D. Karmakar

Long-Term Extreme Load Prediction of Spar and Semisubmersible Floating Wind Turbines Using the Environmental Contour Method

The prediction of extreme loads for the offshore floating wind turbine is analyzed based on the inverse reliability technique. The inverse reliability approach is in general used to establish the design levels associated with the specified probability of failure. The present study is performed using the environmental contour (EC) method to estimate the long-term joint probability distribution of extreme loads for different types of offshore floating wind turbines. The analysis is carried out in order to predict the out-of-plane bending moment (OoPBM) loads at the blade root and tower base moment (TBM) loads for a 5 MW offshore floating wind turbine of different floater configuration. The spar-type and semisubmersible type offshore floating wind turbines are considered for the analysis. The FAST code is used to simulate the wind conditions for various return periods and the design loads of various floating wind turbine configurations. The extreme and operation situation of the spar-type and semisubmersible type offshore floating wind turbine are analyzed using one-dimensional (1D) and two-dimensional (2D)-EC methods for different return periods. The study is useful to predict long-term design loads for offshore wind turbines without requiring excessive computational effort.

Floating Offshore Wind Platforms

The chapter introduces the platform types designed for supporting the tower, nacelle and turbine assembly of floating offshore wind turbines. Basic information on hydrodynamics is presented to provide an understanding of platform behaviour in waves. The classification method that distinguishes the platform types is stabilization. Covering this topic, buoyancy, mooring and ballast stabilized platform types are illustrated and briefly explained. The advantages and disadvantages of each approach are considered and summarized. The areas in which the benefits of each type are the most evident are clarified through various comparative studies. Most platform models are currently still at demonstration or conceptual stages. For this reason, computer codes that aim to capture their motions become particularly significant in design stages, and they are discussed next. A significant number of projects at the initial stages of planning and application are being developed in Japan, Europe and the United States. The platform types observed in these projects are presented. The chapter concludes with newly evolving design standards and a brief discussion on the optimization of platform shapes.

Propagation of Gravity Waves Past Multiple Bottom-Standing Barriers

The interaction of oblique surface gravity waves with multiple bottom-standing flexible porous breakwaters is analyzed based on the linearized theory of water waves. Using the method of eigenfunction expansion and the least square approximation, the wave propagation in the presence of single bottom-standing barriers is analyzed considering the upper edge to be: (i) free and (ii) moored, whereas the lower edge is considered to be clamped at the bottom. The wide-spacing approximation is used to analyze the wave interaction with multiple porous bottom-standing flexible barriers to understand the effect of the submerged flexible barriers as an effective breakwater. A brief comparison of both the upper edge conditions is carried out to analyze the effect of wave dissipation due to the presence of multiple barriers. The numerical results for the reflection and transmission coefficients along with the free surface vertical deflection are obtained for the case of two and three multiple bottom-standing barriers. The attenuation in the wave height due to the presence of porosity, change in barrier depth, and distance between the barriers are analyzed. The present study will be helpful in the analysis of proper functioning of porous bottom-standing barrier as an effective breakwater for the protection of offshore structures.

Shallow water effects on wave energy converters with hydraulic power take-off system

The effect of water depth on the power absorption by a single heaving point absorber wave energy converter, attached to a hydraulic power take-off system, is simulated and analysed. The wave energy flux for changing water depths is presented and the study is carried out at a location in the north-west Portuguese coast, favourable for wave power generation. This analysis is based on a procedure to modify the wave spectrum as the water depth reduces, namely, the TMA spectrum (Transformation spectrum). The present study deals with the effect of water depth on the spectral shape and significant wave heights. The reactive control strategy, which includes an external damping coefficient and a negative spring term, is used to maximize power absorption by the wave energy converter. The presented work can be used for making decisions regarding the best water depth for the installation of point absorber wave energy converters in the Portuguese nearshore.

Flexural Gravity Wave Scattering Due to Variations in Bottom Topography

Oblique flexural gravity wave scattering due to abrupt change in bottom topography is investigated under the assumption of linearized theory of water waves. The problem is studied first for single step in case of finite water depth whose solution is obtained based on the expansion formulae for flexural gravity wavemaker problem and corresponding orthogonal mode-coupling relation. The results for the multiple step topography are obtained from the result of single step using the method of wide-spacing approximation. Energy relation for oblique flexural gravity wave scattering due to change in bottom topography is used to check the accuracy of the computation. Using shallow water approximation the wave scattering due to multiple step topography is derived considering the continuity of mass and energy flux. In this case also the result for single step topography is obtained and then using the wide-spacing approximation the result for multiple steps are derived. Numerical results for reflection and transmission coefficients and deflection of ice sheet are obtained to analyze the effect of multiple step topography on the propagation of flexural gravity waves.Copyright

Hydrodynamic performance of concentric arrays of point absorbers

The behaviour of arrays of 12 heaving point absorbers in concentric arrangements is numerically assessed in a frequency domain model. The floaters are attached to a central cylindrical bottom-mounted structure. Each point absorber is restricted to the heave mode and is assumed to have its own linear power take-off system consisting of an external damping coefficient enabling power extraction and a supplementary mass coefficient tuning the point absorber to the incoming waves. The external damping and supplementary mass coefficients are optimized to maximize the power absorption by each floater in the array, with a restriction on the total control force that can be applied on the floaters. Various concentric arrangements with different radii and number of concentric circles are analysed to determine the most efficient among them. Moreover, the influence of the presence of a central bottom-mounted pillar and the effect of change in its dimension and shape on the power absorption are also studied.

Wave Motion Control Over Submerged Horizontal Plates

The interaction of surface gravity waves with horizontal pitching plate for actively control waves is investigated based on the linearized theory of water waves. The two dimensional problem is formulated for the submerged plate pitching about its middle point and the other plate is considered to be floating above the submerged plate. The submerged plate’s thickness is considered negligible in comparison with the water depth and wavelength of the incident wave. The study is carried out using the matched eigenfunction expansion method and the analytical solution is developed for the interaction of the surface gravity waves with horizontal submerged structure. The numerical results for the reflection coefficient, transmission coefficient and free surface deflection are computed and analyzed. The study is carried to find the optimal value of the length and depth of the submerged plate at which the dissipation of the incident wave energy is observed. The reduction the wave transformation due to the pitching of the plate with the change in angle of incidence is also analyzed. The present study will be helpful in the analysis of proper functioning of submerged pitching plate to control wave motion for the protection of offshore structures.Copyright

Extreme Response Prediction of Offshore Wind Turbine Using Inverse Reliability Technique

In the present study, the environmental contour method is applied for predicting out-of-plane bending moment loads at the blade root and tower base moment loads for 5MW offshore floating wind turbine of spar-type, DeepCWind and WindFloat semi-submersible floater configuration. FAST code is used to simulate the wind conditions for various return periods and a brief comparison on the design loads of the floating wind turbine for I-D, 2-D and 3-D environmental contour method is analyzed. In addition, a brief comparison of design loads with the spar-type, DeepCWind and WindFloat semi-submersible floater is discussed. The study is helpful to improve the turbine design load estimates and is useful in predicting accurate long-term design loads for wind turbines without requiring excessive computational effort.Copyright