LeadingEDGE used FINE™/Marine for a full scale ship performance of the Lloyd's Register benchmark test case, which aimed at accelerating the ships hull design process and providing time and cost-efficient alternatives to model testing. The calculations were performed in one day and results accuracy was clearly proven.
According to the 2017 United Nation Conference on Trade and Development (UNCTAD), seaborne trade of goods and raw materials accounts for about 90% of the global trade. One of the main reasons for such a preferred solution is that long-range ocean freight is one of the least expensive methods of transporting goods, due to the high volumes that can be moved in one single trip.
With an overall fleet of such a considerable size, the environmental impact of those ships on sea and atmosphere can no longer be neglected. At the time of writing, it is estimated that over 100,000 merchant ships are trading internationally and sailing around the globe which, according to the International Maritime Organization (IMO), makes up around 3% of global carbon dioxide (CO2) emissions. This trend is not expected to decrease considering world population growth statistics and the related steep rise in the need for supplies in the near future.
With the intent of tackling this potential increase in pollutant emittance, the IMO has suggested the application of stringent regulations for ship emissions, aiming for a 30% reduction of present values by 2025. Ship designers and builders are free to use the most cost-efficient solutions for the ship to comply with these regulations. From a technical perspective, this can be translated into the need for more efficient and accurate design methods in the hands of engineers when predicting the expected ship performances already in the early design phases. Accurate forecasts reduce the risks of exceeding compliance thresholds later, hence avoiding post-built countermeasures which might involve expensive retrofitting solutions.
Computational Fluid Dynamics (CFD) software is increasingly covering a major role in the prediction of ship performance and the optimization of its initial design. Extensive international workshops, aiming at the constant verification and validation of the data produced by those tools, are regularly organized among different institutions, such as research centers and universities, deeply involved in the further development of this numerical alternative to experimental model testing. In 2016 the Technical Investigation Department from Lloyd’s Register organized a workshop on ship scale hydrodynamic computer simulation. The workshop could count on worldwide participation and turned out to be an excellent study case for benchmarking purposes.
Lloyd’s Register test case description
The scope of the workshop was to validate a numerical method to accurately predict full-scale ship performances, aiming at accelerating the design process of ships and providing time- and cost-efficient alternatives to model testing. Participants had to submit numerical results of predicted vessel speed, shaft torque, dynamic equilibrium and propeller cavitation extension at given conditions. These results were then compared against on-board measurements and observations carried out on the cargo ship ‘REGAL’ during sea trials.
The ship under consideration was a 138m general cargo vessel built in 1994 in Poland, with a gross weight of 11542 tons and equipped with one four-bladed, right handed, fixed-pitch propeller.
LeadingEDGE Marine Engineering’s method was mainly built around an advanced Actuator Disk model implemented in the CFD software FINE™/Marine v7.1 which is reading the open water performance of the real propeller. This approach allowed for the removal of the actual propeller geometry from the numerical computations, hence simplifying the physics and considerably speeding up the numerical process, while keeping the focus on the right elements which govern such simulations. Only the bare hull resistance and the self-propulsion cases were needed to validate the method, since the cavitation and the open water propeller tests proposed in the workshop were out of scope for this exercise.
On the contrary from what is customary for hull resistance simulations, instead of half hull geometry the full one was employed. This allowed for the correct inclusion of the small asymmetries due to both the initial hydrostatic equilibrium of the vessel and of the 3D scanned geometry as provided by the workshop organizer.
The process led to accurate results, within a day, thus proving the method is viable for the commercial services currently offered by LeadingEDGE.
The measurements carried out by Lloyd’s Register on the 138m general cargo “Regal”, were used as a reference for the scope. Further analysis of the numerically predicted self-propulsion points has shown excellent agreement with the actually measured vessel speeds (discrepancies lower than 1%). It also highlighted a slight overprediction of the shaft power values (up to 5.5%), which was expected due to the inherent simplifications in the numerical method here presented.
The resolution of Computational Fluid Dynamics (CFD) problems involves three main steps:
• Spatial discretization of the flow domain,
• Flow computation,
• Visualization of the results.
To perform these steps, LeadingEDGE used the FINE™/Marine v7.1 solver with the HEXPRESS™ all-hexahedral unstructured grid generation system and the interactive Computational Field Visualization system CFView™.