Construction, installation and retrieval of offshore platforms are carried out in exposed areas and at still growing scales. Not only entire top-sides can be lifted but also the installation of large jackets can now be carried out by vessels with dual cranes.To take full advantage of the large cost-savings related to such operations, the feasibility, workability and risk level have to be evaluated accurately. To this end computer simulations of the dynamic behaviour of the total heavy lift system due to wind, waves, current and hoisting / de-ballasting are required.A general purpose 18 degrees of freedom simulation model covering the mechanism of motion has been developed. The computer model is able to provide the motions of the crane vessel, load and transportation barge and the related tensions in hoisting wires, slings and mooring lines as well as the barge-load impact forces. Special attention is paid to the simulation of partly submerged jacket-type structures. The computer program has been validated with results of an extensive model test program.