope of this document.

Site access to the Remote Operations Controllers also require site specific address and group numbers to be broadcast before a response is transmitted. Additionally, security is provided by the nature of the ROC protocol and special features of software to RTU matching that can be implemented.

CDPD Benefits

• CDPD is fast -- the airlink data rate of CDPD is 19.2 Kbps compared to the 9.6 Kbps of competitive solutions. This airlink data rate ensures fast transaction execution;

• CDPD is offered by multiple carriers -- in many markets, CDPD will be offered by both the A-side and the B-side cellular carriers;

• CDPD is open -- the architecture is based upon common standards like TCP/IP, not on one vendor's proprietary technology;

• CDPD is secure -- RSA data security is built-in to the CDPD implementation;

• CDPD uses the existing infrastructure -- since the existing cellular network is already established, CDPD does not require the tremendous up-front costs of installing a new physical network. In addition, the infrastructure to service and to maintain the network is already in place; and,

• CDPD is scheduled for nationwide coverage

Specific CDPD Communications Requirements with the Fisher ROC

This project was dependent upon the ability of the ROC to perform its remote communication utilizing the Cellular Data Packet Data system provided in Western Oklahoma. The customer chose CDPD to minimize the start up costs for the communications system. The cellular carrier will initially charge a flat rate per site for communications and the modem, antenna and coax were billed to the customer per site. Ultimately, the CDPD charges will be by the byte of data sent and the anticipated charges are in the $15.00 per 100Kbyte range, with additional data at lower rates per 100Kbyte.

CDPD service in central Oklahoma and the link to Western Oklahoma at the time of the project start, was not yet completed. This facilitated installing a leased line communication link between Oklahoma City and the nearest tower that serves Western Oklahoma, about 45 miles away. This extra communication link added its own problems at first because of excessive noise on the leased line. Subsequent testing has shown it also adds additional time lags that must be compensated for in timing settings in the software.

Ultimately, the communication system will grow to a direct link to the CDPD network via router and leased line. As software driver improvements are made, the ROCs will be accessible via the internet when the entire system is completely networked. Due to the fact that the access to the sites is via internet protocol addressing scheme, the communication driver will have to allow for internet addressing schemes to be attached to each individual polling record. ROC addressing is still configured as unique to allow for data sorting, but since each CDPD modem has its own individual address, ROC's with identical address and group do not respond.

Modem Setup

CDPD modems by such companies as Cincinnati Microwave, Novatel, Sierra Wireless and others, utilize AT command configurations similar to those found in standard Hayes compatible modems. The user must be familiar with the instruction, configuration manual provided with the modem. Although the commands are sent to profiles via standard communications software, the CDPD configurations are unique and must be customized to the particular installation and the existing cellular coverage. The standard factory defaults must be modified for use with the Fisher ROC. Personnel working with the modem setup must be somewhat familiar with AT command sets and how they affect the communications. The instruction manual provided with the modem explains all of the commands in detail. The primary concerns during configuration were timing issues and automatic reset of the modem should a communication session fail.

Modem Operation

Once the modem is configured properly, it can be attached to the ROC with a straight through 9 pin DB9 cable. The communications driver is configured for telephone dial out. Although the bulk of the communications will occur with the Intellution FIX software, it was desirable to have Fisher GV101 software operational as well. Initialization strings should recall the saved profile and the telephone number is substituted with the Internet Protocol address such as ATDT 204.20.145.365/2100. Hang-up commands are similar as well, typically invoking the command mode with the escape sequence and then a hang up command, +++ATH0.

Keep in mind that there is no actual ringing and modem connection tone like in phone modems, the connection is very quick, hence the term connection less. The modems have to be recognized or "registered" by the cellular network. Modem testing can be accomplished by what is called "pinging" . The modem is connected to standard communication software such as PROCOMM or Windows Terminal. The command AT*P, then the IP address of the switch or the modem you are calling from is entered. This will send out a signal to itself or the CDPD switch. Timing information is returned as well as number of packets sent and received. This is a very valuable tool to diagnose modem troubles and to assist in timing issues. For example, in this application, the round trip ping from the host computer, to the switch , to the ROC and then back to the switch, then back to the host averaged about 1.1 second. This delay had to be incorporated in the retry delays so as not to re-transmit prematurely. Additional information can be found in the user manual and from the local cellular provider.

Timing Issues

Timing issues with the system are especially important and can vary with each system. Critical to the operation in this system was setting the time before retries long enough to inhibit retries before the message made it to the ROC and then answered back to the host. Direct modem to modem communication called for 2.5 seconds delay before retries. Any time shorter than this allowed for small data exchanges but for larger database transfers such as historical database retrieval or in the case of GV101, the AGA report, the longer time before retries prevented the polling device from retrying too quickly and interfering with the ROC response. When this system was installed and the added delays of the leased line were considered, the retry delay was extended to 3.5 seconds for adequate data exchange especially for database and AGA report downloads .

Allowance should be made to allow for enough retries so as to not fail the poll cycle. Testing should be performed to determine how robust the communications are. Signal strength, tower location, communications traffic, and communications equipment glitches can all lead to intermittent comm failures and the need for retries.

Power Description

Most of the sites in this project utilized solar electric generators. Sites vary in power requirements and use either 60 watt or 83 watts of solar cells and either one or two 98 amp/hour gell batteries. Typically, it is desirable to have 10-15 days operation without sun, so appropriate battery reserves must be available. Future sites will also use wind generators charging batteries. Wind generators from Southwest Windpower are small, simple units with built in battery charging regulators. These units have few moving parts and feather themselves under varying wind speeds.

One of the operational characteristics of the CDPD modem transceivers that must be considered in power sizing is their power draw. CDPD transceivers are available with a 3 watt RF output or .6 watt RF output. The power requirements of the 3 watt unit on transmit vary depending upon signal strength but can attain 2.5 amps at 13.8 volts DC. The standby or receive current draw is typically 200-300 ma. The lower RF wattage units more typically draw current in the 200 ma to the 1 amp range. Due to the typically small transmission times, one must consider the duty cycles and size the power accordingly or consider the lower power unit if the conditions warrant. The units can be power cycled, but rapid on off times are not recommended. Polling access times must be programmed into the host application to coincide with the on cycles of the remote sites if power cycling is used. Depending upon the cellular area, each device must be "recognized" or be registered by the cellular network and this can take up to a few minutes. Therefore, the rapid power cycling familiar to those using radio must be modified accordingly to longer on and off cycles.

Host Interface

This project incorporates the Intellution host software and ROC driver. The host system is configured to auto poll the sites over the CDPD network and data will be viewed at the host station as well as be transferred into an existing company database via DDE links and ODBC database linking. Demand polling can also be used for valve control, demand polling of site data, and historical, events and alarm log downloads. A feature of the ROC and Intellutions driver, configurable opcode tables, also allowed for data packets to be custom configured and optimized, which also improves communications.

The ROC driver was configured much the same as a telephone application. In the individual ROC configurations, the IP address was substituted for the telephone number. Timing parameters had to be adjusted to take in account the turnaround of the CDPD packet transmission, much the same as radio transmissions. The typical access and data acquisition per site took about 10 to 15 seconds. This will vary from site to site and may be considerably longer for demand polls of complete historical databases. The Fisher ROC maintains daily, hourly and a minute database for all points configured. The daily and hourly datebase supports up to 35 days of data archiving and the minute database stores minute datapoints for the previous hour which can be polled hourly for increased resolution for data analysis.

Remote Terminal Unit

The Fisher Controls' Remote Operations Controller (ROC) Floboss 407 is the remote terminal unit, electronic flow measurement unit used for this project. The FloBoss 407 provided for multiple meter run capabilities per site, input/output for valve control and a Highway Addressable Remote Transducer (Digital Communications, HART) interface module which provides up to 10 smart inputs for pressures, and tank level monitoring. The keypad on the Floboss will allow for local interface at the site by operations personnel. Data set point changes, data retrieval are just a few of the benefits of this keypad. Laptop computers also can be used locally for complete setup and establishment of security logins to help in management of access of the site by appropriate personnel. Additionally, the configuration software provided with the units, was loaded onto Hewlitt Packard Palmtop computers for an additional savings and a more compact local user interface

Measurement & Control Elements

This project incorporates the latest aspects of "smart" transmitter technology. The limited I/O of the Floboss required the use of the HART Interface Module. This module allows for up to 10 smart Rosemount transmitters to be used for measure tank levels and pressures. Rosemount 1151 smart transmitters were used for tank level measurement, tubing pressure, and casing pressure. The Floboss supports up to four Multivariable Sensors for the orifice meter runs, and the largest site will have three meter runs configured.

Valve control will vary from site to site but will mostly be performed with a latching 12VDC air solenoid. Two discrete outputs apply forward and reverse polarity to the solenoid causing it to latch in either the open or vented mode. This will in turn, control diaphragm pressure to an on off valve on the gas production unit.

Some sites will use the sophisticated Proportional Integral Derivative Loop Control available with the ROC. This control algorithm can be set up for simple single condition applications as well as dual, override modes which allow for a secondary set of control conditions when parameters are met.

Nomination control for daily production control can utilize the PID loop control as well as the Function Sequence Table custom programming tool available with each ROC. This programming language allows for rapid on line custom logic programming to meet a wide range of operator requirements.

Typical Automation Benefits

Although this particular project is in its early stages, a number of automation projects have shown many benefits that justify their investment. In similar ROC automation projects producers have seen the following benefits:

In the Mid-Continent Area

• Information gathered electronically from each well has increased the overall flow and improved the flow balance for the field.

• Automating the State of Kansas Well Test has improved allowables over manual testing

• Short-term gains are being achieved by increasing or decreasing production for the spot market using the flow control capability of the ROC

• The ability to control production from each well and of the overall field assures the producers customers of consistent gas delivery, which has gained it new customers.

• Field operators are now responsible for 50% more sites than before automation, saving the company in contracted services.

• Automated reporting saves operators from unnecessary travel to sites, reducing their "windshield" time and the company's costs for road clearing and maintenance in winter months.

• Production information used by the accounting department increases billing accuracy and reduces the billing cycle time by up to two weeks.

• The engineering department use production information to analyze well performance for maintenance scheduling and for gas reserve analysis.

• In February of 1996, when temperatures were below zero for days, a production company in Southwest Kansas was able to deliver 95% of their planned production while less automated producers delivered only 65% of planned production. Also, they could document compliance to contractual agreements with gas distribution customers. This event demonstrated the value of the system in minimizing lost revenue and operating costs, and achieving a high level of customer satisfaction.

In Canada

• Production increases of 2% to 30%

• Power consumption reduced by 11% to 20%

• Down - hole equipment failure decreases by 17% to 44%

• Surface maintenance and repair costs dropping by 5% to 40%

• Reduced driving and vehicle costs of 31% to 85%

• Overtime and contract labor reductions of 36% to 65%

Summary

This project was successful by maximizing the usefulness of the FloBoss and incorporating it into the CDPD communication system. Each communication system is unique, but this is somewhat typical of what one may find. More work is needed in developing communications drivers as this technology evolves. All indication from other EFM and RTU manufacturers indicate that this method of communication will be a valid player in the growth of the SCADA, EFM, RTU market.

(A version of this article was published in the The American Oil & Gas Reporter.)