It is well know that there is a difference between “marine crane” and “offshore crane”.
The former is allowed to operate in sheltered waters while the latter in open sea. A marine crane is mostly intended for harbour operation and are mainly subject to ILO certification. An offshore crane is subject to dynamic forces due to waves, ship motion, vertical spring rate, type of operation (ship-to-ship, handling of submerged equipment) and are more and more require to be provided with proper load chart by offshore stakeholders.
Many ship-operators, involved with offshore market, have their vessels fitted with a crane wich sometime is not certified for offshore.
Medea has established consolidated and approved method to derate a “pure” marine crane in an offshore crane. This is a sample: http://www.medeadesign.com/project/convertion-marine-cranes-offshore-cranes/
Conversion of “marine” cranes in “offshore” cranes