A new approach to Brownfield meteringby Arabian Oil & Gas Staff on May 20, 2014
As the ‘cash register’ of upstream operations, metering systems ideally need to achieve optimum measurement uncertainty for the entirety of a plant’s lifetime. This is a growing concern for brownfield operators in the Middle East, where ambiguity over reserve volumes means economic margins are always under scrutiny.
Since flow rates shift during a plant’s lifetime, original metering systems on brownfield sites may no longer be fit for purpose. Meters tend to perform better when dealing with flow rates at the higher end of the spectrum for which they are designed.
So as production rates decline, they are likely to have diminishing performance from a metering uncertainty perspective.
Good oilfield practice dictates that when the operating range changes, a review should be undertaken to ensure the existing metering system remains suitable. Repeated system failure or poor performance in audits usually indicates that urgent attention is needed.
However, selecting the most appropriate course of action is not a simple task. Practical challenges are compounded by the fact that original sales agreements need to be considered. These contracts often involve measurement principles that don’t dovetail with today’s best available technologies.
Any equipment upgrade or replacement decisions are partly dictated by OpEx costs relative to the asset. This is particularly complex when it comes to metering technologies.
The expense of upgrading or replacing equipment is just one part of the equation. Repercussions of mis-measurement are another important consideration, as are costs associated with taking a plant offline to facilitate changes. Each of these factors has a bearing on the decision-making process.
Ideally, any work that requires the plant to be offline should be scheduled during a planned shutdown. This presents a defined window of opportunity, as short as seven days, which brings significant challenges. Achieving this is dependent on precision planning and exceptional logistics expertise, as well as reliable equipment.
Intelligent, risk-based decision-making is the cornerstone of best practice in this scenario. This
can be facilitated by a front-loaded strategy led by metering solutions experts.
At the outset, a detailed site survey ensures existing installations and systems are fully understood. Assessments should include analysis of historic metering reports and log files.
It is useful for engineers to see how productivity fluctuates as well as the upper and lower flow rates a system is likely to encounter. Understanding flow rates and overlaying this with data surrounding temperature and pressure changes facilitates evidence-led predictions on how production trends may alter over time.
These forecasts enable engineers to develop solutions equipped for likely future operating conditions. The approach informs technology choices, then influences system design and configuration to help deliver optimum performance.
Selecting the most appropriate metering technology requires a consideration of various factors. Viscosity of process medium, pressure vacillation, temperature ranges and fluctuation in flow rates all have a bearing.
Good repeatability, the ability of an instrument to produce the same result when measuring the same quantity, is another consideration. Sound design and metrology principles can go a long way towards achieving this. However, repeatability should not be confused with accuracy, since an instrument can be repeatedly wrong.
Decisions should also take sampling and analysis technologies into account (eg to determine basic sediment and water contamination in crude). These need to be precisely integrated to enhance overall metering system performance.
Different meter types each have their own benefits and special features. Many sites in the Middle East traditionally used orifice meters, but in the decades since these projects started-up the available technologies have progressed.
Latest generation, proven technologies include both ultrasonic and coriolis meters. Coriolis meters are generally more tolerant of gas entrainment and provide a direct mass output.
Ultrasonic meters have superior flow turndown ratios and measurement accuracies that are many times better than orifice meters, as well as offering better diagnostic capabilities. This enables operators to plan and conduct predictive maintenance schedules more effectively, to keep metering performance at an optimum level.
When reviewing technology choices, the dissemination of data must also be considered. Metering control systems can provide a wealth of information that goes far beyond the totalised flow.
With access to the entire spectrum of data from metering instruments, control systems can provide full traceability of all alarms and events that occur in operation. If required, they can also record operator comments and actions to provide a complete audit trail.
There are many ways to make this information available and early decisions surrounding distribution methods allow the operator to achieve the highest value from a system. Advancements in flow computers and metering supervisory computers mean that metering data can now be reported online in real-time. This facilitates faster, more dynamic management of the measurement cycle, from both a local and a global perspective.
Oil & gas metering is on the cusp of a significant new phase in its development to secure the extended life of plants. Long-term performance and traceability remain paramount, but pinpointing the right time to intervene where reduced measurement uncertainty is evidenced or predicted is also a key factor. The science of measurement is always evolving.
Intelligent, expert-led metering strategies can bring significant bottom-line benefits to forward-thinking operators seeking to optimise brownfield assets.