Putting pressure on the seabed

April 1, 2012

Three Norwegian projects are racing to deliver a subsea technology solution that promises to yield major rewards for the industry. Terry Knott provides a thumbnail sketch of the subsea gas compression ambitions for the Åsgard, Gullfaks and Ormen Lange fields. 

It was just a little over a year ago that operator Statoil took the decision to opt for subsea gas compression to recover the remaining large gas reserves from the Åsgard field in the Norwegian Sea. A bold move in itself, but by adding to that a startup date of 2015 and an investment of some NKr15 billion, no-one can be in doubt that a concept that was a twinkle in the eye of engineers in the 1980s is about to become a firm offshore reality in the near future. In so doing, Åsgard is expected to become the world’s first subsea gas compression project to come onstream.

Layout of the Asgard field where subsea compression is expected to boost recovery by 278 million barrels oil equivalent.

The prize for being bold is clear. Production of gas and condensate from Åsgard and its two subsea satellites, Midgard and Mikkel located some 40-50km away in 250-325m of water, has declined and by around 2014 the fields’ reservoirs will have insufficient gas pressure to produce steadily. To maintain stable production rates, gas output pressure from the satellites must be boosted, and a minimum gas flow is also necessary to avoid the accumulation of MEG (hydrate inhibitor) in the flowlines – MEG flows from Åsgard A to Mikkel and Midgard and then back to Åsgard B (OE February 2011).

By installing subsea compression, Statoil intends to increase gas pressure from the satellites to maintain production, and estimates that an additional 278 million barrels of oil equivalent could be recovered.

MAN Turbo and Diesel has been selected as the supplier of the subsea gas compressors, which will deliver 21 million m3/d of gas. Two 11.5MW MAN subsea compressors will be installed in a 75m x 45m x 20m high subsea station weighing around 4800t, with electric power coming via subsea cable from the Åsgard A floating production vessel. The subsea station will incorporate two compression trains, with a common inlet scrubber and condensate pump, two coolers and an anti-surge system. 

The inlet scrubber will remove liquids from the arriving gas stream prior to compression – the liquids will then be returned to the gas stream downstream of the compressors.,

The compact horizontal MAN centrifugal compressor has magnetic bearings, the motor and compressor are housed in the same casing, and the motor is cooled by the process gas. The two machines will initially be operated in parallel and will be capable of boosting gas pressure by up to 50bar.

A MAN compressor was put though extensive qualification testing by Statoil at the operator’s K-lab in Kårstø to prove the concept of subsea gas compression. Now Statoil is about to begin tests of a pilot compressor.

Torstein Vintersto‘For the Åsgard trials, we are developing new test facilities at K-lab,’ says Torstein Vintersto, Statoil’s VP for project management.

‘These include a shallow water test pit to run a full-sized 11.5MW pilot compressor under water, a flow of 17 million standard m3/d of hydrocarbon gas, plus condensate and water/MEG injection.

‘In 2013 there will submerged tests of the two units to be used in the field, and the onshore spare.’

Several major contracts have been awarded in connection with the project. Among them, Aker Solutions has a contract valued at around NKr3.4 billion for design and construction of the subsea compression system, and a second contract worth an estimated NKr650 million for modifications on Åsgard A to provide electrical power to the compressor station, involving the construction and installation of a new 800t module.

Saipem has a contract for installation of the compressor station and a 900t manifold station on the seabed, as well as lifting the new power module onto Åsgard A.

 

The contract for marine operations, valued above €150 million, has been awarded to Technip, covering the installation of control and power umbilicals, structures, diverless tie-ins and connection to existing subsea infrastructure.

 

Wet gas contender

The Åsgard project looks to be firmly on track to bring subsea compression technology onto the industry’s main stage in the near term, offering an attractive and cost effective solution not only for extending late life production from mature fields, but also as an approach for new field developments.

But will Åsgard actually be the first subsea gas compression project to come onstream, or could there be another contender in the subsea compression race coming up on the rails?

In parallel with the Åsgard project, Statoil is also progressing plans for installing subsea compression in the Gullfaks South field. The goal is to boost overall gas recovery by some 3 billion m3, about 6% more than the current estimate Statoil holds for Gullfaks South.

‘Here we are targeting the recovery of an additional 10 million m3/d of gas from the Gullfaks South subsea satellite, tied back to the Gullfaks C platform some 15km away,’ says Vinterstø. ‘The compressor station would be located in 135m of water near the existing L and M subsea templates on Gullfaks South.’

Wet gas compression is being considered for the Gullfaks South field to capture an additional three billion cubic metres of gas. Inset: Modularised compression station housin two 5MW machines.

However, while the overall concept of boosting gas output pressure to encourage more production from the reservoir is the same as that for Åsgard, there are some fundamental differences in approach. Most notable is that the compressors being considered for Gullfaks are wet gas compressors that can handle a gas stream containing high percentages of condensate and water – if successful, this will eliminate the need for the upstream separation and anti-surge systems required for dry gas compressors, such as those being used for Åsgard, thereby reducing the overall size and cost of the subsea compressor station. And if a compact, lightweight subsea gas compression technology that is also mechanically robust can be realised, it would be attractive for a wide range of applications.

In May 2009, Statoil entered into an agreement with Framo Engineering for a technology qualification programme, aimed at developing a subsea wet gas compressor which could possibly be in operation in the field by 2015. The company – with long experience in the design and manufacture of subsea pumping systems, having notched up over 1.2 million running hours to date – has developed a compact contra-rotating gas compressor which it claims can tolerate 100% liquid flow.

Jon Arve Svaeren‘We have been investing in and working on wet gas compression designs and prototypes for many years,’ explains Jon Arve Svaeren, Framo sales director. ‘For Gullfaks we have developed and qualified a 5MW machine that we have tested submerged in a tank for over 3300 hours in our purpose-built hydrocarbon test loop at Fusa near Bergen. The test loop has been operating at flowrates up to 6000m3/h using a gas of identical composition to that at Gullfaks. What’s unique about this machine is that it can operate on 100% liquid flow for periods of time and so does not require any upstream separation of the liquids or anti-surge protection systems.’

That fact has a significant impact when it comes to the scale of the equipment needed on the seabed. According to Svaeren, the two 5MW compressors required for Gullfaks South would result in a modularised station weighing some 950t and measuring 34m x 20m x 12m – considerably lighter and smaller than the 4500t Åsgard station. The compressors, each one driven by two contra-rotating 2.5MW electric motors, would weigh 70t each, making them and the two associated 60t coolers retrievable from the station by a standard North Sea intervention vessel.

Framo's compact contra-rotating wet gas compressor.The equipment required upstream of the compressors includes coolers to ensure well fluids temperature does not exceed the limits of downstream equipment, and a well fluids mixer to mix the incoming multiphase fluids and provide equal feeds to the two compressors working in parallel.

The Framo wet gas compressor is a multi-stage contra-rotating, vertically mounted axial compressor, which for Gullfaks would be capable of boosting pressure by 30bar. 

Power would be supplied by seabed electrical cable running from Gullfaks C. 

"The compressor has a design shut in pressure of 390bar, a dry gas polytropic efficiency around 84%, and is very robust, incorporating all the same elements we have proven in our pumps, such as motors, seals, bearings and control systems," Svaeren points out. 

"The compressor motors run at relatively low speeds, 1200-4500rpm, which offers advantages in the design of variable speed drives located on the host platform, and the compressors can be operated in series or parallel to deliver a range of flow and pressure outputs."

Statoil’s final investment decision for the Gullfaks subsea compression project is expected shortly.

12.5MW pilot subsea compression system for ORmen Lange readying for submergence testing at Nyhamna.

Testing times

At Nyhamna on Norway’s west coast, testing of a full-scale pilot subsea gas compression system for the Norske Shell-operated Ormen Lange field has been under way since August last year – pressure at the giant gas field is steadily declining and towards the end of this decade pressure boosting will be required to maintain gas deliveries. The Nyhamna pilot trials will help Shell decide if gas compression can be reliably achieved subsea, or whether a floating compression platform will be needed. That decision is expected this year. 

Ormen Lange, being located 120km offshore in 8+60m of water and having no offshore facilities on the surface, presents more challenging conditions than those at Asgard or Gullfaks. So too does Ormen Lange's gas compression requirements some 60 million m3/d of gas must be raised in pressure form 80bar to 140bar. IF full subsea gas compression facilities were installed to meet this challenge it would require 58MW of power to be transmitted at high voltage from shore, complex electrical contreol systems on the seabed, gas/liquid/sand separation, condesate pumping and some hefty subsea compressors. Gas from Ormen Lange comes directly by pipeline to the Nyhamna terminal. Here, some of the gas is being used to test a 12.5MW GE Oil & Gas centrifugal compressor, housed within a full compressor system complete with power supply, designed and built by Aker Solutions (OE January 2008). The pilot plant, equivalent to one subsea compression train for Ormen Lange, is being tested in a water-filled 42m x 28m x1 4m deep pit to simulate the subsea environment. Statoil is conducting the tests on behalf of Norske Shell.

If the pilot can meet the demanding performance and reliability criteria set for it, subsea compression for Ormen Lange would be a more economical solution than would a new deepwater compression platform. However, should the subsea option not fulfil its promise, as its ‘base case’ Norkse Shell has been developing a design for a manned tension leg platform with a topsides weighing around 32,000t as the alternative to a subsea compression station – the platform concept could be powered from shore or assisted by gas turbines onboard.

As OE went to press, Norske Shell confirmed a shift has been made in its overall approach to providing the required compression. For the past few years it has been generally assumed that a subsea compression station for Ormen Lange would consist of four 12.5MW compression trains weighing around 8000t in total. But Norske Shell has modified its base case such that part of the compression capacity will be located onshore at Nyhamna based on a single 30MW compressor, with two 12.5MW trains offshore, be these on the seabed or on a platform. The onshore compressor will be installed prior to the offshore facilities being installed.

But for now, the jury remains out on the way the Ormen Lange compression challenge will finally be met.OE



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