In the design of Floating Production Storage and Offloading facilities (FPSO), it is important to predict the seakeeping parameters like FPSO motions. This paper focusses on the prediction of FPSO motions and wave impact loads in various irregular sea waves and proposes a Computational Fluid Dynamics (CFD) model. Accurate prediction of FPSO motions and wave impact loads requires the accurate prediction of wave kinematics. Waves generated by winds blowing over the water surface in real oceans are short-crested, so the dynamic responses of the moving structure will be very different in long crested and short-crested waves. In this paper, we have verified the pure propagation of oblique short-crested random waves in numerical wave tank with the analytical results, which serves as the basis of more complex analysis including ship hull interaction with waves. The proposed CFD model uses Multiphase Volume of Fluid Model (VOF) along with Six Degrees of Freedom Model (6DOF) to predict the hydrodynamic forces and the ship responses. Long crested and short-crested random waves are simulated in a numerical wave tank, where boundary conditions for free surface profiles and velocities are imposed at the velocity inlet boundary using Jonswap frequency spectrum and cosine-2s directional spreading function. Wave slamming loads and the structural responses depend on many factors, like sea state, FPSO speed, draft, wave heading angle, local structure etc. In this paper, we have analyzed the wave slamming on a moving FPSO for different sea states (unidirectional, multi directional with 45 deg and 90deg angular spread) and studied the resulting hydrodynamic forces and related structural interactions. Ship motions (heave, pitch and roll), vertical accelerations, pitch velocities, roll velocities and the wave impact pressures are compared among different sea states.

Fluid flow and structural deformation of a floating platform during extreme wave slamming events are simulated by Computational Fluid Dynamics (CFD) and Finite Element Method (FEM) in coupled ways, in both model and full scale. The objective of the simulations is to verify correlation between slamming impact pressure on scaled platform model in the wave basin set up and full-scale platform in the real sea when the rigidity of the hull surface is considered. The slamming load and structural response simulated by CFD-FE analysis is first validated against analytic solutions and model tests for the following two cases: Wave slamming on rigid wall by breaking wave and water hammer Drop test of rigid and elastic structure on calm water The validated CFD-FE simulation set up is then applied to simulate wave slamming on three different realizations of a Spar hull structure: Rigid hull at model scale Rigid hull at full scale Pressure panel at model scale In the CFD-FE modeling of the wave slamming at full- and model scale, compressibility of water/air and elasticity of the hull are properly modeled to simulate the difference between the hydroelastic effects at model- and full-scale set up. It has been found that the hydroelastic effect is minimal on full-scale Spar structure but can be significant on pressure panels at model scale. Further CFD investigations on other physical parameters that might affect the difference between wave slaming load at full- and model scale, such as surface tension, air entrapment and air-water mixture properties are made.

In ship hull design, it is important to predict the seakeeping parameters like ship motions, vertical accelerations, wave impact loads, deck wetness, etc. when the ship is subjected to waves. According to the ABS Guide for Building and Classing High Speed Naval Craft (HSNC 2007) slamming impact load is one of the most critical factors for the scantling design of hull structures[7]. Accurate prediction of wave impact loads requires solution of three problems. First one is the prediction of wave kinematics, second one is the prediction of the pressure and viscous forces and the third one is the prediction of ship motion during the wave impact. This paper focuses on the wet deck slamming analysis of twin-hulled offshore ship in irregular sea waves and proposes a Computational Fluid Dynamics (CFD) solution using Volume of Fluid (VOF) method and Six Degrees of Freedom Model (6DOF). Irregular waves were modeled using JONSWAP wave spectrum. The wave profile obtained from the numerical simulation was validated with analytical results. The wave impact load and the hull motions depend on many factors, such as sea state, ship speed, draft, wave heading angle, hull structure, ship loading condition etc. In this paper, we have analyzed the wet deck slamming for a survival condition with sea state of significant wave height of 9m with two different ship loadings and two different ship speeds. CFD analyses were also carried out for calm sea conditions with different ship loadings. The heave and pitch motion of the ship in wave conditions are compared with calm sea conditions. The ship motions, vertical accelerations, pitch velocities and the wet deck slamming pressures are compared for the two ship loadings and two ship speeds. This paper describes these results in detail.

Abstract The sloshing impact loads is one of the major design factors for tank containment system in LNG FPSO (Floating production storage and of?oading). With the development of offshore natural gas exploration and LNG transportation, the application of FLNG is growing rapidly. Sloshing of LNG cargo results in large impact loads on the containment system and the structure needs to be designed to withstand these peak loads. Liquid sloshing is a complex phenomenon of fluid movement, showing a strong nonlinearity and randomness. Partially filled container which is subjected to external excitation can undergo intense movement, and large impacts on container wall, resulting in structure damage. The earlier sloshing study used experimental methods, but the number and size of tests are subject to great restrictions due to high cost, long period, and complicated operation. In view of this, the development of numerical simulation is necessary. CFD modeling can be reliably used to find the sloshing loads at different sea states, liquid levels and vapor properties, as well optimizing internal baffles to dampen sloshing. The current paper proposes Computation Fluid Dynamics (CFD) method to predict the sloshing loads in FLNG during the ship motion. The proposed CFD method is validated with experiments for the free surface level and the impact pressure during the sloshing in various scenarios. Series of CFD studies were performed to investigate effect of mesh sensitivity, solver options and time step size. A separate study was performed to investigate effect of density ratio on impact pressure. Volume of Fluid (VOF) method along with rigid body motion is used to model the liquid sloshing in a moving container. Validation case studies are chosen in accordance with literature.

Abstract Sloshing in LNG tanks has gained increasing attention over the past period of time. This is mainly caused by developments in the LNG market, changes in the design and operation of LNG ships and an increasing interest in floating gas field exploitation. The issue of sloshing in partially filled tanks is relevant for spot trading and offshore loading/off loading of LNG ships as well as for FPSOs with LNG capacity. DNV has developed a step-by-step experimental procedure to determine sloshing loads for structural analysis of the insulation system and tank support structure. Of key importance for a reliable evaluation is the step-by-step approach, putting emphasis on an accurate treatment of every step. This means careful modelling of operational and environmental conditions, accurate ship motion calculations, a well-defined procedure for identifying design sea states, a proper experimental set-up and an accurate treatment of the statistics involved in every step in order to determine reliable and realistic design sloshing pressures. To study sloshing loads in partially filled LNG tanks irregular sloshing experiments have been conducted for head and beam seas for different filling levels and sea severities. A 1/20 scale model of a tank from a 138.000 m3 membrane type LNG ship was used for the tests. Measurements have been conducted using pressure transducers and pressure transducers mounted in clusters. An overview of the tests is given with an analysis of the impact pressure statistics, the pressure pulses and the associated subjected area. From these analyses a discussion is presented on the effect of different filling levels, sea severity, ship speed and heading. Introduction The gas market is in an upswing and will in the future provide an increasing part of the world energy demand. A large part of the natural gas reserves will be transported by sea from well to customer. Several changes are seen in the offshore production of gas and the sea-borne transportation of LNG. Floating installations to produce offshore gas is an upcoming market. In case of liquefied storage, present filling restrictions will be violated implicitly. In the LNG shipping industry three main changes are seen or expected: The LNG market is expected to develop more into a spot-market instead of long-term contracts. This view is supported by the fact that LNG carriers have beenordered without having a first transportation contract. As a consequence of spot-market trading shipowners prefer to increase their trading flexibility by having the possibility to operate with not-fully loaded tanks, which would imply a reduction of the upper filling restriction. A second change is foreseen in the maximum size of LNG carriers. By a market-push to reduce transportation costs the maximum size of LNG vessels will increase. A third change is foreseen in the location of loading and offloading. With an increasing focus on safety, supported by the threat of terrorism, it is becoming difficult to build land-based terminals for loading and off-loading, especially in the US. Offshore terminals, far away from dense populated areas are the logical solution, but with the implication of more severe environmental conditions when loading or discharging.

ABSTRACT This paper describes and analyzes a series of experiments conducted on the impact at a water surface of horizontal cylinders. The cylinders (0.2-, 0.4-, and 0.6-m diameter) were dropped from various heights, the maximum impact velocity being 3.88 m/second. Local pressures and accelerations were recorded during penetration in the water. A total of 185 drops were executed, five parameters being recorded for each of them. Pressure measurements at different points of the cylinder circumference led to a precise description of the impact mechanism. Acceleration measurements furnished an estimation of impact forces intensity and duration. The experiments were conducted in full scale; this is of importance considering the difficulties met in scaling down the phenomena involved: elastic vibrations, air-cushion effect, and transition between the impact and hydrodynamic flows. Local impact pressures lasting less than 1 ms propagate along the cylinder circumference starting from the initial impact point, until the cylinder has penetrated a distance about half its radius. Evidence of an air-cushion effect is given by pressure recordings. Local pressure maxima are well fit by a linear function of impact velocity. Combining pressure and acceleration results leads to a simple model of dynamic loading on cylinders during penetration. Values of impact forces thus calculated are comparable with those of Faltinsen et al. 3 although obtained in a quite different way. Starting from that model, the design engineer can determine the response to wave impact for a given structure whose elastic characteristics are known. INTRODUCTION The horizontal cylindrical members of offshore platforms situated near the sea surface are submerged regularly by waves. In addition to hydrostatic and hydrodynamic forces thus generated, very brief and intense impact forces occur when the structure hits the water surface. These loadings generate very important and destructive vibrations in horizontal members, every time they cross the water surface. These vibrations often are attributed to impact forces whose main characteristics are not well known (duration, intensity, exposed area, etc.). Several theoretical and experimental studies have been devoted to the encounter of cones, V shaped bodies, and flat plates with water, simulating the penetration of shells or ship stems. The slamming phenomenon, being a succession of different flow regimes, cannot be resolved as a whole by analytical methods. As for the experimental results, nothing indicates that they can be extended to different geometries. Only three studies have been found in the literature concerning the impact due to waves on horizontal cylindrical tubes. These studies 1,3,4 insert the impact forces in a hydrodynamic-type formulation. Dalton and Nash, working at a reduced scale, furnish an experimental determination of an "impact coefficient" defined by the following relation. (Mathematical equation available in full paper) where F s is the impact force and S the projected area on a plane normal to velocity.

I. ABSTRACT This paper presents a method for predicting the magnitude and distribution of the slamming loads and pressures acting along the bottom of a catamaran cross structure. Comparisons with experimental model and full-scale data show good agreement. It is shown that, for a given cross structure clearance above the mean water level, there are specific ship speeds and wave conditions which will give rise to cross-structure impacts or slams. The method should provide an extremely useful tool for the design of catamaran cross structure plating. II. INTRODUCTION The problem of slamming of the cross structure of a catamaran or twin-hull ship is a serious concern for the designer. The phenomenon results when the cross-structure hits the surface of the water at small or moderate impact angles giving rise to dynamic pressures and loads which can reach extremely high levels. These pressures and loads must be known since they can induce severe local damage to the cross-structure bottom plating. They can also have significant effects on the overall hull in the form of increased bending moments and shears and induced vibratory motions. The latter willusually make the master of the ship intuitively reduce the forward speed and/or change heading. Whatever the effects of slamming may be, it is extremely important that the naval architect have a design tool which will allow him to predict the magnitude and distribution of these loads. In addition to the magnitude and distribution of the pressures and loads associated with a single impact, it is also important to know what to expect over a long period of ship operation time as far as structural and human tolerances to slamming are concerned. Thus, while the approach to the single impact situation is deterministic in nature, the long range prediction of the slamming loads must be statistical. It must be realized that one should not only be interested in the structural survivability of the ship when exposed to a large number of severe slams taking place within a small period of time. Certainly, it is just as important to be concerned with the comfort and well-being of the crew, and attempts should be made to prevent such a situation from taking place by changing speed or heading or both. Thus, both structural and human elements should be considered simultaneously when establishing the limits for operating under slamming conditions. For example, if the tolerable speed for a slam impact is below the speed free from slam damage, then the ship will be completely safe as far as slamming is concerned. If the situation is reversed, then there exists the possibility that the ship will be operated at speeds where damage may take place. In that case consideration must be given to the cross-structure bottom plating thickness to the extent that speed free from slam damage will be above the tolerable speed.

ABSTRACT A laboratory investigation of the impact pressures that often result when waves break against coastal structures was conducted using periodic waves in a channel 2 ft × 2 ft in cross-section and 100ft in length. The model structure used in the study, a vertical wall 6 in. wide supported in the center of the channel, was 'instrumented with six miniature piezoelectric transducers capable of measuring rapidly applied pressures. Preliminary photographic studies of breaker geometry and kinematics revealed that the presence of the relatively narrow test structure did not significantly alter the breaker form and the data thus obtained could be used to quantitatively characterize the eight wave forms investigated. Pressure measurements verified the variability of peak pressure magnitude observed by other investigators. The presence of air in significant quantities, entrapped between the advancing wave and' the structure, was also observed. Two classifications of impact pressure, termed "ordinary" and "significant", could be defined, hence raising a question about the nature of the impact pressures studied by previous investigators. Significant pressures were relatively large in magnitude and acted over a considerable portion of the structure simultaneously. Ordinary pressures, while at times quite large, occurred more frequently but acted over only a small area at any instant. For significant impact pressures it was found that the average value of the dimensionless maximum pressure, − p max /?Y b increasing steepness of the incident wave and that for any given incident wave form the maximum pressure was inversely proportional to the time required for the pressure to reach a maximum. A comparison of the instantaneous spatial pressure distribution, determined using the six pressure transducers, with the pressure distribution currently in use for breakwater design reveals that the two differ in shape. The difference may arise because previous investigators have been limited by using only one, or at most three, movable transducers and have therefore measured the pressures caused by a number of successive waves rather than the distribution of pressure resulting from a single wave. INTRODUCTION Seawalls and vertical wall breakwaters subjected to breaking wave action occasionally experience high pressures of short duration often termed "shock pressures". The occurrence of these pressures depends critically upon the location of a structure relative to the breaking position of the wave and, because of this dependence, occurs only infrequently against full scale breakwaters. Measurements made by de Rouville, Besson and Petry on a full scale breakwater at. Dieppe revealed that only 2 to 3 percent of the waves hitting the structure resulted in impact pressures 11 [The term impact pressure rather than shock pressure will be used. herein to describe the high pressures caused by breaking waves when they encounter a structure.] Early full scale measurements of breaking wave forces by Stevenson l2 and Gaillard 13 failed to detect the short duration impact pressures because of the slow response of their measuring equipment. Larras in a laboratory study subsequent to the full scale measurements by de Rouville, Besson and Petry ll at Dieppe, was the first to identify the impact pressure as an elastic phenomenon.