ADVANCED LNG CARRIER WITH ENERGY SAVING PROPULSION SYSTEMS: TWO OPTIONS

Transcription

1 ADVANCED LNG CARRIER WITH ENERGY SAVING PROPULSION SYSTEMS: TWO OPTIONS METHANIER DE POINTE AVEC SYSTEMES DE PROPULSION A ECONOMIE D ENERGIE : DEUX OPTIONS Hiroyuki Ohira Yoshihiro Suetake Masaru Oka Mitsubishi Heavy Industries, Ltd. (MHI) Tokyo, Japan ABSTRACT Reduction of ocean transportation costs is a never-ceasing aim in the case of LNG transportation chain operations. To further this goal, such methods as enlarging the size of carriers to raise efficiency and reduction of fuel costs through the use of advanced plant systems are being studied around the world. In this paper, two options for low fuel cost propulsion systems that have been developed and their use for next-generation LNG carriers are presented: 1. DRL: two stroke Diesel main engine + boil off gas Re-Liquefaction plant: Concerning re-liquefaction plant, MHI has developed a next-generation plant and is using on-going feedback based on the actual performance of this plant, which has been installed and is in actual commercial use, for the purpose of establishing the level of improvement in both efficiency gained from using a re-liquefaction plant and in safety and operating reliability. 2. SST: Simplified and Superior efficiency steam Turbine propulsion; Steam turbine plants have been used for most LNG carriers because of their high operating reliability and flexibility of available fuel. However, compared to land-based power plants, the rate of development in this area has been lagging. MHI has recently developed and is offering a new steam turbine plant with vastly improved fuel consumption efficiency while retaining the important features of high reliability and ease of operation. RESUME La réducution des coûts de transport maritime est un but perpétuel en ce qui concerne les activités en série du transport de GNL. Afin d atteindre cet objectif, les méthodes consistant à agrandir la taille des méthaniers pour augmenter le rendement et réduire les coûts inhérents au carburant à travers l utilisation de systèmes motopropulseurs de pointe sont à l étude à travers le monde entier. Dans cet exposé, deux options concernant des systèmes de propulsion à faible coût en carburant qui ont déjà été développés et leur utilisation au niveau des méthaniers de la nouvelle génération sont présentées et analysées : PO-31.1

2 1. DRL: moteur principal Diesel à deux temps + groupe motopropulseur à Re-Liquéfaction de gaz d évaporation : En ce qui concerne le groupe motopropulseur à re-liquéfaction, MHI a développé un moteur de la nouvelle generation et utilise le feedback permanent basé sur les performances réelles de ce groupe qui a été mis en place et est en utilisation industrielle effective, dans le but d instaurer un niveau d amélioration dans l efficacité obtenue par l utilisation du groupe motopropulseur à re-liquéfaction ainsi que dans les domaines de la sécurité et de la fiabilité opérationnelle. 2. SST: propulsion à Turbine à vapeur Simplifiée et de rendement Supérieur : Les groupes motopropulseurs à turbine à vapeur sont utilisés par la plupart des méthaniers du fait de leur grande fiabilité opérationnelle et de leur flexibilité d utilisation vis-à-vis des carburants disponibles. Néanmoins, en comparaison avec les groupes motopropulseurs terrestres, le rythme de développement dans le domaine accuse un retard. Récemment, MHI a mis au point et propose un tout nouveau groupe motopropulseur à turbine à vapeur d une rentabilité fortement accrue en consommation de carburant tout en gardant les caractéristiques fondamentales de haute fiabilité et de facilité d opération. 1. BACKGROUND Reduction of ocean transportation costs is a never-ceasing aim in the case of LNG transportation chain operations. In recent years, various efforts have been made to raise transportation efficiency by using larger carriers and, in connections with these efforts, a growing number of projects have been carried out to study alternate propulsion systems. The main purposes to alternate the propulsion system are the reduction of fuel costs, and preparation for the growing output in propulsion power. In addition, those actions are encouraged by the recent rapid progress in the field of BOG processing, such as re-liquefaction, gas firing internal combustion engine etc. This paper focuses on the lowering of fuel costs, and the results of studies to ascertain the economic merits of various systems related to use cargo LNG. 2. COMPARISONS OF THE DIFFERENT PLANTS 2.1 Features of an LNGC plant Figure 1 indicates the various technologies involved in this study that MHI is developing. The alternatives for propulsion can be characterized by three categories, direct steam turbine driven (ST), direct diesel engine driven (DD) and electric propulsion systems (EP). However, the design features of the plants are different among them even in a category, depending on the method used to process the BOG. Only LNG carrier uses the cargo for fuel, as Boil off gas (BOG) and/or forcing-vaporized LNG (FVG). PO-31.2

3 This approach reflects the reality that the BOG can be reduced but not entirely eliminated so far. Also, a special circumstance has made it possible to find the economical merit in use of LNG as fuel, but the outcome depends on the price of LNG chosen to calculate the cost of the fuel consumed. 1) Steam turbine propulsion In the case of a steam turbine plant, compared to a reciprocating internal combustion engine such as a diesel engine, etc., the level of reliability and the ease of maintenance can be seen to merit high evaluations. In addition, in the case of dual firing boilers, it is possible to a use a wide variety of different fuels ranging from low quality bunker oil to NG and a wide range in ratio of mixed firing using gas and oil is possible. This flexibility in fuel selection is a special feature of steam turbine ship, and no other new option alternative plants that can take full advantage of this flexibility have been available. Up to the present time, the fuel for LNG carriers has tended to be selected based on conditions in the LNG and bunker fuel oil markets and transportation planning. Recently, a carrier with re-liquefaction plants has appeared in order to increase the level of this type of flexibility. Figure 1: Alternative plants under development by MHI Compared to other plants, a weak point of steam turbine plants is their relatively low thermal efficiency. Steam turbine plants can attain only less than approximately 2/3 of thermal efficiency, of 2-stroke diesel propulsion that has attained highest levels of efficiency. However, possibilities still remain to improve the efficiency of steam plants and MHI has been making continuous efforts on new plant of improving the efficiency. Figure 2 shows the different stages in the development of MHI steam turbine plants. The top part of the diagram shows the 4-stage feed water regenerating plant developed for LNG carrier in the early period. In order to get high boiler efficiency, a gas air heater that used the heat from the boiler exhaust gas, was Figure 2: Development of MHI turbine plant PO-31.3

4 adopted. On the other hand, the 2-stage feed system also exists at the same period to aim at achieving simplicity in configuration, but it sacrificed about a 5% loss in efficiency. And the gas air heater, helped to raise the boiler efficiency, was regarded as being effective for use on LNG carriers with gas-fired boilers in the beginning. However, when put into actual use, since the HFO or its mixture with BOG are mainly fired all the time, there were problems with corrosion caused by sulfuric acid so that the use of this approach dropped off. In 1993, for a ship built by MHI, a second-generation plant was used. Subsequently, this type was selected for the fleets of Qatar-Japan Project and is currently the world standard type of plant. Key features of this plant are the high steam condition of superheated, use of a steam-heated air heater and a three-stage feed water regenerating system. This design achieves simplicity while achieving high-level efficiency equivalent to that of a 4-stage feed plant. Concerning new systems that are seen as next-generation, the steam condition of superheated have been raised even further and 2-stage water feed regeneration, etc. have been adopted. The new system promise further increases in thermal efficiency of 5% or more, than the second generation. 2) Direct Diesel propulsion 2-stroke diesel engines (Below, 2st-DE) have become the standard for large-scale energy carriers and achieved high reliability and high thermal efficiency. However, LNG carriers have not been able to have their superiority, because of the method for efficient processing of BOG. Gas-fired diesel engines exists, however, they have not become a new solution because of several problems, including reliability and the feature to handle extremely high-pressured flammable gas, etc. Figure 3 indicates alternative plant adopted 2st-DE, involving the hybrid idea that 2st-DE and propulsion assist electric motor (POD, for example) are used in combination. In any case, the system can be used effectively for the ship increase the performance of the cargo insulating system, however, it is also necessary to optimize these benefits in balance with initial investment costs. Only CODAG has been adopted the system which uses BOG for fuel and others uses re-liquefaction system. MHI has developed the world s first re-liquefaction plant for use on a carrier and the plant has been in commercial use for three years. With Figure 3: Example of Direct Diesel propulsion the adoption of the plant, the largest problem affecting the use of 2st-D has been eliminated, and now the main issue is the evaluation such as redundancy and operating ratios. When considering an engine itself, a turbine is superior in reliability and maintainability, and this is something that cannot be PO-31.4

5 avoided. Thus, MHI is offering the ideas of twin engine and hybrid propulsion options. (Refer to Section 3) 3) Electric propulsion The impetus driving the adoption of electric propulsion lies in the use of LNG for fuel. In addition, when the matter of flexibility of fuel selection is also considered, it becomes necessary to move to development of dual fuel systems. Two representative examples of such systems are shown in Figure 4. Below, the features of these two types of plants will be presented. Figure 4: Electric propulsion (Example) E+DFE The Wartsila 32/50DF dual fuel 4 stroke engine(below, DFE )can be used in two different operating modes that make it possible to use mixed firing (fuel oil and gas firing), by switching to either diesel mode of MDO fuel and lean-burn mode of NG + pilot fuel (MGO), for flexibility of fuel choice. Especially when mixed-firing is taking place, the gas feeding pressure is low (approximately 5 barg) and type of approach has been evaluated as being superior compared to existing high gas pressure type DFD engines (Dual fuel diesel of gas injection diesel type). This system requires the multi-engine configuration, 4 to 6 engines for example, so its redundancy overcomes the steam turbine plant, and higher thermal efficiency in wide range of propulsion load can be expected. Meanwhile, when the matter of reduction of fuel costs is being considered, the possibility of using HFO fuel is attracting a high level of interest. At this point, we are placing expectations on the development efforts of related engine manufacturers. E+GTCC Gas turbines have a level of reliability that is almost equal to steam turbines, and they require relatively few auxiliary equipments and their operation is also simple. Considering maintenance requirements, they are superior to reciprocating type of internal combustion engines with many cylinders, involving previously mentioned type. Proposed power generating system is the combined cycle of gas turbine and heat recovery steam turbine. Considering the high level of efficiency of the gas turbine itself, there were cases where the merits of using a heat recovery steam turbine plant came into PO-31.5

6 question, however, a steam turbine combined circuit involved dual fuel boiler contributes the operability, redundancy and flexibility of selection of fuel. Most of marine gas turbine fuels consist of MGO firing or NG firing fuel types. By connecting the dual fuel auxiliary boiler to a heat recovery steam turbine system, LNG, MDO and HFO mixed firing becomes possible. It means that LNG saving operation is realized by assistance from oil firing with natural BOG, just as steam turbine ship. 2.2 Examination of the economic factors The contributions to lowering transportation costs that can be made by the propulsion plant are in the areas of t he initial investment and the area of fuel costs. The fuel cost depends on the efficiency of the plant and unit fuel price, especially for BOG price ratio against fuel oil. Here, the fuel cost levels for the various plants are compared. Figure 5 indicates for each plant the fuel costs for a round voyage, compared by converted values to HFO quantity as the basis of evaluation. The costs for the other fuels were converted equivalent amounts of HFO. The horizontal axis is used for a LNG/HFO price ratio. As the price of LNG varies, the variations in fuel cost were taken into account. (In the case, the price for MGO and MDO were held constant). In addition, the value of the LNG conserved by re-liquefaction was also subtracted from the total fuel cost in order to evaluate the merit Figure 5: Fuel costs for each plant of BOG saving. There is a need to optimize the boil off rate of cargo tank based on the characteristics of the given plant, but for the purpose of this evaluation, the same value was assumed for all types of plants. The solid lines on the figure indicate the fuel cost with assumption of BOG + fuel oil, and the dotted lines indicate the same one for BOG + FVG. The economics of these two types of systems reverse on a price ratio less than equal, between the re-liquefaction-related plant and the gas-fired plant, as can be seen from the graph, the type of plant employing re-liquefaction is superior over a broad range of price ratio. On the other hand, the gas-fired plant can reduces the fuel cost drastically in cases where the price ratio was relatively low. In actuality, the form of LNG purchasing contract may affect the value evaluation of LNG, and the prices for LNG/HFO fluctuate every time in the market. The plant provided proper flexibility of selection of fuel can select the lowermost curve to minimize the fuel cost. PO-31.6

7 3. INTRODUCTION OF ALTERNATIVE PLANT PLANS From the various types of plants that have been discussed above, two examples, DRL: two stroke Diesel engine propulsion with boil off gas Re-Liquefaction plant, and SST: Simplified and Superior efficiency steam Turbine propulsion, have been chosen and their distinguishing features will be discussed. 3.1 DRL: Two stroke diesel engine propulsion with boil off gas re-liquefaction plant 1) BOG re-liquefaction plant The world first re-liquefaction plant for LNG carrier, installed on S/S LNG JAMAL began to commercial use in November of 2000, when the carrier delivered, and as of October 2003, the plant has been in commercial use for three years. The propulsion plant for that ship is a steam turbine. So the ship can use either BOG to fire in the boiler or re-liquefy BOG and return to cargo tank. Or, it can carry out both at the same time. In other words, if for some reason the BOG is not re-liquefied, it possible to switch over and dispose of BOG by using it for boiler fuel. If a N2 compressor ceased operating by some reason, the ship can switch over to gas firing and the heat is utilized for propulsion. On the other hand, DRL ship, which can t use the combustion heat effectively for ship s propulsion, shall be required to install redundant number of equipment for the re-liquefaction plant, to avoid the disposal of BOG. Figure 6 shows the examples of plant configuration for the LNG JAMAL and the DRL ship. The probability of a re-liquefaction failure drops drastically by application of the rotating equipment and plate-type heat exchangers they are known to be high level of reliability, and by the installation of plural number of rotating equipment such as nitrogen compressor, BOG compressor, etc. with their capacity of 100% or above. Figure 6: Redundant re-liquefaction PO-31.7

8 2) Power generating plant The nitrogen compressor is the major energy consumer of re-liquefaction plant, moreover, it is the largest consumer except for propulsion. (Around 2-4MW) The steam turbine driven compressor is applied to minimize power conversion loss, for the steam turbine ship, LNG JAMAL. As for the DRL ship, MHI proposes the steam turbine combined with electric motor drive, to improve the thermal efficiency, by utilization of auxiliary power coming from the main engine gas waste heat. (Refer to figure 8.) In addition, the heat recovery cycle and dual fuel boiler enables Figure 7: Peak shaving based on parallel operation to operate the re-liquefaction and gas firing in parallel, with recovering the heat generated by BOG combustion. This function can realize a flexibility of fuel for re-liquefaction duty, and a self-sufficient process of BOG treatment. And this approach is effective in making possible peak shaving with regard to the variations in the generation of BOG by ship s operation. As shown on Figure 7, the plant can be operated at nearly optimized load level for most of its operation period, without the restriction of re-liquefaction capacity associated with the gas firing function as a method of BOG treatment. Figure 8: Hybrid DRL PO-31.8

9 3) Propulsion system A multi-engine design is adopted to give the system the adequate redundancy and operating ratio as the same of steam turbine. This idea can provide proper opportunity of the periodic maintenance of cylinder overhaul, which is the particular for two-stroke diesel engine, during the voyages. The opportunity when the ship is stopping in port for cargo loading or discharging is available for that work, by assumption of a stand-by engine, as the most of the ports requires, which enable to leave the shore rapidly in emergencies. In Section 2, the concepts of twin engines and hybrid engine propulsion are proposed from the point of view mentioned above, and of the effectiveness for low draft and/or high-speed ship. Figure 8 shows one of the ideas of hybrid propulsion. This type of the ship can go maneuvering in port without main diesel engine drive, and have added propulsion power when ocean going. Figure 9: A ferry of hybrid propulsion (Ship owner: SHIN NIHONKAI FERRY Co., Ltd.) This arrangement has already been adopted for a ferry, additionally expecting the CRP effect to improve propulsion efficiency, and currently being built by MHI, as a world s first case.(refer to Figure.9) 3.2 SST: Simplified and Superior efficiency steam Turbine plant In the steam-driven land base power plants, higher efficiency has been achieved by many technical improvements such as shown below for many years. 1) Higher pressure and higher temperature of superheated steam 2) Reheat-cycles 3) Increase the number of feed water regeneration stage 4) Improvement of turbine performance (Turbine blades, seal mechanism, etc ) Some of these technologies are applied for the marine use, taking into account the avoidance of complexity that does not suit for onboard system, and it became possible to develop plants for onboard system with more simple designs and then with more than 5% of efficiency improvement. 1) Outline of the plant The major improved features are indicated in the Table below. (Also refer to Figure.2) PO-31.9

10 Together with an improved efficiency, simplification was also achieved and reliability was given maximum attention. In the case of plants for use on ships, with regard to auxiliary equipment, there were no new technologies or changes in basic design used. 2) Dealing with corrosion caused by high temperatures In recent years, there has been a tendency for heavy bunker fuel (HFO) to contain impurities related with corrosion, such as sulfur, vanadium, etc. HFO is a residual oil from refinery process, so the physical properties of residual oil appear to have worsened with improvement of refining facilities. High temperature corrosion is one of the most important issues related to these tendencies, and the issue is represented by vanadium attack. This phenomenon processes by the vanadium compounds that appear in ash adhered to metal surfaces, where they melt together and cause metal materials to corrode. It is pronounced above the melting temperature of vanadium compounds (approx.590 ), at the Na2SO4:V2O5 ratio is 2:8. In the case of LNG ships, the combustion of BOG can help to avoid this kind of corrosion. However, recently, there has been a tendency for the proportion of HFO combustion to increase so that the high temperature corrosion is becoming more prominent. The Figure 10 shows the new advanced type of MHI main boiler. As shown on result of metal corrosion test, the corrosion rate is growing along with the metal temperature go up for existing material, while there has been a significant improvement for the new material, over a wide range of temperatures. This feature is one of indispensables for our next generation plant, because of its higher shift in superheated steam temperature. Figure 10: New material for superheater of main boiler PO-31.10

11 4. SUMMARY All of the propulsion plants introduced have been proven to provide significant fuel cost savings in when selected to match the commercial environments in which they will be used. For reduction of transportation costs, considerations were given not only for the efficiency of the system, but also the fuel mode making the best by its flexibility of fuel selection. The next generation steam turbine plant is using the established technology, so that it is the most orthodox solution for improvement in economy, while keeping other factors, such reliability and maintenance requirements, as they are. The DRL makes it possible to achieve the maximum thermal efficiency and carry the largest amount of LNG. DRL ship can drive the ship by HFO without affection from the cargo condition, as is the case for other energy carriers as well. By switching from all BOG re-liquefaction to self-sufficient re-liquefaction, flexible operations and high economic efficiency can be achieved. E+DFE, E+GTCC and similar systems for electric propulsion can save the fuel cost drastically by using LNG as fuel with provision that its price is almost zero. For the present, this approach can be anticipated to be a superior solution for dedicated fleets of a particular project that can take the type of LNG sale transaction contract being considered as proper. Marine industry has challenged with the fuel cost saving for many years, and achieved a type of diesel direct propulsion, and availability of residual oil wherever in the world. Recently, the issues of emissions from engine are growing an important factor. From the point of CO 2 emission, fuel cost saving contributed intermediately to saving the amount of the emission by high efficiency plant. However, the industry is struggling to respond that the emissions of other components such as NOx/SOx, they are coming to hot. Marine industry will find the solution to alternate LNGC propulsion under the new circumstances in the near future. MHI promises to concern and assist the activities related to this issue, as a ship builder and a marine machinery supplier. PO-31.11