Offshore and Onshore Flow Metering Systems – Final Chapter
Chapter Seven- “The Problem with Master Metering is…
… that it is not a prover.” That’s usually how that sentence ends.
Ball and piston provers are effectively discontinuous positive displacement flow meters. An accurate swept volume divided by the displacement period (measured accurately) gives you a PD meter. Albeit for an average fixed volume batch. It can be very accurate and very repeatable indeed, and we accept that all without argument. But, this does not change the discussion above (chapter four). Namely, in the world of the $50.00 barrel, costs, sizes and weights must drop, and simplicity and operability must rise.
The most common argument against master metering is that it can mask common-mode errors if one uses the same meter principle for the duty meter and for the master meter. For example, let us imagine two similar turbine meters in series. When there is a sudden drop in oil viscosity, then both will probably suffer the same effects on accuracy. This ‘common mode’ error may not be discovered for some time, particularly if the real flow rate is also changing. Additional information, such as an inline density meter, viscosity meter, or a different principle of meter (eg PD meter), or some sophisticated computer processing analysis may be added to improve the situation.
Some early UFM experienced common mode errors in series operation caused by a swirl and/or asymmetric flow profile. This affected both meters in a similar manner. To try to avoid this, new multi-path meter designs, and new processing detection methods, had to be developed – but the concern does remain in some quarters.
The Coriolis principle is perhaps the most immune to external influences. Viscosity changes affected all early Coriolis meters. With the latest meters, though, the changes must be significant enough to have an effect. Even then the resultant error is of low absolute magnitude.
Statistically (and I am definitely not a statistician), the master meter proving technique must carry the risk of uncertainty added to the final flow metering results. But we talk of ‘uncertainty’ and ‘confidence levels’, and the difficulty lies in making a definite absolute measure of this addition.
Chapter Eight- “The Best Thing About Master Metering is…
… that it is not a prover.” That’s usually how that sentence ends. Sound familiar?
And why? Well, because any proving (or verification, or calibration) is better than no proving. As I discussed above, cost, weight, size and complexity must be reduced in the $50.00 barrel world, or the projects will not proceed. A master meter arrangement is usually something like 50% of the cost of an equivalent pipe or piston prover. Perhaps the weight and size are both 25%.
Custody flow meters are unlike almost everything else on an Oil & Gas installation – they are the ‘cash registers’ of the oil company. To operate the rig, they may need an electric generator, they need pumps, they even need emergency lifeboats …But custody oil flow meters? Well, they do not just need them, they want them. Without these, they cannot produce an invoice for payment by the buyer. Without them, the tax-man will impose more tax than is necessary “…Just to be sure…” I cannot think of anything else more significant that they really want:
Flow Metering = Revenue.
Most Oil & Gas owner/operators/field partners, and in most cases the government will want the ‘best’ accuracy for their rigs. That usually means they immediately rush out for a volumetric prover. That can make the scheme so big and expensive that many question whole metering topic. Sometimes the project may not even proceed.
Master metering can (‘does’ not) add extra uncertainty in the measuring process’ proving. However, we cannot overlook the fact that it is more ‘available’. It is an extremely simple form of proving which, in many cases, requires only a start and stop signal. If the master meter is a mass meter, there are no rotating parts to wear-out and to maintain. No sensor faces or transducer windows to contaminate. However, there is a simple traceability directly to mass standards. This means that master metering is more available and easier to achieve than any volumetric proving with fluid stability issues. Certainly, one that requires specialist management. The ease and availability allow it to be carried out whenever required and that must be set against the concerns of additional uncertainty.
“Any proving is better than no proving”
If this helps a project proceed due to lower overall capex and opex, then so be it. We are all in C&I engineering, and we would all like these projects to proceed… Please!
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