A mooring arrangement for a semi-submersible offshore structure having an outer hull with a lower (2) and an upper (1) water line, comprises a mooring line (4) extending from a fixed point on the seabed to an attachment on the offshore structure, at least a portion of the mooring line closest to the attachment being a chain. The attachment comprises a hawse (10) and a chain stopper (22) located below the lower water line. The chain stopper (22) is located in a dry space inside the outer hull and is sealingly attached at one end to the hawse and at the opposite end to a chain pipe (9) which extends inside the hull to above the upper water line. The hawse extends outwards from the chain stopper and is sealed to an opening in the outer hull.