The advent of large crane vessels with lifting capacity up to 14000 ton set a new philosophy on platform design and offshore construction projects. Heavy lift operations has given industry the possibility to apply new solutions to offshore engineering representing substantial cost-savings in field development at exposed offshore locations. Performance of offshore operations is a function of the motion characteristics of the vessels and of the environmental conditions. A method is presented for the operability evaluation of crane vessels and related weather downtime. Barge and semisubmersible vessels are investigated in a comparative basis through a linear frequency-domain analysis. The dynamics of lift operations is analysed and emphasis is given to the coupled motions of the load and the vessels during the stationary pendulum condition of the load after lifting off the transportation barge. Hydrodynamic characteristics of the vessels are obtained through a 3-~sink-source distribution method. The environmental data is derived from a spectra ocean wave model to obtain the wave height exceedance probability which establishes the limiting sea states for performing the operation. Different criteria in terms of confidence levels for the exceedance probability of vessel motions are discussed to evaluate the operability. Further, a joint probability distribution of wave height and period is used to obtain the workability of the vessels in a monthly basis. The aim of the paper is to provide a comparison of different types of crane vessels with a methodology for long-term analysis of offshore operations.
Crane ship operations in offshore activities has increased considerably during the last years for many different purposes. Installation of liftable jackets, integrated topsides and subsea equipments, and platform removal, are some examples of the works performed by a crane vessel. The dimensions and lift capacity of crane vessels can vary considerably among the different types of vessels.
