In this study, installation process of pipe-type structure is conducted experimentally and numerically using dual floating crane vessels. Because installation structure is a long pipe, two crane vessels are required for the installation process. Two crane vessels are facing each other holding each end of the installation structure. In the experiment, two vessels are moored by soft spring, and various wave condition are considered. Measured items are 6DOF motions of two vessels and pipe, wire tensions at the end of the crane and mooring line tensions. In numerical analysis, motions of crane vessels are based on floating body dynamics using convolution integral, and crane wire is treated as simple spring. Installation structure is assumed as a rigid body with 6DOF motion. As in the experiment, numerical simulations in regular wave are performed. Characteristics of vessel motion and wire tension according to wave conditions are checked, and computation results are compared with experimental data. It was checked which frequency range is critical by reviewing the motions of dual crane vessels, and near the yaw motion resonance point of the pipe-type structure has the worst effect on crane vessel's motion.
Floating crane vessels are used for various purposes such as translating, installation and decommissioning of offshore structures, and a high level of safety and precision is required during crane operation. Recently, to maximize the efficiency of the top side installation process, more than 5,000tons of a mega block is manufactured and installed using dual crane vessels (Jung et al. 2016). For these kinds of crane operations, a thorough review of the installation work should be made before the real-sea operation.
Model tests related to floating crane vessel operation are a few. Nam et al. (2016) studied installation work of deep-water structures using single crane vessels with model test and numerical computation. Through the experiment, motion performance of the crane vessel and subsea structure and tensions of crane wire were examined, and change of the installation performance according to the presence of passive heave compensator was checked. Ha et al. (2018) analyzed the mating operation of a topside module by single floating crane vessel using model test and numerical calculation. Jung et al. (2016) built a new system to synchronize dual crane vessels into a single crane unit and carried out a model test to confirm the safety of lifting operation. In addition, they conducted sea trials of mega-module lifting operation using dual crane vessels and reported the results. Velema and Bokhorst (2015) undertook the real sea installation of a 9,500tons oil storage tank on the seafloor using a dual crane lift sequence. Important parameters such as ballast volume, compartment fill rate and line tensions of crane wire were monitored during installation work. China National Offshore Oil Corporation (CNOOC) launched mega jacket in South China Sea in August 2012. Corresponding model test and field test which were related with installation process were conducted. (He et al., 2013, Zhang et al., 2013, Yu et al., 2013)