Using CFD to enhance the energy efficiency of new and existing commercial shipping vessels

The silent green marine revolution

The brainchild of Dr.-Ing. Rodrigo Azcueta, a naval architect and computational fluid dynamics (CFD) expert, Cape Horn Engineering is a world-renowned marine consultancy based in Portsmouth, United Kingdom with a serious racing pedigree. Azcueta led a team of CFD experts for four America’s Cup Challenges and conducted performance analysis for yachts in the world’s most prestigious races, including the Volvo Ocean Race, the Vendeé Globe and the Transat Jacques Vabre. As industry leaders in CFD and marine technology solutions, Cape Horn Engineering specializes in predicting performance for racing yachts, sailing yachts, superyachts, motor boats, commercial ships, renewable energy structures and more.

Although the marine industry was a little slow to join the green revolution, it has now fully embraced it. Cargo vessels and cruise ships are being refitted with more sustainable propulsion systems, and next-generation vessel designs are incorporating new wind and solar technology. With new regulations on the horizon, energy efficiency and improved performance are top priorities for naval architects who design vessels of all shapes and sizes.

Creating a vessel, especially a complex one, introduces myriad engineering challenges. There could be millions of extremely large and complex parts, components and systems to integrate on something as immense as a cruise ship or specialized marine vessel like a liquified natural gas (LNG) carrier, a polar research ship or a crew transfer vessel (CTV). Ships can require an investment of hundreds of millions of euros for a lifespan of more than 30 years. When you start to design complex and expensive vessels like these, every cent that you can shave off by enhancing energy efficiency contributes to reducing the overall cost of operation and long-term sustainability – not to mention the overall environmental impact of reduced emissions and global warming.

CFD for a greener marine future

The specialist team at Cape Horn Engineering is dedicated to achieving sustainability throughout the marine sector, helping naval architects successfully reduce emissions and improve energy efficiency by implementing new, more ecologically friendly energy sources and analyzing design options.

One of the key new technologies they use is a simulation process to make sure a ship design meets or exceeds the upcoming Carbon Intensity Indicator (CII) and Energy Efficiency Existing Ship Index (EEXI) regulations. The International Maritime Organization (IMO), a specialized United Nations agency, has set a policy framework to gradually reduce carbon intensity by at least 40 percent by 2030, and total greenhouse gas emissions by 50 percent by 2050.

Although CII is an operational measure that assesses how efficiently a ship transports its cargo according to real-time fuel consumption, EEXI is a technical measurement that considers the vessel’s design parameters. It is comparable to the Energy Efficiency Design Index (EEDI) for new builds. The EEXI measures the design carbon dioxide (CO2) emissions relative to the vessel’s size and speed and translates this to emissions per cargo ton per mile. The IMO has set limits regarding the allowable EEXI according to vessel size and type. Every ship will be required to comply with the EEXI in 2023.

“Embracing CFD technology gives ship owners a cost-effective means to explore a range of solutions to ensure they are on target to meet the upcoming EEXI regulations,” says Azcueta, who is managing director of Cape Horn Engineering. “It also provides the ideal environment to test and optimize novel energy saving devices, such as wing sails. There are several possible solutions that must be considered to reduce the environmental impact of the shipping industry, each of which has a very important role to play.”

Improving efficiency of existing vessels

Simcenter™ STAR-CCM+™ software, which is a CFD solution that is part of the Siemens Xcelerator business platform of software, hardware and services, can be used to help calculate the EEXI for older vessels by developing speed power curves to update existing documentation. This is a much faster and more efficient method than traditional towing tank testing. Simcenter STAR-CCM+ can also be helpful when engineers need to investigate potential energy efficiency improvements, such as adding wind-assisted ship propulsion (WASP) devices and calculating their impact on the EEXI. To meet the new regulations, ship owners consider WASP devices such as wing sails, suction sails, Flettner rotors, etc.

“WASP devices can potentially cut fuel costs by 10 to 30 percent, but they are highly complex systems to model,” says Azcueta. “Ideally, both the hydrodynamic and aerodynamic effects need to be modeled simultaneously in a single simulation. We have developed a simulation workflow to directly compare WASP device efficiency and determine potential savings. Wind conditions above the water surface are modeled with an accurate wind profile taking into account the atmospheric boundary layer wind gradient.”

Validating WASP technology with simulation workflow

With CFD simulation technology, naval architects and engineers can investigate the performance of WASP options virtually to make the most effective choice according to the vessel’s requirements. Now in a single simulation the team can model both hydrodynamics of the water flow experienced on the hull and the aerodynamics of the air flow experienced on the top sides of the hull, superstructure and WASP devices at certain vessel and wind speeds.

Of course, accuracy is always important, which is why engineers need to be sure to quantify and understand the simulated CFD results via a solid verification and validation process. Today, with increased computer power and more accurate software and processes, digitalized versions of vessels are reflecting real-world performance quite accurately. This is what is referred to as a digital twin.

To showcase this solution, Cape Horn Engineering developed an example comparing two WASP configurations on a 138-meter (m) general cargo vessel, the MV Regal. Benchmark data was readily available for the MV Regal, making it an obvious choice for verifying and validating the process.

The first option examined consisted of two three-wing, three-flap wing sails mounted on the deck. The second option replaced the two wing sails with two similar-sized Flettner rotors.

“Our aim was to demonstrate our simulation workflow as a feasible way to compare different types of WASP devices with a highly accurate, all-in-one, 6 degrees-of-freedom hydrodynamic and aerodynamic simulation based on Simcenter STAR-CCM+,” explains Azcueta. “Even unoptimized, there was a 14 and 24 percent power reduction thanks to the WASP devices. Simcenter STAR-CCM+ can clearly help ship owners make an informed decision when it comes to improving efficiency to achieve energy savings and meet EEXI regulations.”

Enter artificial intelligence

Modeling wing sails can be accurately performed in realistic conditions, but it still takes supercomputer capacity to run these types of simulations, especially when optimization loops are included. In an ideal world, the simulation runs for a study like this could hit the hundreds or even thousands of simulations. To address this issue, Cape Horn Engineering is working together with several companies and universities to train AI models to use reduced order modeling and design neural networks to compute the aerodynamic forces in seconds rather than hours.

“Incorporating AI into the picture opens vast possibilities to optimize the wing sail design and develop intelligent control systems to ensure the largest possible reduction in emissions,” adds Azcueta.

“Our niche area of CFD expertise has so much potential to offer greener shipping solutions. It is not often that your interests align in such a way, so it is an exceptional and exciting opportunity for everyone involved. Collectively we can make a difference on a global scale.”