Indirect load measurements for large floating horizontal-axis tidal current turbines

Highlights

• A novel indirect load measurement method for large floating tidal current turbines was developed.

• The method was based on the numerical relationship between the platform responses and turbine loads.

• A load measurement experiment for a 300-kW tidal current turbine was conducted.

• Measurement results show good agreement with simulation results.

Abstract

Tidal current turbines have undergone tremendous development on their scales and capacities in recent decades, but their further commercial application has encountered great challenges, including their high energy cost and short lifetime. The optimization design for cost reduction requires accurate and reliable turbine load information, and the load reduction control for life extension requires recognizable real-time fatigue load feedback. To avoid the difficulty and complication of direct load measurements, an indirect load measurement method for a large floating horizontal-axis tidal current turbine was proposed in this study based on the numerical relationship between the turbine loads and platform responses. The operation principle of the proposed method was defined in detail, and a turbine load measurement system was designed. To validate the feasibility of this measurement system, a measurement experiment for a 300-kW tidal current turbine on a floating platform was conducted. The turbine loads, consisting of the turbine thrust and fatigue loads, were estimated based on the measured information. In a comparison with the load information predicted by software simulation, good agreement, especially on the turbine thrust, was obtained. The estimated fatigue load signal indicated a high signal-to-noise ratio, which can provide reliable support for load control.

Introduction

Tidal current energy is an attractive renewable energy resource because of its high predictability and stability. Both the size and the generation power of tidal current turbines (TCTs) have been continuously increasing in recent years. The capacity of the world's largest TCT has reached 2 MW (Zhou et al., 2017). However, the further development of TCTs is facing some problems associated with their high energy cost and severe fatigue load.

In the current process of TCT design, the operational load performance of TCTs is investigated mainly based on theoretical analysis using blade element momentum (BEM) theory and CFD numerical simulation (ed-Dı̂n Fertahi et al., 2018, Li et al., 2016). However, the CFD method strongly relies on the computing ability and costs a substantial amount of time. Thus, it is difficult to calculate the load performance of turbines during the control process. In the BEM method, many parameters of blade hydrofoils are transferred from wind turbines, but the two systems have quite different Reynolds numbers, stall characteristics and potential for cavitation (Bahaj et al., 2007, Winter, 2011). In general, the load results predicted by the current theoretical analysis methods are not accurate or reliable enough for the precise designs of TCTs and their support structures. Hence, due to the complex marine conditions and the lack of accurate load estimation of operating TCTs, high reliability indices are often adopted in the design process with additional safety margins that increase the energy cost. This cost disadvantage has seriously impeded the industrialization process of the TCTs.

Additionally, another problem of software simulations and theoretical calculations is that they cannot provide real-time load data of an operating TCT for the load reduction control. With the increase in TCT size, the fatigue load arising from current shear becomes more severe (Li et al., 2019). The absence of effective load reduction control, such as individual pitch control (IPC), reduces the lifetime of TCTs and has led to some unexpected failures (Bahaj, 2011, Zhou et al., 2017). In addition to the pitch control actuators, a reliable real-time fatigue load monitor method is required for IPC. It is therefore important to establish an effective method to measure or monitor the operating load of TCTs.

Thus, developing a load measurement system for TCTs is an efficient approach to meet the requirements of the cost reduction design and lifetime extension. To measure or monitor the dynamic load of wind turbines, strain gauge sensors (Liu et al., 2015, Liu et al., 2016) and fiber Bragg grating sensors (Schroeder et al., 2006) have been proposed and demonstrated for wind turbine blade load measurement. However, because the TCTs operate in deep water, probably in a high-pressure environment, the seal problems and signal transmission problems of these direct load measurement methods affect their longtime and efficient operation and restrict their applications for TCTs. Moreover, some indirect methods have also been studied. Platform pitching motion (Wakui et al., 2017) and tower vibrations (Lio et al., 2018) were used as feedback for power control and load reduction. The effects of platform motion on turbine performance were also studied (Wen et al., 2018, Zhang et al., 2015). However, these studies cannot provide direct reference or feedback for turbine design. The 6-degree-of-freedom (DOF) motions of the Hywind Demo floating wind turbine were measured in a related study that focused on the wave loads and the comparative analysis of measurement and simulation (Lin et al., 2018, Skaare et al., 2015). However, the numerical relationship between the response of platform motions and the turbine loads remains to be studied and experimentally validated.

Although the load measurements were studied for wind turbines, little work has been devoted to the load measurements for TCTs. Until recently, load measuring for TCTs had been a largely under explored domain, and existing methods in wind energy research areas could not provide available direct reference for the load measurement method for TCTs. Thus, this paper presents a novel load measurement method for the large floating TCTs regarding the whole mooring system as a part of a load measurement sensor.

Floating platforms provide convenient maintenance for TCTs and thus are widely used to support TCTs and other equipment (Gu et al., 2018, Sheng et al., 2016, Xu et al., 2016). Compared with the towers of wind turbines, the mooring systems have smaller stiffness so that the motions of the platforms can be easily observed and measured. Hence, avoiding the complicated and difficult direct load measurement, the proposed method in this study is to investigate the loads of TCTs by measuring both the static and the dynamic responses of floating platform motion. It is the focus of this research to obtain the numerical relationship between the turbine loads and the mooring system motion responses as shown in Fig. 1. The main contributions of this method is presenting a new solution for the load measurements for TCTs to support the cost reduction design for both the TCTs and the floating platforms and to provide real-time load feedback for load reduction control.

In this study, a mooring system model was firstly established by analyzing all the loads acting on the platform to obtain the numerical relationship between the turbine loads and the mooring system motion responses. Then, a floating platform motion measurement method based on Euler's angles was proposed and introduced. To validate the feasibility of proposed method, a measurement experiment for a 300-kW floating TCT was subsequently carried out in real sea state. Good agreement was obtained between the measured results and the theoretically calculated results based on BEM theory.