Through its 50/50 joint venture with Prisco, it chartered out the 29-loa Polar Baikal and the 38-loa Polar Piltun (both built 2000) to Sakhalin Energy Investment Company.
The operations under the latest agreement will start in June in the sea of Okhotsk in Russia.
Both vessels were converted into purpose-built crewboats in 2009 and can carry up to 70 people each.
Rieber did not disclose the value of the deal.
