The residual strength of denting- and fracture-damaged box girders were experimentally and numerically investigated. The experiments were conducted under a pure bending moment using the four-point bending test method. The load, deflection, and strain were measured. The strains of the extension structures were also measured, and the frictional forces between the model and the supported round bar were estimated. Test models consisting of two groups were fabricated. These groups were designed to estimate the residual strength of denting- and fracture-damaged models. The damage was induced by releasing a striker using a drop-testing machine to consider the dynamic effect. Additionally, numerical analyses were performed via a nonlinear finite element analysis, where the measured initial imperfection data and welding residual stresses were considered. The ultimate longitudinal moments that considered the frictional force of the round bars were reduced by 12% (on average) compared with those obtained by neglecting the frictional forces.

As the number of offshore wind-power installations increases, collision accidents with vessels occur more frequently. This study investigates the risk of collision damage with operating vessels that may occur during the operation of an offshore wind turbine. The floater used in the collision study is a 15 MW UMaine VolturnUS-S (semi-submersible type), and the colliding ships are selected as multi-purpose vessels, service operation vessels, or anchor-handling tug ships based on their operational purpose. Collision analysis is performed using ABAQUS and substantiation is performed via a drop impact test. The collision analyses are conducted by varying the ship velocity, displacement, collision angle, and ship shape. By applying this numerical model, the extent of damage and deformation of the collision area is confirmed. The analysis results show that a vessel with a bulbous bow can cause flooding, depending on the collision conditions. For damage caused by collision, various collision angles must be considered based on the internal stiffener arrangement. Additionally, the floater can be flooded with relatively small collision energy when the colliding vessel has a bulbous bow.