Müller, Nathalie (Fraunhofer-Institut für Windenergie und Energiesystemtechnik (IWES)) | Kraemer, Peter (University of Siegen) | Leduc, Dominique (Research Institute of Civil Engineering and Mechanics (GeM)) | Schoefs, Franck (Research Institute of Civil Engineering and Mechanics (GeM))
A fatigue test has been conducted on a large-scale offshore wind turbine grouted connection specimen at the Leibniz University of Hannover. For detecting damages in the grouted joint, a structural health monitoring (SHM) system based on fiber optic sensor-type fiber Bragg grating (FBG) has been implemented. By extracting the features of the FBG signal responses using the Wigner–Ville distribution (WVD) and one of its marginal properties, the energy spectral density (ESD), it is possible to detect the occurrence and the global severity of the damage. Some information about the local severity of the damage has also been obtained.
The grouted connection consists of the high-performance grout-filled space between the two structural steel components of respectively the sleeve and the pile of offshore wind turbines (OWTs). For monopile OWTs, it is located around the water level between the transition piece and the pile, whereas for jacket and tripod OWTs, it is located just above the seabed, between substructure and foundation pile. While grouted joints for monopiles are exposed to bending moments, grouted joints for latticed substructures (tripods and jackets) are exposed to predominant axial loadings and low torsional moments (Schaumann and Böker, 2005; Schaumann, Lochte-Holtgreven et al., 2010). It is a critical structural part of OWTs. In 2009–2010, engineers reported grouted connection failures causing slight and progressive settlement of turbines. The problem affected approximately 600 of the 988 monopile wind turbines in the North Sea, requiring further investigations concerning the design of the grouted connection (Rajgor, 2012). Since then, two grouted connection designs reducing the axial forces in this area have been recommended by Det Norske Veritas (2014): using a conical grouted connection (first design) or a tubular connection with shear keys (second design).
Sun, Xiao-Qian (Zhong Neng Power-tech Development Co. Ltd.) | Cao, Shu-Gang (Zhong Neng Power-tech Development Co. Ltd.) | Chi, Yan (Zhong Neng Power-tech Development Co. Ltd.) | Zhu, Zhi-Cheng (Zhong Neng Power-tech Development Co. Ltd.)
This study investigated a vibration and tilt monitoring system for an offshore wind turbine constructed using a high-rise-pile- cap supporting foundation, which is the first offshore wind power project in South China with a batholith seabed. The analysis of data collected by the system during the 2016 typhoon Meranti showed that the typhoon significantly affected vibration and instantaneous tilt of the supporting system without any significant change to the first natural frequency. Additionally, it did not produce any permanent inclination, indicating that no serious structural failure occurred under the influence of the typhoon. However, during the typhoon, the vibration acceleration, vibration intensity, and the effective inclination of the high-rise-pile-cap supporting system using rock-socketed piles were smaller than those with driven frictional piles, indicating that the former is better than the latter in terms of resistance to vibration and tilt.
The construction of offshore wind power plants in China faces many challenges, including the raging typhoons in the East and South Seas. Each year, the Guangdong province experiences typhoons three times on average, accounting for 33% of the annual typhoons in China’s coastal areas. The proportions of typhoon episodes in Taiwan, the Hainan province, the Fujian province, and the Zhejiang province are 19%, 17%, 16%, and 10%, respectively (Wu and Li, 2012). The extreme vibration and abnormal inclination of the offshore wind turbine supporting system as a result of typhoons sometimes lead to structural failures and can even result in the collapse of the wind turbine structure into the ocean.
The support structure of offshore wind turbines is working in harsh ocean environments, where uncertainties exist and affect the performance of the whole system. This work presents an efficient methodology for the Reliability Based Design Optimization (RBDO) of the support structure of offshore wind turbines considering uncertainties. Reliability analysis is a feasible option in the absence of field measurement data. Monte Carlo simulations are robust and used as reliability analysis benchmark, but they are very computationally demanding for offshore wind turbine cases. Efficient Fractional Moment reliability analysis method was proposed. The results show that the proposed methodology can obtain a reliable design with better dynamic performance and less weight. Compared with the deterministic optimization, the presented dynamic RBDO of offshore wind turbines is more practical, and this methodology can be applied in the design of other similar offshore structures.
The support structure of offshore wind turbines is working in harsh ocean environments, reliability analysis is a feasible option in absence of field measurement data (Yang et al., 2017). To ensure that the proposed offshore wind turbine design is cost effective, it is necessary to check whether the decided support structures provides optimal life cycle cost.
For a reliable design, it is essential to consider various uncertainties in the dynamic analysis of offshore wind turbine (Xiao and Yang, 2014; Zhang et al., 2017). Due to the random nature of environmental parameters, wave, wind and currents must be modelled as stochastic process (Zhang and Yang, 2014). Hence, there is a need of stochastic dynamic analysis on one hand and the need of developing performance assessment, maintenance and optimization of the offshore wind turbine system with uncertainties. We try to answer the following questions: a) Can we formulate an efficient and accurate method for reliability analysis to replace Monte Carlo simulations which are robust but too time consuming; b) How to overcome computational challenges associated with reliability-based optimization methodology of offshore wind turbine system?
This paper presents the application of a risk- and reliability-based inspection planning framework for the InnWind 20 MW reference wind turbine jacket substructure. A detailed fracture mechanics-based fatigue crack growth model is developed and used as a basis to derive optimal inspection plans for the jacket substructure. Inspection plans for different inspection techniques are proposed, and recommendations on how to optimize inspection intervals are discussed.
Upscaling current wind turbines to very large wind turbines is considered as one of the important ways to decrease the levelized cost of energy (LCoE) of wind energy. Steel jacket structures are one possible type of support structure for very large offshore wind turbines and have been considered in the EU InnWind project, INNWIND.EU (http://www.innwind.eu). Reliability with respect to fatigue failure is generally driving the design of offshore wind turbine jacket structures and is being considered in this paper in combination with applications of reliability-based inspection planning.
In this work, we present four different methodologies for reducing the computational effort of fatigue assessment for offshore wind turbine support structures. To test these methods, we use them to predict the total fatigue damage of several modified support structure designs based on subsets that represent a reduction of about 6-17 times the original size of the load case set. Three of the methods are able to give quite accurate predictions, with expected errors of no more than 4-8%, though there are some limitations due to the variance inherent in some of the methods.
One of the main challenges for the design of offshore wind turbines support structures is the complexity of both the structure itself and the offshore environment. This complexity means that assessing the performance of the structure requires not only the use of detailed models, but also investigating a large number of different scenarios. Specifically, with reference to the standards that the design must conform to (e.g. International Electrotechnical Commision (2009)), there are literally thousands of different design load cases (DLCs) that must be assessed for any given structure, covering both all the various environmental states one expects to encounter at a given site and all the various scenarios that the structure is likely to experience. To summarize, we need to run detailed models and we need to run them many times. For one single assessment of a design, this can be accommodated by ever improving computer hardware and increased access to computer clusters for both institutions and individuals. However, for those wishing to run either probabilistic assessments or to optimize the design (or worse still, both of these at the same time), the large number of DLCs remains an important challenge. One that should be addressed not just by improved hardware, but by improved methodology. This is the main topic of the work to be presented below.
As it stands, it is not possible to completely replace the standard assessment with something new. Rather, one seeks to approximate the results of such full assessments by a less computationally demanding procedure. If the approximation is good enough, it may then serve well as a replacement for the conventional procedure when small deviations from the true estimates (e.g., fatigue damage) are allowable. Especially in a context like optimization, simplifications leading to such small deviations are often expected and, if the size of the deviations can be estimated, one may even incorporate these as modeling errors in a probabilistic analysis. Previous work attempting to find approximate simplified assessments have encountered some success, but have tended to be very simplified (for example in terms of the types of DLCs studied), have struggled to get a sufficiently accurate approximation while also getting a sufficient decrease in analysis time or have faced a combination of these issues. One approach is to completely abandon the time domain and instead attempt to analyze the structure in the frequency domain (see e.g. van der Tempel (2006)), but this approach has its own set of issues and we will here focus on methods in the time domain.
This paper presents an analytical study on the risk and vulnerability assessment of an Offshore Wind Turbine (OWT) subjected to coupled hydrodynamic and aerodynamic loads. The Computer Aided Engineering (CAE) tool FAST v8 simulator, developed by National Renewable Energy Laboratory (NREL), is used for the multi-hazard simulation of a “NREL offshore 5-MW baseline wind turbine”. FAST is able to incorporate non-linearity coupled with both hydro and aero dynamic effects resulting from wind-and-wave loading scenarios. Site characteristics of the OWT are considered based on Nantucket Sound, Massachusetts, the United States, which is an ideal site for a future U.S. wind farm. The target site that belongs to the east coast is regarded to be a more hurricane-prone region; thus, this paper utilizes an extreme turbulent model (ETM) coupled with irregular waves determined based on Pierson-Moskowitz spectrum. The OWT supported by a fixed-bottom foundation is modeled with multi-degree-of-freedom modules enabling the time-domain coupled analysis. The OWT is simulated, considering the extreme loading scenarios specified by the International Electrotechnical Commission (IEC 61400-3) design standard that takes variability of both wind and waves into consideration. Structural responses of the OWT subjected to coupled wind and wave loads are captured at various critical locations across the overall system, and the flexural demands of the OWT at the mudline are found to be critical in evaluating its failure mechanism. Peak flexural demand quantities are then employed for the development of vulnerability functions for variations in wind and wave characteristics, including wind speed and wave height. The limit state function pertaining to flexural failure mode used for the vulnerability determination is based on First Order Reliability Method (FORM). The analysis of the resulting vulnerability data reveals that the exceeding probability increases due to increase in both wind speed and wave height, especially beyond 12m/s, while the wave height has less impact on the probability than the wind speed until the wave height of 10 m is reached.
The Markov approach to estimate fatigue damage for a monopile-based offshore wind turbine exposed to aerodynamic and hydrodynamic loading is investigated in this study. The focus of this study is on obtaining the rainflow-counting intensity from a peak-trough counting using the Markov method proposed by Frendahl & Rychlik. The fatigue damage estimated from the rainflow-counting intensity is compared to fatigue damage estimated from the original time-series using the rainflow-counting algorithm. The comparison is performed for different load situations. The study shows that the Markov approach performs the best for load situations where wave loading is dominating the response, making it interesting for load calculations of large-diameter monopiles and monopiles in parked or idling conditions.
Offshore wind turbines are prone to failure from fatigue damage due to their exposure to a significant source of quasi-periodic excitations from wind and waves. A detailed fatigue assessment is based on cycle counting (the rainflow-counting algorithm is widely used) from a large number of load simulations in the time-domain, typically in the order of a few thousand. Detailed fatigue assessment is therefore primarily performed in the final stage of the design process (Seidel et al. 2016), since it is inefficient for sensitivity studies or conceptual design phases where several designs have to be assessed. Simplified and/or reduced models of wind turbines or loads (Muskulus, 2015; Schløer et al., 2016; Ong et al., 2017) or calculations in frequency-domain (Ragan & Manuel, 2007; Seidel 2014; Ziegler et al., 2015) are commonly used to estimate fatigue damage in these situations.
Another method to compute the expected damage was proposed by Frendahl & Rychlik in 1993. Fatigue damage is estimated by assuming that the sequence of local extrema forms a Markov chain. This allows to obtain the rainflow-counting intensity directly from the spectrum of the input loads (referred to load spectrum throughout this paper) without the need for response time series in the time-domain. The fatigue damage can be estimated from the rainflow-counting intensity afterwards. The benefit of this method is the possibility to estimate the total damage from a load spectrum without the need to perform lengthy and computationally demanding simulations in the time-domain. The flowchart for both methods are shown in Fig. 1.
Heinonen, Jaakko (VTT Technical Research Centre of Finland Ltd) | Tikanmäki, Maria (VTT Technical Research Centre of Finland Ltd) | Kurkela, Juha (VTT Technical Research Centre of Finland Ltd) | Klinge, Paul (VTT Technical Research Centre of Finland Ltd) | Hekkala, Toni (VTT Technical Research Centre of Finland Ltd) | Koskela, Jussi (Semantum Oy) | Montonen, Anni (Finnish Meteorological Institute) | Eriksson, Patrick B. (Finnish Meteorological Institute)
The Gulf of Bothnia has the potential for large capacity wind farms because of relatively high and constant wind velocities. However, the sea freezes annually, introducing the most significant uncertainties in the support structure design for offshore wind turbines. This presentation introduces an ice load portal that simplifies and speeds up the preliminary design process for offshore wind farms. The design portal integrates all necessary information (site-specific environmental data, structural design, and environmental loads) into a single tool. This study demonstrates the functionality of the ice load design portal with case studies and introduces some validation studies.
The total capacity of wind energy in Finland at the end of 2017 was 2044 MW, consisting of 700 wind turbines. To meet the European target for renewable energy, Finland has set a target to increase the share of renewables above 50% of the total energy production in the 2020s. This can only be realized with large-scale investments in sea areas. This development is also strongly supported by the European Union’s Blue Growth strategy.
The shallow coastline and the consistent wind conditions in the northern part of the Baltic Sea provide a good environment for wind energy production. However, the sea freezes annually, introducing the most significant uncertainties in the support structure design for offshore wind turbines. Drifting ice introduces major design load case for offshore wind turbines. The magnitude and time variation of the sea ice load depends on various factors like the thickness and velocity of the ice as well as the size and shape of the structure. The main engineering challenges regarding ice in this area have been introduced by Määttänen (2010).
The capacity for wind energy production in the southern part of the Baltic Sea is currently 1.5 GW. This area represents the second largest basin (after the North Sea) for offshore wind in the near future. Potentially 8-12 GW of wind power could be installed by 2030 (Wind Europe in Europe, 2017). Due to the relatively high and constant wind velocities in the sea areas, as well as the mostly shallow coastal areas that enable costefficient foundation and grid connection, the interest in offshore wind is also increasing in the Gulf of Bothnia. The first offshore wind farm built on offshore foundations in Finland Tahkoluoto 1 owned by Suomen Hyö tytuuli Oy - was launched in late August, 2017. The wind farm is located on the west coast of Finland (Tahkoluoto in Pori), where, every year, the sub-structures are confronted with very challenging ice conditions. For this reason, a gravity-based foundation has been designed with a conical shape at the waterline to minimize ice loads and to mitigate ice-induced vibrations.
In this work, advanced reliability assessment of OWT (offshore wind turbine) monopiles is proposed by combining reliability analysis method and SHM (structural health monitoring) / CM (condition monitoring) technology. A 3D (three-dimensional) parametric FEA (finite element analysis) model of OWT monopiles is developed, considering soil-structure interactions. A number of stochastic FEA simulations of OWT monopiles are performed, taking account of stochastic variables, such as wind loads, wave loads and soil properties. Multivariate regression is then used to post-process the FEA results, obtaining the performance functions expressed in terms of stochastic variables. After that, FORM (first order reliability method) is used to calculate the reliability index, evaluating the reliability of the OWT monopiles. In the presence of SHM/CM data, the reliability of monopile structures is reassessed and updated. The updated reliability index provides valuable information for decision making for inspection and maintenance of OWT monopiles. The application of the proposed advance reliability assessment method to a 45m-length OWT monopile is presented, showing great potential to reduce the OPEX (operating expenditure) of OWT monopiles by using the proposed method.
Wind power is capable of providing a competitive solution to battle the energy crisis and global climate change, making it the most promising renewable energy resource. Currently, the vast majority of wind power are generated from onshore wind farms. However, the growth of onshore wind farms is limited to some extent by the visual pollution caused by large wind turbines and the limited available space to deploy onshore wind turbines. Compared to the land, there is more available space to deploy wind turbines at sea and the wind is stronger and steadier in offshore locations, driving wind industry move to offshore. According to European Wind Energy Association (EWEA, 2015), offshore wind in Europe is expected to reach 64.8 GW, supplying around 8.4% of total electricity demand in Europe in 2030.
Representing around 80.1% of overall EU's installation in 2015 (Wilkes et al., 2016), Monopiles are currently the most widely used foundation for OWTs (offshore wind turbines), due to their ease of both manufacturing and installation. They are well suitable for water depths shallower than 30m (Maciel, 2010).
Lai, Wen-Jeng (Institute of Nuclear Energy Research, National Central University) | Su, Wei-Nian (Institute of Nuclear Energy Research) | Huang, Chin-Cheng (Institute of Nuclear Energy Research) | Huang, Yi-Mei (National Central University)
The dynamic load calculation of offshore wind turbine is an important technology in the future development of wind turbine industry in Taiwan. This paper presents a simplified equivalent-monopile model of reducing the dynamic load simulation time for offshore wind turbines with jacket support structures under seismic conditions. The FAST v7 code that only simulates monopile substructure is chosen as the analysis tool and is further recompiled to include the effect of seismic-pile-soil interaction in the simplified model. The numerical analysis results are then validated by comparing with the results from the fully coupled model, calculated by Bladed software, in a NREL 5MW offshore wind turbine. Two seismic load scenarios, operational and idling conditions, are studied in this work. The results show that usage of simplified model may offer advantages in preliminary support structure design. The advantages are apparent in seismic simulation since the simplified equivalent-monopile modeling allows current FAST v.7 code constraints relaxed for simulating jacket structures and leads to quicker and more convenient dynamic load calculation. The influence of earthquake and soil flexibility is especially important in design considerations since Taiwan is located in a seismically active region. Thus, the developed simplified numerical model will be helpful to shorten the simulation time of a series of seismic design load cases in the preliminary design phase.
Wind energy is considered as one of the most important renewable energies which is being developed and applied all over the world. In recent decades, exploiting the wind resource has already become a mature and feasible technology, such as the development of offshore wind turbine (OWT). An offshore wind farm is usually with higher average wind speed and lower turbulence, and it can provide more wind power compared with the onshore wind farm. In other words, the wind field over the sea is much more attractive than that on the land, both in terms of quantity and quality of the wind energy. In this aspect, some OWTs might be constructed in seismically active regions. The impact of earthquake load on design and operation of OWTs has to be taken into consideration. Especially for Taiwan, it is located in an active seismic zone where earthquake occurs frequently. By contrast, wind turbine industries have mainly established in North Europe which are not earthquake-prone regions and thus they may not pay much attention to the seismic impact on wind turbine systems. Therefore, the earthquake excitation makes a challenging problem when designing the support structure and foundation in seismically active regions.