The exploration and development of oil and natural gas in offshore (offshore and deep-sea) oilfields has given birth to the birth and development of multiphase mixed transportation technology for oil and gas.
This is due to the separate laying of pipelines for oil and gas. The investment cost is too huge. After adopting the multi-phase mixed transportation technology, oil and gas transportation can share a pipeline, thereby greatly reducing the construction investment cost of pipelines and equipment, and can also reduce pipeline operation and monitoring costs after commissioning.
The development of this multiphase hybrid transmission technology is economically significant due to the long-distance transmission involved. A multiphase pump is key equipment for conveying a multiphase flow of oil and gas. In the process of multiphase flow transportation, the working conditions of the multiphase pump are very harsh and harsh. The transported medium is a mixed flow of crude oil, seawater, and natural gas. It also contains solid sand particles, so this is a veritable and real meaning “Gas-liquid-solid” three-phase flow pump.
Development overview of multiphase pump
Although the multi-phase mixed pump technology has been developed for more than 20 years, it is still considered a young technology because of the complex and changeable natural conditions of the subsea structure.
In the field operation of multiphase pumps, even at a moderate gas hold-up at the subsea wellhead (gas hold-up not exceeding 90), it may encounter a gas phase of 100 for a longer period of time (for example, no more than 2 hours) several times a day. , that is, running under dry rotation. At this time, the dynamic and static parts inside the pump will be damaged due to high-temperature rise caused by dry friction, resulting in a shutdown. The mechanical seals at the inlet and outlet of the multiphase pump are directly subjected to heterogeneous (oil and gas) multiphase flow. Temperature difference changes (thermal shock), resulting in temporary leakage of the seal face or even seal failure. These seemingly unsolvable technical problems have forced major oil companies to include breakthroughs in oil and gas mixed flow transportation technology into their development strategies.
Types of Multiphase Pumps
There are two main types of mainstream pumps that can be successfully put into industrial production: one is a helical axial flow multiphase pump belonging to a co-rotating power pump, and the other is a twin-screw multiphase pump belonging to a positive displacement pump. These two fundamentally different types of multiphase pumps have their own advantages and disadvantages, and they need to be analyzed in detail when applied.
The discharge flow of the screw axial flow multiphase pump is much larger than that of the twin-screw multiphase pump, and the structure is compact; it can operate under high sand content and is not sensitive to the solid sand contained in the multiphase flow. (that is, the sand content) can exceed 5%; the gas content can range from 10% to 90%, which is not as large as that of the twin-screw multiphase pump (up to 97%); the viscosity range of the medium is not as wide as that of the twin-screw multiphase pump, and the medium Too much viscosity can drastically reduce pump efficiency or even make it impossible to run. At present, the helical axial flow multiphase pump with the largest displacement in the world can reach a flow rate of 3300m’/h and a single machine power of up to 6000kW.
Due to the inherent characteristics of the twin-screw multiphase pump, it has some unparalleled unique advantages:
①It has the characteristics of the forced output of gas and liquid. No matter how the gas content changes, it can be forced to discharge, and the gas content of the medium can reach 97%;
②It can transport oil and gas mixed flow with high viscosity, and the dynamic viscosity of the medium can range from 2 to 2000cP;
③Because the master and slave screws are not driven by mutual meshing but driven by external synchronous gears, the master and slave screws and the screw and the stator do not contact each other, so they can withstand “dry rotation”, that is, 100% of the gas is delivered in a short time; ④ The twin-screw multiphase pump has high volumetric efficiency.
In 2008, a helical axial flow multiphase pump prototype test jointly developed by British Shell Oil Company, French Total Company, British Petroleum Company (BP), and Petrobras Company reached 15000kPa (150bar), which is equivalent to a 1500m water column of clean water. . When the inlet gas content is 50%, this head value is a breakthrough, which means that if two pumps are used in series in the subsea pumping station, they can produce oil at a depth of 2000-3000m underwater. This is the largest head (pressure difference) achieved by the helical axial flow multiphase pump in the world.
In a newly designed twin-screw multiphase pump from Bornemann Pump Company in Germany, the lift (pressure difference) obtained by the test is 172001cPa (172bar). This is an unprecedented important milestone in the development of twin-screw multiphase pumps, which means that the subsea depths that twin-screw multiphase pumps can now reach have exceeded that of screw axial flow multiphase pumps.
The weight of a polyphase pump unit (including the pump itself, the subsea high-voltage motor, the high-voltage electrical control system, etc.) installed in the subsea multiphase pumping station is about 20t, which shows that this kind of subsea technology is very difficult. In the “dual-purpose pumping station” under the sea, the multiphase pumps can be connected in series (pumping 3000m deep) or in parallel. If the displacement of the multiphase pump (referring to the helical axial flow multiphase pump) is large, one pump can serve several oil wells by connecting in parallel. Up to one multiphase pump can now be seen for four wells to pump a mixed flow of oil and gas.