Customers benefit from simulations for mega yachts when scope and procedure are agreed upon at an early stage. Software, however, is not enough, but also highly-skilled staff using it.
1. Introduction
The word »simulation« is derived from the Latin word »simulare« which can be translated as »to[ds_preview] reproduce« or »to mimic«. According to VDI (Society of German Engineers), »simulation is the reproduction of a system with its dynamic processes in a running model to achieve cognition which can be referred to reality«. The Oxford dictionary defines »to simulate« as »to imitate conditions of a situation or process«, specifically »to produce a computer model of a process«. In this sense virtually all computer models used in the design, construction or operation of ships would qualify as simulations.
The scope and depth of simulations guiding our decisions in design and operation of ships have developed very dynamically over the past decade. In previous publications, we have described in detail the scope and underlying techniques of simulations, e.g. Fach (2006), Fach and Bertram (2006, 2009). These publications focussed on the technical aspects of simulation. In the present article, we want to focus instead on the benefit of simulations from the customer’s point of view. The »customer« may be the shipyard or the ship owner, buying services based on the special competence of simulation experts.
In such a customer-centric approach, simulations serve to support business processes through:
• Ensuring comfort
• Increasing design freedom
• Achieving better designs in shorter time
• Ensuring fast and efficient trouble-
shooting
These items are discussed in the following sections in the context of large and unconventional yachts.
2. Ensuring comfort
Comfort is a key expectation for mega yachts. Therefore, ensuring comfort (by suitable and reliable prediction methods) is an important issue in mega yacht design. Simulations play a particular role in this aspect, as innovative and unusual designs appeal to customers, but pose risks as design cannot be based on experience as for conventional cargo ships. »Comfort« encompasses many aspects. The most important of which, from an engineering point of view, are discussed below.
Seakeeping is closely related to passenger comfort and, as such, is always an important topic for mega yachtss. For many seakeeping issues, linear analyses (assuming small wave height) are appropriate and frequently applied due to their efficiency. The advantage of this approach is that it is very fast and thus allows the investigation of many parameters (frequency, wave direction, ship speed, etc.). Non-linear CFD simulations are usually necessary for the treatment of extreme motions, particularly those involving slamming or green water on deck. These simulations require significant computer resources and allow only the simulation of relatively short periods (seconds to minutes). Combining intelligently linear frequency-domain methods with nonlinear time-domain simulations enables the respective strengths of each approach to be exploited, Bertram and Gualeni (2011). The approach starts with a linear analysis to identify the most critical parameter combination for a ship response. Then non-linear CFD analyses determine the design values of interest (motions, loads, speed loss in waves, etc.) for these critical cases. Such detailed analyses can lead to designs with considerably improved seakeeping behaviour and resulting benefits for passenger comfort.
Aerodynamics play a role for mega yachts, not so much for resistance, but for noise, smoke propagation, and local flow conditions on helicopter decks, sunbathing decks and similar locations, Bertram et al. (2011). Although wind tunnel tests are still popular and widely used, CFD offers the advantage of overcoming scale effects which can be significant if thermodynamic processes are involved, El Moctar and Bertram (2002). CFD can be coupled with formal optimisation for ship or funnel designs to minimize smoke dispersion on decks, Harries and Vesting (2010).
Ventilation systems are still largely designed based on simple estimates, often resulting in over-dimensioned and energy inefficient systems, sometimes in insufficient performance. CFD can guide more intelligent layout, based on reliable detailed insight.
Noise and vibration limits are standard parts of building specifications for mega yachtss. High comfort expectations are common and must be implemented at the earliest design stage of the project. With our software GL.NoiseFEM, we are able to trace and evaluate major structure-borne noise transmission paths, which helps to locate and economize areas for noise abatement measures. In the hands of experts, simulations yield reliable predictions as validated in detailed measurements, e.g. Wilken et al. (2004). Most recently, underwater noise has also become an issue for mega yachts. Our procedures to predict underwater noise were extensively validated in 2010. Underwater noise is expected to become a design issue for mega yachtss operating in environmentally sensitive regions. The simulation tools to support design for low noise are already in place. In fact, simulations have become a standard tool to support designers in predicting vibration and noise levels.
3. Increasing design freedom
Traditionally, ship design is based on experience. This is still true to some extent, but increasingly we rely on »virtual experience« from appropriate simulations. The rapid development of fast and unconventional ships after the 1980s was only feasible with advanced simulations. Mega yachts have grown to beyond 200 m in length, but this is just one example of how designers push the limits for mega yachtss. Whenever we leave our »comfort zone« of experience moving to new designs, simulations ensure that these designs are efficient and not only feasible, but safe.
Several projects illustrate how modern simulation-based design increases the design freedom for shipyards and owners. The lightweight design of the record-breaking trimaran »Earthrace« was only made possible using sophisticated CFD simulations for the loads and subsequent finite-element analyses for the composite hull structure, Ziegler et al. (2006). For the »Turanor PlanetSolar«, the focus was the obtainable speed in waves which was predicted at the conceptual design stage for this record-breaking solar catamaran. Yet another example concerns the largest mega yachts at that time, which, due to size and speed, set new standards. In all these cases, the very innovative designs were made feasible by high-performance simulations, both for hydrodynamic and structural aspects.
Composite materials are increasingly used in high-performance vessels, including mega yachts. The combination of light weight, high strength and mouldability make these materials highly attractive for designers. The variety of possibilities to design a particular component to suit its purpose is manifold and is often only bound to development budgets or creativity. However, classical »cook-book« approaches in structural design do not work for high-performance light-weight designs, Bertram et al. (2010). Prescriptive rules are often too inflexible; Especially for advanced composite designs. The »Earthrace«, »Turanor PlanetSolar« and »LAMU III« are just three high-profile examples of many advanced yacht projects that have proven THAT characteristic tailored solutions for composite structures require particular attention and understanding. Besides strength and hydrodynamic aspects, environmental aspects should also be considered, namely the different recycling properties of composite materials, Gramann et al. (2008).
4. Achieving better designs in shorter time
The design of hull lines for high-performance yachts has been based to some extent on CFD simulations for many years now. However, formal optimization for industry projects have been reported only relatively recently, e.g. Oossanen et al. (2009). Such formal optimizations may offer 4-5% improvement in required power (and associated weight) for moderate to fast yachts. Alternatively, the optimization may be used to increase the speed of the boat for given installed power. Optimization requires considerable computer resources due to the complex flow and the thousands of design alternatives investigated. However, in our experience the effort is justified by the good results obtained and formal optimization of hull lines is quickly becoming a standard option in high-performance yacht design. Such optimization can also be applied to planing hulls if appropriate CFD tools are used, e.g. Kaufmann et al. (2010).
Simulations can be used similarly for the design of propulsors, rudders and other appendages. For fast ships, these are in principle susceptible to cavitation problems. Unfortunately, problems are often detected late, in sea trials or after some time in operation. Simulations can then guide efficient trouble-shooting and re-design, but it is preferable to avoid cavitation related problems by appropriate checks during the design stage. Computational fluid dynamics (CFD) coupled with cavitation models (describing the formation and collapse of cavitation bubbles) can guide the design of waterjets, propellers and rudders in terms of low cavitation, El Moctar (2007), Brehm et al. (2011).
Traditionally, naval architects have resorted to similar baseline designs, statistical regression models, and systematic series data in initial design. Most of such series, however, are hopelessly outdated. Today, tailored series can be created using numerical simulations. For a parametric design, a family of designs can be created by systematic variation of the parameters describing the design. The key benefit of the proposed method is that it allows the designers to build up a knowledge base ahead of an anticipated project. Once this has been done, they can quickly extract data from the numerical model series to substantiate their design during the bidding and tendering process (which normally takes place under considerable time pressure). The approach is described in more detail in Harries (2010), Couser et al. (2011).
5. Ensuring fast and efficient trouble-shooting
Sea trials often give the first indication of design problems, for example in terms of noise and vibrations. But in some cases, problems occur only after months or years of operation (as in fatigue cracks or cavitation erosion on rudders) or in rare off-design conditions. Defining the cause of the problem is the first step towards solving it. Frequently, dedicated measurements give a detailed diagnosis to characterize the problem, for example severity of noise and vibration levels and associated frequency.
In some cases, the diagnosis of the problem is straight-forward, in other cases a more extensive analysis is required to determine unequivocally the root cause of a problem. Once the cause of the problem is defined, the re-design of structure or operational procedures is straightforward. Simulations are employed both in the diagnosis phase and in assessing the effectiveness of potential re-design options.
Mega yachtss may encounter problems with vortex induced vibrations (VIV). Ships have many degrees of freedom for local vibrations and these may be excited by vortex shedding at many local structures, typically at appendages or hull openings (e.g. sea chest). The traditional trial-and-error approach to localise the source of vortex-induced vibrations may today be replaced by a much more time and cost efficient search guided by CFD and vibration analyses, Menzel et al. (2008), Köhlmoos and Bertram (2009).
In time, vibrations (induced by vortices, engine or propeller) may lead to fatigue cracks despite relatively small amplitude. Fatigue cracks may become an issue of discussion between owners and yards sometimes after years of operation. They occur in the hull structure or in equipment, for example gears or shaft bearings. Light-weight designs and high engine power contribute to their increased occurrence in mega yachts. While appropriate care in the design stage is preferable, practice shows that fatigue problems appear in operation and then require efficient trouble-shooting. Here finite-element analyses are the appropriate tool. The approach may differ depending on the source of excitation causing the fatigue and the affected part of the ship (structure or equipment).
6. Requirements
So far, we have shown the manifold benefits of using simulations for mega yachts shipyards and owners, contributing to better designs (supporting the architect’s vision) and decreased costs (risk cost, repair cost).
Having discussed rather extensively the benefits of advanced simulations, let us have a brief look at the requirements for such simulations:
• Software
It is generally preferable to use commercial software from large, well established vendors. This software is extensively validated and widely used, with known scope and limits of applications. This adds transparency for both sides, supplier and buyer of engineering services. The commercial software still needs to be adapted for fast and specific model generation, in our case to ships. Assorted macros, pre- and processing software has been developed and maintained. The model generation is often crucial for time, costs and quality of results in simulation projects. See e.g. Bertram and Couser (2010) for a more extensive discussion.
• Hardware
State-of-the-art computations frequently employ parallel computers. The costs for hardware within a simulation project are still relatively small, but care has to be taken to exploit the paral¬lel computing hardware efficiently.
• Trained experts
In short, »the pilot is more important than the plane«. Experience is needed in modelling and modelling determines response time, cost and quality of result. The combination of research and extensive industry experience with feedback from large full-scale measurements, model tests and several codes ensures best value.
7. Concluding remarks
Simulation is a powerful tool that can support better business processes within the maritime industry. Simulations for numerous applications now often aid decisions, sometimes »just« for qualitative ranking of solutions, sometimes for quantitative »optimization« of advanced engineering solutions. Continued validation feedback serves to further improve the simulation tools as well as build confidence in their results. Our many projects have proven time and time again that the appropriate use of simulations saves time and money for advanced designs. Ideally, for all parties involved, the scope and procedure for simulations are agreed upon at an early stage. Unless the ship owner (as main benefactor) includes simulations for design optimization in the specifications, time and budget constraints will often prevent them from being utilised.
Finally, the use of advanced simulation software alone is not enough. Engineering is more than ever the art of modelling, finding the right balance between level of detail and resources (time, man-power). This modelling often requires intelligence and considerable (collective) experience. The true value offered by advanced engineering service providers thus lies not in software licenses or hardware, but in the combination of highly skilled staff putting these resources to best use.
Acknowledgements
Many colleagues at Germanischer Lloyd have supported this article with their special expertise, namely (in alphabetical order) Andreas Brehm, Stefan Harries, Karsten Hochkirch, Bettar El Moctar, Wolfgang Menzel, Holger Mumm, Vladimir Shigunov, Stefan Semrau, Gundula Stadie-Frobös, and Tobias Zorn.
Axel Köhlmoos, Volker Bertram
