AN INTEGRATED OPTIMISATION PROCEDURE FOR THE DESIGN OF RO-RO PASSENGER SHIPS OF ENHANCED SAFETY AND EFFICIENCY

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1 AN INTEGRATED OPTIMISATION PROCEDURE FOR THE DESIGN OF RO-RO PASSENGER SHIPS OF ENHANCED SAFETY AND EFFICIENCY George ZARAPHONITIS 1, Evangelos BOULOUGOURIS 1, Apostolos PAPANIKOLAOU 1 1 National Technical University of Athens, School of Naval Architecture and Marine Engineering Ship Design Laboratory zar@deslab.ntua.gr, vboulg@deslab.ntua.gr, papa@deslab.ntua.gr ABSTRACT The development of a formalised multi-objective optimisation procedure for the internal compartmentation of Ro-Ro Passenger ships is presented. The objectives of the optimisation are the maximization of ship s survivability after damage, expressed by the Attained Subdivision Index and the vessel s efficiency, in terms of transport capacity and building cost. The developed procedure is based on the integration of modefrontier, a software package for Multi Objective and Collaborative Design Optimisation with NAPA, a well-known naval architecture design software package. It is used herein to generate the internal watertight subdivision based on a set of design variables and to perform the stability assessment after damage based on the probabilistic concept as well as all other necessary naval architectural calculations, including transport capacity and structural weight estimation. Case studies for a Ro-Ro passenger ship were performed to demonstrate the applicability of the above procedure, and characteristic results of these studies are herein presented. 1. INTRODUCTION The introduction of the probabilistic damaged stability regulatory concept (A.265 [1]), about thirty years ago, as an alternative to the deterministic SOLAS 74 requirements, has been considered as a major step towards the rationalization of the procedure of assessing ship s stability following damage. However, the use of this new approach has been until recently very limited, the main reason being that its application is rather complicated and not transparent to traditional naval architects, compared to the traditional deterministic method. The related computational effort is quite significant and can be carried only by use of specialised software programmes. In the meantime, a very important development towards the application of the probabilistic method in daily practice was the introduction of the probabilistic approach in the SOLAS 1990, Part B Regulations for the assessment of the damage stability of dry cargo vessels built after February The next major development towards full acceptance of this method in practice is the forthcoming IMO harmonisation of damage stability rules for all types of ships on the basis of the probabilistic concept, expected to be approved by the SLF46 committee of IMO in autumn Since the design of Ro-Ro Passenger vessels continued over the years to be based on the deterministic rules, there is a lack of experience regarding the use of the probabilistic approach and the eventual impact on ship design (see, however, [2], [3]). This lack of design experience and systematic research, along with the forthcoming harmonization of the damaged stability regulations on the basis of the probabilistic concept by IMO motivated the set-up of an EU funded project on the Probabilistic Rules-Based Optimal Design of Ro-Ro Passenger Ships -ROROPROB [4]. The project, due to be finished in early 2003, aims to develop and implement an integrated design

2 methodology for the optimal subdivision of Ro-Ro passenger ships, based on the probabilistic damage stability regulations. The present paper is related to the work of NTUA-Ship Design Laboratory within the ROROPROB project and refers to the development of a formalised multi-objective optimisation procedure for the internal compartmentation of Ro-Ro Passenger ships, based on the probabilistic approach for the damage stability assessment [5]. The objectives of the optimisation are the maximization of ship s resistance against capsize, expressed by the Attained Subdivision Index and of her transport capacity, in terms of both increased deadweight and garage deck space. Alternatively, the Attained Subdivision Index may be treated as a constraint (in the form Attained Subdivision Index Required Subdivision Index) and the optimisation may be performed with respect to the maximisation of the transport capacity and minimisation of the building cost, an approach closer to a ship-owner s perspective. Building cost reduction is herein considered mainly as the result of steel weight minimization. The reduction of the number of watertight compartments below the subdivision deck is also considered to have a significant impact besides structural weight, also on equipment costs. 2. OUTLINE OF THE PROCEDURE The adopted procedure is based on the integration of a well-known commercial ship design software package (NAPA) and a general-purpose optimisation software package (modefrontier). The vessel s watertight subdivision is automatically generated, assuming the hull form and the main layout concept given, based on a number of design variables and design parameters. For each design variant the Attained Subdivision Index, along with the total vehicles lane length in the lower hold and main garage deck, and the steel weight up to the top of the main garage deck are calculated. The main features of the adopted procedure are outlined in the following. 2.1 Parametric Development of Internal Arrangement Appropriate NAPA macros have been created for the generation of the ship s internal watertight arrangement based on a set of design variables, forming the so-called design space, and in addition on a set of design parameters supplied by the user. The design variables are systematically updated during the optimisation, using appropriate utilities within modefrontier to perform the design space exploration. The user-supplied design parameters are used to define the vessel s intact loading conditions in partial and full draught, and to provide necessary data for a variety of calculations (specific weights for the structural weight calculation, vehicle dimensions for the lanes length calculation etc.). The design parameters are kept constant during the optimisation. Selected quantities may be treated either as design variables or parameters, depending on the user s intentions or the specific requirements of each design case. For example, in the special case were the watertight subdivision optimisation is restricted to the area of the vessel forward of the Main Engine Room, without affecting the aft area compartmentation, the user may treat the corresponding design variables (i.e. the variables defining the aft ship compartmentation) as parameters. Following the generation of the internal layout, the procedure continues with the assessment of each design variant, making full use of the calculation capabilities available within NAPA. Appropriate NAPA macros have been developed to control the damage stability analysis, to calculate the structural weight and transport capacity (both in terms of DWT and lanes length) and to verify the

3 consistency of each design. The optimisation procedure has been developed under the following assumptions: The vessel s main dimensions (length, beam and draught) and the hull form are kept constant during the optimisation. Since the vessel s displacement is fixed, Light Ship variations are compensated by corresponding variations of DWT. The depth up to the bulkhead deck is treated as a free variable. The variation of the vertical centre of gravity VCG in the full load and partial draught condition is taken into account by use of relevant coefficients supplied by the user. A percentage VCG variation between 50% and 60% of the depth variation δd is assumed suitable in usual cases. The structural weight and the corresponding centre of gravity position estimation are based on user-supplied specific weight coefficients pertaining to the various ship zones. A lower hold intended for vehicles transportation may be generated forward of the Main Engine Room (MER). A second lower hold (not intended for vehicles transportation) may be created aft of MER. The existence and extent of both lower holds is controlled by appropriate design variables and design parameters. The user may select a main deck configuration with either central or side casings. In either case, a small aft casing on each side is always generated, to accommodate the passengers staircases, storerooms, auxiliary rooms, etc. usually located in this area (see Figure 1 and 2). The vessel s transport capacity is expressed by the vehicles lanes length, calculated separately for the main deck and the lower hold. The user is expected to define the typical size of the vehicles carried on the main deck and in the lower hold. The final transport capacity is calculated adding the main deck and lower hold lanes length. To account for the possibility that different kinds of vehicles are carried on the main deck and in the lower hold, the lower hold lanes length is multiplied by a user-supplied equivalence coefficient. Downflooding openings may be defined to limit the range of positive stability after damage. To simplify the use of the procedure and to keep the necessary input at this early design stage as simple as possible, only the height of the downflooding openings above the subdivision deck is needed. The user does not have to supply their longitudinal position, or to specify the actual compartments connected by these openings. A number of openings are automatically distributed along the vessel at pre-selected positions so that they are effective in all damage cases. Characteristic designs with both central and side casing arrangements on the main deck, generated by the above procedure, are shown in Figure 1 and 2. Apart of the main casing, small side at the aft end of the main deck can be seen in both arrangements. The position of the transverse bulkheads, the longitudinal and transverse extent of both lower holds, the vertical position and the extent of double bottom and all the other details of the internal arrangement are controlled by the set of design variables. In the studies presented herein 43 design variables are used to define the vessel s internal layout along with 28 design parameters. According to the user s selection, a subset of the design variables is used to define the design space, while the remaining variables are kept fixed during the optimisation. 2.2 Damage Stability Calculations The calculations for the attained subdivision index have been performed according to the probabilistic damage stability concept. Although we are herein dealing with the design of Ro-Ro Passenger vessels, the results presented are based on Regulation 25 of SOLAS Part B-1, actually applicable to cargo ships, instead of using Resolution A-265. Regulation 25 has been selected following a decision by the ROROPROB partners, since it has been considered that the framework of this Regulation is closer to the expected Harmonized Damage Stability Regulations. The

4 developed optimisation procedure can be easily extended to use Resolution A-265 or other probabilistic damage stability formulations considered by the NAPA software package. Figure 1: Design variant with aft lower hold and central casing Figure 2: Design variant without aft lower hold and side casings 2.3 Optimisation Procedure Implementation The coordination of the optimisation procedure is performed using the modefrontier software package, providing the means for the definition and control of the calculation chain and for the integration of the necessary external software packages. A graphical user interface is used for the implementation and review of the optimisation logical scheme (see Figure 3). The input variables,

5 along with their variation interval and the necessary design parameters are defined in relevant input files. Links to the appropriate external applications are established with the help of batch files, permitting modefrontier to control the procedure s execution and to perform the required data transfer between the various directories and/or computers on the network. The selection of the appropriate optimisation scheduler depends on the particular problem to be solved. In our case studies both SIMPLEX and Multiple Objectives Genetic Algorithm (MOGA) have been used. For the analysis of the output of the optimisation procedure, the various options provided by modefrontier (parallel graphs, scattered diagrams and Student plots) were used. The latter are used to evaluate the importance of each input variable with respect to the output values. Figure 3: Logical scheme of the applied procedure for the multi-objective optimisation of the watertight compartmentation of Ro-Ro Passenger ships for enhanced efficiency and safety 3. CASE STUDIES Case studies have been performed applying the above procedure to a sample Ro-Ro Passenger ship (Figure 4). The vessel s particulars and the definition of the two initial loading conditions are presented in Table 1. The calculation of the heeling angles has been limited to 30. No downflooding openings have been defined in the case studies presented herein. The permeability of the garage spaces is set equal to 0.90, for the engine rooms is 0.85 and for the rest of the spaces is set equal to On the main vehicle deck the central casing configuration has been herein selected. 3.1 Multi-objective Optimisation for Maximum A and Transport Capacity As a first case study, the multiobjective optimisation of the above ship for maximum attained subdivision index A, maximum total lanes length and DWT is presented. Due to the assumption of constant draught and displacement, an increase of DWT corresponds trivially to an equal decrease of Light Ship and in particular of Structural Weight. The first two objectives (maximization of

index A and Lanes Length) are actually contradictory, because maximisation of index A requires a dense compartmentation, resulting to shorter lower hold (and lanes length).

0m Breadth 25.0m Depth (reference) 9.100m Design draught 6.550m Full load draught 6.520m Full load displacement 17520t Full load reference GM 2.440m Partial load draught 5.

6 index A and Lanes Length) are actually contradictory, because maximisation of index A requires a dense compartmentation, resulting to shorter lower hold (and lanes length). Minimisation of Structural Weight is also a contradictory objective against the maximisation of A. Figure 4: Hull form of the selected Ro-Ro Pass. Ferry Table 1 Length o.a m Length b.p m Breadth 25.0m Depth (reference) 9.100m Design draught 6.550m Full load draught 6.520m Full load displacement 17520t Full load reference GM 2.440m Partial load draught 5.884m Partial load displacement 14880t Partial load reference GM 1.830m For this particular example the vessel s internal arrangement optimisation is restricted to the area forward of the Main Engine Room, keeping the aft part arrangement fixed. The logical scheme of the optimisation procedure is shown in Figure 5. Seven design variables, describing the compartmentation forward of the Main Engine Room where selected to define the design space (free variables). A constraint for the minimum acceptable value of the attained subdivision index was imposed. The Multiple Objectives Genetic Algorithm (MOGA) optimisation scheduler has been selected for the actual optimisation, and an initial population of 42 designs was randomly generated. The optimisation process was subsequently initiated for 12 generations with a probability of directional crossover of 0.5, probability of selection 0.5 and a probability of mutation of 0.1. A total of 387 designs were created and evaluated. The results of this study are presented in Figures 6 to 10. A gradual increase of the resulting average Attained Subdivision Index for every new design generation can be observed in Figure 9. Simultaneously, a decrease of the resulting average Structural Weight can be seen in Figure 10. Figure 5: Logical scheme of the developed procedure for Case Study 1

7 Figure 6: Scatter diagram of index A vs. Structural Weight Figure 7: Scatter diagram of index A vs. Lanes Length

8 Fig. 8. Scatter diagram of Lanes Length vs. Structural Weight Fig. 9. History diagram of the attained index A

Fig. 10. History diagram of Structural Weight 3.2 Lanes Length Maximization For the second case study, the maximization of Lanes Length is attempted, subject to the constraint A R.

9 Fig. 10. History diagram of Structural Weight 3.2 Lanes Length Maximization For the second case study, the maximization of Lanes Length is attempted, subject to the constraint A R. This case appears formally simpler than the previous one, since there is only one objective function to take care of. However, this type of optimisation scenario is considered to be closer to the classical design requirements of a potential ship owner, as the lanes length has an immediate impact on the economic value of the ship, while the Attained Subdivision Index is required to be just on the limit set by the safety regulations. In this case the compartmentation of both the aft and fwd ship areas were subject to optimisation. The SIMPLEX method (Nelder and Mead [6]) was herein used for the optimisation process, starting with an initial population of 12 designs. The obtained results are presented in Figures 11 to 13: Commenting on the results of this study, it is observed that the procedure converges after about 160 iterations leading to a design solution of maximum total Lanes Length, while the index A converges to it s the set lower limit R.

10 Figure 11: Lanes Length History Chart Figure 12: Index A History Chart

11 Figure 13: Scatter diagram of index A vs. Lanes Length 4. CONCLUSIONS A multi-objective optimisation procedure has been presented, aiming to assist the designer of Ro- Ro Passenger ships in the preliminary design stage, when the lay-out of the internal watertight subdivision is investigated, considering the impact of recent damage stability regulations and aspects of efficiency and building cost. The developed procedure is based on the integration of modefrontier, an environment for Multi Objective and Collaborative Design Optimisation with NAPA, a well know and versatile naval architectural design software package. Results from the application of the above procedure revealed its potential as a useful and practical design tool, enabling the designer to assess systematically and in very short time hundreds of alternative layouts, subject to a variety of constraints and objective functions related to ship s efficiency and safety. The developed procedure can be used either to generate from scratch a vessel s internal subdivision, or to improve significantly an existing design. It allows the designer to gain a better overview of the design space and to obtain a better compromise of the contradicting design objectives. Despite the huge amount of calculations for a vessel s damage stability assessment according to the probabilistic approach, the required calculation time is not greater than 1.5 minutes (even for the larger vessels) using a PC with Pentium IV microprocessor at 2.4GHz, with a total of about 2.5 minutes calculation time for the evaluation of each vessel. Given that in some cases convergence requires the evaluation of more than 500 designs, application of the above procedure may take about one day of continuous calculations. The extension of the above procedure to other types of ships appears straightforward, especially to ships with less complicated arrangements, like cargo ships, as their compartmentation can be generated by significantly less design parameters.

12 ACKNOWLEDGMENTS This work has been partially supported by the EU funded project on the Probabilistic Rules-Based Optimal Design of Ro-Ro Passenger Ships ROROPROB, Contract Number G3RD-CT The authors are solely responsible for the contents of the paper, which does not represent the opinion of the Community. The Community is not responsible of any use that might be made of data appearing therein. REFERENCES 1. RESOLUTION A. 265 (VIII), "Regulations on subdivision and stability of passenger ships as an equivalent to Part B of Chapter II of the Int. Convention for the Safety of Life at Sea, 1960", IMO Sen, P., Gerigk, M.K., "Some aspects of a knowledge-based system for preliminary ship subdivision design for safety", Proc. PRADS 92 Conference, Vol. 2, pp , Newcastle upon Tyne, Sen, P., Wimalsiri, W.K., "Ro-Ro cargo ship design and IMO subdivision regulations", Proc. 2 nd Henri Kummerman Foundation Conference on Ro-Ro Safety and Vulnerability, The Royal Institution of Naval Architects, May Probabilistic Rules-Based Optimal Design of Ro-Ro Passenger Ships ROROPROB, Contract Number G3RD-CT Zaraphonitis, G., Boulougouris, E., Papanikolaou, A., "Development of an Optimisation Procedure for Passenger Ro-Ro Ship Subdivision ", ROROPROB Project Report, NTUA-SDL, Athens, Aug Nelder, J.A., Mead, R., "A Simplex Method for Function Minimization", Computer Journal 7 (1965) NAPA Release , NAPA Oy, Helsinki, Finland 8. modefrontier, Version 2.5.0, ES.TE.CO., Trieste, Italy

Optimization of Arrangements of Ro-Ro Passenger Ships with Genetic Algorithms

1 Introduction Optimization of Arrangements of Ro-Ro Passenger Ships with Genetic Algorithms Evangelos K. Boulougouris, Apostolos D. Papanikolaou, George Zaraphonitis NTUA-SDL 1 Ships are complex and multifunctional