Dynamic Lifting Analysis Of Offshore Structures

ABSTRACT Lifting operations at the offshore job site are greatly affected by the action of waves. In the past, deck structures were fabricated in several sections so that they could be lifted by low-capacity cranes used at the job site. The frequency of lifting operations was therefore high, and the construction period long. Recently, however, large-capacity crane vessels have been developed that can lift relatively large, heavy deck sections onto jacket structures -vessels that significantly reduce the amount of lifting work and improve the efficiency and safety of offshore construction projects. Such developments have required more rigorous analyses to estimate the appropriate design for the sling system and the operability of the crane vessel. This paper discusses unified analysis -- a method that considers the interaction between a crane barge and its hoisted load -- as applied both to the motion analysis of the barge-load system subjected to waves and to the transient motion analysis of the barge crawling while lifting a load. The calculated results are compared with model test results. The paper also clarifies the conditions necessary for carrying out unified analysis and discloses the occurrence of super harmonic oscillations in sling tension. The analytical procedure described below was applied to regulate the operational limits for lifting a 1900-st deck structure in an actual offshore construction project in the Sea of Japan. The validity of unified analysis was confirmed by the results of field measurements taken at the above job site. INTRODUCTION Recently, large-capacity derrick crane vessels have been constructed and often used to lift heavy deck structures. These operations have called for a careful design of the sling system. Bunce et al1) assessed the lifting criteria in current use and offered comprehensive suggestions concerning weight escalation, dynamic amplification and sling tension redundancy. Based on field measurements, the authors of the present paper once studied the relationship between the motions of a barge and its hoisted load and proposed operational limits.2) Nojiri et a1 3) tackled the problems of coupled motion between vessel and hook load. Recently, the authors have developed a procedure for unified motion analysis -- one that takes account of the interaction between a barge and its hoisted load.4) This analytical procedure has, for a number of years, been successfully applied to heavy lift operations in actual installation work at offshore job sites. UNIFIED MOTION ANALYSIS OF BARGE AND HOISTED LOAD Equation of barge motion On the assumption that the wave height and barge oscillation amplitudes are small enough to allow linear theory to be applied, the coordinate system X-Y-Z in Fig. 1 (b) is established with the center of gravity of the barge (excluding the weight of the hoisted load) as its origin. The barge motion vector {x} consists of translational motions -- surge (X), sway (Y) and heave (Z) -- and rotational motions -- roll (), pitch (6) and yaw (¢).