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CONTENTS
Volume 6, Number 1, March 2016
 

Abstract
The global performance of the 5 MW OC4 semisubmersible floating wind turbine in random waves with or without steady/dynamic winds is numerically simulated by using the turbine-floater-mooring fully coupled dynamic analysis program FAST-CHARM3D in time domain. The numerical simulations are based on the complete second-order diffraction/radiation potential formulations along with nonlinear viscous-drag force estimations at the body\'s instantaneous position. The sensitivity of hull motions and mooring dynamics with varying wave-kinematics extrapolation methods above MWL(mean-water level) and column drag coefficients is investigated. The effects of steady and dynamic winds are also illustrated. When dynamic wind is added to the irregular waves, it additionally introduces low-frequency wind loading and aerodynamic damping. The numerically simulated results for the 5 MW OC4 semisubmersible floating wind turbine by FAST-CHARM3D are also extensively compared with the DeepCWind model-test results by Technip/NREL/UMaine. Those numerical-simulation results have good correlation with experimental results for all the cases considered.

Key Words
wind energy; FOWT (Floating Offshore Wind Turbine); OC4 semi-submersible; turbine-hull-mooring fully coupled dynamics; second-order wave diffraction QTF; FAST-CHARM3D; 5MW wind-turbine; viscous drag; wave-crest kinematics; Irregular waves; steady/dynamic wind

Address
H.C. Kim and M.H. Kim: Civil Engineering, Texas A&M University, College Station, TX 77843, USA

Abstract
The major objective of this study was to de¬velop further understanding of 3D nearshore hydrodynamics under a variety of wave and tidal forcing conditions. The main tool used was a com¬prehensive 3D numerical model – combining the flow module of Delft3D with the WAVE solver of XBeach – of nearshore hydro- and morphodynamics that can simulate flow, sediment transport, and morphological evolution. Surf-swash zone hydrodynamics were modeled using the 3D Navier-Stokes equations, combined with various turbulence models (k-e, k-L, ATM and H-LES). Sediment transport and resulting foreshore profile changes were approximated using different sediment transport relations that consider both bed- and suspended-load transport of non-cohesive sediments. The numerical set-up was tested against field data, with good agreement found. Different numerical experiments under a range of bed characteristics and incident wave and tidal conditions were run to test the model\'s capability to reproduce 3D flow, wave propagation, sediment transport and morphodynamics in the nearshore at the field scale. The results were interpreted according to existing understanding of surf and swash zone processes. Our numerical experiments confirm that the angle between the crest line of the approaching wave and the shoreline defines the direction and strength of the longshore current, while the longshore current velocity varies across the nearshore zone. The model simulates the undertow, hydraulic cell and rip-current patterns generated by radiation stresses and longshore variability in wave heights. Numerical results show that a non-uniform seabed is crucial for generation of rip currents in the nearshore (when bed slope is uniform, rips are not generated). Increasing the wave height increases the peaks of eddy viscosity and TKE (turbulent kinetic energy), while increasing the tidal amplitude reduces these peaks. Wave and tide interaction has most striking effects on the foreshore profile with the formation of the intertidal bar. High values of eddy viscosity, TKE and wave set-up are spread offshore for coarser grain sizes. Beach profile steepness modifies the nearshore circulation pattern, significantly enhancing the vertical component of the flow. The local recirculation within the longshore current in the inshore region causes a transient offshore shift and strengthening of the longshore current. Overall, the analysis shows that, with reasonable hypotheses, it is possible to simulate the nearshore hydrodynamics subjected to oceanic forcing, consistent with existing understanding of this area. Part II of this work presents 3D nearshore morphodynamics induced by the tides and waves.

Key Words
beach types; longshore flow; process-based model; rip current; turbulence; wave characteristics

Address
R. Bakhtyar: Davidson Laboratory, Stevens Institute of Technology, Hoboken, NJ 07030, USA
A. Dastgheib: UNESCO-IHE, PO Box 3015, 2601 DA Delft, The Netherlands
D. Roelvink: UNESCO-IHE, PO Box 3015, 2601 DA Delft, The Netherlands;
Technical University of Delft, PO Box 5048, 2600 GA, Delft, The Netherlands, and Deltares, PO Box 177, 2600 MH, Delft, The Netherlands
D.A. Barry: Laboratoire de technologie écologique, Institut d\'ingénierie de l\'environnement, Faculté de
l\'environnement naturel, architectural et construit (ENAC), Station 2, Ecole polytechnique fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland


Abstract
This is the second of two papers on the 3D numerical modeling of nearshore hydro- and morphodynamics. In Part I, the focus was on surf and swash zone hydrodynamics in the cross-shore and longshore directions. Here, we consider nearshore processes with an emphasis on the effects of oceanic forcing and beach characteristics on sediment transport in the cross- and longshore directions, as well as on foreshore bathymetry changes. The Delft3D and XBeach models were used with four turbulence closures (viz., k-e, k-L, ATM and H-LES) to solve the 3D Navier-Stokes equations for incompressible flow as well as the beach morphology. The sediment transport module simulates both bed load and suspended load transport of non-cohesive sediments. Twenty sets of numerical experiments combining nine control parameters under a range of bed characteristics and incident wave and tidal conditions were simulated. For each case, the general morphological response in shore-normal and shore-parallel directions was presented. Numerical results showed that the k-e and H-LES closure models yield similar results that are in better agreement with existing morphodynamic observations than the results of the other turbulence models. The simulations showed that wave forcing drives a sediment circulation pattern that results in bar and berm formation. However, together with wave forcing, tides modulate the predicted nearshore sediment dynamics. The combination of tides and wave action has a notable effect on longshore suspended sediment transport fluxes, relative to wave action alone. The model\'s ability to predict sediment transport under propagation of obliquely incident wave conditions underscores its potential for understanding the evolution of beach morphology at field scale. For example, the results of the model confirmed that the wave characteristics have a considerable effect on the cumulative erosion/deposition, cross-shore distribution of longshore sediment transport and transport rate across and along the beach face. In addition, for the same type of oceanic forcing, the beach morphology exhibits different erosive characteristics depending on grain size (e.g., foreshore profile evolution is erosive or accretive on fine or coarse sand beaches, respectively). Decreasing wave height increases the proportion of onshore to offshore fluxes, almost reaching a neutral net balance. The sediment movement increases with wave height, which is the dominant factor controlling the beach face shape.

Key Words
beach profile changes; longshore sediment transport; bed load; suspended load; on/offshore sediment transport

Address
R. Bakhtyar: Davidson Laboratory, Stevens Institute of Technology, Hoboken, NJ 07030, USA
A. Dastgheib: UNESCO-IHE, PO Box 3015, 2601 DA Delft, The Netherlands
D. Roelvink : UNESCO-IHE, PO Box 3015, 2601 DA Delft, The Netherlands;
Technical University of Delft, PO Box 5048, 2600 GA, Delft, The Netherlands, and Deltares, PO Box 177, 2600 MH, Delft, The Netherlands
D.A. Barry: Laboratoire de technologie écologique, Institut d\'ingénierie de l\'environnement, Faculté de l\'environnement naturel, architectural et construit (ENAC), Station 2, Ecole polytechnique fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland

Abstract
In meeting the technical needs for deepwater conditions and overcoming the shortfalls of single-layer pipes for deepwater applications, pipe-in-pipe (PIP) systems have been developed. While, for PIP pipelines directly laid on the seabed or with partial embedment, one of the primary service risks is lateral buckling. The critical axial force is a key factor governing the global lateral buckling response that has been paid much more attention. It is influenced by global imperfections, submerged weight, stiffness, pipe-soil interaction characteristics, et al. In this study, Finite Element Models for imperfect PIP systems are established on the basis of 3D beam element and tube-to-tube element in Abaqus. A parameter study was conducted to investigate the effects of these parameters on the critical axial force and post-buckling forms. These parameters include structural parameters such as imperfections, clearance, and bulkhead spacing, pipe/soil interaction parameter, for instance, axial and lateral friction properties between pipeline and seabed, and load parameter submerged weight. Python as a programming language is been used to realize parametric modeling in Abaqus. Some conclusions are obtained which can provide a guide for the design of PIP pipelines.

Key Words
lateral buckling; PIP; parameter analysis; numerical model

Address
Xinhu Zhang, Menglan Duan, Yingying Wang and Tongtong Li: Offshore Oil and Gas Research Center, China University of Petroleum-Beijing, 18 Fuxue Road, Changping, Republic of China;
College of Mechanical and Transportation Engineering, China University of Petroleum-Beijing, 18 Fuxue Road, Changping, Republic of China


Abstract
A theoretical study of Boussinesq equations (BEs) for internal waves propagating in a two-fluid system is presented in this paper. The two-fluid system is assumed to be bounded by two rigid plates. A set of three equations is firstly derived which has three main unknowns, the interfacial displacement and two velocity potentials at arbitrary elevations for upper and lower fluids, respectively. The determination of the optimal BEs requires a solution of depth parameters which can be uniquely solved by applying the Padé approximation to dispersion relation. Some wave properties predicted by the optimal BEs are examined. The optimal model not only increases the applicable range of traditional BEs but also provides a novel aspect of internal wave studies.

Key Words
internal wave; Boussinesq equations; rigid-lid boundary; two-fluid system

Address
Chi-Min Liu: Division of mathematics, General Education Center, Chienkuo Technology University, Changhua City, Taiwan


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