They're drag-chains used to slow down the the ship and it's momentum during launching. The weight and drag of the chains (which look like used anchor cables) creates enough friction to prevent the ship from sailing across the harbour, out of control. Once she's slowed down, the tugs move in and take over to the fitting out dock for final completion. This method of slowing a ship down can be seen in such films as "A Night to Remember" about the TITANIC. Her launching used the same process.