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 lob 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.
Lifting heavy deck structures by crane constitutes on important phase of an offshore installation project. Bunce et al assessed the lifting criteria in current use and offered a comprehensive suggestion concerning weight escalation, dynamic amplification and sling tension redundancy. However, little consideration was given to operational limits and other aspects of installation, such as the movement of a hoisted object caused by wave-induced vessel oscillations and criteria for regulating crane operations. We once studied the relationship between the motions of the barge and the hoisted load based on field measurements and proposed operational limits.
More recently, we made a unified analysis of the motions of a barge and a load being hoisted — similar to the study made by Nojiri et al of coupled motion between vessel and hook load — and compared the calculated results with model test results. We thus clarified the conditions necessary for unified analysis.
These analyses, along with sling analyses for crane operations, are described in this paper.
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