Damen develops fuel-flexible tug designs

Damen Shipyards has developed naval architecture for harbour and coastal tugs ready for the adoption of alternative fuels to diesel as the towage industry aims to reduce greenhouse gas emissions in ports.

The Dutch shipbuilding group is generating designs for fuel-flexible, azimuth stern-drive tugboats that will be able to run on methanol along with marine gasoil using dual-fuel engines, providing the flexibility for owners to switch between fuels.

Damen system owner for alternative fuels, Pieter Spruijt, explains why methanol fuels could be used by tug owners to cut emissions in harbours, and the challenges of designing tugs to operate on these fuels.

“Methanol is widely available, and ports are gaining experience in its bunkering, and are slowly going to green methanol,” he says. “Challenges with methanol include its energy density is half of diesel; it is more flammable than diesel; and it is toxic to inhale at high concentrations for a long period of time.”

Ships built to use methanol fuel need to follow IMO rules and regulations for its storage, handling and hazardous zones. 

For example, methanol must be stored in tanks with cofferdams to act as a secondary barrier, which leads to a 50% smaller tank which is challenging for harbour tugs. Another challenge is positioning venting and over-pressure mitigation, requiring 10-m hazardous zones on deck or around the wheelhouse.

These are difficult to incorporate in tugboat designs due to their compact size.

“Our design philosophy is it has to be future proof and fool proof, and needs to stay an efficient design,” says Mr Spruijt. “We could not design a fuel-flexible tug to prescriptive rules, so we needed to do a risk-based approach, which takes time and energy.”

Damen worked with classification society Bureau Veritas, and Dutch and Belgian flag states, using a risk-based approach to develop its fuel-flexible tug designs. Under prescriptive rules, cofferdams are required to mitigate impact of a leak from a methanol tank, ie to contain the leak. Damen’s alternative design focuses on avoiding the likelihood of such a leak occurring. 

By incorporating gas detection in compartments adjacent to methanol tanks, this solution is assessed to be as safe as the prescriptive design using appropriate risk assessment tools. 

“The design reduces leakage probability through more stringent requirements for allowable stresses and fatigue analyses, coupled with enhanced production measures such as full-penetration welding, among others,” says Mr Spruijt.  

“These strict design and production methods maximise tank volume, increase vessel autonomy, and decrease size, keeping vessels compact, and therefore efficient in fuel consumption.” This arrangement, including the safety measures, has been class approved in principle and is awaiting patent approval. 

In overcoming the next challenge - venting and hazard zones - Damen focused on removing oxygen from the tanks and handling areas. “This involves inerting enclosed spaces by filling them with nitrogen, which is approved in principle,” says Mr Spruijt. “Nitrogen replaces the air and prevents methanol leaks from forming an ignitable mixture with oxygen. As a result, the risk of fire or explosion is mitigated.”

By keeping the space sealed, no methanol vapour is released into the atmosphere, eliminating the need for hazardous zones and allowing the crew to operate without restrictions. “For maintenance purposes, a ventilation system is installed which can purge the space before entering,” says Mr Spruijt. “Hazardous zones are prevented by sealing and inerting the fuel preparation space, and by keeping the tank closed in all pressure scenarios.”