In this study, an Al₂O₃-40TiO₂ coating was deposited on 20MnNiMo steel via atmospheric plasma spraying. The corrosion behavior of the coating was investigated in both artificial seawater and a simulated environment with hydrogen sulfide and high pressure. Additionally, ion dissolution experiments were conducted to evaluate the coating’s bio-friendliness. In artificial seawater, the corrosion rate (based on the corrosion current) of the Al₂O₃-40TiO₂ coating initially decreased before increasing. It was speculated that the blocking of corrosion products in the defect channels was helpful in delaying the progress of corrosion in the early stage. The coating had a corrosion current on the order of 10⁻⁶ A/cm² in artificial seawater, suggesting good protection in conventional seawater environments. In the simulated environment, the corrosion rate (based on the weight loss) of the Al₂O₃-40TiO₂ coating showed a continuously declining trend. It was deduced that, unlike corrosion products in artificial seawater, the corrosion products in the simulated environment (e.g., metal sulfide) might be more chemically stable, leading to a longer blocking effect. Therefore, a minimal corrosion rate of 0.0030 mm/a was obtained after the coating was immersed for 30 days. The amount of dissolved coated elements was negligible and there were only small amounts of dissolved non-coated elements such as Ni and Mo. The developed coating can be considered to be highly biofriendly if the non-coated area of the specimen is well sealed.

It is crucial to accurately characterize the static and dynamic properties of seals for the design of deep-sea equipment. This study investigated the movement resistance of O-rings under high pressure (up to 30 MPa) and low temperature (2 °C). Numerical simulation models were set up to investigate the effects of material properties and seal structure dimensions on the reciprocating friction and installation force of O-rings. Furthermore, a novel resistance testing rig was developed to facilitate the measurement of reciprocating friction and installation force of O-rings under high pressure and low temperature. The results of this study indicate that nitrile rubber (NBR) has a reduced sensitivity to temperature variations, and the hardening of the material due to the low temperatures encountered in deep-sea environments does not substantially increase its movement resistance. Conversely, fluororubber (FKM) exhibits superior static sealing reliability. We optimized the sealing structure of the pressure-maintaining trapping instrument (PMTI) and successfully conducted field tests at the South China Sea. The results of this study may serve as a valuable reference for the sealing design of deep-sea equipment.

In this study, the Mo/C276 coating was deposited on Q690E steel by high velocity oxy-fuel spraying (HVOF), and the coating was treated with an organosilicon sealer. Further, the corrosion behaviors of the coating in the simulated deep-sea cold spring environment with hydrogen sulfide and high pressure were studied. The results show that when the oxygen flow rate is 220 NL/min, the coating has the lowest porosity of 1.71% and excellent mechanical properties. Combined with the micromorphology and elemental analysis of the coating, it was assumed that the Fe element generated by the corrosion of the Q690E substrate migrates to the surface of the coating. The corrosion tests in the simulated deep-sea cold showed that before the failure of the coating in the edge and corner areas, the corrosion rate of the coating was less than 0.002 mm/a, which could meet the long-term use requirements in the real cold spring environment.

Hydrodynamic actions on ships may excite hull-girder whipping and generate short-duration global dynamic bending effects in the structure. To investigate the dynamic ultimate bearing capacity of box girders under such hydrodynamically induced whipping-type dynamic bending loads, a simplified box-girder structural segment is studied in this paper. A nonlinear dynamic finite element model is established under the combined action of hydrostatic pressure and equivalent whipping-type dynamic bending loads. Instead of directly applying localized slamming pressure, opposite rotational velocities with equal magnitudes are prescribed at the end reference points to equivalently represent the global bending response associated with whipping. This treatment allows the load-carrying characteristics and failure behavior of the box girder under transient dynamic bending to be examined. Geometric nonlinearity, material nonlinearity, the Cowper–Symonds strain-rate effect, and initial geometric imperfections are considered in the model. Stochastic finite element analysis and Monte Carlo simulation are further used to evaluate the influence of the randomness of Young’s modulus and loading strain rate on the probability distribution of the dynamic ultimate bearing capacity and structural reliability. The results show that the dynamic ultimate bearing capacity of the box girder increases with increasing strain rate, while its sensitivity to the strain rate decreases markedly when the strain rate exceeds 2.306 s⁻¹. A larger initial geometric imperfection amplitude leads to a more evident reduction in the ultimate capacity. The reliability analysis shows that an increase in the mean load effect significantly increases the failure probability; when the mean load effect is lower than the mean ultimate bending moment, an increase in the load standard deviation reduces structural reliability. This study provides a fundamental numerical reference for predicting the dynamic ultimate bearing capacity and conducting probabilistic safety assessment of box-girder structures subjected to whipping-type global dynamic bending.