With LNG now established as a viable alternative fuel and the refueling infrastructure growing, Wärtsilä has launched its four-stroke 31SG pure gas engine for the marine and offshore sectors
“Floating storage and regasification units (FSRUs), ro-pax vessels, smaller coastal ferries and cruise ships could benefit from the pure gas 31SG,” explains Wärtsilä Marine product manager Rasmus Teir. “Those segments,” he notes, “have had a strong uptake in using LNG as a fuel. In the cruise sector, for example, there are now 20 new LNG-fuelled cruise ships on order, with all the major cruise companies – Carnival, MSC, Disney, Royal Caribbean and TUI – literally betting billions of dollars on LNG as a fuel.”
Mr Teir says that in many ways an FSRU is much like a floating power plant: “Their operating profiles are similar to those of land-based power plants, requiring electricity production and multi-engine installations.
“All the development we have put into the 31SG makes it fit [the FSRU segment] like a glove.” He says a particularly attractive feature is that the engines utilise the FSRU’s cargo as fuel, allowing operators to avoid bunkering other fuels: “You can avoid all those complexities by going pure gas.”
Mr Teir also sees the possibility of the 31SG finding its way into the offshore support vessel (OSV) sector. Widening adoption of battery-hybrid propulsion offers additional opportunities for pure gas engines. “Our Wärtsilä 31DF dual-fuel engine was used in pioneering LNG-fuelled applications in the OSV market,” he notes. “Now, there are also a number of examples of our dual fuel-powered vessels being retrofitted with batteries. A 31SG, used in combination with a battery-hybrid, could offer a very low carbon footprint.”
He says that the Wärtsilä 31SG’s outstanding thermal efficiency makes it well suited for battery-hybrid applications, while also offering lower investment costs that help offset the additional cost of an energy storage system. Pairing the Wärtsilä 31SG with battery packs also enables redundancy to be designed in, while securing the highest total system efficiency.
Introduced in the energy market in 2017, some 1,800 Wärtsilä 31SG engines have accumulated 37M running hours in land-based power applications. One of the key issues the 31SG aims to address is methane slip. “All gas engines have some degree of methane slip,” says Mr Teir. “It was one of the drivers of releasing the 31SG to the marine market. It has the advantages of lower CO2 emissions and a significant reduction in methane slip.” He estimates the engine reduces methane slip by as much as 50% as compared with dual-fuel engines.
While there is a high degree of commonality across the 31 engine platform, Mr Teir says the difference between the dual-fuel engine and the pure gas model is in the ignition technology: “Instead of having a pilot liquid fuel injector, it has a pre-chamber with a spark plug. In a dual-fuel engine, a small amount of distillate fuel is required in the ignition process.”
Mr Teir continues: “The highly efficient Wärtsilä 31SG moves the industry forward by providing owners and operators with a practical means for lowering costs and enhancing environmental sustainability. The gas-only focus and lean-burn spark ignition technology allows for further optimisation of thermal efficiency, while lowering greenhouse gas emissions and facilitating adaptations for alternative heavier gas fuels, such as LPG.”.
Wärtsilä plans to make the 31SG engine available in the marine market in mid-2021.