For analysing the safety of the installation, the gas pipeline has to be considered. The steam turbine running on low pressure gas gives a relative advantage concerning the installation of gas piping, less expensive and with less potential risk of gas leakage. The systems with high-pressure gas could involve a larger risk for the safety of the plant. However, the double wall concept is designed to withstand a burst of the high pressure inner pipe and Type Approved in this respect. Based on Chiba GIDE experience, the double wall pipe system is safe and it uses well proven systems, so it does not involve a higher risk even using high-pressure gas.
Reliability is one of the key factors when selecting between propulsion alternatives. The general requirement of the reliability for modern LNG carriers are that the time between docks should be longer than 30 months and that maximum 3 days off-hire can be allowed in a year. This can be matched today by the large reliability of the steam turbine system but at least the same reliability can be reached by a twin diesel engine installation.
A single diesel engine may seem to have less reliability than the steam turbines. However, as proven by the service experience of Chiba GIDE all trouble experienced related to the gas injection technology have been minor troubles which can be remedied without off-hire during the operation by one cylinder cut out. In case of Chiba GIDE one cylinder cut out operation can not be applied due to the big synchronous generator where electrical unbalance swing will appear.
The time between docks of 30 months (15,000 working hours) can be met by the diesel engine based on Chiba GIDE experience where the liner wear will be 0.0388*15 = 0.582 mm which is far below the limit of replacement (3.2 mm). Some regular maintenance will be necessary between docks according to the life time of parts like sealing rings and o-rings, however this can be performed without off-hire during normal operation thanks to high redundancy of multi-cylinder diesel engines and auxiliary equipment.
Propulsion Redundancy: The examples with twin engines and two shaft lines will indubitably have an excellent redundancy, even in cases like a total failure of one of the engines, or the unlikely case of a failure in the shaft line or the propeller.
Final Conclusion: The analysis considering all the factors previously mentioned, both from the owner's and from the yard's point of view, confirm that the dual fuel diesel engines are a clear and very favourable alternative to the conventional propulsion with steam turbine. The high degree of necessary specialisation and the limited current production of vessels with steam turbines also have to be considered compared to the extensive experience and worldwide application of propulsion plants based on 2-stroke diesel engines. This is an important factor in favour of the reliability of the systems with 2-stroke diesel engines for LNG carriers.
6. New possibility of Chiba GIDE
Based on the current development and merits using clean natural gas in the Chiba GIDE, some new possibilities for increasing the total efficiency could be as follows.
6.1 Exhaust gas re-circulation (EGR)
EGR is well known way of reducing NOx emission, which is widely used for automotive applications. The effect of EGR on NOx formation is partly due to a reduction of the O2 concentration in the combustion zone, partly due to the content of water and CO2 in the exhaust gas. These constituents have high specific heats, so the peak combustion temperature is reduced which, in turn, reduces the formation of NOx.
On the Chiba GIDE plant the exhaust gas is much cleaner than on a normal HFO operating engine- particulate matter and SOx emission is almost zero. So EGR can most likely be performed without cleaning especially if the EGR system is only engaged at high load.
We assume that 25% EGR rate will reduce NOx emissions more than 50% with only a very slight increase of fuel consumption (some 1%). This would give an economical benefit by reducing ammonia consumption because the ammonia cost today is equivalent to 4.3% of the total fuel cost.
6.2 Turbo compound system
Turbo Compound Systems (TCS) and bottoming cycle steam turbines are very often used in plants optimised for electricity production. Chiba GIDE has also the possibility of applying these two systems, however only TCS can easily be applied because as shown in Fig.2, turbine by-pass is already applied. The TCS system flow diagram is shown in Fig.11.
6.3 Intelligent Engine and mechatronic systems
The intelligent engine concept5), 6) has been developed and been commercialised by MAN B&W Diesel. This is a computer-controlled concept that replaces the camshaft system of the conventional engine. The system is illustrated in Fig.12. It consists of an engine-driven high-pressure servo oil system, providing power supply for the hydraulically operated fuel injection and exhaust valve actuation units on each cylinder.
