Monitoring Offshore Lift Dynamics

ABSTRACT This paper illustrates the significance of lift dynamic aspects observed during two major offshore heavy lift operations performed in 1991. Extensive offshore measurements provided further knowledge related to the dynamic behaviour of heavy lift systems offshore. The contribution of lift dynamics to the overall response in the medium frequency range was found to be of similar magnitude as the response in the wave frequency range. Initial correlation studies with computer models show that this aspect was underestimated by the analyses. INTRODUCTION The large semi submersible crane vessels (SSCVS), currently active on the heavy lift market, have been specifically designed and equipped to perform very heavy lift operations at sea, even in rather severe environmental conditions. The lift records of the SSCVS show clearly the trend of increasing lift weight versus time [1,2]. This fact is not surprising as the market takes full advantage of the available lifting capacity. Simultaneously with the lift weight, the dimensions of the lifted structures have also increased significantly, resulting in minimal clearances between the load and the crane vessel. For loads which are relatively light when compared with the displacement of the crane vessel, the wave induced motion behaviour of the SSCV is almost independent of the motion behaviour of the load suspended from the cranes, Furthermore, the relative motions of the load with respect to the crane vessel, thus also the clearance may be controlled to some extent by means of control lines operated from tug winches. For relatively heavy loads, however, the wave induced motion behaviour of the SSCV can be strongly affected by the motion behaviour of the load, and vice versa [23]. The dynamic motion behaviour of both the crane vessel and the load, as well as of a cargo barge in case it is involved in the lifting operations, are in fact coupled. The crane vessel, the load and the cargo barge together form one integrated dynamic system. Considering the phenomenon described above, it becomes evident that lift dynamics play an important role in the feasibility and workability of a lift operation involving a relatively heavy load, Workability restrictions related to motions of the crane vessel and the load are therefore dependent upon the effects of lift dynamics. Furthermore, the clearance between load and crane vessel or the tension variation in the hoisting wires or the slings, can become limiting factors of a lift operation also due to the phenomenon lift dynamics. Maintaining the minimal clearance between crane vessel and lifted load throughout the lift operation has become of vital importance with respect to the overall safety. In 1985, The Netherlands, Shell Imitational Petroleum Maatschappij (SJ.PM), The Hague, and the Maritime Research Institute Netherlands (MARIN), Wageningen, commenced research work with respect to lift dynamics, which was focused on the development of a lift simulation computer program (LIFSIM) able to calculate the dynamic motion behaviour of crane vessel, load and cargo barge during the lift operation [4,5,6]. Extensive model tests in MARIN?s sea keeping basins were carried out, additionally sponsored by Heerema Engineering Service BV, Leiden, for verification and validation of the LIFSDVI program.