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Whenever I am considering a topic or a theme for the paper selection, I realize that certain topics may generate some instant reaction that is counterproductive. I say counterproductive because it may lead to a decision not to read the synopses, let alone the papers themselves, even though there may be good reasons to read them. The topic for this selection falls in that category: offshore wind energy. Now, if you are tempted to stop reading, please bear with me for a few minutes. I did not select this topic because I want to promote renewable energy or because I believe that oil and gas production will end in the foreseeable future.
The industry requires the ability to test system concepts in a realistic environment on a pilot-scale size. While several units up to 10 MW are being tested onshore, existing electrolyzer technology to convert power to H2 has not yet been applied offshore. However, many suppliers are developing innovative concepts and are scaling up to larger units. It has been estimated that at least 1 MW of electrolyzer capacity can be placed on an offshore platform with existing technology.
For any offshore development, especially an ORE project, a specific site investigation is required to qualify environmental, geophysical, metocean-related, and geopolitical issues. Most ORE developments will cover a significant area of ocean or seabed and will require investigation to ensure that marine life, antiquities, unexploded ordinance, and other ocean users will not be put at risk when installation and operation activities are performed. Obtaining the required permits and approvals from all those potentially affected by an ORE development is a complex and time-consuming operation. All stakeholders connected with a development must be considered because the installation could be in position for 30 or more years. A summary of the types of wind and MHK devices is presented in Table 1 of the complete paper.
Offshore wind is a rapidly maturing sector, increasingly seen as a major contributor to electricity supply in states with coastal demand centers and good wind resources. While an almost 3-decade history exists in European experience, the US only recently is beginning to move forward with grid-scale projects on national and state levels. As floating wind is scaled up, to minimize technical risks experienced in the past, formal processes will help to identify the novel features, novel applications, and highest-risk components. Large offshore wind farms have been built by all countries with coastlines on the southern North Sea, the area with the most favorable conditions: strong, consistent winds; water depths of less than 40 m; sand or clay deeper than 70 m; and close proximity to onshore electrical distribution networks and centers of high demand. Rapid reductions have been realized in the cost of electricity, calculated over the full project lifetime, from well over 200 Euros/MWhr for the first large-scale wind farms to 50/MWhr.
Yua, Songchen (College of Shipbuilding Engineering, Harbin Engineering University) | Li, Peng (College of Shipbuilding Engineering, Harbin Engineering University / Peng Cheng Laboratory) | Qina, Hongde (College of Shipbuilding Engineering, Harbin Engineering University) | Xua, Zhijing (College of Shipbuilding Engineering, Harbin Engineering University)
Marine fishery is gradually developing from the coastal area to the deep sea. The semi-submersible offshore fish farm is one of the most foreseeable aquaculture equipment. However, the vast and abundant deep sea also means that the fish farm will be in a harsh marine environment. A model test of a semi-submersible fish farm was carried out in this paper. In order to obtain the hydrodynamic response characteristics of the fish farm, the accelerations on five different positions and mooring forces of the model under different wave conditions and current were measured, the steady-state amplitudes of the first five harmonics experimental acceleration and mooring tension were obtained. The steady-state amplitudes of non-dimensional first-harmonic acceleration at different positions with different wave periods were compared, and the amplitude trends were analyzed. The influence of higher harmonics acceleration on the first ones was investigated. Besides, the amplitude trend of different harmonics acceleration was analyzed under different conditions. Furthermore, the trend and fluctuation range of mooring tension at front and aft positions under different conditions were compared, and the causes of occurrence of the peak value of the instantaneous amplitude were figured out.
New types of aquaculture equipment are gradually put into the ocean to relieve the environmental pressure in coastal area. As a typical new type of aquaculture equipment, semi-submersible fish farm is in a harsher marine environment than the traditional offshore cage. In order to ensure the normal operation of semi-submersible fish farm and the normal survival of aquaculture products, the hydrodynamic performance of which will be the focus of attention. The hydrodynamic analysis of semisubmersible fish farm is similar to that of traditional cage, but there are also differences.
For traditional net cage, Lader et al. (2005, 2007) carried out a series of experiments on different solid ratio of net, and the hydrodynamic forces induced by different current velocity were compared. Fredriksson et al. (2007) compared the experimental data and numerical results of static deformation of a floating collar. By numerical simulation, Huang et al. (2008) studied the hydrodynamic performance of gravity cages induced by wave and current. Taking rigid and flexible floating collars as the object of study, Li and Faltinsen (2016, 2018) carried out model tests of single floating collar under different wave steepness. By comparing the results with that of numerical simulation by WAMIT, the importance of 3D flow, hydroelasticity and strong hydrodynamic frequency dependency was demonstrated; the changes of additional mass and damping coefficients were emphatically analyzed. Xu et al. (2020) studied the drag and wake of an individual long-line mussel dropper using computational fluid dynamics approaches. They found that surface roughness and sharp crowns on the rough cylinder resulted in larger drag coefficients and Strouhal numbers. By numerical simulation, Qin et al. (2020) addressed the probability distribution of the heave motion under irregular waves and extended the conditions of the most probable maximum normalized bending stress distribution of the floating collar. Qin et al. (2020) devised an experimental study of an offshore aquaculture cage induced by wave-structure interactions. The relationship between the first-, second-harmonics acceleration and wave amplitude were analyzed, and the effects of mooring loads were assessed. Concerning the safety and economic efficiency, Liu et al. (2019) conducted a series experiments to obtain the optimum submergence depth of a fish cage. By model test and numerical simulation, Yang et al. (2020) studied the hydrodynamic responses and optimize mooring design factors of a floating rope enclosure. Li et al. (2012) studied the deformation of the floating collar, and they found the flexible effects on the dynamic responses. Kristiansen and Faltinsen (2015) conducted an experiment of a net cage. Results with only waves as well as combined waves and current were obtained.
TAN, Lei (College of Science and Technology, Nihon University.) | Ikoma, Tomoki (College of Science and Technology, Nihon University.) | Fujishima, Katsuhide (College of Science and Technology, Nihon University.) | Aida, Yasuhiro (College of Science and Technology, Nihon University.) | Masuda, Koichi (College of Science and Technology, Nihon University.)
In this paper the motions of a barge-type floating foundation installed with four moonpools and a VAWT are investigated through physical model tests and numerical calculations. The characteristics of motion responses and mooring tether tensions under various wave conditions are examined. The gyroscopic effects of turbine rotations are studied by varying the mass and the rotational speed of wind turbine. Linear potential calculations are carried out using WAMIT. It is found that the gyroscopic effect due to turbine rotations can be significant. The firstorder motions of the floating system are substantially reduced by the gyroscopic effect, while the second-order motions and tether tensions may be significantly increased. The viscous damping of water motions in moonpools is found not negligible to produce reasonable predictions.
To exploit ocean wind energy, varied types of FOWTs (Floating Offshore Wind Turbines) have been proposed over the past decades. In terms of floating foundation, there are mainly Spar, semi-submersible, TLP (Tension Leg Platform), and pontoon (or barge) types of FOWTs. Among these types, the pontoon type of FOWTs are simple to design, construct and install, and they are generally placed in shallow water. With regard to wind turbines, there are mainly two types: HAWTs (Horizontal Axis Wind Turbines) and VAWTs (Vertical Axis Wind Turbines). When multiple HAWTs are installed on a floater, it is necessary to consider the wake effects of air flow and the motions of the floating foundation due to the turbulence around the turbines. The distance between two HAWTs should be no less than one third of the diameter of a turbine rotor. Hence a large area is usually required for the floating systems installed with an array of HAWTs. So far, the concept of floating systems installed with multiple HAWTs has not been commercialized
By contrast, VAWTs have advantages over HAWTs in multi-turbine installed floating systems. For example, VAWT has no requirement for wind direction and its installation and maintenance costs are relatively low. The concept of VAWTs installed on a floating barge may go back to the report of Shigeo (2002). So far, various floating VAWT concepts have been proposed (e.g., Akimoto et al., 2011; Collu et al., 2012; Shires, 2013; Paulsen et al., 2015). The floating systems installed with multiple wind turbines have several advantages, such as reducing mooring tethers, causing less disturbance to sailing and fishery activities, improving the maintainability and workability of the facilities, and potentially enhancing power extraction (Jin et al., 2020). In addition to these advantages, for the development of the floating systems, it is necessary to consider the performance of floating systems in safety and stability.
Liu, Yingyi (Research Institute for Applied Mechanics, Kyushu University Kasuga) | Hu, Changhong (Research Institute for Applied Mechanics, Kyushu University Kasuga) | Yoshida, Shigeo (Research Institute for Applied Mechanics, Kyushu University Kasuga)
A time-domain method is developed for modeling the dynamics of a floating truss-structure wind turbine with multiple rotors mounted on the deck of the platform. In its hydrodynamic aspect, a hybrid panel-stick model is built up incorporating the potential flow theory to evaluate the wave inertia force and a Morison strip method to evaluate the wave drag force. The proposed analysis model is validated against a 1/50 scale test of a semi-submersible floating wind turbine, which was carried out in Kyushu University. Good agreement between the simulation results and the experimental data confirms the validity of the developed method. Further numerical simulations are performed in a set of wind and wave conditions to investigate the effect of wave drag force on the dynamics of the floating wind turbine. The results show that applying a hybrid panel-stick model is fairly effective to reduce the unphysical large resonant responses.
Semisubmersible type is one mainstream type of FWTs (floating wind turbines). An important issue of the semisubmersible foundations is to predict their motion responses, among which the heave response may be of a particular concern. At the natural frequencies, when using a potential flow based method, the semisubmersible normally endures remarkable resonant responses. The reason lies in that, near the boundary layer of the submerged part of the floating structure, the waveinduced drag force (which is a viscous force) has not been accounted for as that is usually done in a Navier-Stokes equation based solver. To include the viscous effect, the potential flow based method needs to be modified to some extent. On the other hand, although there have been some good works on determining the wave-induced loads upon a semisubmersible platform (e.g., Hooft, 1972; Mathisen et al., 1982), they are primarily based on the strip theory (or pure Morison equation) neglecting three-dimensional wave interactions between the Morison elements. Considering the co-existence of large-diameter columns and small-diameter members, it is advisable to employ a hybrid approach combining the potential flow theory and the Morison equation (Li and Yu, 2012). Liu et al. (2016) have done such a hybrid modeling in the frequency domain. The present work extends the idea to the time domain for a complex floating structure.
Yu, Yang (State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University / Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration ) | Li, Zhenmian (State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University / Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration ) | Yu, Jiangxin (State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University / Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration ) | Xu, Lixin (State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University / Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration )
Multi-use platform designs have been focused on in recent years with the evolution of offshore platforms. Based on the classic tension leg platforms (TLP), a multiple production offshore platform was designed by combining the TLP body with an embedded oscillating water column energy converters (EOWC). The contributions of the work include the development of a novel concept of the TLP-EOWC, a preliminary design scheme for the TLP-EOWC, implementation of a multifold nonlinear time-domain analytical model, real-valued simulation for the TLP-EOWC, and a sensitivity analysis of the design parameters. Sensitivity analysis was completed for different orifice ratios and wave heading angles in seven sea states from the LIUHUA site monitoring data. Results show that the system gains low electricity productivity in calm sea conditions and provides considerable power output in rough seas. As per this research, the offshore platform would act as a power-producing wave energy farm and contribute to the energy mix and even help achieve power self-sufficiency
As the world's energy needs continue increasing and onshore resources are reaching their limit, the seas and oceans gain extensive attention since they potentially provide an opportunity for economic growth and resource use. The European Commission indicated ocean resources, including ocean energy, aquaculture, biotechnology, deep-sea mining, and coastal tourism, also called Blue Growth sectors, as high potential components. The evolution of offshore platforms has made it possible to think of other opportunities parallel to the traditional sole function. As a pioneer, the European Union (EU) launched "The Ocean of Tomorrow" (Chandrasekaran, 2015; Koundouri, 2017), a call for proposals for multiuse offshore platforms in 2011. In this call, three projects were selected – H2OCEAN, MERMAID, and TROPOS. In 2014, Maribe developed the H2020 project to determine if there is a future for investment in combining Blue Growth sectors. Two new EU projects, Space@Sea and Blue Growth Farm, began in 2018. As a result of these projects, new concepts for the next generation of offshore platforms were proposed and examined. They included offshore wind (floating or fixed) sharing with aquaculture and shellfish farms, offshore wind sharing with wave energy, wave energy sharing with aquaculture, fixed and floating wind sharing with oil and gas, and desalination combined with other Blue Growth sectors. More related details can be found in the reference (Dalton, Bardócz, Blanch, Campbell, Johnson and Lawrence, 2019).
The Spar-type FOWT, which is a kind of the stable offshore wind generator, has been widely adopted and investigated in recent years. As a permanent mooring structure, it faces the issue on mooring line fracture. In the present work, the simulations are conducted in time domain to investigate its transient response in scenarios with fractured mooring lines. Towards this end, our in-house code SFND, which is a coupled aero-hydro-elastic numerical model is adopted to perform the simulations. The methodology includes a blade-element-momentum model for aerodynamics, a nonlinear model for hydrodynamics, a nonlinear restoring model of SPAR buoy, and a fully nonlinear dynamic algorithm for intact and fractured mooring lines. The simulations are conducted under both stochastic and freak wave scenarios. The motions of platform, the tensions in the mooring lines and the power generation performance are documented in different cases. According to the results, the large drift motion is observed and the transient response is discussed.
During the recent decades, the wind energy has attracted more and more attention because of its advantages and features, such as no pollution, no carbon emission, and so on. However, with the issues on the land limitation and the noise, the installation of the onshore wind turbines nearly reaches the bottleneck. Therefore, the wind turbines are designed to be supported by the offshore foundations, in order to catch the offshore wind energy, which is less turbulence and more strength than the onshore one. Generally, the fixed foundations, including pile, gravity, jacket, etc., are widely adopted. Nevertheless, according to previous research, the costs and difficulties of the installation and maintenance increase exponentially when the water depth exceeds 50m (Leimeister et.al, 2020). To overcome this situation, the floating offshore wind turbines (FOWTs) are proposed.
The conceptual designs of the floating foundation are basically based on the experiences from the oil and gas industry (Hsu, 2017). Hereby, the different types of the floating foundations can be majorly divided into three types, which contains the Spar type, the Semi-submersible type and the Tension Leg Platform (TLP) type. Among these innovative designs, the Spar buoy shows both well hydrodynamic performance and robustness according to numerical simulations and wave basin tests (Yang et.al, 2020, Salehyar et.al, 2017, Li et.al, 2018a, Duan et.al, 2016). Even more, the first floating wind farm, Hywind Scotland, also adopted five Spar-type FOWT and successfully generate power more than two years.