High-tech tools deliver efficient propulsion design
Investment in cutting edge design tools are helping Teignbridge Propellers in its investigation into efficient propulsion systems as part of a UK-funded project.
In February 2016 the company began a project for the Energy Technologies Institute, a UK government agency charged with aiding emissions reductions across several industrial sectors. The £3 million project investigates high-efficiency propulsion systems that can be used to reduce greenhouse gas emissions from a wide range of vessels including handysize bulk carriers and product tankers as well as ferries, offshore service vessels and container feeders.
The principal aims of the project are to develop a commercially-viable system that can be retrofitted to most vessels to deliver fuel efficiency benefits. The target is for a reduction in fuel consumption and corresponding CO2 emissions by an average of 8% across the target vessel types.
The first step for Teignbridge was to develop a suite of integrated design tools. As a result, the company has created an arsenal of state of the art numerical and physical design tools that it says can provide ship owner with a complete analysis of a vessel’s potential performance. These tools and the skills of the engineers behind them ensure that Teignbridge’s propellers and underwater equipment designs are fully optimised to deliver the prime combination of performance, fuel economy and reduced emissions.
Among these tools, newly developed capabilities in computational fluid dynamics (CFD) enable Teignbridge engineers to optimise propeller design to deliver maximum hydrodynamic effciency, whilst ensuring safe cavitation performance in the vessel wake field. Teignbridge uses industry-leading CFD software to run open water and transient wake field simulations in addition to full hull flow simulations as required.
Teignbridge’s CFD propeller simulation methodology has been validated in blind tests against tank test data as well as established industry case studies to ensure simulation accuracy. The company has also developed in-house 2D and 3D panel code routines for rapid simulation and coarse design optimisation work - the 2D panel code is used for blade section design and optimisation, whilst the 3D panel code is used for full propeller optimisation.
Alongside CFD, ship system simulation enables the propulsion system to be modelled within the context of the wider ship system, simulating the relationship between hull, engine, control system and propeller. Adding historical and statistical mission profile (including speed, draft, heading and port calls) and metocean data enables long-term performance metrics such as fuel efficiency to be predicted and optimised for realistic ship operating conditions.
Teignbridge has developed a purpose-built ship system simulator - HEPS Sim - to model propulsion system performance and specifically long-term fuel economy. HEPS Sim is built and run in MATLAB + Simulink to enable best practice time domain modelling, meaning accelerations and other time varying characteristics can be simulated to provide an accurate prediction of performance.
Hull performance can be predicted either by the Holtrop-Mennen calculation method or by full-scale CFD simulation where sufficient detail of the vessel geometry is available. Historical, statistical and future scenario mission profile data, including vessel speed, draft, vessel control approach and more can be used to model performance over days, months or years. Time varying metocean profiles can be included to approximate real world conditions on a given shipping route, enabling simulation of wave and wind impact on vessel resistance and subsequent performance.
Detailed engine simulation is used to faithfully replicate the engine installation on the given vessel ensuring accurate feedback between the propeller and engine, and prediction of fuel consumption.
To ensure that Teignbridge’s innovative designs are structurally fit for purpose under operational, fatigue loading - as well as crash stop and accidental loading – the company performs structural analysis by classification society approved finite element analysis (FEA) methods. Teignbridge uses STAR CCM+ for direct coupling of hydrodynamic loads to propeller blade structural analysis by finite element (FEA) methods. Where simulation of complex assemblies or materials with non-linear properties is required, Nastran is used.
FEA methodologies are used to ensure that innovative propeller, rudder and other stern gear designs that fall outside of standard classification society rules and guidelines can be proven to be fit for purpose and accepted for approval. Where structures deform to the point of affecting hydrodynamic properties, such that hydrodynamic loads and structural response are inter-related, Teignbridge has the facility to undertake fully coupled, fluid structure interaction (FSI) studies.
In order to thoroughly explore the complex design space associated with propeller geometry, the company has developed an algorithm driven design optimisation process, ensuring that every last bit of performance improvement is identified.
The complex 3D geometry associated with propeller design and the subsequent, transient flow field that develops over the geometry during operation creates an expansive design space with multiple performance parameters. Design based on this knowledge and experience can be enhanced through the use of algorithm s to ensure that no stone is left unturned in the search for peak performance. Where propeller performance is critical, Teignbridge uses genetic algorithm driven design optimisation to automatically develop and test (by CFD) design variations to hone in on the optimum design.
Whilst Teignbridge makes extensive use of numerical analysis methods, such methods still require verification by physical testing to instil customer confidence and to minimise human error related to model setup. Over the past two years, Teignbridge has developed a floating prototype propulsion system laboratory – HRV1 (Hydrodynamic Research Vessel 1) - for the purpose of physical model testing and rapid prototyping of new ideas.
HRV1’s key feature is her 375kW retractable pod drive system, which provides a highly instrumented test cell. Once fitted with a propeller or other shaftline component, this test cell is lowered through a moon pool in the centre of the vessel’s cabin. An onboard gantry crane enables propellers to be quickly changed at sea, enabling multiple propeller trials to be completed in a day.
Propeller designer creativity is typically hampered by a lack of visibility on the exact performance of a new design at full-scale and in a real-world deployment. Tank testing and cavitation tunnel work provide accurate results, but with challenging hydrodynamic scaling effects (particularly in relation to novel designs), high costs, and long test cycle times.
HRV1 is a different kind of tool, providing the facility to sea trial propellers of up to 1.2m in diameter (compared to typical 0.25m in a tank test), and enabling Teignbridge to rapid-prototype new concepts, working from design to prototype to test results in just a few days.
A wealth of data on the marine environment is collected to minimise the impact of changing environmental conditions associated with model testing outside of a laboratory environment, and all performance assessment work carried out on HRV1 is compared against in-house CFD simulation.
HRV1 is currently configured to support the HEPS project, with a slow-speed, high-torque shaft driven pod configuration, incorporating a six-speed automotive gearbox for speed and power output flexibility. The setup is capable of testing quarter-scale, open water propellers for small to medium sized merchant vessels.
To accurately capture propeller performance data, HRV1 is fitted with a sophisticated array of sensors including gyroscope + accelerometer modules to measure vessel motions, and a propeller shaft mounted fibre optic thrust and torque sensor array, key to establishing hydrodynamic efficiency. Speed through water (by Doppler Velocity Log), GPS, engine data and more, are gathered from the onboard NMEA 2000 system using LabVIEW software which collates and pre-processes performance data before communicating through a wireless link with Teignbridge HQ back on dry land. HRV1 operates out of Torquay harbour in south Devon and uses the sheltered waters of Tor Bay as a test ground.
An early output of the HEPS project is an innovative (international patent pending) concept in propeller design and construction that provides a flexible alternative to traditional monobloc propellers and bolt-on blade designs. The CNC precision machined components are designed to facilitate ease of transportation, storage, installation, repair and replacement.
The modular propeller has several advantages compared to existing mono-bloc and detachable blade designs. A smaller hub increases the working area of the propeller, increasing thrust and reducing drag. The unit can be retrofitted to any shaft (hydraulic or keyed) while its modular construction enables ease of transportation in a container. The low component weight of each part of the system increases ease of fitting. Further, blades can be replaced without dry docking and the vessel operator can carry individual spares for emergency replacement.
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