An Assessment of Prediction Methods for Waves Inside the Moonpool of a Vessel (Comparisons of Numerical Solutions With Experiments)

This paper presents a preliminary assessment of the computational accuracy and efficiency of three different prediction methods for the water motion inside the moonpool of a rectangular box with forced vertical motion in a water tank. The first method is a linear solution method based on the linear wave diffraction/radiation theory (WAMIT). The second one is a method based on a CFD simulation (STAR-CCM+), the third method is a hybrid method combining a potential flow solver and a viscous flow solver (PVC3D). The accuracy of each method is assessed by comparing the prediction with the physical test data. The computational efficiency (complexity of setting up the computation and the computation speed) of the methods is discussed.

Benchmarking Experiments for Wave Absorption Modeling

Passive wave absorption is usually employed at the downstream end of a wave flume or basin to avoid the build-up of wave energy in the tank. However, absorption of waves is not perfect in physical tanks. A beach of different shape and/or composition can result in different absorption efficiency. Laboratory investigations of various passive beach configurations are costly and time-consuming. A more efficient approach is to perform studies using a numerical wave tank (NWT), which in turn requires empirical data to tune the dissipative effects. This study attempts to better understand the quality of waves simulated in a laboratory flume with a uniformly inclined porous beach and a parabolic-shaped solid beach. The data will be used to validate a newly-developed NWT with passive wave absorption. Different incident wave properties are examined and the reflection coefficient is calculated primarily with the two-probe method proposed by Goda & Suzuki (1976) and compared with other methods. An overview of the experiments, absorption analysis and numerical simulation is presented and discussed.

Optimizing the Performance of a Dual Coaxial-Cylinder Wave-Energy Extractor

This study evaluates two significant design modifications of a dual coaxial-cylinder system as a wave-energy extractor reported in Son and Yeung (2014, OMAE2014-#24582). First, a new and stronger power take-off (PTO) unit for a permanent magnet linear generator (PMLG) was built, along with an appropriate supporting structure, so as to match optimality conditions in terms of impedance matching and mechanical to electrical conversion efficiency. Based on a series of (dry-)bench tests, the properties of the PTO were obtained and the optimal operating conditions were determined. Second, the flat-bottom shape of the outer toroidal floater was modified according to “The Berkeley Wedge design” (Madhi et al, 2014, “The Berkeley Wedge: an asymmetrical energy-capturing floating breakwater of high performance,” Journal of Marine Systems and Ocean Technology, vol. 9(1), pp. 5–16). The new bottom shape led to reduction of the floater damping by almost 70%, which yielded a 3-fold increase in the floater motion response. Experiments in a wave-tank validated the response behavior of the dual-cylinder system with the use of the new PTO. The Berkeley-Wedge shape allowed more than 3 times more energy be extracted compared to the flat-bottom geometry, while the new generator also improved the energy conversion efficiency. As a result, the overall system efficiency of the device was enhanced remarkably five times over that of the previous design.

A Velocity Decomposition Approach for Unsteady External Flow

In this work we address the development of the velocity decomposition algorithm, a numerical flow solution method that incorporates both velocity potential and Navier-Stokes-based solution procedures. The motivation for this is so that the field discretization required by the Navier-Stokes solver can be reduced to the region of the flow domain in which the flow is vortical. Specific advantages are that the computational cost is reduced, it is easier to discretize the flow domain, and difficult problems such as the simulation of ships maneuvering in a seaway are closer to being within reach. The target applications are broad, ranging from vortex shedding on bluff objects such as risers, to the wave induced loads on a platform in a current and irregular seas. In previous work, the algorithm has been successfully applied to address steady flows of 3-D non-lifting bodies without water waves, or 2-D bodies that can have lift and be near a water surface. In this paper, the velocity decomposition approach is extended to numerically solve for the unsteady flow of single-phase viscous flows. The velocity vector is decomposed into irrotational and vortical components. A boundary element method is used to solve for the irrotational component (designated as the viscous potential) by applying a viscous boundary condition to the body boundary. A field method is used to solve for the total velocity on a reduced domain where the flow is vortical. The new algorithm investigates two approaches to solve the unsteady problem based on different types of time-dependence exhibited by the solution. The unsteady velocity decomposition method is demonstrated on two cases, and the solutions are compared to those generated by a conventional viscous flow solver. The results by the new algorithm agree well with the benchmark solutions and exhibit a reduction in time.

Development of a Ship Performance Simulator in Actual Seas

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) were entry into force from 2013. In order to improve energy efficiency of ships in service it is necessary to predict the fuel consumption in actual seas. In order to reduce GHG emission from ships, a Vessel Performance Simulator in Actual Seas has been developed. It simulates ship speed and fuel consumption at steady condition by using weather data and designated engine revolution. Physical models for hull, propeller, rudder and engine are used in the simulator. Especially steady wave forces, wind forces, drift forces, steering forces and engine/governor model are important factor for the estimation. The fuel consumption should be evaluated combined the ship hydrodynamic performance with the engine/governor characteristics. Considering the external forces by winds and waves, the operation point of the main engine is important for the estimation, since the torque limit and the other limit of the engine/governor are affected to the ship hydrodynamic performance. To prevent the increase of fuel consumption in service, the engine control system by the Fuel Index has been applied to present ships. In rough weather condition the revolution of the main engine is reduced to lower revolution by the Fuel Index limit. It causes the large decrease of ship speed but reduces the fuel consumption due to reduction of engine revolution. Using the simulator the navigation performance of a container ship, a RoRo vehicle carrier and a bulk carrier is simulated along the route. In this paper following contents are discussed; 1) evaluation of the physical model; steady wave forces, wind forces, drift forces, steering forces and engine/governor model, 2) simulation and validation of the physical model by tank tests and on-board measurements and 3) effectiveness of the ship performance simulator for GHG reduction.

Experimental Reconstruction of the Hull Sectional Forces Under Slamming Events

An analysis of the slamming load distribution along a slender floating body on the basis of experimental measurements is presented. The dataset is provided by the (lumped) vertical forces measured on the hull portions of a segmented scaled model in seakeeping tests. Using the proper orthogonal decomposition the vector time-history of the hydrodynamic forces is first decomposed into the summation of few terms, each one retaining separately time and space information. This decomposition allows highlighting interesting features regarding variations in the load distribution under different test conditions. The analysis has exploited also the use of pre-filtering for separating the load components at different frequencies to be further decomposed with POD. Then, a polynomial spline approximation under integral constraints is used to approximate the shape functions and then to obtain the continuous distribution of the sectional force. The considered tests were carried out in both irregular sea and regular waves with a choice of model speed and wave parameters so as slamming occurs. The identified distribution of force per unit length over the impacting hull segments is then compared with a modified Von-Karman model (with 3D correction) which accounts for the water uprise in the expression of the slamming force and variable entry velocity.

Vessel Performance Analysis and Fuel Management

A prototype Vessel Performance Monitoring and Analysis System (VPMAS) was deployed on a ferry to acquire needed performance data, to help improve vessel performance and reduce fuel consumption. A paper published in 2014 described preliminary data trends observed, key performance indicators computed, data products explored and exploratory tools developed for crews to gain insight into their vessel operation. The current paper describes further analysis of the operational data for speed optimization in calm sea states and the preliminary development of trim optimization software. It was found that trip durations around 7 hours (13.3 knots) use the least amount of fuel. The least amount of fuel is used when the excess distance travelled is minimized and the voyage time is optimized. There is a lot of leeway in terms of voyage time and excess distance travel by the ship before there is a heavy penalty on fuel consumption. Considering only a mean draft of 6 m and an average speed of 14 knots in the current paper, the optimal trim condition for the ferry is around −0.6 m (bow down), which reduces the resistance by 15% compared to the even keel condition. Positive trim causes the considerable increase of the total resistance consistently.

Experimental Studies of Hydrodynamic Interaction of Two Bodies in Waves

There are challenges in the prediction of low-frequency load and especially the resonant free surface elevation between two bodies in close proximity. Most of the linear potential-flow based seakeeping programs currently used by the industry over-predict the free surface elevation between the vessels/bodies and hence the low-frequency loadings on the hulls. Various methods, such as the lid technique, have been developed to suppress the unrealistic values of low-frequency forces by introducing artificial damping coefficients. However, without the experimental data, it is challenging to specify the coefficients. This paper presents the experimental studies of motions of two bodies with various gaps and the wave elevations between bodies. Model tests were performed at the towing tank of Memorial University. The objective was to provide benchmark data for further numerical studies of the viscous effect on the free surface predictions. The experimental data were compared with numerical solutions based on potential flow methods. The effect of tank walls were examined. Preliminary uncertainty analysis was also carried out.

A Real-Time System for Forecasting Extreme Waves and Vessel Motions

The University of Michigan is leading a team that includes subcontractors Ohio State University, Aquaveo, LLC, and Woods Hole Oceanographic Institute to design, implement, and test an Environmental and Ship Motion Forecasting (ESMF) system. The system has application to many challenges associated with offshore operations, including skin-to-skin transfer of cargo/personnel and extreme wave/response prediction. Briefly, the system uses a modified commercial-off-the-shelf (COTS) Doppler marine radar to determine the wave field surrounding the vessel; nonlinear wave theory to propagate the wave surface forward in time; and seakeeping theory to predict future vessel motions. A major challenge is that all computations must be done in real time. This paper will briefly describe the system and show an example application of predicting extreme waves and motions for a floating offshore type platform.

Dynamic Domain Decomposition Strategy Coupling Lattice Boltzmann Methods With Finite Differences Approximations of the Navier-Stokes Equations to Study Bodies in Current

A Domain-Decomposition (DD) strategy is proposed for problems involving regions with slow variations of the flow (A) and others where the fluid features undergo rapid changes (B), like in the case of steady current past bodies with pronounced local unsteadiness connected with the vortex shedding from the structures. For an efficient and accurate solution of such problems, the DD couples a Finite Difference solver of the Navier-Stokes equations (FD-NS) with a Multiple Relaxation Time Lattice Boltzmann method (MRT-LBM). Regions A are handled by FD-NS, while zones B are solved by MRT-LBM and the two solvers exchange information within a strong coupling strategy. Present DD strategy is able to deal with a dynamic change of the sub-domains topology. This feature is needed when regions with vorticity shed from the body vary in time for a more flexible and reliable solution strategy. Its performances in terms of accuracy and efficiency have been successfully assessed by comparing the hybrid solver against a full FD-NS solution and experimental data for a 2D circular cylinder in an impulsively started flow.