Fourtakas, Georgios (University of Manchester) | Stansby, Peter K. (University of Manchester) | Rogers, Benedict D. (University of Manchester) | Lind, Steven J. (University of Manchester) | Yan, Shiqiang (City University of London) | Ma, Qingwei (City University of London)
This paper presents a two-dimensional, one-way coupling methodology between the quasi-arbitrary Lagrange–Euler finite element method (QALE-FEM) nonlinear potential flow solver and the incompressible smoothed particle hydrodynamics (ISPH) Navier-Stokes equations solver. Nonlinear potential flow solvers such as the QALE-FEM are highly efficient solvers for propagating waves in large domains; however, when extreme nonlinearity takes place, such as fragmentation, breaking waves, and violent interaction with marine structures, the methodology becomes incapable of dealing with these flow features. The particle method ISPH is known to be accurate for such highly nonlinear fragmentized flows and provides near-noise-free pressures. ISPH is thus ideal for near-field flows involving overturning, splashing, and slamming. Herein, we propose a one-way coupling methodology between QALE-FEM and ISPH where the methods are used for the far-field and inner/local regimes, respectively. To validate the one-way coupling algorithm, two sinusoidal waves have been used with satisfactory results. The intention is to extend this approach to the strong coupling of the potential flow solver with ISPH using a two-phase (air–water) solver. The aim is to reliably predict extreme wave forces and slamming on offshore structures such as decks and platforms for marine renewable energy and the oil and gas industry.
A numerical study has been undertaken to investigate focusing wave impact on a fixed FPSO-type offshore structure in this paper. The linear wave theory is used to generate a focusing wave from the inlet whereas a two-phase flow model has been employed to study the details of wave-structure interactions. The large-eddy simulation approach has been adopted in this study, where the model is based on the filtered Navier-Stokes equations with the dynamic Smagorinsky sub-grid model being used for the unresolved scales of turbulence. The governing equations have been discretized using the finite volume method, with the air-water interface being captured using a volume of fluid method and the cut cell method being implemented to deal with complex geometry in the Cartesian grid. Numerical results have been presented and compared with the experimental measurements and other numerical simulations using QALE-FEM+OpenFOAM in terms of the wave run-up and pressure on the structure.
It is noticed that extreme waves will become more common in coastal and offshore region due to the impact of climate change. Wave- structure interaction is a key aspect in the safe and cost-effective design of coastal and offshore structures, and marine renewable devices. Understanding the characteristics of the extreme wave climate, its variability, and survivability is an important consideration for sustainable development of coastal and offshore infrastructure.
In order to roughly predict the hydrodynamic loads on structures, the Morison equation and potential flow theory (Ma, et al., 2015) have been widely used in the literatures. However, it is challenged to consider wave impact on the structures by using these two approaches during wave breaking, especially when there are splash-up and air entrainment.
With developments of CFD (computational fluid dynamics) and increases in computer power, recent models for studying wave-structure interaction, solve the Navier-Stokes equations coupled with a free surface calculation. Several methods have been developed by solving Navier-Stokes model by using mesh-based methods (Chen, et al., 2014; Hu, et al., 2016; Martínez Ferrer, et al., 2016a; Xie et al., 2017), or alternatively, meshless smoothed particle hydrodynamics (SPH) (Lind, et al., 2012) and the meshless local Petrov-Galerkin (MLPG_R) method (Ma, 2005).
Hybrid wind-wave systems are considered to have advantages in harnessing wind and wave energy by using one structural unit. This paper will study a floating wind-wave system with point wave energy absorbers. The system analyzed in this paper consists of a typical 5MW wind turbine with a semi-submerged floater to support the whole system. Specifically, the influence of different sizes of the wave energy absorbers on the overall hydrodynamic performance of the system will be investigated by combining SESAM and FAST. The results are assumed to aid the conceptional design of floating hybrid wind-wave systems.
For response to the worldwide growing of energy needs and the negative impact of traditional petrochemical energy on the environment, people are beginning to look for and utilize renewable energy sources (Hsu et al 2015), such as wind and wave energy in seas. The wind conditions in the deep sea are better than those on land, so the offshore wind power plants have received more and more attention. According to the form of floating foundation platform, the offshore wind turbine can be mainly divided into three categories as shown in figure 1: semisubmersible, Spar and TLP.
So far, there have been many achievements in the study of offshore wind turbines. Mafalda Seixas et al (2016) analyzed the performance of power generation of the offshore wind turbine. You Young-Jun et al (2016) studied the overturning stability of offshore wind power system. Lindenburg et al (2016) and Marit I. Kvittem et al (2015) researched fatigue of wind turbine. Similar to offshore wind Turbine, there have been many achievements in wave energy for several years. Chatzigiannakou et al (2014) put forward a new way to install point wave absorber. Jeremiah Pastor et al (2014) used AWAQ analyzed power output for a point absorber wave energy converter in time domain. Jarred Canning et al (2017) analysis long-term reliability of point absorber wave energy converter. In order to improve the efficiency of power generation, the concept of combined use of wind and wave energy to generate electricity has been proposed in recent years. Until now, the mature floating hybrid wind-wave system has not be created yet, and more research on it is needed. Aim at this problem, Wang Qi et al (2016) designed and analyzed a floating hybrid Wind- wave system which combines wind turbine and a spherical wave energy absorber in his work.
Zhang, Ningbo (Harbin Engineering University) | Zheng, Xing (Harbin Engineering University) | Ma, Qingwei (City University London, Harbin Engineering University) | Hao, Hongbin (Harbin Engineering University)
In this paper, a numerical procedure based on SPH is presented to analyze the failure process of ice. The Drucker-Prager model is implemented into the SPH code to simulate four point bending and uniaxial compression failure of sea ice in a local scale. The procedure for implementing softening elastoplastic model is used in the SPH framework. To validate the model, the numerical results are compared with finite element simulations and experimental results. The good agreement has demonstrated that the presented SPH procedure can be a useful numerical tool for the simulation of failure progress of ice.
With the increasing activities in Arctic regions, the numerical simulation of ice-structure and ice-ship interactions became in the past decade of increasing significance due to its high importance in the design process and better accessibility to test data. In order to simulate ice-structure and ice-ship interactions effectively, it is necessary to have a proper understanding of the ice failure behavior. The bending failure of ice is of high significance for ships in ice due to the inclined contact interfaces with the ice (Valanto, 2001). And while bending as main failure mechanism, the initial contact causes local compressive failure (crushing). The more vertical the contact area or structure becomes, the more compressive features are included in the failure process. This underlines the importance to represent the failure in bending and compression with the same model.
To have a proper understanding of the ice failure behavior, different full-scale tests and model tests were carried out which are readily available in literature (Kujala et al., 1990). However, the experimental results are dispersed because of different testing approaches, the measurements of the test specimens and ice properties. Thus, it is very important to develop a reliable numerical ice model to simulate sea ice failure in bending and compression, especially the current studies on the behavior of sea ice failure are not adequate.
The paper reports a progress on the development of a hybrid approach coupling the Meshless Local Petrov-Galerkin Method based on Rankine Solution (MLPG-R) and the Quasi Arbitrary Langangian-Eulerian Finite Element Method (QALE-FEM) for modelling nonlinear water waves. The former is to solve the one-phase incompressible Naiver-Stokes model using a fractional step method (projection method), whereas the latter is to solve the Fully Nonlinear Potential Theory (FNPT) using a time-marching procedure. They are fully coupled using a zonal approach. The hybrid approach takes the advantage of the QALE-FEM on modelling fully nonlinear water waves with relatively higher computational efficiency and that of the MLPG-R on its capacity on dealing with viscous effects and breaking waves. The model is validated by comparing the numerical prediction with the experimental data for a unidirectional focusing wave. A good agreement has been achieved.
Wave-structure interaction has been a focus for the researches on offshore, coastal and ocean engineering for many years. For safety and survivability of the structures, extreme wave condition must be considered. Accurately modelling such extreme wave condition usually requires a large-scale (~ 10s km) and long-duration (e.g. 3-hour sea state) numerical simulation to capture the spatial-temporal propagation of the ocean wave. On the other hand, the response of the structure in extreme condition is considerably influenced by small-to micro-scale physics, such as the viscous/turbulent effect, hydro elasticity and so on. This implies that an effective numerical model shall be able to deal with both large-scale oceans wave and small-scale near-field physics simultaneously. The presence of the extreme waves invalids the routine wave diffraction analysis based on linear and second-order potential theory in frequency domain and a fully nonlinear analysis shall be considered using time domain analysis.
Advances have been made on the development of fully nonlinear potential theory (FNPT) on modelling highly nonlinear wave waves in large scale and for long duration, e.g. 3-hour sea state. Various numerical models based on the FNPT, e.g. the quasi-arbitrary Lagrange-Euler finite element method (QALE-FEM, Ma and Yan, 2006; Yan and Ma, 2010a) and Spectral Boundary Integral methods (e.g. Wang and Ma, 2015; Wang et al, 2016), have been developed and proven to be robust and highly efficient for modelling extreme water waves without breaking. The FNPT assumes that the flow is inviscid and irrotational, therefore, it cannot deal with breaking waves, slamming and other small-scale physics near structures.
This paper is to investigate and compare hydrodynamics of four different floating platforms for wind turbines, including the natural frequencies, RAOs and standard deviations of motions subjected to the possible sea states of East China Sea at five different water depths (30-200 m) based on potential wave theory. The results show that the semisubmersible platform has smaller natural frequencies and standard deviations than the barges in heave and pitch and that the water depth has significant effects on the floating platforms when it is less than 62.5m in the tested cases. The effects of a heave plate with different diameters on heave RAOs is also studied by using the dipole method with viscosity estimated by an empirical formula, and find that the heave RAOs may not be reduced by the heave plate and the increase of its diameter.
Compared with onshore wind resource, offshore wind energy has much more potential and becomes increasingly popular. Currently, most of the offshore wind turbines are installed on the fixed support platforms, which are based on the mature technology of the onshore wind turbines. However, the technology for fixed offshore platforms may not be economical when water is deeper than 50m (Robertson and Jonkman, 2011). In contrast, the floating offshore wind turbines (FOWT) will be economically better in the deeper sea areas. Previous studies by Withee (2004) and Lee (2005) have revealed some encouraging results for some floating wind turbine systems.
Different from oil & gas offshore floating structures, the floating structure of offshore wind turbine must withstand the vast overturning moment from the wind turbine thrust load. In terms of how FOWT achieve its hydrostatic stability in roll and pitch, there are three primary categories. The tension leg platform (TLP) obtains restoring moments mainly by the integration of the mooring system and surplus buoyancy in the platform, spar buoy primarily depends a deep draft combined with ballast, and semi-submersibles rely on the large water plane area. Jonkman and Matha (2011) analyzed the dynamic response of three primary floating platforms mentioned above and compared with the onshore wind turbine. The increased loads on floating wind turbine were found in their work in contrast to land-based wind turbine. The characteristics of dynamic response for the TLP, Spar and semisubmersible platforms with vertical axis rotor were investigated by Cheng (2015).
Although floating offshore wind turbine (FOWT) can extract large amount of wind energy in deep water region, it often suffers from large amplitude motion in heavy sea states. The large amplitude motion can reduce aerodynamic performance of wind turbine and result in additional structural loads. Therefore, suppression of floating body motion in waves is an crucial task in designing a FOWT system. In this paper, heaving plate effect on the hydrodynamic performance of semisubmersible FOWT is studied by using a computational fluid dynamic tool. The 5MW semi-submersible FOWT model test has been carried out by Zhao (2012) in HEU. The benchmark case based on the above experiment FOWT1 without heaving plate and FOWT2 with heaving plate have been tested. This benchmark case is investigated in detail at the beginning to validate the numerical method. Then, a set of heaving plates with different size and shape are studied based on numerical simulation. Response amplitude operator (RAO) is analyzed in detail to investigate the heaving plate effect on the hydrodynamic performance of semi-submersible FOWT.
Offshore wind energy has been one of the most important renewable resources in the world. In order to fully utilize the wave energy resource, the critical technology of wind turbine has been developed extensively. Many wind turbines which have bottom fixed on seabed are well established products in the onshore industry and transported to appointed place in shallow water. However, the wind energy resource in shallow water can't meet the increasing demand, as a result, the further wind energy exploitation has been moved to deep water. Compared to shallow water, the sea area is large and the winds in deep water are strong and steady which is very suitable for wind energy development.
In deep water, the concept of bottom-fixed can't be adopted for economic consideration which makes floating wind turbine comes into the people's vision. Compared to TLP platform and Spar platform, the semi-submersible floating wind turbine has the highest ability on windresistance and large water plane area to insure its high stability. The detail design of a semi-submersible foundation came up with a Mini-Float design and was applied to a WINDFLOAT DESiGn. In 2008, a WINDFLOAT project is coordinate with the support by NCNS and Vergnet, their foundation is a semi-submersible form. However, this kind of floating foundation is easy to suffer the impact of wave resulting in its big motion, especially heave motion, which seriously affecting the productivity of wind turbine.
Hao, Hongbin (College of Shipbuilding Engineering, Harbin Engineering University) | Ma, Qingwei (School of Engineering and Mathematical Sciences, City University London, College of Shipbuilding Engineering, Harbin Engineering University) | Liao, Kangping (College of Shipbuilding Engineering, Harbin Engineering University) | Zheng, Xing (College of Shipbuilding Engineering, Harbin Engineering University)
Wave propulsion device (WPD) is a device that can directly absorb wave energy and convert it into forward thrust by using an oscillating hydrofoil. The concept of WPD has been proposed for many years and has been proved to be feasible as the dominating or assistant propulsion of ships. The main problem of such device is its low efficiency. A new idea to improve its efficiency is investigated in this paper by using small floaters to adjust the attack angle of the hydrofoil, which is halfactive and named as floater-adjusted wave propulsion device (FAWPD). The experimental tests show that the FAWPD can generate significantly more thrust compared with conventional designs.
New technologies have been proposed widely in industrial practices in order to promote green energy and reduce emission. For example, green ship technologies have received more and more attention and the EEDI (Energy Efficiency Design Index) becomes an international convention in 2011 and takes into effect in 2013. It is enforced since 2015 and the ships failed to meet the EEDI standard will not be allowed by classification society or IMO. Thus developing new green ship technologies can be very helpful in the concept or initial design.
WPD is one of the green ship technologies, which can directly absorb wave energy and convert it into forward thrust. Generally speaking, the resistance of ship will increase when it navigates in waves, which leads to a speed loss and needs more power, meanwhile the responses of the ship due to waves will become violent.
However, some sea animals (i.e. whale and dolphin) can utilize the wave energy to swim, which was found firstly in the middle of the 18th century by an English whale catcher. They found that the floating dead whale swims faster than the ship in waves. This inspired many researchers investigate how to utilize wave energy to drive ships.
When an air cushion supported vessel navigates in the waves, the air pressure in the cushion pulsates due to the pumping effect of waves. The pulsating pressure could induce significant waves to have an impact on the aerodynamics of air cushion. To evaluate the waves, a linear 3D potential method has been proposed by Doctors (1974) and Kim and Tsakonas (1981). However, the algorithm of the 3D method is too complicated and so far the 3D method has not been widely utilized in the air cushion hydrodynamics. In this paper, a 2.5D method for calculating the waves due to the pulsating pressure was firstly presented. The 2.5D method is much simpler and more efficient than the 3D method. The numerical results suggest the sufficient conditions for the 2.5D method to perform as well as the 3D method.
The problem of pulsating pressure induced waves, which is also known as the free surface Dirichlet problem, is common in air cushion hydrodynamics. When an ACV (air cushion vehicle) or a SES (surface effect ship) navigates in waves, the air cushion pressure pulsates due to the wave pumping effect. The pulsating air cushion pressure inversely reacts on the free surface in the cushion and makes waves that could influence the dynamics of the air cushion by changing the cushion volume or air leakage area. Therefore, it is important to evaluate the air cushion hydrodynamics.
The pulsating pressure induced waves or the free surface Dirichlet problems were concerned long time ago. Stoker (1957) and Wehausen et al. (1960) deduced the mathmatic models for solving the free surface Dirichlet problems in 2D case and 3D case, respectively. The 2D model is not practical since the air cushion is always three-dimensional. Meanwhile, it is difficult to directly perform the calculation under the 3D model given by Wehausen. Later Doctors (1974), Kim and Tsakonas (1981) made great efforts to develop a more solvable 3D linear potential method for evaluating the free surface elevation due to a rectangular uniformly-distributed pulsating pressure patch. In order to calculate the waves due to a non-rectangular pulsating pressure patch under an air-lifted vessel, Xie et al. (2008) discretized the irregular pressure patch to a set of rectangular ones, using the method developed by Kim and Tsakonas (1981) to obtain the free surface elevation due to each rectangular patch, and then linearly superimposed them as the final results. In another case, Guo et al. (2016) discretized the nonuniformly-distributed pulsating pressure patch to a series of approximately uniformly-distributed pulsating pressure patches, and then adopted similar procedures to get the free surface elevation. However, it is still not trivial to solve the free surface Dirichlet problem using the upper mentioned 3D linear potential method, since there exist some singular and oscillating integrals.
This paper presents a numerical investigation on the significance of the role of the compressibility of the fluids associated with water entry problems using a multi-phase solver OpenFOAM, in which the water and air are treated as either compressible (compressible solver) or incompressible (incompressible solver). The models are validated by using the experimental data of a 3D plate dropping case, whereas the detailed investigations focus on 2D wedge dropping with different dead-rise angles and/or tilting angles. The effects of the compressibility are examined by comparing the results of the compressible solver and that of the incompressible solver. It is concluded that the free surface profiles during the impact are significantly influenced by the compressibility of the fluids, leading to different patterns of impacts (convective motion between fluids and dropping wedge); even in a case with large dead-rise angle, the incompressible solver may lead to incorrect predictions on the peak pressure and the force acting on the wedge surface.
Large impulsive pressure and slamming forces may lead to the damage of the offshore structure, and are of interest for the engineering purposes. Typical examples include breaking wave impacts on quay walls/breakwaters, slamming of the ship bow during extreme weather condition. The experimental (e.g. Miyamoto and Tanizawa, 1985; MOERI, 2013; Mai et al, 2015), numerical or analytical studies (either based on the potential theory, e.g. Zhao and Faltinsen, 1993; Zhao et al. 1996, or viscous flow theories such as Gao et al., 2012; Oger et al., 2007; Skillen et al., 2013) on the water entry problems, initiated by Von Karman (1929) and Wagner (1932), provides useful references for reliably predicting the slamming loads and exploring associated small-scale physics, such as the air trapping, spray and extreme free surface deformation. Significant advances have been recently made on computational fluid dynamics (CFD) modelling on such problems. Both single-phase (e.g. Gao et al., 2012; Oger et al., 2007; Skillen et al., 2013) and multiphase models (Kleefsman et al 2005; Sussman et al, 1994; Soulhal et al, 2014) have been attempted, and a promising accuracy was demonstrated on predicting slamming loads.