Hybrid Design Reworked After Tug Fire

Carolyn Dorothy
Carolyn Dorothy

By MarEx 2016-07-05 18:46:15

The U.S. Maritime Administration has released its findings on a project to reconstruct the power plant of a hybrid tug after it caught fire.  

The assessment, which was conducted after a battery-related fire on board Foss Maritime’s Campbell Foss, looks at the benefits and risks associated with the reinstallation of batteries as part of the vessel’s hybrid power system. The assessment showed that a refined design with explosion protection, structural separation from occupied spaces, specialized battery controls and shutdown protocols improved the risk profile for the hybrid power system.  

The assessment also demonstrated that without the battery array, the hybrid system would not achieve the tug’s full performance requirements nor the emissions and fuel consumption reductions made possible by hybrid technology. 

In August 2012, an explosion and fire occurred in one of the lithium‐polymer batteries on the Campbell Foss. Subsequent to that fire, Foss removed the remaining batteries from the tug and all of the lead‐acid batteries on Foss’ other hybrid tug, Carolyn Dorothy. Campbell Foss was returned to service in diesel configuration without batteries, and Carolyn Dorothy was returned to service in a modified hybrid configuration that did not require the use of batteries. 

The Maritime Administration (MARAD) then provided $600,000 in funding to put batteries back into the two tugs, and a risk assessment incorporating lessons learned from the fire was carried out by Elliott Bay Design Group. 

The risk assessment showed that with a properly designed and installed battery system, with structural separation from occupied spaces, proper explosion protection and correct and functioning battery control and shutdown protocols, the hybrid boats would be able to realize their full potential in performance, emissions and fuel consumption reductions.  

Conversely, the cost and risk of not reinstalling batteries was high, based on increased fuel consumption and emissions, and decreased usability of the Carolyn Dorothy due to lower bollard pull. The lower bollard pull is a direct result of the fact this vessel has smaller main engines than her sister tugs; therefore requiring a large array of batteries to supplement the installed diesel power in high load conditions.

The modifications made to the tugs included:
•    Installation of bulkheads, inserts, structure and doors to create the new battery compartments
•    Installation of rupture discs between the battery compartment and the adjacent void, and the void and the exterior of the boat.  In the event of a battery explosion, these discs would blow out at a low pressure to allow the escaping gasses to expand without damaging the structure of the compartment
•    Installation of air conditioning and air handling units to keep the compartments cooled to approximately 65F
•    Installation of fire suppression systems to smother any potential battery fires
•    Containment boxes in stern bulwarks to direct any potential flame ball or hot escaping gasses out the stern of the vessel and away from personnel.
•    Insulation of new battery compartment surfaces.

The Carolyn Dorothy, the world’s first hybrid tug, and the Campbell Foss are now back operating in San Pedro harbor in full hybrid mode with batteries functioning and fully integrated into the system. The vessels continue to show a significant reduction in emissions compared with the conventional tugs operating in this same harbor.

The report can be found here.