A maintenance system for a tower located in the sea comprising: • a maintenance capsule for transporting cargo and maintenance people to and from the tower, the maintenance capsule including: a seaworthy watertight, floatable container enabling the cargo and maintenance people to be loaded and unloaded from the container; • a swivelable crane assembly extendable from the tower, said crane assembly including: a boom for extending from the tower over the sea; • and a capsule attachment line holding-and-releasing apparatus for selectively holding and releasing a capsule attachment line for selectively enabling said maintenance capsule to be moved to or from the tower, characterized in • that said tower is a wind turbine tower and that the cargo to be transported by the maintenance capsule includes windturbine tools and maintenance parts, • that the crane assembly includes a trolley (124, 800) for engaging said maintenance capsule via said capsule attachment line for movement to and from the wind turbine tower, • that said trolley comprises a trolley movement structure for moving along said boom, said trolley movement structure moving said trolley along said boom to and from the wind turbine tower, • that said maintenance capsule has an elongated configuration and can be oriented in an upright position for containing the maintenance people in a safe and comfortable position, and a capsule support apparatus for being carried by a maintenance vessel for supporting respective maintenance capsules in an upright position, said capsule support apparatus comprising a launch/recovery cradle, • that the floatable container is able to safely survive being disposed in the sea vertically to float on its side and protect the people and cargo located in said container, • and that the maintenance capsule has a thruster for moving said maintenance capsule in the sea when said maintenance capsule (10 is disposed floating in the sea on its side in a desired direction. • a maintenance capsule for transporting cargo and maintenance people to and from the tower, the maintenance capsule including: a seaworthy watertight, floatable container enabling the cargo and maintenance people to be loaded and unloaded from the container; • a swivelable crane assembly extendable from the tower, said crane assembly including: a boom for extending from the tower over the sea; • and a capsule attachment line holding-and-releasing apparatus for selectively holding and releasing a capsule attachment line for selectively enabling said maintenance capsule to be moved to or from the tower, characterized in • that said tower is a wind turbine tower and that the cargo to be transported by the maintenance capsule includes windturbine tools and maintenance parts, • that the crane assembly includes a trolley (124, 800) for engaging said maintenance capsule via said capsule attachment line for movement to and from the wind turbine tower, • that said trolley comprises a trolley movement structure for moving along said boom, said trolley movement structure moving said trolley along said boom to and from the wind turbine tower, • that said maintenance capsule has an elongated configuration and can be oriented in an upright position for containing the maintenance people in a safe and comfortable position, and a capsule support apparatus for being carried by a maintenance vessel for supporting respective maintenance capsules in an upright position, said capsule support apparatus comprising a launch/recovery cradle, • that the floatable container is able to safely survive being disposed in the sea vertically to float on its side and protect the people and cargo located in said container, • and that the maintenance capsule has a thruster for moving said maintenance capsule in the sea when said maintenance capsule (10 is disposed floating in the sea on its side in a desired direction.