## 1 Introduction  
Large diameter steel ring and/or stringer-stiffened cylinders are widely used in marine structures as a major structural component of floating offshore installations, main legs of tension leg platforms, submarine hulls, and spars. Recently, these structures are also applied extensively in floating offshore wind turbine foundations. The major advantages of stiffened cylinders are good axial compressive and external pressure loading, and low drag resistance force by passing fluid. During operation, stiffened cylindrical members may be potentially damaged caused unexpected loading such as ship collisions (such as tugboats, commercial ships, ferryboats, support vessels, and private vessels), groundings, slamming, or the impact of dropped heavy objects. In these accidents, ship collisions have been highlighted as the most significant cause of damage or even catastrophic loss of offshore installations due to loss of structural integrity [1]. A minor collision will result in only repairable local damage to the structure and probably will not call for a stop of operation. However, the major collision will damage cylindrical structures globally, and assuredly require a stop of operations. Therefore, it is a necessity to increase attention on the influences of collision damage on the strength of the structure and serviceability in the initial design step for safety concerns. To evaluate the internal mechanics of cylindrical structures based on collision events, many approaches are applied such as empirical formulation, simplified analytical method, finite element modeling (FEMs), and experiments. However, it is difficult to perform full-scaled cylindrical collision experiments, and sometimes impossible, because of extremely costly and technological requirements. Thus, small-scale laboratory model tests are the most convincing methods for evaluating the impact resistance of cylindrical structures. There are relatively few studies that provided the experiments on ring and/or stringer-stiffened cylinders subjected to dynamic mass impact. [PERSON] et al. [2] presented the drop-weight impact test on two fabricated steel ring-stiffened cylinders. The damage was created by using the striking mass with a knife-edge indenter. [PERSON] et al. [3] provided the experiment on 27 locally dented unstiffened cylinders under compression. The dent imperfections of different depths, locations, and orientations were considered. It was found that the dented area was largely affected by the deformations in the buckling modes and buckling strengths. They also presented the experimental study on 14 thin-walled intact and dented cylindrical shells subjected to external pressure [4], 8 damaged tubular members under bending loads [5], and 6 locally dented conical shell models under axial compression [6]. In general, the ultimate load-carrying capacity of dented structures was strongly decreased when compared with the intact structures. Recently, [PERSON] et al. [7] conducted the dynamic mass impact tests on two stringer-stiffened cylinders (denoted as SS-C-1 and SS-C-2) with local impact at mid-span. The specimens had three bugs each, divided by two T-shaped ring-stiffeners and stiffened with 20 stringer-stiffeners in axial direction, respectively. These models were then performed under external hydrostatic pressure to assess the residual strength of these structures after collision [8]. [PERSON] et al. [9] provided details of the second series on two ring-stiffened cylinders, namely, RS-C-3 and RS-C-4. These models had seven bugs and were separated by six flat-bar ring-stiffeners. The damage was performed by the free-fall testing frame and their residual strengths were checked under external hydrostatic pressure by pressure chamber. In these references, the test results are very valuable for a common understanding of the dynamic impact behaviors of stiffened cylinders. Furthermore, they can be used to validate the numerical analyses or derive the analytical formulations for collision response of stiffened cylinders.