OWEA Loads

OWEA Loads – Probabilistic load description, monitoring and reduction of loads of future offshore wind turbines

Project Coordinator: SWE – Stuttgart Wind Energy at the Institute of Aircraft Design, University of Stuttgart

Description

Offshore wind turbines (OWEA) are exposed to different environmental conditions than those onshore, which lead to challenges for the design of new large turbines. The extensive experience with onshore wind turbines in Germany therefore represents the foundation block that will support the development of reliable offshore ventures. In this respect, the project of “OWEA Loads” deals with the verification of key aspects of the design and operation of future offshore wind turbines.

As part of the Research at “alpha ventus” initiative by the Federal Ministry for Economic Affairs and Energy (BMWi), the project utilizes data collected by the measurement campaign at the offshore meteorological mast FINO 1, as well as the different sensors mounted on the 5MW wind turbines at the offshore park, to look at the different environmental conditions and their effects on the wind turbines.

The joint project, a continuation of the original “OWEA” project (2009-2011), coordinated by Stuttgart Wind Energy (SWE) at the University of Stuttgart, combines partners from industry and research institutes. Together, the researchers from SWE, IAG, ForWind, Adwen and Senvion help characterize the aerodynamic, hydrodynamic and operational loads that the turbines are exposed to.

The main research topics for the project are described by the three work packages, and their scope and objectives are outlined below.

 

AP A: Load analysis and probabilistic load description

The work in AP A.1 includes the further development of considerations and algorithms for checking the plausibility of measured data made in the previous “OWEA” project. This will help the process be easily transferable and applicable to future measurement campaigns. Important are the methods to be used for the correction or exclusion of erroneous data. In addition, the unsteady loads along the rotor blades in turbulent flow for the offshore wind farms are analyzed through the development of a numerical model (LES-RANS) (AP A.2). Part of the work package outlines the need for the development of a probabilistic description of the characteristic extreme and fatigue loads (AP A.3). In contrast to deterministic loads, a design of an offshore wind turbine with this procedure would take into account the probability of occurrence of the loads on the structural components. This should be checked and compared through extrapolation of the data that is recorded from the sensors of the turbines. Here, there is also the opportunity to investigate the assumptions (that are based on computer simulations) of IEC 61400-3 Ed. 1 (Wind turbine generator systems - Part 3: Design requirements for offshore wind turbines) using the available measured data.

AP B: Load-reducing control and load monitoring

This package includes the development and validation of new operation concepts to reduce the aerodynamic and hydrodynamic loads on the support structure and the rotor-nacelle unit (AP B.1). Specific control approaches for certain foundations like tripod, monopile and monotower are another priority to make them economically viable when confronting respective material costs, increasing water depths and larger wind turbines. To evaluate the load reduction concepts used and the determination of the actual remaining lifetime, load monitoring systems can be implemented in future wind turbines. This is important due to the fact that the same turbine in a different location will often see significantly lower loads than assumed in the design. To get reliable estimations on the remaining lifetime from load monitoring, two complementary methods, based on statistical parameters of standard signals (AP B.2) and stochastic transmission behavior of dynamic and intermittent load fluctuations (AP B.3) are refined. Furthermore, due to the complexity of the component and material behavior of the rotor blades, it is necessary to develop specific measurement and analysis methods to monitor the loads and the state of the rotor blade; these methods will also be a requirement for modern control methods such as individual pitch control (AP B.4).

AP C: Design conditions for future generations of wind turbines

To reduce the design and operating risks, the findings from the first “OWEA” project will be taken to formulate site-specific design requirements for future generations of wind turbines in the German exclusive economic zone (EEZ). Both, the wind conditions at higher altitudes as well as the wind profile influencing atmospheric variables (air temperature, humidity, turbulent flows, etc.) are to be measured. The objective then, is to develop conclusions regarding the local wind profile for heights above the measuring mast of the FINO platform. Additionally, based on the collected data, models to describe the wind speed and wind direction should be developed and verified. The risk of icing in the North Sea for future generations of systems should also be investigated. The proposed work will eventually lead to an improved prognosis of the total energy yield and the short-term forecast of the electrical power. In particular, the description of the wind direction change with height should lead to an improved estimation of the expected loads on the system. Based on the measurement data from the mechanical loads of OWEA, recommendations for a simplified and more effective design process should be given. With help of this data, changes in the fatigue loading as a function of various combinations of wind speed, turbulence class, wave height and wave period, is to be examined. Another technical goal is to develop, from the available measurement data, a relationship between the necessary level of detail of the ambient conditions and the resulting dispersion of the calculated / measured mechanical load on different components of the wind turbine. Based on these results, the in “OWEA” validated computing models will be examined according to practical use and their limitations.

Partners and contacts

University of Stuttgart

SWE – Stuttgart Wind Energy at the Institute for Aircraft Design (www.uni-stuttgart.de/windenergie)

Institute of Aerodynamics and Gasdynamics (www.iag.uni-stuttgart.de)

ForWind Center for Wind Energy Research (www.forwind.de)

Carl von Ossietzky University of Oldenburg, Institute of Physic (www.uni-oldenburg.de/physics)

Senvion SE (www.senvion.com)

Adwen GmbH (www.adwenoffshore.com)