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Prime mover for CT unit
Coiled Tubing (CT) power supply units are built in many different configurations, depending on the operating environment. Most are hydraulic-pressure pump systems powered by diesel engines, though a limited few employ electrical power. In general, the prime mover packages used on CT units are equipped with diesel engines and multistage hydraulic pumps that are typically rated for operating pressures of 3,000 to 5,000 psig. The hydraulic drive unit is supplied in the size necessary to operate all of the CT components in use and will vary with the needs of the hydraulic circuits employed.
Hydraulic power pack
The most common hydraulic power pack system is described as an “open loop” circuit, in which the fluid is discharged from the prescribed motor and returned to the hydraulic reservoir at atmospheric pressure. In general, open-loop power packs are equipped with vane-type hydraulic pumps and are rated for a maximum 3,000 psig service pressure applied to the hydraulic circuit. The pumps in these power packs provide source power for the:
- Service reel
- Well-control stack accumulators
- Console priority
- Auxiliary panels as needed
Where additional power to the injector circuit is needed, the hydraulic power pack may be designed as a “high-pressure, open-loop” system or as a “closed-loop” system. In both of these enhanced hydraulic power systems, the high-pressure circuit is limited to the injector hydraulics, with the remaining circuits powered by the vane-type pumps. The increased pressure in the hydraulic circuit for the injector provides the means for generating higher force loads within the injector motors as compared to the vane pumps, which are limited to 3,000 psig service. The high-pressure open-loop system typically uses a piston pump to provide hydraulic pressure as high as 5,000 psig to the injector circuit. The hydraulic fluid is discharged from the injector motors to the hydraulic reservoir tank at atmospheric pressure. The closed-loop hydraulic system also provides injector pressure to a maximum of 5,000 psig, with the distinction being that the hydraulic fluid is recirculated to the injector without returning to the hydraulic reservoir. The hydraulic fluid losses experienced through the injector motors are compensated by a charge pump incorporated into the closed loop circuit.
In general, the hydraulic pumps on the power pack are equipped with pressure-relief valves(or unloader valves) that limit the amount of hydraulic pressure the pump can deliver to the prescribed circuit. These unloader valves are set at the desired pressure for the respective circuit and must be checked periodically to ensure that they are functioning properly.
Specifically, the unloader valve on the injector circuit should be set at a pressure that limits the amount of force that can be applied to the tubing in tension (pulling) and compression (thrust). Before dispatch of CT service equipment from the vendor facility, the unloader valve on the injector circuit (either on the power pack or in the console) should be set to a pressure which does not exceed the safe load limit of the CT in service. Tests should be performed before equipment load-out to verify the sustained pressure output and fluid flow rate for the hydraulic pumps.
In current power pack design, an accumulator circuit is typically included to provide fluid volume and pressure for the well-control stack operation. The number of accumulator bottles typically ranges from one to six, depending on the size and pressure rating of the well-control stack in service. The accumulator package for well-control operation must have sufficient volume and pressure capacity to complete three complete function cycles of all the rams incorporated within the well-control stack without recharge from the power pack. These function cycles are typically described as “close-open-close” cycles and should be performed periodically to ensure that the accumulators are pre-charged to the appropriate pressure, and that the circuit is free of hydraulic leaks.
Noteworthy papers in OnePetro
Asif Ehtesham ,Robert Howard, and Sharlene Lindsay 2011. Coiled Tubing and Jointed Tubing Hybrid-String Well Intervention: Design, Testing, and Case History, SPE Asia Pacific Oil and Gas Conference and Exhibition, 20-22 September. 145793-MS. http://dx.doi.org/10.2118/145793-MS.
Prestridge, M.L. and Mahoney, A.J. 1994. 3 1/2-in. Coiled Tubing: A First Run, SPE/IADC Drilling Conference, 15-18 February. 27432-MS. http://dx.doi.org/10.2118/27432-MS.