The charter period is expected to begin in 2018-19 and has a maximum 25 years. Ship owners are subsidiaries of Mitsui OSK Lines Ltd. and Nippon Yusen Kabushiki Kaisha.
The LNG carriers each have capacities of 177,000 cu m and were built by Mitsubishi Heavy Industries Ltd.
Mitsui said contracts for five other vessels were signed in 2014. A contract for an eighth vessel is expected.