Matthew E. Wilks
BP Exploration
Dimlington, Hull, U.K.
The Southern North Sea (SNS) gas pipeline hydrocarbon accounting system (SHAS), a computer program, was developed to allocate and attribute supplied gas to British Gas plc in accordance with the Dimlington allocation deed (Part 1, OGJ, July 15, p. 49).
The SNS system delivers gas and condensate from several offshore fields to different customers, according to varying demand. It has been operating successfully in multi-user mode since August 1990.
The gas from each field is of different quality and price and must be allocated to sellers and purchasers on an equitable basis.
Gas sales at the shore terminal are allocated on the basis of offshore production. Substitution of gas sales by one field group on behalf of another allows considerable operational flexibility but causes complex problems of hydrocarbon accountancy.
The logic and rules of allocation and substitution are governed by the Dimlington (terminal) allocation deed (see accompanying box) and were covered in Part 1 of this two-part series. SHAS manages these rules.
SHAS PROGRAM
SHAS is implemented on a DEC VAX computer. it is one of 6 group of software systems developed for the Southern North Sea system. The SNS systems are:
- PRS-Production and reporting system
- ITD-"In-the-Day" (PCbased) system
- SHAS-Hydrocarbon accounting system
- CMI-Contract monitoring and invoicing system.
All of the SNS VAX-based systems have been developed with Ingres, a data base used by both the PRS and SHAS systems. The use of Ingres allows close integration on these systems and provides for the sharing of common data.
In addition there is a standalone computer system for supervisory control and data acquisition (scada). This is responsible for process control and data gathering.
The scada system is interfaced into the SNS systems by a VAX-based module responsible for data transfer, initial validation of data, and storage in the SNS data base (Fig. 1).
Limited data from Ravenspurn North are gathered by scada and transferred into the common data base. The data gathered are daily fiscal-metering figures for gas and condensate, hourly aggregate production data, and hourly chromatograph composition data.
It is likely that additional third-party producers will wish to transport gas via the SNS pipeline into Dimlington.
Therefore, SHAS has been implemented as a flexible system that can be adapted to meet an increase in several producing fields with a minimum of software modifications.
SYSTEM BOUNDARIES
Fig. 2 is a schematic of the parts of the operation important to SHAS. It shows the boundaries of the system.
At one end, product is delivered from producing fields into the pipeline, gas and condensate being metered separately. The other end is the disposal of gas from Dimlington to British Gas and other disposal points.
Contained within these boundaries are all points of information relevant to SHAS.
The main information requirements of SHAS are quantities and qualities.
Quantities are metered amounts of product, either gas or condensate. Qualities are analyses of the percentage presence of different components in the products. Every quantity must have an associated quality.
Quantity and quality information within SHAS falls into one of two main categories: deliveries into the pipeline system (less offshore fuel) and disposals from Dimlington terminal.
Deliveries into the system are the metered amounts of gas and condensate leaving the producing fields and entering SNS gas pipeline. Each field consists of these two product streams and the cumulative production is reported to SHAS hourly, along with the average hourly chromatograph composition for the gas stream. Day-end fiscal production totals are reported at 06:00.
Disposals from Dimlington terminal are fuel, vent, condensate to tanks, and deliveries to British Gas.
The prime source of daily information is the SNS scada system through its interface with the SNS VAX computer. The scada interface provides meter readings from all Dimlington disposal meters and from all Villages and Ravenspurn North delivery meters. The interface also supplies the gas-quality information.
The gas chromatographs on the Cleeton and Ravenspurn North platforms deliver hourly average chromatograph analyses throughout the production day.
Condensate-quality information does not come from the scada system and is input manually.
The condensate analyses are infrequent, usually monthly, and allocation of condensate quantities uses the latest available data.
There is a system of manual input (in conjunction with PRS) for all data. This provides a fallback means of input in case of problems with the scada interface, offshore communications, or spurious data.
British Gas and the pipeline system operator agree on details of the metered receipts which are entered into SHAS and used in the daily reconciliation process.
The other types of data required by the system are of a more static nature. These data comprise the master files and conversion tables required by SHAS. These fields are the infrastructure of SHAS, and it is against these that all variable data are verified.
RUNNING SHAS
SHAS allocates product disposals, such as fuel, vent, condensate, and sales to British Gas, back to the producing fields and groups. This is achieved by taking into account the quantity and composition (quality) of each field's production into the system to work out fields and group entitlements to the disposals.
There are two processing frequencies.
- Field and group allocations are calculated and reported on a daily basis. The allocation deed requires a daily allocation-attribution report to be produced within 36 hr of the production-day end.
- Monthly field and group allocations are reported for invoicing purposes. The reports are mostly summations of the individual days of the month.
The deed requires the monthly report to be produced within 7 days of the month end.
SHAS allocates the production at day end.
A PC-based system was developed, however, to ensure that BG nominations and BP Chemicals requirements were met as they varied during the day. This system, known as "in-the-Day" (ITD), is a simplified version of the Dimlington allocation deed's logic in SHAS.
ITD PROGRAM
During the production day, the ITD system operates on a PC in the Dimlington control room. It is an expert system that advises the control-room operators (CROs) and helps them ensure that the Villages and Ravenspurn North fields meet their individual targets for supply of natural gas to BG (and, in the case of Villages, to BP Chemicals in Saltend).
ITD, in its simplest terms, does this in three ways:
- Calculates the obligation and target for each field group for each customer.
- Predicts end-of-day allocated deliveries for each field group
- Warns the CRO if these two figures predict a shortfall.
In order for ITD to calculate the obligations and targets for each field group, it needs to have up-to-date data for nominations from British Gas, a forecast of offtake by BP Chemicals, and substitution arrangements between the field groups.
- British Gas nominations. The daily target for each field group will be based on weekly nominations if a daily nomination Telex is not received.
ITD calculates the notice periods where renominations are input and works out upper (targets) and lower (objectives) requirements for British Gas based on these notice periods.
- BP Chemicals forecasts. ITD can accept forecasts for the current contract day and the next contract day. It will warn CROs if no forecast has been input for the current contract day.
The forecast is adjusted by a factor based on the different calorific value of gas at the BP Chemicals offtake points and gas delivered by Villages at the British Gas delivery point at Dimlington.
The adjusted forecast is added to the BG nomination for Villages to make up the Villages obligation-target.
- Substitution. ITD can accept substitution for the current contract day and the next contract day. No distinction is made between system and notified substitution. Up to three nonoverlapping substitutions can be entered for a single contract day.
The field group obligations and targets are adjusted depending on which direction substitution is taking place. The targets/obligations for the system in total will not be altered by substitution.
ITD accepts input of production from each platform (Ravenspurn North, Ravenspurn South, Cleeton) and deliveries to British Gas as metered offshore and at the delivery point.
BP Chemicals' offtake is also input to ITD and adjusted (by the factor described previously to account for the calorific value difference).
ITD is designed to accept these data on an hourly basis. However, it will interpolate if input of intervening hours is omitted.
The delivery to BG is allocated to each field in a ratio equal to the production by each field.
The rates for the last hour (production, delivery, and BP Chemicals' offtake) are extrapolated to the end of the day to predict end-of-day allocations and offtake.
ITD compares the predicted end-of-day allocations with the targets, obligations, and forecasts.
It displays an advised production rate for each field group for the remaining hours in the day, which if maintained will cause all targets to be achieved. (This advice does not allow for a changing nomination profile; it recommends a constant rate over the remaining hours.)
ITD will produce warnings if the following events occur:
- The total system end-of-day delivery is less than the sum of system obligations.
- The Villages predicted end-of-day allocation is less than its obligation.
- The Ravenspurn North predicted end-of-day allocation is less than its obligation.
- The Ravenspurn North predicted end-of-day allocation is more than 102% of its target.
- The BP Chemicals predicted end-of-day offtake is greater than the forecast.
ITD produces hardcopy logsheets at the end of each day and on request during the day. These logsheets print data for each hour in the day and show the following:
- BP Chemicals' forecasts and actual offtake
- Targets and obligations for Villages and Ravenspurn North
- Nominations affecting the day's target
- Deliveries, production, and allocation for each field group
- Overall system targets, production, and performance
- Villages and Ravenspurn North performances against the set targets.
A subset of the logsheet information may also be displayed on the screen, both in tabular and graphical form.
ITD uses parameters Specified in the contracts to perform its calculations. If any of these contract parameters change (for example, the daily contract quantity), ITD must be updated accordingly.
OPERATING EXPERIENCE
The SNS pipeline system would not have been able to operate without the ITD and the SHAS systems, which have been operating since August 1990. They have prevented shortfalls and have allowed the pipeline system operator to make full use of all the contract provisions (for example, substitution).
ITD
The ITD program performs many calculations that are impossible by hand in the time available during a production day, especially when substitution and changing nominations occur.
In case of program failure, a manual logsheet is available, although this becomes extremely complex when nomination changes happen.
The PC program prints out all the log sheets which the CROs previously filled in by hand. This has released the operators for other terminal duties.
Because of the short time between completion of the allocation deed and the start of commingled production, the first versions of the program were not fully tested. A few program bugs became apparent which caused the CROs to lose confidence.
This confidence was regained by tailored training sessions which cleared up misconceptions of the system's capabilities.
The introduction of ITD has been a fundamental change in the CRO's job.
SHAS
The SHAS program is a contractual requirement of the pipeline system operator as part of its duties. The main operational problem is the quantity of data involved and its manipulation.
The program uses a large quantity of data transmitted from offshore. These are vulnerable to corruption, loss, and delay.
Initial problems were experienced with incorrect data being transmitted from the fields or, conversely, the correct data being transmitted but being stored in the incorrect area of the VAX memory.
Usually, whenever data corruption occurs, a "-99.9" figure or a blank is stored. The program logic prevents the calculations from running if these exist.
The -99.9's or blanks must be corrected with data faxed from offshore or obtained from the terminal control room (if the problem is at the delivery point). This can be extremely time-consuming if a day's data are missing; that is, 192 entries must be made for a field's composition analyses alone.
The worst-case scenario of data loss has already occurred.
All offshore transmissions are routed via land line to BP's VAX in Aberdeen.
In the second month of commingling, the Scarborough telephone exchange was gutted by fire.
The resulting data loss of 4 days took 3 weeks to correct.
The SHAS program must be able to transmit data to British Gas with a "mailbox" so that British Gas can check the results with a spreadsheet. This has caused problems, mainly because of the spreadsheet being unable to handle the sheer quantity and accuracy of the raw data sent.
When SHAS was developed, a spreadsheet extraction package was added. This package will extract any data which SHAS can access and download them to a PC spreadsheet program.
The ability to obtain at will any production data, substitution data, progress towards contractual thresholds, so forth, has proved invaluable for the monitoring of the pipeline system.
By plotting the individual components in the pipeline, the pipeline system operator can check the metering systems of all parties. Any continual deficit or credit in the pipeline balance will indicate possible metering drift which will be corrected accordingly.
The SHAS program relies on the proficiency of the operator.
The program operator must have thorough knowledge of the allocation deed and be totally impartial. The ability to cope with large quantities of data and to identify anomalies quickly among them is paramount.
SUBSTITUTION
The benefits of substitution have become apparent quickly. Several unplanned shutdowns have occurred due to simops (simultaneous drilling and production operations) restrictions.
Without substitution, the field group would have been unable to meet contractual commitments.
By using notified substitution, the field group was able to make advance requests for substitution and plan with the other producers the best time to continue drilling operations.
Since October 1990, there would have been approximately 30 days when a shortfall would have occurred had substitution not been available.
Since that date, the system has had only two shortfalls connected with simops. The quantities involved were minimized by system substitution, reducing the exposure of the field group involved.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
