GAS CONTROL-1 SYSTEM INTEGRATES SCHEDULING, MODELING, MONITORING

Mark A. Westhoff
Colorado Interstate Gas Co.
Colorado Springs

Tracking and accounting for shippers' gas on Colorado Interstate Gas Co.'s pipeline system have been greatly improved by development of an integrated set of computer programs to schedule, model, and monitor transportation and sales gas.

Based on a DEC VAX 11/780 computer, the system consists of a transportation gas information-management system, a supervisory control and data acquisition (scada) system, and a gas-control planning system.

This first article of two on the system describes the background to the system's development and sets out its components.

The concluding article will cover how the system operates.

CHANGING REGULATIONS

May 1985 marked a turning point in the U.S. interstate natural gas pipeline business. Up to that point pipelines acted as merchants of gas.

Pipelines purchased gas from producers then moved the gas to market and resold it. The U.S. Federal Energy Regulatory Commission (FERC) changed all that with its Notice of Proposed Rule making, Order 436.

The new regulations have transformed most major interstate gas pipelines into common carriers of natural gas. Colorado Interstate Gas Co. (CIG) has made the transition from merchant to transporter, accepting a blanket open-access certificate under FERC Order 500 in 1988.

The change of roles has had a dramatic impact on pipeline operations.

As a merchant of gas, CIG forecast all its loads and scheduled all supplies. Gas transported for others was a relatively small segment of the business.

Supplies were scheduled to meet loads with storage requirements taken into account. The entire process was under virtually complete control of CIG.

Volumes transported by CIG for others tripled from 1984 to 1988. Sales gas fell 60% over the same period (Fig. 1).

Local distribution companies (LDC's) and other traditional sales customers contract with producers to purchase gas, then contract with CIG to transport it.

In addition, volumes moving on to interconnecting pipelines for third parties have increased substantially. Much of the control over where gas moves and in what quantities has passed from CIG to the companies contracting for transportation services.

To understand this control mechanism, we must first review some important terms long familiar to the common-carrier industry but somewhat new to the natural-gas transmission business. As with any business, gas transportation has its own vernacular.

"Receipts" are volumes entering the CIG mainline. "Deliveries" are volumes leaving the mainline.

"Shippers" are those parties who have contracted for transportation service with CIG. " Nominations" are specific requests for transportation service which CIG receives from shippers.

"Transport gas," "transportation gas," and "T&E gas" are all the same thing. It is gas which CIG transports but is owned by someone other than CIG.

OPERATIONS SOFTWARE

A discussion of software which supports T&E functions at the operations level best begins with a description of the pipeline and its operating characteristics.

The operational approach to transportation gas is, in large part, dictated by the pipeline's capabilities and tariff.

Pipelines with large storage capabilities, for instance, may be in a position to accept monthly transportation and exchange (T&E) nominations which are not balanced on a daily basis. That is to say, transactions may be such that receipts and deliveries are even for the month but do not necessarily coincide equally on the same days of the month.

Nomination data must be structured so as to monitor balances on whatever time basis required.

CIG operates a natural-gas pipeline of approximately 3,000 miles. The pipeline extends from the prolific Overthrust Belt in southwestern Wyoming to the Hugoton field in Kansas and Panhandle field in Texas (Fig. 2).

Transmission system compression amounts to nearly 200,000 hp. Line sizes range from a 2-in. lateral to 26-in. mainlines. The majority of transmission lines are between 20 in. and 24 in.

CIG also operates approximately 3,000 miles of gathering facilities located in Utah, Wyoming, Colorado, Kansas, Oklahoma, and Texas. Field compression amounts to nearly 120,000 hp. Almost 3,000 wells are connected to the various gathering systems.

Weather-sensitive loads occur on the pipeline from Cheyenne, Wyo., to Raton, N.M. Gas is sold to distributors such as Public Service Co. of Colorado. Direct sales are also made to customers such as the City of Colorado Springs.

CIG's sources of gas are located in the Panhandle field of Texas, the Keyes field and Mocane-Laverne area of Oklahoma, and the Greenwood and Hugoton fields of Kansas.

A number of fields are located in Wyoming and Utah. Smaller volumes come from sources in eastern Colorado.

Transportation gas is received from gathering systems connected to the mainline as well as from interconnects with other pipelines. Deliveries of transportation gas are made to LDC's and end users connected to the mainline and to interconnecting pipelines.

Major interconnects with other pipelines exist in Wyoming at Green River (Northwest Pipeline), Kanda (Questar), Riner Road (Williams Natural Gas), Elk Basin (Williston Basin Interstate), and Cheyenne (Wyoming Interstate Co.).

In Kansas and Colorado, CIG has interconnects with Panhandle Eastern Pipeline. Major interconnects at Beaver (ANR) and Forgan (NGPL) are located in Oklahoma. Interconnects at Tumbleweed (Transwestern), Big Blue (El Paso Natural Gas), Dumas (Northern Natural Gas), and Bivins (West Texas) are located in the Panhandle of Texas.

OPERATING CHARACTERISTICS

Peak pipeline throughput is approximately 1,800 MMcfd. Average daily throughput is between 900 and 1,000 MMcfd.

Linepack ranges from 1,200 to 1,500 MMcf, i.e., usable linepack amounts to 150 MMcfd. Usable linepack is little more than 15% of average daily throughput.

Keeping transportation gas reasonably balanced on a daily basis (receipts = deliveries) is key to operations at CIG.

Supplementing CIG production in the heating season is nearly 600 MMcfd which can be supplied by four company-operated storage fields. Latigo, Boehm, and Flank storage fields are base-loaded on withdrawal. The Ft. Morgan storage facility is available primarily for peak shaving.

All storage facilities are operated on annual injection and withdrawal schedules. Typically, injection season runs May through October, and withdrawals are made November through April.

Capacity is reached at individual receipt-delivery locations and in a number of segments on the CIG mainline at various times of the year. The Wyoming line from Green River to Cheyenne operates at or near capacity in winter. Gas flowing east from Watkins can approach capacity at any time during the year.

And the line from Kit Carson to Campo is at capacity nearly every day in the summer months.

T&E GAS INFORMATION

Transportation volumes in 1987 nearly doubled those of 1986. And in 1988 the volumes transported were more than two and one-half times 1986 levels. Transportation contracts were increasing in number in a similar fashion.

This growth of transportation activity was not unexpected. Order 436 was issued by the FERC in May 1985.

Interstate pipelines were faced with an emerging business function they had to perform in a new competitive environment. Automated systems were essential for managing transportation gas and are just as essential today.

But developing automated systems to support an emerging business function, such as gas transportation, poses unique challenges. CIG's experience in developing operational tools to manage transportation gas affords a case study in one company's approach to such challenges.

In January 1987, transportation nominations were being managed by way of two spreadsheets which were developed on a personal computer. Nominations were transcribed by hand to forms which in turn were processed by the accounting group for billing. Daily briefing materials were generated mostly by hand.

The existing system was clearly inadequate. An integrated information management system was required quickly.

The objective of the information management system is to provide the gas-control department with a mechanism to manage daily transportation activity. Managing transportation activity at CIG means daily balancing of receipt and delivery volumes for T&E, sales, supplies, and storage while meeting management objectives.

Management objectives include fulfilling our obligations under the tariff, maximizing throughput, managing the weighted average cost of gas, maximizing operating efficiency, and meeting storage injection and withdrawal guidelines.

T&E IMS DESIGN

Design of the transportation and exchange gas information management system (T&E IMS) is predicated on a few simple criteria.

First and foremost the system must be simple from both hardware and software points of view.

At an early stage in the system's design, the system's key elements were defined as:

Single point of entry for nomination data
Efficient transaction processing
On-line contract abstracts
On-line nomination summaries by contract and location
Automated briefing materials and ad hoc reports
Interface to scada system
Interface to the gas-control planning system
Interface to the IBM mainframe accounting system.

The original plans for the T&E IMS called for CIG to install a system similar to one which was under development at ANR which runs on a PC network. (CIG and ANR are both subsidiaries of the Coastal Corp.)

In the spring of 1986, however, CIG acquired two Digital Equipment Corp. (DEC) VAX 11/780 computers as part of a project to upgrade the existing scada system and develop an automated daily operations planning system.

The planning system would become known as the gas-control planning system.

Sufficient capacity existed on the VAX computers to accommodate the T&E IMS. By developing the T&E IMS on the VAX computers, CIG could avoid an expenditure for a PC network. Thus, the decision was made to use the VAX computers for the T&E IMS.

Most applications the size of the T&E IMS require a project during which specifications are written by a systems analyst. The analyst returns to his shop to write the software and in several months returns with the finished product.

With an emerging business function such as gas transportation, the analyst's finished product would likely be in need of substantial enhancement even before it is implemented.

EVOLUTIONARY DEVELOPMENT

The development process for the T&E IMS has not been traditional. The gas transportation business is highly dynamic. The rules of the game are constantly subject to change. These changes seldom occur dramatically; rather most occur incrementally.

The net result is that T&E IMS development must be evolutionary.

The evolutionary approach began with a prototypical system which was quickly developed and implemented by gas-control personnel in the spring of 1987.

Written primarily in DATA-TRIEVE, the prototype met each of its basic design criteria. As the system is used, suggestions for system improvements are taken from the users. Software routines are added, modified, or discarded to accommodate user requests.

At first, system modifications were substantial, including one revision to the data base design. Over a period of about 6 months, suggestions decreased in frequency and substance.

Today, system modifications are made as business needs dictate. The system's evolutionary development has resulted in highly modularized software. Software routines are typically short and very specialized.

Implementing the T&E IMS on the same machine as the scada and planning systems greatly simplifies the process of interfacing the three systems. Further simplifying the interface process was a decision to make file structures in all three systems compatible with DEC VMS utilities, in particular DATA-TRIEVE. Interfacing all three systems is now a matter of routine.

DATA BASE

The T&E IMS data base contains five basic data types (Fig. 3):

Contract abstracts
Nominations
Pipeline data
Scada-system interface cross-references
Gas-control planning system interface cross-references.

All data are maintained by gas-control personnel.

Contract data are provided by the transportation and exchange department. Abstracts of each contract are entered into the data base. The information is used to verify transactions and to assist customers in completing transactions.

Nomination records in the data base can be written and modified only if the transaction can be validated with appropriate contract data.

Gas-control personnel must enter just those nominations which change on any given day. If no change is entered, the nomination is assumed to be the same as that of the preceding day.

Pipeline data in the T&E IMS consist of pipe segment and facility-capacity information. Transportation receipt and delivery points are identified with the segment of the transmission system to which they are connected.

Segments are generally defined from an upstream compressor station to a downstream compressor station. The pipe segment cross-reference can be used to render capacity analyses and to perform curtailment of interruptible transportation.

Individual receipt and delivery-point facility capacities are also maintained in the T&E IMS data base.

Cross-references to the scada system take two forms.

The first involves receipt and delivery points which are identified with specific record keys in the scada data base (e.g., current flow rate, prior day's volume, month-to-date accumulated energy).

And the second involves area keys in the scada data base which are cross-referenced to receipt and delivery points to render all data in the same area as the location of interest.

T&E IMS contract data are irrelevant to the gas-control planning system as they are with the scada system. The planning system simply requires the total volume of T&E nominations at the receipt and delivery points which are being modeled.

Receipt and delivery points in the T&E IMS data base are cross-referenced to the modeled points by means of a simple table. Those responsible for planning can edit the table to change how receipts and deliveries are modeled.

USER INTERFACE

The T&E IMS nomination system is completely menu driven. Written in DATA-TRIEVE, it provides immediate access to all T&E IMS data files as well as direct access to the scada system's data base. From the main menu, more than 35 functions and 50 reports are available.

An effort has been made to reduce keystrokes required to enter and access information. Menus rarely are more than two deep; that is, the desired action is taken within the first two instructions from the user.

Accounting codes for locations, which are integral to the T&E IMS data base, consist of 12 characters. To avoid entering these codes, gas control developed a convention which assigns a three-character location code for all receipt and delivery locations. Typically these codes are the first three characters of a location name.

The three-character codes are cross-referenced to the 12-character accounting codes. The T&E IMS user need only enter the three-character code to enter and access nominations for a given location as well as to access related scada data.

The accountants are left to their own devices.

On a typical day, four gas-control T&E personnel are logged on to the system. Available to them is a variety of information which falls into three categories: contract information, nominations (both scheduled and curtailed), and real-time scada data.

Contract information is used to screen nominations and to assist shippers in making valid requests for service. Contract abstracts in a variety of formats can be called to the screen.

Validity checks are made as nominations are entered. When a nomination fails any validity check, contract information is supplied to identify problems with the nomination.

Nominations are recorded at the contract-location level and can be viewed in a number of different formats. Balance sheets containing all nominations for receipts and deliveries for a contract can be accessed to verify daily and monthly activity.

Balances can be obtained by shipper which are useful in monitoring shippers who move large volumes under many different contracts.

Of greatest interest operationally are summaries of nominations by location. Location summaries form the basis for volume set points at interconnects, monitoring of transportation volumes, and transportation planning.

SCADA INTERFACE

Providing an interface between the T&E IMS and the scada system was relatively straightforward (Fig. 4).

Both systems were developed on and for the same computer, a DEC VAX. File structures in both systems were developed to be DATA-TRIEVE compatible.

Therefore, despite the different languages used to develop each system, accessing data contained in each of the system's data bases is a routine matter.

General scada data queries provide to the T&E IMS user all telemetered data for an area of interest. An area key defined in the scada data base is used in the T&E IMS to gather and report the desired scada data.

The scada data gathered include flow rates, pressures, BTU'S, and any other data which have been assigned to the scada area key. To make it easier for the user, the scada area key is related to the three-character location codes used throughout the T&E IMS.

The user is prompted for a location code, and the T&E IMS performs the data retrieval and reporting.

Specific scada-data queries can be made by use of the scada data record ID. The advantage of such a query is that it is much more efficient than a general query and presents only the data of interest.

For instance, the three-character location code for a receipt or delivery point can be related directly to the corresponding flow-rate record in the scada data base. When the query is made, only the flow rate is retrieved and reported.

The techniques described for general and specific scada data queries were presented in the context of interactive queries. Off-line queries for batch-processed reports use similar approaches.

More elaborate interfaces are certainly possible but have thus far not been required.

PLANNING INTERFACE

The gas-control planning system (Fig. 5) forecasts loads, schedules supplies, and models the transmission system's response to the day's scheduled activity. The planning system was developed by the CIG applied science department.

Final nominations serve as a primary source of input data to the planning system.

The desired resolution in the modeling of the transmission system dictates the format in which nominations are provided to the planning system. Nominations for receipt and delivery locations in close proximity to one another can be summed into a single modeled point.

Each modeled point has a corresponding record in the planning-system data base. A table is used to relate the T&E location codes of the T&E IMS data base to modeling point record ID's in the planning system's data base.

The table is maintained by those responsible for planning. By editing the table, receipt and delivery locations can be summed differently, new locations can be added, and obsolete information deleted.

Nominations are stored in the T&E IMS data base at a contract-location level and are signed positive for receipts and negative for deliveries. Also, the standard unit of measure for nominations is MMBTU (or decatherms).

Transportation arrangements must be balanced on a thermal basis to account for variations in the thermal content of gas. Pipeline operation and planning, however, depend upon volume information.

Each morning, prior to the planning period, a batch job executes the routine which formats the planning system's interface file. The routine must perform several functions.

Nominations which share the same modeled point are gathered.

Each nomination is converted to million cu ft with the appropriate BTU. The routine sums the nominations for the current day and the following day according to the requirements of the planning system.

The results, as well as the times at which the nominations take effect, are written to the interface file.