The problem with pure subsea extraction lies in the restricted accessibility to the machines deployed on the seabed. As a result, reliability and freedom from maintenance are very important. The number of supply and discharge lines required for operation of the machines must also be minimized. In light of these considerations, this application requires a seal-less compressor with an integrated motor. The seal-less concept also ensures that there can be no exchange of material between the extraction unit and the environment.

Because it is hermetically sealed, this compressor model is also suitable for the compression of particularly valuable or toxic gases under conventional conditions.

One invention by Wolfgang Zacharias (58) concerns the cooling of the electric motor, which is subjected to the delivery pressure. A pressure profile in the cooling circuit ensures the mechanical integrity of the stator enclosure.

The equipment used today to extract oil and gas normally comprises two separate units: a motor and a compressor. The compressor’s job is to compress the extracted raw material so that it can be transported onward via pipelines. Difficult weather conditions make the extraction of oil and natural gas from drilling platforms or drillships risky and expensive. Service and repair work, for example, is very dangerous for the technicians.

Moreover, many deposits cannot be exploited because neither ships nor platforms can be used – in regions with icebergs, for example. Because there are often large oil and natural gas deposits in these areas, subsea extraction is being promoted. The aim is to directly pump oil and gas to the mainland through pipelines. However, this is only possible using an entirely newly developed motor and compressor component that has been designed for the special conditions that reign in deep water. Engineers from Siemens Energy Oil & Gas have been working for over five years on the design of this unit. STC-ECO, a large five-meter-square test unit built for use above the water, was completed two years ago. The steel enclosure, which has been designed to protect the combination motor and compressor against the tremendous external pressure in extremely deep water, is suspended in a very robust frame. Next to the steel enclosure is the cooling unit for the motor, whose lines lead into the enclosure. When used underwater, pipes leading out of the unit will connect it directly to the delivery pipe from the borehole and to additional pipes that transport the extracted oil and gas.

“These compressor-motor units can be deployed at depths of up to 3,000 meters,” says Zacharias. The inventor has already fulfilled a key prerequisite for ensuring that they are up to the task: He has come up with a dual cooling circuit for the subsea motor-compressor unit that solves the problem caused by different pressures in the various parts of the machine. In order to accommodate both the motor and the compressor in the protective steel enclosure, the motor must be able to withstand contact with the raw material. This pertains primarily to the copper windings of the motor, which would react chemically upon contact with the gas, resulting in fatal corrosion and erosion.

The copper windings are protected against the gas by a thin separating layer made of a high-strength fiberglass composite that surrounds the shaft like a pipe. “The gap between the shaft, the separating layer, and the copper winding must not exceed a few millimeters, otherwise too much efficiency is lost when force is transferred to the shaft,” says Zacharias explaining the special challenges posed by the design. The cooling oil flows through hoses into and out of the steel enclosure, where it circulates in the copper winding until it reaches this narrow gap. This cooling circuit is necessary because the internal copper windings generate so much heat that even the very cold water temperatures deep below the sea are insufficient to cool the interior of the steel enclosure. Because the gas circulates in the gap between the separating layer and the shaft inside the steel enclosure before being compressed and transported away via the pipes, there are two different pressures in the system – the gas pressure and the pressure of the cooling oil. The gas pressure varies wildly according to how much of the oil-gas mixture is being extracted from the borehole. “To prevent the protective sheath of the copper windings from being destroyed, the cooling oil circuit must have at least the same pressure as the gas,” explains Zacharias. He uses sophisticated control technology to continuously adapt the pressure of the cooling circuit to that of the gas. “These controls must work perfectly, for once the entire unit has been installed down in deep water, it must remain operable for years with no maintenance whatsoever,” he says. A new test unit intended for use under real-world conditions is currently being built.

Zacharias has been working on turbocompressors since completing his degree in mechanical engineering at the University of Paderborn, Germany. He began working as a development designer in Duisburg 36 years ago for what was then Mannesmann DEMAG. After a number of changes in ownership, Siemens acquired the turbocompressor division roughly 10 years ago. “I have experienced a lively corporate history full of changes while staying at the same location the whole time," laughs Zacharias. His field of work has remained constant through the years, however: designing turbocompressors and mechanical components for compressors, such as bearings and gas seals. Zacharias already has 28 invention applications, 34 granted individual patents, and 22 IPR families to show for his work in this field. “Before I started my studies, I did an apprenticeship as a metal forger at my father's company and always wanted to have a practical connection to my designer activity,” he says. Zacharias is the head of the Development Design department, where the standards, rules, and guidelines according to which the planning engineers build tailored compressors for customers are drawn up.

Zacharias’ hobby of making small metal forgings dates back to his metal forger apprenticeship. The married father of two grown children also stays in shape by playing team handball. He enjoys biking through Duisburg, which he feels is no longer the gray industrial city it once was. “Its image is changing from that of an old Ruhr industrial city to a green and innovative city,” he says.