Model Tests on Propulsion Systems for Ultra Large Container Vessel

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1 Proceedings of The Twelfth (2002) International Offshore and Polar Engineering Conference Kitakyushu, Japan, May 26 31, 2002 Copyright 2002 by The International Society of Offshore and Polar Engineers ISBN (Set); ISSN (Set) Model Tests on Propulsion Systems for Ultra Large Container Vessel Se-Eun Kim and Soon-Ho Choi SSMB, Samsung Heavy Industries Co., Ltd. Daejeon, Korea Tomi Veikonheimo ABB Azipod Oy Helsinki, Finland ABSTRACT Model test results for Ultra Large Container Vessel with three different propulsion systems are presented. The single screw, twin screw and CRP Azipod system were tested for 9000TEU container ship at Samsung Ship Model Basin of Samsung Heavy Industries, and power prognosis for 12000TEU container ship was done based on the test results. The CRP Azipod system was proven to be the most efficient propulsion system for ships having this type and size. Power savings to twin screw solution were about 9% and to single screw were about 5%. This propulsion solution can be one serious alternative when selecting propulsion system for Ultra Large Container Vessel. KEY WORDS: Container; 9000TEU; 12000TEU; ULCV; Propulsion System; Single; Twin; CRP Azipod INTRODUCTION Because the operating cost of container vessel decreases according to the increase of container capacity, the growing demand to increase the size and therefore the capacity of container vessels has recently been the reason for so many investigations and research projects (Figure 1). Samsung Heavy Industries Co., Ltd. has already developed 9000TEU container ship and the construction of this ship is now deeply discussed with ship owners. The 9000TEU container ship might be the largest one where the conventional single propeller and single main engine can be installed. It is because the conventional single screw propulsion has technically reached the limits due to engine power, propeller loading, propeller weight, hull vibration, maneuverability, etc. Therefore twin skeg hull with twin screw or CRP Azipod are the future solution. Samsung and ABB agreed to develop the Ultra Large Container Vessel (ULCV) with CRP Azipod propulsion system for 12000TEU container vessel. This co-operation included ship design, market studies, cost evaluations, machinery optimization and model tests. Samsung had already developed 9000TEU container vessel with single screw and twin screw propulsion, now also CRP Azipod was added (Picture 1). It was decided to perform comparison tests with this container vessel size, because the hull designs were already prepared and they were enough to differentiate the performance characteristics for the three systems. This paper deals with model test results for 9000TEU vessel with three different propulsion options. They are single screw, twin screw and CRP Azipod system. All the tests were performed at Samsung Ship Model Basin (SSMB) in Daejeon, Korea Insure Crew Re pair Fue l Port Capital Total USD/TEU 168 5,600 6,200 7,400 9,000 12,000 (CRP Concept) Container Capacity (TEU) ,200 TEU x 2 12,000 TEU x 1 Evaluation Basis - Ship Speed : 25 kts - Duration : One(1) year COST / TEU (%) Figure 1 Operating Cost Comparison of ULCV

2 conventional rudder was replaced with Azipod and the Azipod unit was considered as a separate propulsor. But because of different aft hull design the wetted surface area has somewhat increased in twin screw solution. In the following pictures the propulsion systems and after hull forms are shown. Single Screw hull The hull design is very conventional for this ship type. Picture 1 CRP Azipod system for ULCV And speed-power prognosis for 12000TEU container vessel with these propulsion systems were also done based on the test results of 9000TEU. The main goal of this model test program was to determine the most efficient propulsion system for ULCV. SCOPE OF MODEL TESTS Natural choice was that the tests were going to be made in Samsung's own towing tank at SSMB. The dimensions of the towing tank are 400m(L) 14m(W) 7m(D). SSMB has all the equipment for pod propulsion testing and it is one of the most advanced model basins in the world. It was decided to test all three propulsion alternatives during one short test period in order to make the best possible comparison. The test program included resistance and self-propulsion tests for all three hull variants. Wake survey for designing propellers and open water test for pod and propellers were also performed. This program didn't include cavitation or maneuvering tests. These tests will be done later. Picture 2 Single Screw after ship hull form Twin Screw hull Samsung designed optimum twin screw after ship hull form with two slender skegs. SHIP HULL VARIANTS The hulls tested were designed and optimized for each propulsion alternative. CRP-Azipod solution does not need to have special after hull design because the rudder can be replaced with Azipod propulsion unit and therefore the modifications to the hull will be only local. The hull design work was carried out by Samsung and also the propellers for single screw and twin screw ships were designed by SSMB. The main propeller and Azipod propeller design for CRP Azipod were done by ABB. All the ship and propeller models were manufactured by SSMB. Table 1 Ship and Propeller Main dimensions LPP [m] B [m] T [m] S [%] Dp [m] Z SINGLE CRP /6.4 4/5 TWIN In Table 1 the differences of the hull variants and propellers can be seen. The single screw and CRP Azipod hulls were the same, but the less wetted surface area of CRP Azipod came from the fact that Picture 3 Twin Screw after ship hull form CRP Azipod hull In this option, exactly the same hull form as for single screw has been used and only rudder has been replaced with Azipod unit. 521

3 Picture 4 CRP Azipod after ship hull form self-propulsion tests. In case of the CRP Azipod hull, however, there are two propellers and it is necessary to find the optimum rpm ratio where the two propellers interact the most effectively. The optimum rpm ratio is to be determined in behind ship condition rather than in open water condition, therefore the open water test of CRP Azipod was done after the rpm ratio optimizing test. Because it was incorrect to decide the optimum rpm ratio by direct calculation of power such as 2πNQ from the measured torque, selfpropulsion tests were performed at design speed with different rpm ratio and the optimum value was determined after analyzing the test results which give the best efficiency. Here, the efficiency was deduced as J M /2π K TM /K QM where subscript M means model scale value. Though this efficiency was exaggerated and different from the real full scale value, it could discriminate the optimum point. Figure 2 shows the result of rpm ratio optimization test. When the rpm ratio of Azipod to main propeller was 1.0, the propulsive efficiency was slightly less than that at But because propulsion power ratio for main and Azipod propeller was nearly the same as the designed ratio of 70% and 30%, rpm ratio of 1.0 was selected as an optimum one. DESIGN OF CRP AZIPOD SYSTEM The responsibility of this task was given to ABB, because they have the best experience in this field. Main propeller design was in so close relation with the Azipod propeller design that this was also designed by ABB and Krylov. As a basis the single screw ship was taken. The wake measurement was made for the propeller design of CRP Azipod system. Total installed propulsion power was kept the same as for single screw ships and the power ratio was selected to be 70% for the main propeller and 30% for the Azipod propeller. The reason for this power ratio was to have optimum pod size and type in respect of cost and hydrodynamic efficiency. The optimum power ratio would be 50% for each propeller, but this leads to very high power for Azipod and also the electric losses would be higher, thus reducing the total efficiency of whole system. The propulsive performance is of equal importance with the cost of this whole propulsion system when it is compared to single and twin screw variants. After first calculations and predictions the power ratio and optimum propeller diameters for both propellers were chosen. The blade numbers were 5 for Azipod propeller and 4 for main propeller. Because Azipod propeller is built-in type, it is difficult to have more than 5 blades on the hub, so the Azipod propeller blade number was fixed and best choice for main propeller was the 4 blades. The final design of both propellers was done in Krylov Shipbuilding Institute in St. Petersburg. Because their experience in pod propulsion and CRP systems is very high. Power Ratio (%) Revolution Ratio (Pod/Main) Figure 2 RPM Ratio Optimization Efficiency Azipod Self-propulsion test was performed with the rpm ratio determined above, and propeller open water test was also done with the optimum rpm combination of main propeller and Azipod propeller. Picture 5 shows the set-up of POW test for CRP Azipod. Main Efficiency MODEL TESTS AND RESULTS Though the same hull forms of 9000TEU containers which had been developed two years ago by Samsung were used, model tests for single screw and twin screw hulls were repeated in order to compare their propulsion systems accurately with CRP Azipod. At first, wake survey for single screw hull form was done prior to the main tests in order to design the propellers of CRP Azipod. For single and twin screw hull forms, their own propellers were used, and resistance and self-propulsion tests were performed according to the standard method of SSMB. Standard method of model test and analysis of SSMB is that the ITTC 1957 model-ship correlation line is used and the scale effect is corrected based on the 1978 ITTC performance prediction method. And the Froude s method is utilized for the analysis of resistance. Propeller open-water test is usually performed before the Picture 5 POW test for CRP Azipod 522

4 In Figure 3, the test results have been presented. In NCR condition with 15% sea margin, the total efficiency for CRP Azipod system was 9 % better compared to twin screw and 5 % better compared to single screw option in BHP base. When brake horsepower was evaluated, 8% of electrical loss for Azipod system was considered. In this figure it is clearly seen the difference between delivered power and total propulsion power including all losses. The estimated loss for conventional shaftline in all cases has been estimated to be 1.5%. For Azipod the losses are much larger. All electrical losses including generators, transformers, cabling etc are estimated to be 8 % from the power delivered to Azipod drive. Even if the electrical losses are larger for electric side, they are only affected to the 30% of total installed propulsion power. Therefore the transmission efficiency (η t ) of CRP Azipod was and η t for single or twin screw hulls was The CRP Azipod concept is clearly the most efficient system for this size of vessels and beyond. PROGNOSIS FOR 12000TEU Figure 3 Powering Curves for 9000TEU In Table 2, the speed comparison of the final speed in trial and service condition is shown. Single screw and CRP Azipod ships have the same propulsion power but the engines that are selected for twin screw ship has slightly higher power. Table 2 Speed Comparison Vs trial [kn] Vs service [kn] NCR [%] SINGLE CRP TWIN Based on the test results of 9000TEU, speed-power prognosis for 12000TEU container ship was done. The main dimensions of the ships and the propellers are shown in Table 3. Though the single screw ship for 12000TEU is unrealistic because of engine capacity, prognosis was done here under the assumption that the engine of such capacity exists. The prognosis results of 12000TEU were presented in Figure 5. The speeds for the three different propulsion systems were evaluated in NCR condition with 30% sea margin. 15% sea margin was applied before to 9000TEU container vessel to achieve the service speed of more than 25.5knots with conventional ship hull form and with single engine. Ship owners, however, tend to have more sea margin for large container ships. Therefore 30% sea margin was used for 12000TEU container ship. The difference in total efficiency for each system is similar with 9000TEU. In case of CRP Azipod system, though the same number of blade was assumed for this prognosis, CRP Azipod with the combination of five and six blades seems to be more reasonable considering propeller load on blade. Table 3 Ship and Propeller Main dimensions LPP [m] B [m] T [m] S [%] DP [m] Z SINGLE CRP /6.5 4/5 TWIN In the following Figure 4, the comparison of all propulsion alternatives in regard of propulsive power has been compared at 25.5 kn (NCR) with 15 % sea margin. Figure 4 DHP-BHP Comparison Figure 5 Powering Curves for 12000TEU 523

5 CONCLUSIONS Powering performance of three propulsion systems such as single screw, twin screw and CRP Azipod for ULCV was investigated by a series of model tests for 9000TEU container ships at SSMB. The model test results clearly indicates that the CRP Azipod concept is one serious alternative when propulsion system is selected for Ultra Large Container Vessels. This concept has many advantages compared to single and twin screw alternatives. One clear advantage is improved maneuvering capability in restricted areas such as harbors and coastal areas. Also redundancy aspect will be more pronounced factor in future when propulsion systems for UlCVs are to be considered. Next step what has to be taken in near future is the cavitation phenomenon with small steering angles. According to previous studies there is no problems to be expected, but still careful studies has to be made. SSMB is now preparing similar model tests for 12000TEU container ship and the present prognosis can be confirmed later. And maneuvering test for CRP Azipod will also be done this year. REFERENCES Helasharju, Heikki (1996). Model Tests with Rudder Propulsion System, VTT Symposium 168, Espoo, Finland. ITTC (1999). Final report on recommendations to the 22 nd ITTC, the Special Committee on Unconventional Propulsors, 22 nd ITTC, Seoul and Shanghai. Van, Suak-Ho and Yoon, Hyun-Se (2000). Investigation on the Powering Performance Prediction for Azimuth Thrusters, Journal of Ship & Ocean Engineering, Vol. 30. Kheisin, DE (1992). "Dynamics of Ice Cover Interacting with Ocean and Atmosphere," Int. J. of Offshore and Polar Eng., ISOPE, Vol. 3, No 1, pp Kim, Se-Eun and Choi, Soon-Ho (2001). A study on the Propulsion Systems for Ultra Large Container Vessels, SSMB Report SSMB-S Kim, Se-Eun, Choi, Soon-Ho and Veikonheimo, Tomi (2001). Propulsion System for Ultra Large Container Vessel, Proceedings of the Annual Autumn Meeting, SNAK, Seoul, pp