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The probabilistic analysis of Offshore Wind Turbines (OWT) is not a new practice. The standards for designing OWT (IEC 61400 class) emphasizes that assessing uncertainty is of major importance inside the design chain. Still, major challenges related to the uncertainty and the probabilistic assessment pose to the sector and its development. The analysis of operational loads is one them. The problem of analyzing extreme responses or cumulated damage in operation during the design phase is significantly related to its high computational cost. As we progressively add complexity to the system to account for its uncertainties, the computational effort increases and a perceptive design becomes a heavy task. If an optimization process is then sought, the designing effort grows even further. In the particular case of fatigue analysis, it is frequent to not be able to cover a full lifetime of simulations due to computational cost restrictions. The mentioned difficulties fomented the utilization of surrogate models in the reliability analysis of OWT. From these surrogate approximations the ones based on Kriging models gained a special emphasis recently for structural reliability. It was shown that, for several applications, these models can be efficient and accurate to approximate the response of the system or the limit state surfaces. The presented paper tackles some of the issues related to their applicability to OWT, in a case specific scenario of the tower component subjected to operational fatigue loads. A methodology to assess the reliability of the tower component to fatigue damage is presented. This methodology combines a Kriging model with the theory of extreme values. A one-dimensional Kriging case using the state of art NREL's monopile turbine is presented.The reliability of the OWT tower is calculated for 20 years. The results show that the usage of a Kriging model to calculate the long term damage variation shows a high potential to assess the reliability of OWT towers to fatigue failure.

Driven by the evidence of climate change many governments have decided to reduce carbon emissions associated with the production of electricity. Hence, there has been an increase in the use of renewable energy. Additional benefits include moving towards an energy supply which is more sustainable and more distributed. International targets for decarbonisation of the economy have been agreed at COP21 meetings in Paris in 2015. Of the renewable electricity technologies available, offshore wind allows electricity to be generated at scale and at a reasonable price (as can be seen from the second round of UK contract for difference see [1]). However, for many countries bottom fixed turbines are technically or commercial unfeasible due to water depth and seabed geology at suitable sites. This has been the inspiration for much research into floating wind turbines, see for example Musial et al [2]. Offshore Wind (OSW) is rapidly maturing sector and is increasingly seen as a major contributor to electricity supply in states with coastal demand centres and good wind resource. While there is a 28-year history and set of experiences that has been learned in the European theatre, the U.S. is only recently beginning to move forward with grid scale projects. The U.S. Department of the Interior's Bureau of Ocean and Energy Management (BOEM) has, to date, leased fifteen Wind Energy Areas (WEAs) to developers along the eastern continental shelf of the U.S which to date have auctioned for a total of over $472 million [3]. Furthermore, in October of 2019 the Environmental Business Counsel of New England held a discussion of OSW in the Gulf of Maine which coincides with the new BOEM Gulf of Maine Intergovernmental Renewable Energy Task Force which has been chartered to facilitate coordination and consultation related to renewable energy planning activities in the Gulf of Maine. The first task force was held on December 12, 2019 and BOEM predicts that the new lease areas will be developed within the next ten years. States have also taken steps to support Offshore Wind development. As of 2020, 28 states and the District of Columbia states have mandatory Renewable Portfolio Standards (RPS) which require that a specific percentage of electricity utilities sell come from renewable resources [4] while 7 other states have adopted voluntary RPS programs. Several states have specific consideration for offshore wind by creating Offshore Renewable Energy Credits (ORECs). For example, the Maryland Offshore Wind Energy Act of 2013 established ORECs for sourcing 2.5% of the state's electricity supply from offshore wind [5]. Additionally, in 2019 New Jersey doubled its offshore wind commitment from 3,500 MW to 7,500 MW by 2035 [6]. Furthermore, other states have increased commitments to procure OSW such as Maine (5,000 MW by 2030), Massachusetts (3,200 MW by 2035), New York (9,000 MW by 2025), Connecticut (2,000 MW by 2030) and Virginia (2,500 MW by 2026) [7]. As of 2019, there are over 26 GW of OSW projects in the pipeline with a projection of over 35GW of OSW construction just on the East Coast by 2035 [8]. With dramatically decreasing Levelised Costs of Energy and large scale proposed build out, offshore wind is projected to play a major part in the US electricity grid and economy. The history of offshore wind power has included some notable hurdles and set-backs. Where these were due to fundamentals of design or analysis, large fleets of offshore assets were affected. Examples include failures of gearboxes, grounted connections and accelerated blade surface erosion. As floating wind is scaled up, to minimise similar exposure to technical risks, formal processes will help to identify the novel features, novel applications and components with the highest risk. Assumptions about suitability of existing onshore or offshore technical solutions must be challenged. Also care must be taken when using demonstration projects comprising single units as the basis for scaling up technologies. Currently, although monitored and scrutinised so as to be proven technically, these have comprised small-scale wind farms, under experimental conditions.

This study proposes an analytical methodology to assess the risk to the operation of an offshore wind turbine in order to identify critical assemblies. The gearbox is one of the critical assemblies regarding maintenance costs and downtime. Failure investigation shows that the high speed shaft bearings are one of the gearbox components with a higher probability of fatigue damage. A physics-based model is developed to calculate the accumulated damage and to estimate the remaining useful life. The model is created for a 3 stage gearbox with two planetary and one parallel helical gear. This paper summarises the methodology and results of a damage accumulation calculation under a load spectrum derived from 1 year of SCADA data. The final results are consistent with the damage levels found in the failure investigation.

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