JungHan Lee, Principal Research Engineer, Daewoo Shipbuilding & Marine Engineering Company

Size: px
Start display at page:

Download "JungHan Lee, Principal Research Engineer, Daewoo Shipbuilding & Marine Engineering Company"

Transcription

1 PROGRAMME GASTECH 2OO2 JungHan Lee, Principal Research Engineer, Daewoo Shipbuilding & Marine Engineering Company JungHan Lee is currently the leader of Ship/Plant System R&D team of Daewoo Shipbuilding & Marine Engineering. He has been working for design, commissioning, and R&D in many projects since his first join to the company in He has a BS degree in Naval Architecture from Seoul National University, and an Ocean Engineer s Degree from Massachusetts Institute of Technology. Pierre Michalski, Development Manager, Gaztransport & Technigaz (GTT) Pierre Michalski joined Gaztransport & Technigaz in October 1999 as Development Manager. Before joining GTT he worked for 29 years with ACH Le Havre shipyard as : Engineer, Design Office Manager, Ships in Charge Manager and Technical Director. He has a MSc degree in Shipbuilding from Gdansk Polytechnic High School, where he worked two years as Professor Assistant Fellow.

2 Gas Turbine Propulsion Systems for LNG Carrier Applications Abstract Even though the Gas Turbine (GT) propulsion system has many advantages in power to weight ratio, emission level, flexible machinery arrangement, efficiency and consequential cargo volume increase, it has not been adopted as a new propulsion system in an LNG carrier so far. As GT propulsion system has some characteristic features and limitations compared with the conventional system, detail technical and economic hinders have to be solved in order to implement in an actual LNG carrier. Daewoo Shipbuilding & Marine Engineering (DSME) and Gaztransport & Technigaz (GTT) have carried out extensive technical and economic studies on the GT propulsion system. This paper covers the study results on the GT propulsion system configuration, major equipment specifications, machinery arrangements, cargo volume change, efficiency calculation, economic studies based on initial and operation costs, and environmental issues. Since the applicable LNG and fuel oil costs are important in the economic evaluations, the effects of different LNG prices have been compared. In addition, the cargo volume increase by the GT propulsion system machinery arrangement has been estimated and its economic effects have been described. With the inherent lightweight of the GT system, the cargo volume can be increased by reducing the forward part ballast tank and by eliminating the fuel oil tank. This can bring a favorable economic justification to the GT propulsion alternative. From the study, it can be carefully derived that GT propulsion can be one of the feasible alternative propulsion systems for LNG carriers. The results obtained through this study could be beneficial for the general understanding and feasibility study of the GT propulsion system to the potential ship owners, and could contribute to the environmental friendly and competitive propulsion system applications. 1. Introduction Boiler and steam turbine based propulsion system for an LNG carrier has not been changed fundamentally since the first introduction of LNG carrier decades ago. Recently there are many suggestions and developments on the alternative propulsion system. However, there are no widely accepted propulsion systems other than the conventional boiler and steam turbine system. In LNG carriers, as the boil-off gas is naturally generated from the cargo tanks, the boil-off gas treatment systems have to be provided. This can be done either by consuming the boil-off gas as the fuel for the propulsion system or re-liquefying the gas with a separate system. On the other hand, gas turbine has a limitation in the choice of fuel. The onboard availability of high quality fuels, boil-off gas and LNG, with comparable price naturally attracts the attentions of those who attempt to apply the gas turbines in the LNG carriers. The application of the gas turbine system in an LNG carrier brings many changes in the related system design, weight distributions and machinery arrangement. Due to the inherent lightweight and flexible machinery arrangement, the LNG cargo volume can be increased without much changing the principal dimensions of existing vessels. In this paper, focus is made for the gas turbine propulsion system. Therefore other alternative propulsion systems such as the diesel engine, the diesel electric, and the diesel engine with re-liquefaction plant have not been reviewed in detail. 2. Characteristics of Gas Turbine System Fuel - Fuel for a gas turbine engine is generally confined to diesel oil, gas oil, and natural gas. Due to the high fuel price compared with heavy fuel oil, the gas turbines could not be applied popularly in Lee & Michalski 1

3 commercial vessels. However, in LNG carriers, high quality fuel is available onboard with affordable price. Therefore, in normal situations, the fuel for gas turbine propulsion systems in LNG carriers will be either boil-off gas or vaporized LNG gas. Weight & Dimension - The gas turbine is very light compared with the conventional boiler and steam turbine system and diesel engine system. Regardless of the direct gas turbine driven system or electric propulsion gas turbine systems, the engine room volume can be reduced by efficiently arranging the engines and related systems. Moreover, the lightweight in an LNG carrier after part can bring the reduction of forward ballast tank capacity, which also can be converted to the cargo volume increase. Emission Level The emission level of gas turbine exhaust is very excellent compared with other propulsion systems. It has advantages over the gas and fuel oil fired boiler system, and the diesel engine system. Refer to Table 1 for the typical emission level comparison. Especially when a low NOx burner is applied, the emission level can be reduced further. Propulsion system NOx (PPM) Natural gas Oil Boiler & steam turbine (dual fuel) 2-stroke diesel engine 4-stroke diesel engine (dual fuel) Gas turbine (dual fuel) w/o w/ LNB LNBß ~ ~ (HFO) 1000~1500 (HFO) 1000 () ß LNB: Low NOx Burner Table 1: Emission Level of Each Propulsion Engine 300 (MGO) (MGO) Efficiency GT itself usually has a higher thermal efficiency than that of a boiler and steam turbine system. Therefore, the net fuel consumption is reduced in the GT propulsion system. If combined cycle GT propulsion system is adopted, the overall efficiency can be comparable to that of 4-stroke diesel engine. With the reason, when the naturally generated boil-off gas amount is not counted in the fuel price, the actual required fuel amount for the combined cycle can be about half of the conventional steam propulsion case. 3. LNG Carrier Applications 3.1 Configuration There are many variations in GT applications for LNG carriers. The GTs can be applied as a direct propulsion system or an electric propulsion system. Steam Turbine Propulsion (STP) - For the comparison with other propulsion systems, a simplified diagram for the conventional boiler and steam system is shown in Fig. 1 (STP). Lee & Michalski 2

4 LNG GAS HFO REDUCTION GEAR LD COMPRESSORS WARM-UP HEATERS HP LP AST. TBN FPP BOILER BOILER TBN TBN SYSTEM LOAD FORCING VAPORIZER STM DUMP DIESEL EMG'Y CONDENSER BFWP CARGO TANK Fig. 1: Steam Turbine Propulsion (STP) System Direct GT Propulsion (DGT) The GTs are connected to the propeller shaft through a reduction gear. Depending on the propulsion redundancy scheme, two engines or one engine with a separate electric driven motor input may be adopted. Gas compressors are required for the supply of the boil-off gas or LNG gas to the gas turbines. In this configuration, an excessive boil-off gas treatment system such as thermal oxidizer (flaring system) has to be provided. A separate diesel oil tank is necessary for the case that LNG gas is not available. For the conceptual diagram, refer to Fig. 2 (DGT). Lee & Michalski 3

5 THERMAL OXIDIZER LNG GAS FUEL GAS COMPRESSORS REDUCTION GEAR GT GT CPP FORCING VAPORIZER DUAL FUEL DIESEL AUX. BOILER AUX. BOILER DUAL FUEL DIESEL SYSTEM LOAD DUAL FUEL DIESEL EMG'Y CARGO TANK Fig. 2: Direct GT Propulsion (DGT) System THERMAL OXIDIZER LNG GAS FUEL GAS COMPRESSORS GT MTR REDUCTION GEAR GT MTR FPP HARBOUR & EMG'Y SYSTEM LOAD FORCING VAPORIZER AUX. BOILER AUX. BOILER CARGO TANK Fig. 3: GT Electric Propulsion with Motors (EPM) System GT Electric Propulsion with Motors (EPM) The GTs provide the electric power for the propulsion system and the utility power. The electric motors are connected to the reduction gear to provide the power to the propeller shaft. As either one of the gas turbine generators provides electric power to the utility Lee & Michalski 4

6 system, no separate diesel electric generators are required. When the electric propulsion system is applied, the gas turbine can be located in any position such as the rear part of the accommodation. Refer to Fig. 3 (EPM). GT Electric Propulsion with Pod (EPP) A pod system can be applied for the GT electric propulsion system. Though this system is expensive in the initial capital cost aspect, it can bring more flexible machinery arrangement, which will bring more cargo volume increase. The pod system is known to have benefits in maneuvering performance and maintenance. EPP also can be implemented with combined cycle mode (COGES). Combined GT Electric Propulsion (CGS, CGS+) A combined GT electric propulsion system (COGES) can be applied to achieve higher efficiency. The heat recovery steam generator (HRSG) is installed in the GT exhaust. The steam generated in HRSG is used for the steam turbine generator. It consists of 2 GTs, 2 HRSGs, and 1 steam turbine (CGS). When duct burners are installed, both the excessive boil-off gas treatment and the redundancy power scheme purpose can be achieved. The COGES system can also be implemented by 1 GT and 1 HRSG with duct burners (CGS+). It (CGS+) has LNG GAS FUEL GAS COMP GT DUCT BURNER EXH GAS PROCESS & SERVICE STEAM HRSG STM DUMP TBN MTR REDUCTION GEAR CONDENSER BFWP FORCING VAPORIZER DUCT BURNER HRSG EXH GAS MTR FPP GT SYSTEM LOAD CARGO TANK EMG'Y Fig. 4: Combined GT Electric Propulsion (CGS) System advantages in the initial cost and overall efficiency. However, further studies on the overall system reliability of one GT are necessary. 3.2 System Considerations GT Location - In direct propulsion system (DGT), the GTs have to be installed in the lower part of the engine room, whereas in electric propulsion systems (EPM, EPP, CGS, CGS+), they can be located any convenient locations up to designs. The flexible location of GT ultimately can lead to the cargo space increase. Excessive Boil-off gas treatment In the gas turbine propulsion system, the excessive boil-off gas can be treated in a dedicated boiler, HRSG duct burners, or purpose-made low pressure thermal oxidizer (flare) system. Lee & Michalski 5

7 Duct Burners Duct burners may be used for the excessive boil-off gas treatment and to generate steam for the steam turbine generator when a gas turbine is out of service. The reliable design of the duct burners is very important and the safe operation has to be proved. Refer to Fig 5 for the conceptual diagram. Fig. 5: Duct Burner System Gas Compressor The fuel gas for the gas turbine has to be compressed to around bar. The initial cost effect and related system design have to be investigated. The availability of a screw type gas compressor is known to be very high. Apart from the gas compressor system, the GT is designed to be fueled either gas or diesel oil (MGO). GT Model The selection of the GT model is important in an LNG carrier application. The number of GTs, rated power, thermal efficiency, and dimensions, which meet shipboard application requirements, should be carefully investigated and compared. Electric System When an electric propulsion system is adopted, the overall electric system has to be designed considering the power generator scheme, redundancy, and power management. Contrary to most of the conventional (STP) LNG carriers, where low voltage system is more popular, especially in electric propulsion systems (EPM, EPP, CGS, CGS+), medium voltage systems are applied for large capacity motors including the main propulsion system. 4. Evaluations 4.1 Basis of Comparison Candidates For the comparison of the appropriate GT propulsion system, 6 candidates have been derived: Conventional boiler and Steam Turbine Propulsion (Fig 1), Direct GT Propulsion (Fig 2), GT Electric Propulsion with Motors (Fig 3), GT Electric Propulsion with Pod, COmbined GT Electric Propulsion with 2 GT (Fig 4), and with 1 GT. Each case is coded as STP, DGP, EPM, EPP, CGS, and No. Equipment STP DGT EPM EPP CGS CGS+ 1 Main Boiler 2 2 HRSG Main TBN w/ RG 1 4 Gas Turbine 2 Lee & Michalski 6

8 5 Gas Turbine Gen Gearbox Motor w/ Convert Pod w/ Convert. 2 9 Propeller Shaft & Bearing CP Propeller 1 12 Rudder Steering Gear DG & TG 3 (total) 1 (TG) 1 (TG) 15 DF Diesel Gen LD Compressor 2 17 Fuel Gas Comp Aux. Boiler Thermal Oxidizer HV Elec. 20 (for Propulsion) Table 2: Equipment List for Each Configuration CGS+ respectively for brevity. Initial Cost Initial costs are dependent on the equipment and installation cost for each system. During the studies, we have acquired detail equipment costs for exact comparisons. However, the figures are not included in this paper to avoid any possible misleading or misunderstanding. However, for indirect comparison and understanding of differences among systems, the equipment lists for each of the case are summarized in Table 2. In the economic calculation, some of the typical vessel and equipment prices are used. However, the figures should be understood for relative comparisons only. Operation Cost Operation costs mainly consist of fuel cost, maintenance cost, and other factors. Naturally, the operation costs are much dependent on the fuel efficiencies of each candidate systems. As the fuel cost is much affected by the fuel oil and LNG prices, many case studies for different LNG prices have been made. On the other hand, the fuel oil (FO) price has been fixed to 125 USD per ton to give relative comparisons with different LNG prices. The operation costs for each of short, medium, long voyage ranges also have been made. Revenue For the economic study, a revenue model is suggested. Though the revenue is quite dependent on the long-term contracts and market situations, a very simple model has been used. For each short, medium, and long ranges, daily charter rates per unit cargo volume have been used for the calculation. Therefore, the increased cargo volume can bring favorable revenue. 4.2 Economic Analysis Model Total Cost Comparison - From the first cost, annual capital cost (AA) can be calculated based upon annual interest rate and amortization period. The operating cost (BB) is calculated from the fuel and maintenance costs. In the comparison of the annual total cost (TT), the revenue increase (RR) due to the cargo volume increase can be deducted as credit. (TT = AA + BB RR) Limitations The comparison results should be interpreted with careful discretion. Even though some quantitative evaluations are made, the figures should not be regarded as absolute meanings. It should be noted that some of the other considerations could not be included as quantitative figures. Therefore, the Lee & Michalski 7

9 other factors should also be considered in conjunction with the quantitative evaluations. 4.3 Fuel Cost LNG Price Whereas the heavy fuel oil is well defined as a market price in a specific region, an LNG price is very difficult to determine. Especially the applicable LNG price for the LNG propulsion purpose is very delicate, as the price can be either an ex-production-terminal price (FOB) without any subsequent add-ons, or the consumer market price (CIF). With the reason, the applicable price could be different up to LNG import contracts, shipping company strategies and preferences, and importing countries. However, the economic evaluations are much affected by the LNG prices unfortunately. Comparison with Fuel Oil To avoid any misleading due to different LNG prices, the effects of LNG price have been studied separately. Based upon the LNG prices, different comparison results could be reached. In the evaluating alternative propulsion systems, the relative price ratio between two different fuels is believed more valuable than absolute values. When the economic studies in comparison with diesel electric system or dual fuel (heavy fuel oil and boil-off gas) fired boiler and steam turbine is made, the relative LNG price over the fuel oil can be a deciding factor. In the calculations, the naturally generated boil-off gas (BOG) price has not been included in all cases. In other words, only the FO price and forced LNG price have been calculated in STP and other GT propulsion cases (DGT, EPM, EPP, CGS, and CGS+). 4.4 Cargo Volume Increase The cargo volume increases and corresponding system efficiencies for each case are summarized in Table 3. The cargo volume increase can be brought by the reduction of engine room space, consequent forward ballast tank reduction due to the alternative system designs, some improvements in forward part cargo tank geometry, hull-form modification, and the combinations of these. However, to get direct and conservative results, only the engine room reduction effect is included in some of the economic evaluations. As shown in the table, the GT propulsion systems can increase the cargo volume compared with conventional boiler and steam turbine (STP) case. The cargo volume increase effects can be incorporated as the revenue increase in the economic studies. A typical machinery arrangement for GT propulsion system is shown in Fig. 6. Case Efficiency (%) Cargo Capacity (m 3 ) STP ,000 DGT ,000 EPM ,000 EPP ,000 CGS ,000 CGS ,000 Table 3: Efficiency and Cargo Volume Increase for Each System - Typical 4.5 Comparisons Total Cost Evaluation The economic evaluations are affected by the initial and operating costs, together with the cargo volume increase credit. The operating costs are much dependent on the propulsion system thermal efficiencies. Among many evaluation cases, typical results for medium voyage range at the same LNG and fuel oil prices are shown in Fig. 7A, 7B, and 7C. Fig. 7A is the initial cost graph for each propulsion system. The annual capital cost is calculated from the 6 % interest rate, and 20 years amortization period. Fig 7B is the annual operation cost. Fig 7C shows the total cost comparison, which is the sum of annual capital and operating costs (TT=AA+BB). In this case, the cargo volume increase credit Lee & Michalski 8

10 effects have not been considered in order to get direct effect due to fuel efficiency and to get conservative comparison. The effects of variable LNG prices are shown in Fig. 8. If the cargo volume increase credit is included (TT=AA+BB-RR), more favorable results to GT propulsion systems are derived. From the evaluation of many cases for each of the propulsion systems, followings can be carefully derived: Fig. 6: EPM Machinery Arrangement - The GT propulsion systems have overall benefits over the conventional boiler and steam turbine system (STP) mainly due to the efficiency and cargo volume increase. - The initial cost did not play a major role in the overall evaluation compared with the fuel price and revenue increase by the cargo volume increase except in pod case (EPP). - The GT propulsion system operating cost can be relatively lower than STP case in the reasonable range of the LNG price. - Especially in CGS+ case, the fuel cost saving is very significant. The CGS+ system can be competitive even the LNG price reaches high compared with the fuel oil price due to the high efficiency. Lee & Michalski 9

11 STP DGT EPM EPP CGS CGS Initial Cost (kusd) Cases Fig. 7A: Initial Cost Comparison STP DGT EPM EPP CGS CGS+ 5,000 Operating Cost (kusd/yr) 2,500 0 Cases Fig. 7B: Operating Cost Comparison (Medium, LNG: 125 USD Case) Efficiencies Normally, the GT efficiency is somewhat higher than that of STP case. The efficiency of STP is known around 28%, and that of the GT is around 32-37%. When COGES system is adopted, the overall efficiency can be as high as 40-43%. Therefore, the GT propulsion systems have advantages in the operating cost. However, in actual situation, GT system efficiency is lowered when the GT is not running near the full capacity range. For example when the load is below 80%, the efficiency can be dropped by almost 5%. (For example, from 37% to 32%) This is one of the reasons why the CGS economic evaluation result is not so good as that of CGS+. In this CGS case, the 2 GTs plus combined cycle output exceeds the required propulsion power, so the GTs are operated in de-rated mode. With the reason, if larger propulsion output is required or other appropriate GT model is selected, the overall efficiency can reach to that of CGS+. Lee & Michalski 10

12 STP DGT EPM EPP CGS CGS+ 6,000 Total Cost (kusd/yr) 4,000 2,000 - Cases Fig. 7C: Total Cost Comparison (6 %, 20years, Medium, LNG: 125 USD Case) STP DGT EPM EPP CGS CGS+ 4,000 2,000 Total Cost (kusd/yr) -2, ,000-6,000 LNG Price Fig. 8: LNG Price Effect in Total Cost (FO=125USD/ton) 4.6 Discussions Applicable Fuel Price The LNG price for LNG carrier propulsion system can be dependent on the strategies and decisions between LNG production terminals, shipping companies, and cargo owners. But it could be carefully assumed that the LNG price in production terminals could be within the comparable range with oil price. It also could be considered that LNG production terminals may apply favorable LNG price for the LNG carrier propulsion, so that a fraction of the clean energy can be used for the LNG carrier itself. It might be beneficial for the marketing of LNG producers by accelerating the LNG consumption in the long run. Machinery Arrangement In order to get an appropriate equipment arrangement in engine room area, detail machinery arrangements have been prepared. For the EPM case, more detail 3 dimensional modeling has been made for the design review of the proposed system. The typical views are shown in Lee & Michalski 11

13 Fig. 9. Emission Level Contrary to diesel engines and fuel oil fired systems, the LNG fired GT propulsion systems have superb emission level. The application of the environment friendly system would bring a good reputation to the ship owner, which ultimately contribute to the company s public relations as well. Fig. 9: 3D Model Example Reliability and Redundancy From the initial studies, the GT propulsion systems generally have better evaluation results. However, in order to justify and apply to an actual LNG carrier, the reliability and redundancy subjects have to be investigated further. Preference Even though the numerical results show very favorable to GT propulsion systems, the final adoption is still dependent to the ship operating company s decision and preference. To convince the advantages of the GT propulsion system, more technical and economic studies and verifications are required. 5. Observations and Summaries From this study following observations could be derived. - GT propulsion system could be one of the possible alternative propulsion systems for LNG carriers. The technical problems associated with the new propulsion system could be solved. - GT propulsion system has an advantage in cargo volume increase, which would lead to a favorable economic evaluation. - GT propulsion can be implemented either direct GT propulsion type (DGT) or GT electric propulsion with motors or pod type (EPM, EPP). The application of combined gas turbine electric propulsion system (COGES CGS, CGS+) can have more favorable economic evaluation result with the higher efficiency. - The excessive boil-off gas treatment should be considered in the GT propulsion system design. Lee & Michalski 12

14 - Economic studies for the possible GT propulsion system cases have been made for each systems. - In GT propulsion system, the fuel is based on the boil-off gas and the vaporized LNG gas. Hence the LNG price is sensitive to the economic studies. It is believed that the LNG price at LNG carrier environment could be comparable to the fuel oil price. - The emission level of the GT propulsion system is very good. Hence the application of GT propulsion system would contribute to the environment. - More studies on the GT propulsion system reliability and redundancy scheme are necessary. - Nevertheless, the final adoption on the GT propulsion system application will still be remained to ship owner s preference and decision based on above and more detail operation evaluation, comparing other alternative propulsion systems. In paper, the technical and economic studies on the GT propulsion system have been made. We hope the study results would be a good basis for further studies and contribute to the general understanding of the GT propulsion systems. 6. Acknowledgement During the studies, we have acquired a lot of information from GT, electric propulsion system, and other equipment vendors. We sincerely appreciate their assistance and recommendations together with their active attitudes on the development and preparation of the gas turbine and electric propulsion system application to LNG carriers. We hope our study would reward their efforts by enhancing the GT propulsion system application possibility to a certain extent. Our joint research members appreciate our companies for allowing share the research results with many people who are interested in the subject. 7. References 1) Technical specifications and data sheets from gas turbine vendors (GE, Alstom, Solar, et al.) 2) Technical specifications and data sheets from electric propulsion system vendors (ABB, Alstom, Siemens, et al.) 3) Technical specifications and data sheets from Pod system vendors (Kamewa, Alstom, ABB, Siemens, Schottel, et al.) 4) Technical specifications and data sheets from various equipment vendors (Kobelco, Kaldair, Hamworthy, Wartsila, Deltak, Aalborg, et al.) 5) Everett C. Hunt, Boris S. Butman, Marine Engineering Economic and Cost Analysis, Cornell Maritime Press, 1995 Lee & Michalski 13