Dynamic responses and local load distribution of a twin-barge topside transport system: A numerical and experimental study

Multi-barge floatover operations enable topside installation and decommissioning, but local force-unit loads remain insufficiently understood. Frequency- and time-domain analyses and model-scale experiments investigate coupled motions and local Deck Mating Unit (DMU) forces. Because JONSWAP parameters varied among headings, controlled comparisons used regular waves with a common height and period. Piston-type and first transverse sloshing gap modes at 0.84 and 0.98 rad/s are associated mainly with barge heave–pitch and roll, respectively. Two coupled modes characterized by roll–heave and roll–roll interactions are identified and confirmed by white-noise and free-decay tests. DMU forces reveal mechanisms masked by system-level resultants. Two diagonal Y -direction force pairs act in opposition, producing strong cancellation in the resultant force; among the tested JONSWAP cases, the 135° case has the highest diagonal-force RMS and a cancellation ratio of 0.043. The tested 45° and 135° JONSWAP cases exhibit diagonal Z -direction load redistribution, with local load amplification and partial unloading. Coherence and cross-spectral phase analyses show that Y -direction loads exhibit the clearest phase relationship with inter-barge relative roll, whereas Z -direction loads exhibit the clearest phase relationship with topside roll. Resultant forces alone may underestimate local DMU load demand.