A riser system for transporting slurry from the seabed to the sea surface is provided. The riser system comprises a first and second riser, a slurry pump system (17) to transport slurry up one of the risers, and a waste water pump system (107) to return waste water down one of the risers. The slurry pump system and the waste water pump system are selectively connectable to each of the risers to allow each riser to be either a slurry riser (1) or a waste water riser (2). With this arrangement, if a slurry riser develops a leak partway along its length the waste water riser can be converted into a slurry riser such that operation can continue.