Experimental and numerical studies are carried out to understand the behavior of ore particles in hydraulic collecting. The experiments were conducted in a water tank to measure the critical bottom clearance for the vertical incipient motion of spherical particles, which could accurately represent the collecting features. Four kinds of tests are designed to study the influence of particle size, speed of pipe and particle amount on critical bottom clearance. The characteristics of flow field and vertical force of spherical particle in the hydraulic collecting are simulated by Detached Eddy Simulation (DES) method and the six-degree-of-freedom (6-DOF) motion solver. This study will be useful for mechanism study of hydraulic collecting in the process of deep-ocean mining.
Deep-ocean mining is becoming an alternative way for human to exploit natural resources under the trend that some kind of resources on land are being exhausted. It is widely acknowledged that a vast number of resources are reserved in deep ocean, especially deep-ocean deposits. Therefore, deep-ocean mining is thought to be a key approach to the sustainable development of human beings and its applications have been proposed since 1960s (Mero, 1965; Willums and Bradley, 1974; Chung, 1999; Chung, 2005; Chung, 2009). One of the most important issues in deep-ocean mining is the hydraulic collecting near seabed. Typically, ore particles can be collected to the collector in the upward and gathering flow of water. Significant efforts have been dedicated to the hydraulic collecting system in deep-ocean mining.
A variety of collecting methods such as hydraulic methods, mechanical methods and hybrid collecting methods have been developed. Commercial production must achieve high sweep efficiency (Chung, 1985). OMI Test in 1978 showed that hydraulic methods have higher collecting efficiency than mechanical methods. It was also found that the hydraulic methods have better adaptability to the variation of bottom clearance than other methods (Zhao and Liu, 1995). The mechanism of hydraulic collecting is fairly complicated because the collecting efficiency is affected by a lot of factors such as particle size, bottom clearance, collector structure, flow rate and speed of pipe. The influence of these factors on collecting rate was studied by experiments.
Li, Yang (Shanghai Jiao Tong University) | Xiao, Longfei (Shanghai Jiao Tong University, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Wang, Tongtong (Shanghai Jiao Tong University) | Deng, Boxian (Shanghai Jiao Tong University)
Wave energy, as a kind of renewable resources with large density of power, has been widely studied in decades. Different types of wave energy converters (WECs) have been proposed, one of which is oscillating-body WEC. The wave-induced oscillation of the floating body is converted to electricity by the Power-Take-Off (PTO) system. For the linear PTO system, the resonance may not be excited in real wave conditions, and this makes the WEC less efficient. In order to improve the power capture performance, a novel nonlinear PTO system has been proposed using the bistable impulse mechanism. An array of fixed outer magnets are used to repel the inner magnet away from the stable position in order to make the system bistable. The model of oscillating-body WEC with the nonlinear PTO system is tested in a wave flume to simulate the power capture performance in regular wave conditions. The piece number of small magnets in every outer magnet is denoted by parameter λ to describe the magnitude of the magnetism. This testing program pays more attention to λ, and mainly focuses on how λ affects the power capture performance with input of different-frequency regular waves. The results indicate that the novel style nonlinear PTO system can increase the power capture ratio for low-frequency regular waves in comparison with the linear PTO system. And the system can further enhance the power capture in low-frequency regular waves by selecting the proper λ. In other words, the novel style nonlinear PTO system can capture the wave power in a “broadened bandwidth”, which could be advantageous in future applications.
Ocean-wave energy has attracted more and more attentions because of its higher power density (Falnes, 2007). Many inventors and researchers have studied how to use wave energy effectively, and numerous kinds of wave energy converters (WECs) with different working proposals and principles have been proposed in the past centuries (Antonio, 2010), such as oscillating water column (OWC, including fixed-structure OWC and floating-structure OWC) WECs, oscillating-body (including single-body heaving, two-body heaving, fully submerged heaving, pitching, bottom-hinged and multi-body) WECs, and overtopping WECs. Among these WECs, the oscillating-body WECs have a great application prospect because they could harvest more powerful wave energies available in deep water regions (Henriques et al, 2012). Furthermore, the oscillating-body WECs convert wave energy into electricity by the Power-Take-Off (PTO) system, which is usually mathematically simplified as a linear spring and a linear damper (Zhang et al, 2015).
The characteristics of flow field and force of a single spherical ore particle in the hydraulic collecting have been investigated. Researches of hydraulic collecting spherical particles in various collecting heights and flow velocities have been carried out with numerical method and testing method. It is verified that the vertical force prediction for single ore particle in collecting condition based on numerical simulation is feasible. The study reveals that the variation of the wake vortex is the dominant factor of force vibration. The drag coefficients of the sphere are defined as empirical equations using dimensionless quantities.
The world's growing economy demands more mineral extracted from the ocean. This demand requires the development of the deep-ocean mining technology. Polymetallic nodules are deposited over the ocean floor at water depth of 4,000~6,000 m. They are mostly spherical or ellipsoidal, with their long-axis length varying from 2 cm to 10 cm and a density of 2,100 kg/m3 (Liu et al., 2014). However, a profitable exploitation of deep-ocean mining is feasible on the premise that there is a nodule collector with maximum collecting capacity of 140 kg of wet nodules per second (Herrouin et al., 1989). As a result, effective collecting of manganese nodules out of sediment upper layer of deep seafloor is not only one of the key processes of the deep-ocean mining technology, but also the beginning of an economic and environmentally acceptable mining operation.
To pick up these nodules, a variety of collecting methods such as hydraulic methods, mechanical methods and hybrid collection methods have been developed. Commercial production must achieve high sweep efficiency (Chung, 1985). A sea test (the OMI Test, 1978) showed that hydraulic methods had a higher collecting efficiency than mechanical methods. It is also found that the hydraulic methods have better adaptability to the variation of seabed height than other methods (Zhao et al., 1995).
Wave impact load is of particular concern to ships and offshore structures, especially when they serve in rough seas. Extreme wave impact is catastrophic and may lead to structural failure, crew injuries and economic loss. Therefore, it is very important to determine wave impact load accurately in the design of ships and offshore structures. In this paper, we present the method that we use to measure wave impact force. To validate the reliability of this method, we conducted a well- controlled wedge drop test, and compared the experimental results with CFD simulations and the simplified analytical solution (the Wagner solution). The design of the device in combination with force transducers, the wedge drop test, the analysis of the experimental data, and the comparison with numerical and analytical solutions are discussed in detail. In addition, some shortcomings and corresponding improvements are also presented. It is shown that the device can be used to measure wave impact force over a certain area well.
Wave impact can cause very large and localized force on ships and offshore structures, which may lead to structural failure. Buchner and Voogt (2004) presented two FPSOs experiencing wave impact damage. The accurate determination of wave impact load is very important for the structural design of both deep-water offshore structures and shallow-water coastal structures. Wave impact is a strongly nonlinear transient two phase problem, which is hardly to obtain an analytical solution. It was noted that (Chan and Melville, 1988) air will be trapped when wave impacts structures, which introduces randomness to wave impact load, increasing the difficulty to predict wave impact load theoretically.
The characteristics of wave impact or slamming load are often studied by the problem of wedge entry into water. For the wedge water entry problem, Von Karman (1929) firstly proposed a simplified solution by neglecting the local uprise of the water. Taking into the local uprise of the water, Wagner (1932) developed the Wagner solution, which assumes that the wedge drops into water with constant velocity. In this paper, the Wagner method was modified to be applicable for time- varying velocity. There are a lot of other analytical works for the water entry problem and most of them are based on the potential assumption. Therefore, with these methods, the effect of viscidity and air cannot be considered.
Bracings are essential components of semi-submersible platforms. The existing researches about the bracing of semi-submersible platform mainly focus on its damping and impacting force. Actually, the variation of the hydrostatic force induced by bracings can result in severe nonlinear coupling and is seldom studied. In this paper, an experimental study for a semi-submersible with bracings is firstly carried out. Fresh phenomena are observed about the nonlinear effect induced by bracings, such as the double period and the non-sinusoidal profile of pitch motion. In addition, a new mathematical model considering the effect of the bracings is established to explain those nonlinear effects. Good agreements between the model testing results and the numerical results are achieved.
As the industry’s appetite for oil and gas is growing to be more and more enormous and the production of these resources on the land is stagnating, the exploitation has now spread into deep seas. Deep-water platforms, which are used to exploit or restore the fossil resource, always work on harsh environment. The characteristics of those platforms, such as stability and sea-keeping, hence, should be analyzed before they get into operation. Numerical simulation and physical simulation (model test) are effective approaches to predict the characteristics.
In the prediction of platform response, nonlinear coupling of heave and pitch is one of the most important issues that should be analyzed because it will always result in extreme responses of the platform. Many researches have been done on nonlinear coupled motion of ships and spars. Rho et al. (2002) analyzed the coupled system of heave and pitch of a classic spar model with the method of multiple scales. Experiments of a spar model were carried out, and it turned out that the unstable pitch motion occurred when the heave natural frequency is twice the pitch natural frequency. Nayfeh et al. (1973) presented an analysis for the nonlinear coupling of the pitch (heave) and roll modes of ship motions in regular seas when their frequencies are in the ratio of two to one. It turns out that when the encounter frequency is near the pitch frequency, a saturation phenomenon associated with the response can be observed. Jingrui et al. (2010) determined the first order steady periodic solution of both responses with the method of multiple scales when the wave frequency gets close to the sum of heave natural frequency and pitch natural frequency. A large amplitude sub-harmonic motion of heave and pitch mode are tripped when the wave height exceeds a certain value.
In this paper, a four-layer submerged horizontal porous plate breakwater was proposed, and its wave-dissipating performance was examined experimentally in a wave flume. The design of the geometrical parameters (i.e., submerged depth and plate porosity) is discussed. The experimental results of concerned characteristics in engineering practice (i.e., the effect of layer quantity, plate width, porosity, and submerged depth) are shown and discussed. A relatively larger plate width or a smaller porosity of the upper plate may benefit the wave-dissipating performance, and larger layer depths may increase the wave transmission. Generally, the fourlayer breakwater proposed in the present paper shows satisfactory performance under a wide range of incident wave lengths and has promising application future in coastal engineering.
As an economical and ecologically friendly wave-dissipating structure, the horizontal porous plate breakwater shows quite a few advantages over those of caisson or rubble mound type (Yu, 2002; Liu et al., 2008). Horizontal plates can avoid heavy horizontal wave force impacting on the structure, and the porosity of the plate further reduces the vertical force and attenuates more wave energy. Research on the wave interaction with horizontal porous plate is encouraging about the optimization of its wave dissipating performance and provides for the engineering applications.
The submerged horizontal plate was proposed as a breakwater in 1970s (Hattori, 1975; Siew and Hurley, 1977; Hattori and Matsumoto, 1977). Some succeeding investigations (e.g., Patarapanich, 1984; Liu and Iskandarani, 1991) revealed some limitations of the horizontal solid plate when used as a breakwater. It has little or nothing effect on attenuating wave energy. And long waves which propagate through the breakwater and into the lee side might cause resonance in a sheltered area. Therefore, breakwaters with porous plate were proposed, aiming to attenuate wave energy by the porosity-induced vortices and suppress the vertical force on it (Yu and Chwang, 1994; Chwang and Wu, 1994; Yip and Chwang, 1998). In addition, researches on horizontal porous flexible membrane and submerged inclined plate breakwater were conducted (Cho and Kim, 2000, 2008; Rao et al., 2009). Cho and Kim (2013) investigated the interaction of oblique monochromatic incident waves with a submerged horizontal porous plate, and indicated that the plate porosity near 0.1 and the submerged depth near 0.05h ~ 0.1h might be optimal for the performance, where h is the water depth.
A comprehensive study on the kinematics of freak wave sequences has been conducted with experimental and numerical method. As a preliminary step, an amplitude-phase iteration method has been adopted to generate the deterministic freak wave sequences in the wave flume. Subsequently, a numerical wave tank (NWT) has been set up with the similar sizes of the wave flume. An original wave maker signal used in the wave flume has been replicated in the NWT. The reasonable agreements between the numerical and experimental results indicate the capability of the NWT in simulating the freak wave propagations. Wave kinematics, including the wave energy distributions, wave speeds and horizontal velocity profiles, have been presented and discussed. Some meaningful conclusions regarding the kinematics of freak waves are drawn based on the present study.
In this paper, a nonlinear snap-through Power-Take-Off system is applied to a hemispherical wave energy converter oscillating in heave direction in regular waves. The nonlinear Power-Take-Off system is consisted of two symmetrically oblique springs and a linear damper, which is characteristic of negative stiffness and double-well potential. The dynamic response of the converter with nonlinear Power-Take-Off system is numerically calculated using 4th order Runge-Kutta method. Results shows that the nonlinear snap through mechanism, compared with linear mechanism, can enhance power capture of the converter, especially in low frequency regular waves.
Characteristics of the uniform flow around an oscillating circular disk have been investigated. The circular disk is forced to oscillate sinusoidally along its axis, and a uniform flow is introduced in the direction within the plane of the disk. The incompressible Navier-Stokes equations are solved with direct numerical simulations based on the finite volume method (FVM) using the open source CFD (Computational Fluid Dynamics) code OpenFOAM. Deforming mesh technique is adopted to simulate the oscillation motion of the disk. The thickness ratio (thickness/diameter) of the disk is 0.1. The hydrodynamic force component in the axis direction of the disk is written in a Morison’s equation-like form, and then the coefficients of added mass and damping are calculated using the Fourier analysis. The added mass of the disk decreases as the velocity of the in-plane flow increases. However, the existence of the in-plane flow with a relatively large velocity significantly increases the damping of the disk.
In recent years, Vortex Induced Motion (VIM) of floating structures such as Spar, Semi-submersible, TLP, etc, has proved to be a very relevant subject for the design of mooring and riser systems. The Deep Draft Semi-submersible (DDS) is a typical multi-column floater. With longer exciting length than the conventional Semi-submersible’s, it shows obvious VIM phenomenon. Furthermore, its VIM characteristics are more complicated compared to Spar platform because of the flow interference between columns.
The model test of a DDS with four rectangular columns and four pontoons is carried out in a towing tank to study its VIM response. The 6-DOF motions are measured under different current directions and velocities. Conclusions about the DDS’s VIM characteristics are drawn from the model test. It is shown that a DDS may experience significant VIM in the transverse and in-line direction and its amplitude is strongly dependent not only on the current directions but also on the reduced velocity Ur regularly. The lock-in range is defined as 6<Ur<8 according to response amplitudes and frequencies. In order to better understand the mechanics of this phenomenon, the numerical simulation based on the unsteady Reynolds-averaged Navier-Stokes (RANS) method is done using a simplified two-dimensional model. The model consists of the cross sections of four rectangular columns. The numerical study includes two parts: the cross-flow around four stationary columns and the VIM response of four columns moving freely. The numerical results can help explain the experimental results. And their general agreements on the response amplitudes indicate that the numerical simulation is effective in predicting VIM characteristics of a multi-column floater.