This paper presents a comprehensive on-site decision-making framework for assessing the structural integrity of a jacket-type offshore platform in the Gulf of Mexico, installed at a water depth of 50 m. Six critical analyses-(i) static operation and storm, (ii) dynamic storm, (iii) strength-level seismic, (iv) seismic ductility (pushover), (v) maximum wave resistance (pushover), and (vi) spectral fatigue-are performed using SACS V16 software to capture both linear and nonlinear interactions among the soil, piles, and superstructure. The environmental conditions include multi-directional wind, waves, currents, and seismic loads. In the static linear analyses (i, ii, and iii), the overall results confirm that the unity checks (UCs) for structural members, tubular joints, and piles remain below allowable thresholds (UC < 1.0), thus meeting API RP 2A-WSD, AISC, IMCA, and Pemex P.2.0130.01-2015 standards for different load demands. However, these three analyses also show hydrostatic collapse due to water pressure on submerged elements, which is mitigated by installing stiffening rings in the tubular components. The dynamic analyses (ii and iii) reveal how generalized mass and mass participation factors influence structural behavior by generating various vibration modes with different periods. They also include a load comparison under different damping values, selecting the most unfavorable scenario. The nonlinear analyses (iv and v) provide collapse factors (Cr = 8.53 and RSR = 2.68) that exceed the minimum requirements; these analyses pinpoint the onset of plasticization in specific elements, identify their collapse mechanism, and illustrate corresponding load-displacement curves. Finally, spectral fatigue assessments indicate that most tubular joints meet or exceed their design life, except for one joint (node 370). This joint's service life extends from 9.3 years to 27.0 years by applying a burr grinding weld-profiling technique, making it compliant with the fatigue criteria. By systematically combining linear, nonlinear, and fatigue-based analyses, the proposed framework enables robust multi-hazard verification of marine platforms. It provides operators and engineers with clear strategies for reinforcing existing structures and guiding future developments to ensure safe long-term performance.

The fuel assembly (FA) stands as a pivotal component within the reactor core, impacting safety, reliability, and economic viability of the entire facility during the lifetime. As a newly-designed nuclear heating reactor, the FA of the 200 MW nuclear heating reactor (NHR200-II) embodies a novel configuration, characterized by a 9x9 fuel bundle arrangement inside, which are clamped by three spacer girds along the height direction, and fully enclosed by a square zirconium channel outside for improving effective coolant flow rate. Under the earthquake event, possible deformations of fuel cladding or outer channel, buckling instability of spacer grids, even damage of the structures and control rod insertion failures can occur and further pose unaffordable safety risks. Therefore, it is necessary to implement a comprehensive seismic fluid-structureinteraction (FSI) analysis with regard to the NHR200-II fuel assembly to evaluate the structural integrity. In this paper, a particle finite element method (PFEM) based partitioned two-way FSI scheme is used to capture the seismic responses of NHR200-II FA, that is, implicit finite element method for structure dynamics, PFEM for fluid motion. To accurately solve the FSI phenomenon, it is essential to couple the fluid solver and structural solver advancing with a same time step, i.e., strongly-coupled on the FSI interfaces. The specific fluid solver and structural solver are combined together in the commercial finite element code LS-DYNA to carry out the two-way FSI simulation, which greatly improves computational efficiency. This finite-element based numerical framework has been validated in detail in the previous work on the axial flow-induced vibration analysis of a single NHR200-II fuel rod. At present, the seismic excitations applied to a fuel assembly level are not available, which are usually generated from several analyses of a site-specific earthquake, including sequential analyses of soil-structure-interaction model, the reactor pressure vessel (RPV) internals and FA array models. Thereby the computation employed a time-compressed El-Centro N-S earthquake (1.5 times amplification, similar to 0.36 g peak acceleration) for the preliminary evaluation of the seismic FSI responses of NHR200-II FA. The results present stress distribution of each component (including the top & bottom nozzle, fuel cladding, zirconium channel, spacer grids), channel-spacer grid interactions (transverse impact force), dynamic clamping of the spacer girds, acceleration responses, and response spectra of the FA at typical positions. The results would confirm the reasonable structural design and robust seismic performance of NHR200-II FA and also are broadly applicable to the seismic response of fully submerged fuel assembly with square-type channel outside.