Better battery compartment design – with explosion protection, structural separation from occupied spaces, specialized controls and shutdown protocols – can significantly reduce risk for hybrid power systems, according to a new report.
The findings come out of a U.S. Maritime Administration partnership with Foss Maritime Corp., Seattle. Foss used a $600,000 Maritime Environmental and Technical Assistance (META) grant to document the refit of two hybrid tugboats.
A 2012 lithium battery-related fire in the 78’x34’x15’ Campbell Foss provided an accidental starting point for the project. The Campbell Foss is sister ship to the Carolyn Dorothy, Foss’ first hybrid tug and a newbuild. The Campbell Foss was converted to hybrid power in 2012 using new, more powerful lithium polymer batteries.
After about seven months in service, the Campbell Foss suffered an explosion and fire in one of its batteries. Subsequently, Foss removed all the lithium batteries and returned the Campbell Foss to service with diesel power. The lead-acid batteries on the Carolyn Dorothy were likewise removed, and that vessel returned to service under modified hybrid power using its generators for electric power.
At the time of the fire, Foss had applied to Marad for the META grant, with the intent of converting a third Dolphin-class ship-assist tug to hybrid power. Concerned about the safety of batteries, Foss officials put off that conversion. Instead, they suggested using a META grant to replace the batteries, with a risk assessment and redesign using lessons learned from the fire.
“There is no doubt that hybridization is an excellent way for vessels with variable duty cycles, such as harbor tugs, to reduce all emissions as well as fuel and maintenance costs,” the Marad report notes. Foss’ hybrid technology was verified for diesel reduction by the U.S. Environmental Protection Agency in 2012, “but has yet to gain widespread acceptance in the United States,” the report adds.
One reason may be unease over the safety of lithium batteries, thanks to documented problems of battery fires with laptop computers and other consumer products.
Foss got started in October 2014 by engaging Elliott Bay Design Group, Seattle, to undertake a risk assessment study.
EBDG's naval architects showed that a properly designed and installed battery system, with structural separation from occupied spaces, explosion protection, and the right battery control and shutdown protocols would work. With that, the hybrid tugboats “would be able to realize their full potential in performance, emissions and fuel consumption reductions.”
The battery compartment in Foss' Carolyn Dorothy. Photo courtesy Marad.
Not reinstalling batteries carried its own costs and risks – higher fuel consumption and emissions, and less capability for the Carolyn Dorothy, designed expressly for hybrid power and needing the battery array to achieve its full 65-ton bollard pull for peak loads.
Designers looked for a new battery compartment, and settled on the stern void space as the safest and simplest for engineering the ventilation system. The stern void space was ultimately chosen because it offered the safest alternative and simplified the engineering of the ventilation system.
New FM200 fire suppression, rupture discs to vent explosive gases at low pressure, and vents to release heat and gases away from the crew area were all part of the redesign, along with air conditioning to keep the battery space cooled to 65°F.
The project engineers overcame unexpected problems, like electromagnetic interference from the motor shafts that disrupted communications between the battery controls and software systems. The work was delayed too in May 2015 when airline carriers changed restrictions for air shipping of lithium batteries – requiring the new batteries from EST Technologies to cross the Pacific by ship.
Both tugs re-entered service in February 2016. Foss officials estimated the total cost of the project at around $1.5 million.
