Sadra Amani

Offshore Wind Farms have established themselves as a matured technology to decarbonize energy sources to achieve net-zero. Offshore wind farms are currently being constructed in many seismic-prone zones and the codes/best practice guidelines are not fully developed. The aim of the paper (which is based on the keynote lecture presented at the 7th International Conference on Recent Advances in Geotechnical Earthquake Engineering) is as follows: (a) discuss the potential seismic risks to the offshore wind farm system and its components such as turbines, cables, and offshore substations; (b) present the different analysis and design methods for offshore wind turbine foundations; (c) highlight the performance-based design considerations and how to assess risks. Future research needs are highlighted.

Offshore Wind Turbines are a complex, dynamically sensitive structure owing to their irregular mass and stiffness distribution and complexity of the loading conditions they need to withstand. There are other challenges in particular locations such as typhoon, hurricane, earthquake, sea-bed current, tsunami etc. As offshore wind turbines have stringent Serviceability Limit State (SLS) requirements and need to be installed in variable, and often complex ground conditions, their foundation design is challenging. Foundation design must be robust due to the enormous cost of retrofitting in a challenging environment should any problem occurs during the design lifetime. Traditionally, engineers use conventional types of foundation system such shallow Gravity-Based Foundations (GBF), suction caissons or slender pile or monopile owing to prior experience with designing such foundations for the oil and gas industry. For offshore wind turbine, however, new types of foundations are being considered for which neither prior experience nor guidelines exist. One of the major challenges is to develop a method to de-risk the life cycle of offshore wind turbines in diverse met-ocean and geological conditions. The paper, therefore, has the following aims: (a) Provide an overview of the complexities and the common SLS performance requirements for offshore wind turbine; (b) Discuss the use of physical modelling for verification and validation of innovative design concepts, taking into account all possible angles to de-risk the project. (c) Provide examples on applications of scaled model tests.

Offshore wind power is increasingly becoming a mainstream energysource, and efforts are underway toward their construction in seismiczones. An offshore wind farm consists of generation assets (turbines) andtransmission assets (substations and cables). Wind turbines are dynami-cally sensitive systems due to the proximity of their resonant frequency tothat of loads considered in their analyses. Such farms are consideredlifeline systems and need to remain operational even after largeearthquakes. This study aims to discuss hazard considerations involvedin the resilience assessment of offshore wind farms in seismic regions. Thecomplexity of design increases with larger turbines installed in deeperwaters, resulting in different types of foundations. In addition, Tsunamiinundation is shown to be an important consideration for nearshoreturbines.

Offshore wind farms are a collection of offshore wind turbines (OWTs) and are currently being installed in seismically active regions. An OWT consists of a long slender tower with a top-heavy fixed mass (Nacelle) together with a heavy rotating mass (blades) which makes the structure dynamically sensitive. They are constantly exposed to variable environmental wind and wave loads, and in seismic zones, other loads will also be imposed momentarily. The paper aims to highlight the issues associated with seismic design including liquefaction. The main conclusions are: (a) Liquefaction poses a challenge to monopile supported foundations as there are chances of significant tilt; (b) Jacket is a better option as the load transfer is through push-pull action; (c) Floating systems are better in seismic zones due to the fact that minimum inertia loads are transferred through the cables. However, floating can be potentially vulnerable to large fault ruptures.

Offshore wind farms are currently being constructed worldwide, and most of the Wind Turbine Generator (WTG) structures are supported on single large-diameter steel piles, commonly known as monopile. One of the challenging design aspects is predicting the long-term deformation of the foundation and, in particular, the accumulation of rotation which is a complex Soil-Structure Interaction (SSI) problem. Accumulation of rotation requires the estimation of Load Utilisation (LU) ratio (i.e., ratio of the load-carrying capacity of the foundation to the applied loads from wind and wave). Estimation of LU for monopile is not trivial due to the simultaneous action of lateral load and moments and needs the introduction of interaction diagram concepts. This paper proposes methodologies to obtain LU for monopiles using three types of methods: (a) Simplified method, which is based on closed-form solution (where the load effects are uncoupled) and can be carried out using spreadsheets or pocket calculators; (b) Standard method based on non-linear Winkler spring (also known as p-y method) where the load effects are also uncoupled; (c) Advanced method, which uses Finite Element Method (where the load effects are coupled). Examples of monopiles are taken from European Wind Farms covering different ground profiles: Gunfleet Sands (clay profile), Walney-I (sandy profile), London Array-I (layered profile) and Barrow-II (layered profile) sites are analysed using all three methods. It is hoped that the methodology will be helpful in the design optimisation stage.

An increasing number of offshore wind farms are being constructed in seismic regions over liquefaction susceptible soils. This paper presents a methodology for the analysis and design of monopiles in seismically liquefiable soils by extending the established "10-step methodology" with an additional 7 steps. These additional steps include assimilation of seismic data, site response analysis, stability check of the structure (ULS check through the concept of load-utilization ratio), input motion selection, prediction of permanent tilt/rotation, and ground settlement post liquefaction. A flow chart, which shows the interdependence of the different disciplines, is presented and can be extended to routine design. This proposed method is validated using the observed performance of an offshore and nearshore turbine from the Kamisu wind farm during the 2011 Great East Japan earthquake. Predicted results based on the proposed methodology compare well with the field observation and demarcate the (i) good overall performance of the offshore turbines and (ii) limit state exceedance of the nearshore turbine. It is envisaged that the proposed method will be useful towards the design of monopiles-supported wind turbines in seismic areas.

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.