Severe wave run-ups on a surface-piercing column leading to strong uprush water jets may cause unexpected impact loads on offshore structures. To reveal the underlying mechanism related to the complicated wave–column interaction, this paper investigates the occurrence and evolution of wave run-ups on a fixed surface-piercing square column under focused waves experimentally and numerically by comparing the wave run-up profiles, wave loads on the column, and velocity and pressure fields in the cases of different peak periods and steepnesses. The results manifested that the wave run-up under a very steep focused wave is significantly influenced by the localized nonlinear interaction between the wave crest and the uplifted water mound in front of the column and experiences a different regime from those primarily due to nonlinear wave diffraction. In the cases of breaking or nearly breaking focused waves, the sudden change in the fluid velocity on the wave crest when impacting on the uplifted water mound dramatically increases the peak value and gradient of dynamic pressure in the adjacent region and hence results in critical impact load on the column and strong accelerating effect of the uprush flow. Additionally, a larger peak period can further increase the thickness (or volume) of the uprush flow, potentially increasing the damage risk of offshore structures.

The seafloor soil is characterized by high water content, strong compressibility, and low shear strength. Deep-sea mining vehicles (DSMV) are prone to sinking when walking on the surface of the soil, which will cause significant reduction in traction performance. Therefore, it is necessary to study the sinkage performance. The track is usually considered the travelling mechanism of the DSMV, and the track plate is an important part of the movement system. The study of the interaction between the track plate and the soil is of great significance to the study of the DSMV’s sinkage performance. In this study, firstly, based on the in situ seafloor soil samples of 1000 m in a region of the South China Sea collected by a box sampler, the physical and mechanical parameters of soil were measured by indoor geotechnical instruments. Secondly, an elastoplastic soil numerical model similar to that of in situ soil was established. Based on coupled Eulerian-Lagrangian (CEL) method, a numerical model of the interaction between the track plate and soil was established. Considering the dynamic process, the structure of the track plate and the physical and mechanical properties of the soil, the numerical simulation were carried out under different conditions, such as different dynamic loading, the plate structural parameters and the soil physical and mechanical properties. It is found that the plate-sinkage curve were significantly influenced by these factors. The findings are as follows, firstly, with the increase in the pressure loading rate, the soil sinkage decreasing at the same pressure. On the other hand, with the increase in velocity, soil flow was accelerated, and the nonlinear relationship between resistance and velocity became more obvious; the L/B ratio of different track plates affects the variation law of the curve, and the maximum sinkage gradually decreases as the ratio of L/B increases; with the increase in the grouser height, the maximum sinkage gradually decreases, and the pressure-sinkage curve changes obviously with the grouser type; and different soil physical and mechanical properties affect the variation of pressure-sinkage curve. Innovatively, the heterogeneous soil stress distribution mode was obtained through the fitting function and Python secondary development. This study can provide a reference for studying the sinkage performance of the DSMV.

Three forms of the mooring system in 60 m water depth are proposed for semi-submersible with partially inclined columns (SPIC) concept floating wind turbine (FWT). One is a simple form with only catenary lines, and the other two are hybrid forms including clump weights. The clumps are attached to the suspended section for Hybrid form1 and the bottom section for Hybrid form2. Hybrid form2 achieves the smallest line length and chain weight. Three alternative proposals can be evaluated through mooring line characteristics, dynamic responses, utilization factors, and simple cost analysis. Hybrid form2 allows for smallest pretension, and largest stiffness and nonlinearity only at large offsets. Under operational conditions, the mean surge for Hybrid form1 and Hybrid form2 is similar, but the fairlead tension is significantly smaller for Hybrid form2. Under the survival condition, the clumps of Hybrid form2 are lifted up and put down, leading to small mean offsets of FWT but large wave-frequency components of line tension. Among the three forms of the mooring system, the Hybrid form2 can limit the FWT to the smallest offset range while also controlling the mean mooring line tension to a level similar to the other two forms. Under normal working conditions and accidental conditions with single line broken, the maximal surge motions of FWT under the restraint of three mooring systems all meet the design requirements. The mooring line strength of the three mooring systems meets the requirements in ultimate limit state (ULS) and accidental limit state (ALS) analyses. Among them, the utilization coefficient of Hybrid form2 is closest to 1, demonstrating its best economic performance.