Currently, the development of renewable energy has become a trend with the increasing demand for energy. Wind energy, as a renewable source of energy, is also getting more attention. Increasing effort is devoted to developing floating offshore wind turbines in deep water. In this paper, a V-shaped semisubmersible floating wind turbine was adopted to investigate the dynamic response of the system. Numerical simulations are conducted using aero-hydro coupled analysis in a time domain. The performance of the V-shaped semisubmersible floating wind turbine with respect to global platform motion, mooring line tensions and tower base moment is evaluated in this study. It turns out that the V-shaped semisubmersible offshore wind turbine is a promising concept that provides a good practice for the application of wind energy in deep water in the future.
Currently, due to energy deficiency, many countries are devoted to developing renewable energy to meet energy demands. According to the Chinese 13th renewable energy development five year plan, by 2020, total electric from renewable energy will grow to up to 27% of the total electricity generated(NDRC, 2017). Wind energy, one of the promising renewable energies, has attracted more and more attention because of its low environmental pollution. Compared with onshore wind energy, offshore wind energy has better wind condition, unlimited sites and negligible environmental impact. Especially in China, the area with rich onshore wind resource is far from the energy consumption center, which is located near the eastern coastline (Li et al., 2012). A number of studies have been carried out for offshore wind turbine analysis (Jiang et al., 2015; Shi et al., 2016; Shi et al., 2014). The bottom-fixed wind turbine is not suitable for deep water due to increase in cost (Shi, 2015). Therefore, the floating offshore wind turbine (FOWT) is becoming one of the promising solutions.
According to the offshore oil and gas industry, several different foundations are suitable for FOWT: spar buoy, tension leg platform (TLP), semi-submersible platform and barge. In particular, the semisubmersible platform, compared with spar buoy and TLP, has more feasibility in various water depth, seabed conditions and low installation costs due to the simpler installation (it is fully constructed onshore). The semi-submersible platform can also avoid the main energy range of the waves because of its relatively large natural period. The OC4 semi-submersible offshore wind turbine was simulated by Bayati (2014) to focus on the impact of second-order hydrodynamics on semi-submersible platforms. Moreover, the second-order hydrodynamic force can stimulate the oscillation of the platform and further cause fatigue damage to the structure. How the mooring systems influence the motion of the FOWT (Masciola and Robertson, 2013) determined by using coupled and uncoupled model on DeepCwind semi-submersible FOWT. Luan et al. (2016) employed a braceless semi-submersible platform to establish a numerical model and performed extreme sea states analysis on a braceless semi-submersible platform. The results showed that the platform has good stability under extreme sea and is a good design concept. A 5 WM wind turbine was employed by Kim et al. (2017), and WindFloat and OC4 floating platform were carried out to focus on the motion of FOWT and evaluating the mooring system force by using FAST (Jonkman, 2005) code.
Shi, Wei (Dalian University of Technology) | Tan, Xiang (Nanyang Technological University) | Zhou, Li (Jiangsu University of Science and Technology) | Ning, Dezhi (Dalian University of Technology) | Karimirad, Madjid (Queen's University)
The ice loading process has a clear stochastic nature due to variations in the ice conditions and in the ice-structure interaction processes of offshore wind turbine. In this paper, a numerical method was applied to simulate a monopile fixed-bottom and a spar-type floating wind turbine in either uniform or randomly varying ice conditions, where the thickness of the ice encountered by the spar were assumed to be constant or randomly generated. A theoretical distribution of the ice thickness based on the existing measurements reported in various literatures was formulated to investigate the response characteristics of the monopile wind turbine and spar wind turbine in such ice conditions. The effect of the coupling between the ice-induced and aerodynamic loads and responses for both operational and parked conditions of the rotor was studied. Moreover, the dynamic response of wind turbine in randomly varying ice was compared and verified with that of the wind turbine in constant ice.
So far, more than 80% of the energy all over the world comes from fossil fuels. Excessive and improper use of fossil fuels has caused climate change and threatened human security and development. The Paris Agreement, which entered into force on 4 November, 2016, is a major step forward in the fight against global warming. Due to severe smog, forty Chinese cities reel under heavy air pollution. Air pollution becomes one of the key words in China in 2016 (PTI, 2016). Renewable energies play an important role for reducing greenhouse gas emissions, and thus in mitigating climate change. Offshore wind energy is recognized as one of the world's fastest growing renewable energy resources. By the end of 2015, totally 12,107 MW of offshore wind energy was installed around the world according to Global Wind Energy Council (GWEC) report (Fried, 2016). In Europe, 3230 turbines are now installed and grid-connected, making a cumulative total of 11,027 MW (Ho, 2016). However, governments outside of Europe have set ambitious targets for offshore wind and development is starting to take off in China, Japan, South Korea and the US. The 1.2 GW of capacity installed in Asia as of the end of 2015 was located China and mainly in Japan.
Jiang, Zhiyu (Norwegian University of Science and Technology (NTNU)) | Karimirad, Madjid (Centre for Ships and Ocean Structures and Norwegian Research Centre for Offshore Wind Technology, NTNU) | Moan, Torgeir (Centre for Ships and Ocean Structures, NTNU)
Floating offshore wind turbines experience fault conditions. For a parked wind turbine, if the pitch mechanism fails, the blades cannot be feathered to the maximum pitch set point—the blades are seized. Three parked scenarios are considered: fault with 1 seized blade, fault with 3 seized blades, and normal condition. The responses of a spar-type wind turbine are investigated under turbulent wind and irregular wave conditions. However, only the steady-state (and not the transient) response in the fault condition is estimated. In normal parked conditions, the platform-yaw is sensitive to the blade azimuth while surge and pitch are not. The blade azimuth plays a key role in the roll and yaw motion responses in the parked conditions with 1 seized blade. Fault cases under 1-y environmental conditions are compared to normal cases under 50-y environmental conditions. A fault with 1 seized blade often leads to large roll resonance and yaw motion responses with the extremes exceeding the 50-y reference values by more than 16%. The extreme main-shaft bending moments are more than twice the 50-y reference values. Fault cases with 3 seized blades cause an average rise of 38% and 23% for surge and pitch motion extremes, and more than 10% of the tower-bottom bending moments and blade-root bending moments compared to the 50-y reference of the normal operating case.
Offshore wind energy has witnessed rapid development in recent years. The total installed capacity in 2010 reached approximately 3000 MW, some 1.5% of worldwide wind farm capacity (Burton et al., 2011). In the design of offshore wind turbines, a set of design conditions and load cases with a relevant probability of occurrence shall be considered. The load cases, which are used to verify the structural integrity of an offshore wind turbine, should include both operational and nonoperational design situations such as power production, parked and fault conditions (IEC, 2009; DNV, 2010). Despite the need for defining a possible fault case, the correlation between a likely environmental condition and a fault situation remains virtually unknown for a land-based turbine or an offshore one. Therefore, it is necessary to assume appropriate environmental conditions corresponding to the specified fault scenario in the design case analysis.
The occurrence of the faults and the severity of the end-effects are important for offshore wind turbines. The former can be quantified based on the statistics about the failures experienced by wind farms (Ribrant, 2006). The end-effects, namely the potential harm inflicted on the wind turbines, are the main topic of this paper. It was shown in the recent RELIAWIND project (Wilkinson and Hendriks, 2011) that the blade-pitch system failure contributes 21.3% to the total failure rate. Among the various forms of hydraulic pitch actuator faults, valve blockage is safety critical and leads to an inoperable pitch actuator and a fixed blade (Esbensen and Sloth, 2009). Upon the presence of such a severe fault, either the supervisory controller or the protection system will ensure an immediate shutdown (DNV/Risø, 2002). The rotor is often brought to a standing-still or idling state by aerodynamic brakes. For an offshore floating wind turbine (FWT) with the fault, the outcome is largely decided by the wave and the wind that it is subjected to for a certain period.
Jiang, Zhiyu (Department of Marine Technology, Norwegian University of Science and Technology (NTNU), and Centre for Ships and Ocean Structures, NTNU) | Karimirad, Madjid (Centre for Ships and Ocean Structures, NTNU, and Norwegian Research Centre for Offshore Wind Technology, NTNU) | Moan, Torgeir (Centre for Ships and Ocean Structures, NTNU)