Wave forces and flow field around pile group foundation in China-Maldives Friendship Bridge are investigated with a Computational Fluid Dynamics (CFD) model. A numerical wave tank is developed based on the commercial finite volume package FLUENT. Mesh sensitivities are conducted to validate the numerical model, and an acceptable time-step is adopted to ensure stability in time-marching. Swells are very common in the location of the Bridge, thus in this manuscript, regular waves are simulated acting as swells. The wave interaction with single pile and pile group foundation is studied. The numerically calculated wave forces agree quite well with the predictions from Morison equation. The wave period shows minor influence on the values of maximum wave forces under the condition that the wave amplitude is unchanged. For the pile group foundation, the wave forces on piles of different locations have phase differences due to that the time of wave crest crossing the piles is different. This numerical study confirms that wave height is the key parameter for wave forces on pile group foundation in swell environment.
With the development of economic society, the construction of long-span cross-sea bridges has become much more significant around the whole world, such as HongKong-Zhuhai-Macao Bridge. Pile-slab structures are commonly used in the support structures of cross-sea bridges, offshore wind turbines and other near shore coastal structures. It is of great significance to have a thorough understanding about wave interaction with these structures.
Model test is one of the most common approaches to study the wave-structure interaction problems. The advantage of this method is that the real hydrodynamics of wave-structure interaction can be simulated directly, although physical test is limited by scale restrictions. To overcome this, analytical methods have been developed. In the case of circular cylinder structures, wave forces on these compact structures are divided into two parts: inertia forces and drag forces (Morison et al., 1950). The contribution of inertia and drag forces are determined by the number and diffraction parameter which is the ratio of the cylinder diameter (D) to the incident wave length (λ). When the number is smaller than 2 and the diffraction parameter (D/L > 0.2), the flow field is inertia dominated and wave diffraction effects are important (Sarpkaya et al., 1981). The analytical methods depend on flow coefficients that have to be estimated from experiment data, and it is limited in a small range of structures, such as circular structures.
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.
With the oil and gas exploration stepping into deeper water, taut mooring systems have become a popular solution for station keeping. However, the inclination angle at the anchor point in a taut mooring system may reach up to 40 degree from horizontal, which brings significant uplift load to the offshore foundation. In this scenario, embedded anchors, such as suction anchor, dynamically-embedded plate/torpedo anchor and drag anchors, are always employed to provide adequate holding capacity. The pad-eye of these anchors is designed at a certain depth into the mudline, which causes interaction between the soil and the embedded mooring line. The interaction is complex and introduces challenges in predicting the mooring load acting on the anchor. The present study proposes a dynamic method to predict anchor load caused by the motion of the floating platform. Soil resistance is calculated instantaneously by a simple and robust model. Based on the proposed method, sensitivity studies were conducted to investigate the influence of the oscillation amplitude and period on the anchor load. It is concluded that the maximum anchor load is more sensitive to the oscillation amplitude rather than the period, meaning that for the conditions considered, a dynamic analysis is not required.
Taut mooring systems have become a popular solution for offshore floating platforms in deep water. They have the advantage of reduced self-weight and span radius of mooring system, improving the system’s horizontal restoring ability (Qiao and Ou, 2013). Unlike the traditional catenary mooring system, in which the anchor is mainly subjected to horizontal load, in a taut mooring system, the applied load at the anchor can be up to 400 to the horizontal (Randolph et al., 2005). This results insubstantial uplift loads at the anchor. In this scenario, the embedded anchors, such as the suction anchor, dynamically-embedded plate/torpedo anchor and drag anchors, are widely employed in the taut mooring system to provide enough holding capacity. The padeye in an embedded anchor is beneath the mudline and part of the mooring chain is embedded and interacts with the soil. The complexity of the soil resistance on the mooring chain impedes the accurate estimation of the mooring loads on the embedded anchor, which is a key component for the anchor analysis (Randolph and House, 2002).
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.
Guo, Xiaoxian (Shanghai Jiao Tong University) | Lu, Haining (Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration(CISSE),) | Yang, Jianmin (Shanghai Jiao Tong University) | Peng, Tao (Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration(CISSE),)
As the exploration of offshore oil and gas resources are moving into larger depth, many mobile drillships have been widely applied to the offshore drilling operation, thanks to the good mobility, large variable deck loads and storage volumes. Large motion responses of the drillship and the water motions inside the moonpool were reported, which introduced safety concerns. In this paper, both numerical simulations and physical experiments have been carried out to investigate the hydrodynamic performances of a newly designed deep-water drillship, and the resonant water motions inside the moonpool with submerged recess structure. The three modes of water motions coexist in the moonpool even when there is only one external excitation source provided by a regular wave train. The submerged recess structure clearly introduces the obstacle effects leading to exacerbation of the water motion inside the moonpool.
Moonpools are designed as vertical openings through the decks and the hull structures to support the required underwater operations on marine vessels and offshore platforms. The water inside moonpool is required to provide a moderate environment for operation rather than being exposed to the violent metocean conditions. However large water motions may occur inside the moonpool under some resonant conditions, including the vertical piston motions, and the longitudinal sloshing motions. The effects on 6 degrees of freedom (DOF) RAOs of the drillship which are caused by moonpool also need to be investigated. Recently, the recess type moonpool appears in drillship’s moonpool design, which has advantage on drilling equipment arrangement. The impacts introduced by the recess type structure on relative water motions inside moonpool need to be clarified.
Fukuda (1974) carried out a set of physical experiments to investigate the water behavior inside the moonpool and the effects on the vessel motions in a towing tank. He obtained an empirical formulation which could be used to identify the resonant frequency of the water motions. A mathematical model was developed later to describe the water behavior inside moonpool , and also corresponding model tests with respect to the damping mechanisms and motion behavior were conducted (Aalbers, 1984). Molin (2001) treated this problem in the framework of linear potential theory under the assumption of a infinite length and beam of a barge equipped with a moonpool, and brought out the oscillation natural frequency formulation. It was believed that the water motions response inside the moonpool is overestimated with respect to the experimental results without considering the fluid viscosity (Kristiansen and Faltinsen, 2008). Kristiansen and Faltinsen (2008) set up a fully nonlinear numerical wave tank coupled with an inviscid vortex tracking method to investigate the impacts caused by fluid viscosity and the nonlinear effects that associate with free surface. More recently, computational fluid dynamics (CFD) has been utilized to study the sloshing phenomenon. Zhang (2012) applied Moving Particle Semi-Implicit (MPS) method to study the sloshing phenomenon. Large impact pressure was observed, and it was shown a periodic impact had two pressure peaks in each period.
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.
As the result of an increasing number of accidents that are believed to have happened due to encounters with rogue waves, the physical mechanism of this phenomenon has attracted much research and academic interest. Since this extreme sea event always occurred in the areas with currents, the wave-current interaction was considered to be one of the physical mechanics of the formation of the rogue wave. In this literature, the generation and evolution of the rogue waves are numerically investigated based the current modified fourth order nonlinear Schrödinger (CmNLS) equation. The fourth-order pseudospectral split-step method is used in the iteration process. The effects of the steady current were investigated by comparing with the results without current as well.
In the last a few decades, the physical mechanisms of the rogue wave have been attracted much attention as the more accidents due to such extreme wave events were reported. The rogue waves, which is also known as freak waves, monster waves, killer waves, extreme waves, and abnormal waves, is a type of very rare and extremely large wave that possesses powerful concentrated energy and strong nonlinearity. The rogue wave can cause enormously devastating damage to ships and offshore structures and may even cause significant harm to the onboard staff and valuable property. Studies on the mechanisms of rogue waves are thus of great significance to the vessel and platform design and operation. Some possible mechanisms have been summarized in the review papers (Adcock and Taylor 2014; Broad 2006; Dysthe and others 2008; Kharif and Pelinovsky 2003; Müller and others 2005; Slunyaev and others 2011).
As the rogue waves were frequently observed in the areas with strong currents such as the Agulhas Current, the Gulf Stream and the Kuroshio Current(Lavrenov 1998; Mallory 1974; White and Fornberg 1998), the concentration of wave energy influenced by the wave-current interaction has drawn special attention. The effect of the wave-current interaction on the modulational instability has also been the attractive subject of a number studies over the decade years (Chawla and Kirby 2002; Gerber 1987; Lai and others 1989). The CmNLS equation was developed by Stocker and Peregrine (Stocker and Peregrine 1999). Assuming that the current U is the O(ε) perturbation, Hjelmervik and Trulsen (Hjelmervik and Trulsen 2009) then derived the onedimensional current-modified nonlinear Schrö dinger equation to investigate the statistics of the rogue waves. Compared with the equation derived by Hjelmervik and Trulsen, the equation in (Stocker and Peregrine 1999) is in Dysthe order (Dysthe 1979) while the current is taken to be order O(ε²). They found that the rogue wave events should be rarer than in the open ocean without currents.
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 study, a numerical wave tank is developed based on commercial software package FLUENT. Velocity inlet method is applied to simulate nonlinear wave groups and compared with 1st order and 2nd order wave theories. The horizontal particle velocity of the focused wave group and regular wave with identical crest height and trough-totrough period were calculated. Main efforts are devoted to forces on a horizontal cylinder. The comparison of the vertical forces on horizontal cylinder for both nonlinear focused wave group and regular wave with the equivalent trough-to-trough period and wave crest amplitude is conducted. This simulation confirmed that the kinematics of focused wave group is much severe than that regular wave.
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.