Large scale offshore wind farms are relatively new infrastructures and are being deployed in regions prone to earthquakes. Offshore wind farms comprise of both offshore wind turbines (OWTs) and balance of plants (BOP) facilities, such as inter-array and export cables, grid connection etc. An OWT structure can be either grounded systems (rigidly anchored to the seabed) or floating systems (with tension legs or catenary cables). OWTs are dynamically-sensitive structures made of a long slender tower with a top-heavy mass, known as Nacelle, to which a heavy rotating mass (hub and blades) is attached. These structures, apart from the variable environmental wind and wave loads, may also be subjected to earthquake related hazards in seismic zones. The earthquake hazards that can affect offshore wind farm are fault displacement, seismic shaking, subsurface liquefaction, submarine landslides, tsunami effects and a combination thereof. Procedures for seismic designing OWTs are not explicitly mentioned in current codes of practice. The aim of the paper is to discuss the seismic related challenges in the analysis and design of offshore wind farms and wind turbine structures. Different types of grounded and floating systems are considered to evaluate the seismic related effects. However, emphasis is provided on Tension Leg Platform (TLP) type floating wind turbine. Future research needs are also identified.

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Dynamic ground and ground-structure responses are heavily dependent on the soil shear wave velocity. During seismic excitation, soil stiffness inferred from the shear wave velocity (Vs) might change significantly and affect the overall system response. In this study, an instrumentation and analysis framework was developed to allow for continuous estimation of Vs during dynamic/seismic excitation. The framework is presented along with representative applications during shake table testing of saturated sand strata. For that purpose, results from two different 1-g shake table tests conducted in a laminar soil container are examined and analyzed. In this context, evolution of the soil Vs profile during the shaking event is tracked and documented. The experimental setup, test procedure, and test results are described. Time histories of Vs at different depths within the sand strata are discussed. Overall, the developed techniques can be conveniently included in routine 1-g and centrifuge shake table experimentation efforts, when properly accounting for the differences between sizes of 1 g and centrifuge models.

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Offshore wind power is increasingly becoming a mainstream energy source, and efforts are underway toward their construction in seismic zones. An offshore wind farm consists of generation assets (turbines) and transmission assets (substations and cables). Wind turbines are dynamically sensitive systems due to the proximity of their resonant frequency to that of loads considered in their analyses. Such farms are considered lifeline systems and need to remain operational even after large earthquakes. This study aims to discuss hazard considerations involved in the resilience assessment of offshore wind farms in seismic regions. The complexity of design increases with larger turbines installed in deeper waters, resulting in different types of foundations. In addition, Tsunami inundation is shown to be an important consideration for nearshore turbines.

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