Navigation Maritime Bulgare has acquired two pairs of 37,850dwt and 30,700dwt ships for $115m.
The ice-class 1C ships, Rodina and Bulgaria, are equipped with four cranes each and are capable of lifting up to 30t.
The other two vessels, Vitosha and Strandja, have a service speed of 14kt and an operational range of 14,800 nautical miles.
The company is also planning to purchase another four 30,700dwt and two 17,500dwt vessels with an investment of $145m by the end of 2010.
Navibulgar has set a target to purchase 25 ships for $525m in ten years.