From the very first sail-powered warships to today’s state-of-the-art nuclear supercarriers, there has been significant development in the field of naval propulsion systems. The prevalent pursuit of ever powerful and agile naval vessels has led to the adaptation of gas turbine engines from aircrafts for marine use. In recent years, this propulsion system has been widely installed throughout the US fleet, and adopted for the flagships of several navies globally as well. Despite possessing several favourable statistics, gas turbine engines remain a relatively unpopular choice due to several drawbacks which limits its effectiveness as a power source.
The operating procedure of the gas turbine engine begins with a prime mover rotating an axis which has the turbine blades of the air compressor attached to it. Air at atmospheric pressure is compressed and channelled into the combustion chamber, where fuel is injected and ignited by the spark plug. Gases within the combustion chamber expands substantially and causes a further increase in air pressure. Kinetic energy of the highly-pressurised air flowing through the blades is converted into mechanical energy of the spinning turbines. This process repeats until compressed air of approximately 30 bar is consistently directed into the combustion chamber to complete the gas turbine sequence.
A power turbine is attached to the exit of the gas turbine to transmit power to the ship’s propeller. Fast-flowing exhaust gas from the gas turbine travels through the power turbine, causing the turbines and linked output shaft to spin. The power turbine output shaft connects by coupling to a gearbox which reduces the revolutions per minute of the drive shaft, allowing the drive shaft to exit the gearbox at suitable revolutions required for the propeller.
1-Compact size and light-weight construct.
Gas turbine engines have relatively higher power output compared to the commonly used diesel engines. Coupled with significantly lower weight from fewer engine components, gas turbine engines possess the highest power-to-weight ratio among the different marine propulsion systems .
The low weight of gas turbine engine also allows the engine block to be relocated without severely affecting the balance of the vessel. A higher position of the engine prevents the engine from getting waterlogged in the event of compartment flooding, yet providing sufficient armour above deck to protect against threats from the air. In times of combat, vessels equipped with gas turbine engines at raised positions would possess enhanced survivability as the vessel would be able to maintain propulsion and manoeuvrability for a longer period of time.
Apart from enhanced evasive capabilities, the compact gas turbine engines allow for smaller engine rooms to be constructed. It is estimated that gas turbine engines require 75% less space than its close competitor, the diesel engine . This significant space-saving in supporting machineries for marine gas turbine engines frees up space to store more supplies and weapons, which enhances the war-fighting capabilities of gas-turbine-powered naval vessels.
2-Low emission of pollutants.
The fundamental characteristic of continuous combustion in a gas turbine engine is that the residence time at high temperatures can be controlled. The combustion process in a gas turbine engine is maintained at average temperatures and pressures which are lower than the peak levels in diesel engines. This creates an environment of relatively low temperature and pressure, which prevents nitrogen and sulphur compounds in fuel from reacting in air to form harmful nitrogen and sulphuric oxide compounds . As a result, emission of gaseous pollutants are greatly reduced due to the unfavourable conditions for harmful gases to form.
3-Easy and convenient maintenance processes
Gas turbine engine boasts a highly modular construction which allows for rapid exchange of engine components. For instance, a complete engine change-out can be completed onsite within hours, without dry-docking or extended stays in port . This brings about significant cost savings since vessels can make fewer stops at port or dry-docks. This lowered frequency of stopping at port brings about significant cuts in the fees paid for occupying these spaces. In addition, entire engines and spares can be rapidly and conveniently air-freighted worldwide. This advantage is unique to the compact and light-weight gas-turbine engines, and enhances operational-readiness of gas-turbine-powered vessels by allowing them to function near full capacity more regularly than other engines.
1-Inability to burn abundantly available heavy fuels
There are strict restrictions on the quality of liquid fuel used. Concentrations of vanadium and sulfur must be limited to avoid high temperature corrosion of turbine blades, which causes loss of engine performance . In practice, common residual fuel and cheaper distillates are completely ruled out due to high sulfur content, leaving only refined fuels for suitable for use by gas turbine engines. Since refined fuels are not as readily available as the heavy fuels used in most vessels, gas turbine engines may seem a less attractive option due to the inherent inconvenience of sourcing for suitable fuels required for the operation of the engine.
2-Lack of a broad range of engine models
High costs of converting gas turbine engines from aircrafts into marine variants has led to very few turbine engines producing power between the range of 10,000 to 20,000 HP . In comparison, diesel engines cover almost any power range and does so easily by varying the number of cylinders. This lack of versatility in power output means gas turbine engines are ineffective and unable to cater to power requirements which are out of their limited output range. As a result, manufacturers often turn to alternative power sources such as diesel engines which can produce the required power outputs they desire.
A large volume of air must be compressed in order for combustion in the Gas turbine engines require highly compressed air to cool components in the hot section of the engine, and produce the necessary high power output during combustion. Only about 25% of the air goes into supporting the combustion process while the remaining 75% is required for cooling. For this reason, the gas turbine will never be as fuel efficient as a diesel engine, but produces much cleaner exhaust. However, much progress has been made in recent years by diesel manufacturers in curbing emissions.
1-Corrosion of engine components
The humid sea atmosphere, existence of hideaway sections and imperfect rust-preventive coatings makes it favourable for the metals to rust. Bearing cavities, turbine rotors, compressor assembly attachments and rotating-blade fixtures are particularly vulnerable to corrosion. The loss in structural rigidity of these components due to rust poses as a severe safety hazard as the constant high centrifugal load on these components could cause metal shrapnel to chip off, resulting in collateral damage to the ship or crew. Despite introduction of corrosion-resistant metal alloys, rust formation remains a prevalent issue after extensive usage and exposure out at sea.
Marine diesel fuel used by naval gas turbines also contributes to the issue of corrosion, which heightens the danger level of working in a vessel. Nickel and chromium alloys are extensively used in the hot section of gas turbine engines due to their high heat resistivity. However, these metals react readily with sodium sulfate salts, formed in the combustion chamber, under a condition known as sulfidation. High-temperature corrosion of the remaining nickel-rich but chromium-depleted base material subsequently proceeds. Despite protective coatings introduced to prevent exposure of the chromium, erosive action of carbon particles formed in the combustion areas tends to react away these coatings .
Fuel contamination occurs when solid-contaminants or foreign fluids are mixed with the fuel used to power the engine. The presence of solid contaminants could be due to the inner walls of fuel tanks corroding under exposure with chemicals in the marine diesel, causing pieces of material to mix with the fuel . Foreign fluids are introduced due to leakages in pipes carrying lubrication or black oil, causing foreign fluids to the fuel system. Such contaminated fuel may cause engine misfire or severe damage to the highly-pressurised engine. Mitigation for such situations include using more corrosion-resistant materials such as aluminum and stainless-steel for fuel tanks, or slightly pressurizing fuel tanks to preclude any in-leakage.
3-Multiple safety mechanisms installed to ensure sound operation of engine
During start-up, operation and shut-down, the turbine control system controls fuel management and monitors turbine condition. Any abnormal behaviour detected by the turbine control system triggers an alarm and turbine load is immediately reduced to avoid damage . In the event of extreme situations, the control system will shut down the engine. Main gas turbines are also provided with over-speed protective devices to prevent the turbine speed from exceeding more than 15% of the maximum continuous speed . These systems prevent the gas turbine from incurring severe damage and reduces the chances of any further harm inflicted by the faulty engine on surrounding personnel and equipment.
1-High operation costs
Gas turbine engines lose greater amount of thermal energy during combustion as compared to diesel engines. This reduces the amount of thermal energy being converted to kinetic energy of spinning blades and propeller. Consequently, more fuel has to be used by gas turbine engines to produce the same energy output as a comparable diesel engine. Aero derivative gas turbines in the 20 – 30 MW class consume approximately 20% more fuel in a combustion cycle than a comparable diesel engine . Moreover, operation costs of gas turbines are plagued by expensive maintenance works such as hot section overhauls which generally cost around 1 to 2 million dollars .
2-High initial cost
Gas turbine engines are extremely expensive to produce since components are made from expensive metals, such as titanium air compressor blades . These heat-resistant metals are required in order for components to function normally despite subjection to high pressure and temperatures within the engine. It is estimated that initial investment for a gas turbine engine in the 20 – 30 MW class is approximately 15 – 20% higher than in diesel engines of comparable output .
However, the initial cost of gas turbine enrgy is relatively lower compared to low-carbon emission alternatives like nuclear and renewable energy. This makes gas turbine engines an affordable propulsion units which easily falls within the strict emission and pollutant regulations in today’s global waters.
Current trends suggest that companies are tightly engaged in a competition to build more efficient gas turbine engines. Companies such as Pratt ; Whitney’s claims that its newly-developed engine, which use an internal gearbox to slow down the speed of the fan, could save 20 percent on fuel consumption compared to conventional gas turbine engines. Such competition induces innovations in gas turbine efficiency, which would reduce emission levels by a larger extent in the future. . More durable materials will also be introduced for the manufacturing of engine pieces. Such developments reduce the frequency of maintenance, which cuts cost of replacing damaged parts. The recent introduction of ceramic matrix composites (CMC) for the turbine section of turbine engines, which can survive higher temperatures , is a clear example of improvements of quality of material used over time.
However, the costs of producing gas turbine engines in the future seems to maintain at the higher end of the spectrum. This is because temperatures within gas turbine engines remain significantly higher than internal combustion engines, and this requires the purchase of more durable and costlier materials. Moreover, the extensive hours of research and development invested to produce new materials and cooling processes which enhances engine efficiency also contribute towards raising initial costs of gas turbine engines. With speculations that gas turbine engines might comprise of 50% non-conventional materials sometime in the 2030s , the financial shortcoming of gas turbine engines should remain a prevalent issue which remains unresolved over time.
Overall, gas turbine engines should be capable of reliable, high power output with excellent service life. However, the costs of development and production are likely to remain as shortcomings which can be improved, and their emergence as widely-used propulsion units pivots on the requirements of future naval vessels and if the high longevity of the engine sufficiently outweighs its substantial initial cost.