Some parts of the valve are designed be changed subsea without leaking oil everywhere. For these an ROV (remotely operated vehicle – an underwater robot) can be used to remove the broken parts. The ROV is driven from the ship on the surface. If the valve has broken in a way that the broken parts cannot be changed subsea, then the flow of oil/gas has to be stopped, other valves have to be closed and or plugs have to be pushed into the pipeline to stop oil leaking out and sea water leaking in. Then ROVs are used to disconnect the whole valve and bring it to the surface where it is repaired or replaced. The whole process can take weeks and cost millions of pounds. All the time this is goIng on, it may have been necessary to stop producing the oil. The cost and safety risk of repairing valves (or any other equipment) subsea is huge, so there is a real incentive to design them to be as reliable as possible.