There are also options for two additional non-FiFi tugs.
The 30-metre by 13-metre vessels have a maximum bollard pull of 80 tonnes.
Gulf Island President Kirk J Meche said the follow-on change orders were from Bay-Houston Towing Company and Suderman and Young Towing Company for the additional tugs.
Primarily designed as a port/terminal ship-handling design to include escort duties, the tugs are powered by Caterpillar (EPA Tier 4) engines and Schottel Z-drive units that allow the tug to generate bollard pulls of 80 tonnes and minimum speeds of 13 knots.
Deliveries begin in late 2018.