Dynamics of Heavy Deck Lift-Off From Transportation Barge

Abstract

The size of present day offshore topsides are growing rapidly, with large capacity, complex modules being transported and lifted offshore. This increase in size results in offshore lift weights which are near crane capacity of HLVs (heavy lift vessels). The offshore installation operation involves lift-off of heavy modules from a transportation barge moored perpendicular to the stern of the HLV. Previous experiences with lift analyses indicate that most significant hook load occurs while the module is being lifted off the transportation barge. This can be attributed to the fact that the crane vessel, topside and the transportation barge are all dynamically coupled together and during the “lift-off” condition the module experiences upward and downward impulsive forces from the transportation barge, which in turn increase the apparent weight of the module and hence the hook loads. This paper studies the dynamics of heavy deck lift-off from the transportation barge, when the sea fastening from the module has been removed and all three bodies (transportation barge, HLV and Module) are mechanically coupled through slings, lashings and fenders. The effect of various crane line pretensions, wave, current and wind has been investigated and the resulting DAFs (dynamic amplification factors) of the hook loads have been compared. The effect of wind and current on the lift operation has also been investigated. The “lift in air” case has been studied and compared with the “lift off” case. Multi Operational Structural Engineering Simulator (MOSES) has been used to carry out 3D time domain analysis of lift off and lift in air cases. McDermott’s HLV, 300 class transport barge and a topside weight of 1,500MT have been analyzed for the study.