Single And Dual Lift Analyses Of Offshore Structures

At offshore job sites today, decks are lifted onto jacket structures in relatively large, heavy sections. Since lifting operations are greatly affected by the motions of the derrick barge or the crane ship, it is necessary to evaluate the sling system in consideration of vessel motions in order to regulate lifting operations at the job site. This paper proposes a static analysis method for designing sling systems and describes the application of this method to dual-lift operations carried out by two derrick crane barges in Southeast Asian waters. This paper also presents the methods of analyzing the motions of a hoisted load caused by wave-induced vessel motions and resulting dynamic sling tensions in single- and dual-lift operations. Single-lift, unified analysis -- a method that considers the interaction between the barge and the hoisted load -- is discussed, and calculated results are compared with 1/75-scale model test results to clarify the conditions necessary for unified analysis. Finally, the results of transient motion analysis for a barge crawling while lifting a load are compared with model test results.