Compact compression

An artist's illustration of the Asgard subsea compression facilities. Image from Aker Solutions.

Subsea gas compression is being built on a large scale offshore Norway. But is the future in smaller projects Aker Solutions thinks so. Elaine Maslin reports.

Subsea gas compression technology has been making the headlines. Statoil’s giant Åsgard project offshore Norway is getting closer to first production, in 2015, and the Norwegian major’s second subsea gas compression project, at Gullfaks, is also underway.

Gas compression is used to boost the reservoir flow on gas fields as res- ervoir pressure depletes. Subsea gas compression is beneficial because it is closer to the well, giving additional recovery due to a lower pressure drop in the pipeline, compared to topsides compression.

Rystad Energy, a Norwegian oil and gas consultancy, has predicted sub-
sea gas compression technology will take an increasing role in the growing subsea processing market, which the firm says could be worth US$8 billion by 2020, from about $500 million today.

But will it be the mega projects that make the majority of that market? Bjørn Søgård, business development leader, wells subsea and risers, at DNV GL, suggests that, while focus is on the mega projects, the potential could be unleashed in making smaller units and through industrialization— making them more affordable.

Aker Solutions, which has been leading the Åsgard project, is now doing just that. Marco Gabelloni, senior engineer, subsea power and process, at Aker Solutions, presented the firm’s compact subsea compression concept at Subsea Expo in Aberdeen.

The active cooler module

The aim, he says, is to offer a solu
tion with a smaller footprint and higher flexibility, so that it can provide compression throughout the life of field, adapting to changes in the well stream and or reservoir.

“To do that we are developing a compact subsea gas compression solution,” Gabelloni says. “Based on our qualified subsea compression system design, our target is to introduce new technologies, in order to develop a compact option targeting medium to small size gas and gas condensate fields. We are especially looking also at long step-out distances and deep water.”

Aker Solutions started this development project nearly two years ago and is now in the concept selection and feasibility study phase. Proof-of-concept tests are now on-going, and they will be followed by the identification of related technology qualification programs.

The concept so far comprises three modules, to allow for flexibility, as field requirements change. These will be cooling, compression and power modules. The wellstream will enter via an inlet, pass through an active cooling system, then a compact scrubber, suitable for deep water and to minimize size, before going through gas compression.

Liquid removed by the scrubber will be boosted using a gas pressure pump, utilizing two vessels, which are emptied in turn utilizing gas coming from the compressor. Power supply will be via a subsea transformer and Aker Solutions’ RotoConverter, which is a frequency step up device to enable long distance AC power transmission at low-frequency.

Three core elements being developed for the project are the active cooler, the gas pres- sure pump, and the RotoConverter.

Subsea cooler

Today’s subsea coolers are designed based on what is known about a field during the design phase, Gabelloni says.

“They are passive coolers, mostly with a big footprint, and the temperature control is designed in the initial phase of the project.

“On an active cooling system, we
use forced convection cooling, with a small subsea pump to push the seawa
ter through the cooler. The design is similar to Åsgard, but with additional walls around the cooler and some baffles inside, to increase the heat exchange. The main benefit is the increase in the heat transfer coefficient, so we can reduce the weight and the size of the module,” he explained. Aker Solutions is currently in the concept selection phase on the cooler.

The gas pressure pump

Gas pressure pump

“The challenge with current systems is that the subsea pumps needed for the liquid part of the well stream require the use of barrier fluid, with related tubing, and, in the case of a long step-out, a subsea variable speed drive. Also, in some cases, when we have a small liquid content in the wellstream, we need to re-circulate most of the flow to allow the pump to operate.

“To overcome these issues, we are developing a gas pressure pump. It uses pneumatic transport by leveraging the gas coming out of the gas compressor outlet. We use two vessels, and a set of axial valves, which control the gas from the compressor. The two vessels are emptied alternately. The benefits are that we do not need barrier fluids or any dedicated power supply.”

The company has completed concept selection and is moving into a proof-of-concept test phase including sand testing, to make sure there is no issue with sand accumulation.

Roto Converter

To address the issue with transmission of AC power over long distances Aker Solutions has been developing a step-up device which is called the RotoConverter. Gabelloni says, “typical loads in subsea gas compression require quite high frequency to match the compressor rotating speed— up to more than 10,000rpm. But transmitting power over long distances with high frequency is limited due to charging currents. Therefore we need low frequency transmission, then locally, close to the compressor, we can have a frequency step-up device.”

The RotoConverter consists of a motor and generator coupled together with a different number of poles. “In this way we can increase the frequency to match the requirements of the compressor,” Gabelloni says.

“This is a proven concept and has been used in the industry for a number of years. Our program is to further develop this for subsea use. We are planning to have an enclosure that will be oil-filled with a pressure compensator, which will allow the system to be installed in deep water. With this device, we can remove the need for subsea VSDs, enable more power to be transmitted at longer distances and enable better cable performance.”

A scaled-down Rotoconverter prototype.

Aker Solutions is moving to the proof-of-concept test phase, with plans to test a small scale 50kW unit.

Simplification

“When targeting small gas fields, the need for a compact and simplified system is quite clear,” Gabelloni says. “We are using fewer and smaller components with a long track record topside. That helps to maximize the production and provide a reliable solution. The system is reduced in size and weight, we do not need the VSD subsea, we don’t need the barrier fluid and related auxiliaries for the pump, and the cable cost can be reduced by using the RotoConverter unit.

“The modular design with three small units means they will be easy to install, retrieve and change as necessary during field life, as part of a building block strategy. We strongly believe these technologies unlock new possibilities for smaller gas accumulations and fields that currently cannot be exploited with existing technologies.”