Seismic rehabilitation of offshore jacket structures, as an important component of energy production infrastructure, can ensure higher productivity, life extension, and long-term reliability of these structures. This study utilizes probabilistic seismic demand analysis and comparative loss analysis as a decision-making tool to evaluate the efficacy of deck isolation in enhancing the seismic performance of offshore jacket structures. Low-damping rubber bearings are used to isolate the deck, and two deck-isolated systems are achieved by adjusting the isolator stiffness to target fundamental vibration periods of 2 s and 3 s. Three types of dampers: 1) velocity-dependent 1–4 viscous dampers; 2) direction- and displacement-dependent 2–4 dampers; and 3) direction- and displacement- dependent 1–3 dampers, are incorporated into the isolation system. For each structure, ground-motion ensembles containing 20 records are selected using the generalized conditional intensity measure approach to represent site-specific seismic hazard at 13 hazard levels. The baseline and deck-isolated structures are compared in terms of the cumulative distribution function of the jacket cap displacement and total deck acceleration, as well as drift hazard and loss metrics. Deck-isolation resulted in significant reductions, ranging from 83–91%, in the mean annual frequency of exceeding the inelastic limit and a 67–98% reduction in loss hazard across the considered loss levels. This highlights the applicability of the isolation system and energy dissipation devices to offshore jacket structures. Sensitivity analysis on damage state thresholds and the deferred production cost factor showed, although these parameters influence the loss metrics of each structure, their relative performance remains largely unaffected.
This study develops a coupled aero–hydro–mooring–servo–pumping model to investigate the dynamic responses and structural loads of floating offshore wind turbines (FOWTs). The proposed quasi-static ballast-pumping model uses a PID controller with low-pass-filtered pitch feedback to maintain mean platform pitch near zero while ensuring gradual water-level adjustments and low pump-power demand. Ballast pumping primarily reduces the mean tower-base bending moment (TBBM), with associated reductions in peak TBBM and slight improvements in power output. To mitigate the negative damping associated with baseline rotor control, two alternative strategies incorporate gain-scheduled or constant-gain platform-motion feedback. Both suppress low-frequency platform pitch oscillations near rated wind speed, reducing peak TBBM and damage equivalent loads (DELs). TBBM decomposition reveals aerodynamic, gravitational, and inertial contributions, with responses strongly influenced by platform pitch and wave conditions. In contrast, blade-root bending moments are dominated by aerodynamic loads and show less sensitivity to wave conditions. Dynamic mooring loads are governed primarily by platform surge, with limited sensitivity to rotor control, while the quasi-static mooring model substantially underestimates mooring DELs compared with the dynamic model. Overall, the findings highlight the distinct roles of ballast pumping in reducing mean structural loads and of rotor control in mitigating dynamic load fluctuations. Given the rigid-body assumptions and limited experimental validation, the predicted load reductions and fatigue-related indicators should be interpreted as comparative numerical estimates for understanding load mechanisms and assessing control strategies.
