Gas turbines
Gas turbines
Gas turbines have dominated warship propulsion for many years but their potential remains to be fully realized in the commercial shipping sector. Breakthroughs in containerships, a small gas carrier and the Baltic ferry Finnjet during the 1970s promised a deeper penetration that was thwarted by the rise in bunker prices and the success of diesel engine designers in raising specific power outputs and enhancing heavy fuel burning capability.
In recent years, however, gas turbine suppliers with suitable designs have secured propulsion plant contracts from operators of large cruise ships and high speed ferries, reflecting the demand for compact, high output machinery in those tonnage sectors, rises in cycle efficiency and tightening controls on exhaust emissions. A new generation of marine gas turbine—superseding designs with roots in the 1960s— will benefit from the massive investment in aero engine R&D over the past decade, strengthening competitiveness in commercial vessel propulsion.
The main candidates for gas turbine propulsion in commercial shipping are:
Cruise ships: the compactness of gas turbine machinery can be exploited to create extra accommodation or public spaces, and the waste heat can be tapped for onboard services. In a large cruise ship project some 20–100 more cabins can be incorporated within the same hull dimensions, compared with a diesel-electric solution, depending on the arrangement philosophy. Compactness is fostered by the smaller number of prime movers and minimal ancillary systems (roughly around 50 per cent fewer than a diesel-based plant).
Large fast passenger ferries and freight carriers: the extremely high power levels demanded for such vessels is difficult to satisfy with diesel machinery alone; coastal water deployment favours the use of marine diesel oil in meeting emission controls.
LNG carriers: an ability to burn both cargo boil-off gas and liquid fuel at higher efficiency than traditional steam turbine propulsion plant should be appreciated.
Fast containerships: the compact machinery allows space for additional cargo capacity.
Among the merits cited for gas turbine propulsion plant in commercial tonnage are:
High power-to-weight and power-to-volume ratios; aero-derived gas turbines typically exhibit power-to-weight ratios at least four times those of medium speed diesel engines; compactness and weight saving releases machinery space for extra revenue-earning activities; a General Electric LM2500 aero-derived gas turbine unit delivering 25 000 kW, for example, measures 4.75 m long ¥ 1.6 m diameter and weighs 3.5 tonnes.
Low noise and vibration.
Ease of installation and servicing fostered by modular packages
integrated with support systems and controls.
Modest maintenance costs, low spare part requirements and ease of replacement.
Environmental friendliness (lower NOx and SOx emissions than diesel engines).
Reduced manning levels facilitated by full automation and unmanned machinery space capability.
Operational flexibility: swift start-up: no warm-up or idling period required; idle can typically be reached within 30 seconds, followed by acceleration to full power; deceleration can be equally as rapid, after which the turbine can be immediately shut down; subsequent restarts, even after high power shutdown, can be instant with no ‘cool down’ restriction.
High availability, underwritten by high reliability and rapid repair and/or turbine change options.
PLANT CONFIGURATIONS
Optimizing the combination of low power manoeuvring and high power operation is accomplished in many naval applications by using a combined diesel or gas turbine propulsion system (CODOG). Other arrangements can be configured to suit the power demands and/or operational flexibility required for a project: combined diesel and gas turbine (CODAG—Figure 1); combined gas turbine and gas turbine (COGAG); combined gas turbine or gas turbine (COGOG); and combined diesel and gas turbine electric propulsion (CODLAG or CODEG).
Figure 1 CODAG propulsion plant configuration for a large monohull fast ferry
A notable COGAG plant powers the Stena HSS 1500-class high speed passenger/vehicle ferries, whose service speed of 40 knots is secured by twin General Electric LM2500 and twin LM1600 gas turbines arranged in father-and-son configurations with a total output of 68 000 kW. All four turbines are deployed for the maximum speed mode, with the larger or smaller pairs engaged alone for intermediate speeds; this enables the turbines to operate close to their optimum efficiency at different vessel speeds, with consequent benefits in fuel economy (Figure 2).
Figure 2 Final preparation of a General Electric LM2500 propulsion module for a large high speed ferry
An example of a CODEG plant is provided by Queen Mary 2, the world’s largest passenger ship, whose 117 200 kW power station combines twin General Electric LM2500+ sets with four Wärtsilä 16V46 medium speed diesel engines, all driving generators.
Combined-cycle gas turbine and steam turbine electric (COGES) plants embrace gensets driven by gas and steam turbines. Waste heat recovery boilers exploit the gas turbine exhaust and produce superheated steam (at around 30 bar) for the steam turbine genset. Such an arrangement completely changes the properties of the simple cycle gas turbine: while gas turbine efficiency decreases at low load the steam turbine recovers the lost power and feeds it back into the system. The result is a fairly constant fuel consumption over a wide operating range. Heat for ship services is taken directly from steam turbine extraction (condensing-type turbine) or from the steam turbine exhaust (back pressure turbine), and there is thus normally no need to fire auxiliary boilers.
Installations supplied by General Electric Marine Engines for large Royal Caribbean Cruises’ liners pioneered the COGES plant at sea, the first entering service in mid-2000 (Figures 3 and 4). Each 59 000 kWe outfit comprises a pair of GE LM2500+ gas turbinegenerator sets, rated at 25 000 kWe apiece, and a 9000 kWe noncondensing steam turbine-generator. Heat recovery steam generators located in the exhaust ducts of the gas turbines produce the steam to drive the steam turbine and feed auxiliaries such as evaporators and heating systems. Since no additional fuel is consumed to drive the steam turbine, the additional power it generates represents a 15- 18 per cent increase in efficiency with the gas turbines operating at rated power. The auxiliary steam represents a further efficiency improvement.
Figure 3 Schematic layout of COGES plant for a large cruise ship (GE Marine Engines)
Lightweight gas turbine-generator sets (weighing approximately 100 tons) have allowed naval architects to locate them in the base of a cruise ship’s funnel. Such an arrangement replaces the gas turbine inlet and exhaust ducts normally running to the enginerooms with a smaller service trunk housing power lines, fuel and water supplies to the gas turbine package. A significant area on every deck between
Figure 4 Space saving of a COGES plant compared with a diesel-electric installation for a cruise ship (Deltamarin)
the funnel and the machinery spaces is thus released for other purposes.
CYCLES AND EFFICIENCY
Significant progress has been made in enhancing the thermal efficiency of simple-cycle gas turbines for ship propulsion over the years, R&D seeking to improve part-load economy and reduce the fuel cost penalty compared with diesel engines. In 1960 marine gas turbines had an efficiency of around 25 per cent at their rated power, while second generation aero-derivatives were introduced in the 1970s with efficiencies of around 35 per cent. Subsequent advances—design refinements, new materials and cooling techniques, and the appropriate matching of higher compressor pressure ratios—have resulted in some large simple-cycle turbines achieving efficiencies of over 40 per cent, Figure 5.
Figure 5 Comparison of specific fuel consumption curves against load for various gas turbine cycles and a low speed two-stroke diesel engine
More complex gas turbine cycles can deliver specific fuel consumptions closely approaching the very flat curve characteristics of larger diesel engines. Part-load efficiency can be improved in a number of ways, notably through the intercooled recuperated (ICR) cycle which uses the exhaust gas to heat the combustor inlet (Figure 6). This method was chosen for the WR-21 gas turbine (detailed in the Rolls-Royce section below) which has achieved a 42 per cent thermal efficiency across 80 per cent of the operating range. (The first advanced-cycle gas turbine, the Rolls-Royce RM60, was operated in HMS Grey Goose for four years from 1953. Unfortunately, the technical complexity of the intercooling and recuperation was far ahead of the contemporary production techniques, thwarting commercial success, but the turbine proved reliable and efficient during its comparatively brief service.)
The ICR cycle and other performance/efficiency-enhancing solutions were examined by Mitsubishi Heavy Industries, taking as an
Figure 6 Schematic layour of intercooled recuperated (ICR) cycle and simple-cycle gas turbines (Rolls-Royce)
example its own MFT-8 aero-derived gas turbine developed for marine use (see below). The Japanese designer investigated various methods of improving the thermal efficiency and power output to similar levels as those of the diesel engine, focusing on six gas turbine cycle configurations (Figure 7):
Figure 7 Various gas turbine cycle configurations (Mitsubishi)
— Simple cycle : the standard layout comprising compressor, turbine and combustor.
— Intercooled cycle : as for the simple cycle but with an intercooler added to increase performance through a reduction of high pressure compressor power.
— Intercooled recuperated cycle : as for the intercooled cycle but with a recuperator installed to recover heat from the gas turbine exhaust to the combustor inlet.
— Reheat cycle : as for the simple cycle but with a reheat combustor added downstream of the high pressure turbine.
— Intercooled reheat cycle : a combination of intercooled and reheat cycle configurations.
— Combined cycle : a bottoming steam turbine cycle added to a simplecycle gas turbine.
The main conditions that are changed by varying the cycle, MHI explains, are the turbine inlet temperature and the pressure ratio (1,150°C and 20:1 respectively in the case of the MFT-8 simple-cycle gas turbine). Parametric studies were carried out to establish the optimum performance from each cycle, within the bounds of what is practically achievable in terms of temperatures and pressures. With this in mind, a maximum turbine exhaust temperature of 600°C was specified, and pressure ratios for maximum efficiency and maximum power were determined for each cycle.
The highest efficiency and power was obtained with the combined cycle option, which had the further advantage of a pressure ratio lower than even the simple cycle. The major disadvantage of the combined cycle is the fact that the heat recovery steam generator is physically large and contains very heavy components. Although this is not necessarily a problem in industrial applications, where the combined cycle is commonly used, it is a considerable drawback in marine installations.
MHI’s investigations favoured the intercooled recuperated cycle, which showed high efficiency and in power output was second only to the combined cycle. Moreover, maximum efficiency and maximum power were identical, at the achievable pressure ratio figures of 20 in each case. In the other non-simple cycles the two pressure ratio values were far apart, with one or other of the values being higher than 20, a ratio that incurs many practical disadvantages. The only problem with the ICR cycle, according to MHI, is that both the cost and size of the recuperator need to be reduced to make it more applicable to a marine gas turbine.
Although atmospheric pressure variation, an important consideration for aero gas turbines, is relatively small for marine units operating at sea level both temperature and humidity can vary significantly. Both of these parameters influence the specific heat of the air as a working fluid within the gas turbine. Atmospheric variations can have a significant effect on thermal efficiency and specific fuel consumption. Increased air temperature, in particular, reduces efficiency and lowers fuel economy; relative humidity has a less significant influence, though at high inlet temperatures an increase in humidity will have an adverse effect on specific fuel consumption.
EMISSIONS
A lower operating temperature and more controlled combustion process enable gas turbines to deliver exhaust emissions with significantly lower concentrations of nitrogen oxides (NOx) ands ulphur oxides (SOx) than diesel engines. Gas turbines typically take in three times the amount of air required to combust the fuel and the exhaust is highly diluted with fresh air. Along with a continuous combustion process, this yields very low levels of particulate emissionsa nd a cleaner exhaust.
Combustion in gas turbines is a continuous process, with average temperatures and pressures lower than the peak levels in diesel enginest hat foster NOx emissions. The fundamental characteristic of continuous combustion in a gas turbine is that residence time at high flame temperatures (a key cause of NOx formation) is capable of being controlled. In a gas turbine a balance between smoke production and NOx generation can be easily secured.
Staged pre-mixed combustion allows NOx emission levels to be reduced without the need for expensive selective catalytic reduction systems typically required by diesel engines to meet the most stringent controls (Figure 8). Proven technologies are available to reduce NOx emissions from gas turbines even further:
Wet technology based on steam or water injection into a standard combustor can lower NOx emissions to less than 1 g/kWh, exploiting a concept proven in land-based industrial applications. The availability of clean water can restrict operation at sea but the technology may be suitable for limited use in coastal areas.
Dry Low Emissions (DLE) technology yields much lower emissions than current marine engine requirements, well below 1 g/kWh. Lean pre-mix DLE combustion technology maintains a near
Figure 8 Specific NOx emissions for prime movers operating on marine gas oil
optimum fuel-air distribution throughout the combustion zone and the flame temperature in a narrow band favourable both to low NOx and low carbon monoxide production.
SOx emissions are a function of fuel sulphur content. The heavy fuel typically burned by diesel engines may have a sulphur content up to 5 per cent (3-3.5 per cent on average). In contrast, gas turbines fired by marine distillate fuels have a maximum 1–2 per cent sulphur content and the average level in gas turbine fuels is less than 0.5 per cent.
See also section on Rolls-Royce WR-21 gas turbine below.
LUBRICATION
The quantity of lubricating oil in circulation in a gas turbine is much smaller than in a diesel engine of equivalent output, and similarly the oil consumption is significantly less. (The Rolls-Royce Marine Spey, for example, has a recorded in-service average lube oil consumption of 0.1 litres/h.) The smaller charge, however, means the oil is subject to far greater stress than in a diesel engine, and the reduced consumption means it is not refreshed as often.
Apart from lubricating the bearings and other key components of a gas turbine, the bulk of the oil performs an intensive cooling function. Unlike its diesel engine counterpart, the lubricant does not come into contact with the combustion process and does not have to remove products of combustion or neutralize acids formed by burning sulphur-containing fuels.
In aero-derived gas turbines the concentration of power and heat, coupled with the comparatively small quantity of lubrication oil in circulation, results in peak oil temperatures typically over 200°C. Mineral oils are not suitable for this type of gas turbine because they would quickly oxidize at such temperatures: gums, acids and coke deposits would form and the viscosity of the oil would rapidly increase. Synthetic lubricating oils are therefore favoured because of their intrinsic ability to withstand much higher temperatures than mineral oils.
There are two types of lubricating oil generally available for gas turbine applications: ‘standard’ or ‘corrosion inhibiting’. Most turbine operators, according to lube oil supplier BP Marine, have moved towards premium corrosion inhibiting (C/I in US militar y specifications) oils to take advantage of the higher protection afforded to the bearings and other key components.
A low coking propensity, good swell characteristics and compatibility with seals are cited by BP Marine as desirable properties of a synthetic lube oil for aero-derived gas turbines. Coking is the formation of hard, solid particles of carbon due to high temperatures and can result in the blocking of oil ways. The tendency of an oil to break down and produce coke may be exacerbated by turbine operating procedures commonly encountered in fast ferry service, such as rapid acceleration and sudden shut-downs, resulting in the oil being subjected to excessive temperature rises.
Swell is caused by synthetic seals coming into contact with synthetic oils and absorbing the oil. Some swelling is desirable to ensure good sealing but too much can damage the seal and result in leakage. In addition to swell characteristics, the oil and the elastomers which come into contact must be compatible in all other respects so that degradation is avoided.
BP Marine’s Enersyn MGT synthetic lubricant for marine gas turbines, a corrosion inhibiting product, is claimed to offer the desirable properties outlined above, along with good thermal, oxidative and hydrolytic stability, and corrosion resistance. It also conforms to] the commonly required viscosity of most synthetic oils for gas turbines, nominally 5 cSt at 100°C.
A 5 cSt synthetic lube oil from Castrol Marine, Castrol 5000, is also approved for use in a range of aero-derived marine turbines, promising excellent high temperature and oxidation stability as well as superior load-carrying capabilities. Another synthetic lubricant, Castrol 778, is claimed to exhibit excellent anti-wear and rust protection properties supported by superior oxidation stability. The turbine is protected during extreme cold weather starting and during extended high temperature operation, and deposit or sludge formation is prevented over prolonged drain interval periods.
AIR FILTRATION
The use of air filtration systems tailored to the individual application can significantly enhance the reliability and efficiency of a marine gas turbine installation. A typical filtration system comprises three stages:
Vane separator: a static mechanical device that exploits inertia to remove liquid droplets from an airflow passing through it. The intended purpose of this first stage is to remove the bulk of any large quantities of water and coarse spray that may otherwise overload the remaining components of the filtration system. This type of device can remove droplets down to around 12 microns in size, beyond which inertia has little effect. Unfortunately, the damaging salt aerosol experienced within this environment is predominantly below this size and so will pass to the next filtration stage.
Coalescer : this filter-type device—usually of the one-inch depth pleated variety—is specifically designed to coalesce small water (and particularly salt aerosol) droplets; in other words, to capture small liquid droplets and make them form larger droplets. This comparatively easy task for a filter can be achieved with a relatively open filter material which, although a good coalescer, only has a very limited efficiency against dust particulate.
A low efficiency open media filter has a low pressure loss and thus for this application can be run at much higher airflow face velocities than traditional filters in other applications before it has an impact on gas turbine performance. Velocities are typically between 6 m/s and 8 m/s, compared with 2.5 m/s–3.5 m/s for other applications. This enables a much smaller filter system to be designed for naval vessels, where space is at a premium.
High velocity does not come without a price, however. A coalesce operating at a low velocity is likely to be able to drain all of the liquid it collects—through its media—with carry-over into the duct beyond. But a high velocity coalescer will re-entrain the liquid captured as large droplets, and so a third stage is required, particularly as this liquid is heavily laden with salt.
Vane separator : this third stage device is very similar to the first stage and in many systems is exactly the same. Its purpose is to remove droplets being re-entrained into the airflow by the secondstage coalescer; as such, it must be totally reliable since this liquid has the potential to cause significant damage to the gas turbine because of its salt content.
A successful filtration system serving the gas turbine plant of a fast ferry should have the following features and characteristics, suggests UK-based specialist Altair Filters International:
— It should be a three-stage (vane, coalescer/dust filter, vane) system.
— The filter element should have an appropriate degree of dust filtration performance.
— The filter should have a suitable lifetime.
— The system should be of the high velocity, compact type.
— The system should have a low pressure loss that is not significantly affected in wet conditions.
— The aerodynamics of the intake should be carefully considered.
MARINE GAS TURBINE DESIGNS
Most marine gas turbines are derived from aircraft jet engines, whose development from scratch can cost over US$1 billion. Such an investment cannot be justified for the maritime industry but the extensive testing and service experience accumulated by aero engines provides an ideal basis for marine derivatives. Marine gas turbines are available only in specific sizes and ratings, unlike a given diesel engine design which can cover a wide power range with different inline and V-cylinder configurations. The economical application of gas turbines to marine propulsion therefore dictates matching the unit to the project, and hence calls for atechnical and economic analysis of the vessel’s proposed deployment.
Operational costs include availability for service and maintenance requirements, cost of consumables (fuel and lube oil) and manning costs. All these factors have to be considered for military ship designs but not to the extent that is called for in commercial shipping. Tradeoffs among the various parameters can be conducted to determine the right fit of propulsion system to the application.
All marine gas turbines incorporate the same fundamental components (Figure 9):
Figure 9 Main elements of an aero-derived gas turbine, and the pressure, temperature and air/gas speeds during the cycle
A compressor to draw in and compress atmospheric air.
A combustion system into which fuel is injected, mixed with the compressed air, and burned.
A compressor turbine which absorbs sufficient power from the hot gases to drive the compressor.
A power turbine which absorbs the remaining energy in the hot gas stream and converts it into shaft power.
Lightweight aero-derived, rather than heavyweight industrial-derived, gas turbine designs are favoured for marine propulsion applications. The aero jet engine extracts only sufficient energy from the gas stream to drive the compressor and accessories, and releases the surplus gas at high velocity through a convergent nozzle to propel the aircraft by reaction.
When converting the jet engine into a shaft drive machine it is necessary to provide an additional free power turbine to absorb the energy left in the gas stream and to transmit that energy in the form of shaft power. The original jet engine is then termed the Gas Generator and the whole assembly becomes a Gas Turbine. Using a free power turbine reduces the starting power requirement as only the gas generator rotating assemblies have to be turned during the starting cycle. The resulting prime mover benefits from the intensive development and refinement of the original aero engine, both before and during service.
Certain aero engines are already fitted with power turbines and termed turbo-prop or turbo-shaft machines; these are readily adaptable to industrial and marine use. Special technical and production techniques are applied to the re-designed gas turbine to ensure itssuitability for sea level operation at high powers, and for the marine environment.
A basic gas generator has one rotating assembly: the compressor and its turbine coupled together. The characteristics of axial-flow compressors vary considerably, however, over the operating range from starting to full power. On some high compression ratio compressors it is necessary to fit automatic blow-off valves and to alter the angle of the inlet guide vanes and first stages of stator blades to ensure efficient operation.
To achieve the necessary stability in larger gas generators, the compressor is divided into two separate units: the low pressure and high pressure compressors, each driven by its own turbine through co-axial shafts. Each compressor is able to operate at its own optimum speed, giving flexibility of operation and efficient compression throughout the running range. Only the high pressure rotor needs to be turned during starting, and therefore even the largest gas generator can be started by battery.
Although such a two-spool compressor is very flexible, it is still necessary in some cases to adjust the inlet guide vanes to deal with the changing flow of air entering the compressor. This is effected automatically by pressure sensors acting upon rams which alter the angle of the guide vanes.
An axial-flow compressor consisting of alternate rows of fixed and rotating blades draws in atmospheric air through an air intake and forces it through a convergent duct formed by the compressor casing into an intermediate casing, where the compressed air is divided into separate flows for combustion and cooling purposes. A typicalannular combustion chamber receives about 20 per cent of the air flow for combustion into which fuel is injected and burned.
Initial ignition is executed by electrical igniters, which are switched off when combustion becomes self-sustaining. The resulting expanded gas is cooled by the remainder of the air flow which enters the combustion chamber via slots and holes to reduce the temperature to an acceptable level for entry into the one or more axial-flow stages of the turbine. The turbine drives the compressor, to which it is directly coupled. The remaining high velocity gases are exhausted and are available for use in the power turbine.
A power turbine of the correct ‘swallowing’ capacity is required to convert the gas flow into shaft power, Figure 10. Typically of one
Figure 10 Power turbine of a Rolls-Royce Spey SMIC engine
or more axial-flow stages, the power turbine may be arranged separately on a baseframe designed to mount the gas generator, to which the turbine is linked by a bellows joint to avoid the need for very accurate alignment and to allow for differential expansion. In some cases, however, expansion is allowed for in the design of the power turbine and the gas generator is mounted directly on to the inter-turbine duct. The power turbine, usually designed to last the life of the plant, is surrounded by an exhaust volute which passes the final exhaust to atmosphere through a stack.
Most marine gas turbines incorporate a free power turbine, which is free to rotate at whatever speed is dictated by the combination of the power output of the Gas Turbine Change Unit (GTCU) and the drive train behind it. The gas turbine is designed so that there are no critical speeds in the running range, and it can operate continuously with the power turbine at any speed from stopped to the full power rev/min of the power turbine. This facility is especially important under manoeuvring conditions where the combination of a free power turbine and reversing waterjets provides a flexible and effective propulsion system.
Aero- and industrial-derived marine gas turbines are available with unit outputs ranging from around 2000 kW to over 50 000 kW. Only the larger designs (with ratings of 17 000 kW upwards) are reviewed here:
ALSTOM POWER (ABB STAL)
Lower operating costs through burning cheaper fuels than the light gas and diesel oils normally required by aero-derived gas turbines are promised by Alstom Power, which inherited the lightweight industrial-based turbine interests of Sweden’s ABB Stal. Successful marine service experience on IF30 intermediate fuel is cited for its GT35 turbine, supported by a capability to burn IF180 fuel demonstrated in land-based plants.
The GT35 emerged from a post-war aero engine development programme in Sweden involving Stal Laval and Volvo Flygmotor. Small turbojets were designed and built for testing in the late 1940s/early 1950s but never entered commercial production. Stal Laval saw potential in transforming the engine into an industrial gas turbine for power generation and mechanical drive duties. The first GT35, rated at 9000 kW, was tested in 1955 and installed during the following year in a Swedish power station.
Performance and reliability were subsequently demonstrated in land-based and offshore platform power generating installations. The design has since benefited from two upgradings to attain its present output rating of 17 300 kW at 3450 rev/min for mechanical drive applications, with an efficiency of 33 per cent (Figure 11).
Figure 11 One of two 17 000 kW Alstom Power GT35 gas turbines supplied to drive
The gas generator of the GT35 is a two-shaft engine equipped with a 10-stage low pressure compressor, an eight-stage high pressure compressor, seven can-type combustion chambers, a single-stage high pressure turbine and a two-stage low pressure turbine (Figure 12). The power turbine, developed in parallel with the gas generator, is a three-stage design. A refinement made in the 1990s enabled the power turbine to operate at variable speed and enhanced its suitability for mechanical drive and marine applications.
Figure 12 Flow diagram of Alstom Power GT35 gas turbine in a marine propulsion installation
Refining the GT35 for marine applications called for the following measures: modifying the lube oil system to allow for vessel movements; modifying the support arrangement to protect the turbine from hull deflections; and modifying the power turbine by introducing a second combined journal and thrust bearing, as well as a modified active tip clearance control mechanism (achieving a self-supported power turbine and thus eliminating the need for a thrust bearing in the gearbox).
High reliability from the marinized GT35 was sought by exploiting as many standard components as possible from the power generation and mechanical drive versions. An extensive re-calculation of low cycle fatigue and creep properties of certain critical elements of the turbine was also undertaken, leading to subsequent redesign of some parts. High availability and maintainability in arduous marine duty were addressed by adapting the installation to the specific vessel, with the aim of reducing the time for unit replacement and facilitating servicing. The maintenance programme was also adapted to operating conditions imposing only short shutdown times for routine inspections and unit change-outs.
Four key factors contribute to high fuel flexibility, underwriting operation of a GT35 turbine set on IF30 fuel (normally produced from 75 per cent IF180 fuel/25 per cent marine gas oil) after suitable treatment:
A high viscosity tolerance due to the spill flow fuel system in which the amount of fuel injected into the turbine is governed by a regulating valve mounted in the return pipe from the injectors. The system configuration makes it possible to circulate the fuel through the injectors prior to ignition and hence warm them up sufficiently to facilitate starting on fuels with a viscosity of up to 30 cSt/50°C (equivalent approximately to 10 cSt/95°C, the temperature to which the fuel is heated on the vessel).
A maximum inlet temperature of 880°C (fairly low compared with other gas turbine designs) makes it unnecessary to use cooled blades and vanes, yielding such benefits as insensitivity to high temperature corrosion from sodium sulphate; no need for blade coating; and no risk of plugging cooling holes and channels with ash deposits and reducing the lifetime of blading.
Large combustion chambers (Figure13) secure low sensitivity to variation in radiation heat depending on the type of fuel that is burned. The size and design of the chambers ensure their lifetime matches that of the complete turbine unit.
A simple on-line ‘soft blast’ turbine cleaning procedure—with ground nutshells injected directly into the turbine section via inspection windows in the combustion chambers—results in sustained high turbine efficiency and power output combined with low outage time for cleaning. Cleaning can be effected with the turbine operating at close to full load; depending on the fuel properties, the procedure is necessary at intervals varying between 50 hours and several months’ operation.
Retaining a high output to weight ratio dictated special attention to the marine GT35’s auxiliary systems, the designers reportedly achieving a reduction in weight of approximately 25 per cent over a land-based unit. Among the measures was a redesign of the starting system from a pneumatic to an electric system and a reconfigured baseframe. Quiet running and high safety are fostered by solid turbine casings which allow sets to be installed without an enclosure. Packages can thus be better tailored to individual hulls, while the free-standing auxiliary systems can be located in the most convenient position.
Figure 13 Seven large can-type combustion chambers (foreground) contribute to the lower
GE MARINE ENGINES
General Electric (GE) introduced its first aero-derivative gas turbine, the LM100, in 1959 and in the same year the LM1500, derived from the successful J79 aircraft engine. The first LM1500 installation, serving the hydrofoil vessel H.S. Denison, was followed by over 160 applications in land-based catapults, pipeline pumping systems, marine propulsion and power generation. The mainstay of the US designer’s programme, the LM2500, arrived in 1969 and has since benefited from developments which have doubled its initial rating of around 15 000 kW. Other models subsequently joined the LM-series portfolio, which now embraces five simple-cycle designs with maximum power ratings from around 4500 kW to 42 750 kW and thermal efficiencies up to 42 per cent
Valuable experience from LM-series gas turbines in naval propulsion applications ranging from patrol craft to aircraft carriers was tapped by GE Marine Engines in targeting commercial vessel projects during the 1990s, earning a number of large cruise ship and fast ferry references.
Table: GE Marine Engines’ aero-derived LM gas turbine family
The LM500 is a two-shaft gas turbine derived from GE’s TF34 aircraft engine; the LM1600 is a three-shaft gas turbine derived from the F404 engine; and the LM6000 is a two-shaft gas turbine derived from the CF6-80C2 engine. The most successful models in the commercial and military marine markets are the LM2500 and LM2500+ models, described below.
LM2500 and LM2500+
Derived from the GE military TF39 and commercial turbofan aircraft engines, the LM2500 marine gas turbine is a simple-cycle, two-shaft engine comprising a gas generator, a power turbine, attached fuel and lube oil pumps, a fuel control and speed-governing system, associated inlet and exhaust sections, lube and scavenge systems, and controls and devices for starting and monitoring engine operation. The four main elements (Figure14) are:
A 16-stage compressor, with an 18:1 pressure ratio; the first seven stages feature variable stators and inlet guide vanes promoting easy starting, good part-load performance, high efficiency and a high stall margin over the entire operating range.
Fully annular combustor, with externally mounted fuel nozzles supporting liquid fuel combustion; virtually smokeless operation is assured through the complete power range even when burning heavy distillate fuels.
Two-stage high pressure turbine, air cooled for a long life, which drives the compressor and accessory-drive gearbox.
Figure 14 Cutaway of GE Marine Engines' LM2500 aero-derived gas turbine showing (from left to right the compressor, annular combustor, high pressure turbines and low pressure power turbine
Six-stage low pressure power turbine; a low speed, low stress machine aerodynamically coupled to the gas generator and driven by its high energy release exhaust flow.
Lower installed and life-cycle costs per unit kW were sought from the LM2500+ derivative released in the late 1990s with a 25 per cent higher power rating than its precursor. An introductory rating of 27 600 kW was achieved with a thermal efficiency of more than 37 per cent; release at the design rating of 29 000 kW and a thermal efficiency of 38 per cent followed early sets in service demonstrating the traditional reliability and availability of the LM2500. The current rating is 30 200 kW with a thermal efficiency of 39 per cent. The design refinements applied to the LM2500+ focused on:
— The compressor rotor: air flow was increased by approximately 20 per cent; Stage 1 blades were redesigned to a wide chord configuration eliminating the mid-span dampers; an LM6000 rotor airfoil design was added to Stages 2 and 3; LM6000 stages were incorporated to improve compressor efficiency; and a new inlet guide vane assembly was specified.
— High pressure turbine rotor and stator: redesigned to reduce maintenance costs and new materials exploited for improved oxidation life; Stage 1 and 2 contours optimized for higher flows.
— Power turbine: redesigned in line with the higher output; Stage 1 and 6 blades optimized for aerodynamic efficiency; and the rotor strengthened for the higher torque and potential energy.
No changes were made to either the standard or dry low emissions (DLE) combustion system.
Only slightly longer and heavier than the LM2500, the LM2500+ offers dual-fuel capability (distillate and gas), rapid start-up and loading, variable speed operation and excellent part-load efficiency, GE claims.
MITSUBISHI
A Japanese challenge in the aero-derived marine gas turbine market was spurred by the country’s Techno-Superliner (TSL) fast freight carrier project of the early 1990s which called for an indigenous high output prime mover with a very low weight. In response, Mitsubishi created the MFT-8 from the marriage of its own power turbine and the GG-8 gas generator from Turbo Power and Marine Systems, a subsidiary of Pratt & Whitney (Figure 15).
Figure 15 Major engine groups and sections of Mitsubishi MFT-8 aero-derived marine
The GG-8 is a land and marine derivative of the US group’s JT8D turbofan aero engine, the design modified to replace the front fan with the forward part of the front compressor; otherwise it is fully compatible with the core engine. The key elements of the gas generator are: an eight-stage low pressure compressor; seven-stage high pressure compressor; nine burner cans; a single-stage air cooled high pressure turbine; and a two-stage low pressure turbine. Gas is fed to a high performance power turbine developed by Mitsubishi’s project team as a three-stage design with lightweight, overhung ball and roller bearings.
The main modules and components are designed for ease of removal and replacement during inspection and maintenance; and provision is made for borescope inspection of the hot parts and rotating parts of the gas generator and power turbine without disassembly.
After the prototype achieved a full power rating of 24 300 kW during shop trials, two production sets were delivered for installation in the TSL craft built by Mitsubishi. The Japanese designer subsequently offered the MFT-8 turbine to the wider market with a propulsion plant rating of 25 790 kW and a thermal efficiency of just under 39 per cent.
ROLLS-ROYCE
Marine Spey
The Marine Spey first went to sea with the UK Royal Navy in 1985, and the latest C-rated version entered service in 1990. Numerous warship propulsion references have been logged and commercial opportunities targeted, notably for high speed ferry and freight vessel projects (Figure 16). The twin-spool design is released with
Figure 16 Rolls-Royce Marine Spey machinery arrangement for a waterjet-powered fast ferry
respective commercial and military marine ratings of 18 000 kW and 19 500 kW, and specific fuel consumptions of 230 and 226 g/kWh.
Gas generator, power turbine and all ancillary systems are mounted on a common bedplate to form a self-contained unit for ease of installation (Figure 17). Interfaces for the shipbuilder are limited to the intake and uptake flanges, the mountings to the hull and the output flange. A complete propulsion module, including power turbine, has a length of just over 7.5 m and weighs 17.5 tonnes in its commercial vessel configuration; the gas generator weighs only 1.8 tonnes. An 80 per cent weight saving and 60 per cent space saving over a medium speed diesel installation of equivalent output are reported.
Figure 17 A fully packaged Rolls-Royce Marine Spey propulsion module
The gas generator is a high efficiency, second generation unit of twin-spool design designated the SM1C, which is a development of the SM1A and incorporates technology from the Rolls-Royce Tay and RB211 aero engines. The core achieves a 20:1 pressure ratio and good overall efficiency from a compact unit with relatively few stages of blading. The designers sought excellent operational flexibility, rapid acceleration and good aerodynamic stability, with no visible exhaust smoke at any power level. The modular construction allows gas generator sections to be exchanged on site, the engine comprising five maintenance assembly change units (MACUs).
Specifically designed for the marine environment, the power turbine is a high efficiency, two-stage axial flow design with short and rugged shrouded blades capable of withstanding significant foreign body damage. Its rotor system is supported on hydrodynamic bearings housed in a rigid centre body, and is available with clockwise or anti-clockwise output shaft rotation. The power turbine is designed for the life of the ship but is removable in the event of serious damage (Figure 10).
Marine Spey data
Marine Trent 30
A simple-cycle marine gas turbine with a thermal efficiency exceeding 40 per cent, the Rolls-Royce Marine Trent 30 was scheduled for commercial availability from early 2004 offering outputs up to 36 000 kW from a package weight of 25 tonnes. The MT30 is the eleventh engine type derived from the British designer’s core aero technology.
The aero Trent 800 was selected from a diverse Rolls-Royce engine family as the parent for this new generation of marine gas turbine because it best matched current and emerging customer requirements with margins in hand. The three-spool marine derivative allows variable speed operation, and its ‘turn-down’ facility secures low power harbor manoeuvring capability. Furthermore, the aero engine’s track record (99.9 per cent dispatch reliability) gave confidence for commercial marine applications with high annual usage.
Availability, reliability and maintainability were prime goals in setting the design parameters for the MT30, with an overhaul target of 12 000 hours for the hot end and 24 000 hours between major overhauls. The design reportedly features some 50–60 per cent fewer parts than other aero-derived turbines in its class. A sound basis is provided by an 80 per cent commonality of parts with the aero Trent. Specialized coatings are applied where necessary for protection against the marine environment. Internal temperatures and pressures are substantially lower than those at the aero engine take-off rating to foster component longevity.
A rating of 36 000 kW is available at the power turbine output shaft at ambient air temperatures up to 26°C, with a corresponding specific fuel consumption (sfc) of 207 g/kWh; under tropical conditions (32°C air temperature) the output is 34 100 kW. Competitive efficiency is sustained down to 25 000 kW and thermal efficiency is similar to that of high speed diesel engines, Rolls-Royce claims. The MT30 is designed to burn the widely available distillate marine fuel grade DMA.
Essentially the core unit of an aero Trent engine minus the fan section, the gas generator of the MT30 benefited from the latest design techniques, materials and production technology during its refinement for marine applications. An eight-stage variable geometry axial intermediate pressure (IP) compressor driven by a single-stage IP turbine feeds air to the high pressure (HP) spool, which comprises a six-stage compressor and a single-stage HP turbine. An annular combustion chamber is arranged between the HP compressor and the HP turbine. The IP and HP spools rotate in opposite directions. A two-spool compressor design yields reduced start time, good balance retention and no lock-up problems, says Rolls-Royce, resulting in greater operational flexibility and no need for in-situ balancing.
Hot gas from the gas generator section is supplied to a four-stage free power turbine whose full speed of 3300 rev/min was selected for mechanical drives to yield the lowest sfc. The design addresses the demands of waterjet operation, notably sudden large load changes if air is drawn into the jet unit in severe sea conditions.
The MT30 is supplied as a package with an acoustic enclosure housing auxiliaries and inlet and exhaust diffusers, and incorporating automatic fire detection and extinguishing systems (Figure 18). A complete unit—measuring 9.1 m long ¥ 3.8 m wide ¥ 4 m high— weighs around 22 tonnes dry, depending on the options selected.
The gas turbine change unit, including power turbine, weighs 6200 kg dry and is air-freightable. Installation of the main package can be
Figure 18 Rolls-Royce MT30 in mechanical drive package (dimensions in mm)
effected in a single lift at the shipyard, while the control panel, starter and lube oil modules can be mounted where convenient in the engineroom.
A key element of the package—the integrated control and monitoring system—is housed in a free-standing cabinet with an uninterrupted power supply for protection in the event of mains failure. An engine health monitoring system is also offered. Conditionbased maintenance is facilitated by internal condition sensors; routine servicing is limited to checking fluid levels and visual examinations. Maintenance contracts covering servicing, overhauls and spares supply, long practised in the airline industry, are available to support operators and foster competitive through-life costs.
MT30 gas turbine data
The MT30 development will serve as the basis for a more powerful aero Trent marine derivative, the 50 000 kW Marine Trent 50 (Figure 19). This MT50 is also a modular engine embracing eight prebalanced units, allowing major assembly changes to be carried out locally if required. It retains the same three-spool aero core IP and HP compressors and IP and HP turbines, but with a two-stage LP compressor mounted on the third spool replacing the aero fan. The combustion system comprises an annular (donut ring) combustor with 24 individual burners through which the fuel enters the combustion chamber.
Three co-axially independent running shafts are contained within a carcass of axially-joined circular cases. The low pressure shaft features a two-stage LP compressor driven by a five-stage LP turbine; the intermediate pressure shaft features an eight-stage IP compressor driven by a single-stage IP turbine; and the high pressure shaft incorporates a six-stage HP compressor driven by a single-stage HP turbine. A full power specific fuel consumption of around 200 g/ kWh is anticipated for the MT50, reflecting the decrease in the
Figure 19 Cross-section of Rolls-Royce MT50 gas turbine
proportional blade losses in an axial turbine as the size of the turbine increases.
WR-21
The only advanced-cycle marine gas turbine currently on the market, the WR-21 was developed by Northrop Grumman in the USA and Rolls-Royce during the 1990s with funding from the US Navy, UK Royal Navy and French Navy. It is the first aero-derived gas turbine to incorporate compressor intercooling and exhaust heat recuperation
Figure 20 Intercooled recuperator (ICR) system applied to the Rolls- Royce WR-21 gas Turbine
operating range: a fuel burn reduction up to 30 per cent over simplecycle turbines is reported.
Rated at 25 000 kW, the WR-21 is based on the successful Rolls- Royce aero RB211 and Trent engines, with modifications to marinize the components and effectively integrate the heat exchangers and variable geometry. The first seagoing installations (featuring twin sets) will enter service from 2007 in the UK Royal Navy’s new Type 45 D class destroyers but commercial propulsion opportunities, particularly cruise ships, are also targeted. The fuel consumption characteristics—approximately 205 g/kWh from full power down to around 30 per cent power—enable the WR-21 to fulfil the role of both cruise and boost engines.
Compared with simple-cycle turbines, the benefits cited for the WR-21 include an improvement in fuel efficiency over the entire operating range (with a radical improvement at low power), easier maintenance through enhanced modularization, and the facility to retrofit ultra low emission reduction systems. The intercooled recuperated (ICR) propulsion system package was designed to occupy the same footprint as existing power plant, Figure 21.
Figure 21 Layout of WR-21 gas turbine package
A significant fuel saving over simple-cycle turbines is achieved by using heat exchangers to improve the part-load cycle efficiency. Recuperation alone improves the thermal efficiency of low pressure ratio cycles where the exhaust temperature of the turbine is significantly higher than that of the air leaving the compressor. Heat is transferred to the compressed air before it enters the combustion system, reducing the amount of fuel required to attain the cycle turbine entry temperature.
The recuperator thus pre-heats the combustion air by recovering waste energy from the exhaust, improving cycle efficiency and reducing fuel consumption. Low power efficiency is improved still further by power turbine variable area nozzles; these maintain a constant power turbine entry temperature, which in turn maintains recuperator gas side entry conditions and improves recuperator effectiveness as power reduces.
The intercooler cools air entering the high pressure compressor, reducing the work required to compress the air; the intercooler also reduces the HP compressor discharge temperature, which increases the effectiveness of the recuperator.
Air enters the compressors via a composite radial intake designed to maintain uniform circumferential velocity at entry to the gas generator. The IP compressor (so called because it reflects commonality with the parent three-spool turbofan aero engine) includes six stages of compression; stages two to six are in common with the RB211 engine, while the first stage is modified to suit the increased flow requirements of the ICR cycle.
The intercase between the compressors transmits structural load from the engine through two support legs to the sub-base; it also houses the five intercooler segments in an outer casing and the internal gearbox within an inner casing. Both casings combine to form the air flow path between compression stages. Incorporating an intercooler between the compression stages on a twin-spool cycle increases the specific power of the engine. The amount of work needed to drive the compressor is reduced, thereby increasing the net power available. Bypass valves on the intercooler are modulated depending on the pressure and relative humidity of the air so that condensation formation can be avoided.
Cycle thermal efficiency is approximately the same as that of a simple-cycle engine, as additional fuel is required to offset the drop in compressor exit temperature. Combining an intercooler with a recuperator, however, is attractive for higher pressure ratio engines, leading to high specific power outputs and good thermal efficiency.
The HP compressor also has six compression stages and is aerodynamically identical to its aero origin. Compression is split 30:70 between IP and HP compressors, and both stages incorporate additional boroscope holes to allow greater flexibility for inspection.
The Marine Spey SM1C combustor was adopted as the basis for the WR-21 combustor design as a proven system in use worldwide. Although otherwise conventional in its construction, the combustor features a Reflex Airspray Burner method of fuel injection developed specifically for the marine versions of the Spey. This achieves a controlled mixing of fuel and air, allowing a high burner exit air-fuel ratio (AFR) to be maintained with adequate flame stability. Based on previous experience, high (lean) AFR was considered an important factor in reducing visible smoke when burning diesel fuel.
Preservation of the proven aero RB211 HP and IP spool lengths, which are characterized as short, rigid high integrity structures, was a principal design objective. The requirement to remove compressor delivery air and return recuperated air within the length constraint dictated that the annular RB211 combustor be replaced by a radiallyorientated turbo-annular system.
The manifold designs underwent extensive analysis and testing to confirm structural integrity, aerodynamic flow distributions, ease of manufacture and maintainability. The inner casing carries the structural load from the HP compressor outer casing to the turbine casing; the component is designed so that it provides the minimum of blockage for HP compressor exit air entering the combustion manifold and also transfers HP compressor stage 4 air to the power turbine for sealing and cooling. In-situ removal of the combustor and burner is a key element of the maintainability strategy. The design addresses ease of life monitoring and timely repair or replacement of hardware (without engine removal) in the event of premature failure.
The HP nozzle guide vanes and rotor blade airfoils maintain commonality with the RB211-524 parent engine, with slight modifications to provide a smooth gas path from the radically-swept discharge nozzles. The disc seals and bearing arrangements are essentially unchanged from their aero origins. The IP nozzle guide vane is skewed relative to the aero RB211-535 vane and incorporates the addition of a cast boss to facilitate a boroscope inspection hole. The blade is uncooled and manufactured from a single nimonic to extend creep life.
Consistent with the pedigree of the gas generator, the power turbine also originates from an aero parent, the Trent. Stages two-throughfive incorporate three-dimensional orthogonal blade geometry to maximize turbine efficiency, but the main difference is the incorporation of the Variable Area Nozzles (VAN) which control flow area. The VANs are hydraulically actuated via a single geared ring designed to maintain VAN-to-VAN throat areas within a specified tolerance. The VAN is fully open at full power and closes at part power; this has the effect of retaining the efficiency benefits across the whole power range by maintaining the high exhaust temperature at part powers. The recuperator can thus be exploited fully to give the characteristic flat fuel consumption curve for the WR-21 (Figure 22).
Figure 22 Fuel savings from ICR cycle WR-21 over the power range compared with simple-cycle gas turbine
The gas generator and power turbine comprise twelve interchangeable, pre-balanced modules, whose small size and low weight facilitate removal via simple routes; new or leased modules can be fitted in-situ, reducing maintenance costs and downtime.
The enclosure panels are structurally designed to meet blast overpressure, shock and air tightness, with insulation and acoustic damping treatment applied to satisfy thermal and noise requirements. All the panels are removable, except those welded to the recuperator housing. The engine can be withdrawn sideways by removing vertical supports at its front. The enclosure is cooled by ship’s ventilation, air entering it via a connection above the recuperator ducts. An Electronic Engine Controller (EEC), incorporating a Full
Authority Digital Engine Control (FADEC) system, is responsible for controlling the system in response to commands from either the ship’s monitoring and control system or from the local Man Machinery Interface (MMI). The controller covers sequencing, steady state and transient control, surveillance, fault detection and protection.
Easy control of NOx emissions when entering controlled waters is fostered by the flexibility of the ICR cycle. Punitive charges can be avoided by switching to recuperator bypass mode, lowering the combustion temperature beneath the threshold for NOx generation. Additionally, the high combustor inlet temperatures over the power range in recuperated operation ensure complete combustion with extremely low levels of carbon monoxide and unburned hydrocarbons.
GAS TURBINE MAINTENANCE
Gas turbines have nominal power ratings established for marketing purposes. A power rating at particular ambient conditions determines the internal engine temperatures and the resulting expected service life of components exposed to these temperatures. These ‘hot section’ components include the combustor and the high pressure turbine blades and vanes. The life of the hot section components will determine the interval between major maintenance actions, resulting in an estimated average maintenance cost.
The normal relationship between internal parts life and a representative internal gas turbine temperature, in this case the power turbine inlet temperature, is indicated in Figure23. At higher internal temperatures the component life is determined by creeprupture and decreases rapidly with increased temperature; at lower internal temperatures the component lives are determined by corrosion and do not vary as rapidly. (Turbine blades operate at high temperature and rotate at high speed. The blade material has an overall creep life which is proportional to time at a given power level.)
Figure 23 Relationship between internal parts life and power turbine inlet temperature
The nature of the curve in the figure illustrates the selection of power rating for a particular application. Operation for large percentages of time at power levels in the creep-rupture regime will severely limit component lives in the gas turbine and increase the unit’s average maintenance cost. Conversely, best advantage of a gas turbine’s capability can be taken with respect to maintenance near the top of the corrosion controlled regime. Establishing a rating around the ‘knee’ of the curve therefore tends to be a good starting point with respect to trade-off between optimum power and reasonable maintenance costs.
Vessel operations will generally involve some representative profile with a mixture of high and low power elements, and a range of ambient operating temperatures. The optimized gas turbine rating makes best use of the engine at the predominant mode of operation, while ensuring that the maintenance costs combined with other operating costs produce a minimum total life-cycle cost.
A second factor affecting maintenance costs is the number of cycles of operation per unit time. Since some internal components may be limited in life to a maximum number of thermal cycles, operation with very short mission lengths will result in higher average maintenance costs. Figure24 illustrates the nominal relationship of average maintenance cost to length of mission. For example, a fast ferry operation with mission lengths of one hour will experience a higher average annual maintenance cost than for five or six hour runs at the same power rating and ambient conditions. As the length of the mission increases, the maintenance requirements approach the level determined by the power level and ambient conditions resulting from the hot section life. For short missions, however, the replacement of cyclic-limited components can have a significant effect on average maintenance costs. Total cycle times for very short applications are usually determined by the turnaround time at each end of a route. The additional speed is less beneficial for the short cycle because a smaller percentage of time is spent in transit. Gas turbine propulsion generally yields greater benefits in longer cycle applications.
Figure 24 Average gas turbine maintenance cost versus length of mission
High reliability is a requirement imposed by military ship applications because of the potentially critical nature of missions. In commercial operations, however, time in service translates into revenue production; factors crucial to the commercial success of the vessel design are therefore minimal downtime and short maintenance intervals.
High availability results from two design considerations. First, the maintenance requirements must be minimized through appropriate rating of the gas turbine, and mission planning must allow for maintenance that minimizes disruption of the operating schedule. Second, appropriate installation design must foster quick removal and replacement of modular components, minimizing time out of service.
Commercial vessels are usually designed with a particular service profile in mind, but the design must be flexible enough for diverse deployment if resale to other operators is planned later. The propulsion plant must therefore allow flexibility in operation over a range of power levels while maintaining operating efficiency with respect to fuel and lube oil consumptions.
Gas turbine engine maintenance may be considered broadly in two groups: routine tasks to be carried out onboard (Figure 25)
Figure 25 Routine maintenance of the Rolls-Royce MT30 gas turbine (shown here on the testbed) is limited to checking fluid levels and visual examinations
and routines that call for service engineer support or scheduled removal of the gas turbine change unit (GTCU).
Routine onboard maintenance is limited, consisting mainly of daily visual checks of the running engine. An internal water wash is executed after a fixed number of operating hours to remove any salt deposits in the compressor which would otherwise eventually result in degraded performance.
The engine itself can be divided into two main elements: the GTCU and the power turbine with ancillaries package. The GTCU can be readily removed from the vessel, within six to 12 hours, when dictated by condition monitoring or scheduled removal. The power turbine and ancillaries are designed to remain for longer periods, up to the life of the vessel.
In general, the operator will regard the repair of the GTCU as repair by replacement; it will be removed and replaced, and the vessel returned to service in hours rather than days. An owner will therefore normally choose to own a calculated number of spare engines or major engine components (MACUs: maintenance assembly change units) to support a particular operation with a required availability.
A maintenance regime can be provided through a power-by-thehour arrangement directly with the gas turbine manufacturer. An indicative figure for such an arrangement is often requested by operators, the figure typically including scheduled removal and overhaul of the engine and its replacement by a spare during the overhaul cycle. It is invariably misleading to quote such a figure, which is not a feature purely of the engine but highly dependent on the usage. The calculation will vary strongly with the following factors, notes Rolls-Royce:
The operating profile of the vessel.
The number of starts, stops and idling periods during the route.
The ambient operating temperatures.
The maximum and minimum powers required from the engine.
These factors are fundamental in deciding engine life and therefore the maintenance costs. The resulting calculations are based on creep and cyclic performance. With a knowledge of the operating profile an assessment can be made of the GTCU life at which the blading must be replaced. At one extreme, on a longer ferry route with operation at high power and high ambient temperatures, engine life may be creep limited.
The multiple compressor and turbine stages of the GTCU comprise individual discs with blades attached. Any metal disc that is continuously and repeatedly accelerated from rest to a high speed, then decelerated, will eventually crack. All the discs have a declared cyclic life, a full cycle being defined as from rest to full speed and back to rest. The declared life falls well within that at which a disc might be expected to crack, but when this life is reached the disc must be replaced.
Cyclic life may become a limiting factor on short ferry crossings, particularly if this is carried out at high engine speeds; it may dictate whether the engine can be shut down or should be left running at idle in harbour, largely for economic reasons based on engine lifing.
It is the combination of creep and cyclic life calculations in conjunction with condition monitoring of the engine that decides the periodicity of the required GTCU changes, and therefore impacts directly on any fired-hour maintenance support arrangement. By working with a prospective operator, the engine manufacturer is then able to make an accurate assessment of the maintenance costs. The combination of engine characteristics and selected operation determines these costs rather than the engine alone.
Gas turbine compressor washing is an important maintenance routine in sustaining power output. Since the compressor consumes up to two-thirds of the total engine power even slight blade fouling significantly reduces the net available output and hence the drive power available or increases fuel costs. Off-line washing is an effective countermeasure in restoring performance and overcoming the efficiency drop caused by compressor fouling. On-line washing is available when engines cannot easily be shut down for an off-line wash (as with cruise ships during long voyage legs).
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