Remote oil fields place extreme demands on conveyor technology

Paul Boughton

Robert Kurz describes how the world's largest progressing cavity pump for crude oil beats extreme conditions.

Whether in the tundra of Alaska, in the Arabian desert or miles offshore in the middle of the ocean, ever more remote deposits are being tapped in the constant search for crude oil.

Systems are often at the mercy of wind and weather. Maintenance work is difficult in environments far from human habitation. Robustness is therefore paramount for humans and machines. The changing consistency of the mixture of oil, water, solids and gas also puts immense stress on conveying systems.

That is why the operator of an oil field in the Central Asian desert had a NETZSCH multiphase pump installed. The world's largest progressing cavity pump in the industry is able to continue working even when there are strong fluctuations in the consistency of the material. Integrated heating and a lubricated gasket system were added to the system to beat the extreme weather on site.

Sand, water and gas in particular in the unseparated crude oil repeatedly caused issues in the previous conveying system at the mine site in the middle of the Eurasian desert belt. The fluctuating and changing composition of the pumped material frequently brought the installed centrifugal rotary pumps to a standstill. The high-energy costs were an additional disadvantage.

The pumps were also not technically adapted to handling slugs - the plugs that develop when a mixture alternately has a high solid content and a high gas content; plugs can cause the material flow to break off which damages the technology. The oil field operating company therefore decided to switch to a special multiphase pump to transport their crude oil.[Page Break]

Precise dosing

Most pumps are either suitable for solids or for gases, depending on their design. Progressing cavity pumps, on the other hand, can handle all combinations.

The technology is based on the principle of displacement. The medium moves from the suction housing to the conveying elements - the stator and the rotor.

The helical rotor rotates regularly inside the fixed stator creating closed conveying chambers between the two matched segments. The medium in these chambers is moved continuously from the suction side to the pressure side by the movement of the rotor. The mixture to be transported is therefore conveyed in precisely dosed portions with low pulsation and irrespective of its consistency.

The specific design of the pump is adapted to the specific application. That means that the materials for the housing, gaskets, stator and rotor are selected for the anticipated conditions, and the precise configurations of the parts are adjusted to match. The multiphase pumps can be operated reliably and at stable pressure even with a high-water content and a gas content of up to 90 per cent under this specification.

As a lot of gas was frequently present in the oil mixture in Central Asia, there was a danger that the sealing surfaces of the mechanical seal could occasionally run dry and break. To prevent that, the engineers installed a system with a quench supply. A 5-litre tank with rinsing fluid ensures that the sealing material is always lubricated, and prevents increased wear and tear. As any sealing lip attached externally would have been exposed to low minus temperatures without protection, a double-acting mechanical seal was used instead. This robust design increases the working life and reduces the need for maintenance - an important consideration in terms of operational reliability at the isolated location.[Page Break]

Resistant materials

The temperature gradient in the middle of the Eurasian continental plate is particularly dramatic as a result of the inner-continental climate. In summer, temperatures frequently reach 40°C, whereas the thermometer drops to -40°C in winter. Although the heat of the pumped medium is around 10 to 30°C, the ambient air temperature is typically -20°C on site. In addition to the type and material of the gasket, the materials for the rotor, stator and other components of the pump from the motor to the gear box therefore also had to be selected and adapted specifically. The system was also fitted with trace heating. The pump can then still be used without problems at a temperature low of -40°C.

The NETZSCH multiphase pump has already been in service in the desert for about two years. The system has been conveying up to 160m3/h of oil-water-sand-gas mixture with a viscosity of between 100 and 1,000cP without interruption at up to 36 bar pressure. The manufacturer also supplies systems for flow rates up to 600m3/h and pressure of up to 60 bar where higher volumes are required. The gearbox and the motor have been fitted with an adjusting device to align the elastic coupling optimally. The advantage is that the operator can readjust the drive unit independently as necessary, thereby preventing damage to coupling bearings or to the gasket.

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Robert Kurz is Head of the Business Field Oil & Gas Downstream, NETZSCH Pumpen & Systeme GmbH in Waldkraiburg, Germany.

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