Introduction to Propulsion Methods for Modern LNG Tankers
Modern LNG tankers have hulls similar to oil tankers, but with up to six large LPG gas tanks integral to the hull installed. The LPG must be kept liquefied in these tanks during transit, but despite insulation some of the liquid gas “boils off” and is lost to the atmosphere. This gas is known as boil-off gas or BOG.
The technique of collecting this gas and its use as fuel for the main boilers or for use in dual-fuel marine diesel engines has been established. These means of propulsion for modern LNG tankers have cut fuel costs as well as exhaust gas emissions.
This is an article in Marine Engineering, and here we will examine the use of LNG in ships propulsion of gas that was previously lost to atmosphere due to boiling off and vaporization. We begin then with an overview of the LNG Tanker.
Overview of an LNG Tanker
As we have seen in the introduction, the hull is similar to that of a conventional oil tanker having numerous cofferdams and ballast tanks, along with additional stiffening to secure the LNG tanks, effectively giving the vessel a double skin complying with current regulations.
For transportation by sea the natural gas must be cooled to -163ᴼC. When it liquefies, it contracts to 1/600 of the original volume, allowing for cargo volume of up to 2,000,000 m3.
There are a number of tank designs, the most common type being spherical or prismatic containment tanks, manufactured from high grade stainless steel or aluminum; although membrane types are also used and are becoming a more popular containment system.
It is normal to have between three and six large tanks installed and supported in the ship’s hull; their top half protruding up through and forming part of the deck. These are very visible above the LNG ships main deck as illustrated by a typical LNG carrier shown below.
The tanks are insulated to a very high standard to prevent excessive LNG boil off. However even with high efficiency insulation, boil off still occurs and this gas was formerly vented to atmosphere.
On LNG Tankers, the boil off gas can be disposed of by several methods;
- Used in the ships boilers to produce steam for steam turbine propulsion
- Used as fuel in ships dual fuel main diesel/HFO engines that have been specifically designed to run on gas and/or fuel oil.
- The boil off gas can be collected and re-liquefied using a reliquefaction plant. The newly liquefied gas can then be pumped back into the LNG tanks, ensuring full delivery of cargo to the discharge port. LNG carriers using this technique have modern electronic controlled high efficiency marine diesel /HFO engines installed, running only on fuel oil. In the event of a malfunction of the reliquefaction plant, a thermal oxidizer in the form of a gas combustor is used to burn excess vaporized boil off gas.
- Used to fuel diesel generators, producing electrical power for electric drive propulsion methods.
Use of LNG Boil-off Gas (BOG) to Propel LNG Tankers
Modern LNG Tankers have a boil-off vaporization rate between 0.1% and 0.15% of cargo volume per day during the voyage. This gas must be removed from the tanks otherwise pressurization will result leading to safety valves lifting. (Tanks are rated at 0.25 bar - 3 lbs/sq inch).
This vaporization occurs mainly because of the difference in the storage temperature of -163ᴼC (-261ᴼF) and the ambient temperature, equating to a sizeable total loss of cargo before the gas was utilized for propulsion purposes.
The gas is extracted from the vapor spaces in the tank tops through a common header pipe that links all the LNG tanks. This header runs along the deck parallel to the cargo discharge/loading header, also having connections alongside the discharge/loading manifolds.
Because the use of boil off gas is well established (over twenty years) we shall only examine its use as a dual fuel in marine diesel engines. However before the boil off gas can be used as fuel for the ships engines, it must be compressed, and this is carried out using gas compressors that pressurize the gas to its required working pressure.
On modern LNG tankers the marine two and four stroke diesel/heavy fuel oil engine has been developed to run on fuel oil and/or BOG and is known as a dual fuel engine.
The engine is always started on diesel fuel oil and changed over to gas when warmed through- but not normally before “full away” at start of passage. In my day at sea after dropping off the pilot, the bridge used to give a “double full ahead ring” on the engine room telegraph signifying full away, and we opened up the engine to maximum revs.
This signified start of passage; ending at standby for picking up the pilot for next port, being entered in the bridge and engine room log books.
Anyway, I diverge (a trait of an elderly retired Irish marine engineer). The BOG is compressed using gas compressors located in the gas compressor room, and supplied to the main engine electronic gas control mechanism through a double walled piping system.
Dual Fuel 4-Stroke Engines
The cylinder head contains the exhaust and inlet valves, air start/relief valves, and fuel oil injector incorporating the pilot fuel oil injector, with these having separate fuel pumps.
On the inlet stroke the gas is injected close to the inlet valve where it is mixed with combustion air supplied from the turbo-blowers. The inlet valve opens allowing the mixture of BOG and air to be drawn into the cylinder by the descending piston. As the piston rises to near top dead center, pilot diesel @ 900 Bar is injected via the fuel oil injector on the cylinder head.
Compression combustion of the pilot diesel fuel occurs igniting the BOG. This forces the piston down on its power stroke, the ensuing upward stroke expelling the exhaust gasses to the turbo-blowers. The blowers draw ambient air from the engine room, compressing and cooling it before supplying it as combustion air to the inlet manifold. And so the 4-cycle is completed, suction, compression, combustion, and exhaust; or as I was taught to remember it: suck, squeeze, bang, blow. (Don’t quote this if sitting your orals for BOT Marine Engineering tickets!)
The operating principles of the four-stroke dual fuel engine are therefore:
- BOG compressed to 5 Bar and supplied to the engine control mechanism through double walled steel pipe.
- Diesel fuel oil is supplied to the fuel injector by common rail system, from a dedicated fuel pump at 900 bar being used as pilot oil.
- Diesel fuel/heavy fuel oil is supplied by separate fuel oil pumps used when on standby and maneuvering modes or when the ship is ballasted and BOG supply is limited.
A sketch of a four-stroke dual fuel marine engine is shown below:
Please read on to see how new technology is applied to 2-stroke dual-fuel marine diesel engines;
Dual-Fuel 2-Stroke engines
The BOG gas is passed through a heater before being compressed to 250-300 Bar. It is fed from the gas compression room to the engine room gas control system through double walled stainless steel pipe.
The normal 2-stroke operation is applied to the dual fuel engine, with a few modifications.
The compressed gas is supplied to the gas control system where it is contained in an accumulator. The gas leaves the accumulator, passing through filters, regulators, and an isolating valve (by-passing to vent or recirc) before being supplied to the engine gas admittance valves.
There are two normal fuel oil injectors and two gas admittance valves per cylinder, located in the cylinder head along with the usual air-start, relief valves, and exhaust valve.
My interpretation of the new designed 2-stroke dual fuel marine engine is shown below;
The gas valve is hydraulically/electronically operated and controlled. Gas leakage is avoided by using a high pressure oil seal arrangement (about 25 bar above gas pressure). Although preventing gas leakage, a small amount of the oil is lost by being injected into the engine along with the gas.
The engine is started and operated on fuel oil whilst maneuvering, and once “full away” has been communicated from the bridge, gas is admitted along with the fuel oil.
On the compression stroke a calibrated amount of gas and diesel is injected just before TDC and compression combustion follows, forcing the piston on its power stroke down the cylinder.
As the piston descends it uncovers the scavenge air inlet ports located about two thirds from the top of the cylinder liner, and a fresh charge of air is introduced to the cylinder. As the piston begins to rise back up the cylinder, the exhaust valve opens and discharges combustion gasses to the turbo-blowers. The exhaust valve shuts and compression combustion again occurs completing the 2-stroke cycle.
The operating principles of the two-stroke dual fuel engine are therefore:
- BOG gas is compressed to 250-300 Bar before being supplied to an accumulator
- There are two gas injection valves per cylinder, and two diesel / HFO fuel injectors per cylinder located on the cylinder head.
- Exhaust valve, fuel oil, and gas injectors are operated hydraulically.
- All gas piping in the engine room is double walled, stainless steel pipe.
- Engine can be run on gas, fuel oil or a combination of both.
- Fuel oil is used whilst maneuvering or on standby.
Safety on Board the LNG Tanker
Safety aboard LNG carriers is paramount and it is mandatory that LNG tanks are certified to the latest maritime rules and regulations. These bodies include the International Maritime Organization (IMO), Safety of Life At Sea (SOLAS), and the American Bureau of Shipping (ABB).
Safety Precautions to be Observed When Using LNG as a Fuel
- The crankcase must be purged before entrance for inspection, to ensure there is no gas trapped therein.
- Explosion relief doors must be fitted to exhaust manifolds.
- Gas supply pipe should be double walled, with the space between gas pipe and outer pipe constantly purged to deck and continually monitored for gas leaks.
- Gas leakage detection instruments are to be located throughout the engineroom.
- Air suction points for purging or combustion purposes must be located away from any gas supplies.
- In the event of a gas leak being suspected or alarm systems operating, the gas supply must be shut off and the engine changed over to fuel oil automatically, without loss of power.
1. eagle: Propulsion systems for LNG Carriers (ABB).
2. classnk: 4-stroke dual fuel operating principles.
3. mandieselturbo: 2-stroke dual fuel operating principles.