Gas compression race begins
  from: Offshore Engineer
  by: John Sheehan
  Monday, July 07, 2008

Battle lines have been drawn between MAN Turbo and Siemens in the quest for a subsea gas compression unit designed to be ready for use on StatoilHydro’s Norwegian Sea Åsgard field within five years. John Sheehan reports.

Testing is already under way on MAN Turbo’s Hofim-type unit at the K-lab at Norway’s Kårstø gas terminal and this will be followed next year by trialing of Siemens’ ECO-II compressor.

StatoilHydro is looking to qualify the subsea compression units to increase recovery from the Midgard and Mikkel fields in the Åsgard area. By 2013, the company aims to have two 8MW compressors powered from the Åsgard B platform on the seabed in 250m of water.

Speaking at the launch of the world’s first full-scale testing programme of a compressor for subsea application at K-lab, Rolv Herfjord, Åsgard minimum flow project manager, explained: ‘There are two ways for compression to be carried out.We could have compression on Åsgard B or we could have it locally either on a new compressor platform or it could be on the seabed. Of course, the subsea compressor is the solution for the future. It is tomorrow’s technology.’

Herfjord said that with the subsea compressor solution, StatoilHydro would be aiming to recover 80% of the gas from Midgard and 70% from Mikkel.

‘A subsea compressor station would look just the same as being on a platform. The wellstream passes through a scrubber and is split into gas and liquid. You compress the gas and pump the liquid and downstream of the compressor you mix the gas and the liquid and feed the stream to the Åsgard B platform,’ he explained.

‘It is very simple, but all this would be taking place on the seabed and the challenge is that this is an environment with no eyes, ears or hands. There is noone there to carry out maintenance which would be a costly and time-consuming exercise.’

Five-year cycle

For maintenance work to be carried out, the unit would have to be lifted from the seabed taking out valuable production time.

Herfjord added: ‘Our goal is to have a five-year interval between regular maintenance and so the unit should be flexible, reliable and robust in relation to changing operating conditions.

‘The composition of the well fluids will change over time and maybe the water output and liquid output will increase over time and the machine should be capable of handling these various changes.’

He said the compressor station should also be modularized for easy individual retrieval of components and sub-systems. ‘That is an absolute requirement,’ he declared.

Herfjord warned, however, that there is a major technology gap that needs to be closed before a subsea compressor becomes a reality.

‘There are a number of challenges with having a compressor rotating at speeds up to 10,000-11,000 revolutions per minute subsea and of course a conventional compressor is ruled out.

‘In a conventional compressor you have a motor, you have a gear, a shaft seal system and that is not the sort of compressor you could install subsea. It has to be a compact design, so you have the motor and the compressor on the same shaft rotating at the same speed. There is no gear, no oil seal system and having a compact design you reduce weight as well as the footprint area.’

K-lab upgrade

Herfjord said K-lab had been chosen to carry out the testing because of its years of experience and the abundant availability of natural gas. ‘I don’t believe there is a test site in the world that could compete with K-lab,’ he added.

The facility has undergone a $50 million upgrade to enable the trials to take place and as K-lab manager Trond Austrheim explained: ‘We are going to take a deep dive into the technology to see what it is good for and how robust it really is.

‘We are going to push the compressors to the limit or even over the limit to see how the machinery copes with attacks from the gas.Will it cope with liquid being injected and how will it react. What if something happens subsea, will it be able to cope?’

Tests on the MAN Turbo compressor are expected to last for six or seven months and will run continuously for 3000-4000 hours.

The compact Hofim-type compression system features a newly developed, integral high speed motor and is the product of some 20 years of evolution, as Bernhard Haberthuer, vice president for sales and marketing explained at the test launch. ‘Today we have the latest version ready to undergo its final qualification steps and if all goes well as expected, the technology will be available in the market wherever challenging conditions require it.

‘The concept for the compressor has evolved over the years and has been constantly improved and its application range extended. The design is very simple and compact compared to conventional oil lubricated compressors,’ added Haberthuer.

‘There are only a few key components including a hermetically sealed pressure vessel to hold the casing for the hydraulic compressor and a fully integrated electrical motor that is cooled by the process gas. The rotating arms are levitated by magnetic bearings and these bearings are wear free and electronically controlled.’

Haberthuer said the design is based on a modular concept that can be customised to the requirements of individual fields. He added: ‘The technology has been constantly improved and special design considerations have been made to account for the harsh operating conditions. For instance contact with wet and corrosive gas, contact with liquids and some allowances were made for erosion from sand.’

He said the current design is expected to be robust enough to operate with the required maintenance intervals of five years and longer.

Once testing is complete on the MAN Turbo compressor, Siemens will enter the fray next year with its ECO-II unit which will undergo the same stringent tests as its rival.

The fundamental difference between the two units is that while the MAN Turbo compressor is based on a horizontal design and is cooled by process gas, the Siemens unit stands vertically and is cooled by oil.

Siemens, which has teamed up with FMC Technologies to work on subsea projects, says the unit is based on a recently introduced compressor concept, which is unique in that it is seal-less and canned and contains an integrated electrical motor drive specifically designed for non-clean applications such as sour, acid or toxic gas.

The ECO-II compressor is a multistage centrifugal compressor, with gas-cooled magnetic bearings, driven by a compact variable high speed induction motor. The fact that it has no seals means it is emissions-free. It can also handle wet gas making it a suitable candidate for subsea application.

A first prototype of the ECO-II compressor, a joint development by Siemens and Shell Global Solutions, is currently operational on Nam’s onshore Vries-4 gas field in the Netherlands.

Ormen Lange

The prize for the manufacturer of the winning subsea gas compression concept will be enormous with dozens of stranded gas fields or fields in harsh environments and ultra-deepwater ripe to be tapped with subsea boosting technology.

Other technology suppliers are acutely aware of this, however, and partners in the Ormen Lange field, now operated by Shell, are also qualifying new technology that they hope will help boost output from the field.

Aker Solutions (formerly Aker Kvaerner) and GE Oil & Gas are supplying the technology that will be tested in the $500 million Ormen Lange pilot project.

As Bernt Granaas, project manager for Ormen Lange phase two, explained: ‘The subsea compression concept is currently in the technology qualification stage. A pilot project to construct and test a single train is under way.

‘The compression hardware is at the procurement stage, managed by StatoilHydro and the site construction is currently under preparation at Nyhamna, the Ormen Lange onshore plant site. Detail design and installation of support equipment and hookup begins this summer and will continue through to the second half 2010.’

According to Granaas, engineering and procurement will continue through to mid-2009 and component testing will then be carried out 2009/10. Integrated system testing underwater is anticipated from 2010 to 2012.

A successful outcome to testing of the subsea compressor for Ormen Lange would see four 12.5MW units installed on the seabed at a depth of 860m, with the capacity to process 60 million m3/d of gas and 7200 million m3/d of condensate.

A 25,000t platform would be replaced by a subsea system weighing in at around 3500t, generating significant cost savings. The subsea system would be supplied with 52MW of power via a cable laid from Nyhamna. A final decision on the solution will be taken in 2011.

Granaas added: ‘Testing will be in two phases with initial qualification testing of components and modules in the manufacturing plants. Once assembled and fully proven in dry conditions, the modules will be installed in the Nyhamna test basin and system function testing completed. Following functional testing and qualification, long duration endurance testing is planned.’

He said the testing would be similar to that being carried out on the MAN Turbo and Siemens compressors at K-lab. Costs are at the root of the quest for subsea gas compression and it is estimated that savings of up to 40% could be made over a conventional platformbased system if the equipment can be marininized.

As Granaas stated: ‘The subsea compression system will be selected if it can demonstrate advantages in operability and project economics, compared to alternatives available for Ormen Lange compression. The pilot project has been set up to test this.’OE

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