Platform Pitch Motion Research Articles

Experimental Study on Influence of Pitch Motion on the Wake of a Floating Wind Turbine Model

Wind tunnel experiments were performed, where the development of the wake of a model wind turbine was measured using stereo Particle Image Velocimetry to observe the influence of platform pitch motion. The wakes of a classical bottom fixed turbine and a streamwise oscillating turbine are compared. Results indicate that platform pitch creates an upward shift in all components of the flow and their fluctuations. The vertical flow created by the pitch motion as well as the reduced entrainment of kinetic energy from undisturbed flows above the turbine result in potentially higher loads and less available kinetic energy for a downwind turbine. Experimental results are compared with four wake models. The wake models employed are consistent with experimental results in describing the shapes and magnitudes of the streamwise velocity component of the wake for a fixed turbine. Inconsistencies between the model predictions and experimental results arise in the floating case particularly regarding the vertical displacement of the velocity components of the flow. Furthermore, it is found that the additional degrees of freedom of a floating wind turbine add to the complexity of the wake aerodynamics and improved wake models are needed, considering vertical flows and displacements due to pitch motion.

Unsteady aerodynamics of offshore floating wind turbines in platform pitching motion using vortex lattice method

As the flow states of an offshore floating wind turbine (OFWT) differ from those of an onshore fixed wind turbine, it is questionable as to whether the aerodynamic load prediction of a turbine using conventional blade element momentum theory (BEMT) is accurate. The aim of this paper is to show the characteristics of aerodynamic load predictions using the vortex lattice method (VLM). Washizu's experimental data, which was measured under a similar flow state of a floating wind turbine, is used for validation. The prediction shows good results compared to those of an experiment. To determine the unsteady aerodynamics of a floating wind turbine, the NREL 5 MW wind turbine model is used for the simulation of a floating wind turbine. These results show that a turbulent wake state (TWS), which is undesirable condition and cannot predicted in BEMT simulation, arises when a floating wind turbine is operated at a low-speed inflow condition. In addition, the rotor experiences a TWS when the floating platform undergoes upward pitching motion.

Controlling Platform Motions and Reducing Blade Loads for Floating Wind Turbines

Offshore sites hold great promise for the growth of wind energy. To tap the vast resource in deep water sites, new support structures, such as those that float, are needed. For floating structures to succeed, they must withstand the offshore wind and wave environment. Two new methods for controlling a floating turbine and reducing the platform and blade loads are presented. The first is a method for controlling collective blade pitch and reducing platform pitch motion, a significant problem for floating structures. The rated generator speed is made a function of the platform pitch velocity. When the platform is pitching upwind, the set point generator speed is set to a larger value, and vice versa. For constant generator torque, this approach essentially makes the rated power a variable that depends on the platform pitch velocity. Fundamentally, this control approach trades power variability for platform pitch variability. The results will show substantial reductions in platform pitch motion but minor increases in power variability. Second, an individual blade pitch controller (IPC) designed to reduce blade fatigue loads is implemented for a floating turbine. The IPC approach is commonly utilized for reducing the 1P fatigue loads on the blades. The goal of implementing this IPC approach is to investigate how traditional load reduction control, which is successful for onshore turbines, integrates and performs with floating turbines. The results will demonstrate the unique challenge of reducing blade loads on a floating turbine.

Tether-platform coupled control

From the control point of view, tethered systems pose several challenges, the major one pertaining to the regulation of the unstable system dynamics during the retrieval phase. On the other hand, the system configuration permits design of controllers using length rate, tension and offset schemes, which are not feasible with other satellites. Here “offset” refers to the time dependent variation of the tether attachment point at the platform end. The present paper studies several applications of the offset scheme in controlling the tethered systems. To that end, planar equations of motion of a space platform based Tethered Satellite System (TSS) are derived by the Lagrangian procedure. This is followed by representative results aimed at the offset control of platform pitch, tether attitude and vibration motions. The offset scheme is used for simultaneous control of platform and tether pitch motion. Finally the attention is directed towards simultaneous regulation of the platform pitch and longitudinal tether vibration. The numerical results clearly show considerable promise for the offset control scheme in regulating tether, platform and combined tether-platform dynamics.

Analysis of Brief Tension Loss in TLP Tethers

The avoidance of “slack” tethers is one of the factors which may establish the required tether pretension in a tension leg platform (TLP) design. Selection of an appropriate safety factor on loss of tension depends on how severe the consequences may be. It is sometimes argued that if tethers go slack, the result may be excessive platform pitch or roll motions, tether buckling, or “snap” or “snatch” loading of the tether. The results reported here show that a four-legged TLP would not be susceptible to larger angular motions until two adjacent legs lose tension simultaneously. Even then, this analysis shows that a brief period of tether tension loss (during the passage of a large wave trough) does not lead to excessive platform motion. Similarly, momentary tension loss does not cause large bending stress in the tether or significant tension amplification as the tether undergoes retensioning. This paper presents TLP platform and tether response analysis results for a representative deepwater Gulf of Mexico TLP with large-diameter, self-buoyant tethers. The time-domain, dynamic computer analysis included nonlinear effects and platform/tether coupling.