Shiprepair & Maintenance: January 2019
This year’s RINA President’s Invitation Lecture, which took place at One Whitehall Place in London on the 29 November, saw Navigator Gas’ director of fleet and technical operations Paul Flaherty describe the process of converting ethylene/LPG carrier Navigator Aurora from dual-fuel LNG to ethane operation. The project, undertaken in August, marked the world’s first conversion of a slow-speed dual-fuel/diesel main engine to ethane operation. Conversion of the vessel’s two fuel gas-burning auxiliary engines is planned in the future.
Flaherty began by explaining the history of Navigator Gas, which controls nearly 40% of the world fleet of ethylene and ethane ships. Growing quantities of ethane delivered by shale gas production led the company to order four handysize semi-refrigerated ethylene-capable vessels, one of which was the 37,300cbm, ABS-classed Navigator Aurora. The vessel is currently on a ten-year charter contract with Borealis.
According to Flaherty, Navigator and Borealis made the decision to convert the vessel to ethane following difficulties securing LNG for use a fuel. Besides the practical advantages of using the same fuel carried on board as cargo – including predictable availability, shorter bunkering times and a fixed price – Flaherty cited compliance with the 2020 sulphur cap as a key driver. Ethane contains 99% less SOx than HFO and 85% less than diesel, and also reduces both CO2 and NOx.
The vessel’s MAN B&W 6S50ME-C8.2-GI engine was converted to a 6S50ME-C-GIE unit. Flaherty explained that an earlier relevant HAZID study from 2014, based on a 91k VLEC using ethane as fuel, had to be updated for the Aurora project, to assess the novel conversion of existing systems and components, establish a comparison between burning LNG versus ethane, and ensure compliance with ABS and Statutory requirements. This meant the use of a test engine at Kawasaki Heavy Industries in Japan, which needed significant adjustment to reflect the planned engine setup for Aurora. As such, the test engine’s combustion chamber was changed, and new cylinder liners / piston crowns were fitted.
Following successful testing – which made clear that installed equipment was suitable for ethane operation, and only minor hardware and software modifications would be necessary – the engine was converted over a 15 day period with the vessel alongside at Frederikshavn. Flaherty highlighted the equipment that needed changing: the cylinder heads, the chain piping (preventing potential cracking due to the lower operating temperatures), the gas injection valves, the gas pressure sensor, and seal kites, with upgraded materials to withstand lower temperatures.
The conversion, Flaherty noted, ran without issue, although he explained that sea trials lasted for longer than expected due to issues staying on gas burning mode. The vessel is currently operating on ethane, but further modifications will be necessary to install bigger filtration units, to maintain steady gas flow at higher or fluctuating loads.
Wider lessons learned for conversions of fuel gas systems included keeping the fuel gas system in good condition regardless of its usage profile, as degradation, moisture and rust can severely affect valves and filters, causing stoppages. Flaherty also stressed the need to maintain exchange parts and overhaul kits for all aspects of the system – and to be aware that OPEX is likely to increase at least initially due to the expense of maintain a fuel gas system. Training of crew was further highlighted as a priority with regard to engine operation and safety.
