Adapting a Linear Potential Theory Solver for the Outer Hull to Account for Fluid Dynamics in Tanks

The linear boundary value problem for the wave dynamics inside a tank is very similar to the solution for the outer hull. Because of this, the boundary value solver for the outer hull can be re-used for the tank. The oscillating hydrostatic pressure in the tank may also be calculated in the same way as for the outer hull. Thereby, the hydrostatic coefficients from the tank can also be obtained from the outer solution. This makes it, in principle, easy to adapt outer solution computer code to also account for the inner solutions for all the tanks. The procedure is discussed by Newman (2005). We have used it in a different way, isolating the tank solution into more flexible independent sub-runs. This approach provides part-results for the tanks, like added mass and restoring from the tanks. It also has numerical benefits, with the possibility to reuse the calculations for tanks of equal geometrical shape. We have also extended the procedure to account for full tanks without waves and restoring effects. The linear tank fluid dynamics is programmed into a quite general hydrodynamic frequency domain solver, with the possibility of automatic transferring of local loads to structural (FEM) analysis. Results for local loads are presented. A simpler method of quasi-static loading in tanks is discussed, with comparison to the present method. Effects on global motions and local pressure coming from the tank dynamics contributions are pointed out, such as the shifted resonance of the vessel and the added mass which differs from rigid masses of the tanks.

Development of a Calculation Method for Fuel Consumption of Ships in Actual Seas With Performance Evaluation

Greenhouse gas shall be reduced from shipping sector. For that purpose the regulation of EEDI (energy efficiency design index for new ships) and SEEMP (ship energy efficiency management plan) have been entry into force from 2013. In order to improve the energy efficiency in ship operation it is necessary to predict the fuel consumption accurately. In actual seas the wave effect is the dominant component of the external forces. In particular it is well known the bow shape above water affects the added resistance in waves. To reflect the effect of the bow shape a method which takes into account the result of simplified tank tests is proposed here. Using the results of tank tests the effect of the bow shape above water can be evaluated with accuracy as well as with robustness. Regarding to the fuel consumption it should be evaluated by combining the ship hydrodynamic performance with the engine characteristics. Especially the operating limits of the main engine, such as the torque limit and the over load protection, are affected to the ship hydrodynamic performance. In rough weather condition the revolution of the main engine will be reduced to be below the operating limits of the engine. This causes the large decrease of ship speed. To prevent the increase of fuel consumption, a control system by Fuel Index as an index of fuel injection has been applied to some ships. The calculation method for the fuel consumption by using Fuel Index is presented. In this paper following contents are reported; 1) development of a calculation method for the added resistance due to waves combined with the simplified tank tests in short waves, 2) comparison of the calculation method with onboard measurement, 3) development of a calculation method for the fuel consumption considering the engine operating mode in actual seas and 4) comparison of the method with onboard measurement of a container ship. From these investigations the availability of the present method is confirmed.

Relation Between Flow Velocity and Impact Pressure in a Green Water Flow

Impact pressure due to plunging breaking waves impinging on a simplified model structure was investigated in the laboratory based on two breaking wave conditions: the wall impingement wave condition and the deck impingement wave condition. Pressure, void fraction, and velocities were measured at various locations on the deck surface. Impact pressure was correlated with the mean kinetic energy calculated based on the measured mean velocities and void fraction to obtain the impact coefficient. For the wall impingement wave condition, the relationship between impact pressure and mean kinetic energy is linear with the impact coefficient close to unity. For the deck impingement wave condition, the above relationship does not show good correlation, whereas the impact coefficient was found to be a function of the rate of pressure rise.

Theoretical Modelling of Free Falling Wedges Entering the Water Surface: Part I — Wedge Motions

As the first of two companion papers, theoretical models are proposed to describe the motions of free falling wedges vertically entering the water surface at Froude numbers: 1 ≤ Fn < 9 (Here, the Froude number is defined as Fn=V0/gc0). The time evolutions of the penetration depth, the velocity and the acceleration are analyzed and expressed explicitly The maximum and average accelerations are predicted. The drag (slamming) coefficients are extensively studied. It is found that for the light wedge the transient drag coefficients have slow variation in the first half stage and rapid variation in the last half stage, and for the heavy wedge the transient drag coefficients vary slowly during the whole stage and can be treated as constant. The theoretical results are compared with numerical simulations by nonlinear BEM (Wang & Faltinsen (2010, 2013)), and good agreements are obtained.

Comparison of RANS and Potential Flow Force Computations for One DOF Heave and Pitch of the ONR Tumblehome Hullform

The commercial CFD software STAR-CCM+ is applied as a RANS solver for comparison with several potential flow codes in calculating time varying forces and moments resulting from prescribed heave and pitch motions of the ONRTH hullform. Combinations of amplitude and frequency of oscillation ranging from small to large are considered. A verification study is completed to ensure that the mesh and time step used by the RANS code are acceptable. Differences in the force and moment results from the various potential flow and RANS methods are attributed to particular code characteristics. RANS and nonlinear potential flow methods are shown to agree well for all cases considered.

Hydro-Elastic Modelling of an Electro-Active Wave Energy Converter

Since 2009, SBM Offshore has been developing the S3 Wave Energy Converter (S3 WEC). It consists in a long flexible tube made of an Electro-Active Polymer (EAP). Thus, the structural material is also the Power Take Off (PTO). In order to optimize the S3 WEC, a hydro-elastic numerical model able to predict the device dynamic response has been developed. The inner flow, elastic wall deformations and outer flow are taken into account in the model under the following assumptions: Euler equation is used for the inner flow. The flow is also assumed to be uniform. Elastic deformation of the wall tube is linearized. The outer flow is modeled using linear potential theory. These equations have been combined in order to build the numerical model. First, they are solved in the absence of the outer fluid in order to obtain the modes of response of the device. Secondly, the outer fluid is taken into account and the equation of motion is solved by making use of modal expansion. Meanwhile, experimental validation tests were conducted in the ocean basin at Ecole Centrale De Nantes. The scale model is 10m long tube made of EAP. The tube deformations were measured using the electro-active polymer. The model was also equipped with sensors in order to measure the inner pressure. Comparisons of the deformation rate between the numerical model and experimental results show good agreement, provided that the wall damping is calibrated. Eventually, results of a technico-economical parametric study of the dimensions of the device are presented.

Radiation Hydrodynamics of a Surface Effect Ship

The radiation hydrodynamics of a heaving surface effect ship (SES) is examined including the effect of air compressibility on the hydrodynamic forces and surface waves. Of particular focus of the study has been on determining the nonlinear viscous and air compressibility effects at natural frequencies corresponding to the piston and sloshing wave modes between the hulls and at the natural frequency corresponding to the heave motion of a surface effect ship with the restoring force dominated by the compressibility of the air cushion. In the present paper, the air cushion pressure is assumed to be uniform with its variation due to change of volume modeled using the adiabatic gas law pVγ = constant, where p denotes the absolute pressure of the air, V the air volume bounded by the side hulls, the free surface and the wet deck, and γ the ratio of specific heats Cp/Cv which is about 1.4 for air. The incompressible Navier-Stokes equations governing the nonlinear viscous wave-air-body interaction problem is solved in the time domain using a finite-difference method based on boundary fitted coordinates. New results presented in this paper show that air cushion compressibility affects the generation of waves and wave radiation forces significantly even at small amplitude of hull motion. As already well known, the free surface nonlinearity due to hull motion is significant for large amplitude of oscillation. At small amplitude of body oscillation, significant nonlinearity can be caused by air compressibility resulting in the generation of higher harmonic waves and forces. The results also highlight the significance of viscosity and flow separation, in conjunction with air compressibility, in the case of large amplitude hull motion with a small draft.

Maneuvering Behavior of Ships in Irregular Waves

Ship maneuvering in waves is analyzed by using a unified seakeeping and maneuvering two-time scale model in irregular sea that has been applied by Skejic and Faltinsen [1] for regular waves. The irregular wave effects are accounted for by Newman’s [2] approximation of the slow-drift 2nd order wave loads valid for deep water (Faltinsen [3], Pinkster [29]). The modular type maneuvering model (MMG model) based on Söding’s [4] nonlinear slender-body theory is used for the maneuvering analysis. Forces and moments due to rudder, propeller, and viscous cross-flow are accounted for as presented by Skejic and Faltinsen [1] and Yasukawa [5, 6]. In particular, the behavior of the propulsive coefficients (the thrust deduction and wake fraction) in waves (Faltinsen et al. [7], Faltinsen and Minsaas [8]) are discussed from the perspective of ship maneuvering characteristics in both regular and irregular wave environments. The unified model of seakeeping and maneuvering for deep-water irregular waves is validated for the ‘S7-175’ (‘SR 108’) container ship in calm water and regular deep-water wave scenarios by comparison with experimental results by Yasukawa [5, 6]. The maneuvering model is applied to a ‘MARINER’ ship performing turning maneuver in irregular waves. The obtained results of the ships main maneuvering parameters are discussed from a statistical point of view.

Study on Ship Structural Hydroelasticity and Fatigue Assessment in Irregular Seaways

This paper introduces an analysis of ship hydroelasticity for a fatigue assessment of ship structural design. In this study, the hydroelastic analysis for springing and whipping is carried out by using a fully coupled three-dimensional BEM-FEM approach with two-dimensional slamming theoreis, and a sequential fatigue assessment is performed. The fatigue damage is decomposed to wave frequency and high frequency components. Furthermore, the high frequency component is again decomposed to 1st harmonic springing, super harmonic springing and whipping contributions. This decomposition is achieved using a modal superposition principle. Amount of the contributions are compared in irregular sea states.

Impact of Elastic Body on the Deep and Shallow Water

Two-dimensional unsteady problem of elastic body impact on liquid free surface is considered. The water is either of infinite depth or shallow. We are concerned with the effect of the water depth on the bending stresses in the structure caused by the fluid-structure interaction. The Wagner model is used for infinite water depth. In the case of shallow water impact, the hydrodynamic problem is one-dimensional but nonlinear. Both problems for deep and shallow waters are solved numerically by the normal mode method. Two shapes of the body, cylindrical shell and elastic wedge, are considered. The impact conditions and the structural characteristic are identical. The bending stresses in the structure are investigated. It is shown that the bending stresses for impact on shallow water are greater than those for the infinite water depth. The developed methods and approaches can be combined with FFM to include complex structures.