ABSTRACT This study considers the prediction of speed loss of ship in waves by using time-domain seakeeping-maneuvering coupled approach. To this end, the time-domain seakeeping analysis based on the three dimensional Rankine panel method is coupled with the modular type 4- DOF maneuvering equation. In the coupling procedure, the seakeeping performances, which are dependent on direction and encounter frequency of wave, and the maneuvering motions, which are influenced by wave-induced drift force, are directly interacted with each other in time domain. Especially, the drift force, which is the main cause of speed loss in waves, is calculated by the near-field method, namely, the direct pressure integration method. In order to validate the developed coupling method, the computed drift force and turning trajectory with the presence of an incident wave are compared with existing experimental data. Then, by implementing the trajectory tracking method based on rudder control, the free-running simulation for ship keeping a certain course in wind and wave is performed. The effects of environmental loads on speed loss are investigated by conducting the simulations for various environmental conditions including irregular seas and winds in different directions. Through this process, it is confirmed that the present free-running simulation can be used for evaluation of ship operation performances such as operation efficiency and course keeping. INTRODUCTION In recent years, the importance of ship efficiency and safety in the real sea state has been growing, and related regulations have been actively established. In particular, the Marine Environmental Protection Committee (MEPC), one of the specialized committees of the International Maritime Organization (IMO) introduced the regulation related with energy efficiency to reduce the amount of greenhouse gas emitted by ship. According to the regulation, the calculation of the Energy Efficiency Design Index (EEDI) becomes mandatory for ships sailing internationally. The EEDI includes prediction of the weather factor ( f w ), which means the speed loss of ship in actual operation conditions. Therefore, in order to predict the weather factor, accurate computations of the added resistance induced by environmental loads such as wind and wave are required as well as the calm-water resistance.

ABSTRACT In the present study, the two-dimensional tidal current and oil spill numerical model was established by using MIKE21/SA (Spill Analysis). After validation, the model was applied to simulate the diffusion of the oil spill under the second phases of the reclamation project in Bodaozui sea area. The influence of the wind and tide conditions were also analyzed. The results showed that the particles reached to the observation points faster during the ebb tide than the flood tide. The diffusion area of the oil spill was also larger during the flood tide. Under the condition of calm wind and flood tide, the particles reached Yangshan temple and Yangshan scenic zone after 19-21 hours. The diffusion area can increase to the maximum value of 1040.58 km 2 after 72 hours. While under the wind directions of SE, NNE and NNW during flood and ebb tides, the oil particles were unable to spread out of the bay due to the interception of the breakwater, but adhered to the south break water and the reclamation area instead. The surrounding area outside the breakwater was not affected by the oil spill. INTRODUCTION Oil spill has become a worldwide and the most destructive form of pollution to the marine ecological environment over the past few decades (Zhang et al., 2006). Oil spill is accidental and can have disastrous ecological and social consequences(Xu, 2006). The oil can penetrate into the structure of the plumage and threat the survival of seabirds. In addition, the direct toxicity of the oil spill can cause the death of fish, shrimp and shellfish in the surrounding area. When the oil spill enters to the shore area, it will affect the reserved wetland and coastal scenery. Besides, serious oil spill can cause fire and explosion, which cause damage to the ship or offshore facilities, and even cause injuries and deaths (Yu et al., 2017). Therefore, it is necessary to establish numerical and physical models to simulate, analyze and forecast the polluted range of the oil spill. The modeling work are also has great significance to the planning for the scientific emergency rescue and cleanup after an oil spill accident.

ABSTRACT In this study, installation process of pipe-type structure is conducted experimentally and numerically using dual floating crane vessels. Because installation structure is a long pipe, two crane vessels are required for the installation process. Two crane vessels are facing each other holding each end of the installation structure. In the experiment, two vessels are moored by soft spring, and various wave condition are considered. Measured items are 6DOF motions of two vessels and pipe, wire tensions at the end of the crane and mooring line tensions. In numerical analysis, motions of crane vessels are based on floating body dynamics using convolution integral, and crane wire is treated as simple spring. Installation structure is assumed as a rigid body with 6DOF motion. As in the experiment, numerical simulations in regular wave are performed. Characteristics of vessel motion and wire tension according to wave conditions are checked, and computation results are compared with experimental data. It was checked which frequency range is critical by reviewing the motions of dual crane vessels, and near the yaw motion resonance point of the pipe-type structure has the worst effect on crane vessel's motion. INTRODUCTION Floating crane vessels are used for various purposes such as translating, installation and decommissioning of offshore structures, and a high level of safety and precision is required during crane operation. Recently, to maximize the efficiency of the top side installation process, more than 5,000tons of a mega block is manufactured and installed using dual crane vessels (Jung et al. 2016). For these kinds of crane operations, a thorough review of the installation work should be made before the real-sea operation. Model tests related to floating crane vessel operation are a few. Nam et al. (2016) studied installation work of deep-water structures using single crane vessels with model test and numerical computation. Through the experiment, motion performance of the crane vessel and subsea structure and tensions of crane wire were examined, and change of the installation performance according to the presence of passive heave compensator was checked. Ha et al. (2018) analyzed the mating operation of a topside module by single floating crane vessel using model test and numerical calculation. Jung et al. (2016) built a new system to synchronize dual crane vessels into a single crane unit and carried out a model test to confirm the safety of lifting operation. In addition, they conducted sea trials of mega-module lifting operation using dual crane vessels and reported the results. Velema and Bokhorst (2015) undertook the real sea installation of a 9,500tons oil storage tank on the seafloor using a dual crane lift sequence. Important parameters such as ballast volume, compartment fill rate and line tensions of crane wire were monitored during installation work. China National Offshore Oil Corporation (CNOOC) launched mega jacket in South China Sea in August 2012. Corresponding model test and field test which were related with installation process were conducted. (He et al., 2013, Zhang et al., 2013, Yu et al., 2013)

ABSTRACT Based on a series of physical property tests and static triaxial tests of marine soft soil near shore in Zhejiang Province, China, the hyperbolic relationship is gained between the mean stress and the generalized shear stress value. Then the modified Rhodes stress angle is considered in the model. The Cambridge model is modified and make it more responsive to the static characteristics of marine soft soil. The UMAT material constitutive subroutine of finite element software Abaqus is compiled, and a modified Cambridge model of marine soft soil is developed. The static triaxial simulation test is carried out by Abaqus, and the rationality of the improved model is verified. The results show that the modified Cambridge model of marine soft soil effectively simulates the stress-strain characteristics of marine soft soil, the nonlinear characteristics of the soft soil and the characteristics of the plastic flow. INTRODUCTION As one of the stress-strain models describing incremental theory of plasticity (Roscoe, Schofield and Thurairajah, 1963), the Cambridge model (Roscoe, Schofield and Wroth, 1958; Roscoe, Barland, 1968; Zienkiewicz, Taylor and Zhu, 2013) is widely used in soil mechanics. Due to the differential soil properties, this model is modified in various specific applications, based on which, the cambridge model of marine soft soil should be modified on its shortcomings and the particular characteristics of marine soft soil. Y Chen (2002) considered the rheology of soil, based on the modified Cambridge model present a new modified Cambridge visco elastic plasticity model. YH Guo (2015) improved Cambridge model and got the accuracy of the elastic-plastic matrix improvement model, by using the Cambridge model to calculate the volumetric strain and shear strain of soil theoretically. There are also some researchers have improved the Modified Cam-Clay Model to make it compatible for overconsolidated soils (Amerasinghe, Kraft, 1983; Xu, Qi and Gao, 2008). In this paper, based on the physical mechanics experiments, by fitting the mean stress and the generalized shear stress value, the state line of marine soft soil is hyperbolic, and the modified Rhodes stress angle are considered in the model, the further improvement of the Cambridge model and make it more responsive to the static characteristics of marine soft soil, which is validated by the secondary development of UMAT material constitutive subroutine in Abaqus.

ABSTRACT With the development and utilization of ocean resources, maritime transportation is becoming increasingly busy. Stopping ability has great effect on the safety of ship maneuvering for those large ships. In this paper, naoe-FOAM-SJTU solver with 6DOF motion module with a hierarchy of bodies developed by Wan Decheng's research team in Shanghai Jiao Tong University is used to numerically investigate complex ship motion problem in stopping maneuver. The simulation starts from the steady state of self-propulsion and the propeller is controlled to a reverse speed to achieve the ship stopping condition. Detail information of the flow field during stopping maneuver are presented and analyzed to explain the stopping effect. The predicted results for the stopping maneuver in calm water are compared with the corresponding experimental data. The comparison is satisfactory and shows that the naoe-FOAM-SJTU solver is feasible for the direct simulation of stopping maneuvers. And the calculation results can provide suggestions when designing a ship or choosing stopping method. INTRODUCTION In recent years, ships tend to become larger for reducing the cost of shipping and improving the transport efficiency. Since the large size worsens the maneuverability, accidents can easily occur in the crowded ports and channels. Therefore, it is significant to study the stopping ability of large ships to prevent the ship from collision and to ensure the safety of the ship sailing near the port. Generally, reversing the propeller is still the most common operation when a large ship needs to brake to prevent collision. In the procedure of stopping manuever, the bow will turn left or right because of the side forces at the aft caused by reversing propeller. The existence of the transversal force caused by reversing propeller is determined by Chislett and Smitt (1972) through a ship model test. The stopping trajectory is shown in Fig. 1. Good stopping ability means minimum stopping distance, horizontal distance and yaw angle.

ABSTRACT This study carries out a dynamic analysis of the installation operation using offshore cranes in an ocean environment. The analysis is based on multibody dynamics considering various types of external forces, such as wave, wind and current in an ocean environment. The target operation is an installation of offshore structure using dual offshore cranes in the ocean. During the operation, two offshore cranes lift, turn over, and install offshore structure. The motions of offshore structure and the loads acting on the wires from the analysis were verified by comparing with the model test. INTRODUCTION Background Offshore cranes are essential for heavy equipment related offshore operations such as offshore plant and subsea equipment installation. As offshore operations take place in extreme environments, the weight of the equipment targeted at sea operations is increasing. Thus, to overcome this, operations may be carried out using large offshore crane or multiple offshore cranes as shown in Fig. 1. Large offshore cranes are advantageous in that they can lift heavy objects by using one offshore crane, but they are influenced by the movement conditions of the offshore crane and the seabed conditions of the offshore installation for mooring. Therefore, if it is difficult to perform an operation with a large offshore crane, it is necessary to perform a combination operation using several smaller offshore cranes. In the case of combination operation using multiple offshore cranes, there is a need to synchronize the operation of two or more cranes, which reduces the safety of the operation compared to using one offshore crane. Therefore, we must perform an analysis to confirm the stability of the operation in advance. In this paper, we have developed a simulation for offshore installation operation based on a prototype of an integrated framework for evaluating and verifying stability to prepare marine operations using multiple cranes. The developed prototype confirms the problems that may happen in the offshore operation process using multiple crane such as crane collision, disconnections of wire rope due to excessive dynamic load, and position fixation of crane barge due to mooring design error. To apply various offshore tasks in the future, the framework extensibility is considered. To this end, a framework is designed to expand and reduce functions using a plug-in modular design.

ABSTRACT Sinusoidal motion of a cylinder in viscous flow has been extensively studied in the past decades. Distinction of flow patterns exists between cylinders in cross-flow freedom restricted and freely vibrating conditions when experiencing oscillatory flow. In this paper, a series of numerical simulations are carried out by the in-house CFD code naoe- FOAM-SJTU, which is developed basing on the open source code OpenFOAM with overset grid capability. The diameter of the cylinder is 0.02m and the KC numbers varies from 3 to 12 corresponding to the attached vortices regime and the transverse street regime. Results of vortex evolution, flow regimes and hydrodynamic force coefficients are compared. INTRODUCTION In actual production, offshore floating structures subject to waves, currents or winds will cause the platform to move periodically in the water. Then relatively oscillatory flow is generated between the riser and the water. In recent decades, researches of the sinusoidal motion of a cylinder in viscous fluid have been extensively studied by Bearman (1984, 1985), Sarpkaya (1986,1995) and Williamson (1985). Williamson (1985) conducted a series of experiments to investigate development of vortices around a single cylinder in relative oscillatory flow. And several vortex regimes were identified within particular ranges of Keulegan-Carpenter (KC) Numbers: the attached vortices regime (0#x003C;KC<7), where no major vortices shed during a cycle; the single pair regime (7#x003C;KC<15); the double pairs regime (15#x003C;KC<24); the three pairs regime (24#x003C;KC<32) and the four pairs regime (32#x003C;KC<40). For further KC regimes, the number of vortices pairs shed in each oscillating period would be increased by one each time the KC regime changed to a higher one. Kozakiewicz et al., (1996) conducted experiments of a cylinder exposed to oscillatory flow for two Keulegan-Carpenter numbers, KC=10 and 20. Then numerical simulations of a cylinder freely vibrating in the cross-flow direction were carried out at the same KC numbers. Comparisons showed that the number of vortices generated over one oscillating cycle increased when the cylinder was freely vibrating in the cross-flow direction. The vortex shedding direction changed to the opposite side of the cylinder in the transverse street regime when KC=10.

ABSTRACT Based on the CFD-CSD (Computational Fluid Dynamics-Computational Structure Dynamics) coupled model, the antiliquefaction effect of stone layers on liquefiable seabed is studied. Under the extreme waves, the dynamic response of the seabed around the wind turbine in the Xiangshui area of Jiangsu Province was simulated. By comparing the seabed response results of four tests with cover stones under the same wave conditions, it shows that the thickness and porosity of the cover stones are two important parameters of anti-liquefaction capacity. INTRODUCTION The stability of composite bucket foundation of offshore wind turbine under wave action is very important for the development of offshore wind power technology (Zhang et al., 2016). Under the action of wave, the seabed liquefaction occurs due to periodic changes in pore water pressure and effective stress in the seabed. It is of great scientific significance and great engineering value to study the liquefaction and anti-liquefaction measures of the seabed soil under the action of waves for the stability of the offshore wind turbines, especially for the steady development of the offshore wind power foundation. Based on the analytical method, experimental study and numerical simulation, the study of liquefaction and stability of seabed under wave action is mainly concentrated on three aspects: the pore water pressure in the time and space, the effective stress state and shear strength in the seabed are analyzed (Hsu and Jeng, 1994; Jeng, 1997; Jeng, 2013; Ye et al., 2018); the wave- structure-seabed interaction is investigated (Sumer, 2014; Ye et al., 2015; Zhang et al., 2016; Sui et al., 2017); in order to accurately analyze the liquefaction of seabed, a series of seabed liquefaction standards have been put forward (Ye, 2012), and a series of anti-liquefaction methods have been studied (Yang et al., 2004; Susana and Rafeal, 2006; Sumer et al., 2010; Zhang et al., 2014; Huang et al., 2015).

ABSTRACT Response in a porous seabed under dynamic environmental loading is a vital engineering issue in marine geotechnics. Lots of investigations for seabed response under dynamic loading have been developed through mathematical, numerical and experimental approaches. Most previous numerical models for seabed response in marine environments were based on finite element models. In this paper, based on local radial basis function collection method (LRBFCM), a meshfree model is proposed for the seabed response in the marine environments. In the present model, partial dynamic approximation ( u-p approximation) will be used, and three different types of natural loading will be considered, i.e., wave, current and earthquake loading. INTRODUCTION In the last twenty years, more and more marine structures are constructed with the deeper exploration and study for the offshore area. The most important aspect to be considered in engineering practice is the stability after putting in use of those marine structures under the complicated environment loading. In general, three types of the environmental loading needs to be taken into account for the design of marine structures, which are ocean waves, currents, and probable earthquake respectively. The dynamic response under these loading has attracted great attention among coastal and geotechnical engineers due to the growth of activities in marine environments. As the conventional loading, how ocean wave and current affect the marine structure stability is a vital problem for coastal engineers. In general, the propagating ocean wave will generate the dynamic pressure in the sea floor, which may trigger soil liquefaction of the seabed as reported in the laboratory test (Sassa and Sekiguchi, 1999). Meanwhile, the effect of earthquake is also important for engineering design. Although the probability of earthquake occurred nearby the marine structures is not so high, once the earthquake happened, the damage would be devastating. As one of the major natural disasters need to be considered in structure design, earthquake is also able to liquefy the saturated soil through seismic shaking effect. The liquefaction phenomena induced by seismic wave was fully aware by the public from the Niigata earthquake in 1964 in Japan, which caused unprecedented damage. The problem of earthquake-induced liquefaction attracted a great deal of attention of geotechnical researcher and great achievements have been made in the past (Seed et al ., 2003). However, as pointed by Ye and Wang (2015), most of the studies for earthquake loading are concerned with onshore structures, while only a few studies considered offshore structures whatever by experiment or numerical simulation. For the earthquake loading, Chen et al . (2018) has developed the analytical solution for layered porous seabed under vertical seismic motion.

ABSTRACT A series of numerical simulations were carried out to investigate the current generation performance in deepwater ocean engineering basin (DOEB) of KRISO. In the numerical simulations, unsteady Reynolds averaged Navier-Stokes (RANS) equations were solved to study the flow characteristics of the current generated in the basin. Three different current profiles, i.e. uniform, Gulf of Mexico (GoM) and Brazil profiles, were directly generated by applying the same current generation system as the DOEB and their accuracy was examined. In addition, an optimization algorithm based on cost function is newly proposed to control the inlet and outlet velocities of the multiple culverts for the desired current velocity profile. Through various numerical simulations, discussion are made on free surface elevation, effect of initial ramping operation, effect of false bottom plate, and full 3D simulation. INTRODUCTION Nowadays, large amount of the total global production of oil and gas is composite of offshore resources which only may be reached with the help of new floating structures such as Floating Production Storage and Offloading (FPSO), Tension Leg Platform TLP, and SPAR platform, among others. Offshore systems are exposed to extreme environmental conditions for example wind, current and waves. Current accounted largely in the total load of offshore systems with deep draft such as SPAR platforms. Risers experience vortex induced vibrations (VIV) due to current loads. Floating systems are affected by different current loads based on its depth and location in the world. Current velocity profile in Brazil, North Sea, Gulf of Mexico, Caspian Sea, Persian Gulf, and Yellow Sea among others have been monitored and studied. Characteristics of current velocity profiles were recognized at each particular site. Hydrodynamics performance of offshore structures due to current loads are investigated through scaled model test. Current loads are modeled mainly in towing tank by towing the model at constant speed and ocean basin through a local jet and other techniques; however, previous method may model only uniform current profile and realistic current profiles cannot be generated. Therefore, some institutions and universities built a new kind of facilities which can generate more realistic environmental loads. Characteristics of these new facilities are described in Alho et al. (2008), Lu et al. (2010), and Bucher and de Wilde (2008). Representative numerical and experimental studies performed are presented next.

ABSTRACT In recent years, there has been several cases of mooring line failures on various floating offshore units. In several of those cases, the failures were identified months or years after they initially occurred. Most assets being designed for single line failure only, this means that the risk of a catastrophic failure of the whole mooring system is quite high if the failure of a single line cannot be detected reliably. To mitigate that risk, class societies such as DNV GL have introduced requirements for the use of lines tensions monitoring systems as part of the mooring class notations (POSMOOR). However, most of the line tension monitoring systems available on the market today have proven unable to remain functional after more than 2 years in operation, due to the harsh conditions and loads they are exposed to. For that reason, an alternative system for line failure detection is needed. In this paper, a system is developed to detect reliably a single line failure based solely on GPS and motions sensors data installed on the asset. The GPS and motion time series are used to train a neural network which can then reproduce any motion signal as a function of the others, capturing all the complex nonlinear correlations between the wave frequency and drift motions of the asset along its 6 degrees of freedom. Any change in the mooring system properties such as a line break has an impact on those correlations, this change is captured by the neural network, therefore enabling it to detect a line failure. The accuracy of the system is demonstrated using numerical simulations for an FPSO in various sea states, where a line break occurs at one time instant during the simulation. INTRODUCTION A moored floater is a 6-DOFs system where the loads are stochastic environmental loads, wind waves and current. The behavior of the system in a given sea state is governed by its inertia, damping and stiffness properties. The failure of one or several mooring lines has an obvious effect on the stiffness, but also on the damping of the whole system since the drag on the lines is one of the damping contributions. The floater displacement in the same sea state will therefore be different for an intact or damaged system. Those differences might not be easily detectable since depending on the mooring system configuration, and the sea state, a line failure might not change significantly the basic statistics of the motions such as mean and standard deviation. In those cases, changes in the higher order statistics, as well as in the correlations between different degrees of freedom motions need to be detected to identify a line failure.

ABSTRACT Because of the continuous advances in computational fluid dynamics, the use of numerical simulation to assist ship and marine structure hydrodynamic performance prediction has become common. This paper presents a numerical study of the generation and propagation of long- crested wave. ITTC double-parameter wave spectrum is chosen for the study, and the wave spectrum is spilt based on equal-frequency method. The self-developed MPI based in-house code is used for the study. The computations are carried out by a PC cluster. In this study, the finite difference method is used to discretize the partial differential equations, and the conservative level set method is used to capture the free surface. The behavior of the free surface elevation along the basin and over time has been evaluated by time domain waveform comparison with theoretical solutions, frequency domain and statistical analysis. The results obtained for the free surface elevation for numerical wave tank indicated that the numerical model could correctly capture the wave profile. The results in this study show the in-house code is reliable and could be used to simulate the complex 3D wave and flow fields. INTRODUCTION At present, there are two kinds of irregular waves studied. One is the long-crested wave, which can be understood as the combination of different regular waves with the same direction. Therefore, in recent years, the simulation of wave environment is mostly aimed at regular wave, and the simulation of long-crested wave is not much. Therefore, it is necessary to conduct more simulation research on long-crested wave. Numerical wave tank is a very effective method to study wave in recent years. Since the calculation of the numerical wave tank is carried out by computer, the numerical wave tank can save a lot of cost compared with the traditional physics wave tank, and it is also easier to monitor and observe the features of wave. In recent years, many researchers have devoted themselves to the development and construction of numerical wave tank. In this paper, based on self-developed MPI code, the simulation of long-crested wave is carried out by numerical wave tank, so as to have more understanding of irregular waves, and evaluate the reliability of the numerical wave tank developed.

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 This study presents results of numerical simulations on the effects of the presence of breakwaters on the OWC (Oscillating Water Column) performance. CFD (Computational Fluid Dynamics) simulations using a commercial code, Star-CCM+, are done for evaluating performances of the OWC without and with considering the breakwaters in regular waves. Flow fields of the OWC in regular waves are shown as continuous contours, and the effects of the breakwater on the performance of OWC are evaluated. Finally, the cause of the performance improvement and reduction of OWC due to breakwater is discussed. INTRODUCTION Economic feasibility of the OWC (Oscillating Water Column) WEC (Wave Energy Converter) can be improved by combining it with existing or newly constructed breakwaters. The OWC WEC, which has a sloped chamber with same inclination as the breakwaters, has been developed for installation in a breakwater on isolated islands since 2016 by the R&D Project funded by the Korean government (Park et al., 2018). Demonstration tests of the developed breakwater OWC will be done in the future after completion of construction for the OWC with modifying the existing breakwater in Chuja Island, Korea, based on the design for the integrated system composed of OWC chamber, turbine, generator, controller, energy storage system, and micro-grid system. Fig. 1 shows a conceptual diagram of the present OWC system integrated with the breakwater. The OWC chamber producing a reciprocating airflow that operates a turbine from ocean waves functions as the device for primary energy conversion. Earlier studies had performed experimental and numerical analysis for OWC chamber without considering the presence of the breakwater (Elhanafi et al., 2017; Vyzikas et al., 2017; López et al., 2015; 2014; Iturrioz et al., 2015; Kamath et al., 2015; 2015). They evaluated the performance of the independent OWC by measuring or calculating wave elevations in the chamber, the differential pressure between the inside of the chamber and atmosphere, and airflow speed through the duct. In addition, interactions between water surfaces in the chamber and the turbine were considered by modeling it as the orifice representing a damping effect for an impulse turbine of the present OWC system.

ABSTRACT This paper focuses on the inline force and bow wave height on a surface-piercing vertical cylinder moving at a constant forward speed in calm water and in waves. A Computational Fluid Dynamics (CFD) model has been developed using Caelus ® open source code. A stationary mesh was developed and the wave libraries were modified to generate current of a speed equal to the velocity of the moving cylinder while still prescribing the desired wave profile. The capability of the CFD model has been illustrated by the very good agreement achieved against results from physical experiments performed in a towing tank for various conditions of increasing complexity. INTRODUCTION Cylindrical structural elements are commonly used in offshore and coastal engineering structures such as jacket, jackup and tension-leg platforms and marine pipelines. Therefore, extensive studies on such structures have been conducted over recent years for multiple objectives. For instance, Xiong et al. (2015) experimentally studied the inline force acting on a stationary truncated vertical cylinder of a constant diameter subjected to regular waves. They found that the measured force is influenced by the submerged depth of the cylinder, wave steepness and scatter parameter (function of wavelength and cylinder diameter). Li and Ye (1990) investigated lift and inline forces on a vertical cylinder due to random waves and currents using the Morison equation and found relationships between drag, lift inertia coefficients and Keulegan-Carpenter number. Similarity, Koterayama (1984) studied the hydrodynamic forces and coefficients of a submerged circular cylinder moving forward with a constant velocity in regular waves. With advances in computational resources, Computational Fluid Dynamics (CFD) modelling has drawn the attention of several researchers interested in drag forces and flow behaviour around cylindrical structures. These models upon validation represent a viable and feasible tool to examine various designs and capture detailed physics. For example, Shao et al. (2013) compared the drag coefficient of a surface-piercing cylinder yawed at different angles in a steady flow and concluded that the drag coefficient differs significantly from that on a vertical cylinder, especially for yaw angles larger than 15°. Yan et al. (2015) investigated the forces acting on fixed and moving cylinders subjected to focused waves. In their study, a CFD model using OpenFOAM ® open source code was used to mimic the experiments for the case where the cylinder was fixed. To avoid any negative effects a current might have on wave focusing time and/or location when replicating the more complex experiments of the moving cylinder in waves, they replaced the CFD model by a numerical model based on fully nonlinear potential theory. Suh et al. (2011) developed and validated, in very good agreement, a CFD model using CFDShip-Iowa to capture detailed flow field and vortex shedding for an interface piercing circular cylinder subjected to a uniform flow of medium Reynolds and Froude numbers. Using the validated model, Koo et al. (2010) extended this work to investigate the effect of Reynolds and Froude numbers. Kim et al. (2015) presented validated CFD results for hydrodynamic forces and pressure coefficients of flow past a circular cylinder at subcritical Reynolds numbers, then examined the effects of Reynolds number on the statistical characteristics of the cylinder wake and the shear-layer instability.

ABSTRACT Floating Production Storage and Offloading (FPSO) vessels for offshore operations use a Dynamic Positioning system (DP), which includes a controller to correct the position variation of the FPSO subject to internal and external disturbances. Most of these positioning systems use a classic Proportional Integral Derivative controllers (PID) and their deferent variants, where the control law is determined adjusting three control gains: proportional, integral and derivative, usually heuristic techniques are employed to determine the control gains. In this study we propose a theoretical tuning procedure in order to determine the control gains in a simple way, analyzing the boundary conditions in the matrices of the FPSO dynamic model, as well as the relation they have with the control gains for the FPSO motion in an adjust domain. In order to guarantee the semi-global stability of the closed-loop system, a stability proof in the Lyapunov sense is carried out. The theoretical results were validated in numerical simulations using Matlab. These results show that the methodology presented in this work is highly satisfactory for the control gain selection in the trajectory tracking control problem for FPSO motion. INTRODUCTION A Floating Production, Storage and Offloading (FPSO) system is considered in this paper. In the last two decades FPSOs have been the dominant offshore platforms used in oil and gas fields. Fig. 1 shows the Ta'Kuntah FPSO, which was the first FPSO operated in the Cantarell Field in the Gulf of Mexico. A FPSO can be operated in deep water and sometimes must perform maneuvering movements and marine activities with other vessels as shown in Fig. 2 (Tamuri, 2009), these activities involve huge risks due to the environmental random forces presence, affecting the normal operation and sometimes causing severe accidents (Moan, 2002) and lack stability (Chen, 2008). In offshore basically there are two ways to maintain the FPSO position, the first is by Mooring Positioning (MP) and the second by Dynamic Positioning (DP) (Sorensen, 1996; Sorensen, 1997). In DP the principal advantage is the immediate positioning on a required set point, in other hand the MP systems are limited to operate about 500m (Veksler, 2016).

Abstract This paper focuses on an innovative modeling approach of a floating body in a continuous wave current. This approach is Lagrange-based providing finite particles, which represent discrete hydrodynamic properties in contrast to continuous hydrodynamics. It is implemented into a computationally efficient numerical multi-body simulation program, which is flexible in geometry and dynamic load spectrum. It features a particle-based liquid model and provides time accurate hydrodynamic pressure results in three dimensions and 6 Degrees-of- Freedom. Based on the results of dam break tests and sloshing analysis, unsteady impact loads are computed for a floating body in a closed-loop simulation. The wave current is modeled as a continuous flux of particles with velocity-based boundary conditions at the inlet and outlet. Thereby, the floating body geometry is flexible in terms of wall friction, elasticity laws and damping coefficients. A special focus is set on the wave interaction characteristics and the resulting acceleration of the floating body due to the hydrodynamic forces. Introduction Dynamic loading of floating bodies is one of the key issues for an improved efficiency in offshore installations and ship operations. Especially the understanding of the hydrodynamic load distribution over the floating body surface in a wave current and the peak pressures during that interaction are of importance. A scientific approach to the floating body dynamics consists of a step-by-step analysis of the physical phenomena involved. First, the flow pattern around a fixed reference body needs to be understood before a floating body is envisaged. In addition to that, in that early step a submerged body undergoing pure hydrodynamic forces is considered, which does not consider buoyancy effects of the real floating body. For that case Arnold et al. (2015) have proposed an experimental setup to analyze submerged body dynamics with the help of a submerged pendulum.

Abstract RANKINE source method applying continuous free surface panels is developed to examine two-dimensional linear wave-current coexist problems. The wave-current interaction problem is solved by combination of steady and unsteady potential problem respectively. The boundary integral method can be directly applied to examine the effects of current. Numerical results for both radiation and diffraction problems with current effect are compared with available data and reasonably good agreement is obtained. Apart from some well-known conclusions about the current effect, the pressure distribution of wetted body surface are extensively studied and some new phenomena are observed from our numerical prediction. Introduction Accurate predictions of hydrodynamic loads on floating structures subjected to wave and current are of practical importance in offshore engineering. When the current speed is small the effects of flow separation are usually unimportant and therefore the wave-current interaction problem can be investigated within potential flow framework. The radiation and diffraction problems in the absence of a current have been investigated extensively by a free surface Green function model or a Rankine source approach. The former model involves with a complicated free surface Green function which requires the singular integral on the free surface boundary (Wehausen and Laitone, 1960). In contrast, the latter (Beck, 1994) only needs to calculate the integration of Rankine sources on free surface and body boundaries. In the presence of current or forward speed, the free surface Green function model needs to be modified to include the effects of current or forward speed. By following the forward speed Green function originally derived by Haskind (1944), Grue and Palm (1985) studied wave radiation and diffraction problems for a two-dimensional submerged circular cylinder in frequency domain. Wu and Eatock Taylor (1990) developed a new mathematical formulation based on a perturbation series in terms of forward speed and the hydrodynamic force can be directly obtained without forward speed involved. Nossen et al. (1991) derived the Green function for small forward speed in the form of zero-speed Green function and its derivatives. Generally significantly computational difficulties arise due to the complexity in evacuating the forward speed Green function (Sen, 2002).

Abstract In this study, three-dimensional numerical simulations are performed to study the VIV of two side-by-side cylinders of different diameters in a steady flow. The two cylinders are rigidly connected together and elastically mounted as a single body. The main aim of the study is to identify the difference between the response of the two cylinder system and that of a single cylinder. It was found that the lock-in range of the reduced velocity of the two cylinder system is similar to that of a single cylinder. However, if the sum of the two cylinder diameter is used as the representative dimension of the system to calculate the reduced velocity, the lock-in regime is narrower that of a single cylinder. Introduction Vortex-induced vibration of a circular cylinder has been studied extensively due to its engineering significance. It is well known that the vortex shedding frequency and the vibration frequency of the cylinder synchronize in a range of reduced velocity, and this range of reduced velocity is commonly called the lock-in range. The reduced velocity is defined as V r= U /( f n D ), where U is the fluid velocity, f n is the natural frequency of the cylinder and D is the cylinder diameter. Comprehensive reviews of VIV of a cylinder can be found in Sumer and Fredsøe (1997) Sarpkaya (2004), Bearman (1984) and Williamson and Govardhan (2004, 2008). Recently, many numerical studies have been conducted to study VIV of cylinders. Due to the intrinsic three-dimensionality of the wake flow behind a vibrating cylinder, it is preferable to perform threedimensional simulations to study VIV. Recently, numerical studies of VIV of a circular cylinder have been extended from two-dimensional (2D) to three-dimensional (3D) simulations. Kondo (2012), Lucar et al. (2005), Navrose and Mittal (2013) and Zhao et al. (2014) studied VIV of a circular cylinder by solving the 3D Navier-Stokes equations and found some mechanisms of VIV that could not be found using 2D numerical models. For example, Lucor et al (2005) found that the correlation of the lift force along the span of the cylinder was poor at the reduced velocities in the hysteretic range between the upper and lower branches, and Zhao et al. (2014) found that the flow in the wake of a vibrating cylinder at Re=1000 is dominated by the streamwise vortices in the lock-in regime. Due to the limitation of the computer power, three-dimensional numerical studies of VIV are still performed at relatively low Reynolds numbers in the turbulent wake flow regime. For example, all the above examples of 3D numerical studies have been performed at Reynolds numbers less or equal to 1000.

Abstract The flow around a circular cylinder at Reynolds numbers near the drag crisis was numerically simulated based on a dynamic Smagorinsky Large Eddy Simulation (LES) model at two Reynolds of Re = 1.0 × 10 5 and 6.0 × 10 5 . The flow structures and the hydrodynamic force on the cylinder were analyzed. The numerical results show that the results at Re = 1.0 × 10 5 follow the key feature of subcritical turbulent flow and the results with Re = 6.0 × 105 capture the main features of the supercritical flow. This demonstrates that the Dynamic Smagorinsky LES model is a suitable numerical method for exploring high Reynolds flow around a circular cylinder. Introduction Slender structures are commonly used in civil, mechanical and offshore engineering, such as skyscrapers, chimneys, tubes in heat exchangers, bridge piers, subsea pipelines, risers and supporting frames of offshore platforms. Due to the strong engineering application background, hydro-/aerodynamics associated with slender structures have been widely researched. The flow around an isolated cylinder involves most of the generic flow features (flow separation, vortex shedding, recirculation, transition of turbulence within the wake and in the boundary layer), thus providing an excellent model for gaining insight into fluid mechanics around structures. As an important flow-structure interaction model, large amount of research about flow around circular cylinders has been published (Sumer and Fredsøe 1997, Zdravkovich 2003).