Effective Estimation of Water Waves Scattering by Double-Row Pile Breakwaters

This paper describes various hydraulic characteristic of double-row pile breakwaters (DPB). Applying an eigenfunction expansion method, a numerical method have been developed that can compute wave transmission, reflection, and other hydraulic characteristics. To verify the validity of developed prediction, laboratory experiments of Isaascson et al. (1999) have been utilized. Then for an efficient calculation, optimum number of necessary evanescent waves for an effective and efficient prediction is discussed for various hydraulic quantities of interest. In a nutshell, for an effective and efficient performance of the DPB, intermediate water wave and porosity range of [0.2 0.3] are recommended. Relative distance between two barriers must be set depending on significant wave length of design.

Motion Response Control of the DWSC Spar Based on State-Space Model

In 2007, the Office of Naval Research (ONR) started a technology development program called STLVAST (Small to Large Vessel At-Sea Transfer), in order to develop ‘enabling capabilities’ in the realm of logistic transfer (i.e. stores, equipment, vehicles) between a large transport vessel and a smaller T-craft ship, using a Deep Water Stable Crane (DWSC) spar between them. In this paper, the equation of motions of the single DWSC spar is initially expressed as the standard state-space model. Then the ODE solver of Matlab is directly employed to obtain the motion responses at each time step. Two levels of approximation of hydrodynamic coefficients are considered in this study. One is the Constant Coefficient Method (CCM), and the other one is the Impulse Response Function (IRF) method, with fluid memory effects considered. WAMIT software is used to calculate the hydrodynamic coefficients, including the added mass, radiation damping, IRF, the first order and second order waves loads transfer functions, etc. The motion response control is achieved by assuming the thrusters can provide the optimal feedback force derived from Linear Quadratic Regulator (LQR) method.

Thrust Enhancement From Radial Velocity in Squid Inspired Thrusters

Squid and jellyfish generate propulsive forces by successively taking in and expelling high momentum jets of water. This method of propulsion offers several advantages to underwater vehicles/robots. The driving mechanism can be placed internal to the vehicle, reducing the drag associated with an abundance of external thrusters and control surfaces. The thrusters can generate accurate predictable forcing in the low thrust range, while still generating thrust nearly instantaneously over the entire force range. Vortex ring formation dynamics play an important role in creating thrust. It is observed that squid and jellyfish eject fluid jets which are not exactly parallel, and have a contracting velocity in the radial direction. A prototype thruster was developed which generates both parallel and converging propulsive jets. The total impulse of the jet is determined from DPIV techniques to determine the effect a non-zero radial velocity had on thrust production. The radial velocity was observed to increase the total impulse of the jet by 70% for low stroke ratio jets, and 75% for large stroke ratio jets.

Two- and Three-Dimensional Simulations of the Flow Around a Cylinder Fitted With Strakes

Two- and three-dimensional simulations of the flow around straked cylinders are presented. For the two-dimensional simulations we used the Spectral/hp Element Method, and carried out simulations for five different angles of rotation of the cylinder with respect to the free stream. Fixed and elastically-mounted cylinders were tested, and the Reynolds number was kept constant and equal to 150. The results were compared to those obtained from the simulation of the flow around a bare cylinder under the same conditions. We observed that the two-dimensional strakes are not effective in suppressing the vibration of the cylinders, but also noticed that the responses were completely different even with a slight change in the angle of rotation of the body. The three-dimensional results showed that there are two mechanisms of suppression: the main one is the decrease in the vortex shedding correlation along the span, whilst a secondary one is the vortex wake formation farther downstream.

Computational Fluid Dynamics Study for Modeling Vortex Induced Vibrations Using High Fidelity Turbulence Models

Flow over smooth cylinder at very high Reynolds number, ReD = 2×106, is simulated using the unsteady Scale Adaptive Simulation (SAS) turbulence model. Flow structures and vortex shedding were accurately captured. Grid sensitivity study was performed to compare averaged drag coefficient for a conformal fine mesh as well as non-conformal coarse mesh. Predicted value of drag coefficient was within 8% of the experimental value and Strouhal number compared well with the experimental observations.

CFD Virtual Testing for Resistance, Wind and Current Loads on a Supply Boat

Ship and platforms environmental loads are often predicted by model scale experiments or empirical calculations only. This paper presents the application of a commercial CFD (Computational Fluid Dynamics) software as a numerical approach to calculate the flow around a supply boat considering current, wind loads and fluid resistance determination. Since, owing to many practical situations, free surface effects can be neglected, the above and underwater problems can be uncoupled and independently evaluated, although for higher Froude number cases in the resistance determination the interfacial effects have to be considered. Throughout this paper, the assumptions adopted and the boundary conditions applied are discussed. All meshes were developed on ICEM CFD® and appropriate mesh density studies indicate that meshes with approximately 2 million nodes can represent the experiments accurately. The simulations were conducted using the ANSYS CFX® solver, using Perl scripting for automatic evaluation of multiple run settings and simulation execution at Petrobras Research Center Clusters. The results for each case are compared with either towing tank or wind tunnel experimental data, both harvested at IPT (Instituto de Pesquisas Tecnológicas). The comparison between simulation results and experiments allows the analysis of the present CFD models benefits and limitations, providing guidelines for future similar studies. The overall match between laboratory and virtual tests results supports the expansion of this procedure to other vessels and offshore floating units.

Workshop on Verification and Validation of CFD for Offshore Flows

This document introduces the Workshop on Verification and Validation (V&V) of CFD for Offshore Flows, to be held during OMAE2012. It presents a brief introduction to the purpose of Verification and Validation with the identification of the goals of code and solution verification and validation. Within this context, three test-cases are proposed: Case-I of code verification, Case-II of solution verification and Case-III of solution verification and validation. Case-I consists on a 3D manufactured solution of an unsteady turbulent flow. Case-II is an exercise on the canonical problem of the infinite smooth circular cylinder flow at different Reynolds numbers. Case-III is a more complex flow around a straked-riser. The participants are asked to perform at least one of these test-cases. The objectives for the three proposed test-cases are presented, together with a detailed description of the numerical settings to be used, and the results to be obtained and sent to the Workshop organization. At the end some considerations on general conditions, paper submission, deadlines, and encouragements are stated.

Wind Effect Estimation in Side by Side Offloading Operation for FLNG and LNG Carrier Ships

Liquid Natural Gas resource development is often conducted worldwide. Recently the drilling area has gradually expanded from shallow sea area to the deep ocean. A Floating LNG facility (FLNG) and a LNG carrier ship (LNG) are assumed to operate in the open sea expected to wind, wave and current. In this situation, an operational capability evaluation of the LNG would be needed to grasp the operational weather limitation. The effect of each weather element, i.e. wind, wave and current, giving manoeuvring effect to ships, is expected to assess exactly as external loads. In such a situation, wind interaction effect under the operating condition that a FLNG and a LNG are in same closed area is not clearly understood. This paper treats and proposes one estimation method of wind load for the operation of side-by-side offloading including interaction effect of a FLNG and a LNG. The proposed wind load estimation method based on the wind tunnel experiments represents the shielding effect of the LNG behind the FLNG. Operational assessment on ship manoeuvring under strong wind is calculated using the proposed wind load method in the final stage.

Experimental Analysis of Surface Geometry, External Damping and Waves on Semisubmersible Vortex Induced Motions

Semisubmersible Vortex Induced Motions have been studied extensively at model scales, and observed under field conditions. VIM response is of particular concern in evaluations of Steel Catenary Riser (SCR) fatigue damage. Historically, most of this experimental work was done with bare models, without much effort to represent actual hull surface characteristics. In this paper, the effect of these appurtenances, tow direction and wave action is investigated. There is also an examination of vortex induced yaw response. VIM response is assessed using motion trajectories, dominant response direction, and amplitudes and periods of response. Variations in these quantities are also investigated.

Flow Induced Fluttering and Autorotation of a Hinged Vertical Flat Plate

This paper addresses the investigations on fluttering and autorotation motions which may occur in the interaction of uniform current and freely rotating plate about a fixed vertical axis. The autorotation is a name given to the case that the plate turns continuously about the vertical axis and the fluttering motion is the periodic or chaotic oscillations of the plate around the vertical axis. According to the dimensional analysis the motion in flow induced rotation motion is governed essentially by dimensionless moment of inertia, Reynolds number and initial conditions. Certain combinations define the stability boundaries between fluttering and autorotation. Hence, a bifurcation diagram was prepared by the experiments to classify different states observed the small fluttering, fluttering and autorotation based on different Reynolds number and dimensionless moment of inertia.