On-site measurement and numerical modelling of a lifting operation for caissons using floating crane

Floating cranes are often used for the installation and maintenance operations for offshore wind turbines when they are located in deep waters. As the motions of the vessel being easily affected by external environments, accurate prediction of the motion of the system is important. In this study, first a numerical system based on basic multibody dynamics system is developed for estimation of the behaviour of the lifting operations using floating cranes. Next, on-site measurements are carried out for a lifting operation of a caisson using floating crane. Two numerical models were built with flexible and fixed connectors and validation was performed for the static displacement during lifting operation and the dynamic characteristics of the caisson in the stand-by condition. The static displacements during the lifting operation calculated with de developed tool agreed well with the measurement. The peak frequency of the cargo acceleration in y direction calculated with the flexible model was lower than that calculated with the fixed model. Both models approximately agreed with the measurement. Calculated acceleration of the cargo in x direction largely underestimated the peak frequency of the measurement for both flexible and fixed models. The use of the linear spring to model the wire rope including friction at the end is a possible contributor to the difference.