The first tugs with pure gas propulsion

December 16, 2013

This vessel, named Borgøy, is the first of two tugs ordered by Buksér for operation as an escort tug at the Kårstø gas terminal, under contract to Statoil. The second tug, Bokn, is scheduled for delivery in 2014.

The choice of LNG as fuel for gas terminal tugs seems logical, but there were several obstacles to be overcome before the project could be finalised. Statoil had asked for high environmental credentials when the contract to provide escort tugs services was put out to tender, and Buksér had proposed gas fuel in its offer, based on the virtual absence of sulphur, NOx, visible smoke and particulate emissions and the favorable greenhouse gas (GHG) exhaust gas figures. But to turn the proposal into reality, the company had to make sense of the economics and design challenges.

Economically, Buksér says that bunkering costs for LNG and MGO are almost equal at present, and long-term forecasts suggest that LNG will become cheaper in comparison. Counting against this is the cost factor, with a gas-fuelled tug running at a premium of some 40% to 50% more than a conventional vessel. The gas engines are more expensive, but most of the extra cost comes from the fuel tanks and fuel system. There is an increased explosion risk, which has to be considered in the ship design. The liquid gaseous fuel has to be stored under refrigeration (-163° C) and pressure, and needs heating and processing before it can be burned in the engine. And although there are plenty of gas-fuelled engines in service, at sea as well as on land, the demands of a tug operating profile set new challenges.

The tugs were designed by Buksér og Berging’s in-house team, with the assistance of Norwegian company Marine Design and with further input from Sanmar, the chosen builder. At 35m long, they are of ASD type, with 70t bollard pull and with escort capabilities of 100t steering force at 10 knots. The design meets the requirements of DNV’s Gas Fuelled Ships rules and the International Code of Safety for gas fuelled ships, involving such requirements as automatic shutdown of gas supply and electrical equipment in case a leak is detected, 30 changes of air per hour in the machinery spaces, ventilated double sheathed piping and excess flow shutdown.

The designers turned to Rolls-Royce for the propulsion and gas systems. Each tug has two Bergen C26:33L6PG engines, driving the latest design 3,000mm diameter US35 azimuthing thrusters. The Rolls-Royce scope of supply included the C-type double-walled Aga Cryro 80m³ gas tank and the gas supply systems.

Rolls-Royce believes that, as well as the obvious emission and fuel cost benefits, the use of LNG will mean lower maintenance costs, particularly on the fuel system where spark plugs are used instead of high pressure diesel oil pumps, injectors and filters; superior cleanliness; reduced lube oil consumption, with no purifier required; and, in Norway in particular, tax benefits arising from the much-reduced emissions. The question arises of course as to the choice between a dual-fuel propulsion plant and pure LNG. Rolls-Royce believes that the spark-ignited lean burn pure gas option is well-suited, as the all-important (for tugs) transient response is equivalent to diesel for the single-fuel engine, and it can meet IMO Tier II, Tier III, EPA and all ECA emissions limits, plus make a significant impact on the ship’s EEDI. The single-fuel solution is claimed by Rolls-Royce to be simpler, less prone to methane slip (and thus higher GHG emissions) and to be able to operate on gas of 70 methane number without derating, whereas dual fuel engines need at least 80 methane number.

As far as methane emissions are concerned, Rolls-Royce claims advantages for its lean-burn combustion over dual fuel engines, particularly at lower load levels. According to fibures produced by Norwegian research institude Marintek, the Bergen C26:33L emits 5g/kWh of methane at 25% load, dropping gradually to just under 4g/kWh at 100%, whereas the best dual fuel engine is over 40g/kWh at 25% load, fropping to around 8g/kWh at 50% then reducing to around 6g/kWh at full load. Methane is said to be 21 to 25 times more aggressive as a greenhouse gas than CO2.

Safety remains a concern with all gas engine installations, and measures implemented on the two tugs include double-walled gas supply pipes between the tank and combustion chamber, with the outer pipe ventilated 30 times per hour and protected by gas detectors, with a further external gas detection system above the engine in accordance with class requirements. In the event of a leakage from any inner pipe, the gas is collected and ventilated to the mast. Depending on the size of the leak, the engine is either shut down or an alarm sent to the operator. ‘Block and bleed’ valves are fitted in the fuel system so that in the event of an unexpected engine stop the entire gas supply is cut. The gas detectors and shut-off valves are all linked through the alarm and control system, which includes crossover valves to allow both engines to keep running in the event of a problem with the tank connection space on one side.

The relative simplicity of the fuel system is another benefit cited by Rolls-Royce for its lean-burn single-fuel engine over the dual fuel solution, while the direct mechanical governing system for the gas offers robustness and simplicity. The design is claimed to be well proven, with Bergen gas engines of similar design having operated for a number of years in different environments, the first marine gas engine having achieved well over 45,000 operating hours.

The gas system is based on a vertically-mounted C-type double-layer tank – the double skin provides a degree of redundancy. Because the pressure in the tank ensures the gas feed to the engines, no additional pumps are needed. The gas is fed to the engines via tank connection spaces – which allow LNG to vaporise to form gaseous fuel, and gas regulatory units, which heat the gas to the correct temperature (+35° C) for the engines and regulate the gas pressure. The top support of the tank is designed to slide, accommodating thermal expansion.

The gas tank provides autonomy of 4-5 days operation at 10 knots. Obviously the delivery voyage will pose a challenge, but road LNG tankers will be made available at ports en route between Turkey and Norway. We understand that road tankers will be employed for bunkering at the Kårstø terminal until the necessary shoreside infrastructure is built.

The tugs feature Rolls-Royce’s new design of azimuthing thruster, designed for a higher level of efficiency. The US35CP is physically smaller for the equivalent power handling capacity in comparison with its predecessors, with a shorter stem length and a compact sensor and pitch box design. The lower gear housing is more streamlined, and the lower leg is redesigned to produce lower wake, while the units feature energised oil circulation for higher efficiency. The two tugs are each equipped with two units, featuring direct mechanical drive to ducted CP propellers of 3,000mm diameter. Power handling capability is 1,705kW at 1,000rpm each, which gives each thruster a 65ton BP capability. Thruster steering is controlled by the company’s Aquapilot system.

For additional capabilities, the tugs feature a Heila deck crane, FiFi and oil recovery equipment, while a Schottel 333kW bow thruster provides extra manoeuvrability. The onboard accommodation comprises two single and two twin cabins, mess room and galley, all sound proofed to 65dB noise levels at 85% engine load.

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