In order to improve the performance of Darrieus turbine, the pitch angles of the blades are varied and tested in a low-speed wind tunnel. First, the pitch angles of the blades are varied with an interval of 15 degrees, to evaluate the possibility of the improvement. The tested tip-speed ratio is ranged from 0.5 to 2.8, which covers the peak of the power, and the generated torque is measured by a torque meter. However, none of the blades exceed the performance of zero-pitched blade. Then, the pitch angles of the blades are varied but with an interval of 2 degrees. As a result, it is found that the blades with pitch angle of 4 and 6 degrees give the highest torque, especially in a tip-speed ratio over 1. By changing the pitch angle of the blade from 0 to 6 degrees, the peak value of the power coefficients is improved by more than 30 percent.
An ocean current is an attractive renewable energy source for a country surrounded by sea, like Japan. In the ocean currents, a tidal current can reach to a speed above five meters per second, which has an equivalent energy with a wind of typhoon, nearly 47 meters per second. Moreover, the occurrence of a tidal current is highly periodical and easily predicted. A tidal current changes its flow direction by 180 degrees with regular intervals. The Darrieus-type water turbine is suitable for such situation because of its independency to a current direction, as well as simplicity. Actually, the Darrieus turbine is adopted in the demonstration of power generation from a tidal current at Kanmon Strait (Hiraki, et. al. 2010).
There are some drawbacks for the Darrieus-type turbine. One of them is the lower efficiency of the power generation, as compared to the horizontal-axis turbines. This is one of the reason why the Darrieus turbine is not commonly used in real situations. The attachment of a device that collects a current into the turbine is an option to enhance the efficiency (Hiraki, et. al. 2013). The incapability of self-starting is also a known problem for a turbine that uses a lift force, which is also true for Darriues turbine.
The next large scale exploitation of wind energy will gradually move to the seas with the depth of 30-100m, in which only the semi- submersible and barge type foundation are suitable. Compared with the semi-submersible foundation, the barge type has simpler structure and is more adaptable to water depth, however, suffers larger seakeeping motions in waves. In order to improve the seakeeping performance of the barge foundation for offshore wind turbines, the present work proposes a concept of Air-cushion Supported Floating Platform (ASFP), and integrates the air cushion into barge foundations, which can buffer the wave loads acting on the foundation and reduce the motions. The air cushion makes the new floating foundation very different, and this paper presents a method to estimate the initial stability of the air- cushioned floating offshore wind turbine foundation
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The next generation of wind energy exploitation in China will move to the seas with the depth of 30-100m. Generally, the fixed offshore wind turbine is used in shallow water, and the cost increases very quickly with the increase of water depth. It is considered that the fixed one is not suitable for the water of depth more than 30m (Zhou, 2013), in which the floating one should be considered. Besides, the floating one could be built and assembled in shipyard, which is very useful to reduce the cost. So the floating offshore wind turbine should be used when the water depth is within 30-100m.
Some types of platforms have been employed for floating offshore wind turbines (Ewea, et al, 2013), which can mainly be classified into four types (Wang, et al, 2010): Spar-buoy type, Tension-leg platform (TLP) type, Semi-submersible type and Pontoon type (Barge type). The Spar-buoy type needs a long body to lower the center of gravity and the required water depth should be more than 100m. The TLP type needs a certain water depth to adapt the tidal range and the required water depth should be more than 70m. So only the Semi-submersible and Barge type platforms are suitable for the seas with depth of 30-100m. Compared with the Semi-submersible platforms, the Barge type is more adaptable to water depth, and the simpler structure makes it possible to be built by concrete, which can reduce the cost and overcome the seawater corrosion effectively. But it suffers larger seakeeping motions in waves. So if the motion response of Barge type platforms in waves can be reduced, it will be very desirable to be used in the seas with depth of 30-100m.
Wang, Wenjin (Huazhong University of Science and Technology) | Zheng, Yu (Huazhong University of Science and Technology) | Xu, Guohua (Huazhong University of Science and Technology) | Li, Wang (Wuhan Second Ship Design and Research Institute) | Ma, Xiaolong (Huazhong University of Science and Technology)
Since the self-propelled model doesn’t have the function of adjusting buoyancy, predetermined positive buoyancy is necessary for the normal operation of the Wireless Communication Systems between the mother ship and self-propelled model. Positive buoyancy is effectively a kind of continuing disturbance forcing self-propelled model to float. Consequently, depth control in this condition seems to be more difficult. In addition, the state of self-propelled model’s is unsteady because of surface capture when it dives from the surface into the deep water. In order to solve problems above, preprogrammed diving strategy with appropriate surge velocity and elevator angle is adopted and the combination of the velocity and angle is determined by extensive experiment. Furthermore, a depth-keeping algorithm aiming to eliminate the effect of the positive buoyancy at approximate 13 knots is proposed. The effectiveness of the diving strategy, as well as the depth-keeping algorithm, has been completely proved in trial tests.
The self-propelled model can simulate the actual ship's maneuver under an unsteady voyage state such as course-changing, depth-changing and speed-changing (Eng, Lau, Low and Seet, 2008). Hydrodynamic and noise characters measured by this method are more practical, accurate and credible than those obtained by constraint model test or wind tunnel test. Therefore, the self-propelled model is a new experimental platform for hydrodynamic research which has already been adopted to deal with complex problems and difficulties in the existing technology of modular construction of hydrodynamic, noise and structure (McDonald and Whitfield, 1996). What’s more, the experimental technique of self-propelled model is supplementation of traditional research method. It’s a new method that can be used to study hydrodynamic comprehensive performance and a new technique which promises well and is worth to be developed.
The Self-propelled model in this paper adopts a common streamlined shape, equipped with a propeller, an orientation rudder and an elevator. The self-propelled model has the function of WIFI radio and underwater acoustic communication, equipped with PHINS, depth meter, DVL, anti-collision sonar and altimeter. Based on these navigation sensors, the self-propelled model is able to complete several manuvers automatically. The principle of the control system is shown in Fig. 1 and Fig. 2.
Kim, Jihoon (Korea Institute of Ocean Science & Technology) | Park, Hyeju (Korea Institute of Ocean Science & Technology) | Ko, Jin Hwan (Korea Institute of Ocean Science & Technology) | Won, Bo Reum (Korea Institute of Ocean Science & Technology) | Sitorus, Patar Ebenezer (Korea Institute of Ocean Science & Technology) | Park, Jin-Soon (Korea Institute of Ocean Science & Technology) | Lee, Kwang-Soo (Korea Institute of Ocean Science & Technology) | Kang, Taesam (Konkuk University) | Park, Hoon Cheol (Konkuk University)
Up to now, a pitch controller has been used as a typical solution for adapting the variation of flow speed in horizontal axis tidal current turbines. This study was mainly about the development procedure of a pitch controller for a horizontal axis tidal current turbine throughout analyzing thrust, torque, and rotational speed which measured from multiple-step experiments. First, we conducted indoor experiments using a scale turbine model with a pitch-change module to explore the effect of pitch variation at the beginning of the development. In the next step, the different extracted powers by the pitch angle variation is measured from the indoor experiments of a scaled-up model. In the last step, a re-scaled-up turbine model with a pitch controller was fabricated based on the results of the indoor experiments and used for consecutive outdoor experiments. In a broad range of the flow speed from 1 to 4 m/s, the developed pitch controller successfully regulated the rotational speed within 80±8 rev/min; hence, the extracted power was maintained within 6.73±0.55 kW. Moreover, the thrust, which is a big burden to the tidal current turbine and its supporting structure, was stayed from 10.5 to 7.1 kN. Namely, it was recognized from the outdoor experiments that the developed pitch controller could well regulate the power as well as the thrust of the tidal turbine model in the unpredictable flow speed variation of an offshore condition. Eventually, based on the results and experiences of the three-step experiments, the pitch controller for a 200 kW tidal current turbine was designed and fabricated.
As the problems of fossil fuel, which are depletion of resource, environmental pollution, and economic damage, are rising, renewable energy has been developed to reduce fossil fuel usage. Among them, tidal energy is the most reliable and alternating energy resource from the ocean because it is predictable, regular and higher energy density. Moreover, leading countries such as the United Kingdom, Canada have vast resources of tidal energy and they then continue to do research about tidal energy extraction(Rourke, Boyle, & Reynolds, 2010) In the Republic of Korea, the studies of tidal energy extraction was actually starting with a helical-bladed Darrius turbine at the Uldolmok in Jindo, 2003.(Han, Lee, Yum, Park, & Park, 2010)
Simulation of the impact about the motion response of different fairlead positions for 6MW spar-type floating offshore wind turbine (FOWT) using aero-hydro-servo-elastic simulation code-FAST software is researched. Simulation results show that the platform with the higher fairlead position has better initial stability and dynamic motion response in operational sea state. The platform with higher fairlead position also has more ideal dynamic motion response and lower extreme value of mooring force when adding the 2nd slow drift force into calculation under extreme sea state. Considering these global performance and mooring force, the design with the highest fairlead position of the three designs is accepted in the 6MW spar-type FOWT For further research, this paper can provide some references about the influence of different fairlead positions on hydrodynamic performance under different ocean environmental conditions.
As the reduction of fossil energy resources and pollution problems brought by energy, the development of renewable and clean energy sources have already become a hot issue around the world. Wind energy is exactly a kind of rich, low-cost and renewable energy. With the 17% increase over the previous year, the total global wind power generation capacity has reached 433 GW around the world by the end of 2015 (GWEC, 2016). The development and utilization of onshore wind turbine (OWT) is relatively mature, but OWT is subject to land space and environmental impacts. Moreover, the region with rich wind resource has sparse population and is away from the power consumption area. To meet the increasing energy demand in the future, offshore wind turbine can be a prospective solution. Floating offshore wind turbines (FOWT) are adopted more than fixed types in depths of more than 60 meters due to the rapid rise in costs of the platform. The research about platforms of FOWT includes: barge type, semi-submersible type, TLP type, spar type, etc.
Hibernia Management and Development Company Ltd. (HMDC) sponsored a R&D initiative in 2012 with the objective of obtaining high quality three dimensional iceberg profiles off the east coast of Newfoundland and Labrador. The three dimensional iceberg profiles collected during that program have been used in this work to illustrate the potential benefits to iceberg management which could be realized if towing vessels were outfitted with the equipment required to obtain three dimensional iceberg shapes quickly in the field. Tools have been developed under the further financial support of HMDC to assess the stability of the iceberg in order to identify the preferential towing directions to reduce the likelihood of rolling the iceberg during a tow. A tool has also been developed to allow the user to assess the local shape of the iceberg relative to the towing net to help avoid unfavorable iceberg shapes and slopes. The iceberg shapes have also been used to assess the current iceberg towing net design with recommendations for potential improvements to reduce net slippage during a tow.
Summary One of the key challenges in using the Distributed Acoustic Sensing (DAS) method for seismic survey in exploration wells is to achieve sufficient coupling between the wireline cable and the vertical borehole to ensure the data quality. Many numerical simulations have been conducted to understand the behavior of the cable when it is lowered in a well. However, this simulation method requires too much time and processing power to be effective during Vertical Seismic Profile (VSP) survey and is not an interactive check on the state of the cable. This paper presents a new method to monitor the strain distribution along the cable and to estimate the depth distribution of the cable using a Brillouin backscattering interpretation technique in real time. Consequently, the field engineer has the ability to perform a real time quality check to determine the right amount of slack to improve the DAS data quality.
Wang, Zhilin (Huazhong University of Science and Technology) | Dong, Zhenwei (Huazhong University of Science and Technology) | He, Ran (Huazhong University of Science and Technology) | Wang, Xianzhou (Huazhong University of Science and Technology)
Submarine surfacing in currents is three-dimensional unsteady motion and includes complex coupling between force and movement. This paper uses computational fluid dynamics (CFD) to solve RANS equation coupled with six degrees of freedom solid body motion equations .The CFD code used is an in-house developed code .RANS equations are solved by finite difference method and PISO arithmetic. Level-set method is used to simulate the free surface. Computations were performed for the standard DARPA SUBOFF model. The structured dynamic overset grid is applied to the numerical simulation of submarine surfacing in regular currents and computation cases include surfacing in the static water, and steady, uniform flows with different speed (v=0.2, 0.4, 0.6 m/s) and surfacing with different value between gravity and buoyancy(n=2%, 4%) . The asymmetric vortices in the process of submarine surfacing can be captured. It is shown that roll instability is caused by the destabilizing hydrodynamic rolling moment overcoming the static righting moment. Relations among maximum roll angle, surfacing velocity fluctuation and current parameters are concluded by comparison with variation trend of submarine motion attitude and velocity of surfacing in different current conditions. Simulation results confirm that current speed has a significant effect on surfacing velocity fluctuation. Maximum pitch angle decreases with the increase of current speed. Especially the law of pitch angle decreasing with the currents speed presents approximate linear relationship. Maximum pitch angle with current speed at 0.2m/s can reach to 12.89° while 3.07°at 0.6m/s. According to the above conclusions, maneuverability can be guided in the process of submarine surfacing in currents in order to avoid potential safety hazard.
Safety is a very important consideration in the design and operation of submarines. To reduce risk at depth, submarines are usually equipped with emergency systems designed to rapidly blow the ballast tanks rising to free surface quickly. In the case of emergency maneuvers, this requires simulating extreme and unsteady motions. In the process of submarine surfacing, motion, force, moment and free surface have complex coupling with each other. Especially in currents with high speed, submarine maneuvering conditions become more unpredicted and roll angle will be negatively affected while surfacing.
With the development of the offs hore wind turbine technology, the offshore wind turbine is going to deeper waters, and the scale of the offshore wind turbine is becoming larger, in which case the numerical computation of aerodynamic performances of large-scale floating offshore wind turbines is becoming more and more important. In this paper, NREL offshore 5-MW baseline wind turbine of the Phase II of the Offshore international Code Comparison Collaboration Continuation (OC4) project has had been selected as the object as its detailed data and widely concern, the multiple re ference frame(MRF) method based on open source code OpenFOAM is used, the aerodynamic performance of the 5-MW baseline wind turbine under different wind speeds without considering the impact of the floating structure is simulated, and the rotor thrust, torque, pressure coe fficient and wake vortex aerodynamic data and flow field information were obtained in detail.
In dynamic positioning systems, the horizontal plane motions, including surge, sway and yaw, are commonly taken into consideration. However, the ver tical plane motions, especially roll and pitch, which are induced by thruster force may have influence on positioning of marine constructions as well. Horizontal plane motion control with roll and pitch control might be a solution to this problem. Roll and pitch velocity and acceleration are feedback signals to generate the control forces in the horizontal plane. The e ffect of the additional roll and pitch motion control law on the positioning accuracy of the horizontal plane remains uncertain. In this paper the analysis of roll and pitch motion of a dynamic positioned semi-submersible with roll and pitch motion control is made. The mechanism of the effect under the roll and pitch control law is presented in the time domain. It is shown that the effect is determined by the attitude of the semi-submersible, as well as the horizontal control force at that moment. The analysis of the influence on positioning accuracy is conducted based on the statistical analysis of the time-domain motions.