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A study on availability and safety of new propulsion systems for LNG carriers
ARTICLE IN PRESS Reliability Engineering and System Safety 93 (2008) 1877– 1885
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Reliability Engineering and System Safety journal homepage: www.elsevier.com/locate/ress
A study on availability and safety of new propulsion systems for LNG carriers Daejun Chang , Taejin Rhee, Kiil Nam, Kwangpil Chang, Donghun Lee, Samheon Jeong Hyundai Heavy Industries Co. Ltd., Ulsan, South Korea
a r t i c l e in fo
abstract
Available online 21 March 2008
This study investigated the availability and safety concerns of the conventional and prospective propulsion systems for LNG carriers: dual-fuel steam turbine mechanical (DFSM) propulsion, dual-fuel diesel electric (DFDE) propulsion, dual-fuel gas turbine electric (DFGE) propulsion, dual-fuel diesel mechanical (DFDM) propulsion, and diesel mechanical propulsion with reliquefaction (SFDM+R).
Keywords: LNG carrier Propulsion Availability Safety Hazard
The two prospective candidates, the DFDM and DFGE, exhibited the availabilities of design and emergency propulsion loads as high as the newly adopted DFDE and SFDM+R, while the DFSM demonstrated the highest. All the propulsion systems achieved a satisfactory level of the availability for the BOG utilization. The newly introduced dual-fuel systems of DFDE, DFDM, and DFGE accompanied new hazards due to their need for pressurized fuel gas supply. The failure modes caused by these hazards were identified, and feasible safe guards were suggested. The hazards of fire and explosion stemming from flammable gas leak were considered to be acceptably mitigated by the safety requirements from the current industrial standards and classification society. & 2008 Published by Elsevier Ltd.
1. Introduction The LNG shipping industry had been tremendously cautious in choosing the propulsion system, and the steam turbine had been practically an exclusive option for LNG carriers over the last several decades. The cargo tanks of the LNG carriers are insulated heavily to contain the cryogenic LNG at around 160 1C under the atmospheric pressure. Nevertheless, heat ingress into them inevitably gives birth to the naturally generated boil-off gas (usually called the natural BOG). In the steam turbine-based propulsion the BOG fires the boilers to produce steam, which drives the steam turbines. When the natural BOG is not sufficient, the forcing vaporizer may generate additional BOG, or the liquid fuel may make up. Recently, the cargo capacity has jumped from 150,000 to 250,000 m3 or larger with the transport distance stretched. Further increase in cargo capacity is expected, nearly doubling the capacity of the conventional carriers. The steam turbine-based propulsion system is losing its higher ground for its low efficiency. Alternative propulsion systems have been suggested to replace the steam turbine type [1]. The natural BOG, the unavoidable offspring of the carrier on voyage, keeps the new comers from
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E-mail address: [email protected] (D. Chang). 0951-8320/$ - see front matter & 2008 Published by Elsevier Ltd. doi:10.1016/j.ress.2008.03.013
entering straightforwardly into the commercial arena. Burning of the BOG without energy recovery is not economical since more than 0.1% of the cargo evaporates everyday. If the primary mover of the propulsion system is of dual-fuel type, the BOG can be used as fuel. Otherwise, it should be recovered or reliquefied in a separate plant, called the BOG reliquefaction plant [2]. Two of the new concepts are on the verge of service after construction and sea trial: the low-speed diesel engine propulsion with reliquefaction [2] and the medium-speed dual-fuel diesel electric (DFDE) propulsion [3]. The other two, the low-speed dualfuel diesel engine propulsion [4] and the gas turbine electric propulsion [5], are trying to find their commercial opportunities. No system is superior to the others; selection of the propulsion system should be determined with the specific characteristics of the individual ship such as the cargo capacity, voyage conditions, and port environmental regulations taken into account. However, their availability and safety should be demonstrated as a minimum. A limited number of comparative studies were carried out, mainly in terms of economic or qualitative comparison [6]. Few studies were dedicated to their safety and reliability, which are the most critical measures judging their commercialization. Above all, the BOG handling system should have deserved proper consideration since it is not the economical determinant, but also the central source of hazards and malfunctions due to its high-pressure piping and rotating machines with high failure rates.
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Nomenclature ABOG AE AP BOG LBOG DFDE DFDM DFGE
availability of BOG utilization availability of emergency propulsion load availability of design propulsion load boil-off gas liquefied boil-off gas dual-fuel (medium-speed) diesel electric propulsion dual-fuel (low-speed) diesel mechanical propulsion dual-fuel gas turbine electric propulsion
The present study deals with two principal issues of the new propulsion systems: the availability and safety. The BOG handling system will be considered in parallel. First of all, the general features of the propulsion systems are described with their current and future statistics. To avoid comparing simply the system availability, three availabilities are considered: one for design propulsion load, the second for emergency propulsion load and the last for BOG utilization. For safety issue, significant hazards are identified for each propulsion system, and safety regulations are reviewed.
2. General features of propulsion systems 2.1. Categorization of propulsion systems The propulsion system for LNG carriers is closely related with the BOG utilization and electric power generation. Most propulsion systems use the BOG for fuel as well as liquid fuels. To the contrary, the diesel electric propulsion generates electric power to drive the electric motor connected to propeller shaft. In ships with the BOG reliquefaction unit, the BOG is liquefied in a dedicated plant consuming a huge amount of power. Consequently, it is reasonable to consider that the propulsion system should include the basic propulsion system itself, the main power generation, and the BOG utilization. One way to categorize the propulsion systems for LNG carriers is to follow their nature to handle the BOG, as shown in Fig. 1. The acronym in Fig. 1 will be used extensively throughout this paper. There are two classes depending on whether the BOG is recovered or consumed as fuel. The diesel mechanical propulsion with reliquefaction (SFDM+R) is a unique solution for the BOG recovery branch. The other branch bifurcates depending on the
BOG recovered
DFSM GCU HRSG LEL LNG RBD SFDM SFDM+R
completeness of the dual-fuel utilization; whether the engine can burn the BOG with the liquid fuel simultaneously or separately. The medium-speed DFDE propulsion corresponds to the latter. There are three propulsion systems that can use the BOG and the liquid fuel at once: the dual-fuel steam turbine mechanical (DFSM) propulsion, the (low-speed) dual-fuel diesel mechanical (DFDM) propulsion, and the dual-fuel gas turbine electric (DFGE) propulsion. For the past several decades, the DFSM has dominated the market of the propulsion system in the field of the LNG shipping industry [7]. Exceptionally, several small LNG carriers of more than 200 were equipped with diesel engines. Recently, two new alternatives, SFDM+R and DFDE, have entered into the commercial arena. Roughly speaking, each of these new types accounts for a quarter of the LNG carriers on the order books. Still, half of the LNG carriers will be equipped with steam turbines. Most of the LNG carriers with the SFDM+R type have the cargo capacity greater than 200,000 m3 while the carriers with the DFDE are comparable to the DFSM in the cargo capacity. 2.2. Principal features of propulsion system The DFSM shown in Fig. 2 burns the BOG in boilers to produce high-pressure steam, which drives the steam turbines connected to the propeller. In spite of its low fuel efficiency, the system had dominated for the past several decades because of its simple operability and intrinsic safety. One of advantages of this type is ease of handling the BOG to be capable of burning the BOG with the liquid fuel simultaneously. When the cargo tank pressure is elevated, the boilers burn the excessive BOG and the generated steam is dumped into the main condenser. The simple philosophy for the BOG handling eliminates the need of the gas combustion unit, which is mandatory for its followers.
SFDM+R Electric
Simultaneously burnable
dual-fuel steam turbine mechanical propulsion gas combustion unit heat recovery steam generator lower explosive limit liquefied natural gas reliability block diagram single-fuel (low-speed) diesel mechanical propulsion single-fuel (low-speed) diesel mechanical propulsion with reliquefaction
Single-fuel (low speed) diesel mechanical propulsion with reliquefaction
DFGE
Dual-fuel gas turbine electric propulsion
DFDM
Dual-fuel (low-speed) diesel mechanical propulsion
DFSM
Dual-fuel steam turbine mechanical propulsion
DFDE
Dual-fuel (medium speed) diesel electric propulsion
Diesel engine
Mechanical BOG as fuel
Separately burnable
Steam turbine
Fig. 1. Categorization of propulsion systems for LNG carriers in terms of BOG utilization.
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Boilers BOG BOG Compression Compression System System 11
Steam Turbines
Gear Box
Turbine Generator
Liquid Liquid Fuel Fuel System System
1879
Diesel Generator
Pump Turbine Main Pumps
Consumers Condenser Cooling Water
Exchangers
Fig. 2. System schematic of dual-fuel steam turbine mechanical propulsion (DFSM).
GCU Dual Fuel Generator BOG Compression System 2
Electric Motor
Gas Valve System System
Gear Box Dual Fuel Generator
Liquid Fuel Tank
Dual Fuel Generator Gas Valve System
Other Consumers
Diesel Generator Dual Fuel Generator
Fig. 3. System schematic of dual-fuel (medium-speed) diesel electric propulsion (DFDE).
The DFDE in Fig. 3 is a modified version of the diesel enginebased propulsion of other carriers in order to burn the BOG as well as diesel oil [3,8]. The dual-fuel engines generate electric power and propel the ship with electric motors. The engines, however, cannot burn the two fuels at once and should shift the fuel mode for different fuels. Additionally, it requires relatively low pressure (about 6 bara) for the BOG to be used as fuel. Note that this propulsion system contains a GCU for the case where the BOG is greater than the fuel gas demand of the engines. Fig. 4 illustrates the SFDM+R, where the BOG is liquefied in a dedicated system called the reliquefaction plant, instead of being used as fuel [2]. The low-speed diesel engine is connected directly to a propeller shaft. Conceptually, the propulsion is separated from the BOG treatment. The reliquefaction unit is huge containing several machineries and cryogenic exchangers. Note that this propulsion system is also equipped with a GCU for the case where the BOG is greater than the capacity of the reliquefaction plant. As shown in Fig. 5, the DFGE combines a gas turbine with a heat recovery steam generator (HRSG), using the hot exhaust gas from the gas turbine to drive a steam turbine to generate electrical power [5,9]. The HRSG is also fitted with burners for auxiliary firing with the liquid fuel or BOG. The fuel gas supply system equipped with screw compressors boosts the BOG pressure and feeds it into the main gas turbine. The liquid fuel enters the main gas turbine or the HRSG burners for combustion. An auxiliary gas turbine is optionally installed to adapt to low load demands during various operations as well as to perform the propulsion
redundancy when required. A GCU is installed for the disposal of BOG as a necessary back-up method of providing cargo tank pressure control when all of the BOG cannot be consumed in the main gas turbine or the HRSG. The HRSG generates superheated steam to drive a steam turbine with the hot exhaust gas from the gas turbine. As the gas turbine load reduces to match low load demands, the HRSG steam production decreases correspondingly. The DFDM in Fig. 6 combines the advantages of the DFDE and SFDM+R systems [10]. It is of dual-fuel type like the DFDE. Moreover, it is capable of burning both the BOG and liquid oil simultaneously. It is equipped with the low-speed diesel engines. This system, however, has a new problem that the fuel gas should be compressed up to around 250 bara, which has been never faced in the LNG shipping industry. Some wonder that this highpressure gas may cause serious trouble in the actual operation. As described later, the risk due to high-pressure gas is mitigated by a set of safety layers such as double-wall pipes and gas detection systems.
3. Comparison of availability 3.1. Functional availabilities The reliability is the key to the commercialization of the new propulsion systems. Approximated from their precise definition, reliability is regarded as the failure-free probability under a given
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Diesel Generator
GCU
BOG BOG Compression Compression System 6
Diesel Generator
Reliquefaction
Diesel Generator BOG from Tanks
LBOG to Tanks Diesel Generator Other Consumers Emergency Generator Slow Speed Two-Stroke Diesel Engine
Liquid Fuel Tank
Slow Speed Two-Stroke Diesel Engine Fig. 4. System schematic of single-fuel (low-speed) diesel mechanical propulsion with reliquefaction (SFDM+R).
BOG Compression System 1
BOG Compression System 3
Fuel Gas Accumulator
Main GT Generator
Electric Motor Gear Box
Liquid Fuel System
Aux. GT Generator
Electric Motor Gear Box
HRSG & ST Generator
GCU
Diesel Generator Other Consumers
Fig. 5. System schematic for dual-fuel gas turbine electric propulsion (DFGE).
condition for a fixed time period, while availability is the asymptotic ratio of uptime (operating time) to the total time with maintenance taken into account. Consequently, the availability is the preferred parameter to evaluate the confidence of operation. Depending on the required function, three availabilities are defined [11]:
AP: availability of design propulsion load; AE: availability of emergency propulsion load; ABOG: availability of BOG utilization. The availability of design propulsion load, AP, is the probability that the propulsion system is able to generate the design
propulsion load. The availability of emergency propulsion load, AE, corresponds to the probability that the propulsion system is able to generate the minimum required propulsion load, taking into account the failure of the prime mover. The last availability, ABOG, is for BOG utilization or a measure of the capability to use the BOG as fuel or recover it. For the SFDM+R propulsion, ABOG corresponds to the availability of the reliquefaction plant. 3.2. Study assumptions and methodology Availability analysis for the propulsion system is bounded by some constraints. The first constraint is that the system design, typically shown in Figs. 2–6, is different ship by ship not only for
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Diesel Generator
GCU
Diesel Generator
BOG Compression System Other Consumers
Diesel Generator BOG from Tanks Diesel Generator
Emergency Generator
Slow Speed Two-Stroke Diesel Engine
Liquid Fuel Tank
Slow Speed Two-Stroke Diesel Engine
Fig. 6. System schematic for dual-fuel (low-speed) diesel mechanical propulsion (DFDM).
the prospective systems waiting for commercialization, but also for the conventional ones. Second, the detailed logistic and maintenance action, which affect the repair time, is variant depending on shipping companies. The availability estimation without consideration on theses factors should be less than its actual value. Lastly, the extent of the automation and detailed features of the control system are difficult to generalize. For example, any of interlocks, spurious trips, random hardware failure of logic solvers and so forth may stop the system operation and reduce the availability. Since these control instruments and hardware is relatively low failure rates compared with the machinery part, neglecting them does not have significant impact on the availability estimation. Under the above-mentioned constraints, the following assumptions are employed. 1. All the components comply with the exponential failure model. 2. Only the critical failures are considered over the operational time. 3. Repair time is only for active repair, and repair adjustment is complete. 4. Redundancy items do not require any switch delay time. 5. Control instruments including transmitters, switches and indicators are not considered. 6. The compressors, pumps and diesel generators have a back-up system. 7. The BOG Compression, the major electricity consumer, is not available if the power supply caused by the generator failure is less than its demand. 8. The fuel supply of dual-fuel types is available if one of the two fuel sources is available. 9. For the design propulsion load, the primary movers do not have any redundant systems. 10. For the emergency propulsion load, failure of one of the primary movers leads the rest to propel the carrier at a reduced speed.
Table 1 Failure rates and active repair times for selected major items Equipment
Failure rate, per 106 h
Active repair time, h
Steam turbine Boiler Gas turbine Diesel engine Electric generator Screw compressor Centrifugal compressor Oil-processing pump Electric motor Process control valve
40.0 82.2 756.8 324.7 48.9 47.4 256.4 98.5 94.5 3.9
16.0 2.8 23.7 78.8 18.0 22.8 25.7 14.6 9.5 22.9
Source: Steam turbine from [13] and the others from [12].
Since the failure rate and repair time data for the components for either the conventional or new propulsion systems are rare in the ship operation industry, they are extracted mainly from the OREDA handbook [12], which contains corresponding data for the offshore industry. Data of steam turbines, which are not available in the OREDA handbook, are obtained from another reference [13]. The failure rates and active repair times are shown in Table 1 for selected major items. The availabilities are estimated by constructing reliability block diagrams (RBDs). The identical systems have the same RBD with the same failure rate. For example, the BOG Compression system, labeled BOG Compression System 1, has the same RBD and failure rates for the DFSM and DFGE. The diesel generator is another example, which is common to all the propulsion options. The availability at its steady state is taken for the comparison study. Theoretically, the availability begins at unity, decreases with time and approaches its asymptotic value with time. Roughly speaking, the availability has a steady-state value in 10 days, which is far less than the ship’s life.
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1.00
1.00
0.95
0.95
AE
AP
1882
0.90
0.90
0.85
0.85
0.80
0.80 DFSM
DFDE
SFDM+R DFDM Propulsion system
DFGE
DFSM
For the design load to be available, all of the component systems that do not have any redundant or back-up system should be operating. The fuel gas system to feed the BOG into the dualfuel engines does not have to be available since the liquid fuel system can supply the back-up fuel. The duality in the fuel supply indicates that the availability of the design propulsion load is governed by the availability of the primary movers. Fig. 7 compares the availability of the design propulsion load, AP. The conventional DFSM attains the highest level due to the lowest failure rate of the steam turbine. The DFDE including four diesel engines demonstrates the lowest availability since the availability is inversely proportional to the number of components, given the same failure rate for all components. The availability of the emergency propulsion measures the capability that the carrier is capable of sailing with one of the primary movers failed. Analogous to the availability of the design propulsion load, the availability of the emergency propulsion load, AE, is not affected significantly by the availability of the fuel gas supply system. As demonstrated in Fig. 8, the SFDM+R propulsion demonstrates the highest level. Note that the frequency of the emergency voyage is extremely low and the availability of emergency propulsion should not be taken seriously unless it is significantly low. The availability of the BOG utilization, ABOG, is a measure to use the BOG as fuel for the gas-consuming engines or recover the BOG through reliquefaction for the SFDM+R system. This availability is important since the economics of the BOG utilization has profound impact on that of the LNG carrier itself. The ABOG is affected not only by the availability of the BOG compression system (the supplying part) but also by that of the propulsion system (the consuming part). Since the former is equipped with redundancy, the ABOG is roughly proportional to the availability of the propulsion load, resulting in that ABOG looks similar to AP. 4. Hazard identification and safety requirements 4.1. Significant potential hazards Stringent safety practice has led the LNG shipping industry to show a remarkable safety record. Maritime Business Strategies presented a list of serious 25 accidents in LNG carriers from 1965 to 2005, only with one accident ending up with six deaths. By the way, four of the accidents were attributed to the failure of
SFDM+R
DFDM
DFGE
Propulsion system
Fig. 7. Availability of design propulsion load, AP.
3.3. Comparison of availabilities
DFDE
Fig. 8. Availability of emergency propulsion load, AE.
Table 2 Required BOG pressure depending on propulsion systems Propulsion
Compressor type
Pressure, bara
DFSM DFDE SFDM+R DFGE DFDM
Centrifugal Centrifugal Centrifugal Screw Reciprocating
2 6 4– 8a 40 250
a
BOG pressure for reliquefaction plants.
propulsion systems such as engine breakdown and gear box vibration [7]. The new dual-fuel propulsion systems of DFDE, DFDM, and DFGE are introducing new hazards due to their need for pressurized fuel gas supply, as shown in Table 2. The fuel gas pressure around 2 bara is sufficient for the DFSM to burn the fuel gas in the boiler. Higher pressure levels are required for the DFDE to run in the gas mode and for the SFDM+R to liquefy the BOG at elevated temperature. Further pressurization is necessary for the DFGE and DFDM to simultaneously burn the BOG and liquid fuel. The failure modes caused by these hazards need to be identified, assessed and resolved systematically. Typically, a hazard identification meeting comprising shipbuilders, engine manufacturers, and other interested parties was held for each propulsion system to make a HAZID report. In usual, detailed features of the reports are not available publicly for their nature. The common and significant failure modes anticipated during the operation of the dual-fuel propulsion systems with severe consequence are summarized as Table 3. All the dual-fuel propulsion systems require the pilot oil injection to ignite the gas in the engine combustion chamber. They can instantaneously change their operation mode from gas mode to liquid fuel mode against the pilot oil injection failure. The high-pressure gas leakage from the fuel gas supply system may result in such serious consequences as fire or explosion, especially if it happens in the engine room. In the event of gas leakage in the engine compartment, the propulsion systems are required to be shutdown when the gas concentration reaches 60% of LEL. The DFDE and DFDM have the double-wall piping against the high-pressure gas leakage. The DFGE propulsion system is slightly different from the other two systems since it is installed in the turbine enclosure. The failure to control the fuel gas supply is also classified as significant failure mode. The failure of the control-actuating
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Table 3 Failure modes of dual-fuel (DFDE, DFDM, and DFGE) propulsion systems Failure mode Pilot oil injection failure
Cause
Injection pump failed Pilot valve stuck
Consequence
Fuel gas supply stopped Cargo tank pressure increased
Loss of liquid fuel supply to both pilot and main system
Fuel pipe ruptured Circulation pump failed Filter clogged
Gas leak from fuel gas piping system
Pipe leakage Loose connection
Engine shutdown
Flammable gas in
Fuel gas supply failure
Compressor failed Gas pressure out of limit due to
compartment Fire or explosion
Safeguard
Switchover to liquid fuel mode (DFDE, DFDM) HRSG running (DFGE) BOG burning in GCU Preventive maintenance
Isolation and shutdown of engine at low gas concentration (oLEL 60%)
Fuel gas system shutdown Recirculation of gas to cargo tank Loss of fuel gas supply Switchover to liquid fuel mode
bad control
Valve failed Vaporizer leakage in tube side
Oil mist in crankcase (DFDE, DFDM)
Excessive BOG generation
Potential inflow with ice to cargo tank
Bearing failure Poor maintenance
Mechanical damage Fire or explosion
Engine’s low demand
Cargo tank pressure
Gas ingress in exhaust system
Misfiring Exhaust valve leakage
Excessive vibration
Degradation Bad alignment
medium of the block and bleed valves or the closure of master valve cause the gas supply system including the compressors to be subject to sudden interruptions. The pressurized gas that is not sent to the engines can be directed temporarily to the cargo tanks or the GCU. This indicates that the actual availabilities would be affected by the failure from the control system even though the availability analysis of the current study neglects it for its low frequency. It is recommended that any rigorous availability analysis of a specific propulsion system should include the control system failure mode and its maintenance. 4.2. Minimum safety requirements from classification society The safety requirements for the new dual-fuel propulsion systems are specified by the international codes and classification rules:
IMO IGC code, international code for the construction and equipment of ships carrying liquefied gases in bulk.
ABS, rules for building and classing steel vessel (Section 5-816).
DNV, gas fuelled engine installations (Part 6, Chapter 13). ABS, guide for propulsion systems for LNG carriers. A set of safeguards stated by the requirements to prevent the presence of high-pressure gas in the engine room are as a minimum as follows:
Double-wall piping or ventilated pipe or ducts. Automatic shut-off valves and master gas valves.
Gas vent drain tank with gas detector Switchover to LNG vaporizer Engine shutdown Preventive maintenance BOG burning in GCU
increased BOG loss
Explosion
Mechanical damage Fuel gas leakage
Switchover to liquid fuel mode
Engine shutdown Condition-based maintenance
Gas detection system with independent redundancy. Ventilation system with independent redundancy. Engine exhaust and scavenge system. The fuel gas piping installed in the engine room for the dualfuel propulsion systems should be either a double-wall piping or a ventilated pipe/duct. The double-wall piping consists of concentric pipes with the fuel gas in the inner pipe and the pressurized inert gas between pipe walls. The pressure of inert gas should be controlled at the pressure greater than the gas fuel pressure in the inner pipe. A loss or leakage of the inert gas between the pipes should be detected by suitable alarm systems. Where the gas fuel piping is installed within a ventilated pipe or duct, the capacity of ventilation system should be designed to provide at least 30 air changes per hour for air space. The ventilation system should operate at a pressure less than the atmospheric pressure [14]. For the DFDM propulsion system, the double-wall piping is applicable to the fuel gas supply piping system. On the other hand, the single-wall fuel gas piping within the entire gas turbine enclosure is sufficient for the DFGE propulsion system if the enclosure satisfies the requirement of the gas-tight type (Fig. 9). The master valve and automatic shut-off valves are also the essential safety devices in the fuel gas supply system. The master valve should be installed in the part of the fuel gas piping outside the engine room. In addition to the master valve, a set of automatic shut-off valves called ‘‘double block and bleed’’ valves should be installed in the fuel supply piping to each gas utilization unit. Fig. 10 shows a typical configuration of the master valve and automatic shut-off valves for a dual-fuel propulsion system. If an unwanted event (e.g. the loss of pressure in the space between
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concentric pipes) occurs, the master valve and two of three automatic shut-off valves installed in series close the gas supply automatically, and the third valve vents the fuel gas that is between the two valves in series at the same time [14,15]. The gas concentration in ducts and engine rooms should be monitored continuously by the gas detection systems. The gas detection system installed in the dual-fuel propulsion systems should provide functional redundancy against the failure of its components. Alarm is supposed to issue at 30% of LEL. The DFDM propulsion system should be shut down when the gas concentration reaches 60% of LEL in the engine compartment. The gas detection criteria in the space containing the dual-fuel gas turbine are stricter than those of the low-speed dual-fuel propulsion system. Alarm is to be activated at the gas fuel leakage detection of 5% of LEL with automatic fuel switching to liquid oil. If the gas concentration in the space containing the gas turbine reaches 10% of LEL, the enclosure and machinery space are to be shut down [14]. The ventilation system of the new propulsion system or the conventional dual-fuel propulsion system is to be entirely separated from all other ventilation systems. If there is a loss of ventilation in the engine room, the fuel gas supply is to be shut
1.00
ABOG
0.95
0.90
0.85
0.80 DFSM
DFDE
SFDM+R
DFDM
DFGE
Propulsion system Fig. 9. Availability of BOG utilization, ABOG.
Automatic vent valves
down automatically with alarm. The capacity of one ventilation fan is to maintain not less than 100% of the total required in case the other is out of service. Explosion relief devices such as rupture disks for the engine exhaust system against explosion are to be provided in the exhaust manifolds. The exhaust gas pipes from the dual-fuel propulsion systems should be installed independently. The exhaust gas pipes are not be connected to the exhaust pipes of other engines or systems [14].
6. Conclusions The primary movers governed the availability of BOG utilization as well as that of design propulsion load since they had no redundant system for them. Note that most of the other subsystems had relatively high availability for the redundancy back-up for the machinery parts prone to failure. For the design propulsion load to be available, all of the primary movers should be operating for any propulsion options. In terms of BOG utilization, the primary movers played a role of the BOG consumer burning the BOG as fuel. Failure of one of them meant the reduction in the BOG utilization to that degree. It was no coincidence that two availabilities had the similar levels and relative values, as shown in Figs. 7 and 9. The availability for emergency propulsion load attained high levels compared with the other two availabilities since all the subsystems including the primary movers were dual. New propulsion systems commonly required the supply of pressurized fuel gas, which was the dominating source of the most failure modes and hazards. Leak of explosive gas was considered the most disastrous hazard. Through the systematic qualitative approaches to hazard identification and risk reduction, there were available the safety requirements to mitigate the hazards from the international and classification society. Selection of the propulsion option should take into account factors other than availability. The DFSM, the conventional steam turbine-based system, demonstrated availability around 0.99. The propulsion system, however, has demerits such as low efficiency and bulky installation space. Actually, many LNG carriers took systems other than the DFSM for economic reason.
Double block and bleed
Master valve
Automatic shut-off valves
Fig. 10. Automatic shut-off valve arrangement of fuel gas supply system for dual-fuel diesel engines.
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Further studies are recommended for detailed availability assessment, quantitative risk analysis, and the life cycle cost comparison. The availability estimation of the current study was constrained by some factors, as taken into account in some of the assumptions. The reliability information of the LNG shipping industry is required. Moreover, the maintenance policy and consideration of the control system should be considered for improved estimation. The quantitative risk analysis should focus on leak, fire and explosion hazards. The fuel gas supply system for the DFGE and DFDM boosts the gas pressure up to considerable levels, which has not been faced in the LNG shipping industry. In spite of the ventilation system some large leaks may cause the flammable concentration to stay above the LEL for a significant period if the gas detection is delayed. Quantitative analysis will help to verify whether the probability and consequence of the hazards are acceptable. Comparison of the life cycle cost may be effective to tell which propulsion option would be best for a fixed transportation path. The reliability and availability aspects, however, should be considered in the cost comparison. In other words, the maintenance cost for the failed items and logistic cost for spare parts should deserve proper attention. References [1] Chang D, Rhee TJ, Nam KI, Chang KP, Lee DH, Kwak BJ, et al. Challenges of LNG carriers propelled by new propulsion systems: ship builder’s viewpoint. Abu Dhabi, United Arab Emirates: Gastech; 4–7 December 2006. p. 1–24.
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[2] Sørensen RI, Christiansen P. Development of shipboard reliquefaction systems for LNG carriers. Abu Dhabi, United Arab Emirates: Gastech; 4–7 December 2006. p. 1–20. [3] Thijssen B. Dual-fuel-electric LNG carriers. Abu Dhabi, United Arab Emirates: Gastech; 4–7 December 2006. p. 1–11. [4] Linwood J, Ha JP, Aabo K, Laursen RS. LNG gas carrier with high-pressure gas engine propulsion application. Abu Dhabi, United Arab Emirates: Gastech; 4–7 December 2006. p. 1–30. [5] Harsema-Mensonides A, Dalston CM. Gas turbine electric propulsion for LNG carriers: myth or money spinner. Abu Dhabi, United Arab Emirates: Gastech; 4–7 December 2006. p. 1–17. [6] Kuever M, Clucas C, Fuhrman N. Evaluation of propulsion options for LNG carriers. Gastech 2002, Qatar, 13–16 October 2002. [7] Maritime Business Strategies web pages, /http://www.coltoncompany.comS, 2007. [8] Hansen JF, Lysebo R. Electric propulsion for LNG carriers. LNG Journal 2004;September/October:11–2. [9] Nam KI, Rhee TJ, Chang KP, Chang D. An Analysis of gas turbine propulsion system for LNG carriers based on reliability and economic evaluation. In: The Society of Naval Architects and Marine Engineers: maritime technology conference & expo and ship production symposium, Paper no. B1, 10–13 October 2006, Fort Lauderdale, USA. [10] Skjoldager P, Lunde T, Melaaen E. Two-stroke diesel engines and reliquefaction systems for LNG carriers. In: Motorship conference, Hamburg, 2003. [11] Chang D, Rhee TJ, Nam KI, Lee DH, Jeong SH, Chang KP. Comparison of availabilities of new and conventional propulsion systems for liquefied natural gas carriers. In: The European safety & reliability conference, Estoril, Portugal, 18–22 September 2006, p. 2771–6. [12] OREDA Participants. Offshore reliability (OREDA) handbook, ed. Høvik, Det Norske Veritas, 2002. [13] Ayyub BM. Risk analysis in engineering and economics. London: CRC Press Company; 2003. [14] ABS (American Bureau of Shipping). Guide for propulsion systems for LNG carriers. ABS Plaza, Houston, 2005. [15] DNV (Det Norske Veritas), Gas fuelled engine installations. Høvik, Det Norske Veritas, 2005.
Contents lists available at ScienceDirect
Reliability Engineering and System Safety journal homepage: www.elsevier.com/locate/ress
A study on availability and safety of new propulsion systems for LNG carriers Daejun Chang , Taejin Rhee, Kiil Nam, Kwangpil Chang, Donghun Lee, Samheon Jeong Hyundai Heavy Industries Co. Ltd., Ulsan, South Korea
a r t i c l e in fo
abstract
Available online 21 March 2008
This study investigated the availability and safety concerns of the conventional and prospective propulsion systems for LNG carriers: dual-fuel steam turbine mechanical (DFSM) propulsion, dual-fuel diesel electric (DFDE) propulsion, dual-fuel gas turbine electric (DFGE) propulsion, dual-fuel diesel mechanical (DFDM) propulsion, and diesel mechanical propulsion with reliquefaction (SFDM+R).
Keywords: LNG carrier Propulsion Availability Safety Hazard
The two prospective candidates, the DFDM and DFGE, exhibited the availabilities of design and emergency propulsion loads as high as the newly adopted DFDE and SFDM+R, while the DFSM demonstrated the highest. All the propulsion systems achieved a satisfactory level of the availability for the BOG utilization. The newly introduced dual-fuel systems of DFDE, DFDM, and DFGE accompanied new hazards due to their need for pressurized fuel gas supply. The failure modes caused by these hazards were identified, and feasible safe guards were suggested. The hazards of fire and explosion stemming from flammable gas leak were considered to be acceptably mitigated by the safety requirements from the current industrial standards and classification society. & 2008 Published by Elsevier Ltd.
1. Introduction The LNG shipping industry had been tremendously cautious in choosing the propulsion system, and the steam turbine had been practically an exclusive option for LNG carriers over the last several decades. The cargo tanks of the LNG carriers are insulated heavily to contain the cryogenic LNG at around 160 1C under the atmospheric pressure. Nevertheless, heat ingress into them inevitably gives birth to the naturally generated boil-off gas (usually called the natural BOG). In the steam turbine-based propulsion the BOG fires the boilers to produce steam, which drives the steam turbines. When the natural BOG is not sufficient, the forcing vaporizer may generate additional BOG, or the liquid fuel may make up. Recently, the cargo capacity has jumped from 150,000 to 250,000 m3 or larger with the transport distance stretched. Further increase in cargo capacity is expected, nearly doubling the capacity of the conventional carriers. The steam turbine-based propulsion system is losing its higher ground for its low efficiency. Alternative propulsion systems have been suggested to replace the steam turbine type [1]. The natural BOG, the unavoidable offspring of the carrier on voyage, keeps the new comers from
Corresponding author. Tel.: +82 52 2023236; fax: +82 52 2509588.
E-mail address: [email protected] (D. Chang). 0951-8320/$ - see front matter & 2008 Published by Elsevier Ltd. doi:10.1016/j.ress.2008.03.013
entering straightforwardly into the commercial arena. Burning of the BOG without energy recovery is not economical since more than 0.1% of the cargo evaporates everyday. If the primary mover of the propulsion system is of dual-fuel type, the BOG can be used as fuel. Otherwise, it should be recovered or reliquefied in a separate plant, called the BOG reliquefaction plant [2]. Two of the new concepts are on the verge of service after construction and sea trial: the low-speed diesel engine propulsion with reliquefaction [2] and the medium-speed dual-fuel diesel electric (DFDE) propulsion [3]. The other two, the low-speed dualfuel diesel engine propulsion [4] and the gas turbine electric propulsion [5], are trying to find their commercial opportunities. No system is superior to the others; selection of the propulsion system should be determined with the specific characteristics of the individual ship such as the cargo capacity, voyage conditions, and port environmental regulations taken into account. However, their availability and safety should be demonstrated as a minimum. A limited number of comparative studies were carried out, mainly in terms of economic or qualitative comparison [6]. Few studies were dedicated to their safety and reliability, which are the most critical measures judging their commercialization. Above all, the BOG handling system should have deserved proper consideration since it is not the economical determinant, but also the central source of hazards and malfunctions due to its high-pressure piping and rotating machines with high failure rates.
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Nomenclature ABOG AE AP BOG LBOG DFDE DFDM DFGE
availability of BOG utilization availability of emergency propulsion load availability of design propulsion load boil-off gas liquefied boil-off gas dual-fuel (medium-speed) diesel electric propulsion dual-fuel (low-speed) diesel mechanical propulsion dual-fuel gas turbine electric propulsion
The present study deals with two principal issues of the new propulsion systems: the availability and safety. The BOG handling system will be considered in parallel. First of all, the general features of the propulsion systems are described with their current and future statistics. To avoid comparing simply the system availability, three availabilities are considered: one for design propulsion load, the second for emergency propulsion load and the last for BOG utilization. For safety issue, significant hazards are identified for each propulsion system, and safety regulations are reviewed.
2. General features of propulsion systems 2.1. Categorization of propulsion systems The propulsion system for LNG carriers is closely related with the BOG utilization and electric power generation. Most propulsion systems use the BOG for fuel as well as liquid fuels. To the contrary, the diesel electric propulsion generates electric power to drive the electric motor connected to propeller shaft. In ships with the BOG reliquefaction unit, the BOG is liquefied in a dedicated plant consuming a huge amount of power. Consequently, it is reasonable to consider that the propulsion system should include the basic propulsion system itself, the main power generation, and the BOG utilization. One way to categorize the propulsion systems for LNG carriers is to follow their nature to handle the BOG, as shown in Fig. 1. The acronym in Fig. 1 will be used extensively throughout this paper. There are two classes depending on whether the BOG is recovered or consumed as fuel. The diesel mechanical propulsion with reliquefaction (SFDM+R) is a unique solution for the BOG recovery branch. The other branch bifurcates depending on the
BOG recovered
DFSM GCU HRSG LEL LNG RBD SFDM SFDM+R
completeness of the dual-fuel utilization; whether the engine can burn the BOG with the liquid fuel simultaneously or separately. The medium-speed DFDE propulsion corresponds to the latter. There are three propulsion systems that can use the BOG and the liquid fuel at once: the dual-fuel steam turbine mechanical (DFSM) propulsion, the (low-speed) dual-fuel diesel mechanical (DFDM) propulsion, and the dual-fuel gas turbine electric (DFGE) propulsion. For the past several decades, the DFSM has dominated the market of the propulsion system in the field of the LNG shipping industry [7]. Exceptionally, several small LNG carriers of more than 200 were equipped with diesel engines. Recently, two new alternatives, SFDM+R and DFDE, have entered into the commercial arena. Roughly speaking, each of these new types accounts for a quarter of the LNG carriers on the order books. Still, half of the LNG carriers will be equipped with steam turbines. Most of the LNG carriers with the SFDM+R type have the cargo capacity greater than 200,000 m3 while the carriers with the DFDE are comparable to the DFSM in the cargo capacity. 2.2. Principal features of propulsion system The DFSM shown in Fig. 2 burns the BOG in boilers to produce high-pressure steam, which drives the steam turbines connected to the propeller. In spite of its low fuel efficiency, the system had dominated for the past several decades because of its simple operability and intrinsic safety. One of advantages of this type is ease of handling the BOG to be capable of burning the BOG with the liquid fuel simultaneously. When the cargo tank pressure is elevated, the boilers burn the excessive BOG and the generated steam is dumped into the main condenser. The simple philosophy for the BOG handling eliminates the need of the gas combustion unit, which is mandatory for its followers.
SFDM+R Electric
Simultaneously burnable
dual-fuel steam turbine mechanical propulsion gas combustion unit heat recovery steam generator lower explosive limit liquefied natural gas reliability block diagram single-fuel (low-speed) diesel mechanical propulsion single-fuel (low-speed) diesel mechanical propulsion with reliquefaction
Single-fuel (low speed) diesel mechanical propulsion with reliquefaction
DFGE
Dual-fuel gas turbine electric propulsion
DFDM
Dual-fuel (low-speed) diesel mechanical propulsion
DFSM
Dual-fuel steam turbine mechanical propulsion
DFDE
Dual-fuel (medium speed) diesel electric propulsion
Diesel engine
Mechanical BOG as fuel
Separately burnable
Steam turbine
Fig. 1. Categorization of propulsion systems for LNG carriers in terms of BOG utilization.
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Boilers BOG BOG Compression Compression System System 11
Steam Turbines
Gear Box
Turbine Generator
Liquid Liquid Fuel Fuel System System
1879
Diesel Generator
Pump Turbine Main Pumps
Consumers Condenser Cooling Water
Exchangers
Fig. 2. System schematic of dual-fuel steam turbine mechanical propulsion (DFSM).
GCU Dual Fuel Generator BOG Compression System 2
Electric Motor
Gas Valve System System
Gear Box Dual Fuel Generator
Liquid Fuel Tank
Dual Fuel Generator Gas Valve System
Other Consumers
Diesel Generator Dual Fuel Generator
Fig. 3. System schematic of dual-fuel (medium-speed) diesel electric propulsion (DFDE).
The DFDE in Fig. 3 is a modified version of the diesel enginebased propulsion of other carriers in order to burn the BOG as well as diesel oil [3,8]. The dual-fuel engines generate electric power and propel the ship with electric motors. The engines, however, cannot burn the two fuels at once and should shift the fuel mode for different fuels. Additionally, it requires relatively low pressure (about 6 bara) for the BOG to be used as fuel. Note that this propulsion system contains a GCU for the case where the BOG is greater than the fuel gas demand of the engines. Fig. 4 illustrates the SFDM+R, where the BOG is liquefied in a dedicated system called the reliquefaction plant, instead of being used as fuel [2]. The low-speed diesel engine is connected directly to a propeller shaft. Conceptually, the propulsion is separated from the BOG treatment. The reliquefaction unit is huge containing several machineries and cryogenic exchangers. Note that this propulsion system is also equipped with a GCU for the case where the BOG is greater than the capacity of the reliquefaction plant. As shown in Fig. 5, the DFGE combines a gas turbine with a heat recovery steam generator (HRSG), using the hot exhaust gas from the gas turbine to drive a steam turbine to generate electrical power [5,9]. The HRSG is also fitted with burners for auxiliary firing with the liquid fuel or BOG. The fuel gas supply system equipped with screw compressors boosts the BOG pressure and feeds it into the main gas turbine. The liquid fuel enters the main gas turbine or the HRSG burners for combustion. An auxiliary gas turbine is optionally installed to adapt to low load demands during various operations as well as to perform the propulsion
redundancy when required. A GCU is installed for the disposal of BOG as a necessary back-up method of providing cargo tank pressure control when all of the BOG cannot be consumed in the main gas turbine or the HRSG. The HRSG generates superheated steam to drive a steam turbine with the hot exhaust gas from the gas turbine. As the gas turbine load reduces to match low load demands, the HRSG steam production decreases correspondingly. The DFDM in Fig. 6 combines the advantages of the DFDE and SFDM+R systems [10]. It is of dual-fuel type like the DFDE. Moreover, it is capable of burning both the BOG and liquid oil simultaneously. It is equipped with the low-speed diesel engines. This system, however, has a new problem that the fuel gas should be compressed up to around 250 bara, which has been never faced in the LNG shipping industry. Some wonder that this highpressure gas may cause serious trouble in the actual operation. As described later, the risk due to high-pressure gas is mitigated by a set of safety layers such as double-wall pipes and gas detection systems.
3. Comparison of availability 3.1. Functional availabilities The reliability is the key to the commercialization of the new propulsion systems. Approximated from their precise definition, reliability is regarded as the failure-free probability under a given
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Diesel Generator
GCU
BOG BOG Compression Compression System 6
Diesel Generator
Reliquefaction
Diesel Generator BOG from Tanks
LBOG to Tanks Diesel Generator Other Consumers Emergency Generator Slow Speed Two-Stroke Diesel Engine
Liquid Fuel Tank
Slow Speed Two-Stroke Diesel Engine Fig. 4. System schematic of single-fuel (low-speed) diesel mechanical propulsion with reliquefaction (SFDM+R).
BOG Compression System 1
BOG Compression System 3
Fuel Gas Accumulator
Main GT Generator
Electric Motor Gear Box
Liquid Fuel System
Aux. GT Generator
Electric Motor Gear Box
HRSG & ST Generator
GCU
Diesel Generator Other Consumers
Fig. 5. System schematic for dual-fuel gas turbine electric propulsion (DFGE).
condition for a fixed time period, while availability is the asymptotic ratio of uptime (operating time) to the total time with maintenance taken into account. Consequently, the availability is the preferred parameter to evaluate the confidence of operation. Depending on the required function, three availabilities are defined [11]:
AP: availability of design propulsion load; AE: availability of emergency propulsion load; ABOG: availability of BOG utilization. The availability of design propulsion load, AP, is the probability that the propulsion system is able to generate the design
propulsion load. The availability of emergency propulsion load, AE, corresponds to the probability that the propulsion system is able to generate the minimum required propulsion load, taking into account the failure of the prime mover. The last availability, ABOG, is for BOG utilization or a measure of the capability to use the BOG as fuel or recover it. For the SFDM+R propulsion, ABOG corresponds to the availability of the reliquefaction plant. 3.2. Study assumptions and methodology Availability analysis for the propulsion system is bounded by some constraints. The first constraint is that the system design, typically shown in Figs. 2–6, is different ship by ship not only for
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Diesel Generator
GCU
Diesel Generator
BOG Compression System Other Consumers
Diesel Generator BOG from Tanks Diesel Generator
Emergency Generator
Slow Speed Two-Stroke Diesel Engine
Liquid Fuel Tank
Slow Speed Two-Stroke Diesel Engine
Fig. 6. System schematic for dual-fuel (low-speed) diesel mechanical propulsion (DFDM).
the prospective systems waiting for commercialization, but also for the conventional ones. Second, the detailed logistic and maintenance action, which affect the repair time, is variant depending on shipping companies. The availability estimation without consideration on theses factors should be less than its actual value. Lastly, the extent of the automation and detailed features of the control system are difficult to generalize. For example, any of interlocks, spurious trips, random hardware failure of logic solvers and so forth may stop the system operation and reduce the availability. Since these control instruments and hardware is relatively low failure rates compared with the machinery part, neglecting them does not have significant impact on the availability estimation. Under the above-mentioned constraints, the following assumptions are employed. 1. All the components comply with the exponential failure model. 2. Only the critical failures are considered over the operational time. 3. Repair time is only for active repair, and repair adjustment is complete. 4. Redundancy items do not require any switch delay time. 5. Control instruments including transmitters, switches and indicators are not considered. 6. The compressors, pumps and diesel generators have a back-up system. 7. The BOG Compression, the major electricity consumer, is not available if the power supply caused by the generator failure is less than its demand. 8. The fuel supply of dual-fuel types is available if one of the two fuel sources is available. 9. For the design propulsion load, the primary movers do not have any redundant systems. 10. For the emergency propulsion load, failure of one of the primary movers leads the rest to propel the carrier at a reduced speed.
Table 1 Failure rates and active repair times for selected major items Equipment
Failure rate, per 106 h
Active repair time, h
Steam turbine Boiler Gas turbine Diesel engine Electric generator Screw compressor Centrifugal compressor Oil-processing pump Electric motor Process control valve
40.0 82.2 756.8 324.7 48.9 47.4 256.4 98.5 94.5 3.9
16.0 2.8 23.7 78.8 18.0 22.8 25.7 14.6 9.5 22.9
Source: Steam turbine from [13] and the others from [12].
Since the failure rate and repair time data for the components for either the conventional or new propulsion systems are rare in the ship operation industry, they are extracted mainly from the OREDA handbook [12], which contains corresponding data for the offshore industry. Data of steam turbines, which are not available in the OREDA handbook, are obtained from another reference [13]. The failure rates and active repair times are shown in Table 1 for selected major items. The availabilities are estimated by constructing reliability block diagrams (RBDs). The identical systems have the same RBD with the same failure rate. For example, the BOG Compression system, labeled BOG Compression System 1, has the same RBD and failure rates for the DFSM and DFGE. The diesel generator is another example, which is common to all the propulsion options. The availability at its steady state is taken for the comparison study. Theoretically, the availability begins at unity, decreases with time and approaches its asymptotic value with time. Roughly speaking, the availability has a steady-state value in 10 days, which is far less than the ship’s life.
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1.00
1.00
0.95
0.95
AE
AP
1882
0.90
0.90
0.85
0.85
0.80
0.80 DFSM
DFDE
SFDM+R DFDM Propulsion system
DFGE
DFSM
For the design load to be available, all of the component systems that do not have any redundant or back-up system should be operating. The fuel gas system to feed the BOG into the dualfuel engines does not have to be available since the liquid fuel system can supply the back-up fuel. The duality in the fuel supply indicates that the availability of the design propulsion load is governed by the availability of the primary movers. Fig. 7 compares the availability of the design propulsion load, AP. The conventional DFSM attains the highest level due to the lowest failure rate of the steam turbine. The DFDE including four diesel engines demonstrates the lowest availability since the availability is inversely proportional to the number of components, given the same failure rate for all components. The availability of the emergency propulsion measures the capability that the carrier is capable of sailing with one of the primary movers failed. Analogous to the availability of the design propulsion load, the availability of the emergency propulsion load, AE, is not affected significantly by the availability of the fuel gas supply system. As demonstrated in Fig. 8, the SFDM+R propulsion demonstrates the highest level. Note that the frequency of the emergency voyage is extremely low and the availability of emergency propulsion should not be taken seriously unless it is significantly low. The availability of the BOG utilization, ABOG, is a measure to use the BOG as fuel for the gas-consuming engines or recover the BOG through reliquefaction for the SFDM+R system. This availability is important since the economics of the BOG utilization has profound impact on that of the LNG carrier itself. The ABOG is affected not only by the availability of the BOG compression system (the supplying part) but also by that of the propulsion system (the consuming part). Since the former is equipped with redundancy, the ABOG is roughly proportional to the availability of the propulsion load, resulting in that ABOG looks similar to AP. 4. Hazard identification and safety requirements 4.1. Significant potential hazards Stringent safety practice has led the LNG shipping industry to show a remarkable safety record. Maritime Business Strategies presented a list of serious 25 accidents in LNG carriers from 1965 to 2005, only with one accident ending up with six deaths. By the way, four of the accidents were attributed to the failure of
SFDM+R
DFDM
DFGE
Propulsion system
Fig. 7. Availability of design propulsion load, AP.
3.3. Comparison of availabilities
DFDE
Fig. 8. Availability of emergency propulsion load, AE.
Table 2 Required BOG pressure depending on propulsion systems Propulsion
Compressor type
Pressure, bara
DFSM DFDE SFDM+R DFGE DFDM
Centrifugal Centrifugal Centrifugal Screw Reciprocating
2 6 4– 8a 40 250
a
BOG pressure for reliquefaction plants.
propulsion systems such as engine breakdown and gear box vibration [7]. The new dual-fuel propulsion systems of DFDE, DFDM, and DFGE are introducing new hazards due to their need for pressurized fuel gas supply, as shown in Table 2. The fuel gas pressure around 2 bara is sufficient for the DFSM to burn the fuel gas in the boiler. Higher pressure levels are required for the DFDE to run in the gas mode and for the SFDM+R to liquefy the BOG at elevated temperature. Further pressurization is necessary for the DFGE and DFDM to simultaneously burn the BOG and liquid fuel. The failure modes caused by these hazards need to be identified, assessed and resolved systematically. Typically, a hazard identification meeting comprising shipbuilders, engine manufacturers, and other interested parties was held for each propulsion system to make a HAZID report. In usual, detailed features of the reports are not available publicly for their nature. The common and significant failure modes anticipated during the operation of the dual-fuel propulsion systems with severe consequence are summarized as Table 3. All the dual-fuel propulsion systems require the pilot oil injection to ignite the gas in the engine combustion chamber. They can instantaneously change their operation mode from gas mode to liquid fuel mode against the pilot oil injection failure. The high-pressure gas leakage from the fuel gas supply system may result in such serious consequences as fire or explosion, especially if it happens in the engine room. In the event of gas leakage in the engine compartment, the propulsion systems are required to be shutdown when the gas concentration reaches 60% of LEL. The DFDE and DFDM have the double-wall piping against the high-pressure gas leakage. The DFGE propulsion system is slightly different from the other two systems since it is installed in the turbine enclosure. The failure to control the fuel gas supply is also classified as significant failure mode. The failure of the control-actuating
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Table 3 Failure modes of dual-fuel (DFDE, DFDM, and DFGE) propulsion systems Failure mode Pilot oil injection failure
Cause
Injection pump failed Pilot valve stuck
Consequence
Fuel gas supply stopped Cargo tank pressure increased
Loss of liquid fuel supply to both pilot and main system
Fuel pipe ruptured Circulation pump failed Filter clogged
Gas leak from fuel gas piping system
Pipe leakage Loose connection
Engine shutdown
Flammable gas in
Fuel gas supply failure
Compressor failed Gas pressure out of limit due to
compartment Fire or explosion
Safeguard
Switchover to liquid fuel mode (DFDE, DFDM) HRSG running (DFGE) BOG burning in GCU Preventive maintenance
Isolation and shutdown of engine at low gas concentration (oLEL 60%)
Fuel gas system shutdown Recirculation of gas to cargo tank Loss of fuel gas supply Switchover to liquid fuel mode
bad control
Valve failed Vaporizer leakage in tube side
Oil mist in crankcase (DFDE, DFDM)
Excessive BOG generation
Potential inflow with ice to cargo tank
Bearing failure Poor maintenance
Mechanical damage Fire or explosion
Engine’s low demand
Cargo tank pressure
Gas ingress in exhaust system
Misfiring Exhaust valve leakage
Excessive vibration
Degradation Bad alignment
medium of the block and bleed valves or the closure of master valve cause the gas supply system including the compressors to be subject to sudden interruptions. The pressurized gas that is not sent to the engines can be directed temporarily to the cargo tanks or the GCU. This indicates that the actual availabilities would be affected by the failure from the control system even though the availability analysis of the current study neglects it for its low frequency. It is recommended that any rigorous availability analysis of a specific propulsion system should include the control system failure mode and its maintenance. 4.2. Minimum safety requirements from classification society The safety requirements for the new dual-fuel propulsion systems are specified by the international codes and classification rules:
IMO IGC code, international code for the construction and equipment of ships carrying liquefied gases in bulk.
ABS, rules for building and classing steel vessel (Section 5-816).
DNV, gas fuelled engine installations (Part 6, Chapter 13). ABS, guide for propulsion systems for LNG carriers. A set of safeguards stated by the requirements to prevent the presence of high-pressure gas in the engine room are as a minimum as follows:
Double-wall piping or ventilated pipe or ducts. Automatic shut-off valves and master gas valves.
Gas vent drain tank with gas detector Switchover to LNG vaporizer Engine shutdown Preventive maintenance BOG burning in GCU
increased BOG loss
Explosion
Mechanical damage Fuel gas leakage
Switchover to liquid fuel mode
Engine shutdown Condition-based maintenance
Gas detection system with independent redundancy. Ventilation system with independent redundancy. Engine exhaust and scavenge system. The fuel gas piping installed in the engine room for the dualfuel propulsion systems should be either a double-wall piping or a ventilated pipe/duct. The double-wall piping consists of concentric pipes with the fuel gas in the inner pipe and the pressurized inert gas between pipe walls. The pressure of inert gas should be controlled at the pressure greater than the gas fuel pressure in the inner pipe. A loss or leakage of the inert gas between the pipes should be detected by suitable alarm systems. Where the gas fuel piping is installed within a ventilated pipe or duct, the capacity of ventilation system should be designed to provide at least 30 air changes per hour for air space. The ventilation system should operate at a pressure less than the atmospheric pressure [14]. For the DFDM propulsion system, the double-wall piping is applicable to the fuel gas supply piping system. On the other hand, the single-wall fuel gas piping within the entire gas turbine enclosure is sufficient for the DFGE propulsion system if the enclosure satisfies the requirement of the gas-tight type (Fig. 9). The master valve and automatic shut-off valves are also the essential safety devices in the fuel gas supply system. The master valve should be installed in the part of the fuel gas piping outside the engine room. In addition to the master valve, a set of automatic shut-off valves called ‘‘double block and bleed’’ valves should be installed in the fuel supply piping to each gas utilization unit. Fig. 10 shows a typical configuration of the master valve and automatic shut-off valves for a dual-fuel propulsion system. If an unwanted event (e.g. the loss of pressure in the space between
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concentric pipes) occurs, the master valve and two of three automatic shut-off valves installed in series close the gas supply automatically, and the third valve vents the fuel gas that is between the two valves in series at the same time [14,15]. The gas concentration in ducts and engine rooms should be monitored continuously by the gas detection systems. The gas detection system installed in the dual-fuel propulsion systems should provide functional redundancy against the failure of its components. Alarm is supposed to issue at 30% of LEL. The DFDM propulsion system should be shut down when the gas concentration reaches 60% of LEL in the engine compartment. The gas detection criteria in the space containing the dual-fuel gas turbine are stricter than those of the low-speed dual-fuel propulsion system. Alarm is to be activated at the gas fuel leakage detection of 5% of LEL with automatic fuel switching to liquid oil. If the gas concentration in the space containing the gas turbine reaches 10% of LEL, the enclosure and machinery space are to be shut down [14]. The ventilation system of the new propulsion system or the conventional dual-fuel propulsion system is to be entirely separated from all other ventilation systems. If there is a loss of ventilation in the engine room, the fuel gas supply is to be shut
1.00
ABOG
0.95
0.90
0.85
0.80 DFSM
DFDE
SFDM+R
DFDM
DFGE
Propulsion system Fig. 9. Availability of BOG utilization, ABOG.
Automatic vent valves
down automatically with alarm. The capacity of one ventilation fan is to maintain not less than 100% of the total required in case the other is out of service. Explosion relief devices such as rupture disks for the engine exhaust system against explosion are to be provided in the exhaust manifolds. The exhaust gas pipes from the dual-fuel propulsion systems should be installed independently. The exhaust gas pipes are not be connected to the exhaust pipes of other engines or systems [14].
6. Conclusions The primary movers governed the availability of BOG utilization as well as that of design propulsion load since they had no redundant system for them. Note that most of the other subsystems had relatively high availability for the redundancy back-up for the machinery parts prone to failure. For the design propulsion load to be available, all of the primary movers should be operating for any propulsion options. In terms of BOG utilization, the primary movers played a role of the BOG consumer burning the BOG as fuel. Failure of one of them meant the reduction in the BOG utilization to that degree. It was no coincidence that two availabilities had the similar levels and relative values, as shown in Figs. 7 and 9. The availability for emergency propulsion load attained high levels compared with the other two availabilities since all the subsystems including the primary movers were dual. New propulsion systems commonly required the supply of pressurized fuel gas, which was the dominating source of the most failure modes and hazards. Leak of explosive gas was considered the most disastrous hazard. Through the systematic qualitative approaches to hazard identification and risk reduction, there were available the safety requirements to mitigate the hazards from the international and classification society. Selection of the propulsion option should take into account factors other than availability. The DFSM, the conventional steam turbine-based system, demonstrated availability around 0.99. The propulsion system, however, has demerits such as low efficiency and bulky installation space. Actually, many LNG carriers took systems other than the DFSM for economic reason.
Double block and bleed
Master valve
Automatic shut-off valves
Fig. 10. Automatic shut-off valve arrangement of fuel gas supply system for dual-fuel diesel engines.
ARTICLE IN PRESS D. Chang et al. / Reliability Engineering and System Safety 93 (2008) 1877–1885
Further studies are recommended for detailed availability assessment, quantitative risk analysis, and the life cycle cost comparison. The availability estimation of the current study was constrained by some factors, as taken into account in some of the assumptions. The reliability information of the LNG shipping industry is required. Moreover, the maintenance policy and consideration of the control system should be considered for improved estimation. The quantitative risk analysis should focus on leak, fire and explosion hazards. The fuel gas supply system for the DFGE and DFDM boosts the gas pressure up to considerable levels, which has not been faced in the LNG shipping industry. In spite of the ventilation system some large leaks may cause the flammable concentration to stay above the LEL for a significant period if the gas detection is delayed. Quantitative analysis will help to verify whether the probability and consequence of the hazards are acceptable. Comparison of the life cycle cost may be effective to tell which propulsion option would be best for a fixed transportation path. The reliability and availability aspects, however, should be considered in the cost comparison. In other words, the maintenance cost for the failed items and logistic cost for spare parts should deserve proper attention. References [1] Chang D, Rhee TJ, Nam KI, Chang KP, Lee DH, Kwak BJ, et al. Challenges of LNG carriers propelled by new propulsion systems: ship builder’s viewpoint. Abu Dhabi, United Arab Emirates: Gastech; 4–7 December 2006. p. 1–24.
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[2] Sørensen RI, Christiansen P. Development of shipboard reliquefaction systems for LNG carriers. Abu Dhabi, United Arab Emirates: Gastech; 4–7 December 2006. p. 1–20. [3] Thijssen B. Dual-fuel-electric LNG carriers. Abu Dhabi, United Arab Emirates: Gastech; 4–7 December 2006. p. 1–11. [4] Linwood J, Ha JP, Aabo K, Laursen RS. LNG gas carrier with high-pressure gas engine propulsion application. Abu Dhabi, United Arab Emirates: Gastech; 4–7 December 2006. p. 1–30. [5] Harsema-Mensonides A, Dalston CM. Gas turbine electric propulsion for LNG carriers: myth or money spinner. Abu Dhabi, United Arab Emirates: Gastech; 4–7 December 2006. p. 1–17. [6] Kuever M, Clucas C, Fuhrman N. Evaluation of propulsion options for LNG carriers. Gastech 2002, Qatar, 13–16 October 2002. [7] Maritime Business Strategies web pages, /http://www.coltoncompany.comS, 2007. [8] Hansen JF, Lysebo R. Electric propulsion for LNG carriers. LNG Journal 2004;September/October:11–2. [9] Nam KI, Rhee TJ, Chang KP, Chang D. An Analysis of gas turbine propulsion system for LNG carriers based on reliability and economic evaluation. In: The Society of Naval Architects and Marine Engineers: maritime technology conference & expo and ship production symposium, Paper no. B1, 10–13 October 2006, Fort Lauderdale, USA. [10] Skjoldager P, Lunde T, Melaaen E. Two-stroke diesel engines and reliquefaction systems for LNG carriers. In: Motorship conference, Hamburg, 2003. [11] Chang D, Rhee TJ, Nam KI, Lee DH, Jeong SH, Chang KP. Comparison of availabilities of new and conventional propulsion systems for liquefied natural gas carriers. In: The European safety & reliability conference, Estoril, Portugal, 18–22 September 2006, p. 2771–6. [12] OREDA Participants. Offshore reliability (OREDA) handbook, ed. Høvik, Det Norske Veritas, 2002. [13] Ayyub BM. Risk analysis in engineering and economics. London: CRC Press Company; 2003. [14] ABS (American Bureau of Shipping). Guide for propulsion systems for LNG carriers. ABS Plaza, Houston, 2005. [15] DNV (Det Norske Veritas), Gas fuelled engine installations. Høvik, Det Norske Veritas, 2005.