Liu, Lei (Shanghai Jiao Tong University, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Lu, Haining (Shanghai Jiao Tong University, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Yang, Jianmin (Shanghai Jiao Tong University, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Peng, Tao (Shanghai Jiao Tong University, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Tian, Xinliang (Shanghai Jiao Tong University, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration)
Numerical study of the free-fall of a single sphere at different Reynolds numbers has been conducted with Computational Fluid Dynamics(CFD) method based on the engineering concerns of the dynamics of ore particles in vertical pipes in deep sea mining. A combination of Detached Eddy Simulation (DES) and the six-degree-of-freedom (6-DOF) motion solver was adopted. The sphere motion, the hydrodynamic forces on the sphere and the characteristics of the surrounding flow field were analyzed in detail. Different falling trajectories of the sphere were observed. The surrounding flow field gradually lost the symmetry with the increase of Reynolds number. The results of this article would provide a basic reference for the further investigation on motion of the multiple ore particles.
As the increasing demand of the natural sources in the world, deep sea deposits are considered as the most valuable alternative sources. Deep sea mining applications has been proposed since 1960s (Mero, 1965; Willums and Bradley, 1974; Chung, 1999; Chung, 2005; Chung, 2009). One of the most important issues in deep sea mining is the ore transportation from seafloor. Typically, ore particles can be transported vertically to the support vessels in the upward flow of water in a riser. Significant efforts have been dedicated to the vertical hydraulic transport system in deep sea mining (Engelmann, 1978; Bournaski et al., 2001; Xia et al., 2004; Chung et al., 2007; van Wijk, 2016).
Engelmann (1978) conducted experimental investigation on the hydrodynamic behaviors of ore particles in a vertical tube, and established the empirical equations for designing the hydraulic transport system in deep sea mining. Chung et al. (1998), Chung et al. (2001) and Chung et al. (2007) had a thorough investigation on the vertically upward transport in deep sea mining, including the transportation of spherical bead and non-spherical particles, the effects of particle shape and size, different particle behaviors over a wide range of Reynolds number in both Newtonian fluid and non-Newtonian fluids. Yoon et al. (1999), Yoon et al. (2001) and Yoon et al. (2008) studied the flow characteristics of the solid-liquid two-phase mixture in both vertical tubes and flexible hoses. Bournaski et al. (2001) and Xia et al. (2004) studied the hydraulic gradient caused by the fluid, the coarse particles and the collisions in the vertical pipes. Parenteau (2010) carried out numerical simulations to investigate the transient behaviors and pressure predictions for the risers by using Computational Fluid Dynamics (CFD) methods. Talmon and Rhee (2011) designed a close-loop system in the laboratory to conduct experiments on ore transport over large vertical distances. Sobota et al. (2013) experimentally investigated the velocities of ore particles and carrier liquid to determine the slip velocities for the artificial nodules in the vertical pipe. Vlasak et al. (2014) studied the influence of pipe inclination, solid concentration and mixture velocity on the characteristics of particle-water mixtures by using a pipe loop system. van Wijk (2016) carried out a study into flow assurance of the hydraulic transport system in deep sea mining and proposed a onedimensional flow model to investigate the mechanisms leading to the riser blockage.
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.
Deng, Yanfei (Shanghai Jiao Tong University;Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration (CISSE)) | Yang, Jianmin (Shanghai Jiao Tong University;Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration (CISSE)) | Li, Xin (Shanghai Jiao Tong University;Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration (CISSE)) | Tian, Xinliang (Shanghai Jiao Tong University;Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration (CISSE))
An experimental investigation of the nonlinear wave forces on vertical cylinders induced by freak wave trains has been presented. A series of freak wave trains were modeled in a wave flume. The corresponding wave forces on vertical cylinders with different sizes were measured. The experimental wave forces were also compared with the Morison predictions adopting different stretch models. It shows that not only the strong asymmetric waves but also the weak asymmetric waves could result in strong asymmetric wave forces. The particle velocity distributions under the freak wave crest were measured with Particle Image Velocimetry (PIV) technique. It shows that the modified Wheeler model is capable to provide satisfactory predictions on the wave kinematics and wave forces of freak waves. By comparing the wave kinematics and wave forces results, it is concluded that the wave kinematics is the primary factors in predicting the strong asymmetric wave forces.
Extreme waves have long been a major threat for the safety of marine structures and offshore workers. The extreme waves could result in significant wave run-ups, tremendous wave loads as well as the motion responses. Freak wave is one special appearance of extreme waves with abnormal wave height and asymmetrical wave profile. In recent decades, the reports on the damages and shipwrecks caused by freak waves emerge in an endless stream. By the statistics, there are more than 22 super carriers missing due to the attacks of freak waves between 1969 and 1994 (Kharif and Pelinovsky, 2003). Freak waves were observed at both deep sea and nearshore, in both stormy and calm seas (Chien et al., 2002; Mori et al., 2002). In view of this, interactions between freak waves and marine structures have been receiving more and more attention.
Since the formation of freak wave is often accompanied with a huge volume of water, rapid concentration of wave energy (Rudman and Cleary, 2013), it probably results in strongly nonlinear wave forces when rushing at marine structures. Cylindrical members are very common on offshore structures, and the wave forces on these structures have been widely studied. Among them, Morison formula (Morison et al., 1950) is the most famous and popular approach for predicting the wave forces on slender bodies. MacCamy and Fuchs (1954) presented the theoretical solution of linear diffraction problem for large diameter vertical cylinder. Kriebel (1998) studied the second-order wave forces on large diameter cylinders based on semi-analytical diffraction theory. In order to investigate the ringing phenomenon of marine structures in steep waves, multiple theories were proposed to calculate the thirdorder wave forces (Faltinsen et al., 1995; Malenica and Molin, 1995; Rainey, 1989). Being restricted by various kinds of assumptions, theoretical approaches are insufficient to predict the extreme wave loads due to freak waves.
Ge, Xiaona (Shanghai Jiao Tong University) | Tian, Xinliang (Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Yang, Jianmin (Shanghai Jiao Tong University) | Kou, Yufeng (Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration)
Air gap performance has been a key issue in the design of semisubmersible platform. In this paper, a comprehensive study on air-gap is carried out based on the field measurement data of a semisubmersible in South China Sea using statistical analysis. The analysis shows that the energy extremes of air gap mainly appear in the wave frequency range. By fitting the probability density and cumulative probability distribution curve, the air gap extreme can be predicted. And the air gap performance of the platform is relatively stable in different months because the curves in different months appear very close. Moreover, the correlation between air gap and vertical motion of the platform is analyzed, which indicates that parts of the lowfrequency data are unreliable, and a method to evaluate the reliability of the field measurements is proposed.
Semi-submersible offshore drilling platform has drawn a wide range of attention in the offshore community because of its large deck area, large displacement, strong wind resistance and excellent motion performance. One of the key issues regarding the performance of the semi-submersible platform is the air gap which is defined as the vertical distance between the lower deck of platform and the wave surface. Sufficient air gap should be ensured to reduce the possibility of the damage from the wave impact on the lower deck. On the other hand, a larger air gap would result in a great change in draft and gravity center of the platform, which may directly affect the overall design and cost of the platform. Thus, the air gap is often determined based on the compromise among those concerns.
Air gap is closely related to two aspects, the wave elevation at the specific position of the platform and the vertical motion response of the platform.
Currently most requirement and restrictions of the air gap are on the minimum value during the service life. In fact, its characteristics and probability distribution are also important in addition to extreme values. Simply elevating deck of the platform to reduce the wave impacts on the deck is neither a good idea nor economical. Sweetman (2004) proposed a method to reduce the cost. The principle of the method is to reinforce the structures which are more likely to be impacted by the waves.
Liu, Lei (Shanghai Jiao Tong University) | Yuan, Hongtao (Shanghai Waigaoqiao Shipbuilding Co., Ltd.) | Yang, Jianmin (Shanghai Jiao Tong University) | Tian, Xinliang (Shanghai Jiao Tong University) | Li, Chunhui (Shanghai Waigaoqiao Shipbuilding Co., Ltd.) | Lu, Haining (Shanghai Jiao Tong University)
Offshore platforms under construction are normally moored on the dock during the outfitting stage. The safety of the platforms must be guaranteed during the whole stage of outfitting which may last for several months. This paper presents a wave basin model test study of a jackup moored on the dock in Shanghai Waigaoqiao shipyard in China. In the model test, the jackup and the sea states were scaled based on the Froude similarity law. The dynami c responses of the system, including the six degrees of freedom (6DOF) moti ons of the jackup and the barge, tensions on the mooring lines and the collision forces on the fenders, were measured in various sea states. Meanwhile, the current-and-wind-only sea states were simula ted and the dynamic responses were measured for comparison with those in the wave conditions. The mooring line tensions were found to excee d the strength of the lines in offshore wind conditions. And this phenomenon may be attributed to the decrease of the jackup's yaw m otion stiffness. In addition, several suggestions are proposed for optimizing the mooring system performance.
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.