Custody Metering for electricity generation
16 February 2015
When the Dubai Electricity and Water Authority (DEWA) commissioned a new natural gas-powered electricity generation plant in Aweer, a key consideration when ordering the Diagnostic Performance and Monitoring (DPM) software was its Custody Metering capabilities – where the mass of the gas and its calorific value is measured as it is supplied from the linked offshore gas field.
A consortium of Siemens, Mott McDonald, and Process Group subcontracted the metering skid to the integrator in instrumentation and metering skids: Emphor Fzo. That company had been looking for several years for the best DPM (Diagnostic Performance and Monitoring) software for metering skid projects that could satisfy DEWA (Dubai Electricity Water Authority) specifications. In 2007, it chose zenon technology from COPA-DATA with implementation by KD Engineering.
KD Engineering is an automation integration company that provides specialised highly integrated solutions for custody transfer measurement applications for the oil and gas industry. Custody Metering is an essential tool for the profitable operation of offshore platforms. Metering systems are in constant evolution to meet the exacting standards for accuracy, reliability and safety required for the operation of these increasingly sophisticated and automated platforms and, given their extreme space constraints, the need is for compact, light equipment.
Why metering matters
Simply put, metering is important for cost saving. If a typical offshore oil production facility produces 150,000 bpd (barrels per day) and incurs a random $5 barrel extraction cost, an under-reading error of 0.5 % in flow can result in a $1.3m yearly loss, assuming an oil price of $35 a barrel. Since a custody metering station only costs around $1m, it will have paid for itself in a little over nine months.
However, the primary reason for offshore metering is security, because the only way to know the correct output is by metering it at the source. It is possible to offload crude by shuttle tanker to a shore terminal for custody metering. However, in practice this opens the way to disagreements with production partners and the tax authorities over issues such as ‘bill of lading’.
Fiscal, custody and allocation
In the metering business, the terms ‘fiscal’ and ‘custody’ are essentially interchangeable, although fiscal means ‘concerned with government finance and policy’ and custody means ‘safekeeping or guardianship’. Neither term defines a level of metering accuracy in itself, although both are taken to mean the ‘best accuracy’ in normal oil field practice.
Metering of gas flow alone does not reveal the actual value of the gas being supplied. Flow and density will vary according to temperature, whereas Custody Metering is centred upon mass as opposed to volume. Additionally, the calorific value of the gas will vary according to the chemical variation of the gas as it is extracted. This requires a solution that is called ‘Diagnostic & Performance of gas quality and Metering’ (DPM) - used in the application for the Dubai Electricity & Water Authority.
Towards a solution
Most authorities insist upon ‘stand alone’ flow computers for each metering stream. This allows individual units to continue operating even if a neighbour is damaged.
A supervisory computer can ‘manage’ the disparate stream, as well as prover flow computers, alongside additional tasks such as flow scheduling and automatic flow sampler control. These are now most often ‘dual redundant’ arrangements rather than the earlier and arguably much more complex ‘hot standby’ systems. Many now prefer a single station supervisory function since the primary metering data is retained in the separate flow computers.
A flow computer is an electronic computational device that implements the required algorithms to convert the raw data received from flow meters, to which it is connected, into volumes at base conditions. A flow computer also audits changes that have been made to any of the parameters required to turn the raw flow meter data into volumetric information. It records events and alarms related to the flow meter (for example, loss of flow, loss of required electrical signals from measurement transducers, or transition of these electrical signals near their upper or lower range). It will keep a running tally of the volume for each flow meter it monitors and perform a ‘gauge off’ of this volume on an hourly, daily or monthly basis.
The flow data is made available externally through an electronic interface so that other computers can download the information for the purposes of supervision, accounting or auditing. A gas chromatograph is a laboratory device that is used to analyse gas or light liquid streams to determine their composition by component. Gas stream compositions are typically reported in ‘mole’ or volume percent. Liquid stream compositions are typically reported in liquid volume percent.
zenon offers both functionality and value for such applications, key issues being one-click redundancy, open networking and a vast library of drivers to connect to various instruments and devices that comprise a metering system. zenon is a high performance SCADA system, therefore all aspects of visualisation were to be found within the basic package and could be easily configured to provide displays of plant status and detailed reports and alarms.
zenon in application
The DPM SCADA software supervises a two stream line metering system (duty and standby/master meter system) configured in ‘Z pattern’ arrangement. The ‘two streams’ arrangement is called a ‘Train configuration’.
This comprises of a multi-path Ultra-Sonic Flow Meter (USFM) to measure the flow in each of the two streams, a ‘prover’ flow-computer that reads flow from the USFM flow-meter and performs the following calculations in each of the two streams, correction to give the flow rate under standard condition (volume of gas differs according to temperature and pressure), calculation of total flow in terms of energy, mass and volume, and it also performs the AGA10 calculation to check performance of the USFM and the ISO 6976 calculation to check performance of the Gas.
zenon screen - Image: Copa Data
Advanced and very complex thermodynamics calculation has been made possible by zenon’s automatic multithreading, ‘mathematical driver’ and the straton softPLC that provided real-time capabilities and C++ encapsulating container for the thermodynamic real-time calculation.
For example, each of the two metering streams has two Flow Meters, two Flow Computers and a Gas Chromatograph requires only a zenon license of 512 tags. For four stream systems it requires 1024 tags; and for a four stream system where GC composition has to be transferred to another DPM, 2048 Tags.
Sometimes only one station has real access to the Gas Chromatograph, and this station then acts as a Modbus slave gateway to several other stations several kilometers away. Thanks to straton, this allows a zenon station to act as a Modbus slave to the other stations even with the Modbus serial protocol.
It is also note-worthy that the duty/standby server redundancy is easily configured in zenon with one mouse click. A most important feature of DPM software is its capability to interface as a gateway to several DCS interfaces; information such as totalising flow and alarms must be sent safely to the DCS system. Using the zenon process gateway and straton, multi link to several DCS channels is made easy.
zenon fully handles system diagnostics, metering overview, totals, reports, trending, alarm history & summary, gas quality and also a continuous monitoring of meter diagnostics (swirl, asymmetry), ultrasonic foot print and ratio, velocity of sound verification and performance monitoring.
zenon handles Gas Calculations in accordance with international standards such as API/AGA 10/ ISO as well as customer-specific logic. zenon’s embedded communication drivers with integrated protocol analyzers are fully utilised.
Experience in development
According to Karim Saikali of KD Engineering, “in zenon we have a very powerful platform to build application solutions that are platform independent with a modern network technology with distributed client-server structure and very sophisticated circular redundancy; and, most of all, more than 250 different driver connections allow connection to all kind of protocol standards in the oil and gas industry. Working in close partnership with COPA-DATA, a specially developed ASCII Modbus was created for this application, which was also needed to be configured with a backup channel. Mr Saikali also stated, “We provided the best support to our client and were very flexible to their demands, this is the strongest reason why the client switched to us.”
At the core of the application is an accurate and auditable history. The daily and monthly reports are stored between six months and a year. The reports are used by customer representatives and operators. The detailed reports are used for daily operation (instead of using DCS reports); moreover if disagreement with suppliers occurs they can check the totalizing flow from the DPM at any time and over any period. This important information is held in the zenon archive database.
An important product of the DPM software is the Totalizing flow that is read from the Flow Computer. A representative of the customer came from time to time to compare flow computer values and DPM values – they were identical. In developing a solution based upon zenon KD Engineering found that it was easy to use – allowing attention to be focussed upon the application rather than the intricacies of the SCADA system.
This project is large and ongoing. KD Engineering has seen that zenon has a key role to play in this application area – in future stages straton, the embedded SCADA-logic of zenon, will be used to provide additional functionality, using the robust tools that straton offers.
“In the next three projects we will use straton,” added Mr Saikali. “As a matter of fact only one of the stations will communicate with the G.C. and act as a Modbus slave to the others stations using the serial Modbus protocol, so straton is needed to configure 3 Modbus serial driver slaves.”
Utilisation of zenon with straton means that risk is further reduced for all stakeholders in the project. KD Engineering sees an efficient way to the future using zenon & straton-based solutions; whilst not everything was required on this project they have additional functionality for Diagnostic and Performance Metering applications that will be required in future projects such as:
• Inbuilt embedded communication drivers with integrated protocol analysers.
• Gas calculations in accordance with international standards such as API/AGA 10/ ISO as well as customer specific logic.
• Calculation of the velocity of sound based on the gas com position and the pressure and temperature of each meter run in accordance with AGA-10 and monitor differences between the calculated value and the VOS per path of each connected ultrasonic meter.
• Validation of the meter cross section (Z) of the master meters and check meters.