For a second time, Ineos has increased the number of vessels required to transport US shale-gas derived ethane to its planned terminals at Grangemouth, Scotland, and Rafnes, Norway.

In May of this year, the Swiss-based group said it would order two additional "Dragon Ships" from supplier Evergas of Copenhagen, Denmark, raising the number then from four to six.

The vessels are also capable of transporting LPG, LNG and petrochemical gases including ethylene.

Ineos' terminal and ethane storage tank at Rafnes is nearing completion and expected to be fully operational some time next year. The tank and terminal at Grangemouth is due to be completed in 2016.

David Thompson, chief operating officer of Ineos Trading & Shipping, called the top-up ship order "another important milestone in our plan to bring the benefits of shale US gas economics to our European sites." Ethane is "the most competitive feedstock in Europe," Thompson remarked.

In October 2012, Ineos signed the first-ever agreement to ship US ethane to Europe. In the mean time, a number of other European companies have followed suit.

The formerly British chemical group also has taken stakes in two shale gas exploration licenses covering areas near its Grangemouth production complex.