Practical Method to Estimate Foundation Stiffness for Design of Offshore Wind Turbines

Offshore wind turbine structures (OWTs) are dynamically sensitive due to their shape and form (slender column supporting a heavy rotation mass) and also due to the different forcing functions (wind, wave, and turbine loading) acting on the structures. Designers need to ensure that the first Eigen natural frequency is not close to forcing frequencies to avoid dynamic associated effects such as resonance and fatigue damage. Such damages may result in higher maintenance costs and a lower service life. Therefore, it is crucial to get the best prediction of the first natural frequency during the early stages of a project. Other design requirements include the serviceability limit state (SLS) criteria which imposes strict pile head deflection and rotation limits. These calculations require foundation stiffness and the aim of this chapter is to provide practical methods to predict the stiffness of the foundations for any ground profile (nonuniform or layered soils) through the use of standard methods. The foundation stiffness values can then be used as an input to predict the first natural frequency of OWT system as well as checking SLS requirements. An example problem is taken to show the application of the method

Impedance Functions for Rigid Skirted Caissons Supporting Offshore Wind Turbines

Large diameter caissons are being considered as plausible foundations for supporting offshore wind turbines (OWTs) where reductions in overall cost and environmentally friendly installation methods are expected. The design calculations required for optimization of dimensions/sizing of such caissons are critically dependent on the foundation stiffness as it is necessary for SLS (Serviceability Limit State), FLS (Fatigue Limit State), and natural frequency predictions. This paper derives closed form expressions for the 3 stiffness terms (Lateral stiffness KL, Rotational Stiffness KR and Cross-Coupling term KLR) for suction caissons having aspect ratio between 0.5 and 2 (i.e. 0.5 <L/D<2) which are based on extensive finite element analysis followed by non-linear regression. The derived stiffness terms are then validated and verified using studies available in literature. An example problem is taken to demonstrate the application of the methodology

Physical modelling of Offshore Wind Turbine Foundations for TRL studies

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