Offshore Wind Turbines (OWTs) are dynamically sensitive structures and as a result estimating the natural frequency of the whole system taking into effect the flexibility of the foundation is one of major design considerations. The natural frequency is necessary to predict the long-term performance as well as the fatigue life. Currently, jackets supported on multiple foundations (such as piles or suction caissons) are being considered to support WTG (Wind Turbine Generators) for deeper water developments. This paper presents a practical method to compute the natural frequency of a jacket supporting WTG by incorporating Soil-Structure-Interaction (SSI) based on closed form solutions. The formulation presented can be easily programmed in a spreadsheet type program and can serve as a convenient way to obtain natural frequency with least amount of input. The basis of this method is the Euler-Bernoulli beam theory where the foundations are idealized with a set of linear springs. In this method, a 3-Dimensional jacket is first converted into a two 2-Dimensional problem along the orthogonal planes of vibration which are essentially the principle axes of the foundation geometry. Subsequently, the jacket is converted into an equivalent beam representing its stiffness and a formulation is presented to find an equivalent beam for entire tower-jacket system. Using energy methods, an equivalent mass of the RNA (Rotor Nacelle Assembly)-tower-jacket system is also calculated and fixed base frequency of the jacket is estimated. To consider the flexibility effects of the foundation, a formulation for an equivalent rotational spring of the foundation is developed. A method to incorporate the mass of the transition piece is also presented. Finally, a step-by-step application of the methodology is presented by taking example problems from the literature which also serves the purpose of validation and verification.
Monopiles are currently the preferred option for supporting offshore wind turbines (OWTs) in water depths up to about 40 m. Whilst there have been significant advancements in the understanding of the behaviour of monopiles, the guidelines on the prediction of long term tilt (Serviceability Limit State, SLS) under millions of cycles of loads are still limited. Observations and analysis of scaled model tests identify two main parameters that governs the progressive tilt of monopiles: (a) Loading type (one-way or two-way) which can be quantified by the ratio of the minimum to maximum mudline bending moments (Mmin/Mmax); (b) factor of safety against overturning i.e. the ratio of the maximum applied moment (Mmax) to the moment carrying capacity of the pile or Moment of Resistance (MR) and therefore the ratio Mmax/MR. Due to the nature of the environmental loads (wind and wave) and the operating conditions of the turbine, the ratio Mmin/Mmax changes. The aim of this paper is to develop a practical method that can predict the nature of loading for the following governing load cases: Normal Operating Conditions, Extreme Wave Load scenario, and Extreme Wind Load scenario. The proposed method is applied to 15 existing wind farms in Europe where (Mmin/Mmax) and (Mmax/MR) are evaluated. The results show that the loading ratio is sensitive to the water depth and turbine size. Furthermore, under normal operating conditions, most of the wind turbine foundations in shallow waters are subjected to one-way loading and in deeper waters and under extreme conditions the loading is marginally two-way. Predictions for the nature of loading for large wind turbines (8MW and 10MW) in deeper waters are also presented. The results from this paper can be used for planning scaled model tests and element tests of the soil.
Foundations are one of the most expensive items in the capital cost break down of an offshore wind farm and the design is a challenging and multidisciplinary task that requires an understanding of the aerodynamics, hydrodynamics, structural dynamics, and soil-structure interaction. Though there has been extensive research, foundation codes are still not fully developed and are heavily dependent on the principles developed for oil & gas platforms which have distinct differences with offshore wind installations. Furthermore, current offshore wind turbines are becoming larger in size and installed in deeper waters, thus jackets are becoming a more attractive option when compared to the conventional monopiles.
However, the current design methods for jackets are computationally challenging and time consuming, and often is the case, require data that is unavailable in the public domain which makes concept designs a difficult process. Due to the lack of simple integrated approaches, this thesis focuses on developing an integrated and modular design approach which can be easily implemented on spreadsheet type software and result in a conservative foundation size with adequate accuracy. The design criteria covered in this thesis are the Ultimate Limit State (ULS), Serviceability Limit State (SLS), and the natural Frequency requirements.
For the dynamic analysis of foundations, a novel approach is proposed that expresses the natural frequency of the system in terms of mechanics based approaches considering the flexibility of the jackets and supporting foundations. The analysis steps from the thesis are compiled into an integrated design approach applied to jackets and their supporting foundations named as the ?10-step method?. It is shown that following these steps will result in a similar jacket and foundation size as detailed designs. The approaches of this thesis are expected to be a very powerful tool in the concept design stage when the financial viability of a wind farm is assessed. The work of this thesis also sets ?templates? of appropriate jacket and foundation sizes in the detailed design stage. Finally, future works and enhancement to the work of this thesis are also provided.