i I I High-Performance Simulations for High-Performance Vessels by Prof. Dr Volker Bertram and Axel Köhimoos, Germanischer Lloyd megayachts extra

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1 extra megayachts High-Performance Simulations for High-Performance Vessels.. If i I I by Prof. Dr Volker Bertram and Axel Köhimoos, Germanischer Lloyd

2 CFD simulation of smoke dispersion. Introduction provide a fast method to investigate numerous parameters (wave length, wave direction, ship speed, etc.). Studying extreme motion patterns, particularly those involving slam- ming, usually requires non-linear computational fluid dynam- Ics (CFD) simulations, which place high demands on computer resources and are limited to relatively short time periods (seconds to minutes). Combining intelligently linear frequen- cy-domain methods with non-linear time-domain simulations takes advantage of the strengths of both approaches. This detailed analyses enables designs with considerably improved seakeeping behaviour and passenger comfort. Aerodynamics affect noise and smoke propagation as well as local flow conditions on helicopter decks, sunbathing decks and similar locations. While wind tunnel tests are stil widely used, CFD offers the advantage of overcoming scale effects, which can be significant when thermodynamic processes are involved. CFD can be combined with formal optimisation, e.g. to minimise smoke dispersion on decks. Ventilation system designs are usually based on simple estimates, which often result in over-dimensioned and energy-inefficient systems and sometimes poor performance. CFD, on the other hand, delivers detailed insight, leading the way to a smarter layout. Noise and vibration limits are standard elements of building specifications for megayachts. High comfort expectations must be accounted for during the initial design stages. The simulation tools to support low-noise design are currently Ship design has always been primarily an experience-based skill. While this is still true to some extent today, we increasingly rely on "virtual experience" drawn from computer simulation. The rapid development of fast and unconventional ships since the 1980s would have been impossible without advanced simulation technology. Simulation is especially crucial in megayacht design where innovative and unusual concepts requested by customers often involve additional risks, Megayachts have grown beyond 200 metres in length; this is just one example of how designers push the limits. Whenever we leave the "comfort zone" of our experience and venture Into new designs, simulations ensure that these designs are efficient and not only viable, but also safe. Simulation has become a powerful tool to support better business processes within the megayacht industry by: ensuring comfort; increasing freedom of design; achieving better designs in less time and enabling fast and efficient trouble-shooting. Ensuring Comfort Comfort, a key requirement for a megayacht, comprises many aspects. The most important ones are discussed below from an engineer's perspective. Seakeeping is vital for passenger comfort. For many seakeeping issues, linear analyses (assuming small wave height) 36 nonstop

3 "TURANOR PLANETSOLAR". The obtainable speed in waves was predicted at the conceptual design stage. being expanded to also include underwater noise, an aspect receiving growing attention in the megayacht industry. Increasing Freedom of Design Several projects illustrate how advanced simulation-based design methods increase the freedom of design for shipyards and owners. The lightweight design of the record-breaking trimaran "Earthrace" was only made possible by using sophisticated CFD simulations for the load and subsequent finite-element analyses of the composite hull structure. In another pioneering project, the solar catamaran "Turanor PlanetSolar", the obtainable speed in waves - crucial for the first circumnavigation of the world by a solar-powered vessel - was predicted as early as during the conceptual design stage. Yet another example is the largest megayacht of its time, which set new standards in terms of size and speed. In all these cases, highly innovative designs were implemented based on high-performance simulations of both hydrodynamic and structural behaviour. Composite materials are increasingly used in high-performance vessels, including megayachts. It is the combination of low weight, high strength and mouldability that makes these materials attractive for designers. The choices in designing a particular component to suit its purpose are often only limited by the development budget or the > STERN OF A MEGAYACHT. Original design (left) and modified design for reduced slamming impact (right); slamming pressures were reduced by almost 30 per cent. 01/

4 COMPOSITES. Structural analyses of trimaran "LAMU II > designer's creative imagination. Classical, "by-the-book" approaches to structural design do not work for high-performance, light-weight concepts. Prescriptive rules are often too inflexible, especially for advanced composite elements. In addition, it is necessary today to give due attention to the recycling properties of composite materials, besides their strength aspects. puter resources due to the complex flow patterns and the enormous number of design alternatives to be investigated. However, in our experience the effort is justified by the good results obtained, and CFD simulation is quickly becoming a standard tool in high-performance yacht design. Simulations are similarly useful when designing prope - Iers, water jets, rudders and other appendages. On fast ships Achieving Better Designs in Less Time Using CFD simulation to develop the hull lines of high-performance yachts has been standard procedure for years. Formal optimisation, however, is a relatively new field of application. CFD simulation may improve the required power (and associated weight). Alternatively, the gain in efficiency may be used to increase the speed of the boat at the given installed power. Optimisation requires considerable com- especially, these components are frequently affected by cavitation issues. Unfortunately, problems often remain undetected until the ship undergoes sea trials or has been in operation for some time. Even then, simulation can guide efficient troubleshooting and redesign, but avoiding cavitationrelated problems in the first place by performing appropriate checks during the design stage is clearly preferable. Traditionally, naval architects have resorted to standard baseline designs, statistical regression models and systematic series data as a basis for their design. Most of these data series. 38

5 however, are hopelessly outdated. Today, the FRIENDSHIP- Framework software offers an environnnent for compiling custom data series for numerical simulation. To develop a parametric design, a family of design variants can be created by systematically modifying the parameters. The key benefit of this method is that it allows the designer to build a knowledge base ahead of an anticipated project. Once this has been done, the designer can quickly extract data from the numerical model series during the conceptual design stage. The first indication of a design problem, such as noise or vibration, is often encountered during sea trials. In some cases, however, it can take months or even years of operation, or a rare off-design situation, for a design flaw to become apparent (e.g. fatigue cracks or cavitation erosion on rudders). In some cases, the diagnosis of the problem is straight-forward, in others a more extensive analysis is required to pinpoint the root cause of a problem. Once this is known, redesigning the affected structure or operational procedure is comparatively easy. Simulation techniques are employed in both the diagnosis of the problem and the effective assessment of the redesign options. Vibration, including vortex-induced vibration, is an area where simulation has advanced our processes considerably. Enabling Fast and Efficient Trouble-Shooting Highly Skilled Experts Make the Difference Time and time again our advanced design projects have shown that simulation, when used skilfully, saves time and money. Ideally, the parties involved should agree on the scope and procedure for simulations at the earliest time possible. > FLOW. Simulation for a fast twin-screw vessel. FRIENDSHIP FRAMEWORK. Tailored design series for megayachts. r» 01/

6 Vibration Level in Owner's Cabin Ï t - Vortex shedding-induced excitation depending on speed through water t I I 1 1 i I mm/s! "1 "1 ^ Vibration Level in Owner's Cabin - After Modification mm/s m *,. p Resonancejconditioh T t ^ r E. I I ^..1. i I I) i t.l l 1 Natural frequency at 3.6 Hz : Machinery-induced excitation depending on speed through water Hz 8 10 Hz > Unless the shipowner (as the main benefactor) specifically asks for simulations for design optimisation in his project specifications, time and budget constraints will often prevent them from being carried out. Furthermore, the mere use of advanced simulation software does not guarantee that true optimisation will be achieved. More than ever before, engineering involves the art of modelling and finding the right balance between the level of detail and the aval - able resources (time, manpower). This requires significant experience. The true value offered by engineering services thus lies not in software licenses or hardware but in highly skilled staff putting these resources to best use. Bibliography Bertram, v.; Brehm, A.; Kohlmoos. A (2011), Smoke assessment for megayachts, Via MafeXin,pp,49-5ï Bertram, v.; Gualenï, R (2011), An overview of seakeeping tools for maritime applka* tions, IMAM 2011 Conf, Genoa Bertram, V.; Hoffmeister, H,; Miiller, S.; Moltschaniwsky], A, (2010), Technological challenges for construction and classificatron 'Türanor PlanetSolar', 7th Int, Conf. on High- Performance Marine Vehicles (HIPER), Melbourne, pp Brehm, A.; Bertram, V,: El Moctar, 0, (2011), CFD guided improvement of a cavitationprone rudder, 14lh Numerical Towing Tank Symp,(NuTTS), Poole Couser, P.; Harries, S,;Tillig, F. (2011), Numerical hull series for calm water and seakeeping, 10th Int. Conf Computer and IT Applications in the Maritime Industries (COMPIT), Berlin, pp El Moctar, O.M. (2007), How to avoid or minimize rudder cavitation, 10th Numerical Towing Tank Symp. (NuTTS), Hamburg El Moctar, O.M.; Bertram, V. (2002), Computation of viscous flow around fast ship superstructures, 24th Symp. fjaval HycJrodyn., Fukuoka Fach, K. (2006), Advanced simulation in the work of a modern classification society, 5th Ifït. Conf, Computer and IT Applications in the Maritime Industries (COMPIT), Oegstgeest, pp. 34^ Fach, K.; Bertram, V. (2006), High-performance simulations for high-performance ships, 5th Int, Conf. High-Performance Marine Vehides (HIPER), Launceston, pp Fach, K.; Bertram, V. (2009), Advanced simulations as design dedsion support, 10th Int. Marine Desigfi Co:if, Trondheim Gramann, H.; Krapp, R.; Bertram, V, (2008), Disposal and recycling of HSC materials, 6lh Int, Conf. High-Performance Marine Vehides (HIPER). Naples, pp, Harries, S. (2010), Investigating multi-dimensional design spaces using first principle methods, 7th Int. Conf on High-Performance Marine Vehides (HIPER), Melbourne, pp Harries, S.; Vesting, F. (2010), Aerodynamic optimization of superstructures and com* ponents, 9th Int, Conf Computer and IT Applications in the Maritime Industries (COMPIT), Gubbio, pp Kaufmann, J.; Kohlmoos, A.; Bertram, V, (2010), CFD guided hydrodynamic design of a rigid-inflatable boat, Via Mare X, August Kohlmoos, A; Bertram, V. (2009), Vibration analysis for a mega yacht, RIfJA Conf, Design, Construction & Operation of Super & Mega Yachts, Genoa Menzel, W.; El Moctar, 0. M,; Mumm, H. (2008), Advanced thinking on tricky excitations. The Naval Architect, March, pp Oossanen, P. van; Helmann, I: Henrichs, J.; Hochkirch, K. (2009), Motor yacht hull form design for the displacement to semi-displacement speed range, 10th Inl Conf Fast Sea Transportation (FAST), Athens Ziegler, W.; Fach, K.; Hoffmeister, H.; El Moctar, 0.M.; Bethane, R (2006), Advanced analyses for the EARTHRACE project, 5th Conf High-Performance Marine Vehides (HIPER), Launceston, pp nonstop