Coupling of Heave and Roll for High-Speed Planing Hulls

For planing hulls, dynamic lift reduces the submergence of the hull, allowing small motions to result in large changes in hydrodynamic forces and moments. The dynamic lift forces acting on the bottom of a planing hull dominate the hydrodynamics and these lift forces are known to depend on speed and wetted surface. As a planing boat rolls the wetted surface changes, which affects the dynamic lift. A series of tests using a wooden prismatic planing hull model with a constant deadrise of 20 degrees were done at static heel and heave positions as well as oscillating heave conditions. This paper presents the results from these experiments, primarily looking at the hydrodynamic coefficients in heave as a function of heel angle and exploring the coupling between these motions for a prismatic high-speed planing hull.

Waves’ Numerical Dispersion and Damping due to Discrete Dispersion Relation

In linear Rankine panel method, the discrete linear dispersion relation is solved on a discrete free-surface to capture the free-surface waves generated due to wave-body interactions. Discretization introduces numerical damping and dispersion, which depend on the discretization order and the chosen methods for differentiation in time and space. The numerical properties of a linear Rankine panel method, based on a direct boundary integral formulation, for capturing two and three dimensional free-surface waves were studied. Different discretization orders and differentiation methods were considered, focusing on the linear distribution and finite difference schemes. The possible sources for numerical instabilities were addressed. A series of cases with and without forward speed was selected, and numerical investigations are presented. For the waves in three dimensions, the influence of the panels’ aspect ratio and the waves’ angle were considered. It has been shown that using the cancellation effects of different differentiation schemes the accuracy of the numerical method could be improved.

Numerical Study of Two Vessels Seakeeping in Waves

This paper presents a numerical study of seakeeping in regular waves for two vessels in close proximity using commercial seakeeping software HydroStar and an in-house code MOTSIM. The objective was to study the possible sheltering effect of the larger vessel (FPSO) on the smaller one (OSV) during personnel transfer between the two vessels, where one vessel was at some angle relative to the other vessel and there was no connection line between them. The study mainly focused on the OSV motion resulting from the interaction of the FPSO when the OSV was at different headings and wave directions. Initially the OSV motions close to the FPSO (and parallel) were compared with those for the OSV alone. For an un-parallel position of the two vessels, an objective function based on the OSV RAOs motion in roll, pitch and heave directions was used to optimize the OSV position. Finally comparisons between HydroStar and MOTSIM results are provided. The main conclusions are: 1) When the FPSO and OSV are located in parallel, the OSV motions in sway, roll and yaw are larger than the single OSV motions in head waves while surge, heave and pitch are almost the same. The OSV motions in most of the six degrees of freedom are smaller than the single OSV motions when the waves are from other directions (always on the port side of the FPSO), which means that there is a sheltering effect. 2) The simulation results from different OSV rotation angles show that the hydrodynamic interaction between the FPSO and OSV e.g. the sheltering effect is related to the OSV angle and the wave heading. The objective function in roll, pitch and heave RAOs indicates that the OSV should maintain a close to parallel position with the FPSO to minimize motion when the waves come from the port side of the FPSO from 180 to 240 degrees. When the wave direction is around 240 degrees the OSV should have relatively small motion in waves for any OSV rotation angle. 3) A comparison of HydroStar and MOTSIM results shows that the MOTSIM results of a single vessel seakeeping simulation is in a good agreement with HydroStar. In two vessels situation more validation work needs to be done.

A Parametric Study of Low-Frequency Damping of Mooring System

Mooring line damping plays an important role to the body motion of moored floating platforms. Meanwhile, it can also make contributions to optimize the mooring line system. Accurate assessment of mooring line damping is thus an essential issue for offshore structure design. However, it is difficult to determine the mooring line damping based on theoretical methods. This study considers the parameters which have impact on mooring-induced damping. In the paper, applying Morison formula to calculate the drag and initial force on the mooring line, its dynamic response is computed in the time domain. The energy dissipation of the mooring line due to the viscosity was used to calculate mooring-induced damping. A mooring line is performed with low-frequency oscillation only, the low-frequency oscillation superimposed with regular and irregular wave-frequency motions. In addition, the influences of current velocity, mooring line pretension and different water depths are taken into account.

Heave Induced Internal Flow Fluctuations in Vertical Transport Systems for Deep Ocean Mining

Deep ocean mining systems will have to operate often in harsh weather conditions with heavy sea states. A typical mining system consists of a Mining Support Vessel (MSV) with a Vertical Transport System (VTS) attached to it. The transport system is a pump pipeline system using centrifugal pumps. The heave motions of the ship are transferred to the pump system due to the riser-ship coupling. Ship motions thus will have a significant influence on the internal flow in the VTS. In this paper, the influence of heave motions on the internal flow in the VTS for a typical mining system for Seafloor Massive Sulfide (SMS) deposits in Papua New Guinea is analyzed. Data on the wave climate in the PNG region is used to compute the ship motions of a coupled MSV-VTS. The ship motions then are translated into forces acting on the internal flow in order to compute fluctuations in the internal flow. In this way, the workability of the mining system with respect to the system’s production can be assessed. Based on a detailed analysis of the internal flow in relation to ship motions, the relevance of a coupled analysis for the design of VTS is made clear. This paper provides a method for performing such analyses.

Influence of the Waterline Integral on the Solution of the Frequency-Domain Forward-Speed Radiation-Diffraction Problem of a Ship Advancing in Waves

The radiation-diffraction potential of a ship advancing in waves is studied using the three-dimensional frequency-domain forward-speed free-surface Green function (Brard 1948) and the forward-speed Green integral equation (Hong 2000). Numerical solutions are obtained by making use of a second-order inner collocation boundary element method which makes it possible to take account of the line integral along the waterline in a rigorous manner (Hong et al. 2008). The present forward-speed Green integral equation includes not only the usual free surface condition for the potential but also the adjoint free surface condition for the forward-speed free-surface Green function as indicated by Brard (1972). Comparison of the present numerical results of the heave-heave wave damping coefficients and the experimental results for the Wigley ship models I, II and III (Journee 1992) has been presented. These coefficients are compared with those calculated without taking into account of the line integral along the waterline in order to show the forward speed effect represented by the waterline integral when it is properly included in the free-surface Green integral equation. Comparison of the present numerical results and the equivalent time-domain results (Hong et al. 2013) has also been presented.

Numerical Study of Wave Slamming on an Oscillating Flap

Bottom hinged Oscillating Wave Surge Converters (OWSCs) are efficient devices for extracting power from ocean waves. There is limited knowledge about wave slamming on such devices. This paper deals with numerical studies of wave slamming on an oscillating flap to investigate the mechanism of slamming events. In our model, the Navier–Stokes equations are discretized using the Finite Volume method with the Volume of Fluid (VOF) approach for interface capturing. Waves are generated by a flap-type wave maker in the numerical wave tank, and the dynamic mesh method is applied to model the motion of the oscillating flap. Basic mesh and time step refinement studies are performed. The flow characteristics in a slamming event are analysed based on numerical results. Various simulations with different flap densities, water depths and wave amplitudes are performed for a better understanding of the slamming.

Analysis of the German Navy Stability Standard BV 1030 With Respect to Operability in Heavy Weather

The stability standard of the German Navy — the BV 1030 — was developed in the mid-sixties of the last century in close cooperation with the German Navy Authorities (now BAIINBw) and the University of Hamburg. Other than the stability standards used for commercial shipping, the BV 1030 is based on righting and heeling lever balances for each individual loading condition of each individual ship. Different types of heeling moments have to be assumed in several combinations and they have to be balanced against the righting levers of the ship. Not only the still water stability curve is subject to this lever balance, but also wave crest and wave trough situations are subject to the stability analysis. The BV 1030 stability standard further requires a minimum stability if the ship is on the wave crest. Since this stability standard is in force, the German Navy never experienced a stability accident. The development of new hull forms with the focus on fuel efficiency has widely brought up new problems in heavy weather, for example the vulnerability for parametric rolling. It was therefore of interest for the German Authorities (BAIIN) whether the existing stability standard has sufficient safety to cover also these phenomena connected to more modern hull forms. Therefore an analysis was carried out in close cooperation between BAIINBw, MARS and TUHH where the operability of several ships of the German Navy was analyzed with numerical sea keeping computations. The nonlinear sea keeping code E4ROLLS was used which allows the computation of time series of the ship motions in irregular, short crested seas. From these computations, operational limits could be derived, or, vice versa, the required stability to guarantee a certain operability. The results showed that the concept of the German BV 1030 stability standard provides a significantly higher safety level compared to IMO standard for commercial ships. The results did also show that for modern hull forms, some adjustments to the existing safety standard were found to be useful to better cope with righting arm fluctuations in longitudinal waves.

Numerical Simulation of Tsunami Run-Up and Inundation for the 2011 Tōhuku-Oki Tsunami: A Parametric Analysis for Tsunami Run-Up and Wave Height

This paper presents computational results for predicting earthquake-generated tsunami from a developed integrated computational framework. The computational framework encompasses the entire spectrum of modeling the earthquake-generated tsunami source, open-sea wave propagation, and wave run-up including inundation and on-shore effects. The present work develops a simplified source model based on pertinent local geologic and tectonic processes, observed seismic data (i.e., data obtained by inversion of seismic waves from seismographic measurements), and geodetic data (i.e., directly measured seafloor and land deformations). These source models estimated configurations of seafloor deformation used as initial waveforms in tsunami simulations. Together with sufficiently accurate and resolved bathymetric and topographic data, they provided the inputs needed to numerically simulate tsunami wave propagation, inundation and coastal impact. The present work systematically analyzes the effect of the tsunami source model on predicted tsunami behavior and the associated variability for the 2011 Tōhuku-Oki tsunami. Simulations were carried out for the 2011 Tōhuku -Oki Tsunami that took place on March 11, 2011, from an MW 9.1 earthquake. The numerical simulations were performed using the fully nonlinear Boussinesq hydrodynamics code, FUNWAVE-TVD (distributed by the University of Delaware). In addition, a sensitivity analysis was also carried out to study the effect of earthquake magnitude on the predicted wave height. The effect of coastal structure on the wave amplification at the shore is also studied. Simulated tsunami results for wave heights are compared to the available observational data from GPS (Global Positioning System) at the central Miyagi location.

Numerical Study on Coastal Wave and Near-Shore Current Interaction

Coastal waves and near-shore currents have been investigated by many researchers. This paper developed a two-dimensional numerical model of near-shore waves and currents to study breaking wave induced current. In the model, near-shore water wave was simulated by a parabolic mild slope equation incorporating current effect and wave energy dissipation due to breaking, and current was simulated by a nonlinear shallow water equation incorporating wave exerted radiation stress. Wave radiation stress was calculated based on complex wave amplitude in the parabolic mild slope equation, and this result in an effective method for calculating wave radiation stress using an intrinsic wave propagation angle that differs from the ones of using explicit wave propagation angle. Wave and current interactions were considered by cycling the wave and current equation to a steady state. The model was used to study waves and wave-induced longshore currents at the Obaköy coastal water which is located at the Mediterranean coast of Turkey. The numerical results for water wave induced longshore current were validated by measured data to demonstrate the efficiency of the numerical model, and water waves and longshore currents were analyzed based on the numerical results.