Equinor and its partners in the huge new Arctic oilfield Johan Castberg will invest $396 million (4 billion Norwegian crowns) to construct the first tie-back of another discovery to the Castberg hub that would boost oil production from the area. The investment decision by Equinor, Vår Energi, and Petoro was made just eight months after Johan Castberg came on stream, the Norwegian energy major said on Thursday. Equinor announced in the summer that Johan Castberg hit full capacity of 220,000 barrels per day…