Designers are using computational fluid dynamics (CFD) and other modelling programs to optimise the interaction of hull lines with the propulsion system. They are adding wave resistance features to their design analysis programs to enhance hull optimisation in different sea conditions. These services are increasingly in demand as owners are looking to reduce their fuel costs and emissions.
FutureShip has developed software for optimising hull and propulsion systems. It has the FS-Flow software for meshing of ship designs and analysis of boundary conditions in fluid flow. The FS-Equilibrium program is a workbench for analysis of fluid flow and stresses on floating structures. It provides equilibrium condition analysis in up to six degrees of freedom. The FS-Waves program uses CFD to compute the spectral wave spectrum.
FutureShip’s senior project engineer, Volker Bertram, says ship design software is becoming more complex with greater integration of CFD with 3D meshing and wave action analysis. He adds, “Hull designs can be optimised through CFD and using parametric modelling, mesh models and analysis software. What is coming during this decade is designing, analysing and then optimising the whole structure of the hull with the propulsion system and fuel-saving appendages – such as twisted rudders, weight equalising ducts and saver fins. We expect to see more propulsion shapes and efficiency devices in the future. Also, we can estimate the size of the resistance in a scale of sea conditions, from calm to rough waters. We have new software computing the 3D flow around the ship to compute the wave action on the hull in these conditions.”
One of Mr Bertram’s colleagues at FutureShip, Karsten Hochkirch, expects more developments will come in CFD and design analysis. He says, “There is still significant potential for future improvements in hydrodynamics and design, such as for minimising lifetime fuel consumption. Today’s computer capacity allows us to complete complex design environments, integrating panel modelling and free surface CFD. Massive parallel computers can be used in designs for better refined and tuned optimisation, and to connect propulsion with hull optimisation.”
In a paper presented at the Computer and IT Applications in the Maritime Industries (COMPIT) 2012 conference, Mr Hochkirch explained how FutureShip expanded its computer capacity. “In 2011, our parallel computing cluster was extended to a total of 6,600 cores [processing units]. Correspondingly, storage capacity was extended to 36 terabytes. Rapid internal network communication was ensured by a network with a data rate of 40 Gbps. With increased computing resources, the door is open to more sophisticated applications.”
The exponential growth in computer hardware continues to drive innovation in ship design software. Mr Hochkirch expects developments in cloud computing and processor performance will facilitate the development of more applications. “The growth in computing power lays the foundation for more advanced optimisation applications. However, the desire for ever more sophisticated applications is likely to mean computational requirements will grow faster than hardware unless optimisation approaches are changed as well. It is likely that we will see various techniques to make optimisation projects more efficient on the software side as well,” he explains.
He expects developers to introduce hybrid computing that mixes low-fidelity CFD for coarse meshes with high-fidelity CFD and fine meshes. Another design advancement is likely to be the development of self-learning meta-modelling that produces hydrodynamic response surfaces based on variants from CFD evaluations.
Mr Hochkirch continues, “The CFD software appears to be mature enough to change little for the hull optimisation applications. However, more sophisticated models should evolve over the next decade. Such models may add complexity in various forms – more sophisticated seakeeping models, especially for added resistance in waves. Such computations, coupled with meta-modelling and using coarse meshes, may drift gradually into industry applications over the next decade.”
In her COMPIT presentation, ABS senior engineer, Yi-Fang (Yvonne) Hsieh, explained how CFD can be used to reduce energy losses from propulsion systems. “CFD really opens up a new possibility for the future design of energy-saving devices to maximise the energy loss reutilisation for propulsion. CFD simulations provide detailed flow information around a propeller in both the temporal and spatial domain, which is usually very difficult and expensive to measure in the traditional model tests.”
ABS worked with China’s Ship Design and Research Institute (SDARI) in analysing propeller energy-loss reutilisation for a Capesize bulk carrier. In her COMPIT paper, Ms Hsieh described the results. “In energy-loss analysis, CFD simulations of the open water propeller, bare hull, and propeller/hull interaction situations were carried out and compared with model test results. The propeller energy loss was evaluated based on the validated CFD results of the propeller/hull interaction simulation.
“According to the propeller energy loss analysis, 52 per cent of the shaft power was usable for ship propulsion, and the axial kinetic energy loss was 37 per cent. Other energy losses included the friction and tangential energy losses of the shaft power. Since the axial kinetic energy gain was responsible for the primary propeller energy loss, devices that can reduce axial energy loss should be considered in the first priority.”
Ms Hsieh said friction losses could be reduced through propeller design optimisation, such as using a smaller expanded area ratio, and larger diameter with a lower rotational speed. She added in the paper, “Based on the validated CFD results, the propeller energy loss evaluation was performed and used to guide the energy-saving device design to maximise the energy-loss reutilisation for propulsion. Devices may be used to recover the tangential kinetic energy.”
In another example, CFD analysis was mixed with computer-aided engineering (CAE), finite element modelling (FEM), batch meshing and morphing tools to provide more streamline hull structures. Pre- and post-processor software helped facilitate the FEM processing and analysis. Time consuming tasks like meshing, model simplification and mesh quality improvement were replaced by automated processes. This saved engineering working hours and helped to avoid any human errors that might have occurred during the process.
This was the case on a project involving Greek design software company Beta CAE Systems. Its pre- and post-processor programs were used for designing and optimising a rudder on an unnamed 169m-long Handysize bulk carrier. During the project, it was assumed that the main force that strains the rudder is produced by the water flow around it. The maximum force appears at the vessel’s full speed of 15 knots, when the rudder turns to the maximum angle of 35 degrees.
Beta CAE Systems’s supervisor, George Korbetis, described in his COMPIT paper how these programs were used. He demonstrated a process of defining strength analysis, and the application of the multi-objective optimisation on the rudder. The first stage of the process was to set up two different FE models for structural and CFD analysis. The ANSA Fluent CFD software calculated the maximum force applied on the rudder’s surface. The calculated force was taken as the loading condition for the rudder’s structural analysis. The CFD results, such as streamlines, vectors and contours, were also viewed and evaluated in Beta CAE’s uETA program.
The static analysis calculated the strength of the rudder assembly and the contact pressure between the rudder bearing and stock. The rudder’s surface and assembly was batch meshed using ANSA. Model shape parameterisation was performed by the ANSA morphing tool and the optimisation software. During this process, different design variables were modelled and analysed.
One area of research is the use of CFD to improve hull optimisation by minimising calm-water resistance. For this research, Italian shipbuilder Fincantieri Cantieri Navali used Engys software for hull design, combining a flow solver with multi-variate analysis. CFD that combines free-surface effects with a viscous solution was employed for accurate drag prediction with fast turnaround times. This was ideally suited for the hull optimisation study Fincantieri undertook for a passenger ferry hull, says Engys’s Paolo Geremia.
He adds, “The innovative CFD solver is coupled to an optimisation method, based on an efficient multi-variate analysis approach. This allows the designer to perform a fast and broad search on the global design space by identifying the set of the optimal designs, using a clustering method, in order to find out the optimal layout of the design solution for the given operating conditions applied.
“A large number of geometrical-design variables were considered in early-stage design. Different methods are used to optimise the hull with respect to resistance over a range of different speeds for a given displacement. The optimal solution can be retrieved with far fewer evaluations with respect to the traditional methods, thus saving a considerable amount of computational time. So it is possible to use high-fidelity CFD simulations. The application of multi-variate analysis allows the designer to have a better insight into the design space, which becomes crucial in the decision-making phase of the design process.”
Further research in France has focused on the computation of turbulent flow around a container ship’s propulsion system, in all operating conditions. The Laboratory for Hydrodynamics, Energy and the Environment Atmosphere (LHEEA) research at central Nantes University (ECN) has been working with Numeca’s Fine Marine to develop a suite of design software for computing viscous flows around ships. According to LHEEA ECN lecturer, Patrick Queutey, turbulence modelling is a crucial aspect to hull design. But more physical modelling is involved when simulating propeller motions.
Mr Queutey adds, “Propeller-hull interaction is an important topic for ship design. Accurate prediction for ships is a challenging task for CFD computation. It requires not only an accurate prediction of resistance and wake flow, but also a good prediction for the propeller. While turbulence modelling for the bilge vortex is a crucial issue to ensure an accurate prediction for the wake flow, more physical modelling is involved in a propeller simulation: transition, cavitation, turbulence, ventilation, etc. More advanced numerical techniques are also required to capture very small scale flow motion such as tip vortex.”
One of the main goals of Mr Queutey’s research team is the simulation of ship propellers in extreme operating conditions, with accurate modelling of all the physics involved. “This requires the capability to simulate a rotating propeller behind a ship hull, combined with effects of free-surface deformation, ventilation, and cavitation. These capabilities are being developed for inclusion in ISIS-CFD, the unstructured finite-volume flow solver,” Mr Queutey adds.
“This flow solver code has been fully validated for resistance and wake flow prediction using advanced turbulence models, such as the explicit algebraic stress model with rotation correction. An essential building block for the simulations is a sliding grid technique, which allows a part of the grid (containing the propeller) to rotate within the main part of the grid, while keeping a connection between the two parts. Also, since many of the phenomena to be studied originate from highly localised low pressure zones, the accurate simulation of these phenomena can be obtained by automatic adaptive grid refinement.”
During LHEEA’s modelling study, the researchers first analysed an isolated propeller in order to evaluate the minimum settings for the generation of the mesh. The same propeller was used to check the sliding grid approach. Starting from a coarse mesh, the adaptive grid refinement method was applied to a rotating propeller. It automatically improved the prediction of the tip vortex, with a significant consequence on the prediction of the forces. “The study of the flow around the container ship gave us the opportunity to assess the flow solver to predict hull/propeller coupling in self-propulsion conditions. Considering the complexity of this exercise, the results obtained for this first try are very promising,” Mr Queutey says.
Changing face of design analysis
The following will have the potential to change ship design during this decade, according to FutureShip’s Karsten Hochkirch:
1) Operational profiles will be considered not just as a single design point. The consideration of operational profiles (where for example 80 per cent of design speed is the most frequent operating speed) has been shown to offer fuel saving potential of several percentage points.
2) Consideration of matrices of load conditions and speeds is theoretically feasible, and likely to become standard as computing power becomes available and design for actual fuel consumption will gain in importance.
3) Hull optimisations will eventually also allow changes in the geometries of propellers and appendages.
4) Some design aspects, such as manoeuvring and production, will be further considered, as more sophisticated methods to quantify these aspects are added to optimisation models.
New design software for seakeeping analysis
Maestro has released a new version of its ship design software that includes seakeeping analysis for the first time. The Maestro Wave program can apply linear seakeeping panel pressure analysis to complete ship models. The software was made commercially available for the first time in April, says Nick Danese, a consultant and agent for Maestro.
“Maestro has introduced seakeeping into the software this year for extreme load and fatigue analysis on vessels,” he said at the COMPIT conference in Liege. “Maestro can transfer hydrodynamic loads to a dense mesh finite element model. It is an integrated solution that provides structural, hydrodynamic and extreme load analysis. It supports any shape or mesh density, so the mesh can be as coarse or fine as needed.”
Mr Danese says panel-based hydrodynamic analysis was suited for transferring seakeeping loads to 3D finite element structural models, but can use huge volumes of computer power. Maestro’s software maps the source of forces on ship hull forms in the structural mesh instead of using the hydrodynamic pressure. This reduces the computer memory required for analysis. Other software suppliers have released new software that will enhance shipyard productivity and quality, and improve model generation and 3D imaging.
Dream product tanker designed
Romanian shipbuilder Santierul Naval Constanta (SNC) has worked with designers SDC Ship & Consult, using CFD to design the ‘Dream’ series of tankers. The ships are designed for Mediterranean and Black Sea operations, with efficient propulsion, optimised hull lines and lower deck heights.
The hull optimisation was done with CFD analysis and in conjunction with the Hamburg ship model basin (HSVA), says SNC’s managing director, Radu Rusen. The Dream tankers were designed for flexible trade patterns, multiple port characteristics and tightening environmental regulations. Mr Rusen says the tankers would be compliant with the Energy Efficiency Design Index (EEDI) up to 2025, delivering fuel savings of 30 per cent compared with competing vessels.
“SNC analysed design options for different hull forms. We used CFD analysis to improve the hull performance and reduce wave resistance, then confirmed our findings by tank testing at HSVA,” says Mr Rusen. “The designs have a shallow draught for use in ports with depth restrictions, and they have larger cargo capacity than their competitors. We have done this without compromising the mild steel content and scantlings.” The 50,000 dwt medium range tankers would be built at Constanta Shipyard when SNC receives orders.
Software aids Brazilian tanker design
Rolls-Royce used Napa and ShipX software to help design bunker tankers for the Brazilian coastal trade. The NVC 604 BT-type tankers are under construction in Brazil for owner Navegação São Miguel. The Det Norske Veritas-classed, 4,350 dwt vessels have a design draught of 4.5m and a service speed of 10 knots. Rolls-Royce used Napa for stability analysis and ShipX for vessel speed simulation and propulsion estimates, says Rolls-Royce’s manager of ship technology and design, Aage Milde.
“Operators need high technology and optimised propulsion to get good performance and better efficiency. Hull lines need to be designed with CFD and FEM (finite element modelling), integrating their design with the propeller. We also model ship systems, including the pipework, in 3D, so owners can access this on the Internet at an early stage to provide their feedback.” Rolls-Royce will also supply propulsion and machinery, propeller, rudder, steering gear and thrusters, controls, automation and switchboards.MEC