Y bwriad yw ei gwblhau erbyn yr Hydref, a rhagwelir y bydd y budd i economi Cymru yn golygu cynnydd o £4 biliwn mewn gwerth ychwanegol gros.
It is scheduled to be completed in the autumn and it is predicted that the benefit to the Welsh economy could be as much as a £4 billion GVA increase.