Register for a free trial
LNG World Shipping

LNG World Shipping

GCU makes light work of boil-off gas

Thu 24 Jan 2019 by Ed Martin

GCU makes light work of boil-off gas
The GCU contains few components and moving parts, making for easy integration and maintenance (credit: Alfa Laval)

Newbuild LNG carriers are looking to thermal oxidation as a means of handling boil-off gas, while a new deepwell cargo pump designed specifically for LNG-fuelled engines will help reduce maintenance costs

Boil-off gas is an inevitability on board LNG vessels and must be dealt with effectively to avoid adverse effects on gas pressure in the LNG tanks. Reliquefaction, whereby LNG is cooled and reverted to a liquid state, is one option; another is feeding the boil-off gas to the vessel’s engines, either at tank pressure in the case of four-stroke engines, or in compressed form for two-strokes. But increasingly, straightforward thermal oxidation, in the form of burning off the gas, is being employed as either a supplemental or standalone solution.

More than half of the LNG carriers built during 2018 utilised Alfa Laval’s Gas Combustion Unit (GCU) for boil-off gas management, according to the company itself.

Over the past decade the trend in newbuild LNG carrier propulsion has shifted from steam turbines to dual-fuel diesel electric propulsion, both of which are economically and environmentally sound compared to traditional systems. However, such engines require additional means to regulate LNG cargo tank pressure. The GCU provides this by burning excess boil-off gas that the ship’s propulsion system does not use.

Available in four standard sizes, to accommodate flow rates of 1.5-3 tonnes per hour (t/h), 3-4.5 t/h, 4.5-6 t/h and 6-9 t/h of methane, the GCU can safely dispose of excess boil-off gas without the need for pilot oil burners.

“Increasingly, straightforward thermal oxidation, in the form of burning off gas, is being employed as either a supplemental or standalone solution to boil-off gas”

Monaco-based GasLog LNG, manager of one of the world’s largest fleets of LNG carriers, acquired the 100th GCU from Alfa Laval and its innovation and technology manager noted, “Between its compactness and simplicity, the GCU is easily integrated into our LNG carriers and provides the high reliability we demand.”

Elsewhere, the Yamal LNG project’s 15 Arctic ice-class tankers will each carry specially engineered Alfa Laval machinery, including GCUs, to cope with the extreme cold conditions in which they will operate.

As well as a GCU, each vessel will also carry a thermal-fluid-based secondary heating system, six Aalborg XS-TC7A economisers and two 35-tonne Aalborg OL boilers with dual-fuel burners from Alfa Laval. All this equipment must be able to handle temperatures of minus 50°C or lower.

The GCU comprises three levels: a fan deck, which contains combined dilution and combustion fans and an air chamber; a combustion chamber and utility deck, which along with the combustion chamber and burner arrangement contains other equipment such as the gas valve train, power cabinets and the control cabinet; and the top casing deck/stack, which contains the exhaust gas outlet and temperature sensors. The GCU is available in either single or dual combustion chamber configurations, depending on the required methane flow capacity.

The relatively small size and lack of components, interfaces and moving parts makes the system simple, reliable, and easy to integrate, according to Alfa Laval.

The technology behind the GCU was acquired in 2013 by Alfa Laval from Snecma, now known as Safran Aircraft Engines, a Courcouronnes, France-based aerospace engine manufacturer. Since then, the company has continued to develop the technology in partnership with customers and at the Aalborg, Denmark-based Alfa Laval Test & Training Centre., which was expanded to cover gas applications in 2017. Since the expansion, a full-size GCU has been at the centre of Alfa Laval’s 1,350 m2 testing space.
Alfa Laval’s exhaust and combustion systems manager Pieter Borg said: “Numerous GCU advances have been made at the Alfa Laval Test & Training Centre, such as a full free-flow application that enables full-capacity burning at low inlet pressure without the need for gas compressors.

“We have also implemented Alfa Laval Touch Control, which is the same control system used on other Alfa Laval products, such as Aalborg boilers and Smit inert gas generators.”
This year, a smaller version of the GCU is expected to be released. Unlike the current system, which is most suited to large vessels, the new unit will be optimised for 100–1000 kg/h of boil-off gas.
Mr Borg said: “The smaller GCU will be an excellent fit for small LNG carriers, floating storage and regasification units, LNG bunker vessels and vessels using LNG as fuel, which also need an option for burning boil-off gas.”

Growing demand for pump innovation

Aalborg, Denmark-based pump manufacturer Svanehøj Group is optimistic about going it alone following its divestment from Wärtsilä, which was announced in October 2018.

Svanehøj was acquired by Copenhagen-based investment house Solix, which has committed to support the deepwell and in-line pump specialist by strengthening its sales and service arm and investing in new production development.

“Even though we have been very pleased being a part of the Wärtsilä Family, we are confident that we will also benefit positively from the transfer to Solix,” said Svanehøj’s sales director Jens Peter Lund.

One of the company’s most recent developments is the ECA Fuel Pump (EFP), a deepwell cargo pump developed in cooperation with LNG fuel-gas system designers specifically for LNG-fuelled engines. The EFP first came to market in 2017 and the first vessels fitted with the pump, which include a DEME dredger, have recently entered operation. Wärtsilä is one of the main customers for the EFP, according to Mr Lund, who noted that the design underwent a year and a half of assessment at Svanehøj’s former parent’s testing facility in Finland.

The EFP is a cryogenic multi-stage centrifugal pump and is designed for continuous variable frequency operation. The deepwell design sets the EFP apart from most other LNG fuel pumps, which are of the submerged-type, said Mr Lund. This design makes it easier to monitor the pump, as all electrical service parts sit outside of the fuel tank itself, there is no heat transfer from the motor to liquid gas and the motor does not increase pressure and boil-off gas.

For a small company like us, we are dependent on lean decision making and an agile structure.

Another advantage over competing designs is the EFP’s long maintenance interval of five years. The pump also features a hermetically sealed MagDrive coupling, which separates gas in the tank system from its surroundings, systematically removing the risk of gas leakages through shaft sealings. The EFP’s foot valve can be activated from outside of the fuel tank, and when closed, allows the caisson pipe to be purged with nitrogen and the pump to be retracted for repair, even when there is still liquid gas in the fuel tank.

There are six EFP models available, split between three nominal flow sizes of 5, 11 and 24 m3 per hour and two pressure versions, 8 and 18 bar, designed for standard four- and two-stroke engines. As well as LNG, the pump is also suitable for LPG and ethane.

“We see that [the EFP pump] has high levels of attention from major shipyards both in Japan and Korea,” said Mr Lund.

“It is one of the pumps that we are expecting a lot from, but it is still very early days – and it is very early days for LNG as a fuel.”

Svanehøj has seen a lot of interest in its cargo and offloading pumps for LNG bunker barges, but investment in these vessels seems to be lagging other vessels. “It’s been a little bit like the chicken and the egg,” said Mr Lund, with development behind both in terms of bunkering infrastructure and dual-fuel vessels. While there are some bunker barges coming to market, it is often the case that potential clients looking for modern vessels with high-tech equipment opt instead for floating platforms and trucks.

Mr Lund is also dismayed by the slow uptake of LNG as a marine fuel. He attributes this to the availability of alternatives, such as scrubbers, that allow owners to comply with IMO’s 2020 sulphur cap requirements, and the relative scarcity of LNG bunkering. But Svanehøj expects this to change in the near future: “We do expect that both small LNG vessels, bunkering and the LNG fuel market will grow enormously over the next years,” said Mr Lund.

Another area of growth he foresees involves ultra-large LNG carriers. While Svanehøj’s current range does not see use on these vessels – the target market is LNG carriers with less than 100,000 m3 capacity – an increase in such vessels is likely to precede growth in the rest of the LNG sector, with smaller vessels such as coastal LNG carriers and bunker vessels following on.

“I think that we can also tell from the Clarkson numbers that everybody expects that the gas market will become quite lucrative over the next years,” added Mr Lund, noting that with China keen to take ethane, LNG and LPG, and the potential for growth from by-products of shale gas in the US, there is cause for optimism.

Recent whitepapers

Related articles





Knowledge bank

View all