A method for retrofitting an existing pedestal mounted crane with a flywheel energy storage system, wherein the existing pedestal mounted crane comprises: - an existing pedestal, the pedestal extending between a foot thereof that is adapted to mount or mounts the pedestal to a structure, for example an offshore structure or a hull of a vessel, and a head thereof, wherein the head is configured to mount a slew bearing thereon; - an existing slew bearing mounted on the head of the pedestal, the slew bearing having an inner diameter and a vertical slew axis, - an existing revolving crane superstructure, which revolving crane superstructure is mounted on the slew bearing, the slew bearing enabling slewing of the crane superstructure relative to the pedestal about the slew axis, wherein the revolving crane superstructure comprises a crane boom, - an existing hoisting system configured for lifting and lowering a load, the hoisting system comprising a motor driven hoisting winch and an associated hoisting wire, as well as one or more sheaves configured to guide the hoisting wire, at least one of said sheaves being mounted on the boom and the hoisting wire depending from said sheave to a load connector of the hoisting system, wherein the method comprises: - providing a flywheel energy storage system comprising: - a flywheel having an outer diameter and a rotational axis, - a flywheel support structure provided with one or more bearings that support the flywheel rotatably relative to the flywheel support structure, - an electric motor-generator coupled to the flywheel, - accommodating at least the flywheel and at least a part of the flywheel support structure in the pedestal, e.g. in the existing pedestal and/or in a pedestal extender that is optionally mounted on the head of the existing pedestal with the existing slew bearing being removed from said head and a slew bearing then being mounted on a head of the pedestal extender, - providing an electrical circuit between the motor-generator of the flywheel energy storage system and an electric motor that drives the hoisting winch. - an existing pedestal, the pedestal extending between a foot thereof that is adapted to mount or mounts the pedestal to a structure, for example an offshore structure or a hull of a vessel, and a head thereof, wherein the head is configured to mount a slew bearing thereon; - an existing slew bearing mounted on the head of the pedestal, the slew bearing having an inner diameter and a vertical slew axis, - an existing revolving crane superstructure, which revolving crane superstructure is mounted on the slew bearing, the slew bearing enabling slewing of the crane superstructure relative to the pedestal about the slew axis, wherein the revolving crane superstructure comprises a crane boom, - an existing hoisting system configured for lifting and lowering a load, the hoisting system comprising a motor driven hoisting winch and an associated hoisting wire, as well as one or more sheaves configured to guide the hoisting wire, at least one of said sheaves being mounted on the boom and the hoisting wire depending from said sheave to a load connector of the hoisting system, - providing a flywheel energy storage system comprising: - a flywheel having an outer diameter and a rotational axis, - a flywheel support structure provided with one or more bearings that support the flywheel rotatably relative to the flywheel support structure, - an electric motor-generator coupled to the flywheel, - a flywheel having an outer diameter and a rotational axis, - a flywheel support structure provided with one or more bearings that support the flywheel rotatably relative to the flywheel support structure, - an electric motor-generator coupled to the flywheel, - accommodating at least the flywheel and at least a part of the flywheel support structure in the pedestal, e.g. in the existing pedestal and/or in a pedestal extender that is optionally mounted on the head of the existing pedestal with the existing slew bearing being removed from said head and a slew bearing then being mounted on a head of the pedestal extender, - providing an electrical circuit between the motor-generator of the flywheel energy storage system and an electric motor that drives the hoisting winch.