QatarEnergy has signed a deal with China State Shipbuilding Corporation to buy six additional ultra-large ships to carry liquefied natural gas, it said on Monday, raising such vessels it has on order to 128 as part of a fleet-expansion programme.

The ships, QC-Max, are the largest LNG vessels ever built, with a capacity of 271,000 cubic metres each, QatarEnergy said. They will be built at China's Hudong-Zhonghua shipyard, with delivery expected between 2028 and 2031.

QatarEnergy, already among the world's top LNG exporters, will boost its position with its North Field expansion project, which will ramp up Qatar's liquefaction capacity from 77 million tons per annum to 142 mtpa by 2030.

The agreement with CSSC takes the total number of QC-Max vessels on order by QatarEnergy to 24, worth a total of about $8 billion, QatarEnergy said.