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About 200 kilometers off the coast of Norway, 300 meters below the sea, dim shafts of light reveal a gigantic structure of yellow-painted steel struts. A gas compressor station the size of a soccer field is firmly anchored to the seafloor, ready to extract up to 1.28 billion cubic feet of gas per day.
The colossal high-tech setup ensures that gas will flow from the Statoil-operated Åsgard reservoir to the Åsgard B processing platform, 40 kilometers away, for years to come. The centrepiece of the underwater facility consists of two HOFIM ™ subsea compressors developed by MAN Diesel & Turbo in Zurich and delivered to Aker Solutions. As the general contractor, the company integrates and interconnects the high-tech elements for Statoil ASA. The stateowned Norwegian energy company is among the leading drivers in developing subsea extraction technology.
Without the underwater compressor module, Statoil would have had to cease its platform operations in the Åsgard field in the foreseeable future. Like many others, the Norwegian oil and gas company had to face a problem common to the industry. The further the extraction of hydrocarbons (oil and gas) progresses, the greater the decrease in natural pressure within a reservoir. Low pressure and the associated unstable flow in the underwater pipeline pose a risk of damage to processing facilities and jeopardize operations. That is why, to this day, only about half the volume of natural gas fields can be tapped into using conventional recovery methods. The solution to the problem lies in compressing the gas as closely to the source as possible, in other words, before flow instability occurs in the pipeline. “Thanks to the new compressor solution, we will achieve high recovery rates and extend the field’s productive service life until 2032,” says Siri Espedal Kindem, Senior Vice President for Åsgard operations, Statoil.
“Our compressor is designed in such a manner that it can be set up in close proximity to the borehole – in other words underwater,” explains Patrik Meli, Head of Engineering at MAN Diesel & Turbo Schweiz AG and the project director who oversaw the development of the subsea compressor. “The advantage is that the pressure in the pipeline is high enough again to avoid undesired flow conditions.
As a result, one can tap the existing fields for a longer period of time.” In addition, the higher pressure allows for higher extraction rates. In the case of Statoil, this means the new technology will enable the company to draw about 300 million more barrels of oil equivalent from the reservoir, representing a maximum recovery capacity of 21 million standard cubic meters of gas per day. At the same time, the innovative technology decreases energy consumption and, thereby, CO2 emissions. Meli mentions another benefit,
stating, “Building a platform costs a lot of money. However, installing the facility directly on the seabed also decreases additional costs.”
In the long run, the revolutionary technology could change the whole industry, given that a complete “subsea factory” is the production concept of the future. Statoil has been doing research to realize such a venture for decades. Ultimately, their goal is not only to relocate compressor systems onto the seafloor, but also the entire production facility, including gas treatment and distribution. MAN’s Diesel & Turbo subsea compressor has brought Statoil a significant step forward in realising their vision. “The Åsgard project is highly significant for us,” says Terje Steingrimsen, Senior Vice President, Emerging Subsea Technologies at Aker Solutions. “At Åsgard, together with our subcontractors, we have succeeded in creating added value for the long term.”
The particularities of underwater conditions, of course, initially required innovative solutions. Installations on the seabed are exposed to extreme stresses, and require a substantial amount of electronics to facilitate the inevitable remote control systems. Another key point is that the machines must be configured for a minimum amount of maintenance, given that maintenance means raising them to the surface from a depth of 300 meters at significant cost. To avoid having to dismantle the entire structure in such a case, Aker Solutions developed a special concept.
“The system is subdivided into single modules, comparable to a Lego system. In this way, each machine can be uninstalled individually,” explains Knut Olaf Nyborg, Vice President, Concept & Product Line Management, Aker Solutions. The utilized MAN Diesel & Turbo compressors have a critical advantage in this regard. Equipped with a highspeed induction motor and active magnetic bearings, the hermetically sealed HOFIM ™ (high-speed oil-free with integrated motor) compressors operate without the complex interaction of lubrication and dry gas seal systems, gearboxes, and couplings. They are, thus, less prone to breaking down, encapsulation makes them leak-tight, and maintenance efforts are reduced to a minimum because the compressors run almost without any mechanical friction.
The HOFIM™ compressor, also used in other applications like onshore gas storage, was specifically adapted for underwater use. To counteract the risk of corrosion inside the machine, all components are designed to be as robust as possible. “The moment there is any residual moisture, and hydrogen sulfide and carbon dioxide are involved, as is the case in subsea gas and oil recovery, corrosion can result,” explains Patrik Meli. Engineers substituted the original rotor consisting of many small disks with a “solid rotor” forged out of a single block of steel. To prevent machine overloads and associated frequent maintenance tasks, the engineers also designed a somewhat conservative machine layout that operates at lower speeds.
To ensure that the system functions reliably under such conditions, specialists tested the compressor over many years in Statoil’s water basin, built especially for this purpose on the premises of the company’s research laboratory in Karstö, Norway. When operated underwater, the machine’s thermal boundary conditions change and these can only be simulated prior to actual implementation. The system’s other components also underwent intensive testing. “There are no technical regulatory authorities for subsea installations. For that reason, we had to address, evaluate, and cover all uncertainties and risks thoroughly,” says Anders Storstenvik, Engineering Manager at Aker Solutions.
MAN Diesel & Turbo and Aker Solutions have now agreed on a long-term partnership to advance the development of next-generation subsea systems. The objective is to make the units smaller, lighter, and more cost-effective, enabling smaller oil and gas fields to profitably use the technology, as well. These units will then be ready to take the next ground-breaking step in the oil-and-gas recovery industry, namely a fully immersed subsea factory.
down, Statoil's revolutionary gas compressor station for subsea gas extraction is firmly anchored to the seafloor and ready to extract up to
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