Practice-oriented seismic retrofit decision-making for offshore jacket structures based on comparative loss metrics

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.