Currently, with the saturation of the site selection of onshore wind farms and the negative impact on residents, exploiting the offshore wind energy at deep water has drawn the attention of the industry and governments. As a result, it is necessary for us researchers to study the interaction between multiple floating wind turbines at offshore wind farms. In this paper, the mutual interference of two floating offshore wind turbines is presented. Compared to the onshore wind turbine at the same wind condition, the energy efficiency of single floating offshore wind turbine decreases about 5%. In addition to the rotational motion of wind turbine itself, some other forms of motions, such as roll, pitch, heave motions of the plate, must be taken into account. The existence of all these motions constitutes the biggest difficulty in studying floating offshore wind turbines. We studied the effects of different lateral distances and longitudinal distances on the performances of two floating offshore wind turbines. The change of lateral distance has little effect on pitch motion and energy efficiency, but has a great impact on roll motion. However, the change of longitudinal distance has a great impact on the energy efficiency. INTRODUCTION As the industry comes to the 21st centenary, with the continuous improvement of onshore wind energy development technology and because of the fewer and fewer alternative onshore wind farm sites, people have begun to set their sights on the vast ocean (Peter E et al, 2016). Compared with onshore wind energy, the offshore wind energy has many advantages, such as more stable wind situations, less impact on residents and more available locations(Strachsonsalla M et al, 2016). However, each coin has two sides, the offshore wind energy also has its disadvantages, such as higher structural strength requirements, more complex load action and more freedom of movement. During the past decades, the installed capacity of offshore wind power has grown rapidly, with an average growth rate of more than 80% (U.S. EIA et al, 2018). In April 2019, the Global Wind Energy Council released the global wind energy report of year 2018 (GWEC, 2019). In this report, by the end of 2018, global offshore wind power installed capacity was about 23140MW, an increase of approximately 462% compared to 2011. In addition, it is expected that by 2023, the global offshore wind installed capacity will reach 54,000 MW. On October 18, 2017, Hywind , the world’s first floating wind farm, was officially put into operation on the east coast of Scotland, whose average wind speed is about 10m/s (Amanda Saint, 2017). It occupied an area of about 4 square kilometers, and its installed capacity is 30MW, composed of five floating offshore wind turbines. The largest floating wind farm under construction in the world is Hywind Tampen in Norway, with an installed capacity of 88MW (ALOWPCE, 2019). It is scheduled to be operated in 2022, and will directly power offshore oil platforms instead of being integrated into the onshore power grid.

To solve the problem of large roll motion of mono-hull ship of 1000-ton class under high sea conditions, three kinds of ship types are designed including Angle, Round Bilge and Mixed hull types. Roll damping of these ship types are calculated by CFD method, meanwhile, model tests are carried out to verify the numerical results. The results showed that, the numerical method is in good agreement with the model tests. The angle hull type has the faster roll decay speed and larger roll damping than other two ship types. Through the analysis of roll damping components, the proportion of damping components of three types of ship is obtained. This research provides a reference for the development of mono-hull ship type. INTRODUCTION About 1000 tons of mono-hull ship is widely applied in practical ship and offshore engineering, including patrol ships, deep sea fishing boats, small and medium-sized rescue ships, small transport ships and so on. Compared to the ship of large displacement, because of the limitation of tonnage measurement of this kind of ship, the seakeeping especially rolling performance under the high sea condition is bad. Anti-rolling devices such as bilge keels and fin are often used, but the space of the ship is reduced. Good ship shape design optimization can improve the roll performance from the inherent characteristics of the hull on the basis of adding anti-roll measures. Referring to the influence of different hull types of mono-hull ship on roll damping (Zhang and Liu, 2001) and ship design experience, three ship types of widely varying in transverse section for the design of an ocean rescue ship of about 1000 tons are proposed in this paper, namely Round bilge, Angle and Mixed ship type. The study of roll damping has always been a hot and complicated research field of ship hydrodynamics. The study of roll damping characteristics of three hull types with large variations is the premise to study their roll motion performance, which is of great significance. Model test and empirical formula are the conventional methods of roll damping prediction. In the past two decades, CFD technology has been applied to calculate roll damping (Korpus and Falzarano, 1997), which can better reflect roll damping nonlinearity caused by viscosity of water. Yu et al. (2008) simulated forced roll motion of four different hull sections with 2D solution by CFD, and gave the time history of roll moment and roll damping coefficient under different roll Angles. The same method was used to simulate the rolling motion of a mono-hull ship by Zhu (2009). Zhang et al. (2017) studied the property of nonlinear roll damping of a trimaran ship. Metin (2018) and Simone et al. (2018) used dynamic mesh and overlapping mesh technology to simulate roll attenuation of DTMB5415 ship, and uncertainty analysis was carried out.

ABSTRACT The paper puts forward a theoretical capsizing probability calculation model that is suitable for the preliminary design stage. Based on the risk-based design concepts, the capsizing probability calculation model is nested into the traditional ship form optimization process. Thus, the hull form modification model based on capsizing risk forecasting is obtained. In this paper, the cruise ship of the Yangtze River trunk line is considered as a parent ship. Ship form parameters are the main optimization objects that are constrained within a certain range. The maximum roll angle and capsizing probability in regular waves are used as objective functions. Optimization decision is made between design factors such as intact stability indicators and economic indicators. Therefore, the ship's capsizing rick can be effectively evaluated and forecasted during the preliminary design stage. INTRODUCTION In recent years, cruise tourism has become more and more popular. Taking the Yangtze River Basin as an example, by 2018, more than 40 luxury cruise ships with four-star or above will have a combined capacity of about 14,000 passengers and a total reception capacity of about 700,000 passengers per year(Feng, 2018). As the cruise ship of the inland river is developing towards luxury and large scale, ensuring cruise's safety performance is becoming a topic of general interest. For cruise ships, the intact stability is especially important. Once the ship is capsized, it will cause heavy losses to the lives and property of the passengers. Unlike the small ferry, the inland river luxury tourist cruise ship has large dimensions, and has the characteristics of shallow draft, developed superstructure and high center of gravity. When encountering wind and waves, it is prone to occur large roll motion or even overturning accidents. Over the years, people have been persistently exploring the intact stability criteria that can guarantee the safety of ships. Huang et al. (2001) studied the calculation of ship capsizing probability under the combined action of crosswind and wave, and solved the equivalent linearized differential equation of roll motion. Liu (2004) studied the ship stability under the combined action of crosswind, beam wave and wind-wave, simplified the non-linear roll equation by harmonic balance method and multi-scale method, and derived the higher-order equation of roll angle and encounter frequency. Hoon (2006) studied the calculation of ship capsizing probability by solving the joint probability density function. Arena et al. (2013) considered the maximum roll motion of a ship in confused sea waves, and showed that a reliable estimate of the maximum roll motion is found considering the ship response to an approximate deterministic representation of an appropriately large and adequately rich(frequency-wise)load.

ABSTRACT To improve survivability on damaged ship, cross-flooding devices have been widely used. In this work, the flooding process of a damaged cruise ship with cross-flooding devices is investigated using CFD method. The numerical simulations are carried out on naoe-FOAM-SJTU solver developed based on the open source code OpenFOAM. The incompressible URANS equations are adopted as the governing equations. The VOF method, dynamic grid technique and a six degree of freedom module are used in simulating two cases of coupled heave-roll motion and pure roll motion of flooding. The coupled heave-roll motion effect on flooding process is discussed. Detailed flow field information such as velocity distribution around the flooding holes and cross-flooding devices is presented. INTRODUCTION A variety of maritime tragedies such as flooding, collision, stranding, fire and explosion, have caused irreparable damage to human life and cargo safety (Zhang et al., 1998). As a modern maritime excursion ship, the cruise integrates navigation, entertainment and leisure functions. With the development of the scale, the safety of cruise ships has attracted much attention. International Maritime Organization (IMO) has imposed strict demand on stability and damaged stability of the cruise in SOLAS(2013). The European research project SAFEDOR (2005–2009) shows that the risk to human life caused by flooding is the dominant issue for passenger ship safety. Therefore, studying the stability and motion characteristics of the damaged cruise can provide a reference for effectively reducing the loss. Compared with the intact stability, the dynamic stability of the damaged ship is more complicated. Therefore, it is of vital importance to establish appropriate life-saving measures and evacuation procedures to do the time domain simulation of the dynamic stability of the damaged ship. In the design of cruise ships, the cross-flooding devices are often used to reduce the roll angle in an emergency situation. The cross-flooding holes are mainly used for the connection of adjacent tanks. When the ship is damaged on one side, the cross-flooding devices can quickly introduce seawater into the unbroken side so that the ship could restore rapidly. The cross-flooding devices can increase the survival probability after the ship is damaged, thereby increasing the achieved subdivision index.

ABSTRACT In general, the motions of floating structure are evaluated based on linear potential theory. However, because viscous effects are ignored in the linear potential theory, roll motion through potential theory may be significantly different with actual roll motion, especially at the adjacent of resonance. In this study, the estimation techniques of roll damping using free decay and harmonic excited roll motion were studied. To design roll motion excitation device (RMED), numerical simulations are conducted for the design variables such as moving weight, moving distance and period. The test conditions are also chosen considering the results of numerical simulations. To study the roll damping of 30k LNG-BS(Bunkering Shuttle), experiments using RMED are carried out and results are analyzed by three different analysis methods for determining equivalent roll damping moment studied by Handschel et al.(2015). In addition, free decay model tests were performed, and the roll damping results were compared with the results of RMED. INTRODUCTION Increasing demand for LNG as a fuel of ships has allowed to propose a new concept of bunkering system, known as a floating LNG bunkering terminal, FLBT which can distribute LNG to LNG fueled vessel safely and economically. The FLBT moored at a distance from a port with a turret mooring system, can load LNG from a LNG carrier, as well as storing approximately 200k of LNG capacity. The FLBT also involves LNG Bunkering shuttles (LNG-BS) which can have various storage capacities. Since LNG-BS loads LNG from the FLBT and supplies to LNG fueled vessel where they are usually berthed at a port to load and unload cargo, LNG fueled vessel can save their time in berthing and unberthing process at the typical on-land LNG bunkering terminal to load LNG fuel. The storage capacity for LNG BS can be changed depending on the required amount of LNG fuel and FLBT system configuration. The LNG-BS loads LNG from the FLBT in side-by-side mooring configuration and then LNG-BS moves to a port and supply LNG to LNG fueled vessel. Since the LNG-BS and LNG fueled vessel also moored with side-by-side configuration, the roll motion characteristic of LNG-BS should be studied to avoid collision between ships.

Abstract Roll motion is a major concern of ship and offshore operators because it has a direct impact on the boarding comfort of crews, downtime of the process plant and structural integrity of the hull and appurtenance system. Nevertheless, the nonlinear effects involved in the roll motion of ship and offshore structures are not only incompletely understood, but also inaccurately predicted during the design stage. Therefore, efforts have been made to identify the dynamic characteristics of the roll motion of a FPSO using a novel dynamic system identification scheme based upon the Hilbert transform. First, the system identification scheme using the Hilbert transform was applied to the idealized single DOF nonlinear oscillator problem with quadratic damping and quintic stiffness terms. The solution of the single DOF problem was solved numerically using the 4 th order Runge-Kutta method and the target damping and stiffness coefficients were determined by applying the Hilbert transform to the numerically obtained free decay signal. A comparison was also made with traditional logarithmic decrement technique for the damping coefficient. Second, the system identification method was applied to the model test results of a FPSO with a bilge keel, where the nonlinear effect of roll damping and restoring is expected to be dominant. This methodology can be applied to full scale measurement data to achieve a clearer understanding on the nonlinear effect of roll motion. Introduction Roll motion of a ship-like floating structure under wave excitation is one of the most important factors that affects the production downtime, boarding comfort of crews and the structural safety of moorings and risers. Therefore, an accurate prediction of roll motion is very important during the design stage of a structure, which has prompted extensive research efforts. Recently, the joint industry project named Nonlinear Roll JIP(Oliveira et al., 2014) was initiated and has attracted considerable attention from ship owners and builders. The project covers a broad range of topics, which are closely related to the accurate prediction of roll motion under wave excitation, such as roll damping estimation, influence of the riser on roll motion, second order effect and structural assessments. Model basin tests, computation fluid dynamics simulations and full scale measurements were considered as methodologies. The major technical difficulties related to the roll motion of a floating body are the nonlinear effects of roll damping. Unlike other motions, where damping is dominated significantly by the propagation of radiated waves, the roll damping is influenced mainly by the viscosity induced vortexes between the hull surface and water particles. Normally, the viscous damping force acting on the rolling structure is known to be proportional to both the angular velocity of the body and the product with its modulus eventually making the problem nonlinear. In addition, when roll motion becomes considerably large, beyond the limit of linearity, the restoring moment, which is no longer proportional to the roll angle itself, is influenced by the higher order terms of odd numbers.

ABSTRACT: Parametric roll resonance is a ship stability related phenomenon that generates sudden large amplitude oscillations up to 30–40 degrees of roll. This can cause severe damage, and it can put the crew in serious danger. The need for a parametric rolling real time prediction system has been acknowledged in the last few years. This work proposes a prediction system based on a multilayer perceptron (MP) neural network. The training and testing of the MP network is accomplished by feeding it with simulated data of a three degrees-of-freedom nonlinear model of a fishing vessel. The neural network is shown to be capable of forecasting the ship's roll motion in realistic scenarios. INTRODUCTION Parametric roll resonance is a well known phenomenon that has gathered great attention in the last years due to the real threat suffered by container ships in common passage conditions, which were generally considered of no danger. The resonance is most likely to happen when the ship is sailing in head or stern seas under some specific conditions as: the wave encounter frequency is approximately twice the ship's natural roll frequency; the wavelength is almost equal to the ship length; the wave amplitude is larger than a ship dependent threshold. When these conditions are fulfilled, the periodic alternation of wave crests and troughs amidships brings about dramatic changes in ship transverse stability, which in turn determines a sudden and quick growth of roll oscillations. Modern container carriers are particularly prone to parametric roll resonance due to their hull shapes - large bow flare, overhanging stern, wall-sided midship sections - which are designed to achieve an optimal trade-off between high service speed and maximum container payload above deck (Shin et al., 2004, France et al., 2001, Nielsen et al., 2006).

ABSTRACT: Purse seine fishing vessel almost has more serious roll motions in rough sea due to its transverse lateral hauling feature. In this paper, we focus on a fishing vessel in beam waves and make a comprehensive stability analysis by using the nonlinear dynamic theory, including series model tests. Accordingly, the safety of ship in beam wave and crosswind are analyzed according to the Multi-scale method and Melnikov approach. A series of roll tests for one tuna purse seine ship are implemented for validation of our results under the regular waves and irregular long-crest waves, according to sea state V and VI. INTRODUCTION Recently, the ship stability and safety problems become more serious problems, as the new ship type, large-scale vessel and high-speed ships increase significantly. Only limited information is available on this subject. In particular, the events concerning sea accidents constantly occur. For capsizing problems related with the loss of transverse, therefore, the further study in extreme condition is important and necessary. For the ocean purse seine vessels, as expected, there're many variety of fishing equipments, characterized with heavy. They are often piled on the deck of the ship. One problem is with the effects of the gravity position. Also, the transverse lateral hauling feature could lead to heavy roll more easily, especially even more serious in rough weather. Consequently, the above factors should be fully considered in ship design stage, especially in the case involving the beam wave, which can ensure good transverse stability for the fishing vessel. The ship rolling motion involves the complex nonlinear and chaos phenomena. Within the context of the rolling motion of nonlinear dynamic model, various theoretical models in the literature have been applied for achievement of this particular problem, e.g. the differential dynamical system theory, bifurcation theory and chaos theory of various methods.

ABSTRACT: Weather routing system is a quite useful system for reduction of fuel consumption, keeping a schedule and safe navigation. In this paper, the authors developed weather routing system that is the supporting system for the selection of optimum route of a ship under way in stormy weather based on atmospheric and oceanic information in order to improve efficiency and safety of ship navigation. The effectiveness of the proposed system is verified by numerical simulations. INTRODUCTION In general, a navigation route is planned before starting a voyage. When a ship encounter bad weather under way, it is necessary to change her route because bad weather condition will cause not only increase of fuel consumption but also delay of schedule or damage of cargo for a ship. Weather routing service is useful for such case. It provides an optimum route based on atmospheric and oceanic information considering fuel consumption, delay of schedule or safe ship operation including damage of cargo. That is to say, an optimum route depends on prediction of atmospheric and oceanic condition and estimation of ship performance in actual sea. It is difficult for average ship operators without much knowledge or experience to find out an optimum route though weather routing is existing technology and completed at some level as practical technology. Utilizing such weather information, weather routing system using which even average ship operators could automatically obtain an optimum route has been studied (e.g. Hagiwara, et al., 1991). On the other hand, ship performance in actual sea could be estimated with accuracy based on many studies (e.g. Kashiwagi, 1995 and Takahashi, 1987) and weather routing system which propose an optimum route based on the estimation method of ship performance in actual sea has been studied (e.g. Tsujimoto, et al., 2004).

Long time ago, people began to study the performance of ships in still water. But, varieties of facts suggest that ships with excellent performances in still water would not function well in waves. Seakeeping performance is becoming of increasing importance. There are two basic ways to study seakeeping performance of ships: theoretical method and experimental method. The method based on theory focus on the foundation and resolution of various differential equations of ships in regular waves, and prediction of ship motion from the resolution of equations. The introduction of strip theory had greatly promoted the theoretical analysis of heaving and pitching motion, but it cannot give satisfactory results for rolling motion. Beside theoretical analysis, doing experiment is another important way of seakeeping research. Especially for certain problems without theoretical answers, experimentation is the only way for analysis. This paper did some research on the rolling motion of pentamaran through a method combined both theoretical analysis and experimentation. A method for analyzing the rolling motion of multi-hull ships has been put forward. Firstly, an automatic controlled free-running multi-hull model experimental platform has been designed and built. With the prior knowledge, the equation of non-linear rolling motion with undetermined coefficients has been established. Approximate equation for rolling motion of pentamaran has been given through identification with the data collected from model tests. Identification method is an optimization method based on Genetic Algorithm, and it would search the best results with given ranges of parameters. At last, the method has been proved feasible after analysis of the results, and provided some experiences and suggestions on the seakeeping analysis of multi-hull ships. INTRODUCTION In recent years ship seakeeping was paid more and more attention, so the ship with better seakeeping was becoming research hotspots. Ship seakeeping is a subject to study ship movement rules in wave.

An experimental and numerical study of the free roll decay motion for a vessel of the Italian Navy is presented. Experimentally both sea trials and model scale tests have been carried out; numerical analysis has been performed by means of unsteady RANS simulations. The focus is on the analysis of the roll motion coefficients (damping and period of oscillations) at different Froude and Reynolds numbers. To this aim, experiments and numerical simulations were carried out at different Froude and Reynolds numbers. Sea trials were carried out on the Italian Navy ship Comandante Bettica in the Mediterranean Sea, close to the coast of Sicily; models experiments were performed in the towing tank at INSEAN. Computations were carried out by means of an inhouse unsteady RANS solver. INTRODUCTION The analysis of the roll motion of a ship is of practical interest for both safety and comfort reasons. In this paper an experimental and numerical analysis of the roll decay for patrol boat of the Italian Navy is carried out. Full scale trials in the Mediterranean sea in cooperation with NSWCCD (Naval Surface Warfare Center, Carderock Division) and model scale experiments at the INSEAN towing tank have been performed. For a proper comparison, hull in fully appended configuration, (i.e. with the rudders, bilge keels, fins, and propeller apparatus, including struts, A-brackets and the propeller shaft) has been considered. To properly understand the effect of the rotating propeller on the roll damping, model scale experiments have been performed with and without the rotating propeller. Several Froude numbers have been considered, both in full and model scale, to highlight the effect of the ship speed on the roll damping. Numerical simulations have been carried out for three different Froude numbers; the steady flow around the vessel with a fixed heel angle, and the unsteady free roll decay of the vessel from an initial of heel angle of 10 degrees are computed.

ABSTRACT This paper addresses the hydrodynamic interaction of ship-shaped hull sections in prescribed roll motion and also in transient roll-decay motion. The flow around 2-D hull-section with four different model geometries are studied using a Finite Volume Method (FVM) based numerical model, and the results from the FVM scheme are compared with experimental data and other numerical results. For the prescribed roll motion, the effects of different hull geometries and roll angle amplitudes on the moment histories and corresponding hydrodynamic coefficients are presented, and the highest amplitude of motion studied has been increased to 20 degrees. The relation between the hydrodynamic coefficients and the corresponding roll damping rate and frequency in the case of transient roll response for different hull geometries and different initial displacements is also discussed. INTRODUCTION Ship-shaped hulls have often been found to be subject to excessive roll motions that inhibit their use as a stable production platform. Bilge keels have been widely used as an effective and economic way of mitigating the roll motions, and their effectiveness lies in their ability to damp out roll motions over a range of frequencies. The performance of roll motion, in a conventional sense, is measured in terms of hydrodynamic added-mass and damping coefficients. (Vugts 1968) was the first to calculate the hydrodynamic coefficients for sharp-edged sections in roll motions and observed the importance of the viscous effect. The estimation of these coefficients through various numerical methods and validation through experiments has been the focus of research over the years, (Na et al. 2002; Wilson et al. 2006; Yeung et al. 1998; Yeung et al. 2000). The estimation of the damping coefficient on a non-conventional hull-section is investigated experimentally by (Yuck et al. 2003), where they found that the roll damping could increase significantly due to the hull geometry.

ABSTRACT Parametric roll resonance of modern containerships was predicted at initial design phase using numerical simulations and model scale experiments in a deep towing tank. Dynamic instabilities of modern containerships vulnerable to large but bounded roll motions when either floating or advancing in certain frequencies of wave encounter were successfully simulated using three-dimensional time-domain seakeeping computations based on Rankine source panel method. In the hydrodynamic computations geometric nonlinearity was accounted for in Froude-Krylov and restoring forces using simple wave corrections over exact wet hull surface of the containerships. The numerical simulations in the present work were compared with the calculations based on a classification society in-house code specially designed to assess parametric roll motions of commercial vessels. The computations for a mid-size and an ultra-large size containerships were validated through four degree-of-freedom model scale seakeeping experiments in regular head waves at the deep towing tank of Hyundai Heavy Industries. INTRODUCTION In general, the roll response is a matter of no great importance for the ship in head seas. However many shipping agents have reported excessive roll motions when containerships, whose ship shape and size are modern and large, were operating in head seas of the harsh ocean environments. The abrupt accident of the large containership as well as Panamax or post-Panamax class is well known (William et al., 2003; Riberiro e Silva et al., 2004). Recently, the concern for the roll resonance due to the parametric excitation in the head sea and following seas tends to increase with the size of the containership, which is now exceeding 10,000 TEU class. The parametric resonance of the roll motion occurs suddenly in head seas or following seas and causes the roll motions to grow rapidly to an excessive roll angle of 40 degrees.

In the context of the European VIRTUE program dealing with the improvement of CFD models for naval applications, Principia, Bassin d'Essais des Carènes and Ecole Centrale de Nantes work on the sea-keeping work-package of the project. It concerns the development and the validation of CFD codes for the simulation of non-linear hydrodynamic effects such as ship roll damping due to viscosity and wave radiation. The paper shows sea-keeping applications based on comparisons between numerical simulations performed with the CFD code EOLE of Principia and measurements carried out at the Bassin d'Essais des Carènes. INTRODUCTION The sea-keeping numerical simulations are performed out with the CFD code EOLE developed by Principia. URANS equations are solved using multi-blocks curvilinear meshes. Turbulence is included with a k-ε model and the free surface is considered using a VOF implicit model. Harmonic roll motion is imposed to the mesh (fixed to the ship) in the absolute frame to simulate forced oscillations tests, with or without forward speed of the ship. Forced oscillations tests have been carried out in the B600 basin at Val de Rueil, on a hull model test at 1/24 scale. A specific test arrangement is used for the tests. The 3D motion generator from ECN (hexapod), fixed on the towing carriage, allows forced roll motions. Several tests have been performed in calm water considering a large range of periods, roll amplitudes and forward speeds. MODEL TEST Experimental set-up The Hamburg test case model (HTC), coming from HSVA Hamburg, is considered. The scale of the model is 1/24 and hydrostatics of HTC are presented in table 1. Pictures of the model are shown on Fig. 1. The main objective of the test campaign is to provide experimental data to validate numerical tools on the specific issue of viscous roll damping simulation for ship in forced roll motion.

ABSTRACT The objective of this research is to model the physics of the separated unsteady viscous flow over the bilge keels of an FPSO hull subject to roll motions. A cell center based Finite Volume Method (FVM) scheme is implemented for the solution of the 2-D Navier-Stokes equations (NS2D). The scheme is verified in the case of a submerged fixed hull with bilge keels subject to alternating flow. The effects on the pressure distribution along the hull and the moment histories are investigated. Moreover, the inviscid flow results are also compared to those from the Boundary Element Method (BEM) based potential solver for verification purposes. INTRODUCTION FPSO hulls have often been found to be subject to excessive roll motions. The installation of bilge keels has been widely used as an effective and economic way of mitigating the roll motions of hulls. Vugts (1968) was the first to calculate the hydrodynamic coefficients for sharp-edged sections in roll motions and observe the importance of viscous effects. Yeung, Liao, and Roddier (1998) applied the Free- Surface Random Vortex Method (FSRVM), Yeung and Vaidhyanathan (1994), to a rectangular ship-like section oscillating in roll motions and compared the hydrodynamic coefficients obtained from their method with those obtained from their experiments. They also suggested a composite model for extracting the hydrodynamic coefficients from the roll moment history. Yeung, Roddier, Liao, Alessandrini, and Gentaz, (2001) included the effects of bilge keels in the case of an FPSO hull subject to roll motion by using the FSRVM and compared the results to those of a Boundary-Fitted Finite Difference Method (BFFDM, Alessandrini and Delhommeau, 1995) and those measured in their experiments. Their numerical solutions show that an increase in the keel size increases both the added mass and damping coefficients.

ABSTRACT A numerical approach to evaluate the coupling effect on the sloshing load is presented. The motion of liquid cargo inside a LNG tank is solved by a finite-element method under the assumption of potential theory in the time domain. The nonlinear free-surface conditions are satisfied exactly on the actual free surface of the liquid cargo. The hydrodynamic load on the tank wall is decomposed into impulsive and non-impulsive components. The integration of the impulsive pressure results in time-varying added mass for the liquid cargo motion, which is considered as part of the virtual mass when solving the ship motion. The transient motion of LNG carrier is mathematically modeled by the use of impulse response function (IRF) and wave exciting force calculated from the three-dimensional frequency-domain panel method. The calculated ship motion is compared with the experimental result to validate the numerical scheme. The sloshing impact load on the tank wall is investigated for various loading conditions and sea states INTRODUCTION Sloshing Load at Standard Filling Level Ocean going LNG vessels have been designed to operate 20–25 years of service life in the severe wave conditions of the North Atlantic, which is referred to as ‘unrestricted service’. While the design load of the main hull structure is governed by the external wave load and inertial load due to ship motion, the sloshing impact load due to liquid motion inside the tank governs the design load for the LNG containment system and surrounding bulkheads. LNG carriers are fully loaded when they leave the loading terminal. In case of membrane-type LNG containment system, where the pressure and temperature inside the tank is maintained near atmospheric and cryogenic, LNG cargo keeps boiling during the voyage.

ABSTRACT This paper presents laboratory observations of vortex generation and evolution process as regular waves passing a rectangular barge in a two dimensional wave tank. The test barge was mounted at the center of gravity by the hinge that allows the barge to roll only. Two regular waves with periods of 1.0 and 2.0 seconds, respectively, were selected for this study. Neither waves resulted in overtopping on deck. Spatial velocity field in water surrounding the barge and the roll responses of the barge were captured in PIV images for further process. The mean velocity field and its associated turbulence properties were reduced by averaging over sample PIV images recorded from repeated test runs based on phase averaging concept. The test results confirm that the vortex generation and evolution process can be clearly captured by the present technique. It is interesting to note that the size and trajectory of the vortices produced by waves of different period are remarkably different. The flow pattern and kinematic properties observed lead to a better understanding of the mechanism of eddy making and decay. This finding helps clarify near filed pressure distribution around a flat bottomed vessel and provide guideline to roll damping simulations. INTRODUCTION Viscous effect is known to heavily influence roll motion of a blunt shaped floating structure. Potential flow theories, although reproduce heave and pitch motion very well, are much less effective in predicting roll motion due to their negligence of fluid viscosity. This shortfall is normally compensated by introducing a viscous roll damping coefficient. However, a proper damping coefficient is rather difficult to come by owing to its highly nonlinear nature. Current practice often turns to empirical formulas, which require extensive calibrations with laboratory or field measurements. Nevertheless, these tests are time consuming and cost intensive, and results are case dependent.

ABSTRACT A correct estimation for roll motion is needed to evaluate the stability and operational efficiency of offshore vessels like FPSO, shuttle tanker and drill ship. The prediction of roll motion is directly affected by roll damping coefficients. In this work, the roll-damping coefficient for non-conventional cross sections is investigated experimentally. Tank tests are carried out for box-type sections with step. In order to validate the present approach, a typical bilge section is considered and our result is compared with published data. The comparison shows a good agreement. For convenience, the roll damping is divided into wavemaking component and viscous component. The wave component is determined experimentally by far-field momentum method and compared with numerical results obtained from potential theory. The viscous component is estimated by extracting the wave component from the total damping. It is found that the roll damping of the box-type section with step increases significantly due to the enhanced vortex shedding and that the roll arm is an important parameter for the magnitude of the roll damping. INTRODUCTION The stability and operability is an important issue for the design of floating-type offshore structures like FPSO, drill ship and shuttle tanker because these structures are being increasingly deployed into severe deep sea. In order to clarify the operability and stability of vessels in high sea, an accurate motion analysis is needed. It is particularly the case for roll motion because it directly affects the stability and operability of vessels. Accurate estimation of the roll-damping coefficient is of central interest in roll motion analysis because the roll motion is sensitive to the roll damping near resonance. As well known, a rolling ship exhibits large list near the resonance, which may lead to cargo shift, loss of deck cargo and eventually undesirable consequences including capsize.

ABSTRACT A Reynolds-Averaged Navier-Stokes (RANS) numerical method has been employed in conjunction with a chimera domain decomposition approach for time-domain simulation of capsizing behavior of a pontoon barge under large amplitude waves. Calculations were performed first for a fixed rectangular barge in beam sea conditions. The computed wave elevations, velocity vectors, and vorticity contours were compared with the corresponding experimental data obtained from Particle Image Velocimetry (PIV) measurements to verify the accuracy of the simulation results. After successful validations for the fixed barge, the method was generalized for time-domain simulations of barge capsizing in regular waves. To facilitate the simulation of large amplitude barge heave and roll motions, a general chimera domain decomposition approach was developed to handle partial hull submergence and green water on the barge deck. The barge responses were close to harmonic before the green water passed over the barge deck. Once the deck is partially submerged, however, the green water greatly impeded the barge from rolling back to its equilibrium position. INTRODUCTION Pontoon platforms, which were designed primarily for inland uses, have been assuming more important roles offshore in recent years. The attributes of modular construction and large payload capacity make them a low-cost candidate for contingent operations at sea. The development of on-site assembly capability of a floating structure in rough waters (Huang, 1997) further enhances their attractiveness for offshore applications. Although these floating structures exhibit certain functional qualities resulting from the box shape, they also exhibit the weather sensitive motions typical of a large water-plane hull. Dynamic stability is a serious concern of slender platforms. While pontoon platforms normally demonstrate a hefty load carrying capacity due to their large water plane areas, they also suffer from low freeboard under heavy loads due to their shallow depths.

ABSTRACT A hybrid anti-rolling system was equipped on board of the oceanographic research vessel)i MIRAI" (8,672GT). The greatest advantage of the system is capable of reducing the rolling motion by fifty percent. As it is the first system which was applied to a large vessel, it is necessary to examine operational conditions of the system at sea. Rolling and wave heights data acquisition was carried out to investigate not only the system performance but also oceanic conditions of wave heights and wind velocities. Then, the measurement data were analyzed by a statistical method. The system was operated up to Sea State 6 in the North Pacific Ocean. It was proved that the system functioned very well and had high reliability. 1. INTRODUCTION Oceanographic research vessel "Mirai" (Photo 1) (8,687GT) began its operation as the world-biggest class research vessel in 1997. The advantage of the large vessel is to make it possible the observation work even in stormy weather. To support the observation work, the first ship borne type hybrid antirolling system (Photo 2, hereafter called "Antirolling device") was installed on "Mirai". The roll motion is reduced by the antirolling device using a moving mass controlled by the computer even when the vessel is drifting or stopping for oceanographic observation and deck work for samplings. To make a functional survey of the antirolling device in the sea, the mass displacement was continuously recorded, with the wave height, and wind data. The relation between the mass displacement and the roll angle of the ship was examined in this paper. Moreover, the relation between the roll angle and the movable mass displacement was clarified from the operation data and the ability of the antirolling device was confirmed.