Solving subsea’s biggest challenge



Developing technology to compress natural gas on the seabed and send it straight to shore - as an alternative to installing a large offshore platform - has been a long-term goal for the offshore industry. Now that goal is just one step away from becoming reality, as final testing is underway for the full scale Ormen Lange subsea compression pilot, designed and built by Aker Solutions.

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Boosting Åsgard production


Aker Solutions is at the forefront of subsea gas compression. In addition to the Ormen Lange pilot, the company was awarded the contract by operator Statoil to supply a complete subsea compression system for Norway’s Åsgard field.

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The Ormen Lange pilot is the best opportunity the industry has yet had to prove that subsea gas compression is the future.

By Alex Markland

At Nyhamna, on the west coast of Norway, a large pit dug out of the rock is about to play a vital part in a 25-year long saga. Inside the pit and submerged under water sits the world's first full-scale subsea processing and compression system as it begins up to two years of rigorous testing to ensure it can meet the demands of one of the toughest challenges ever set by the offshore industry.

The pioneering subsea system, a large and highly sophisticated equipment package some 120 feet (36m) long and weighing over 1000 tons, is the pilot subsea compression plant for the giant Ormen Lange gas field.

Ormen Lange, operated by Norske Shell and Norway's second largest gas field, came onstream in September 2007 and reached plateau production in November 2009, delivering up to 2.5 billion cubic feet (70 million m3/d) of gas and 32,000b/d of condensate to the Nyhamna processing plant in mid-Norway. Production from the field comes from fifteen wells located some 75 miles (120km) offshore in water depths of 2790-3600 feet (850-1100m) – there are no surface facilities such as a platform, with gas and condensate flowing directly to shore through two 30in (760mm) diameter multiphase pipelines.  

At present, the reservoir has sufficient natural pressure to drive the hydrocarbons to shore, but – as for all reservoirs – that pressure will steadily decline, and later this decade gas compression will be required in the field to boost pressure and maintain the flow. And before then, a critical decision must be made: should gas compression be achieved conventionally by locating compressors on the topsides of a new offshore platform in the field, or could gas boosting be achieved entirely subsea – potentially at far less cost - by using the breakthrough technology promised by the subsea pilot plant?

The benefits of opting for subsea compression for Ormen Lange or any other gas field are numerous. Capital and operating costs of a subsea station are more economical than those for a new deepwater compression platform – for example, for Ormen Lange, Norske Shell is currently developing a design for a manned tension leg platform with a 32,000 tons topsides as the alternative to an unmanned subsea compression station of around a quarter of that weight. Another key advantage is that by locating compressors on the seabed near to the subsea gas wells, the back pressure exerted on the gas reservoir will be much reduced, enabling greater production to be achieved.  But, before these and other benefits can be realised, a series of significant technology challenges must be overcome to ensure that the large-scale rotating equipment involved can be relied upon to operate continuously in deep water for long periods. The availability of the Ormen Lange production system has to be over 97.6%, it must remain in operation for 20-25 years, and the subsea intervention periods for maintenance are targeted at four to five years, a daunting challenge even for compressors based onshore.

Full-scale pilot

Subsea compression is not a new concept – indeed Aker Solutions has a long heritage in the technology which stretches back to 1985 when early ideas for subsea compression were beginning to take shape in the company. But despite the concept being mulled over by the industry for decades, it was not until 2006, following a competitive equipment qualification, development and testing programme for Ormen Lange carried out by many leading suppliers, that Aker Solutions won the $160 million contract to take development forward by designing and building the first full-scale pilot plant.

The Ormen Lange pilot was built throughout 2010 at Aker Solutions' Egersund offshore construction yard, in a new purpose-designed fabrication hall designated as a 'clean space' for the assembly and testing of sensitive subsea equipment.

"The pilot consists of eight large subsea modules, grouped to form process, control and high voltage power systems," says Svenn Ivar Fure, head of subsea processing, power and pumps in Aker Solutions. "Together these make up a single full-sized compression train, based upon a 12.5MW compressor. If the Nyhamna test programme is successful, the pilot could become one of four compression trains of this size, required to boost the pressure of Ormen Lange's gas from around 1160 psi to 2030 psi (80 bar to 140 bar)."

The modules for the pilot, standing some 36 feet (12m) high, contain the essential equipment for solving the technology challenge.  The entire system was put through several months of system testing at Egersund, before being disassembled and shipped to Nyhamna for reassembly in the 138 x 92 x 46 feet (42 x 28 x 14m) deep test pit, where it will be put through its paces on 'live' Ormen Lange gas and condensate. 

At the heart of the pilot is Aker Solutions' GasBooster compression module, housing a compressor, a compact 16 feet (5m) high vertical centrifugal machine which will operate at 11,000rpm. The innovative design of the compressor has reduced the number of moving parts and supporting systems. For example, the bearings are magnetic and do not require a lubrication system and need only low voltage power for their operation. The compressor and its high speed electric drive motor are housed in a single, hermetically sealed enclosure which is pressurised with a barrier system to keep the motor and compressor spaces separate, and also to ensure clean operating conditions for the motor and bearings – these will be cooled by process gas in a closed loop with an external seawater heat exchanger.

Process know-how

As important as the compressor and its reliable operation are to the project, it is the wider expertise of Aker Solutions that has been brought to bear on bringing together all of the many new components necessary to create the overall system. One example is for achieving liquid separation and boosting.

"A characteristic of all gas compressors is that they have a limited tolerance to the volume of liquids in the incoming gas stream, which therefore must be controlled," Fure points out. "The solution for the pilot is to remove the bulk of the liquid condensate before it reaches the compressor, raise its pressure and inject it back into the gas export line downstream of the compressor."

Separation of the condensate is achieved in a 10 feet (3m) diameter vertical separator upstream of the compressor – the separator also acts as a buffer for any liquid slugs coming from the wells. The condensate is routed to an Aker Solutions' LiquidBooster pump which feeds the liquids into the high pressure export gas line for transportation to shore. The separator also contains an Aker Solutions system for removing sand from the incoming well fluids to prevent sand building up inside the vessel and to protect the compressor. 

"The sand is pumped by the LiquidBooster into the gas export line to be carried to shore by the gas flow," notes Fure. "The 400kW multi-stage centrifugal pump has proved it can handle sand in extensive testing at our Tranby subsea facility, where we fed 140 tons of sand through a LiquidBooster as part of its qualification for Statoil's Tyrihans subsea water injection project."

The pump is directly driven by an electric motor, filled with a barrier fluid of monoethylene glycol (MEG) and water. If subsea compression becomes the chosen solution for Ormen Lange, the MEG will be supplied from the existing Ormen Lange subsea production facilities. And as this particular supply of MEG is recycled and regenerated onshore, the LiquidBooster pump has had to be qualified for operation with regenerated MEG, requiring a change in some of the pump's materials of construction.

Should the compressors need to be run in recycle mode, the liquid separator is also equipped with one anti-surge cooler which is designed to dissipate 9MW of heat to the surrounding seawater. Full-scale testing of the coolers was carried out at Tranby, in addition to the selection of the type and thickness of coating required on the heat exchangers to prevent biological growth caused by contact with seawater.

Power play

The Ormen Lange pilot is 'all electric' – there are no hydraulically operated components. The full compression station would be electrically operated and controlled, with around 58MW of power being transmitted from shore through a subsea power cable at 132 kilovolts, which would be stepped down by transformer to 22 kilovolts at the subsea station. The cable would also contain fibre optics for carrying control signals.

In the pilot plant, power is distributed to the compressor and pump through a circuit breaker module. This contains a number of circuit breakers, and also step-down transformers for charging the batteries in the pilot's two uninterrupted power supply (UPS) systems – the UPS will provide a back up for controlled shut-down and black-start, should power from shore fail.  All power components in the pilot are housed in sealed subsea enclosures.

"In operation, the circuit breaker enclosure will be filled with nitrogen at atmospheric pressure," says Fure. "We developed this for Ormen Lange operations in 2950 feet (900m) of water, and gained a lot of knowledge in sealing technology from this experience. Now we are working on an enclosure design for 9840 feet (3000m) water depth."

The list of sophisticated electrical components that had to be developed specifically for the Ormen Lange pilot is long. High on that list are the variable speed drives (VSDs) for the compressor and pump, which would be controlled from shore by signals sent through the fibre optics in the seabed power cable. As gas production rates and pressures from the Ormen Lange reservoir change over time, the VSDs will regulate the speeds of the drive motors by varying the supplied voltage and current frequency, thus enabling the compressor duty to be changed and the LiquidBooster pump speed to be regulated to control the liquid level in the separator. Additional challenges met by the new VSD design are the ability to run for five years without maintenance, and a much more compact size compared with the large VSDs normally used onshore - all the major equipment items in the pilot plant are sized to be retrievable from the seabed by surface lift. 

And so to Nyhamna where all of the ingenuity and engineering effort that has gone into the Ormen Lange pilot is currently being tested. In January 2011, Aker Solutions' Egersund yard shipped to the test site a purpose-built 900t process module which is designed to simulate a range of flow conditions, including slugs, to test the pilot. The Nyhamna test loop has capacity to deliver up to 530 million standard cubic feet (15 million Sm3/d) of gas per day, 11,300 barrels (1800 Sm3/d) per day of condensate, 16MW of electrical power and 580-2250 psi (40-155 bar) operating pressure.

"The Ormen Lange pilot is the best opportunity the industry has yet had to prove that subsea gas compression is the future," concludes Fure. "It has required advances in technology on a number of fronts, and Aker Solutions has combined its capability, knowledge and depth of experience to meet all of those challenges. We are confident that the outcome will be positive."

The 25-year subsea compression saga is set to enter its next chapter.

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