Asgard's case for subsea gas compression

With a Norwegian sector breakthrough in the area of subsea gas compression very much on the cards for the next year or two, Statoil last month reaffirmed the case for compression in the Åsgard area while one the key vendors targeting this technology unveiled a timely enhancement of its motor-compressor units. Meg Chesshyre reports.

Statoil and its Åsgard partners are eyeing subsea gas compression as the best commercial option to help recover remaining reserves from the Åsgard area, with an investment decision due to be reached in 1Q 2011. Subsea gas compression has also been assessed on Gullfaks and a pilot project is under way for the Ormen Lange field together with operator Shell (OE January 2010).

On Åsgard, the Midgard and Mikkel gas deposits have been developed with seabed installations and are tied back to the Åsgard B production floater 40-50km away. Analysis shows that pressure in Midgard and Mikkel will become too low to sustain their ability to produce to the B platform towards the end of 2014.

‘Installing compressors will sustain the gas flow and ensure a long producing life for the platform,’ explains Ståle Tungesvik, Statoil’s head of reserves and business development. ‘This makes it possible to recover large volumes – we’re adding 28 billion cubic metres of gas and 14 million barrels of condensate after a possible investment decision.’

Statoil maintains that many years of development work underpin this choice. ‘Compressing gas on subsea installations represents a considerable technological leap for the industry,’ says Margareth Øvrum, Statoil executive vice president for technology & new energy. ‘With this technology in place, the recovery factor and producing life can be substantially increased for a number of gas fields. Moving compression to the seabed gives both improved energy efficiency and lower costs compared with keeping it on a platform or on land. The closer to the well we compress the gas, the higher the efficiency and the production rates.’

The compression unit will comprise a gas cooler, liquid separator and compressor, with the latter due to be powered from the Åsgard A oil production ship. Installing compressors between the reservoir and the receiving platform will reduce the pressure where the wellstream enters the seabed unit. That in turn will boost production from the field by enhancing the pressure difference between reservoir and seabed receiving installation. Through the compression process, the gas acquires sufficient additional pressure for it to be transported through the pipeline to the receiving platform.

Meanwhile, the Åsgard partners have made an investment decision to develop the Smørbukk North-East field as a satellite to the Åsgard B platform with production expected to start late 2011. Recoverable reserves of oil, gas and condensate on Smørbukk North-East are estimated at around 16 million barrels of oil equivalent. The expected development investment is close to NKr1 billion.

‘From sanctioning to start-up of the project we will need 13-14 months,’ says project manager Helge Rivelsrud. ‘This will be one of the fastest developments ever in the Norwegian Sea.’ The early start-up is possible by using an existing exploration well as a production well. Subsea equipment originally built for the Sigrid field and owned by the licencee will furthermore be modified and applied. ‘The modifications on Åsgard B will be small since we can hook up to an existing template,’ adds Rivelsrud.

M43 piles on the power

One of the companies in the vanguard of enabling technology development for subsea gas compression, MAN Diesel & Turbo, believes its newly developed 18MW high-speed motor will do its prospects no harm at all. The company describes the new motor, which recently gained ATEX certification, as ‘a crucial step in further enhancing its compressor technology for the worldwide oil & gas industry’. This substantial power boost stems from the company’s long-running R&D programme to prepare its Mopico (motor pipeline compressor) and Hofim (high-speed oil-free with integrated motor) motor-compressor series for subsea application.

According to project manager – offshore Thomas Gutjahr, MAN Diesel & Turbo now boasts the most powerful highspeed motor-compressor unit worldwide in its field of operation. ‘A unique motor design allows the unit to handle a wide range of gas qualities including wet and sour gas as found in upstream oil & gas applications,’ he says, adding that the robustness and operational reliability of the motor-compressor unit have been demonstrated after more than 9000 operating hours under severe testing conditions at Norway’s K-Lab for Statoil’s extensive subsea qualification programme.

For the past two years, Hofim M33 units have been equipped with an integrated 10MW high-speed motor design. With the latest development, the Hofim/Mopico family can now be provided with an upscaled motor to boost the power up to 18MW. Dubbed the ‘M43’, the new motor-compressor unit received its ATEX certification operating at 10,000rpm, establishing new industry benchmarks for such machines. Each motorcompressor unit consists of a single or tandem compressor arrangement. The complete rotating assembly levitates in magnetic bearings and is installed in a hermetically sealed housing, thus eliminating the need for shaft seals to atmosphere. The entire unit is oil-free and creates no gas emissions. OE

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