Effectiveness of deck-isolation and viscous dampers supplement on enhancing seismic performance of offshore jacket platforms

Abstract

Offshore structures, mainly used for oil production, are increasingly being adapted for renewable energy applications. Seismic performance assessment of these structures is generally conducted using a limited number of ground motions without taking due account of the site's seismic hazard. In this study, a detailed seismic assessment is performed on a typical offshore jacket platform as well as its two modified versions, equipped with an isolation system comprising low-damping rubber bearings and dampers. These dampers include velocity-dependent 1–4 viscous dampers, direction- and displacement-dependent 2–4 dampers, and direction- and displacement-dependent 1–3 dampers. For each structure, 13 ensembles of ground motions are selected (each containing 20 records) compatible with the site's seismic hazard at 13 exceedance levels using the generalized conditional intensity measure methodology. The results at both the jacket cap and deck level are presented in terms of the reductions in median and maximum displacement and acceleration, as well as the base shear. Application of 1–4 damper with 20% damping ratio in the isolated structure resulted in reductions up to 84% and 34% in median displacement and 56% and 88% in median acceleration at the jacket cap and deck levels, respectively, while also decreasing median base shear by up to 71%.