API RP 85:2003(2013) pdf download
API RP 85:2003(2013) pdf download.Use of Subsea Wet-gas Flowmeters in Allocation Measurement Systems.
Simply substituting the values for the variances on the saks gas neasurement and those of the flare and fuel incasurentents for estimates) into the appropriate places in EquatHIS) (I) yields the cOrres.1 allocation lactors for the gas pan of the system which incorporate the effects of all these unccrtainties. There may be other specific instances like these where a portion of the collected hydrucatlxms are directed elsewhere, such as gas liii. circulahon of pigs. etc. For these cases, the user should develop a simple extension to the techniques shown here.
5.5 APPUCATION OF ThE ALLOCATION EQUATIONS
In order to apply the equations developed here to allocate the System Imbalance hack to the individual production streams, the uncertainties in gas and liquid flow should be knowii at reference meter conditions. Appendix A discusses how this can be done.
It should be noted that with knowledge of composition at both the allocation and reference meters measurements of gas and liquid itiass how rates and tmcenaiiitics can he conserted to component (low rates and uncertainties. These can then be used to allocate the constituent totals hack to the indis idual streams in the general case where the stream compositions differ.
It is anticipated that normally the computation of System Imhnlimix for hoth gas and liquid flows will he in units of mass. This is not absolutely essential. however, as the calcubt.ions can he made an a volumetric basis as long as it can be shown th this is being done properly. Howxver. the System Imbalance is most easily distributed back to the contributing meters on a mass basis, hence this method is stnmgly preferred,
5.6 PERSPECTIVE ON ALLOCATION: THE IMPACT OF SYSTEMATIC ERRORS
What has been developed here has made the assumption that measurement errtws on all meters are unbiased, (i.e., there is no systematic component to deal with). The reason for this was simply that the nuihcmatical derivations became easier, and insight could he gained by formulating equations, such as [quation (I) in 5.4. in this way. However, in the real world this assumption is rarely. if ever, completely valid. Even it the allocation and reference meters were unbiased when installed.
So it is reasonable to ask how useful is the allocation methodology developed here, and how should it be applied. The answer to the second question is that a ngornus program for sensing. correcting (where posihle. and accounting for systematic errors down to the sensor level must be instituted at first calibration and continued throughout the life of the fIeld. Only through continuing diligence will their effects be minimized.
The first question is not so easy to deal with. but can be partially answered by recognizing that any allocation scheme one can envision will be aftected by the presence of undetected systematic errors. Unless the etThrts at detecting bias errors yield fruit, this measurement inequity will be distributed—whether by Uncertainty-based Allocation or by any other method—to all the streams which are commingled.
Furthermore, one can argue that meters with large random measurement uncertainties arc more likely to have large s. tL’Inatw errors than meters with smaller measurement uncertainties. If this is so, Uncertainty-based Allocation would assign the largest portion of an imbalance duc to both systematic and random errors to those streams which arc most deserving. Conversely, if the owner of a stream diligently maintains a low metering uncertainty. keeping control over sources of systematic errors, his reward will be that he receives little or none of the imbalance, whether from random or systematic errors. Obviously this methodology provides a strong incentive for partners to keep their meters in top working order.
In summary. it is worth noting that any form of subsca allocation brings with it risks like those described above. Users must be aware that these risks exist, and that they will probably be greater than those in well-defined, controlled, and predominately single-phase measurement systems (topsides facilities). The user must assess his willingness to accept the measurement exposure outlined here before agreeing to a commingling arrangement using subsea metering systems.
