Perhaps a little more info would help. For instance, is your crane going to be dockside, or will it be on the barge? Also, the maximum height the crane needs to be to facilitate the loading, and do you want the cane to be mobile.
From what you're describing so far, that will be one huge crane. A crane that can lift 50 tons at a 60 foot radius sounds like a 3,000 ton crane if you do the math, (50 x 2000# x 60'). That's short tons @ 2,000 pounds a ton versus the 2,204.6 pounds in a metric tonne, if that matters.
Very basically, the way slewing cranes (rotating cranes) work can be broken down into a formula that uses foot pounds to determine the force and strength needed to move a load. A foot pound is a weight or force that is equal to one pound, one foot from the center of the rotating axis. That's the vertical axis running through the slewing gear, the slewing gear being the part of the crane that allows rotation. So a one pound load 60' from the rotating axis equals 60 foot pounds, and a 60 pound load 1' from the rotating axis equals 60 foot pounds, as well.
Now, if you want to lift 50 tons (100,000 pounds) at a 60' radius, the slewing gear will see 6,000,000 foot pounds (60 X 100,000) of force applied to the slewing gear. That's 3,000 tons of force. That's a BIG crane for a rotating crane. Additionally, the weight of the crane will have to be able to withstand that 3,000 tons of force trying to tip it over. It's a balancing act so you'll need a like-kind force on the opposite side of the slewing axis to balance he load.
Now, a gantry crane (two 'legs' supporting a load beam) is a different animal and can handle those big loads using different geometry and may be something you could consider if your crane had the ability to span the waterway where your barge is to be loaded. Put it on tracks and it's mobile...
All of the above is just food for thought, so any additional info you could provide will no doubt help with answering your question.