Application of Gas Turbine Power to Oilwell Service Pumping

Transcription

1 The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its 70-GT-69 Divisions or Sections, or printed in its publications. $2.00 PER COPY $1.00 TO ASME MEMBERS Discussion is printed only if the paper is published in an ASME journal or Proceedings. Released for general publication upon presentation Copyright 1970 by ASME Application of Gas Turbine Power to Oilwell Service Pumping T. G. THOMETZ Chief Engineer Service Design, Byron Jackson, Inc. Subsidiary of Borg-Warner Corp., Long Beach, Calif. This paper discusses the application of the two-shaft gas turbine to oilwell stimulation pumping through the successful design and manufacture of a mobile mounted 2000-hp pumper, using two 1000-hp turbines. The design is discussed from the standpoint of: functional requirements; operational requirements; power train design; control requirements and design; auxiliary systems; and field operating experience. The pumper was put into operation in October 1968 and after one year of experience it was judged successful in terms of reliability and reduction of maintenance and operating costs. Two more pumpers are now planned and scheduled for completion in March and June Contributed by the Gas Turbine Division of The American Society of Mechanical Engineers for presentation at the ASME Gas Turbine Conference & Products Show, Brussels, Belgium, May 24-28, Manuscript received at ASME Headquarters, January 19, Copies will be available until March 1, THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS, UNITED ENGINEERING CENTER, 345 EAST 47th STREET, NEW YORK, N.Y

2 Application of Gas Turbine Power to Oilwell Service Pumping T. G. THOMETZ INTRODUCTION In oil well pumping service, and more specifically in well cementing and well stimulation using the positive displacement triplex type pump, the gas turbine is now a field proven prime mover. The actual application of the gas turbine as an engine for well cementing service, only, has been somewhat limited, as horsepower requirement per engine beyond 300 hp is really not necessary. Limited use is helicopter portable pumping skids for remote areas where the equipment must break down into less than 4000-lb packages for the standard allowable carrying weights of the helicopter. Some higher powered skids that use minimum space on offshore platforms also are a limited application of turbines. The principal use of the gas turbine as prime movers for triplex pumps, however, is in oil well stimulation pumping--the acidizing of well formations or the hydraulic fracturing of formations with sand-laden slurries at pressure to 10,000 psi and with power levels to 10,000 hp. It is the application of two-shaft gas turbines to this type of well stimulation that this paper will be restricted. The development and design of the turbine powered well stimulation pumping unit, shown in Fig.1, will be discussed from the standpoint of: 1 Functional requirements 2 Operational requirements 3 Power train design and component selection 4 Control requirements and design 5 Auxiliary support design to turbine 6 Field operating experience FUNCTIONAL REQUIREMENTS In oil-well stimulation, the functions of the pumping equipment that must be controlled are horsepower and speed. The ideal power/speed relationship is the ability to control output shaft speed at any horsepower level of the engine. This, in turn, provides the operator the ability to control the pressure/flow range of triplex pump. Fig.1 Byron Jackson Inc. Model 127-T oil well stimulation pumper powered by two Solar T-902 Saturn gas turbines. Photo courtesy of SOLAR Division, International Harvester Co. The concentration of a large quantity of power in a relatively small well site space was another of the functional design requirements. Up to 10,000 hp often is needed at one location. In addition, the selection of the turbine demands its power compatibility with the pump. In this design, the pump was developed for intermittent power input of 1000 hp with variable input speed to 3200 rpm. Extensive pressure/flow ranges of the pump was still another specification. Test pressure of 15,000 psi and pumping pressure to 10,000 psi was listed. Minimum pumping pressures are governed by pump plunger sizing and maximum speed of the pump. Operating conditions of 100 F engine inlet air and 3000-ft elevation had to be met. Dusty atmosphere and frequent starts also were requirements. Initial capital cost was a prime specification. To offset the high initial cost of the turbines, it was felt a minimum of 2000 hp would have to be incorporated into one pumper to replace three existing pumpers which used two gasoline 1

3 NumEll SOLAR T-902 CO; 4-SPEED SLIPJOINTli BYRON JACK ) GAS TURBINE TRANSMISSION COUPLING PACEMAKER. TRIPLEX PUMP E I OPERATING 67% RANGE 600 Fig.2 Diagram of one of two identical power train assemblies used on the Model 127-T pumper powered engines each and were rated at 700 hp each. This 2000-hp unit also had to be truck chassis mounted, not trailer, because of small well sites, and yet had to comply with the gross vehicle weight (GVW) laws and regulations set forth by the states in which it would operate. To meet these GVW rules, a total unit weight of 45,000 lb was set, with a load distribution of 13,000 lb on the front axle, and the remaining 32,000 lb on the rear axle. OPERATIONAL REQUIREMENTS The operational requirements are those affecting the actual control of the pumper. Simplicity in control was stressed from the beginning. This was to include unit mobility, turbine engine starts, ability to meet pump pressure/flow demands with relative ease, automatic primary controls, and limited protective controls. At no time, however, would the simplification of controls reduce the reliability of the unit. The need for unit mobility was resolved with the functional design specification of a truck chassis mounted unit instead of a semi-trailer type carrier. To ensure sound engine starts each time, the truck chassis engine was to be utilized in that it would drive a hydraulic pump to provide an independent power source for hydraulic starters on the turbine. Remote start, control, and shut down was established as an additional operating requirement This was decided to favor operator safety. Constant attendance at the controls was also set as an operational need since few pumping jobs exceed two hours in duration, and, in fact, most lasted less than one hour. This precludes the use of extensive protective controls, but rather the substitution of indicating gages and lights. POWER TRAIN DESIGN AND COMPONENT SELECTION A power train for oil well stimulation pumper includes a prime mover, transmission, drive line or shaft coupling, and the triplex type posi IL z F F ENGINE INLET AIR 3000 FEET ELEVATION 0 I I I 1 0% 20% 40% 60% 80% POWER TURBINE (PT) SPEED -PERCENT 100% Fig.3 Power turbine speed range in which BJ 127-T pumper designed to operate tive displacement pump. This power train must be designed primarily to meet the functional requirements of the unit (Fig.2). As a prime mover, a 1000 hp Solar T-902 Saturn engine was selected. Being a two-shaft engine, its gas producer speed and engine power can be controlled independently of its power turbine output shaft speed, resulting in an engine with variable output shaft speed from stall to maximum for either full or part throttle operation. It is this variable output shaft speed feature of a two-shaft turbine that made it the choice over a single-shaft engine to meet the power/speed requirements. The 1000-hp rating of the T-902 engine is compatible with the Byron Jackson Pacemaker II triplex pump. The extensive pressure/flow range of the pump cannot be met by direct coupling the turbine to the pump alone. Two approaches were considered to match the turbine to the pump to meet specific pumping pressure. The rejected method was to direct drive the pump with the turbine and single reduction gear box and then size the pump fluid end and plunger diameter to give the desired pressure at or near maximum engine power. If pumping pressures change in sufficient magnitude that the torque demands to the engine create a power level situation that is not at or near maximum, then the pump plungers should be changed to the correct diameter. This will restore the engine to power level. The accepted approach was inclusion of a four speed transmission in the power train assembly. Then, proper gear ratio selection in the transmission will provide the required torque to the pump to meet the wide pressure range necessary 2

4 ST - mil ii N ONE Lt) MINI' 7O I!It :0 6 6,,, 6\. /BOO \ 100 F ENGINE INLET AIR I N 3000 FEET ELEVATION 67% PT. 100%PT. 1 I FLOW RATE-BARRELS PER MINUTE A4),, 4., I \ \ \ Fig.4 Pacemaker pumps (two) discharge pressure and pumping rate: in the four different transmission gear ratios with engine power turbine speed range from 67 to 100 percent for well servicing. Since no commercially available transmission was available that met the design specification for correct gear ratios, input speed, weight, and power rating compatibility, a custom transmission was designed and built by Cotta Transmission Company to Byron Jackson's specification. These specifications included direct coupling to the turbine to reduce weight and length of power train. This was done by SOLAR supplying their engines with an internal spline on the output shaft into which the Cotta manufactured transmission mated. The torque required at the pump input shaft was established to be through a range of 5.7 to 1.0 to meet the pressure demands at the pump discharge. This torque range was then divided into four subranges from which the gear ratios of the transmission were determined. Further, to calculate the optimum ratio to the operating speed range of the power turbine was established between 67 and 100 percent as illustrated in Fig.3. The actual ratios are within 0.2 to 4.4 percent of the calculated optimum ratios. From the ratios, and taking in account the torque multiplication of the power turbine output shaft with decreasing speed, a unit pump performance curve was developed as shown in Fig.4. The transmission-to-pump mechanical coupling is an industrial drive line with a spline-type slip joint. The drive line was selected to allow for misalignment between the transmission and pump resulting from the rather flexible truck frame on which the drive train components are mounted, Fig.5 Remote panel from which turbine powered pumper is controlled CONTROL REQUIREMENT AND DESIGN To meet all the operational requirements listed, the control panel was designed for remote location with two 30 conductor cables between it and the two engines. This panel was designed for utmost ease of operation to include the throttle, master switch, automatic start button, transmission shift buttons, plus monitoring gages which are the gas producer and power turbine tachometers and monitoring lights to include turbine oil temperature and pressure, transmission oil pressure, pump oil pressure, and transmission shift release (Fig.5). Since the application of the gas turbine determines the type of primary control, the throttle to gas producer section and, subsequently, the power level of the engine is the prime control in this unit. This throttle is an electrical servo motor operating on the force balance principle. The servo motor is connected to the throttle lever on the fuel control system, and since the angular displacement of this lever is not a linear relationship to the gas producer power level, a nonlinear variable potentiometer was designed and used in the control circuit as is the throttle knob on the panel. The result eras more precise power control between the 90 and 100 percent speed of the gas producer. The only indicating gages on the panel are the tachometers for the gas producer and power turbine speed. The gas producer tachometer is conventional zero to 120 percent indication, but the power turbine tachometer is color indication only. From zero to 67 percent, the indicating 3

5 pointer is in a red zone, from 67 to 100 percent in a green zone, and from 100 to 120 percent in a red zone again. Since the power train and transmission ratios were designed to operate the power turbine in the 67 to 100 percent range, the operator merely has to keep the pointer in the green zone. If it falls below 67 percent he down shifts; if it climbs to 100 percent and the gas producer indicator declines, he selects the next higher gear ratio in the transmission. To continue with design for operating ease, and since the controls were remote located, the transmission was designed to include power shifting. Push-button electrical switches at the panel energized solenoid valving at the transmission to direct high-pressure oil to the selected clutch pack. The shift cycle time lapse is 0.1 to 0.3 sec, during which the turbine is in a no-loaded condition. To prevent power turbine overspeed when the load on the turbine is suddenly released, a shift button/throttle interlock system was developed which locks the shift keyboard whenever the power level of the gas producer is above 67 percent. The operator, therefore, has to manually reduce the engine power level before he negotiates a transmission shift. The protective controls on the design are quite limited. The unit is in constant attendance while operating, so monitor lights are used. The few protective controls are the power turbine overspeed shutdown set at 110 percent for shutdown control, a modulating topping governor, and a combination indicating/shutdown turbine inlet temperature control system developed by SOLAR for the engine. If the power turbine inlet temperature reaches the high set point of 1155 F, a warning light so indicates at the control panel and a five-second time delay relay is energized. If the operator does not react in the five-second period to reduce the power level of the engine, then the engine is automatically shut down. An ultimate shutdown temperature of 1300 F is also included in this control and is independent of time. AUXILIARY SUPPORT DESIGN In order to adequately complete the turbine installation, extra care was given to the design of auxiliary support systems. Hydraulic starting was selected over electrical starting for this reason. A third power source, the truck chassis engine, was available to drive the hydraulic pump. The hydraulic circuit was sized at 50 gpm at 2000 psi, which provides adequate power to drive the turbine up to 50 percent gas producer speed. Only at 50 percent does the starter cut out, controlled by the gas producer speed monitor switch. This system insures against "hung starts" and subsequent internal damage to the turbine. The start cycle is set at 22 sec per SOLAR recommendation. Since the actual time the hydraulic circuit operates is so short, the turbine lubrication oil, which is a 45-gal capacity, is used for the hydraulic fluid. The start cycle is automatic, including the acceleration of the truck engine to increase the circulating rate of the hydraulics, and the return of the truck engine to an idle condition after the turbine start has been completed. The truck engine driven alternator is 24 v, and the battery pack is heavy duty 200 amp-hr capacity which is sufficient to support the turbine controls and monitoring system. With this combination of hydraulic starting and 24-v truck engine drive alternator, the electrical starter/ generator systems were eliminated from both turbines. The air filtration system was designed for a pressure drop of only 0.7 in. of water across the filter. This is far less than the 4 in. of water allowed by the turbine manufacturer. It must be kept in mind that this unit operates in a rather dusty atmosphere, and we have no assurance the filter will be cleaned on schedule. The oil-cooling systems for the turbine, transmission, and triplex pump all use oil-to-air heat exchanger mounted horizontally on top of the unit to minimize the opportunity to recirculate heated air from the exchanger into the turbine inlet. FIELD EXPERIENCE This first pumper was released to field operation in October It was almost immediately recognized that an operating checklist was required to insure consistently successful unit start up and operation. This checklist system is broken down into four major categories: 1 Turbine start and checkout in station 2 Turbine pre-job checkout and start at job site 3 Turbine post-job shut down at job site 4 Turbine post-job checkout in station Since the implementation of this operating method, the engine and, in fact, total unit reliability has been excellent. At the end of the first 100 hr of field operation, the engine/transmission package was given a thorough visual inspection. No indication of problem areas were detected, and the unit was committed to extended service. 4

6 Operating and maintenance costs of the unit considered a successful design, generally meeting is near equal to that of two of the three 700-hp functional and operational specifications. Actual units it replaced. gross vehicle weight was 44,740 lb with a front Substantial savings is realized on the chas- axle weight of 13,450 lb, slightly more than desic cost as we now use only one, as compared to sired. the previous three needed to deliver an equivalent With the implementation of the operating amount of horsepower to a well site. Turbine main- check list program, the total unit reliability has tenance costs have been negligible to date. Trans- exceeded expectations, particularly the electrical mission and accessories costs are acceptable. controls. Savings in operation and maintenance Pump costs have increased, and this was predicted expense is acceptable with an additional savings to occur. In a triplex pump in this type of serv- of $21,000/year in labor, since only one operator ice, the valving in the fluid section of the pump controls the horsepower previously requiring the and the three packing assemblies for the plungers attention of three people. are considered expendable parts. The life of these Two more units are in the manufacturing components is governed by the abrasiveness of process with no major design changes. Projected slurry being pumped, the pressure at which it is completion date of these additional pumpers is pumped, and the rate at which it is pumped. With Parch and June this higher horsepower input to the pump by the Solar T-902 turbine, higher rates of pumping, of course, are established at a set pressure as compared to a unit of lesser ACKNOWLEDGET horsepower. C011CLUSIONS The 2000-hp gas turbine powered pumper is The author wishes to thank the management of Byron Jackson Inc., a subsidiary of Borg-Warner Corporation, for permission to prepare and publish this paper. 5