Hydrodynamic Trends in Ferry Design

Transcription

1 11 th International Conference on Fast Sea Transportation FAST 2011, Honolulu, Hawaii, USA, September 2011 Hydrodynamic Trends in Ferry Design John S. Richards 1, Oliver Reinholz 1 1 Hamburg Ship Model Basin, Hamburg, Germany ABSTRACT In this paper results of hydrodynamic investigations for a number of ferry conversions are presented. In particular the efforts applied to minimizing the power penalty due to an increase in displacement are addressed. Additionally some current trends in new ferry design are presented using as examples a double end ferry with azimuth propulsion and a triple screw shallow water ferry with a hybrid propulsion concept. Motivations for ferry conversions can include increasing the versatility or productivity of an existing vessel, the application of an existing vessel to a new trade or simply to make older ferry designs comply with newer regulations. A number of ferry conversion projects have been conducted in the past in cooperation with HSVA. The lengthening of an existing vessel is a direct approach for achieving an increase in payload. One practical way of improving the stability is to increase the waterline beam in the aftbody by adding sponsons. In most cases this measure is accompanied by an increase in displacement above that which is required for the sponsons alone. Some years ago it was accepted that there would be some power penalty (or speed loss) resulting from the fitting of the sponsons and the increasing of displacement. More recent developments demonstrate however that under certain circumstances no speed loss at all must occur. In the second part of the paper, some findings and results of recent CFD investigations and model test series for fast ferry new buildings are presented which reflect current trends in new ferry designs. Using as an example a fast, double-ended inland waterway ferry which operates at Froude numbers up to about FN = 0.32 the implications of the application of a complex propulsion system on the global hydrodynamic design are considered. As a second example some aspects of the hydrodynamic design of a fast RoPax new building are presented. This vessel is to operate in shallow water at relatively high speeds with depth Froude numbers higher than FN,H = It is shown that some common design experience from deep water applications has to be adapted when significant shallow water effects come into action. Additionally approaches for optimizing this vessel s hybrid propulsion system which consists of a center propeller and wing azimuth drives are presented. KEY WORDS Ferry, Ship lengthening, Sponsons, Ducktail, Interceptor, Azimuth propulsion, Hull form design 1.0 INTRODUCTION Four ferry conversion projects have been considered for this paper. Three of these vessels are sailing in the Mediterranean Sea and one of them in the Baltic. The projects concerning the Mediterranean based vessels are similar in that the purpose of the conversions was primarily stability related. For these vessels the increase in displacement due to the addition of sponsons was only in the range of about 1.5 to 2%. The earlier projects from 2003 and 2004 both ended up with a speed loss of about 0.2 knots due to the measures applied for increasing the stability. The results for the latest project, which was carried out in 2009 was more or less neutral with a power increase of only about 1%. This very good result can at least in part be attributed to the increasing knowledge and experience of the designers. An example for one of these conversions is given below for the Comarit owned vehicle and passenger ferry MV BERKANE. A further project which stands out among the others is the HAMMERODDE. For this vessel a major aspect of the conversion was the addition of about 10% additional displacement to make the vessel fit for a new contract sailing route. This project is described in more detail in section 3 below. Two projects have been chosen for the presentation of newer developments in ferry design. The first is a fast, shallow water ferry for which the hydrodynamic aspects are coupled with considerations for a hybrid triple screw propulsion concept. The second project presented is a double end ferry driven by a set of 4 azimuth propulsors. 2.0 HULL FORM OPTIMISATION FOR THE BERKANE CONVERSION In mid December of 2003 HSVA was contracted by the Moroccan ship owner Comarit to perform a model testing program in conjunction with the modification of the passenger and vehicle ferry MV BERKANE. The BERKANE was built by Chantiers Dubigeon S.A. of Nantes, France and was originally put into service in 1976 under the name of Napoleon for SNCM. Since 2002 the BERKANE is owned by Comarit, and is sailing on a daily basis between the Spanish city of Almeria and Nador in Morocco American Society of Naval Engineers

2 Fig. 1. The BERKANE approaching Tanger prior to the conversion The hull modification consisted of the addition of a ducktail sponson for improving the damage stability and survivability of the BERKANE. In the past few years this type of modification is common for older ferries which, in order to stay in service, are required to meet the SOLAS 90 standard regarding stability and survivability. In the case of the BERKANE the modification allowed the lifetime of the vessel to be extended beyond The hull modification design work was done by the Finnish marine consultant firm Deltamarin Ltd., and the target of the optimisation was to have the performance improvement due to the optimised ducktail compensate as much as possible for the negative effect on the speed of adding about 200 tonnes displacement and increasing the breadth of the transom. The purpose of the model tests was to check a number of ducktail variants in order to quantify the effect of the modification on the calm water performance of the vessel. A reference test with the ship in the as is condition was also performed, the result of which was to serve as a basis for comparison of the modification variants. Fig. 2. The model of the BERKANE with ducktail variant 3 at HSVA s large towing tank As is often the case for commercial projects, the time available for model preparation and testing was less than extravagant. In fact, as the tentative date for docking the vessel was already set, and thus also the deadline for completion of the steel drawings, it was necessary to begin model manufacturing concurrently with Deltamarin s design work. All sponson ducktail components were manufactured and fitted as removable parts (Fig. 2) for easy exchange in order to make optimal use of the time slot in the towing tank. The information flow including the provision and clarification of the original shipyard appendages drawings by Comarit was extremely effective, which helped to minimise the overall throughput time for the project. The model tests demonstrated that with the best ducktail variant, the reduction in speed due to the required modification could be limited to about 0.2 knots. Fig. 3. The BERKANE during conversion in Dock 10 of Blohm + Voss Repair in Hamburg Following the ship s conversion at Blohm + Voss Repair in Hamburg (Fig. 3), the model test based performance rediction has been substantiated by Comarit, who have confirmed that the performance of the BERKANE since the conversion is very much the same as before. She can easily manage 22 knots under normal service conditions and 24 knots at full power. 3.0 HYDRODYNAMIC UPGRADE FOR THE MV HAMMERODDE The MV Hammerodde is a RoRo / Passenger ferry which serves Rønne on the island of Bornholm from the Danish port of Køge near Copenhagen and from the mainland port of Ystad in Sweden. She was built in the Netherlands by Merwede Shipyard in 2005 and has been in service since April of that year. Together with her sister ship, the MV Dueodde, she sails under Danish flag for Bornholmstrafikken A/S, which is headquartered in Rønne. In order to fulfil a new contract with the Danish Ministry of Traffic, the MV Hammerodde is required to increase her cargo carrying capacity from the original 1200 lane meters up to 1500 lane meters. The required space will be made available by adding a further RoRo deck. The corresponding demand for about 10% more displacement will be fulfilled by increasing the draught and at the same time adding a set 2011 American Society of Naval Engineers 383

3 of sponsons and ducktail. The sponsons and ducktail not only provide more displacement but at the same time a larger waterplane area to ensure sufficient stability for the after conversion hull form. A further and somewhat more challenging feature of the new contract is that the present speed of the vessel, which is now 18.5 knots, must be maintained. The idea of fulfilling this requirement via an expensive machinery upgrade was not particularly appealing to the ship owner. Therefore the Finnish marine consultants Foreship Ltd. were requested to apply their art to the task of adding more than 800 m3 to the volume while at the same time maintaining the speed without increasing the power requirement. The first phase of testing was completed with a very encouraging result for the ship owner. In the past, conversions of this magnitude have usually resulted in an overall speed loss for the vessel. In the case of the MV Hammerodde however, the speed loss of about 0.5 knots due to the added volume and increased draught was just compensated by the introduction of the interceptor. Thus the contract point concerning maintaining the 18.5 knots speed was met. Why invest in an expensive machinery upgrade when you can avoid it by upgrading the hydrodynamics instead? Fig. 4. The MV Hammerodde arriving at Rønne on Bornholm In November of 2008 HSVA was contracted to perform the model tests for the MV Hammerodde conversion project. The targets of the investigation included the sponsons/ducktail, an alternative bulbous bow and the introduction of an interceptor plate on the ducktail transom. For this purpose a very time and cost efficient test program was agreed upon and a multi-component ship model was manufactured. Fig. 5. The model of the Hammerodde showing the after conversion condition In phase 1 of the testing work the hull form modifications and interceptor performance were investigated. In a second phase the concentration was placed on the rudder design and also a shift of the rudder position. Here it was expected that the installation of a high efficiency rudder system in conjunction with flap rudders would not only increase the propulsive efficiency but especially also improve the harbour manoeuvring capabilities. The crabbing performance was therefore investigated in a series of dedicated tests. Fig. 6. Transom flow off the ducktail with and without interceptor As a result of the introduction of a tailor-made rudderpropeller package with high-efficiency flap rudders, another impressing 4.5% power reduction could be achieved at the target speed. The MV Hammerodde will thus be fulfilling her new contract with a reduced fuel bill. 4.0 MULTI-OBJECTIVE OPTIMIZATION FOR A FAST SHALLOW WATER FERRY The GR 12 is a new RoPax ferry design which will serve the ports of Rostock and Gedser in the Baltic Sea. The two sister vessels are being built by a major German shipyard. The first of the two vessels will enter service in Together they will sail for a Scandinavian ferry operator. With the two vessels in operation, the service between Rostock and Gedser provides nine daily departures from each port. The ferries are thus operating on a very tight timetable. The most challenging feature of this trade is that the vessels are to sail at speeds of up to 20.0 kts. At first sight, 20.0 kts do not sound like much. However, the ferries will operate in a relatively shallow area of the Baltic Sea with under-keel clearances between 3.0 and 15.0 metres. This results in Depth Froude numbers of up to FN,H = The shallow water effects lead to an increasingly high power consumption which demands a finely tuned and adapted hull shape. Furthermore both the resulting wave system along the ship and the dynamic squat affect the vessel s steering capabilities. This is an important aspect because especially the port of Gedser features confined waters. Thus good manoeuvrability at both low and high speeds must be ensured in the design. In order to fulfil these demands an innovative propulsion concept was introduced. The propulsion plant consists of two off-center steerable wing thruster units that provide propulsion power and manoeuvrability while the ship is en route into and out of port American Society of Naval Engineers

4 Further, a center propeller is foreseen which acts as a booster while the ship is crossing the open stretch of water. Additional manoeuvrability in port is provided by two bow thruster units. In early 2010 HSVA was contracted to perform the model tests for the new ferry design. The objective of the investigations included the minimization of power demand and also the assessment of the ship s manoeuvrability characteristics. At Depth Froude numbers of up to FN,H = 0.81, both the vessel s resistance and manoeuvring characteristics are significantly influenced by the shallow water effects. Hull form design experience acquired for the deep water performance of ferries of similar dimensions cannot be transferred one to one to the conditions in shallow water. Therefore a thorough model test series consisting of dedicated optimization of the hull form and the propulsor arrangement was conceived in order to support the design work. A multi-component ship model was manufactured for this purpose. The model allowed the application of different bulbous bow and trim wedge designs. An important role was assigned to the bulbous bow design. With increasing shallow water effect the wave system changes in shape and height. Typical bulbous bow shapes designed primarily for good deep water performance prove to be detrimental under shallow water conditions. A special, more voluminous bow shape was selected based on the results of comparative model tests. Further, as it was expected that significant aft trim due to the squat effect would occur, special attention was paid to the trim wedge design in order to provide a beneficial dynamic trim condition. The propulsor arrangement was also addressed. In order to reduce the fuel oil bill in future service, different rudder designs which featured rudder bulbs and integrated rudderpropeller devices were conceived. individual propulsors. Each propulsor operates on it s own distinctive open-water curve and is subject to individual inflow situations. Much effort was put into finding the optimal load distribution between the propulsors in order to further minimize the overall power consumption. At the time this paper was prepared, the phase of testing related to the powering issues has been completed with a very encouraging result for the ship owner. Due to the application of the different hull and rudder-propulsor modifications as well as due to the eventual introduction of a tailor-made and high-efficiency design rudder design propulsor package, a reduction in power of about 5.0% at the target speed could be achieved in comparison to the already well-performing initial configuration. Currently, the manoeuvring characteristics are being investigated in shallow water in a thorough test series. 5.0 CFD HELPS TO OPTIMIZE FERRY SERVICES Fjord 1, one of Norway's leading coastal ferry operators run a vast amount of ferry services in inland Norway and along the coast as part of the Norwegian Coastal Highway. Being part of a highly sophisticated transport system, they require high performance and reliability of their operations and especially their vessels. Most of the ships are designed as double ended vessels offering high flexibility during loading / unloading and hence and unrivalled performance when in port. Optimising the design of a double ended ferry for a range of sailing conditions however imposes great challenges on the hydrodynamics of the vessel. Fjord 1 have entrusted Multi Maritime A.S. to design their newest member of the fleet. The design of the new ship features a symmetrical bow / stern configuration with bulbous bow. A propulsion concept based on 4 Rolls-Royce Azimuth thrusters mounted on head boxes has been adopted for the project. Fig. 7. The shallow water ferry model used for the test series, showing the propulsion arrangement The alignment of the thrusters was subject to optimization as well in order to ensure optimal inflow to the wing propulsors. It is inherent to the concept of a triple-propulsor vessel that an optimal load sharing exists between the Fig. 8. Artist Impression of the New Ferry Design The four propulsion units form a prominent part of the underwater hull and proper alignment of the head boxes is of prime importance for the performance of the ship. HSVA was commissioned to optimise the shape of the head boxes in a numerical exercise and perform model tests to find the optimal angle of attack of the Azimuth thrusters as well as to confirm the numerical results. The optimisation targeted a high speed condition at scantling draft American Society of Naval Engineers 385

5 Starting from a given design provided by the client, HSVA used its viscous flow code FreSCo+ to analyse the flow over the complete appended hull including thrusters and head boxes in a VoF based free surface prediction. This led to a detailed insight into flow effects such as local directions and wave elevations which were then used to design different alternative shapes of the head boxes. The challenge of this exercise was to design the head boxes so that minimum resistance and maximum propulsive efficiency can be obtained for operation in both directions. Determining the appropriate entrance and exit angles of the headbox and limiting the flow acceleration in the canal between the main hull and the inner side of the head boxes proved to be a vital aspect. The figure below indicates the complex flow situation around the head boxes at the bow for different designs. elevations along the hull are compared for three selected design alternatives. Fig. 10. Comparison of Wave profiles along the hull for 3 designs, (z-direction amplified) A careful adaptation of the geometry of the head boxes led to a reduction in the predicted effective power of 5.3% for the final version. Model tests were performed in HSVA s large towing tank with a fully appended scale model of the ferry. These confirmed the FreSCo+ overall force predictions as well as flow details such as limiting streamlines as shown in Fig. 12 below. During model tests the optimum angle of attack for the thruster units was also determined. Fig. 9. Flow situation around different head box designs In several iterative steps a geometry following the main flow direction introduced by the ship hull has been derived. This included an initial analysis of a set of geometry variations with HSVA s in-house panel code v-shallo. Following that, complete RANS VoF predictions for the hull form including complex appendages have been performed with FreSCo+. In Fig. 11 below the wave Fig. 11. Comparison of wall streamlines obtained from model test and CFD prediction 6.0 CONCLUSION On the examples of ferry conversion projects it has been shown that adding displacement to an existing vessel for increasing stability does not automatically mean an increase in required power. By applying well designed and optimized stern configurations quite acceptable results can be achieved. In new designs it has been demonstrated that also for complicated hull form and propulsor configurations already good performance can be further improved through the dedicated efforts of designers and builders. A most optimum result for the ship owner can be achieved by his own involvement in the development process American Society of Naval Engineers