Multiphase Pumping
Introduction
Multiphase pumps are most commonly used to add energy to unprocessed fluids for transportation to central processing facilities a long distance downstream. A reduction, consolidation, or elimination of the production infrastructure, such as separation equipment and offshore platforms, also can be achieved. In this way, marginal fields in hostile environments can be developed more economically. In mature fields, MPPs have potential to reduce the backpressure on producing wells, leading to an increase in production rate and recovery efficiency. There also is an environmental advantage of MPP application: the possibility to reduce gas emission and flaring [1]. Also, enhanced oil recovery for fields in deep water or with long stepouts can be achieved using subsea multiphase pump systems. Electric power to the subsea system is supplied through a subsea cable which is terminated to a high voltage - low current connector. This in turn is terminated to a step down transformer. The connection from subsea transformer to the pump motors is made via low voltage - high current jumpers containing wet mateable connectors and penetrators [2].
Multiphase pumps
Oil and gas wells include a wellbore formed in the earth to access hydrocarbon bearing formations. Typically, a borehole is initially formed and thereafter the borehole is lined with steel pipe, or casing in order to prevent cave in and facilitate the isolation of portions of the wellbore. To complete the well, at least one area of the wellbore casing is perforated to form a fluid path for the hydrocarbons that either flow upwards to the surface of the well due to naturally occurring formation pressure or are urged upwards with some form of artificial lift. Regardless of the manner in which the hydrocarbons reach the surface of the well, this flow will arrive as a mixture of oil, gas, dirt and sand which is referred to as a "wellstream" or "fluidstream". The fluidstream is then transported by a flowline to a predetermined location, such as a separator where it may be separated into gas, liquids, and solids. If the fluidstream cannot flow to the separator, it may be pumped by a multiphase pump. These pumps must be capable of moving volumes of the oil, gas, water or other substances making up the fluidstream. The pumps can be located offshore or onshore and can be connected to a single or multiple wellheads through the use of a manifold.
Over the past 20 years, two principle types of rotary pumps have been used as multiphase pumps: the twin screw pump and the helico-axial pump. The twin screw pump is a positive displacement pump constructed basically of two intermeshing screws. The fluidstream enters the pump from the wellhead and is trapped between the screws of the pump. The rotation of two screws forces the fluidstream into the downstream flowline. The helico-axial style pump combines positive displacement with dynamic compression and is basically constructed of turbine blades in combination with a screw driver. This combination imparts energy from turbine blades and the screw drive into the discharged fluids.
The rotary style multiphase pumps have been popular due to their long market exposure but have demonstrated deficiencies. Maintenance problems that usually require more than 24 hours to resolve is one deficiency that affects both the twin screw pump and the helico-axial pump. Many of these problems are associated with erosion or heat that damage the mechanical seals. Sand can also erode the screws and liners of the pumps. Excessive amounts of gas can cause a reduction in the dynamic performance occur in the helico-axial pumps and can lead to build up and gas locking in the twin screw pumps. Conversely, excessively long liquid slugs can affect the efficiency of the helico-axial pumps.
A horizontal, reciprocating pump has been successfully deployed for low to medium gas volume fraction applications. This pump contains horizontal rams that are moved in and out by a rotating crankshaft. The pump has reasonable tolerance for sand in the well stream. It uses replaceable liners to cover and protect the compression cylinders which can be changed in the field. Even though the horizontal reciprocating pump overcomes some of the deficiencies of a rotary style multiphase pump it may experience dynamic problems if the flow is mainly gas.
More recently, vertical reciprocating pumps have been used to transport well stream. These pumps were introduced to overcome deficiencies of rotary pumps. They operate at a slower pace than the rotary pumps, using larger volume chambers and long strokes to attain the flow rates desired. Due to the slow fluid velocities and vertical plunger design, sand and other impurities from a wellbore have little adverse effect on its moving parts. Because they have no rotating mechanical seals; they can handle a full range of fluid mixtures without requiring liquid trapping or re-circulation to insure seal survival. Preferably driving cylinders are placed in line with their respective plungers. Power fluid supplied from a pressure compensated pump is used to drive one plunger fully down, triggering a sudden pressure increase at the end of the stroke. This pressure spike is used to shift a shuttle valve, causing the swash plate of the compensated pump to reverse angle and to redirect the power fluid to the opposite cylinder. Each power circuit is connected to the piston end of one cylinder and also to the rod end of the other cylinder, thus assuring that the opposite plunger will be driven upward when the first plunger is moving downward.
Although the vertical pump overcomes many of the deficiencies in the prior pumps, problems still exist with the use of vertical plungers in a hydraulically driven multiphase pump. For example, if a deficit of hydraulic fluid occurs, the pump will pause, and go to neutral, and may need intervention to restart. In another example, pressure spikes created during the operation of the hydraulically driven pump can cause premature failures in relief valves and hoses at the end fittings. These pressure spikes occur when one of the plungers reaches its preset retracted position and thereby causing the fluid to be further compressed in the hose without any way of escape. This increase pressure is utilized in the system to cause the swash plate in the pressure compensated pump to reverse angle thereby redirecting the flow of hydraulic fluid to the opposite cylinder. Since the swash plate does not change direction instantaneously, the pressure continues to increase in the hoses thereby causing a very high pressure spike resulting in failure of hydraulic components. In yet another example, when an inlet pressure is insufficient to raise the ascending plunger ahead of the descending plunger the pump begins to short stroke on subsequent cycles and ultimately stop pumping. The combination of these problems greatly reduced the functionality of hydraulically driven multiphase pump.
In view of the deficiencies of currently available hydraulically driven multiphase pump a need exists for a hydraulically driven pump that operates effectively and efficiently in pumping multiphase liquids and does not systematically pause during a pumping cycle. There is a further need for a hydraulically driven multiphase pump that is not subject to premature failure of hydraulic components and hoses. There is yet a further need for a hydraulically driven multiphase pump that does not short stroke while operating in various pressure conditions [3].
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