Boosting safety and profitability using CFD for escort tugboat certification

Driving ship design innovations

Robert Allan Ltd. (RAL) is a naval architecture and marine engineering company in Canada. Since 1928 it has been designing vessels such as tugboats, passenger ferries and research ships. It is renowned for its innovative designs and is a world leader in the global tugboat market. With so many potential dangers at sea, RAL uses computer-aided design (CAD) and computational fluid dynamics (CFD) technology to ensure the highest standards of safety and operational integrity for every vessel. For this, RAL engineers rely on Simcenter™ STAR-CCM+™ software, which is a part of the Siemens Xcelerator business platform of software, hardware and services.

Increasing marine safety using escort tugboats

As the frequency of marine traffic increases, it is essential to ensure safe transit through confined waterways using escort tugboats. This is especially important for tankers carrying liquefied natural gas (LNG) and crude oil, where an incident could have high consequences. Since the Exxon Valdez oil spill in Alaska in 1989, it has become mandatory in most ports
worldwide for tankers to use escort tugboats.

These tugboats can exert steering and braking forces at transit speeds of 6 to 12 knots to maneuver the escorted vessel to safety.

However, escorting at these speeds requires significant power and stability to counter the increased heeling movement from the towline.

Brendan Smoker, project manager and hydrodynamics engineer at RAL, explains that engineers design tugs to take advantage of the indirect towing mode. “Rather than relying only on the thrusters to produce a towline force, the escort tug attaches to the tanker’s stern and uses the hydrodynamic lift and drag generated by its hull,” says Smoker. “This leverages the direct thrust available from the thrusters so at speeds above 6 knots, the escort tug can generate steering and braking forces in excess of its rated bollard pull.”

Generating more power with fewer emissions

Smoker highlights the modern challenges of escort tugboat design is generating high escort forces while maintaining a safe stability margin. “Customers want smaller boats but with more power,” says Smoker. “Further, they can’t compromise on safety, and they want vessels to be environmentally friendly. As the demand for de-carbonization increases, we are looking at electric or alternate fuel engines, which also introduces new engineering challenges.”

Using CFD for ship performance prediction engineering

To meet these challenges, RAL engineers rely on Simcenter STAR-CCM+ for CFD simulations. "We have a much larger CFD department and capability than some other naval architecture firms of our size,” says Smoker. “Performance prediction is a vital aspect of the development process as it lets us know if our designs will meet requirements as early as possible.”

In the past, designers used expensive physical testing via small models in towing tanks or full-scale vessels at sea. The emergence of CFD allows designers to carry out more frequent virtual testing earlier in the design phase, faster and at a lower cost. The team doesn’t have to wait until the design is finalized to build the scale model to test. “We still use physical tests; however, it is becoming more common to use CFD validation,” says Smoker. “Our engineers can use simulations to optimize designs faster, which is useful since towing tanks can be expensive and difficult to attain. Using Simcenter STAR-CCM+, we can speed up our processes and save money.”

Further, using Simcenter STAR-CCM+, engineers can carry out full-scale resistance and escort studies. This eliminates scaling effect errors, allows separate pressure and shear force calculations and provides the flexibility to test several hull configurations and conditions without the expense of constructing several physical models and conducting tests.

Certifying and classifying tugboats digitally

Certifying and classifying tugboats is crucial to ensuring safety. RAL engineers, led by Smoker, discovered a potential way to use CFD simulations to enable a fully numerical certification process.

“The traditional method for achieving certification was full-scale testing,” says Smoker. “You had to order a tanker to run a test that was incredibly expensive and offered limited reliability. For example, monitoring instruments might break, or the tug operator might not drive it optimally to create the proper forces and behaviors. You would certify tugboats for certain performances, but you’d need to explain that it was based on trials that varied widely in quality. It made it impossible to accurately compare one tugboat’s performance to another.

“Alternatively, it was possible to use scale model testing in a towing tank. However, same as full-scale testing, it is difficult and costly to make design changes at that stage once identifying issues.”

For RAL engineers, using Simcenter STAR-CCM+ simulation for certification was the obvious way forward. “Thanks to simulation software, we have proved that it can simplify certification processes,” says Smoker. “Using Simcenter STAR-CCM+, our results are more accurate than physical model testing because we can analyze the boat consistently and compare stability,
safety and forces to get a reliable assessment of how each boat will perform.”

After publishing a paper recommending CFD certification, Smoker was first approached by Bureau Veritas, a marine classification society. “By working together, we were able to improve our methods and satisfy all their validation and classification requirements. We were issued an approval in principle (AIP) certificate, meaning they accepted our method of using CFD in place of full-scale trials or model testing.”

Increasing profits and speeding up turnaround time

Over time, RAL engineers received approval for the CFD method from many other marine classification societies such as the American Bureau of Shipping (ABS), Lloyd’s Register and Det Norske Veritas (DNV). “We’ve spent a lot of time with numerous societies to make our reports into standards that everyone will accept,” says Smoker. One crucial factor for convincing the classification societies was the ability to compare CFD data with previously collected model testing data. “That allowed us to show that our methods were just as good, if not better,” explains Smoker.

As a result, the company is submitting as many as 50 reports per year, which is about 200 percent more than they could do with traditional model and full-scale methods. “It is a huge boost to competitiveness,” says Smoker. “We use Simcenter STAR-CCM+ to turn around projects faster, which allows shipyards to increase their capacity and build more tugs. It is a boost for everyone.”

Using an established product like Simcenter STAR-CCM+ made it is easier to get the classification societies on board. Since many of them were already familiar with the software, they understood and accepted the quality of the results. “Proving our work with Simcenter STAR-CCM+ was much easier than if we had developed our own custom code and had to persuade them that it worked,”
says Smoker.

Finding and leading with unique solutions

Thanks to the efforts RAL’s CFD team put into standardizing the CFD method, they are regarded as experts in CFD simulation and escort tug classification in the marine industry. Their work has not only convinced various classification societies to accept CFD instead of model testing but also helped harmonize class rules across them. “We’ve had a big influence on the rules,” says Smoker. “In some instances, we’ve pointed out gaps that our work highlights. We’re constantly pushing for even higher safety standards.”

Further, using Simcenter STAR-CCM+ has led them to previously unexplored designs. Smoker highlights one case with a particular type of tugboat used in Australia. They weren’t sure it would meet all the performance requirements and CFD simulation confirmed this concern. However, the Simcenter STAR-CCM+ analysis suggested a change to the skeg design that wasn’t intuitive. “Using Simcenter STAR-CCM+, we found an unconsidered, unintuitive change. We tried it, validated it with model testing and it did exactly what we needed. It performed well and impressed the customer.”

As RAL leads the way in both design and classification innovations, their engineers work to ensure customers are confident in their solutions and that the boats will meet performance requirements and classification standards. “Knowing that customers are not returning with issues is a great success for us,” says Smoker.

Driving forward innovation

While already achieving a great deal, Smoker and his team are always looking to improve and take advantage of the latest technological developments. “The next step is a solution that gets closer to real-life maneuvers,” says Smoker. “If we could physically drive the boat in a CFD environment including all the physics around it in real time, enabling us to capture the behavior in all condition types, we could simulate more variables and understand more unexpected situations.”