Mark A. Westhoff
Colorado Interstate Gas Co.
Colorado Springs
Colorado Interstate Gas Co. has successfully integrated its transportation and exchange (T&E) gas accounting and tracking into its planning process for daily operations through a set of integrated computer programs called the Transportation and Exchange Gas Information Management System (T&E IMS).
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.
Part 1 (OGJ, Mar. 5, p. 63) described the background of the system's development.
NOMINATION PROCESS
Scheduling transportation gas begins with a nomination from a shipper (Fig. 1).
The nomination consists of contract number, a receipt location and amount in millions of British thermal units, and a delivery location and amount also in MMBTU. Both receipts and deliveries are verified with the appropriate interconnecting pipelines.
With receipts and deliveries confirmed, nominations are ready for input to the T&E Ims.
As they are entered into the system, the nominations are automatically run through a series of checks. These checks include:
- Contract must exist.
- Nomination must be for service which is to occur between the contract's effective and termination dates.
- Receipt and delivery locations must be authorized under the contract.
- Initial service for a contract at a particular receipt or delivery point must be accompanied by appropriate reports to meet regulatory reporting requirements.
Nominations are recorded only after they successfully pass the checks.
With all nominations entered, throughput calculations are made to determine if the system's capacity constraints have been exceeded.
If a constraint is exceeded, curtailment of interruptible transportation services is performed. All interruptible transportation flowing through the constrained point must be queued according to priority.
Priority of interruptible services is based on transportation rate (/thousand cubic feet); the higher the rate, the higher the priority.
Priority of service is obtained from the transportation and exchange department. Interruptible transportation is curtailed until requirements no longer exceed capacity.
Affected shippers and interconnecting pipelines are notified and nominations at receipt and delivery points are adjusted. In addition, a record is kept of those nominations for which complete service cannot be provided.
All nominations in the T&E IMS are available for company-wide dissemination. Daily briefing reports for gas controllers and management are generated.
Transport-rate analyses can be automatically performed for the marketing department.
Capacity analyses are available to engineering and marketing development departments.
And once each month, the entire month's transportation nominations are provided to accounting for billing.
PLANNING PROCESS
The gas-control planning system (Fig. 2) is designed to forecast loads, schedule supplies, schedule storage, and simulate pipeline operations.
On any given day, T&E volumes can amount to between 50 and 95% of system throughput. Therefore, the nominations in the T&E IMS data base are a major source of input data to the planning system.
MODELING
The planning process begins with a transfer of data from the T&E planning system interface file to the planning system's data bases. Any last-minute changes can be made with manual overrides. Other planning system support files are then readied.
With the file set-up complete, modeling of the next 48 hr of transmission system operations can begin.
FORECASTING, SCHEDULING
The first step in the modeling process is forecasting loads.
The gas-load forecaster generates hourly load profiles for every modeled delivery point on the transmission system and estimates fuel to be used in compression.
Most load profiles are generated in one of two ways. Weather-sensitive loads are predicted by a time series. Deliveries at interconnects with other pipelines are modeled as a function of current scada data and nominations.
Before the era of open-access transportation at CIG, weather-sensitive loads were almost entirely sales gas. Today, significant portions of these loads are supplied with gas which is transported on behalf of a local distribution company (LDC).
While this does not alter the predicted load profile which is used in the transient-flow simulation, it does affect the scheduling of supplies.
The time series which is used to forecast weathersensitive loads incorporates past weather conditions and loads as well as weather predictions. The weather predictions include ambient air temperature, wind, and cloud cover.
Predicted total load is then separated into sales and transportation by the subtraction of transportation gas from the total load. CIG must schedule supplies for the sales portion of the load.
Modeling deliveries made to interconnecting pipelines is a two-step process.
In the first step, the gas-load forecaster obtains from the scada system the latest measured flow rate at the interconnect.
The new nomination and the time at which it takes effect are then obtained from the T&E IMS in the second step.
The flow rate at the interconnect is assumed constant at the measured rate until the new nomination is to take effect. For all times after the new nomination takes effect, the flow rate is assumed to be equal to the new nomination.
Company supplies are scheduled after loads are forecasted.
T&E receipt nominations are scheduled first. Profiling T&E receipts is accomplished by following the procedure described for modeling delivery nominations at pipeline interconnects.
On most days at least one or more shippers deliberately nominate receipts in excess of deliveries or deliveries in excess of receipts. Such nominations are intended to correct an imbalance which accrued at an earlier time.
Imbalances are scheduled as pipeline-operating conditions permit. The overall daily imbalance for T&E has a direct effect on the scheduling of CIG supply.
When T&E receipts exceed deliveries, supply requirements are reduced by the amount of the imbalance. Supply requirements increase one for one with the imbalance when T&E deliveries exceed receipts.
The remaining supplies are scheduled to meet prevailing operating requirements and management objectives. Purchased and company-owned production is scheduled in such a way as to hold to a minimum of swings from one level of production to another.
Production rates are based on a list of supplies arranged by priority. Storage injection and withdrawal volumes are scheduled partially to compensate for daily variations in the weather-sensitive loads.
At the same time storage guidelines for injections and withdrawals are followed within a prescribed tolerance.
SIMULATING PIPELINE OPERATION
The pipeline response to forecasted loads, scheduled supplies, and desired control strategy is simulated with a transient-flow model.
The purpose of the simulation is to verify the feasibility of the basic plan and to test alternate control strategies in an effort to determine the most efficient approach to operating the pipeline.
At times, transportation-gas nominations for an LDC exceed the total load that CIG forecasts for its market area. Such instances are the subject of reports which are reviewed during the planning process.
If the amount of the predicted overnomination is relatively small and the LDC owes CIG gas, no action is taken. In extreme cases, the LDC is required to lower delivery nominations and corresponding receipts so that nominations do not exceed the forecasted total load.
Imbalances are a fact of life in the gas-transportation business. Correcting imbalances accrued in prior months is an ongoing effort.
Many factors affect determining the amount of gas CIG can take or deliver to rectify standing imbalances. Among these factors are expected loads, desired field production levels, and storage guidelines.
With the aid of the planning system, quantifying all gas received and delivered is , a daily practice at CIG. When all gas entering and leaving the system is quantified, T&E imbalance gas can be scheduled.
Pipeline capacity can be determined with reasonable accuracy with the use of transient-flow simulations of pipeline operation. As transportation volumes increase, capacity is being reached in various pipeline segments with increasing frequency.
When a segment's capacity is reached, all interruptible transportation gas flowing through that segment must be queued by priority and curtailed until requirements do not exceed capacity.
Accurate capacity estimates for prevailing operating conditions are important. Curtailing too much gas results in a loss of revenue. Overscheduling gas results in imbalances.
MONITORING T&E
As a matter of operational imperative and quality of service, monitoring T&E activity occurs at CIG. For a pipeline of CIG's size and storage capability, large variances between daily nominations and actual flow rates cannot be tolerated.
Quality of service in the gas transportation business can be measured in part by the accuracy with which volume setpoints are met. The size of imbalances also serves as a measure of service quality.
Currently, routine daily reports are generated which compare the previous day's activity at receipt and delivery points, as measured by the scada system, with corresponding nominations.
The previous day's energy, either received or delivered, is calculated at selected points by multiplying the previous day's volume by the volume-weighted average BTU. The result can be compared directly to nominations which are recorded on a thermal basis.
Variances are accumulated for the month and used as a guide in adjustments to setpoints to reduce imbalances.
Production which enters the transmission system from gathering systems is monitored daily. The total volume received from a gathering system is compared to the total of requested CIG supply and nominations for transportation.
Variances are identified by gas control and referred to gas supply. Gas-supply personnel, working with field dispatchers, identify producers who are involved with the variance.
If transportation volumes are involved, the producer is given the option of altering production or nominations.
Previous-day sales calculations are routinely performed for the major LDC's connected to CIG, The automated calculation routines use metered volumes and transportation nominations.
When metered volumes fall short of nominations, an imbalance results because the gas received on CIG's system for the LDC is not physically delivered to the LDC. In such cases, the LDC is notified and, if necessary, receipt volumes are reduced to prevent additional imbalances from occurring.
Nominated imbalances are tracked daily. An unanticipated imbalance can indicate a data-entry error. Monthly accounting statements of actual imbalances are not typically available before the 20th of the following month.
Since the last accounting statement, it is assumed that what is nominated actually flows. Current imbalance estimates can then be used to evaluate shipper requests for imbalance gas.
SYSTEM RESULTS, FUTURE
Compared to 1986, gas control processes three to four times the amount of T&E volumes for approximately eight times as many T&E contracts. The increased workload has been managed without an increase in staff.
The productivity gain is due in large measure to the Transportation and Exchange Gas Information Management System.
Developing software to support an emerging business function requires an approach which differs from that of a standard data processing application. The development of the T&E IMS was an evolutionary one carried out by the eventual users of the system.
The result is a highly modularized dynamic system which can be upgraded quickly and easily to handle the next new requirement of the gas transportation business.
Some of the new requirements which CIG will soon face include open-access storage and transmission-system expansions.
Successful integration of the T&E IMS with the scada system and gas-control planning system is essential if nominations are to be exploited as a source of planning data and T&E activity is to be monitored.
The systems have been successfully integrated with a minimum of effort because of two simple design criteria: The system has been developed on the same machine as the scada and planning systems, and all three systems share file structures which are compatible with VAX utilities, in particular DATATRIEVE.
The future of the system is governed in part by the fact that transportation volumes at a particular location can vary substantially from day to day. And the number of points at which sizable transport volumes are received and delivered is increasing.
Scada system alarm limits must be set manually. As a result, scada-alarm limits for transportation volumes cover a range which is necessarily broad. The range is typically so broad that the associated alarms are of little practical use.
Automatically updating volume alarm limits at transportation receipt and delivery points is actively being pursued. The idea is to calculate tolerance intervals about the total nomination at a particular point.
Scada-system alarm limits would then be automatically updated. The alarms could then serve as a valuable tool in reducing operational imbalances.
ACKNOWLEDGMENTS
This article was possible only with the support of the staffs of various departments at Colorado Interstate Gas Co. Their timely assistance is appreciated. Also appreciated is the permission to develop and present this article which was provided by CIG and Coastal management.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.