Analysis of Operational Effi ciency of the Proposed Propulsion Systems for Selected Large RoPax Vessel

This paper presents the characteristics of ferry shipping with particular emphasis on large RoPax vessels operating in the Baltic Sea. A critical review of main propulsion system used on large RoPax ferries has been done. Optimal propeller parameters and required brake power have been estimated on the basis of total resistance of bare hull and appendages approximated according to Holtrop-Mennen method. Main engines and generating sets have been selected for minimized fuel consumption approximated with quadratic regression. Operational parameters and costs of analysed large RoPax main propulsion systems have been compared.


INTRODUCTION / Uvod
Ferry shipping has been an extremely important component of international transport system for decades. Due to their functions, in particular complementary role in respect of the existing routes of land transport and shore outline, ferry routes are largely limited to sea basin with a highly fragmented shoreline [1]. The Baltic Sea is a leading market for ferry services where approximately 17% of international ferry fl eet is used [2]. These specifi c conditions prejudge local advantage of ferry shipping both over land transport and container shipping which is the most popular on a global scale. Implementation of horizontal loading (Roll-On/Roll-Off ) of passenger cars and trucks, semi-trailers, wagons and roll-trailers, etc. on board has greatly contributed to facilitation of loading and unloading of ferries while indirectly leading to the decrease of the transport distance, cost and time as well as to the elimination of timeconsuming container handling.
The most popular vessels in contemporary ferry shipping are RoPax (Roll-On/Roll-Off -Passenger) vessels with separate decks for passengers and rolling cargo. The fi rst RoPax ferries were constructed as a result of conversion of the existing RoRo ferries with an expansion by a passenger section, whereas the following models were designed bottom-up by adjusting vessel specifi cation to the characteristics of a given route [3]. International trans-Baltic routes are currently dominated by large RoPax vessels with a gross tonnage above 40,000 [2].
Nowadays the development of propulsion systems is aimed primarily at energy effi ciency and reduction in emissions of harmful substances. Chapter 4 of the MARPOL Annex VI, put into eff ect in July 2011, obliged shipowners to use technical solutions to reduce carbon dioxide (CO2) emissions. All vessels over 400 GT built as from January 2013 are subject to the Energy Effi ciency Design Index (EEDI). The standard puts a cap on the amount of CO2 allowed per unit of transport work. Until 2025, ships are required to achieve a 30-percent reduction in their CO2 emissions compared with the average emissions of ships built between 1999 and 2009. The EEDI value calculated in accordance with the procedure shown in Figure 1 must be less than or equal to the value required for the type and size of vessel [4]. In addition, according to the fi ndings of the 75th session of the Marine Environment Protection Committee (MEPC), from 2023 all in-service vessels are planned to be subject to minimum energy effi ciency standards, as defi ned by the EEDI-equivalent Energy Effi ciency Existing Ship Index (EEXI) [5].
However, CO 2 is not the only substance restricted by MARPOL Annex VI. The existing IMO limits on nitrogen oxide NO X emissions, and fuel sulphur content, are shown in Figure 2. Continually decreasing and territorially expanding restrictions also apply to nitrogen oxides NO X and sulphur oxides SO X . They are strictly defi ned in the current IMO Tier II and Tier III standards. Tier II is global, while the range of the more restrictive Tier III is currently limited to the Baltic and North Sea, and the parts of North American coastal waters. The sulphur content reduction to 0.1%, met by e.g. Marine Gas Oil (MGO) and Liquifi ed Natural Gas (LNG), has also applied in the territorial seas of the European Union Member States [6].
Compliance with MARPOL Annex VI regulations is an important reason why propulsion systems are still being modernised. One of the basic issues having fi nancial consequences is the operational effi ciency of the propulsion system. In the design of RoPax vessels, the current priority is the common use of combined propulsion systems [3,7].
The aim of this paper is to compare the operational effi ciency of diff erent main propulsion systems for large RoPax vessels, using certain energy and economic indicators calculated relative to those of the traditional CODAD main propulsion system.
Calculations of resistance based on geometrical data for the m/f Finnstar (45,923 GT) hull were performed by the Holtrop-Mennen method, due to the ease of taking account of diff erences in resistance caused by appendages. Performance parameters of engines and generating sets were approximated by quadratic regression using data provided by the manufacturers.

PROPULSION SYSTEMS OF THE LARGE ROPAX FERRIES / Porivni sustavi velikih RoPax trajekata
All European large RoPax ferries (i.e. over 40,000 GT), including both those in operation and those on order books, are still equipped with one of the following propulsion systems [8]: -Traditional, diesel-mechanical CODAD (Combined Diesel and Diesel); -Diesel-electric CODEL (Combined Diesel-Electric); In each of the aforementioned cases, medium-speed engines are used. Despite having lower power and effi ciency than low-speed engines, these off er signifi cantly lower height, which is particularly important with regard to the need to reduce the engine room height and improve stability. The CODAD propulsion system has been associated with RoPax vessels since the 1960s, when the fi rst such units were  [6] brought into service, and is still the predominant solution. In the most common variant, each pair of main engines (of the same type and with the same or diff erent power) drives one controllable pitch propeller shaft through a reduction gear. Behind the controllable pitch propeller there are a rudder blade and open shaft lines outside the hull separated by a stabilizer keel ( Figure 3) [3,9].
The use of four main engines allows their load to be adjusted in a fl exible manner to the required brake power. As a result, engine run time can be maximised within a load range similar to the optimum range [3]. If brake power demand is reduced, engines 3 and 4, connected to the reduction gear with a long shaft, are shut down, so that engines 1 and 2 ( Figure 4) may be properly loaded and the entire system avoids ineffi cient operation at low load range. CODAD is mainly characterised by [3,11]: -Moderate investment costs; -Adoption to longer (e.g. trans-Baltic) shipping routes during which operation under contract load is predominant; -Diffi culty in maintaining the main engines within their optimum load range -at partial loads specifi c fuel consumption increases; -Necessity to place the entire engine room in aft part of the hull below waterline. The CODEL propulsion system ( Figure 5) is implemented in e.g. two existing large RoPax vessels (m/f Megastar 49,000 GT and m/f Viking Grace 59,565 GT) serving short routes between ports in the Gulf of Finland, where, due to the route characteristics, operation under contract load is restricted to approximately 20% of shipping time [7,13]. Moreover, one of Tallink's large RoPax ferries currently under construction (m/f MyStar 49,000 GT) is also to be equipped with CODEL [17]. In this system, diesel generating sets with medium-speed engines powering synchronous generators produce electrical energy supplying two synchronous motors of a fi xed pitch propeller system with the use of transformers and frequency converters. In comparison with CODAD, the CODEL system is characterised by [14]: -Maintaining optimum load of combustion engines regardless of the vessel's speed; -Easiness of automation; -Application of more effi cient fi xed pitch propellers instead of controllable pitch propellers; -No mechanical connections between the diesel generating sets and the synchronous motors, which allows engine room to be relocated outside of the standard area in aft part of the hull to a suitable place where it is possible e.g. to increase cargo space or reduce the hull's dimensions (and the required brake power) while maintaining current brake power; -Reduction of vibrations on board to considerably increase crew and passenger comfort; -Signifi cant losses (by about 8 ÷ 9%) in electrical energy transmission from generators to the propeller motor; -Installation in a hull of the same form and exterior design. In this paper the CODED-CRP (Combined Diesel-Electric and Diesel-Mechanical -Contra-Rotating Propeller) system is also considered. Although this system has not been used in large RoPax ferries, it has been successfully implemented in smaller twin Coastal Ferry vessels (m/f Akashia and Hamanasu, 16,810 GT). An illustration of a RoPax equipped with CODED-CRP is shown in Figure 6.
In the CODED-CRP hybrid propulsion system, a pair of medium-speed engines drives a controllable pitch propeller through a reduction gear. In its axis is a podded azimuth thruster (Azipod) with a contra-rotating fi xed pitch propeller, which utilises some of the energy of the circular movement of water generated by the controllable pitch propeller. Electrical energy for propulsion purposes is generated by diesel generating sets with medium-speed engines powering the Azipod low-speed synchronous motor through transformers and frequency converters [3]. This system consists of a dieselmechanical part with a design and principle of operation fully equivalent to those of CODAD, and a diesel-electric part with a design and principle of operation corresponding to a CODEL variant used in Azipod-equipped cruise ferries. Both sections may be operated simultaneously or separately as required. Moreover, the hull of a vessel with such a propulsion system has a form similar to the hull of single-propeller vessels. An exemplary CODED-CRP confi guration is shown in Figure 7.
In comparison to both previously discussed propulsion systems, CODED-CRP is characterised by very high investment costs and advantages mentioned in Table 1 [8]: The basic type of RoPax vessel propulsion system and a reference for other system is the CODAD. Alternative design solutions may be introduced with regard to the characteristics of a given route, e.g. low share of shipping at contractual speed in total sailing time or the need to increase cargo space at limited hull dimensions.

RESISTANCE CALCULATIONS AND SELECTION OF THE PROPULSION SYSTEM ELEMENTS / Izračuni otpora i odabir elemenata porivnog sustava
The values of eff ective power and parameters of the propulsion system were estimated using the Holtrop-Mennen method for the three above-mentioned variants. The total resistance is considered to be a sum of the following components [16,17]: (1) Viscous resistance of bare hull and appendages in (1) as a sum of their frictional and viscous pressure resistance were determined through use a form factor [16,17]: (2) and: (3) R B and R S components shown in (1) were omitted in calculations as they represent on average only (0.02÷0.025)% of total resistance [17], thus: where: , kN -Total resistance BH , kN -Viscous pressure resistance of bare hull -Form factor of bare hull BH , kN -Frictional resistance of bare hull according to the ITTC-57 formula , kN -Viscous pressure resistance of the appendages -Form factor of the appendages , kN -Frictional resistance of the appendages according to the ITTC-57 formula , kN -Wave resistance , kN -Model-vessel correlation resistance (incl. such eff ects as hull roughness of k S = 150 μm and air drag in conditions of 2 in the Beaufort scale) , kN -Additional pressure resistance of bulbous bow near the water surface , kN -Additional pressure resistance due to transom immersion The estimation was based on actual parameters of the hull of m/f Finnstar ferry equipped with the CODAD provided in Table 2. External and longitudinal section view of m/f Finnstar are presented in Figure 8.
E stimates were made of the total resistance of the bare hull, which is the same for all of the analysed propulsion system  Block coeffi cient C B -0.5662 11.
Service speed V s kn 25 Source: DNV -GL [12] designs, and the resistances of appendages protruding outside the hull, required for a given propulsion system. These two components were added to obtain the value of the vessel's total resistance. The most important values related to the resistance and eff ective power of a potential vessel with any of the analysed propulsion systems are given in Table 3. Furthermore, values corresponding to sea trial conditions were increased by a sea margin of 15% and an engine margin of 10% to obtain service and maximum values in nominal conditions.
Appendages viscous resistance of a vessel equipped with CODED-CRP is lower by 38.4% (equivalent to 39.68 kN) than the values for CODAD and CODEL, resulting from a smaller wetted surface area and lower form factor of appendages (Table 3), leading to a lower total resistance and effective power for the same service speed under identical sailing conditions. A comparison of appendages viscous resistance is presented in Figure 9.  Using the estimated values of eff ective power, optimum geometric and operating parameters of propellers as well as values of effi ciency of propulsion systems with components and the required brake power were calculated according to the following formulae [16,19]: CODAD propulsion system: where: , kW -Brake power , kW -Eff ective power -Total effi ciency of propulsion system -Hull effi ciency -Open water propeller effi ciency -Propeller rotative effi ciency -Shaft line effi ciency -Gearbox effi ciency Effi ciencies of the components of CODAD propulsion system are shown in Figure 10. where: -Generator effi ciency -Effi ciency of electrical energy transmission from generator to motor Effi ciencies of the components of CODEL propulsion system are shown in Figure 11. Figure 11 Effi ciencies of the CODEL propulsion system components Slika 11. Učinkovitosti komponenti CODEL porivnog sustava Source: Own study on basis [14] CODED-CRP propulsion system: hence: (8) where: -Total effi ciency of diesel-mechanical part of CODED-CRP -Total effi ciency of diesel-electric part of CODED-CRP The eff ective power of a hull equipped with CODED-CRP is split into components to estimate the effi ciency and the required brake power, corresponding to the diesel-mechanical and the diesel-electric parts of the system, which have diff erent parameters. According to Wärtsilä [15], to ensure the highest energy effi ciency, the power should be divided equally between the controllable pitch propeller and fi xed pitch propeller. Due to the impossibility of free adjustment of the required power and rotational speed of the low-speed synchronous motor of the Azipod, however, the maximum benefi ts are not always achievable. Aiming to approach a value of 50% of expected propulsion power and to increase the rotational speed and reduce the propeller diameter by 20% relative to the controllable pitch propeller to avoid cavitation, the Azipod XC1800, with a rated brake power of 13,500 kW and a rotational speed of 195 rpm, was selected [21]. Multiplying the rated brake power of the Azipod by the product of the effi ciencies in this section of the propulsion system, according to equation (6), a correct value of eff ective power was obtained. By subtracting the latter value from the total value, it was possible to obtain a correct value of eff ective power for the diesel-mechanical part (9) and to determine the power split according to formulae (7) and (8).
Major geometric and operational parameters of all propellers are shown in Table 4: Calculated energy indicators of analysed propulsion systems were discussed below and then shown in Table 5. Hull effi ciency is a ratio between eff ective and thrust power which the propeller delivers to the water, determined as a quotient of thrust deduction coeffi cient and wake fraction coeffi cient [9,16,22]: (10) where: t -Thrust deduction coeffi cient w -Wake fraction coeffi cient Open water effi ciency is a ratio of the thrust power to the power absorbed by the propeller operating without a hull attached, i.e. in open water, determined in relation to thrust loading coeffi cient [9,16,22]: (11) where: C Th -Thrust loading coeffi cient Relative rotative effi ciency describes a ratio between effi ciency of the propeller behind hull and in open water conditions. Value of this parameter was obtained from formula for twin-screw vessels [9,16,22]: (12) where: C P -Prismatic coeffi cient (Table 1) C B -Block coeffi cient (Table 1) -Propeller pitch ratio (Table 3) Shaft line and gearbox effi ciency were taken as an average constant values of these parameters for twin-screw vessels. Generator and electrical energy transmission effi ciency ( Figure  9) are values declared by the manufacturers [14,23,24].
Total effi ciency of diesel-mechanical and diesel-electric parts were obtained according to (5) and (6), which are equivalent to CODAD and CODEL total effi ciency. However, CODED-CRP total effi ciency was considered to be a weighted average of the effi ciency of both mentioned parts according to (7).
Main engines and diesel generating sets were selected for the capacity to ensure the required brake power, minimise fuel  oil consumption and, in the case of the controllable pitch propeller powered by the CODAD and linear (L) cylinder alignment, reduce the volume of engine rooms. Selected models and the values of specifi c fuel oil consumption (SFOC) under service load in ISO ambient conditions are presented in Table 6 [23,24].

RESULTS AND DISCUSSION / Rezultati i rasprava
Comparative analysis of operational effi ciency of proposed variants of large RoPax propulsion systems was carried out using the selected energy and economic indicators provided in Table 7 which were referred to the standard CODAD propulsion system [8].  Table 6 and shown in Figure 12, it was concluded that a vessel equipped with the CODED-CRP propulsion system has a total propulsive effi ciency higher by 6.09 p.p. (9.37%) than the effi ciency of CODAD, assuming a value of 0.7107 comparable to that of the direct-drive propulsion system with low-speed engine and fi xed pitch propeller. Moreover, both the diesel-mechanical section and the diesel-electric section of the CODED-CRP system have total effi ciencies higher than the effi ciency of CODAD (Table 4), respectively by 4.81 p.p. (7.4%) and 8.09 p.p. (12.45%), as a result of a considerably higher value of the product of the hull effi ciency and relative-rotative effi ciency (Table 4), which easily off sets the lower effi ciency of the propellers and losses in the generation and transmission of electrical energy. This is due to a more streamlined hull formvery similar to the form used for single-propeller vessels -and the utilisation of part of the energy of the circular movement of water (generated by the controllable pitch propeller) by the Azipod's fi xed pitch propeller. The CODEL system has total propulsive effi ciency lower by 0.64 p.p. (0.98%) than that of CODAD, yet it is still comparable. This results from the identical total resistance (Table 2) and losses in the generation and transmission of electrical energy that slightly exceed both the power transmission losses in the CODAD system and the gain from the fi xed pitch propeller, which has slightly higher effi ciency when installed in this system.
To sail with a service speed of 25 knots, CODED-CRP requires brake power of 30,310 kW, which ensures savings up to 3,746 kW (11%) relative to CODAD, with demand lower by 344 kW (1.01%) than that of CODEL. These results, presented in Figure  13, are obtained directly from the aforementioned diff erences of eff ective power and total effi ciency of the propulsion systems. Tot al installed power is the sum of the rated powers of all engines driving the propellers and generators producing electrical energy for propulsive purposes. On the basis of a comparison of the data given in Table 6 (in accordance with  Table 5) and in Figure 14, it was concluded that CODED-CRP requires lower installed power than CODAD by 4,560 kW (11.88%) for operation under service load while maintaining the engine margin. CODEL requires diesel generating sets with a sum of rated powers equal to 38,720 kW, exceeding the CODAD system's power by 320 kW (0.83%).
CODE L has the lowest specifi c fuel oil consumption for MGO (Marine Gas Oil) in ISO ambient conditions, amounting to 170.05 g/kWh, which is lower by 3.9 g/kWh (2.24%) than the CODAD system's consumption (Figure 15). Such a low value was obtained using Wärtsilä V31 engines (Table 5) powering a synchronous generator. Their record-breaking minimum specifi c fuel oil consumption is equal to 167.7 g/kWh at a rated load of 85%. A pair of engines of this series is used similarly in the CODED-CRP system, for which this parameter, being the weighted average of the values for the diesel-mechanical and diesel-electric sections, amounts to 171.47 g/kWh. This ensures savings up to 2.48 g/kWh (1.63%) relative to the CODAD system, which is based entirely on Wärtsilä 8L46F engines with lower energy effi ciency.
Daily and annual fuel oil consumption are the products of the specifi c fuel oil consumption and attained brake power with time, expressed as a number of hours or days. In the case of the second indicator it was assumed that the vessel is used for 2/3 of the year under service load. The CODED-CRP system consumes 124.73 t/day ( Figure 16) and 30,350.9 t/year of fuel, which allows 17.45 t/day and 4,246.2 t/year (12.27%) to be saved relative to the CODAD system. These diff erences arise from lower values of specifi c fuel oil consumption and service brake power. In the CODEL system the savings result only from   [25] was used to compare costs of fuel oil consumption. Both alternative propulsion systems allow the saving of some part of the amount corresponding to CODAD's fuel oil consumption per hour, which results from the fact that they are its multiples. The CODED-CRP system generates savings up to 9,597.5 USD/day and 2,335,391 USD/year. The CODEL system's savings are nearly tenfold lower, at 995.5 USD/day and 242,238 USD/year.

CONCLUSIONS / Zaključci
The c alculation results presented in Table 7 indicate that the brake power of a large RoPax with CODED-CRP required for operation at service speed will be lower by 11% than for the same vessel equipped with CODAD, due to lower total resistance (by 2.7%) and higher total propulsive effi ciency of the propulsion system (by 9.73%). Therefore, total installed power will also be lower (by 11.88%), as will the consumption and costs of fuel oil (by 12.27%).
What is more, a large RoPax vessel equipped with CODED-CRP also off ers better manoeuvrability (the entire Azipod thruster power may be used for steering) and greater capacity to maintain higher effi ciencies of the propulsion system in the part-load range (the electric propulsion is better adapted to load variation, and generators producing electrical energy for the Azipod may also replace the auxiliary propulsion to avoid ineffi cient operation in a low load range).
Nevertheless, the costs of construction of a ferry equipped with the CODED-CRP system are signifi cantly higher (especially due to the purchase and installation of the Azipod thruster) than the costs of the standard CODAD system, but this type of main propulsion system entails a series of benefi ts, presented in Table 1, which enable investment expenditure to be recovered in the course of long-term service. On this basis it is concluded that the CODED-CRP system, which combines the advantages of CODAD and CODEL, may be the best solution both for ferries serving long routes with a large proportion of operation time under service load, and for ferries serving short routes with a negligible proportion of operation time under service load. The CODEL propulsion system is selected only for ferries serving short routes running mainly under reduced load. Generating low savings under service load, it cannot enable increased investment expenditure to be recovered over a standard 20-year ferry service period on the long routes dominated by CODAD vessels, which still prevail in current order portfolios. It should be expected, however, that the use of CODED-CRP in newly built vessels will become more frequent with the development and growing popularity of hybrid propulsion systems as a consequence of increasingly restrictive emission limits for harmful substances.