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This study evaluated a multi-catenary spread mooring system design of a mobile ocean test berth for wave energy converters (WECs), the Ocean Sentinel (OS) instrumentation buoy, through dynamic simulation, numerical analysis, and comparison with measured field motion data of the OS off the OR coast. First, the accuracy of the numerical employed model based on a fully-coupled method of the OS and its mooring lines was validated by comparing predicted mooring tensions to the field measurements. Then, the anchor movability, fatigue damage, and extreme mooring tension of the OS mooring system were analyzed to assess survivability. Field test results show that the numerical model provided accurate predictions of mooring tensions even under environmental conditions of strong wind, current, and waves. Factors affecting the accuracy are discussed. One mooring anchor was shown to have moved significantly during the ocean field test. Mooring fatigue damage was calculated for different levels of sea states. Design strengths of the mooring lines were calculated and analyzed.

The investigation of innovative macroalgal cultivation is important and needed to optimize farming operations, increase biomass production, reduce the impact on the ecosystem, and lower system and operational costs. However, most macroalgal farming systems (MFSs) are stationary, which need to occupy a substantial coastal area, require extensive investment in farm infrastructure, and cost high fertilizer and anchoring expenses. This study aims to model, analyze, and support a novel binary species free-floating longline macroalgal cultivation concept. The expected outcomes could provide a basis for the design and application of the novel MFS to improve biomass production, decrease costs, and reduce the impact on the local ecosystem. In this paper, Saccharina latissima and Nereocystis luetkeana were modeled and validated, and coupled with longline to simulate the binary species MFS free float in various growth periods and associated locations along the US west coast. The numerical predictions indicated the possibility of failure on the longline and breakage at the kelp holdfasts is low. However, the large forces due to an instantaneous change in dynamic loads caused by loss of hydrostatic buoyancy when the longline stretches out of the water would damage the kelps. Buoy-longline contact interactions could damage the buoy, resulting in the loss of the system by sinking. Furthermore, the kelp-longline and kelp-kelp entanglements could potentially cause kelp damage.

The survivability, safe operation, and design of marine vehicles and wave energy converters are highly dependent on accurate characterization and estimation of the energy content of the ocean wave field. In this study, analytical solutions of the nonlinear Schrödinger equation (NLS) using periodic inverse scattering transformation (IST) and its associated Riemann spectrum are used to obtain the nonlinear wave modes (eigen functions of the nonlinear equation consisting of multiple phase-locked harmonic components). These nonlinear wave modes are used in two approaches to develop a more accurate definition of the energy content. First, in an ad hoc approach, the amplitudes of the nonlinear wave modes are used with a linear energy calculation resulting in a semi-linear energy estimate. Next, a novel, mathematically exact definition of the energy content taking into account the nonlinear effects up to fifth order is introduced in combination with the nonlinear wave modes, the exact energy content of the wave field is computed. Experimental results and numerical simulations were used to compute and analyze the linear, ad hoc, and exact energy contents of the wave field, using both linear and nonlinear spectra. The ratio of the ad hoc and exact energy estimates to the linear energy content was computed to examine the effect of nonlinearity on the energy content. In general, an increasing energy ratio was observed for increasing nonlinearity of the wave field, with larger contributions from higher-order harmonic terms. It was confirmed that the significant increase in nonlinear energy content with respect to its linear counterpart is due to the increase in the number of nonlinear phase-locked (bound wave) modes.2

In this study, effects of damage levels of fiber ropes on the performance of a hybrid taut-wire mooring system are investigated. The analysis is performed using a numerical floating production storage and offloading (FPSO) model with a hybrid mooring system installed in 3000 m of water depth. An in-depth study was conducted using the numerical model, the dynamic stiffness equation of damaged fiber ropes, the time-domain dynamic theory, the rainflow cycle counting method, and the linear damage accumulation rule of Palmgren-Miner. Results indicate that, in a mooring line with an increasing damage level, the maximum tension decreases, while the offset of the FPSO increases. Particularly, when a windward mooring line failure occurs, in addition to the significant increase in the offset of the FPSO, the maximum tension, tension range, and annual fatigue damage levels of the remaining lines adjacent to the failed also increase significantly. The present work can be of great benefit to the evaluation of the offset of the floating platform, the tension response, and the service life of the hybrid mooring systems.

Flow systems with highly nonlinear free/moving surface motion are common in engineering applications, such as wave impact and fluid-structure interaction (FSI) problems. In order to reveal the dynamics of such flows, as well as provide a reduced-order modeling (ROM) for large-scale applications, we propose a proper orthogonal decomposition (POD) technique that couples the velocity flow field and the level-set function field, as well as a proper normalization for the snapshots data so that the low-dimensional components of the flow can be retrieved with a priori knowledge of equal distribution of the total variance between velocity and level-set function data. Through numerical examples of a sloshing problem and a water entry problem, we show that the low-dimensional components obtained provide an efficient and accurate approximation of the flow field. Moreover, we show that the velocity contour and orbits projected on the space of the reduced basis greatly facilitate understanding of the intrinsic dynamics of the flow systems.

This study illustrates a comparison of two numerical methods under a unified computational platform for solving fluid–structure interaction (FSI) problems. The first is an arbitrary Lagrangian–Eulerian (ALE)-based fluid model coupled to a structural finite element (FE) method (ALE-FE/FE), and the second is a smoothed particle hydrodynamics (SPH) method coupled to the same structural FE code (SPH/FE). The predictive capabilities and computational efficiency of both the numerical methods are evaluated and validated against a canonical problem of a rapidly varying flow past an elastic gate for which experimental data are available. In both numerical solutions, the fluid flow is governed by the Navier–Stokes equation, and the elastic gate is modeled as a flexible structure. Numerical simulation results show that the ALE-FE/FE continuum approach not only captures the dynamic behavior properly but also predicts the water-free surface profiles and the elastic gate deformations accurately. On the other hand, the coupled purely Lagrangian approach of the SPH/FE under an identical computational platform is found to be less accurate and efficient in predicting the dynamics of the elastic gate motion and the water-free surface profiles.

Dynamic position (DP) control and pipeline dynamics are the two main parts of the deepwater S-lay simulation model. In this study, a fully coupled analysis tool for deepwater S-lay deployment by dynamically positioned vessels is developed. The method integrates the major aspects related to numerical simulation, including coupled pipeline motion and roller contact forces. The roller–pipe interaction is incorporated in the S-lay pipeline model using a contact search method based on a lumped-mass (LM) formulation in global coordinates. A proportional-integration-differentiation (PID) controller and a Kalman filter are applied in the vessel motion equation to calculate the thrust allocation of the DP system in time domain. Numerical simulation results showed that the dynamic effects add a significant contribution to the tension, but have little influence on the maximum pipe stress and strain. The dynamic response of the coupled S-lay and DP pipeline deployment system increases the demand on the tensioner load carrying capability as well as the maximum DP thruster power.

A two-dimensional fully nonlinear potential flow model is employed to investigate nonlinear stochastic responses of an experimental fluid-structure interaction system that includes both single-degree-of-freedom surge-only and two-degree-of-freedom surge-heave coupled motions. Sources of nonlinearity include free surface boundary, fluid-structure interaction, and large geometry in the structural restoring force. Random waves performed in the tests include nearly periodic, periodic with band-limited noise, and narrow band. The structural responses observed can be categorized as nearly deterministic (harmonic, sub- and super-harmonic), noisy periodic, and random. Transition phenomena between coexisting response attractors are also identified. An implicit boundary condition upholding the instantaneous equilibrium between the fluid and structure using a mixed Eulerian-Lagrangian method is employed. Numerical model predictions are calibrated and validated via the experimental results under the three types of wave conditions. Extensive simulations are conducted to identify the response characteristics and the effects of random perturbations on nonlinear responses near primary and secondary resonances.

An independent-flow-field (IFF) model selected in this study to investigate the nonlinear response behavior of a medium-scale, experimental, submerged, moored structure is validated via parametric studies. Bifurcations in experimental responses are frequently observed, and the associated nonlinear primary and secondary resonances are identified in frequency response diagrams. Distinct from previous investigations, this study intends to identify a set of “best-fit” constant coefficients for predictions and comparisons over the entire wave frequencies examined. It is concluded that the small-body, IFF model predicts, reasonably well, the nonlinear, moored, and submerged structural response subjected to regular waves.

Complex responses observed in an experimental, nonlinear, moored structural system subjected to nearly periodic wave excitations are examined and compared to the simulations of a newly proposed independent-flow-field (IFF) model in this paper. Variations in wave heights are approximated by additive random perturbations to the dominant periodic component. Simulations show good agreement with the experimental results in both time and frequency domains. Noise effects on the experimental results, including bridging and transition phenomena, are investigated and interpreted by comparing to the simulations of its deterministic counterpart. Possible causes of a chaoticlike experimental result as previously observed are also inferred.

A computationally efficient quasi-two-degree-of-freedom (Q2DOF) stochastic model and a stability analysis of barges in random seas are presented in this paper. Based on the deterministic 2DOF coupled roll-heave model with high-degree polynomial approximation of restoring forces and moments developed in Part I, an attempt is made to further reduce the DOF of the model for efficient stochastic stability analysis by decoupling the heave effects on roll motion, resulting in a one-degree-of-freedom (1DOF) roll-only model. Using the Markov assumption, stochastic differential equations governing the evolution of probability densities of roll-heave and roll responses for the two low-DOF models are derived via the Fokker-Planck formulation. Numerical results of roll responses for the 2DOF and 1DOF models, using direct simulation in the time domain and the path integral solution technique in the probability domain, are compared to determine the effects of neglecting the influence of heave on roll motion and assess the relative computational efforts required. It is observed that the 1DOF model is computationally very efficient and the 2DOF model response predictions are quite accurate. However, the nonlinear roll-heave coupling is found to be significant and needs to be directly taken into account, rendering the 1DOF roll-only model inadequate for practical use. The 2DOF model is impractical for long-duration real-time response computation due to the insurmountable computational effort required. By taking advantage of the observed strong correlation between measured heave and wave elevation in the experimental results, an accurate and efficient Q2DOF model is developed by expressing the heave response in the 2DOF model as a function of wave elevation, thus reducing the effective DOF to unity. This Q2DOF model is essential as it reduces the computational effort by a factor of 10−5 compared to that of the 2DOF model, thus making practical stochastic analysis possible. A stochastic stability analysis of the barge under operational and survival sea states specified by the U.S. Navy is presented using the Q2DOF model based on first passage time formulation.

In this first part of a two-part study, the general nonlinear system identification methodology developed earlier by the authors for a single-degree-of-freedom (SDOF) system using the reverse-multi-input/single-output (R-MI/SO) technique is extended to a multi-degree-of-freedom (MDOF), sub-merged, moored structure with surge and heave motions. The physical nonlinear MDOF system model and the formulation of the R-MI/SO system-identification technique are presented. The corresponding numerical algorithm is then developed and applied to the experimental data of the MDOF system using only the subharmonic motion responses to identify the system parameters. The resulting model is then employed in Part 2 for a detailed analysis of both the sub and superharmonic dynamic behavior of the MDOF experimental system and a comparison of the MDOF response results and observations with those of the corresponding SDOF system examined earlier by the authors.

INTRODUCTION This paper focuses on the development of optimal deterministic, nonlinearly coupled barge motion models, identification of their system parameters and calibration of their prediction capability using experimental results. The ultimate objective is to develop accurate yet sufficiently low degree-offreedom stochastic models suitable for efficient probabilistic stability and reliability analyses of US Naval barges for preliminary design and operation guideline development (see Part II). First a three-degree-of-freedom (3DOF) fully coupled Roll-Heave-Sway model, which features realistic and practical high-degree polynomial approximations of rigid body motion relations, hydrostatic and hydrodynamic force-moment specifically suitable for barges, is examined. The hydrostatic force-moment relationship includes effects of the barge’s sharp edge and combined roll-heave states, and the hydrodynamic terms are in a “Morison” type quadratic form. System parameters of the 3DOF model are identified using physical model test results from several regular wave cases. The predictive capability of the model is then calibrated using results from a random wave test case. Recognizing the negligible sway influence on coupled roll and heave motions and overall barge stability, and in an attempt to reduce anticipated stochastic computational efforts in stability analysis, a 2DOF Roll-Heave model is derived by uncoupling sway from the roll-heave governing equations of motion. Time domain simulations are conducted using the (3DOF) Roll-Heave-Sway and the (2DOF) Roll-Heave models for regular and random wave cases to validate the model assumptions and to assess their (numerical) prediction capabilities. In the design of ship-to-shore transport cargo barges, it is essential to determine barge stability for a range of operational and survival sea conditions. In general, the barges will operate in a variety of directional seastates. However, the most unstable scenario is if the barges broach and become broadside to the waves in the so called “beam seas” and may incur large amplitude three-degrees of freedom (3DOF) -roll, heave and sway motions with the possibility of capsizing [1]. Many researchers further reduced the DOF of the systems to that of roll only by taking advantage of the dominant roll behavior [27]. Parameters for the coefficients of nonlinear roll motions were determined [8] and the roll motions characteristics of full scale ships were examined [9]. A stochastic approach to the analysis of noisy periodic roll motions was proposed [10]. This paper presents a deterministic 3DOF Roll-HeaveSway model [7, 11], and a corresponding two-degree-offreedom (2DOF) Roll-Heave model [12-13], to predict barge motion responses. These low DOF models, with high-degree polynomial approximations of force and moment relationships, capable of capturing the important nonlinear characteristics of the coupled nonlinear responses for large roll angle motions, will be used in the development of efficient stochastic models for preliminary design and response predictions under operational and survival conditions (see Part II). In research conducted earlier at Oregon State University, a one-degree-of-freedom (1DOF) system [10] was developed to model pure roll motion of a barge in random beam seas. Nonlinearities of the model include the righting moment and fluid-structure viscous effects. Hydrodynamic and structural damping effects were approximated by a linear term plus a Copyright © 2004 by ASME 1 “Morison” type quadratic term [14]. The righting moment included nonlinear stiffness terms to provide a more accurate restoring moment at larger roll angles. This 1DOF model was compared with measured barge motion data and was found capable of reasonable predictions in terms of statistical moments, spectral densities, and histograms. ( ) ( ) M I dt d F mv dt d = = ω ; (1) An inertial coordinate system is placed at the location of the prescribed body-fixed "roll center" of the barge under static equilibrium. Note the inertial coordinate system coincides with the body-fixed (moving) coordinate system initially. Static roll righting moments and heave buoyant restoring forces are calculated as a function of the position and rotation of the barge about the roll center. Equilibrium of forces and moments are considered about the roll center (the position of which is time dependent with respect to the inertia coordinates) with heave and sway directions respect to the inertial coordinates. In this study, we focus our discussion on a three-degreesof-freedom (3DOF) deterministic model including the nonlinear coupling effects of roll, heave and sway motions [1, 11]. This 3DOF model is expected to improve the predictive capability at large roll angles over the 1DOF system because the heave and sway coupling effects with the roll through hydrostatics and rigid body kinematics are included, and significantly higher degree polynomial approximations are employed. The equations of motion of the rigid barge including hydrostatics are first derived. Waves are then applied and terms modeling the hydrodynamic properties are added. Relative motion effects of the barge with respect to the free surface are included. The effects due to hydrostatics are represented with sufficiently high degree polynomials in the model. Various degree polynomials were examined to identify an optimum fit. Because the edges of the barge are sharp, fairly high degree polynomials are required. The coupling effects of sway on roll-heave response prediction are examined using the Roll-Heave-Sway model and a corresponding (2DOF) rollheave model with similar parameters. The body-fixed coordinates are defined such that X = Surge, Y = Sway, Z = Heave, φ = Roll, Θ = Pitch, and ψ = Yaw (Fig. 1). For the (body-fixed) coordinate system origin, the roll center is at the center of gravity of barge and the coordinate system axes are aligned with the principal axes of inertia. One of the main objectives in this study is to extend the equations of motion for a SDOF system in roll to a multi-DOF system. For a symmetric barge in beam seas, the dominant response will be in sway, heave and roll. The surge, pitch and yaw motions become negligible [11-13]. Equation (1) now becomes,     − − =       − + =     − − =

A system-identification technique based on the Reverse Multiple-Input/Single-Output (RMI/SO) procedure is applied to identify the parameters of an experimental mooring system exhibiting nonlinear behavior. In Part 1, two nonlinear small-body hydrodynamic Morison type formulations: (A) with a relative velocity (RV) model, and (B) with an independent-flow-field (IFF) model, are formulated. Their associated nonlinear systemidentification algorithms based on the R-MI/SO system-identification technique: (A.1) nonlinear-structure linearly damped, and (A.2) nonlinear-structure coupled hydrodynamically damped for the RV model, and (B.1) nonlinear-structure nonlinearly damped for the IFF model, are developed for an experimental submerged-sphere nonlinear mooring system under ocean waves. The analytic models and the associated algorithms for parametric identification are described. In this companion paper (Part 2), we use the experimentally measured input wave and output system response data and apply the algorithms derived based on the multiple-input/single-output linear analysis of the reverse dynamic systems to identify the system parameters. The two nonlinear models are examined in detail and the most suitable physical representative model is selected for the mooring system considered. A sensitive analysis is conducted to investigate the coupled hydrodynamic forces modeled by the Morison equation, the nonlinear stiffness from mooring lines and the nonlinear response. The appropriateness of each model is discussed in detail. @DOI: 10.1115/1.1710874#

The highly nonlinear responses of compliant ocean structures characterized by a largegeometry restoring force and coupled fluid-structure interaction excitation are of great interest to ocean and coastal engineers. Practical modeling, parameter identification, and incorporation of the inherent nonlinear dynamics in the design of these systems are essential and challenging. The general approach of a nonlinear system technique using very simple models has been presented in the literature by Bendat. In Part 1 of this two-part study, two specific nonlinear small-body hydrodynamic Morison type formulations: (A) with a relative-velocity (RV) model, and (B) with an independent flow-field (IFF) model, are formulated. Their associated nonlinear system-identification algorithms based on the reverse multiple-input/single-output (R-MI/SO) system-identification technique: (A.1) nonlinear-structure linearly damped, and (A.2) nonlinear-structure coupled hydrodynamically damped for the RV model, and (B.1) nonlinear-structure nonlinearly damped for the IFF model, are developed for a specific experimental submerged-sphere mooring system under ocean waves exhibiting such highly nonlinear response behaviors. In Part 2, using the measured input wave and output system response data, the algorithms derived based on the MI/SO linear analysis of the reverse dynamic systems are applied to identify the properties of the highly nonlinear system. Practical issues on the application of the R-MI/SO technique based on limited available experimental data are addressed. @DOI: 10.1115/1.1710875#

A numerical study on the dynamic response of a generic rigid water-landing object (WLO) during water impact is presented in this paper. The effect of this impact is often prominent in the design phase of the re-entry project to determine the maximum force for material strength determination to ensure structural and equipment integrity, human safety and comfort. The predictive capability of the explicit finite-element (FE) arbitrary Lagrangian-Eulerian (ALE) and smoothed particle hydrodynamics (SPH) methods of a state-of-the-art nonlinear dynamic finite-element code for simulation of coupled dynamic fluid structure interaction (FSI) responses of the splashdown event of a WLO were evaluated. The numerical predictions are first validated with experimental data for maximum impact accelerations and then used to supplement experimental drop tests to establish trends over a wide range of conditions including variations in vertical velocity, entry angle, and object weight. The numerical results show that the fully coupled FSI models can capture the water-impact response accurately for all range of drop tests considered, and the impact acceleration varies practically linearly with increase in drop height. In view of the good comparison between the experimental and numerical simulations, both models can readily be employed for parametric studies and for studying the prototype splashdown under more realistic field conditions in the oceans.

This paper presents a novel point absorber wave energy converter (WEC), developed by Columbia Power Technologies (COLUMBIA POWER), in addition to the related numerical analysis and scaled wave tank testing. Three hydrodynamic modeling tools are employed to evaluate the performance of the WEC, including WAMIT, GL Garrad Hassan's GH WaveDyn, and OrcaFlex. GH WaveDyn is a specialized numerical code being developed specifically for the wave energy industry. Performance and mooring estimates at full scale are evaluated and optimized, followed by the development of a 1:33 scale physical model. The physical tests of the 1:33 scale model WEC were conducted at the multidirectional wave basin of Oregon State University's O.H. Hinsdale Wave Research Laboratory, in conjunction with the Northwest National Marine Renewable Energy Center (NNMREC). This paper concludes with an overview of the next steps for the modeling program and future experimental test plans.

Complex multidisciplinary physical fields formed by the dynamic interaction between fluid flows, structure motion, and seabed profile evolution are natural in a marine environment. Modeling and analysis of such fluid-structure-sediment interactions are essential for predicting and analyzing the nonlinear behavior of movable structures and their surrounding sediments under wave action. However, no analytical and numerical tools which consider the detailed physics of the entire coupled fluid-structure-sediment system are currently available. In this study, a three-dimensional coupled fluid-structure-sediment interaction model is developed to provide an overarching computational framework for simulating the dynamic behavior of multidisciplinary physical systems. The model consists of an extended Navier-Stokes solver that computes incompressible viscous multiphase flow, a volume-of-fluid module that tracks air-water interface motion, an immersed boundary module that tracks structure motion, and a sediment transport module that tracks suspended sediment motion and seabed profile evolution. For validation, the model is applied to hydraulic experiments on local scouring around a movable short cylinder supported at the base. It is found that the model predicts scour patterns around the cylinder reasonably well, consistent with experimental results measured in the hydraulic experiments. In addition, the computational applicability of the model is demonstrated to predict and analyze a general complex fluid-structure-sediment interaction phenomenon in the marine environment.

This study presents a stochastic approach for the analysis of nonchaotic, chaotic, random and nonchaotic, random and chaotic, and random dynamics of a nonlinear system. The analysis utilizes a Markov process approximation, direct numerical simulations, and a generalized stochastic Melnikov process. The Fokker-Planck equation along with a path integral solution procedure are developed and implemented to illustrate the evolution of probability density functions. Numerical integration is employed to simulate the noise effects on nonlinear responses. In regard to the presence of additive ideal white noise, the generalized stochastic Melnikov process is developed to identify the boundary for noisy chaos. A mathematical representation encompassing all possible dynamical responses is provided. Numerical results indicate that noisy chaos is a possible intermediate state between deterministic and random dynamics. A global picture of the system behavior is demonstrated via the transition of probability density function over its entire evolution. It is observed that the presence of external noise has significant effects over the transition between different response states and between co-existing attractors.

We introduce and demonstrate the applicability of a parameter identification algorithm based on the Hilbert transform to nonlinear ocean mooring systems. The mooring dynamical system consists of a submerged small body and includes a geometrically nonlinear restoring force and a nonlinear dissipation function incorporating both viscous and structural damping. By combining a recently developed methodology with a generalized averaging procedure, parameter estimation from the slowly varying envelope dynamics is enabled. System backbone curves obtained from data generated by numerical simulation are compared to those obtained analytically and are found to be accurate. An example large-scale experiment is also considered.

This paper presents a parallel implementation and validation of an accurate and efficient three-dimensional computational model (3D numerical wave tank), based on fully nonlinear potential flow (FNPF) theory, and its extension to incorporate the motion of a laboratory snake piston wavemaker, as well as an absorbing beach, to simulate experiments in a large-scale 3D wave basin. This work is part of a long-term effort to develop a “virtual” computational wave basin to facilitate and complement large-scale physical wave-basin experiments. The code is based on a higher-order boundary-element method combined with a fast multipole algorithm (FMA). Particular efforts were devoted to making the code efficient for large-scale simulations using high-performance computing platforms. The numerical simulation capability can be tailored to serve as an optimization tool at the planning and detailed design stages of large-scale experiments at a specific basin by duplicating its exact physical and algorithmic features. To date, waves that can be generated in the numerical wave tank (NWT) include solitary, cnoidal, and airy waves. In this paper we detail the wave-basin model, mathematical formulation, wave generation, and analyze the performance of the parallelized FNPF-BEM-FMA code as a function of numerical parameters. Experimental or analytical comparisons with NWT results are provided for several cases to assess the accuracy and applicability of the numerical model to practical engineering problems.

A nonlinear three-dimensional two-way coupled fluid-sediment interaction model is developed in this study. The model is composed of a generalized Navier–Stokes solver (GNS) with a volume of fluid module for air-water interface tracking and a sediment transport module (STM) for fluid-sediment interface tracking. The GNS model is based on the finite difference method with a turbulent stress model of large-eddy simulation to compute incompressible viscous multiphase flows. The STM is used to compute nonlinear sediment bed profile change due to bed-load sediment transport. A two-way coupling scheme connecting GNS with STM is implemented at each time step to ensure the fluid-sediment interaction. For validation, the fluid-sediment interaction model is applied to predict cross-shore profile change of a sloping beach due to breaking solitary waves, and the resulting predictions are examined and compared with the measured data from a set of hydraulic tests. It is found that the fluid-sediment interaction model predicts reasonably well the sediment transport and the resulting beach profile change. The sensitivity of model parameters involving the sediment transport to the beach profile change is analyzed. Finally, the fluid-sediment interaction model is applied to predict local scour in front of a quay wall due to a jet flow to demonstrate its applicability to general three-dimensional problems.

This paper presents an innovative technique for evaluating the performance of direct-drive power take-off systems for wave energy devices using simulated force and velocity profiles. The performance of a linear generator was evaluated in a realistic operating condition using the results from a coupled model of a taut moored, dual body, and wave energy conversion system as position input for Oregon State University’s wave energy linear test bed. The experimental results from the linear test bed can be compared with the predictions of the simulation and used to evaluate the efficiency of the generator.

The predictive capability of a three-dimensional (3D) numerical model for sediment transport and resulting scour around a structure is investigated in this study. Starting with the bed-load and suspended-load sediment transport (reference) model developed by Takahashi et al. (2000, “Modeling Sediment Transport Due to Tsunamis With Exchange Rate Between Bed Load Layer and Suspended Load Layer,” Proceedings of the 27th International Conference on Coastal Engineering, ASCE, Sydney, Australia, pp. 1508–1519), we first introduce an extension to incorporate Nielsen’s modified Shields parameter to account for the effects of infiltration/exfiltration flow velocity across the fluid-sand interface on the sediment transport (the modified Shields-parameter model). We then propose a new model to include the influence of the effective stress to account for the stress fluctuations inside the surface layer of the sand bed (the effective-stress model). The three analytical models are incorporated into a 3D numerical solver developed by Nakamura et al. (2008, “Tsunami Scour Around a Square Structure,” Coast. Eng. Japan, 50(2), pp. 209–246) to analyze the dynamics of fluid-sediment interaction and scour. Their solver is composed of two modules, namely, a finite-difference numerical wave tank and a finite-element coupled sand-skeleton pore-water module. The predictive capability of the three alternative coupled models is calibrated against hydraulic experiments on sediment transport and resulting scour around a fixed rigid structure due to the run-up of a single large wave in terms of the sediment transport process and the final scour profile after the wave run-up. It is found that, among the three models considered, the proposed effective-stress model most accurately predicts the scouring process around the seaward corner of the structure. The results also reveal that the deposition and erosion patterns predicted using the effective-stress model are in good agreement with measured results, while a scour hole at the seaward corner of the structure cannot be always predicted by the other two models.

The predictive capability of two-dimensional (2D) fully-nonlinear-potential-flow (FNPF) models of an experimental submerged moored sphere system subjected to waves is examined in this study. The experimental system considered includes both single-degree-of-freedom (SDOF) surge-only and two-degree-of-freedom (2DOF) surge-heave coupled motions, with main sources of nonlinearity from free surface boundary, large geometry, and coupled fluid-structure interaction. The FNPF models that track the nonlinear free-surface boundary exactly hence can accurately model highly nonlinear (nonbreaking) waves. To examine the predictive capability of the approximate 2D models and keep the computational effort manageable, the structural sphere is converted to an equivalent 2D cylinder. Fluid-structure interaction is coupled through an implicit boundary condition enforcing the instantaneous dynamic equilibrium between the fluid and the structure. The numerical models are first calibrated using free-vibration test results and then employed to investigate the wave-excited experimental responses via comparisons of time history and frequency response diagrams. Under monochromatic wave excitations, both SDOF and 2DOF models exhibit complex nonlinear experimental responses including coexistence, harmonics, subharmonics, and superharmonics. It is found that the numerical models can predict the general qualitative nonlinear behavior, harmonic and subharmonic responses as well as bifurcation structure. However, the predictive capability of the models deteriorates for superharmonic resonance possibly due to three-dimensional (3D) effects including diffraction and reflection. To accurately predict the nonlinear behavior of moored sphere motions in the highly sensitive response region, it is recommended that the more computationally intensive 3D numerical models be employed.