ABSTRACT The on-bottom stability of a submarine pipeline is a flow-pipe-soil coupling problem. For the integrated pipe-soil system under the action of steady current, two correlated instability modes could be involved, i.e., the lateral-instability of the pipeline and the tunnel-erosion of the underlying soil. In the previous studies, the aforementioned two instability modes were investigated separately. In this paper, a finite element (FE) model for the flow-seepage-elastoplasticity sequential coupling is established to reveal the multi-physics coupling effects of such integrated system. This numerical model can realize the simultaneous and coupled simulation of the flow field over the partially-embedded pipe, the seepage-flow field and the elastoplastic stress-strain field of the soil. The proposed model is compared and well verified with the existing experimental results. Numerical simulation shows that both the non-uniform pressure distribution together with the viscous stress along the pipe periphery and the pressure-drop along the seabed are formed synchronously due to the existence of the pipe. The former will generate the hydrodynamic forces on the pipe, which may trigger the lateral-instability; while the latter will induce seepage flow in the underlying soil, which may result in the occurrence of tunnel-erosion. Numerical results also indicate that not only the hydrodynamic forces but the maximum upward hydraulic gradient increase nonlinearly with increasing inflow velocity. Instability mode and mechanism for the pipe-soil coupling system can be finally determined with the instability criteria. INTRODUCTION As the offshore exploitation of gas and oil moves into deeper waters, the on bottom instability of a submarine pipeline induced by ocean currents becomes one of the main causes for the structural failure (see Drumond et al, 2018). For such a flow-pipe-soil interaction issue, two physical processes within the integrated pipe-soil system, i.e. lateral-instability of the pipe and tunnel-erosion of the underlying soil, are involved. The physical mechanisms and criteria for the two different instability modes have been explored intensively in previous studies (e.g., Fredsø e, 2016; Zhang et al., 2016; Gao, 2017; Drumond et al, 2018). In ocean currents, the flow over a pipeline and the seepage-flow within the underlying soil can be generated synchronously. On the one hand, when the lateral soil resistance provided by seabed soil is insufficient to balance the hydrodynamic forces exerted on the pipeline by the flow, the pipeline would breakout laterally from its original position. On the other hand, if soil seepage failure due to the pressure drop between the upstream and downstream of a partially-embedded pipeline is triggered, the tunnel-erosion underneath the pipeline will be initiated immediately (see Chiew, 1990; Sumer et al. 2001; Yang et al., 2014). Note that, the lateral-instability of pipeline and the tunnel-erosion of soil were always investigated separately. Moreover, in the tunnel-erosion numerical simulation, the pipeline was often set as a fixed rigid boundary with assumption of a rigid and permeable seabed (e.g., Zang et al., 2009); and in the lateral-instability studies, the elasto-plastic behaviors of soil and the interfacial characteristics for the pipe-soil interaction were the main focus of attention (e.g., Gao, et al., 2012; Bai et al., 2015).

Abstract Two-dimensional numerical simulations are conducted to examine the effects of tripping wires on the hydrodynamic properties of a stationary cylinder in uniform and shear flows at a fixed Reynolds number of 100. Three sets of numerical tests are carried out: symmetric and asymmetric tripping wire arrangements in a shear flow; symmetric arrangement of tripping wires of various diameters in a uniform flow; and symmetric arrangement of tripping wires placed at a range of distances from the cylinder in a uniform flow. The results of each test are described in terms of hydrodynamic forces and vortex shedding frequency. Introduction Vortex induced vibration (VIV) takes place when a fluid flow passes an elastically mounted bluff body. Earlier studies of VIV focused primarily on the suppression of this phenomenon to minimize its damage caused to engineering structures such as bridges and offshore pipelines. In recent years, a shift towards developing viable renewable energy technologies has led to a technology that harnesses energy from the typically harmful phenomenon of VIV (Raghavan, 2007; Bernitsas et al ., 2008; Nishi et al ., 2012). Since the amount of energy produced by a VIV energy harnessing system is directly related to the amplitude of the transverse oscillations associated with VIV, which in turn depends on the lift force acting on the bluff body, recent research has been focusing on maximizing the amplitude of lift fluctuation and enhancing the motion induced by the fluctuating lift.

Abstract The effect of yaw angle ( α ) on hydrodynamic forces of a yawed square cylinder oscillating in still water is studied experimentally. The independent principle (IP) was examined by evaluating the hydrodynamic coefficients that derived from the measured in-line and lift forces using a 3-component force link at a Keulegan-Carpenter (KC) number range of 3~30 and a Stokes parameter ( β ) range of 500~1600. Significant differences are observed in normalized drag and inertia coefficients for the KC ranges of 8~20 and 10~18, respectively, indicating the invalidity of the IP. The peak on the spectra of the lift force becomes pronounced as α becomes larger, which implies that the yaw angle has an intensified effect on the vortex shedding process around a square cylinder in oscillatory flow. Introduction Oscillatory flow around a cylindrical structure is of importance due to its intrinsic interest in theoretical hydrodynamic research and its relevance to practical engineering applications. Knowledge of the hydrodynamics is essential for both the design and operation of offshore structures, such as semisubmersibles and tension leg platforms. For some cylindrical structures, owning to their directionality, the sea waves and currents can approach the structures obliquely. In these cases, the fluid velocity in the axial direction of the structure is not negligible, which may have profound effect on the vortex instability and force characteristics of the structure.

Abstract The numerical investigation of the passive hydrodynamic performance of the manta ray beneath the water surface is presented. The main aim is to investigate the effect of attack angle on hydrodynamic forces of the manta ray near the water's surface. Three dimensional incompressible Navier-Stokes equations are resolved with RNG k-e turbulence closure model. The undulation of the free surface is captured by the volume of fluid (VOF) method. Numerical simulations are carried out with the manta ray oriented with the longitudinal axis at attack angles of -5, -2.5, 0, +2.5 and +5 degrees with various swimming speeds ranging from 0.5m/s to 4.5m/s. The drag coefficient and the lift coefficient of the manta ray are measured and compared. Results show obvious differences between negative and positive angles of attack. Additionally, the deformation of the free surface induced by the gliding manta ray is also captured and compared.

ABSTRACT: To utilize floating structure under heavy environmental condition in sea safely, motions of the floating structure and the mooring forces have to be reduced. To decrease these motions and mooring forces, fins attached structure has been proposed. In this paper, as a first step of the study, two vortex methods, the Random-Walk method and the Core-Spreading method, are applied to vertical oscillating plate under free surface. Hydrodynamic forces on the plate are calculated and compared with experimental results. INTRODUCTION It has been required that the ocean space is effectively used in Japan surrounded by the sea and various floating structures for development of ocean resources or oceanographic survey, etc. have been proposed. In recent years, the floating structures for utilizing ocean energy such as offshore wind farms or ocean thermal energy conversions are required because of depletion of fossil fuels, global warming and the accident in the nuclear power plant by the eastern Japan earthquake. Moreover, the floating structure seems to be useful as disaster prevention bases or shelters, because the damage of the floating structures in the tsunamis caused by the eastern Japan earthquake were relatively small. Therefore, various types of floating structure will be required in future. To utilize floating structure in safely economically under severe wind and wave conditions around our country, motions of the floating body and mooring forces acting on the body have to be reduced. To decrease these motions and mooring forces, fins attached structure has been proposed (Fig. 1). The attached fin seems to dissipate the energy of the motion as viscous damping and reduce the mooring forces due to so-called "wave devouring propulsion" effect. To put this method to practical use, the calculation method to design optimum shape and arrangement of the fin has to be developed.

ABSTRACT: The hydrodynamic force acting on a fin oscillating beneath a free surface is investigated. To safely utilize floating structures under severe environmental conditions in the sea, the motion of the floating body and the mooring forces acting on the body should be reduced. To decrease these motions and mooring forces, fins attached to the structure are proposed (Yago et.al 2007). In this paper, a flat plate was used as a fin. As a first step, the hydrodynamic forces acting on the fin oscillating near free surface were resolved in new method. This method uses the modified Morison equation added to a wave damping component. INTRODUCTION The amount of natural resources is decreasing because of the industrialization of developing countries. Because the amount of natural resources in Japan is limited and most of them are imported from abroad, the utilization of resources in an exclusive economic zone (EEZ) is planned. Japan has an EEZ domain that spans more than ten times the country area and is considered to have abundant resources under the seabed. Rare metals, minerals and natural gas can be found on the sea bottom. To develop these EEZs, a large-scale platform for natural resources, renewable energy and fisheries is planned. In Japan, the technology for a large-scale platform is developed. In the Mega-Float project in 2000, a platform with a length of 1km was tested in the Tokyo bay, and a feasibility study for a floating airport was performed. Additionally, an offshore platform that is used for the utilization of an EEZ was installed in severe wind and wave conditions as compared to the platform installed in coastal region. The installation of a breakwater that protects the platform from severe wind and waves is difficult in offshore area with deep sea.

ABSTRACT: The second order wave exciting force on a floating ship considering tank liquid dynamic effect is examined. The second order drift forces are quadratic products of the first order quantities. The ship motions in wave considering tank liquid dynamic effect are calculated by linear potential theory. Then the second order drift forces are computed by near field method based on the pressure integration and validity is confirmed by far field method. For a numerical calculation example, the motions and drift forces of 250K LNG FPSO in wave condition are demonstrated. INTRODUCTION To estimate the ship motion in waves containing large liquid tanks are very complex problem. In most seakeeping computations, the liquid inside the tanks has been generally treated as a rigid mass. However, for LNG carriers and FPSO vessels during loading and unloading cargo operations the ship motion is very important topic. Coupled global ship motion and tank liquid dynamic effect have been studied numerically and experimentally by Chen, X. B. et al.(2007), Gaillarde, G. et al.(2004), Rognebakke, O. F.(2003) and Kim J. W. et al (2005). FPSO vessels are operated at a fixed station point by some kind of mooring systems. Therefore in addition to estimate the ship motion ability the evaluation of the drift force acting FPSO is required from the operational view point. Newman (2005) conducted the ship drift force calculation considering tank liquid dynamic effect based on the commercial program WAMIT. The result showed that the tank liquid dynamic effect on the drift force is significant at sloshing resonance wave period. This suggest that during loading and unloading cargo operations between FPSO and shuttle tanker which are considering the tank liquid dynamic effect on the ship motion and drift forces are essential problem.

ABSTRACT: An experimental study has been carried out to investigate impact pressures on a slab and a vertical pile due to regular waves. The pressures on the bottom of the slab are measured by the pressure transducers and hydrodynamic loads acting on the pile are measured by a four-component balance. It has been found that in a wave period, wave pressure undergoes periodic variation along with time. Moreover, the influences of relative clearance height and length of the slab on the impact pressures are discussed. On the other hand, comparison is carried out thoroughly between the hydrodynamic force exerted on the single pile with and without the effect of the slab. 1.INTRODUCTION The problem of wave force acting on structures is of significant importance for the security of piers, platforms and slabs. A particular illustrative case is that the Ekofisk platform (Gebara, Dolan, Pawsey, 2000; Jha, Kiciman, Gebara, 2000). Isaacson and Prasad (1993) found that the slamming phenomenon was of a high frequency in nature. A number of publications related to wave slamming on horizontal cylinder or horizontal plate have been published. Both theoretical analysis and experimental results have been presented and discussed. Kaplan and Silbert (1976) developed a solution for vertical force exerted on a horizontal cylinder from the instant of impact to full immersion. For cylinders, wave slamming force is generally expressed by a slamming coefficient Cs. In recent years, experimental studies on the distribution of pressure along a horizontal plate have been performed in ever increasing numbers, and correspondingly various empirical relationships are proposed (Song, Bai, 1997; Zhou, Chen, 2003; Zhou, Chen, Huang, 2003; Ren, Wang, 2004). Zhou (2004) proposed that the maximum total uplift forces did not necessarily occur with the maximum impact pressure intensity synchronously.

ABSTRACT: New results of numerical hydrodynamic analysis of the novel offshore cross-flow vertical axis wind turbine with large floating rotor (WEMU design) are presented. The research goals are to investigate a spatial movement of rotary pontoon in incoming regular and irregular waves as well as their influence upon the water ring. Waves are launched by two components of flow velocity and free surface elevation at inlet boundary. Irregular wave is presented according to Pierson - Moskowitz spectrum by sum of first order waves with random phases. Wave loads acting on the pontoon are predicted and hydrodynamic power loss due to waves is calculated. INTRODUCTION Offshore wind power engineering is among leaders in the renewable energy sector already for years. And potentially, after solving essential technological and economical problems, its leadership can become absolute. Offshore wind power plants have obvious advantages against located onshore. First, their ecological restrictions are softer; blade speed can be higher, and therefore, an increased efficiency is reachable offshore. Moreover, visual impact of distant offshore wind farms is diminished and territory cost is practically excluded. At the same time, modern offshore turbines inherited their design (propeller installed on tall tower) from onshore and differ only by support structures. So, wellknown drawbacks of propellers could not be excluded. We must indicate here the limited unit power capacity, infrasound emission, and harmful effects on birds and animals. Wind energy cost is still higher than that from conventional power plants. Technological problems quickly grow in large-scale turbines so some research (e.g., Klinger F. et al, 2002) has revealed that an efficient 10MW turbine could not be developed in the one-propeller concept. In this case, wind energy cost will remain relatively high. A technological breakthrough in offshore wind power engineering is still absolutely necessary.

ABSTRACT: The dynamic analysis of the complete integrated deep-ocean mining pilot system is carried out in this paper taking into account the interactions between each subsystem and ocean environment. The selfpropelled seafloor tracked miner is modeled as a single-body vehicle with 6 degrees of freedom which allows real-time simulation and can be applied for the dynamic simulation analysis of the complete integrated mining system. The interaction model between the tracked miner and soft cohesive seafloor soil is built based on the theory of terramechanics and considering the principal dimensions of the tracked miner and the mechanical characteristics of seabed soil. The pipe subsystem including the lift pipe, pumps, the buffer and the flexible hose are built as 3-D discrete element models which are divided into rigid elements linked by flexible connectors with 6-DOF stiffness coefficients. External forces such as the hydrodynamic force of seawater, the buoyancy forces acting at discrete locations on the flexible hose, and other forces are considered. The virtual prototypes of each subsystem are modeled and connected to form the complete integrated mining system and the interaction models between mining system and environment are developed, then the dynamic simulation analysis of the complete integrated deep-ocean mining pilot system is performed and discussed. INTRODUCTION Interest in deep-ocean mining of manganese nodules and other metal deposits has developed as a result of rising metal prices, and out of concern for securing supplies of strategic and critical minerals. A typical and may be the most prospective deep-ocean mining system is an integration of self-propelled seafloor miner, miner-to-buffer flexible hose, buffer, pumps, lift pipe, mining ship and ocean transportation system. Due to the complexity of each subsystem and the coupled interaction between subsystem and ocean environment, the dynamic behavior analysis of the complete integrated mining system is difficult.

ABSTRACT: For these ten years, the author has investigated about unstable phenomena for high-speed planning craft experimentally and theoretically. In this paper, some of the unstable phenomena are introduced. The measured results of the motion of dynamic instabilities are shown and the characteristics of them are explained. The estimation method developed by the author to assess the possibility of their occurrence and some results are shown. And the mechanism of the instabilities is also explained in detail from the point of view based on motion equations. INTRODUCTION Planning craft are sometimes suffered from various unstable motions that are completely different from those for conventional displacement type vessels. Some of them are known as non-zero heel, bow steering, bow diving, chine walking, corkscrew, porpoising and so on. Occurrence of the phenomena has close relation with high forward speed. Therefore, it can be safely said that the phenomena occur only when hydrodynamic forces are dominant comparing with hydrostatic forces like buoyant forces. Then, the phenomena are generally referred to as dynamic instabilities (Blount & Codega, 1992). Although the phenomena are dynamic, the occurrences of them are fundamentally governed by stability characteristics created by hydrodynamic forces at high forward speed. The author has investigated the phenomena experimentally, revealed the mechanisms of the occurrences of some of them, and proposed several methods to assess the occurrences of them using measured hydrodynamic forces acting on a model towed steadily at high forward speed. In this paper, especially, some of the dynamic unstable phenomena for planing craft, that measured by Katayama et al. (1996, 1997a, 1997b & 2001) and Suzuki et al. (2004), are introduced. The measured dynamic instabilities are shown. The unstable phenomena are called porpoising and transverse porpoising, and the characteristics of them are explained.

ABSTRACT: A formulation for calculating the steady hydrodynamic loading on a slender ship moving at a constant forward speed and small attack angle in finite water depth is developed. The study is motivated by the need to predict the manoeuvrability of ship with large draft in relatively shallow water. The slender body assumption is used to reduce the problem into a series of two-dimensional boundary value problems that are solved using the boundary element method. The fully non-linear free water surface conditions are imposed on the instantaneous water level. The numerical calculations are performed for the Wigley hull model ship. Effects of the attack angle and finite depth are assessed in the subcritical region. INTRODUCTION Predictions of ship manoeuvrability have drawn recent attention after the introduction of the IMO interim standards on ship manoeuvrability in 1993. The ship manoeuvrability prediction requires amongst other things, knowledge of the ship resistance and propulsion characteristics. Part of the resistance arises from the hydrodynamic forces acting on the ship body moving at a constant speed in calm seas. The hydrodynamic force computation is further complicated in that the problem is non-linear due to the non-linear boundary conditions at the unknown free water surface. The computation of ship hydrodynamics had received very significant attention in the past. Maruo and Song (1990) had developed a boundary element solution for a slender ship based on Green's functions in three-dimensions and using the Kelvin sources which satisfied the linearized free surface conditions exactly. Results were presented for the cases of a Wigley hull, a sailing yacht hull and a Series 60 C B = 0.60 hull, with good agreement with existing experimental data. They found that the non-linearity had an insignificant effect on the water surface elevation alongside the ship hull but significantly affected wave resistance.

ABSTRACT: A brief overview is given of researches on the prediction methods of hydrodynamic force on a body moving in waves and having a steady -advance speed. Then two methods developed by the present author for computing hydrodynamic forces are presented and their results are discussed. These two methods are based on different assumptions of the steady flow field around the body. OVERVIEW Prediction of hydrodynamic forces on off-shore structures in current as well as in waves or towed in waves must be based on the correct understanding of current-wave interaction effect. There are other less obvious contexts in which this effect must be considered. Slow drift oscillation in waves is of so large amplitude and long period compared with wave motions that it may be considered as a quasi-steady movement. Damping of the motion may be predicted as a steady force when assuming they move at a steady speed in waves backward or forward. Tilt moment on the structures in waves is caused by vertical steady force. This will be affected when they make slow drift oscillations. That means the tilt moment is also to be predicted taking into consideration that the structures are moving in waves at steady speed. In this chapter I give a brief, I never intend it complete, review of recent researches on prediction of hydrodynamic forces on bodies moving at constant speed in waves. Description is restricted to the potential theories. Correct prediction of hydrodynamic forces on a floating body moving at steady velocity in waves has been mainly a concern of ship hydrodynamics. In the contexts-of seakeeping of ship, however, ship" s geometrical form is often assumed very slender. Prediction of those forces on bluff bodies such as off-shore structures is quite a different problem.

ABSTRACT: The characteristics of drag and lift forces on a circular cylinder model near a wavy boundary have been experimentally investigated in a water tunnel. In the range of the Reynolds numbers, based on the diameter of cylinder model, from 10 4 to 1.9 X 10 4 , the drag, lift force on the cylinder and lift force frequencies have been measured at the wave crest and trough and various distances from the wavy boundary. Flow visualization experiments revealed the wake structure varying as gap-to-diameter ratios (G / D) and mechanism Interacting between the cylinder and the boundary. INTRODUCTION In the offshore engineering, pipelines are used to convey oil and gas for power generation, sea-water for desalination, sewage for disposal at sea, and for the protection of communication cables. There are numerous problems associated with the design and installation of pipeline in deep-water offshore. The submerged pipelines are often fixed by concrete saddles and buried by dredging and trenching the sand at a seabed. It has been known from the practice that pipeline spans can occur due to local scour of the bed sediment The proximity of a pipeline near a seabed boundary gives rise to hydrodynamic and environmental problems m offshore engineering. Research on flow field and forces on the cylinder at various distances from the seabed boundary are of a great interest to many. There are many investigations (see Davis,1976) on forces on submerged pipelines In flow or wave. A lot of research works also have been carried out about wave forces on a cylinder near a plane boundary (see Sarpkaya,1981, Shankar et al.,1988). However, researches of hydrodynamic forces on a cylinder near a plane boundary in steady flows are relatively scarce. Second, the plane boundary changes the symmetry of flow about the cylinder nearby.

Abstract: Due to the constantly varying sea-states with which any wave energy conversion device must contend in order to efficiently extract energy the ability to control the device" s position relative to the incident waves is critical in achieving the creation of a truly functional and economical wave energy device. In this paper, the authors will propose methodology based on a variable structure system theory to utilize a three dimensional source distribution as a model to estimate anticipated surge, sway and yaw of a wave energy conversion device relative to varying angles and characteristics of incident waves and there from derive a feedback to a sliding mode controller which would reposition the device so as to maximize its ability to extract energy from waves in constantly varying ocean conditions. Introduction Previous attempt to efficiently extract energy from ocean waves did not achieve the desired results due to the high degree of structural complexity which must be incorporated into any wave energy conversion device attempting to extract energy from the non-linear hydrodynamic phenomena that characterize ocean waves. Ocean waves may come from any direction of incidence and have different characteristics of wave height length etc. The combination of these two sources of complexity make it extremely difficult to estimate or promote extracted wave power. as previously demonstrated both theoretically and experimentally(Kim and Park.1989) (Kudo. 1984). Therefore. a three dimensional source distribution method is used to estimate the hydrodynamic forces on a wave energy conversion device and to calculate the hydrodynamic forces according to the variations in length and direction of incident waves (Xim and Park.1989).(Kim and Park and Xim,1988). And the authors believe is possible to control the position of a wave energy conversion device by using a "water column" model" s three dimensional source distribution.