Why subsea compression is a keystone of the energy transition process

By Mario Vacca

Aker Solutions helps oil and gas operators transition to a more sustainable future.


/tran'ziSH()n, tran'siSH()n/

noun: transition; plural noun: transitions

"the process or a period of changing from one state or condition to another."

Image provided by Aker Solutions

We're facing one of the biggest challenges of humankind: climate change. We must find solutions to counter the change and reduce our CO2 emissions. And we need to find them quickly.

Alas, there isn't a team of superheroes who can save the planet. And there isn't – to date - a single disruptive technology that can prevent us from releasing tons of harmful CO2 into the atmosphere.

It's all about transitioning our ways to produce energy; from technologies that damage the environment to sustainable ones. And transitions happen gradually.

Saving the planet is on us. All of us. At Aker Solutions, engineers take this challenge very seriously. The company supports energy and utilities industry players in their transition towards more sustainable operation and helps them reach their net-zero goals.

Aker Solutions relies on various innovative technologies to achieve these objectives. One of these innovations consists in designing and installing compressors on the seabed to increase the output or extend the lifetime of a subsea well.

Henrik Alfredsson, Managing Director, and Nicolas Barras, Global Pursuit Manager, Subsea Compression, Pumps and Power at Aker Solutions, explain why subsea compression is essential for a successful energy transition process.

Discover subsea gas compression

The technology is relatively new. Subsea gas compression was first introduced in the 2010s. In September 2015, Åsgard became the world's first subsea gas compression facility. A few years later, the technology is one of the most important measures for delivering important gas volumes from existing fields on the Norwegian continental shelf.

How does it work?

In a gas reservoir, the pressure naturally declines over time as gas is taken from the reservoir. Gas is a compressible element, and when the pressure depletes, it naturally occupies more space in the flow lines. As a result, the velocity of the gas through the flow line increases, which results in further pressure drop. At a certain point the flow crawls to a halt and liquids start to accumulate in the flow line. This poses a significant risk to operations, effectively marking the end of field life.

Without gas compression, operators will shut down a well after extracting approximately 40% of the reserve, meaning that simply put 60% remains untouched in the well. Adding a compression station on the seabed helps augment the well's pressure and send the remaining gas onwards through to the host.

The benefits of subsea compression

The technology of topside compression has been in use since the '90s. However, a platform-based compressor demands a lot of energy. The gas must be pulled up first, then compressed, and sent back down through the pipes heading to shore. Placing the compressor on the seabed at essentially the same elevation as the wells minimizes the energy needed for compression power. In fact, compared to conventional compression technology, subsea compression can reduce the compression power by 90% and extensively increase the recovery from aging gas fields with minimum carbon footprint.

In short, subsea compression increases the amount of gas that is possible to extract from a well. It can at the same time increase the maximum production rate (plateau rate) beyond what would be feasible for a pressure sustained reservoir.

Support energy transition efforts

Where does our energy come from? In 2019, coal delivered 162.4 exajoules of energy versus 140.8 from natural gas extraction. However, the process of burning coal to produce energy releases nearly twice as many greenhouse gases into the atmosphere as natural gas power plants do.

Every step towards lower emissions counts. And every effort to decrease our reliance on coal-based energy brings us closer to a net-zero goal, where unavoidable emissions are effectively compensated.

Relying on fossil fuel, be it coal or natural gas, isn't a sustainable option in the longer term. But in the mid-term, burning gas instead of coal to produce our energy contributes to lowering emissions. And it buys us some time to fulfill the much-needed transition towards sustainable energy sources.

Gas reserves are still plentiful on the planet. If owners and operators focus on prolonging the lifetime of already developed wells, they are not only likely to increase profitability by minimizing investments while maintaining revenues. They also deliver an energy source that is less damaging to the environment.

A blend of monetary, political, and environmental benefits

Some nations like China, Malaysia, India, and Bangladesh, are heavily dependent on coal. Coal is the only large fuel source that is available domestically. Being energetically self-sufficient, or at least largely self-reliant, provides a major geopolitical advantage. But unlike coal or oil, natural gas and its sales product liquefied natural gas (LNG) can be purchased on the spot market. It grants nations that do not own large natural gas reserves the flexibility to maintain economic and political independence. For all of these reasons, coal-dependent nations should consider retrofitting their coal-fired power plants to employ LNG as an energy source.

Subsea gas compression is an energy-efficient solution. It generates fewer emissions, owing to the large reduction in required compression power compared to traditional topside solutions, and increases production from existing and new fields. Its environmental footprint is significantly lower than the one of topside or onshore solutions.

Its economic and environmental benefits are clear. But is the technology mature enough? How far can owners and operators today rely on it?

Image provided by Aker Solutions

Why digitalization matters

Why is digitalization so crucial to subsea compression technologies? Take the automotive industry example: digitalization helps cut down lead times and speeds up engineering processes. And, although simulation is widely used to accelerate the product development cycles, automotive manufacturers can always fully test their prototypes before introducing new technology to market.

But subsea process system manufacturers typically don't have that luxury. Think about the Jansz-Io field: it features three 12-megawatt compressors operating in parallel. It would be a daunting task to test for design purposes a system at full scale that weights more than 4000 tons and feeds an LNG plant producing 16 million tons per year. The first opportunity to physically test the full system will occur on the seabed at a depth of 1,300 meters. But, down there, there is no room for error. Failure is simply not an option. All systems need to be verified beforehand – but how?

The Jansz-Io field installation

The world-first subsea compression system was installed off the coast of mid-Norway on the Åsgard field. It has been delivered by Aker Solutions and its partners and is operated by Equinor. Since September 2015, the system operated with a regularity over 99.9%, resulting in an added production value of 220 billion Norwegian Krones (NOK) with a total investment of 19 billion NOK. The Åsgard Subsea Compression System (SCSt) example constitutes a solid proof that subsea compression is a robust, well-qualified technology ready for worldwide implementation.

The Jansz subsea compression system is based on the technology qualified for and field-proven on Åsgard. The Jansz system, however, presents its own technological and operational challenges. The Jansz SCSt Inlet Scrubber is an engineering feat undertaken to reduce the overall system size whilst handling immense gas rates. It is by far the world's largest high-pressure scrubber, with three compressors running through it, operating at a suction pressure in excess of 100 bar. And it is designed to operate autonomously, in a maintenance free environment, at 1400m water depth for 50 years. The single scrubber configuration significantly reduces the overall size of the system, compared to the standard configuration of a single inlet scrubber per compressor as seen on Åsgard. Another noteworthy engineering achievement is the SCSt Discharge Cooler which is certainly the world’s most effective passive subsea cooler. It has a rated heat dissipation in excess of 50-megawatt, operating at an efficiency approaching the one of an onshore process cooler.

Barras explains: "It is both exciting and challenging to know that in Western Australia, the conditions are quite different to what we experienced previously. So far, we've successfully completed three subsea compression projects, all of them in Norway. Norway has relatively shallow waters. In Western Australia, the waters are deeper. And the equipment must be designed to withstand enormous pressures, which is another challenge."

Despite the difficulties, all the complex equipment parts will need to operate flawlessly for the entire lifetime of the field and fulfill the requirements. How can engineering teams ensure the flawless operation of the systems?

Image provided by Aker Solutions

The benefits of integrated, trustworthy digital tools

Enters digitalization. Engineers will need to test all the equipment numerically before it goes down to the seabed. The Aker Solutions teams have spent multiple years testing and validating the numerical tools, with scale-testing methods, medium-pressure and high-pressure applications to validate the simulation processes. The Simcenter™ STAR-CCM+™ software is one of the numerical tools that underwent a thorough validation process. Simcenter STAR-CCM+ is part of Xcelerator™ portfolio, the comprehensive and integrated portfolio of software and services from Siemens Digital Industries Software.

Alfredsson explains: "The engineering of the inlet scrubber demonstrates the effectiveness of computational fluid dynamics (CFD) simulation. The hybrid multiphase model in Simcenter STAR-CCM+ is tailored to our applications, fluids, and pressures to ensure that it replicates real behaviors. We validated the model against test data obtained from separate test loops around the planet. Therefore, we can use our numerical test benches to qualify the technology. So, the first time we start up the station, we are already certain of its performance".

The Aker Solutions engineers used this approach previously to test the Åsgard subsea compression system. The numerical validation confirmed that the individual components were fully functional. Once in real conditions, the Åsgard field operations show a higher regularity, meaning that the operation uptime is higher than that of a field that doesn't rely on compression.

According to Alfredsson, "this is a true testament of a robust system enabled by the successful integration of digital tools and real-life components."

Lower emissions

Do you know which phase of the oil and gas production process is the most damaging to the environment? In fact, most CO2 emissions result from the operating systems' power consumption. A digital twin helps control, monitor, and ultimately optimize production. In that way, it contributes to optimizing or reducing the energy consumption of machinery in every step of the production process: extraction, separation, compression, etc. Ultimately, it minimizes the overall CO2 emissions.

Therefore, the digital twin technology not only helps optimize any process, whether it relies on topside or subsea compression, it also contributes to reducing harmful emissions.

Barras adds: "Depending on the type of application, the energy-saving range from 10 to 70%. This fulfills one of the objectives of Aker Solutions. The complete numerical models of the production systems, our so-called digital twins, support operators in optimizing their production process and reducing the energy consumption of the different systems."

Monitor conditions and minimize maintenance

Aker Solutions uses digital twin technologies based on the Simcenter portfolio as an integrated condition monitoring system for the scrubber and the inherent cooler technology. The digital twin informs how the system will or should operate during a specific timeframe depending on the conditions. It tells whether the system remains inside or falls outside its normal mode of operation. In which case, the operator learns that a potentially damaging event has occurred. This event may not cause an issue yet but could grow into one and jeopardize operations. The digital twin technology helps predict future modes of operation: operators can take preventive or corrective actions without unexpectedly shutting systems down. The digital twin technology and intelligent condition monitoring capabilities are part of the future-proof subsea compression system.

Digital twins partially rely on reduced-order models. Indeed, the condition monitoring system for the subsea compression station has strong computational power. However, it is impossible to have extremely heavy models run in the system. The Aker Solutions engineers characterize the equipment to develop a reduced order of the digital twin that can run online together with the asset.

A digital twin also helps operators visualize the operation of a specific part in terms of a performance map. The current operation mode appears as a small floating ping's ball above the response surface. If the ball moves closer to the response surface, the operator immediately understands that the system might enter a mode outside its design specifications. Operators don't need to be application experts, as they can assess risks at a glance.

Deploying the technology worldwide

Why is the successful deployment of the Jansz-Io subsea compression system a milestone for the oil and gas industry?

Barras explains: "When completed, we can reuse the qualification done for the Australian project to cover approximately 80 to 90% of the oil and gas field application worldwide. Once the subsea compression system is in operation, it becomes a proof of concept and opens the door to setting up future subsea compression projects. And this is quite exciting!"

A technology for smaller and marginal fields

Indeed, the subsea compression technology supports the gas production of large fields. However, it is possible to scale it down and apply it to the production process of smaller fields. The core technology, however, remains similar. The well stream compression is a compact and simplified solution with a wider range of applications. For marginal fields, the added energy from subsea compression overcomes the resistance of long-distance pipelines. Therefore, the technology is particularly attractive in the field development phase as it allows engineers to consider more options.

Image provided by Aker Solutions

Supporting countries with no infrastructures

Aker Solutions expects to deploy subsea compression systems in countries with no gas production infrastructure. For instance, some countries in Western Africa or Latin America may have gas resources that are currently not valued. In some cases, there exists an infrastructure to produce oil, but the remaining gas is just flared out. The compression technology can help maximize the output from existing small wells and open up more possibilities for exploiting resources.

Subsea compression makes it easier to process gas and transport it to shore to convert it into electricity. Countries with a poor electrical network could greatly benefit from the technology and improve their electrical grid while minimizing emissions.

A future-proof technology

"Our world-leading technology improves field recoverability while offering carbon emission efficiencies compared to traditional compression alternatives," said Kjetel Digre, chief executive officer of Aker Solutions. Combined with renewable energy sources, the subsea gas compression technology effectively supports the transition to more sustainable energy production processes.

Subsea compression technologies

There are two technologies currently available for subsea compression.

Conventional Subsea Compression Systems

The conventional subsea compression system includes a subsea separator, a compressor and pump units. The conventional technology involves commingling several wells. Smaller fields may have a single well, but large fields have multiple ones. The technology commingles the output of several wells into one line and sends all constituents into one compression train. An inlet scrubber separates the gas and liquid in the compression train. It sends the liquid through a pump and the gas through a compressor. Eventually, all elements are sent onwards through the pipeline. This technology is the one that is currently installed to operate the Åsgard field. The conventional technology is mostly used for high-capacity applications, as it complies with the most stringent requirements and can tolerate the most challenging upset conditions.

Well Stream Compression Systems

The well stream compression technology is similar but excludes the separation process and dedicated pump. The technology lets an extensive amount of liquids flow from the well stream through the compressor. It doesn't separate liquids but sends the well stream output through the 12-megawatt compressor and immediately downstream. This technology is particularly suited for smaller and marginal fields since it avoids the capital investments affiliated with scrubbers.