Automatic Custody -Transfer System at Lake Pelto, Louisiana
JOHN T. TANSIL SUN OIL CO. LESLIE M. MOOR, JR. BEAUMONT, TEX. JUNIOR MEMBERS AIME Downloaded from http://onepetro.org/JPT/article-pdf/12/10/15/2237652/spe-1452-g.pdf by guest on 27 September 2021
Abstract operation in Louisiana. However, the expected reduction in capital outlay, coupled with the reduction in value of The production system and custody-transfer system for the equipment and product inventories to be exposed to the Sun Oil Co.-Pure Oil Co. installation at Lake Pelto the natural hazards of this relatively unprotected water La., was designed to operate continuously and automat location, prompted a decision in favor of the construction ically to achieve maximum throughput with a minimum and operational proving of the continuous throughput requirement for on-lease storage and manual attendance. design. Suitable equipment was installed in such a manner thaI The installation was planned and constructed early in all the wells can be controlled from a central production the field development and was put into operation in Nov., platform; their produced liquids can be processed con 1957, with only three wells connected. Since that time, the tinuously and delivered to the custody-transfer system installation has been expanded to serve additional wells with the assurance that all oil delivered is of merchantable and has undergone extensive operational refinement and quality. testing. The test period has proved the system both practi The custody-transfer system was designed to deliver cal and reliable. oil automatically from the production system to the gath Following approval by the Louisiana Commissioner of ering line, using positive-displacement meters and related Conservation, automatic custody transfer was commenced equipment for the measurement function. at Lake Pelto on April 14, 1959, and has been used for the transfer of all liquids leaving this property since that date. Introduction Everyone is aware that the cost of oil production and Lease Facilities handling facilities in water locations is quite high. These As shown in Fig. 1, the lease facility consists of two costs have been trimmed a little through the years by main structures-connecting walkways and an auxiliary more careful engineering, by prefabrication, and even by intermediate structure. The larger platform contains the some attempts at mass production; today, however, the production control manifold, the separators and four greatest hope for cost reduction lies in the use of new cr 1,000-bbl tanks. The smaller platform supports the oil improved oil-handling techniques. treating facilities, the pipeline station and the custody-trans For this reason, in planning the facility to serve the fer equipment. The intermediate structure supports the boat Sun Oil Co.-operated Louisiana State Lease 2620 in the landings, tool storage and electrical generating equipment. Lake Pelto field, the design section was instructed to uti lize automatic control systems and equipment wherever the Production System application could be of economic or operational advantage. The full portent of this charge was realized when ini The production system controls the wells and continu tialengineering estimates were summarized to show (1) ously processes the produced liquids for removal of gas that a continuous throughput design, utilizing automatic and impurities for delivery to the custody-transfer system. custody transfer, would reduce oil storage requirements As shown in Fig. 2, wellhead controls consist solely of 60 to 70 per cent compared to a conventional system and safety devices. Each flow-wing is protected by a high-low (2) that the additional cost of the equipment and controls pressure safety valve; where necessary, a pressure-reduc necessary to make the operation continuous and complete ing regulator is used to allow moderate flow-line and mani ly automatic was approximately the same as the cost of fold working pressures. 1,000 bbl of storage in this water location. Rotary-type choke bodies are used in the production At that time, no such system had been approved for control manifold to control the production rates of the
Original manuscript received in Society of Petroleum Engineers of individual wells. This type of choke permits production fice March 4, 1960. Revised manuscript received July 31, 1960. Paper rate changing without shutting-in the wells. Two motor presented at Upper Gulf Coast Drilling and Production Conference of SPE, March 18, 1960, in Beaumont, Tex. valves in each well manifold are used, allowing the wells SPE 1452-G O(:TOBER, 1960 Downloaded from http://onepetro.org/JPT/article-pdf/12/10/15/2237652/spe-1452-g.pdf by guest on 27 September 2021
Fig. I-Physical layout of Lake Pelto installation. to be opened, closed or diverted to test facilities on signal automatic custody transfer in the petroleum industry de from the production-system controller. scribes the measurement and sampling of liquids in mo The produced fluids are routed through separating and tion, rather than at rest, to determine the quantities of treating facilities, as needed for removal of gas and im crude petroleum oils 'transferred from the custody of one purities, and then through a BS&W monitor for quality into the custody of another. inspection before entering the delivery tank. If the moni The automatic custody-transfer system must provide tor detects excess BS&W, control action is taken diverting equipment for the performance of three basic and func the entire production stream through treating facilities tional objectives. First, it must provide equipment to de and routing the contaminated stream to the "wet oil" tank termine from liquids in motion the data necessary to cal for recycling. When the BS&W content of the steam culate the quantity of oil contained in the liquids trans from the treating facilities again meets the monitor's ferred from the property. Second, it must limit the total "merchantable oil" standard, the routing automatically transfer to that quantity permitted at any time by the reverts to the delivery tank. regulations set forth for the particular property in ques tion. Third, it must provide the pumping equipment and Overflow lines, connecting the tanks, allow filling of operational controls necessary for unattended pipeline de all available storage in case of a delay in delivery. All of livery. the tanks and pressure vessels are equipped with high level controls to close the wells and prevent loss of product or It is convenient to regard the equipment necessary to damage to the production equipment. measure and continuously regulate the volume of oil leav ing the property as the "custody-transfer unit". The cus A "well test programer" automatically routes the pro tody-transfer unit, as such, is the single most vital portion duction from individual wells, in sequence, through test of the continuous throughput system. facilities for determination of production rate and oil qual Basic equipment necessary to oil measurement in the ity. The quantities of gas, oil and water produced during Lake Pelto system is the meter, the meter prover and the the test are measured and the data recorded on a strip chart. Representative samples of the well liquids are taken sampler. The custody-transfer unit used in this installation, and stored in individual containers for quality testing. as shown in Fig. 3, includes two identical, parallel meter Dump-type meters are used for the liquid measurement, runs for flexibility and maintenance ease. The meters are of the positive-displacement type, equipped with automatic and an integrating flowmeter is used for gas measurment. temperature compensators, visible registers and ticket Automatic repetition of the test cycle provides an ample printers. The meters determine, and read directly, the in source of timely and reliable information for accurate con trol and allocation of production. dicated volume of liquid at 60°F leaving the property. Each meter is protected from foreign material and en trained gas by a strainer and gas eliminator. The presence Automatic Custody-Transfer System of excess gas in the gas eliminator stops flow through the The term "automatic custody transfer" has come to meter until the condition is corrected, thus eliminating mean, in general, the measurement and transfer of cus this possibility of mis-measurement. tody of liquid hydrocarbons by the use of methods other The meter prover is a 300-gal open-type tank, built than customary gauge tanks. More specifically, however, in accordance with API Code 1101 and precisely cali-
16 JOURNAL OF PETROLE UM TECHNOLOGY bra ted using volumetric standards certified by the National The "set-stop" counter of the monthly transfer control Bureau of Standards. The prover is protected internally ler is adjusted at the beginning of each month, or during by plastic coating and is externally insulated to stabilize the month when required, to read the total volume of the effect of ambient conditions. liquid to be run through the custody-transfer unit during The meter prover is used periodically to determine the the month. The volumes of liquid measured by the two meter factor, which is the ratio of the true volume of meters are combined mechanically and the total contin liquid at 60°F to the indicated volume at 60°F as meas uously subtracted from the reading of the set-stop coun ured by the positive-displacement meter. The product of ter. When the reading reaches zero, a mechanical linkage meter reading and meter factor is the true volume of is released, closing the set-stop valve and stopping the liquid at 60°F. transfer. This controller must be manually re-set before transfer can be resumed. The sampler is one of the proportional type, with the The daily transfer controller is set at the beginning of sampling tube manufactured and installed in accordance each month, or as required, to reflect the volume of liquids with API Standard 2500, Supplement 1. The sample is permitted the custody-transfer system at that time and the stored in a vapor-proof container equipped with a mixer rate of accrual of additional permissible volume. If at and purge pump. The sampler is actuated by impulses from any time during the month the volume transferred equals the meter combinator in such a manner that a small sam the volume permitted the custody-transfer unit at that ple is taken from each barrel of liquid metered. The "API time, the transfer is stopped temporarily until additional gravity" and "deduction for impurities" for the liquid permissible volume accrues. Transfer is automatically re
volume transferred are determined from this sample. Thus, Downloaded from http://onepetro.org/JPT/article-pdf/12/10/15/2237652/spe-1452-g.pdf by guest on 27 September 2021 sumed at the end of this waiting period. the calculation for the volume of oil transferred becomes The pipeline gathering station, as shown in Fig. 3, is Meter Reading X Meter Factor equipped with two gas engine-driven rotary pumping units, - Deduction for Impurities = Volume of Oil at 60°F each capable of handling the anticipated daily pumping Transferred. requirement. The dual installation affords operational flexi The equipment necessary to regulate the volume of oil bility and eliminates possible costly delays for maintenance. transferred is the monthly transfer controller which limits Control equipment used in conjunction with these units the total monthly throughput of the station, and the daily allows the units to be automatically started, warmed up transfer controller which limits the throughput of the sta and placed on the line. Standard operational safety con tion on an accrual basis. trols protect the engines and the station from predictable
Fig. 2-Production system schematic.
OCTOBER, )960 17 Downloaded from http://onepetro.org/JPT/article-pdf/12/10/15/2237652/spe-1452-g.pdf by guest on 27 September 2021
Fig. 3-Custody-transfer system schematic. malfunctions by stopping one or both of the units, ac During the test period, the measurement obtained from cording to the nature of the malfunction. Any malfunc the custody-transfer unit was compared with the official tion which causes complete station shut-down signals for measurement obtained by conventional tank gauging. This operator attention. comparison (Table 1), involving some 290,000 bbl, has The station is automatically started, stopped and flow demonstrated the reliability of the custody-transfer unit. rate controlled according to the quantity of liquid in the To demonstrate the accuracy of the meter measurement, delivery tank. Subject to the regulation of the custody a comprehensive test was made where, for a period of transfer unit and the limitation of the operational controls, nine consecutive days, all tank measurements were made the pipeline station will begin transfer when sufficient by highly skilled gauging personnel under the most ac liquid is available in the delivery tank. Increasing liquid curate gauging conditions possible. Gauge lines were read height in the delivery tank then causes the transfer rate to the nearest 1/ 16 in. and back-gauge volumes were cor to increase until it matches the rate at which liquid is rected for temperature. All readings were checked, re entering the tank. Conversely, a decrease in liquid height checked and any difl:erences averaged. The results for the produces a reduction in flow rate. This allows the pipe 12,425.22 bbl gauged during the test indicated that the line station to handle continuously a wide range of pro metered volume exceeded the gauged volume by 0.0139 duction rates with a minimum product inventory and with per cent. only occasional "starts" and "stops". The custody-transfer unit has performed throughout the The metered oil sump collects the drain-down from the period of operation described with a minimum require prover tank, pump seals, pipeline scraper trap and all other ment for maintenance. As might have been expected, some drain points on the gathering-line side of the meters for of the original regulating devices have been replaced with automatic return to the pipeline downstream of the meters. newer and more dependable equipment developed sub sequent to the construction of this installation. Operation Evaluation Considerably more difficulty has been experienced in the operation of the unattended pipeline station. Dependable More than 1 million bbl have been metered and trans automatic starting of the gas-engine prime movers was ferred by the automatic custody-transfer system at Lake found to require that optimum settings of cranking speed, Pelto since operation began in Nov., 1957. The system was throttle and choke position, ignition timing and spark in semi-automatic operation on a test basis prior to April intensity be determined and maintained. 14, 1959. Since that time, with the approval of the Louisi ana Commissioner of Conservation, 535,000 bbl have Flow-rate control of the engine-driven rotary pumps been delivered continuously and automatically by the cus proved difficult at first because of unexpectedly large var tody-transfer system. The system is now handling approxi iations in station discharge pressure. A pneumatic con mately 2,000 BID with an active storage volume of only troller with proportional and re-set action is now being 1,000 bbl. About 75 per cent of the required active stor used to position the engine throttles and achieve the de age is the result of a routine, pipeline batching problem sired flow rate, regardless of discharge pressure variations which requires a periodic shut-down. Thus, the remaining or the number of units on the line. 3,000 bbl of available storage is always present for emer The automatic testing and processing features of the gency use. production system have performed as intended, but this
18 JOURNAL OF PETROLEUM TECHNOLOGY TABLE 1-CUSTODY-TRANSFER UNIT MEASUREMENTS VS CONVENTIONAL TANK were processed for the identical deliveries described by GAUGING MEASUREMENTS Gross Volume Net Volume 57 (individual tank) "Gathering Receipt Tickets", Further, Measurement at 60°F(bbl) at 60°F(bbl) the "Crude Petroleum Production Record" for that month Meter 289,602.12 288,775.50 contained 134 entries, whereas only 14 were required us Tank 289,288.13 288,472.02 Comparison (per cent) 0.108 0.105 ing the meter measurements.
Conclusion system has not been entirely without its problems. Centralized control of production rate, using moderate The Lake Pelto production system and custody-transfer flow line and manifold working pressures, was complicated system have proved that the continuous-throughput con by the need to take some pressure drop at the wellhead. cept, using automatic custody transfer, is a practical reality Critical flow phenomena so limited the range of control for lease oil-handling facilities. Future, it is evident that when fixed wellhead chokes were used for the initial re the system will result in a significant reduction in initial duction thai pressure-reducing regulators with flow-limit capital investment for production facilities in locations ing, fixed-size seats had to be substituted on the high-pres where working space is at a premium, or where large sure flow wings. The use of regulated pressures against throughput volumes are anticipated. f*** positive chokes for flow control and the frequent, reliable tests made practical by the automatic well test equipment now allow production to be controlled to very close tol
erances. Downloaded from http://onepetro.org/JPT/article-pdf/12/10/15/2237652/spe-1452-g.pdf by guest on 27 September 2021 An over-all evaluation of the Lake Pelto installation makes it clear that successful automatic operation demands that operating and maintenance personnel be thoroughly familiar with the normal function of the system compon ents, and be instructed in procedures for "start up" and other abnormal operating periods. To assure performance reliability, test procedures for detecting and isolating sys tem defects must be devised and incorporated in a vigo JOHN T. TANSIL (left) was graduated in 1949 from Texas rous, scheduled, preventive-maintenance program. A & M College with BS and MS degrees in mechanical An adjunctive feature of the meter method of measure engineering. Since that time he has been employed in Sun ment is the tremendous reduction in the data gathering Oil Co.'s Gulf Coast Div., where he currently is supervisor and data processing necessary to account for and report of the mechanical engineering section. LESLIE M. MOOR, the quantities of oil produced and delivered from a prop JR. (right) is a petroleum engineer with Sun in Beaumont, erty. Tex. He joined Sun in Jan., 1953, following graduation For example, during an average test month at Lake from The U. of Texas where he received a BS in chem Pelto, four meter tickets (two for each of the two meters) ical engineering.
OCTOBER, 1960 19