The potential numerical wave flume is applied in this study to estimate the forces and the moment on a submerged plate in the combined wave-current flow by solving the Laplacian equation based on the higher order boundary element method (HOBEM). The free surface motion is tracked by solving the free surface boundary conditions and is advanced in time using the fourth-order Runge-Kutta scheme. The performance of the potential numerical wave flume is assessed by comparing with the published theoretical results and the experimental measurements. The forces and the moment on the submerged plate alternatively increase to the peak value and then decrease to zero with increasing plate breadth, and they are found to increase with increasing water depth. Additionally, it is found that the use of Doppler shifted solutions is not sufficient for considering the effect of depth-uniform current on waves. The generation of higher harmonics due to a sudden change in water depth and the current-induced form drag are found to make significant contributions to the wave loading.
The submerged plate, used as a breakwater device, is less dependent on the bottom topography, more economical and can assure open scenic views. It allows seawater to exchange freely between the sheltered region and the open sea to prevent stagnation, pollution, transport of sediment to maintain the general partition of the natural seabed. It has been applied as an efficient breakwater in coastal and offshore zones Thus, investigations on submerged plate have been focused on their reflection and transmission characteristics (Stamos and Hajj, 2001.), or the generation of higher order bound and free harmonic waves that affects the sailing conditions (Brossard and Chagdali, 2001; Lin et al., 2014). However, the effects of wave-induced forces and moment on submerged structures are of practical importance as well to assure the strength and stability of the structure over the design life. Extensive researches have shown that the mutual influence of waves and currents is intensified in shallow waters/coastal areas (Isaacson and Cheung, 1993; Chen et al. 1999). Methods that can provide accurate predictions of forces and moment on submerged structures in combined waves and currents are required for the safe and cost efficient design of submerged structures in coastal areas.
An anti-sloshing concept for FLNG tanks is investigated by taking advantage of floating foam layers. Physical experiments are conducted in a rectangular tank to investigate the sloshing properties. Effects of the layer thickness on the sloshing dynamics are analyzed. It shows that the floating foam layer can efficiently damp the free surface oscillations and reduce the pressure amplitude in a tank. Higher-order frequency components of the hydrodynamic pressure in the tank gradually vanishes as the foam thickness increases. Considering the economy in practice, the thickness of the foam layer should be as small as possible, as long as the sloshing mitigation effect can be guaranteed. For the present situation, a suitable choice of the foam thickness is recommended to be one-tenth the mean liquid depth.
A proper design of the anti-sloshing technique for FLNG tanks is important for the operation safety. At present, various anti-sloshing techniques have been investigated for liquid cargo tanks. A great proportion of these techniques was based on energy dissipation effects of internal structures (e.g. fixed baffles, flexible baffles, screens, blocks, bulkheads, etc.) in a liquid tank, which has been comprehensively reviewed by Faltinsen and Timokha (2009). There are also some membrane-based techniques used in space technology, and inflatable membranes in trucks (e.g. Accede company, the Netherlands).
Some more recent studies for FLNG tanks include Jung et al. (2012), Wang et al. (2013), Lu et al. (2015), Cho and Kim (2016), Xue et al. (2017), Yu et al. (2017), Sanapala et al. (2018), and so on. The above baffle-based anti-sloshing techniques have been widely adopted by oil or chemical tankers. However, these techniques still require further verification for the special case of LNG tanks. Because the internal surface of an LNG tank is fully covered by thin invar membranes to prevent gas leakage at an extremely low temperature, installing baffles through the membrane surface may affect mechanical properties of the membrane and bring safety risks. Thus, alternative anti-sloshing ideas keep emerging in the recent decade, trying to avoid damaging the membrane surface.
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.
A semi-passive sloshing mitigation technique that takes advantage of movable structures in the tank is considered. Hydrodynamic characteristics of the 3-D rectangular tank with a cylindrical floating structure are investigated. A numerical model is established based on boundary element method (BEM) to predict natural sloshing frequencies and corresponding modes for these liquid tanks. The influence of the geometry parameters, including the position, draught and radius of the floating cylinder on the natural sloshing frequency of the liquid is analyzed. It is also found that the anti-sloshing floating structure works better for a shallower water tank.
The natural gas with environmental advantages over other fossil fuels has become increasingly significant in meeting the world's energy needs. However, a great proportion of the present natural gas reserves is located below the offshore seabed. The floating liquefied natural gas (FLNG) facility is an attractive technology proposed for the offshore gas exploitation. Compared with conventional natural gas facilities, the FLNG facility has advantages on initial cost and construction time. Many existing designs have taken the FLNG facility as a giant floating vessel moored above an offshore natural gas field. During the operation, the natural gas is chilled onboard into the liquefied natural gas (LNG) and stored in several huge tanks below the deck. Then, the produced LNG is offloaded to LNG carriers and transported to the land. Unless docking for inspection, the FLNG is continuously moored at the location for around 20-25 years, so that it can experience any complex sea state with any fill in tanks during the period of service. The vessel motion induced by external ocean waves could easily cause the violent liquid sloshing in partly filled tanks and threaten the safety of the platform structure. Thus, designing effective sloshing mitigation or anti-sloshing techniques are of great significance for structural safety of LNG tanks.
At present, many existing anti-sloshing techniques are mainly designed for conventional liquid cargo tanks. These techniques can be divided into the active and passive types. The active anti-sloshing technique relies on external power control and sensors for real-time feedback of the liquid state. Bubble injections (e.g., Hara and Shibata, 1986) or dynamic baffles (Hernandez and Santamarina, 2012) may be used to achieve real-time adjustment of hydrodynamic characteristics in the tank and to weaken the effects of sloshing. However, the complex machinery system of active techniques normally indicates high maintenance costs, which should also be considered in practical applications. Especially for the flammable and explosive LNG cargo, using electric power in an LNG tank put the platform in a great danger.
Zhao, Xuanlie (Dalian University of Technology) | Ning, Dezhi (Dalian University of Technology) | Johanning, Lars (Dalian University of Technology, University of Exeter) | Teng, Bin (Dalian University of Technology)
High construction-cost is one of the barriers that limited the developments of wave energy utilization. Integrating wave energy converters (WECs) into other marine structures may reduce the construction cost of WECs effectively. In this paper, an integrated system with a medium array (11 devices) of heaving point absorber WECs (PAWECs) arranged at the weather side of a fixed pontoon-type structure is proposed. The hydrodynamics of the PAWECs are investigated numerically by using higher-order boundary element method (HOBEM) code package (i.e., WAFDUT), which is developed based on linear potential flow theory. The hydrodynamic performance (including interaction factor, wave exciting force and heave response) of the WEC array with the rear pontoon is investigated with focus on the influence of the spacing between the WEC array and the pontoon (WEC-pontoon spacing). For sake of comparisons, the results corresponding to the isolated WEC array, i.e., without the pontoon, are presented. Results show that the performance of the pontoon-integrated WEC array performs better than that without the pontoon.
One obstacle that limits the wave energy utilization is the high construction cost. Integrations of wave energy converters (WECs) and other marine structures (such as breakwaters, offshore wind turbine, offshore platforms, etc.) have attracted much attention for its advantage of cost-sharing (Mustapa et al., 2017; Astariz and Iglesias, 2015; Favaretto et al., 2017). The cost reduction of WECs caused by the integration scheme may enhance the competitiveness of the wave energy converters. Pontoon-type structures are very common in offshore engineering, such as breakwaters, floating docks, ships, etc. In addition to the sharing of the infrastructures of both aspects, the WEC devices can provide power to the offshore operation in a convenient way.
It is understood that, for the pontoon-type structures, the wave conditions at the weather side can be described as the superposition of the incident waves and the reflected waves caused by the pontoon. Thus, it is expected that the energy conversion efficiency of the WECs can be improved. There are some cutting edge studies on improving the efficiency of WECs (mainly including oscillating water column WECs and heaving point absorber WECs) by using the reflection of costal structures. The detailed investigations can be found in Howe and Nader (2017), McIver and Evans (1988), Mavrakos et al. (2004), Schay et al. (2013) and Zhao et al. (2017).
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.
The dual-chamber oscillating water column (OWC) is considered in this study. The device has two sub-chambers with a shared orifice. A fully-nonlinear numerical wave flume based on the potential-flow theory is applied for the simulation. At various wave conditions, effects of the chamber geometry (i.e. the draft and breadth of two chambers) on the hydrodynamic efficiency of the OWC device are investigated numerically. The hydrodynamic efficiency of the dual-chamber OWC is compared with that of the single-chamber one. The dual-chamber device shows a higher efficiency near the resonant frequency. Then, effects of the breadth and draft of two sub-chambers are discussed. It is observed that a proper set of two sub-chambers can increase the general hydrodynamic efficiency of the OWC device.
Due to their non-polluting nature and environment friendliness, renewable energies have gained great deal of attention and deserve a substantial body of research. The wave energy as an important type of renewable energy has drawn people's attention for several decades (Dizadji and Sajadian, 2011). Thousands of prototypes of Wave Energy Converters (WECs) have been developed for many decades now for exploiting the energy of the ocean waves (Vyzikas et al., 2017). Featured by high efficiency and structural simplicity, the OWC device becomes one of the most favorable wave energy converters (Delaure and Lewis, 2003).
In recent decades, a great volume of researches has been carried out to investigate the efficiency of OWCs analytically (McCormick, 1976; Evans, 1978; Falcao and Sarmento, 1980; Evans, 1982), numerically (Zhang et al., 2012; Luo et al., 2014; Ning et al., 2015) and experimentally (Morris-Thomas et al., 2007; Falcao and Henriques, 2014; Murakami et al., 2016; Ning et al., 2016a), most of which focus on the single chamber device. For the single chamber OWC, it has been recognized that the maximum power absorption occurs only when the frequency of incident waves is close to the resonance frequency of the OWC chamber (Morris-Thomas et al., 2007; El Marjani et al., 2008; Sahinkaya et al., 2009; Iturrioz et al., 2015). To enhance the performance of the OWC devices, the multi-chamber OWC concept has been proposed. The principle of double chamber OWC device's operation has been extensively studied by Boccotti, (2007), Boccotti et al. (2007) and Wilbert et al. (2014). They observed that relative opening depth along with asymmetry value have strong effects on hydrodynamic energy conversion capacity of the device. Rezanejad et al. (2013) and Rezanejad et al. (2015) analytically and numerically analyzed the hydrodynamic efficiency of a dual-chamber OWC placed over stepped bottom. They found that by considering dual-chamber OWC device on the stepped sea bottom, the performance of the device can be improved significantly in wide range of frequencies, as compared with the single chamber case.
The hydrodynamic performance of a novel system, which integrates an oscillating buoy wave energy converter with a pile-restrained floating breakwater, is experimentally investigated in a 2-D wave flume. A current controller-magnetic powder brake system is used to simulate the power generation system. Results show that excellent wave attenuation performance and satisfactory capture width ratio of the integrated wave energy system can be achieved at certain ratios between the width of floating breakwater and the wavelength. The wave pressure at the bottom of the breakwater is measured for different values of the excitation current of the current controller, corresponding to the power-take off damping force. The experimental results show that the wave pressure amplitude is affected by the power take-off, and that the installation of a wave energy absorbing system may reduce the horizontal wave loads. Hence, the integration of a wave energy system with the floating breakwater may not only utilize wave energy in a cost-efficient way, but may also improve the breakwater performance and survivability.
The first wave energy converter patent was filed in 1799. Since then, various models for absorbing energy from ocean waves have emerged (Lindroth & Leijon, 2011). For many wave energy developers, hunting subsidiaries is a primary task; high construction cost is one of the vital barriers which limits the commercial application of wave energy technology. In terms of engineering applications, cost-sharing is often effective to achieve the goal of cost-reduction of a special technology. In recent years, some concepts for which wave energy converters were integrated into the design of a breakwater have emerged (Arena, Romolo, Malara, & Ascanelli, 2013; Chen, Liu, & Kang, 2015; He & Huang, 2014; Michailides & Angelides, 2012; Margheritini et al., 2009. etc). Here we propose a another configuration, for which the function of cost-sharing between a floating breakwaters and an oscillating wave energy converters can be realized. The boxtype floating breakwater (FB) is vertical pile-restrained, so the FB is restricted to move in heave, and a power take-off (PTO) system was installed above the floating breakwater.
Zang, Jun (Department of Architecture & Civil Engineering, University of Bath, Bath, UK) | Ning, Dezhi (The State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology Dalian, China) | Liu, Shuxue (The State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology Dalian, China) | Liang, Qiuhua (School of Civil Engineering & Geoscience, Newcastle University, Newcastle upon Tyne, UK) | Taylor, Paul H. (Department of Engineering Science, University of Oxford, Oxford, UK) | Taylor, Rodney Eatock (Department of Engineering Science, University of Oxford, Oxford, UK) | Borthwick, Alistair G.L. (Department of Engineering Science, University of Oxford, Oxford, UK)
This paper presents a Cartesian cut-cell Boussinesq model for simulating nonlinear wave interaction with a curved structure. The Cartesian cut-cell technique permits accurate boundary-fitting of complicated, curved geometries in the numerical domain. A Godunov-type shock capturing scheme is used to solve the Boussinesq-type equations in hyperbolic form in order to provide accurate predictions of strongly nonlinear wave interactions with curved structures in shallow water. The numerical model is used to simulate the interaction of a focused wave with a circular cylinder, and excellent agreement is obtained with data from laboratory experiments conducted in a wave basin.
Numerical models based on the Boussinesq-type equations are becoming widely used to simulate wave transformation in shallow coastal regions and run-up at beaches. Such models can play a key role in design and re-assessment in coastal engineering. Boussinesq (1872) was the first to derive depth-integrated mathematical descriptions of shallow flows that included the influence of vertical accelerations and hydrodynamic pressure. During the past 20 years, considerable efforts have been made to improve the representation of nonlinearity and dispersion by Boussinesq-type formulations (see e.g. Madsen and Sørenson, 1992; Nwogu, 1993; Schäffer and Madsen, 1995; Agnon et al., 1999; Madsen et al., 2002). Many coastal structures, such as offshore wind-turbine foundations and piers, are composed of vertical surface-piercing circular cylinders. In order to estimate the local wave hydrodynamics and loading on such structures, there is a need to develop Boussinesq-type solvers that can model wave interaction with curved structures. The aim of the present work is to develop a simple, accurate and flexible method for modelling the hydrodynamics of coastal flows in the vicinity of surface-piercing structures of arbitrary plan-form geometry. The Cartesian cut-cell technique was implemented for body fitting for compressible flow by Yang et al. (1997) and for shallow water flow by Causon et al. (2000) using piece-wise linear segments.