Andrew Wilde
LICconsult (U.K.) Ltd.
Stockton on Tees, U.K.
Neil J. Bidwell
Amoco Production Co.
London
Amoco (U.K.) Exploration Co., London, has installed a comprehensive pipeline monitoring and nominations system for the Central Area Transmission System (CATS) pipeline and production platforms of its Central Graben North Sea development.
The system, designed and installed by LICconsult (U.K.) Ltd., Stockton-on-Tees, U.K., consists of integrated systems for distributed controls, metering, pipeline monitoring, gas and NGL allocations, and gas nominations.
The 36 in. CATS pipeline runs 256 miles from the CATS riser in the central North Sea to landfall south of the River Tees in northeastern England (OGJ, June 7, 1993, p. 37; Fig. 1). The pipeline operates at a maximum inlet pressure of 179 bar (2,600 psi).
Currently, two gas production platforms operated by Amoco, Lomond and Everest, are connected to the pipeline via the CATS riser with further additions expected. From its landfall, the pipeline passes under the river Tees to Amoco's CATS terminal where NGLs are separated from marketed gas (Fig. 2).
With production from Amoco's Everest and Lomond fields at around 300 MMcfd and the total gas transportation capacity of the system exceeding 1.6 bcfd, there is ample capacity available within the pipeline.
This additional capacity should enable third party gas fields to access a pipeline system for transportation and distribution of their gas.
Among the discovered gas fields that fall within the catchment area of the CATS pipeline are Andrew, Armada, ETAP, J Block, Shearwater and Puffin, Elgin and Franklin, Erskine, and Britannia.
CHANGING TIMES
Demand for natural gas within both the developed and developing countries of the world is increasing at a rate that is placing strains on both operators and users of gas transportation and distribution networks.
The $120 billion/year global market has some countries, such as Italy, forecasting an increase in demand of 3%/year, while the U.S. is predicting an increase in demand of 3.8%/year, with an overall increase in worldwide gas demand of 20%/year by 2000.
Annual consumption in Western Europe has been predicted to rise by 55% over the next two decades (OGJ, May 16, p. 32).
This prospect has led many European and North American gas producing and supply companies to begin linking gas networks at national and international levels. Such links enable the import and export of gas between various systems to cover the increasing customer demands.
Gas transportation "interconnector" systems are also becoming common supplying gas to areas of the world with limited production capacity but a potentially large customer base.
Deregulation (U.S. Federal Energy Regulatory Commission Order 636, for example) is placing increasing demands on both gas transportation and gas distribution systems and on operators who attempt to balance supply with demand needs while maintaining profitability.
Managing these gas networks, which involve multiple suppliers and customers, will become increasingly more difficult without the use of sophisticated computer systems capable of tracking, monitoring, and controlling the multiple transactions taking place.
To attain the most cost-effective operating regimes, operators of major gas transmission and local distribution companies (LDCs) require the capability of fully understanding, controlling, and managing their operations.
One of the most recent major trunk lines to come into service is the Amoco Central Area Transportation System (CATS) in the central North Sea.
As owner and operator of the CATS, Amoco (U.K.) Exploration has insisted on a comprehensive pipeline management system that encompasses all the facilities riser, platforms, pipelines, and terminal and uses current technology and some that required development for the project.
It was envisioned that, by providing the operators with an integrated pipeline monitoring system, optimum management and control of the pipeline under all conditions could be achieved.
MORE PRODUCERS, FIELDS
When the CATS pipeline was in the conceptual design phase of the development, a conscious decision was made to provide a gas transportation system that would have the capacity to transport ashore gas from other fields in the central North Sea.
The specification of the systems and the complexity of the tasks involved required close cooperation between LICconsult and Amoco to establish fully the functionality and interconnectivity of the parts of the system. This involved interaction between different companies. each with its own hardware, software, and ideas, combining resources to produce an effective pipeline monitoring and control system.
Information technology links with individual shippers' and producers' corporate systems ensure efficient reaction to customer demands and billings as well as timely production information.
Amoco has invested in a state of the art, pipeline-monitoring system to provide a cost effective gas management tool for operating and controlling its pipeline and the shippers using it. All users of the transportation network will benefit from the system; these benefits could be important in attracting new shippers of gas to the pipeline.
The transportation capacity of the pipeline was sized accordingly and several subsea tie in points were installed, in addition to the riser platform connection header, during the laying of the pipeline to facilitate easy connection to other fields.
Managing multiple producers of gas who use the same transmission system has its own inherent problems.
Additionally, it is common in the North Sea as well as in other producing areas for operators of a single field to be part of a consortium consisting of many shareholders within a single development. Or, multiple gas fields may access a single production platform, each field operator insisting on adequate monitoring of his share of the product for financial, commercial, and accounting purposes.
With all these concerns in mind, developing an effective pipeline management system was crucial to the success of the project. The system developed for CATS has five main constituents:
- ABB Masterpiece DCS (distributed control system)
- Daniels fiscal metering system
- LICconsult pipeline monitoring system (PMS)
- LICconsult allocation computer system (ACS)
- LICconsult nomination computer system (NCS).
These are interconnected by a common communication system and in turn are linked into the Lomond and Everest platforms via digital tropospheric scatter link. This link provides the system data from offshore platforms.
The ABB master system is used for the monitoring and control of the offshore platforms and also for the monitoring and control of the Teesside gas processing plant. This arrangement ensures continuity of DCS and communications equipment for the project.
Additionally the system is linked to Amoco's corporate wide area network for information transfer around the organization.
The Daniels fiscal metering packages are located on each of the platforms and also at the Teesside terminal and provide data through the ABB master bus communications network.
The monitoring and nomination computer systems are based on a dual redundant DEC microVax platform running VMS operating system, using the DV Draw Graphics package, and communicating with the DCS and the allocation system via Ethernet local area network.
The system software consists of several constituent modules configured to suit the requirements of the CATS pipeline.
The pipeline monitoring and the nominations computer systems run concurrently within the computing environment, each providing separate facilities for the operators.
PIPELINE MONITORING SYSTEM
The pipeline monitoring and inventory control system consists of several standard software modules providing the following functions: data handling, real time pipeline model (Fig. 3), automatic tuning, pipeline leak detection, pipeline leak location, pipeline inventory control, look ahead module, composition tracking, instrument analysis, survival time analysis, and predictive and training model.
- In the system's data-handling function, a data-reception module regularly receives real time process data from the DCS. Data reception runs independently of subsequent modules to allow for differences in data acquisition and message reception.
Preprocessing of the real-time data is essential. This capability evaluates the credibility of incoming data through a series of validity checks specifically designed by LICconsult for real time pipeline modeling.
The output of the data-handling module is a complete set of pipeline data at a given instant in time to be used in subsequent processing; each datum is accompanied by a quality tag.
- In terms of the real time software model, the pipeline is subdivided into several sections.
The software uses end-point data for dynamic real time modeling of each section. Each sectional model produces profiles of pressure, flow, temperature, and density.
The pipeline model is fully dynamic and includes solution of the complete dynamic equations, thus enabling description of the pipeline's complete transient behavior.
- The differences between measured and calculated data are used to tune the model to actual pipeline conditions resulting in improved performance of application modules such as leak detection.
- The output of the pipeline model together with process measurements form the basis of leak detection calculations. Pipeline simulations compare calculated values with measured values to provide round the-clock leak detection.
- A model assisted technique calculates pipeline leak location based on data from end points and intermediate pressure measurements. The model calculates leak location by comparing calculated pressure with measured pressure.
- The pipeline model provides real time linepack data which is displayed by the DCS system. At any instant, it is possible to determine total pipeline volume.
Gas temperature and pressure profiling is automatically applied to the linepack calculations to compensate for the effect of temperature and pressure.
- The look ahead tool is a simulation facility capable of performing "What if?" scenarios. This tool provides the operator with advance information about future pipeline conditions, along with any necessary preventive action.
The look ahead tool performs a faster than real time forced flow analysis to discover the consequences of controlled adjustments, independent of real time pipeline operations.
- Another module uses composition measurements of product
entering the pipeline. These data are being periodically
updated and passed to the pipeline model.
Composition (energy) is tracked down the pipeline noting times of arrival at the gas terminal and actual pipeline conditions.
- The instrument analysis module continuously monitors the
accuracy and drift conditions of the process
instrumentation. This condition monitoring is based on the
parameter tunings performed in the pipeline model.
The module also provides schedules for instruments requiring regular maintenance.
- The purpose of survival time analysis is to assist the
operator in determining operation of the pipeline in the
case of major loss of flow into the pipeline.
Based on calculated current line pack, pipeline flow in, and pipeline flow out, the survival time analysis module will provide information on how long the gas plant can continue to operate.
- The predictive model provides the capability to simulate
pipeline operational strategies independently without
disturbing normal pipeline operations.
The model utilizes any combination of measured, calculated, or artificial process measurements or status. It is executed in time steps and provides an "artificial" simulation of pipeline conditions.
The output is differentiated from the primary pipeline model data and presented to the system operator for analysis. As a training facility, the predictive model enables the testing of new operational strategies, the training of new operators, and operator reaction in emergency situations.
NOMINATION COMPUTER SYSTEM
The NCS is used for data entry related to the intended use of the pipeline for the next 10 days including redelivery to the customer along with information relating to the current inventory of the pipeline.
Shippers book capacity in the pipeline to transport gas. Each day shippers' shares of gas field production may change because each shipper may choose to take his share of a field at a different rate from others producing from the same field.
Each day the shipper notifies customer delivery requirements to the operator. These requirements, called "nominations," are in the form of a rate of delivery profile in gigaJoules/day from a shipper to a customer. Several nominations can be notified for any day and these can be changed in response to changing requirements for gas as the day progresses.
The nominations are entered into the nomination computer system which aggregates them into half-hourly rates in gigaJoules/day and gas required in each half hour for shipper, field, and pipeline.
The inability of a shipper to achieve his customers' nominations can be handled and tracked by use of "substitutions" from other shippers' gas.
These substitutions are either prearranged borrowing agreements or ad hoc short term contracts between shippers. The important functions that document, monitor, track, and predict substitutions are carried out by the nominations system (Fig. 4).
Pipeline operational changes require careful consideration. In particular, changing from one outflow rate to another involves delivery of extra gas to ensure that nominations are achieved.
The maximum extra gas, called "ramp gas," needed in any half hour is calculated by formulas which vary from contract to contract. The NCS transfers the half-hourly figures to the allocation system's data base.
Based on the nomination figures, the allocation system executes the allocation program and produces various reports and other information for Amoco and its partners.
Following are some of the main functions of the nomination system:
- Operator entry of nomination data
- Forward tracking and prediction of production requirements
- Exchange of data with the allocation system
- Calculation of critical volume and energy levels by
shipper or ownership
- Calculation of critical pipeline pressure
- Presentation of nomination data
- Real time fiscal and compositional tracking.
ALLOCATION COMPUTER SYSTEM
The LICconsult allocation system provides a gas stock accounting tool that accommodates deliveries to multiple consumers from multiple shippers.
The system determines physical entitlement to redelivery of gas and NGL for each gas field. It also determines the actual gas redelivery to each customer from each gas field, based on the customer's gas nominations for the day.
The difference between the two is the field's gas redelivery difference. The accumulated differences expressed as energy indicate the balance of redeliveries between the fields. The fields' differences sum to zero.
The NGL entitlement is similarly calculated for each field. The field's redelivery of NCL equates to liftings from the stock tank on the day. The accumulated NGL redelivery differences for all the fields plus the tank stock equal zero.
The field figures are converted to energy using ISO 6976. A standard volume by field is also determined by ISO 6976 from the component redeliveries and utilities.
Each field's closing pipeline stock is then determined from its allocated quantity and its day's production. The closing pipeline stock for 1 day is the opening stock for the next. This can be adjusted for untraceable loss or gain by prorating to the pipeline monitoring system's inventory stock and for traceable loss and transportation loss from riser and pipeline flare meters or manual input.
The NGL stock tank can similarly be adjusted and the field's share of any stock change determined.
Suballocation takes place to shippers on the basis of their pipeline stock at the start of the allocation day, their nominations, and field production share.
The nominations to a customer may include ramp gas. Where this occurs, the allocation system must allocate the shipper, field, or customer nominations equitably.
A field may notify the operator that a certain amount of its production for a given period in time will be deemed, for allocation purposes, to come from another field. This is termed "substitution."
Traceability to meter totalizers is unaffected. Applicable transportation charges for this gas will be to the account of the field from which the substitution gas has been deemed to come.
The ACS applications comprise allocation procedures and several pre and postallocation calculations.
The allocation procedures are daily allocation, reallocation, quick daily allocation, and allocation on demand.
The pre and postallocation calculations are for pipeline untraceable loss and gain, pipeline traceable loss and gain, pipeline transportation loss and gain, pipeline fill, shipper opening stock records, NGL tank loss and gain, and reassignment of redelivery gas and NGL.
Activation of the three types of allocation system operator requested allocations (daily allocation, reallocation, and quick daily allocation) is provided through the allocation menu.
Allocation on demand is activated from the nominations system. Pre and postallocation calculations are activated from various menus listed in the description of the individual applications.
Nominations and substitutions along with shipper shares figure in the allocation procedure and so are stored on the allocation system's data base. The figures are usually input by the nominations system but can be altered through the allocation system if, say, reallocation is taking place.
The allocation system is capable of encompassing up to 25 delivery meters and 25 redelivery meters. Furthermore, the allocation procedure is able to include future metered flows for background fuel gas and for contaminant processing gas (from future gas processing plant).
Allocation of background fuel gas is based on throughput; allocation of plant processing gas, to remove C02 and H2S, is based on the "contaminator pays" principle.
The allocation system is based on a Digital Vax 4000 Model 60 computer with a VMS operating system communicating with the pipeline monitoring system, nominations system, and the DCS via ethernet and ABB Gcom protocol and to the corporate wide area network via ethernet and TCP/IP protocol.
The data base uses Oracle and was developed with SQL*forms and SQL*menu tools.
