bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Simulation of Nonlinear Free Surface Water Waves: Coupling of a Potential Flow Solver to a URANS/VOF Code
Authors: Düz Bülent , Bunnik Tim , Kapsenberg Geert , Vaz Guilherme
Published: October 2016

Numerical simulation of free surface water waves using various wave models is studied. The performance of the wave models is compared in three test cases in 2D, all of which include a focusing event. Then the attention is focused on coupling one of these wave models to a URANS/VOF code, ReFRESCO. Here the strong suits of both solvers are utilized to simulate nonlinear water waves in an accurate and efficient manner. Even though only simulating nonlinear waves is discussed in this paper, the main purpose of the coupling approach is to simulate hydrodynamic wave loading on structures and wave-structure interaction. Computational cost, convergence behavior and effects of grid refinement are some of the topics that are also discussed throughout the paper.
Experimental Study of Wave Induced Loads and Motions on FLNG in Head and Oblique Sea Waves
Authors: Jin Yuting , Chai Shuhong , Duffy Jonathan , Chin Christopher , Bose Neil
Published: October 2016

In the past decade, an innovative concept, the floating liquefied natural gas (FLNG) system has been developed as a more effective solution over conventional pipelines for exploiting offshore natural gas resources. Understanding the hydrodynamic behaviour of such a mega structure in a real seaway is essential for determining its performance as well as evaluating the operabilities of on-board facilities and safe offloading.

In this paper, experimental study on the hydrodynamic performance of a generic FLNG hull form has been presented. The 1:100 scale model was tested in the Australian Maritime College model test basin for head sea and oblique sea conditions at zero forward speed. The wave induced loads and motions were measured by load cells and linear variable differential transducers (LVDTs) respectively. Experimental uncertainties on each of the measured variables were studied by taking partial differentiations on the uncertainty sources. The time history measurements were decomposed by Fourier series for obtaining frequency domain force/moment and motion transfer functions. The results were compared with numerical solutions from potential flow and Reynolds-Averaged Navier-Stokes (RANS) solvers. A good correlation between the experimental and numerical results has been demonstrated.
Weather Window Analysis in Connection With Operation and Maintenance of Ocean Renewable Energy Devices
Authors: Taniguchi Tomoki , Ishida Shigesuke , Fujiwara Toshifumi , Inoue Shunji
Published: October 2016

There is an increasing need for utilization of ocean renewable energy (ORE) around Japanese coast because Japan is surrounded by ocean. Because technologies for harnessing ORE have not been mature enough, Japanese government selects some demonstration sites for ORE devices and some demonstration projects are going. As these projects are progressed, the operation and maintenance (O&M) activities will increase and become essential factors for the success of demonstration projects. Hence, weather window analysis is required to quantify the levels of access for ORE devices in the demonstration projects, and commercial projects in the future.

In this paper, two new parameters are proposed in order to evaluate accessibility to ORE devices. One is the operational probability, and the other is the forecasted waiting time. The operational probability assesses weather duration with considering variability of wave condition. The forecasted waiting time is an expectation value of waiting time before O&M planners get next chance to arrange the O&M activities.

In order to check the effectivity of the proposed 2 parameters, accessibility is evaluated for significant wave height in terms of the 2 proposed parameters, these are

• Operational probability

• Forecasted waiting time

and 3 conventional parameters, these are

• Excess probability

• Persistence probability

• Waiting time between windows

The accessibility is evaluated at two locations along the Japanese coast.

This study reveals that large differences are caused between persistence probability and operational probability when operational wave height limit occurs intermittently and required window length is long. The forecasted waiting time has the same variation tendency as the waiting time between windows.
Impact of Water on the Fatigue Performance of Large-Scale Grouted Connection Tests
Authors: Schaumann Peter , Raba Alexander , Bechtel Anne
Published: October 2016

Grouted connections represent a common joining technique between substructure and foundation piles of offshore oil & gas platforms as well as of offshore wind turbines. Due to cyclic loads arising from wind and wave actions the fatigue performance of the connection has to be considered. In lattice substructures like jackets the grouted connections are located at seabed level being fully submerged during their entire lifetime. Today’s fatigue design regulations are based on investigations neglecting any influence of the surrounding water since they were conducted in dry ambient conditions. So far, only Germanischer Lloyd gives additional recommendations for submerged grouted connections.

At the Institute for Steel Construction, Leibniz Universität Hannover, Germany investigations of the joint research project ‘GROWup’ focus on the fatigue performance of axially loaded grouted connections. The project is funded by the Federal Ministry for Economic Affairs and Energy (BMWi, funding sign: 0325290) and is the third project in a row dealing with grouted connections. As part of this research project, cyclic loading tests on small-scale and large-scale grouted connections with shear keys are conducted. Small-scale fatigue tests showed a reduced number of endurable load cycles for connections when tested in wet ambient conditions.

However, the transferability of these findings to a larger scale was still doubtful due to unknown scale effects. Therefore, the impact of water on the fatigue performance was tested recently at large-scale grouted connections. Previous to the submerged large-scale grouted connection fatigue tests, similar test specimens were exposed to alternating loads at dry ambient conditions. Comparison of both large-scale test results under wet and dry conditions enable to estimate the influence of water on the fatigue performance of grouted connections. Reflection of the small-scale test results gives hints on the scale effect. Test preparation, test results and design recommendations are presented in the paper.
Automated Landing and Mooring in Deep-Sea Environment Using Suction Buckets for Unmanned Vehicles
Authors: Boeck Florin , Golz Matthias , Ritz Sebastian , Holbach Gerd
Published: October 2016

Autonomous underwater vehicles (AUVs) depend primarily on electrical power. Monitoring the deep-sea ocean floor efficiently requires easily accessible recharging techniques. In contrast to conservative ship-dependent charging processes on the surface, the Subsea Monitoring via Intelligent Swarms (SMIS) consortium uses a seabed station (SBS) as a power supply for AUVs. This paper describes the fully automated secure landing procedure of the SBS followed by a ground mooring maneuver on sandy sediments or soft clays. The unmanned seabed station is equipped with various sensors for status detection and environmental observation. In order to control the freefall and grounding without endangering the valuable components, an innovative procedure was implemented. Once landed, the nearly buoyancy-neutral station is moored using down-scaled suctions buckets that are normally used in immobile offshore oil rig foundations.

Laboratory full-scale experiments in modified test tanks for different length-to-diameter ratios endorse the design process. The feasibility of landing, mooring and ascending techniques was proven in field-tests in the Baltic Sea and the Atlantic Ocean.
Estimation of Wave Loads due to Green Water Events in 10000-Year Conditions on a TLP Deck Structure
Authors: Pakozdi Csaba , Östman Anders , Ji Guomin , Stansberg Carl Trygve , Reum Ola , Ovrebo Stig , Vestbøstad Tone , Sorvaag Christian , Ersland Jon
Published: October 2016

In order to provide qualitative and quantitative information on the hydrodynamics loads during green water events on a module on the deck of a TLP in 10000-year conditions, MARINTEK has carried out CFD simulations. This paper presents extreme wave events and corresponding hydrodynamics loads on the module which can be directly linked to the extreme events observed in model tests. This means that the simulated extreme events can be related to a probability of occurrence, found from the model test.

A prerequisite for the structural design is that reliable estimates of hydrodynamic loads during a green water event can be made. Measured time series of waves from existing model test data are compared with CFD generated synthetic numerical waves. The selection of steep wave events are based on two physical parameters: the wave crest height and the rise velocity (time derivative of the free surface elevation at a given location). These parameters are relevant for green water and corresponding loads. The comparison of the measured free surface elevation of the calibrated waves with the time series of the numerical waves, as well as the measured and simulated relative wave probes time series shows that the applied numerical wave events have similar physical conditions as those observed in the model test.

In a new procedure developed by MARINTEK one identifies observed steep wave events, which are similar to existing numerical wave events, instead of trying to reproduce measured events. This procedure reduces the computational time, as well as computational costs, to an industrially acceptable level.

Traditional load estimation is not able to provide such reliable detailed local load history for structural design purpose at areas exposed to wave impacts. Therefore, topside modules are currently not installed in such areas. This new procedure, where CFD simulates realistic breaking waves with coupling to structural analysis tools, offers new possibilities for the design of structures subject to risk of green water loading.
Neural Networks for Tubular Joint Optimization in Offshore Jacket Structures
Authors: Christiansen Niels Hørbye , Tang Benny Korsholm
Published: October 2016

The use of jacket structures to support offshore installations has for a long time been a popular choice in places with appropriate water depths. In recent years the use of jacket structures as offshore wind turbine foundations has also attracted increasing attention and is becoming a feasible alternative to traditional monopile foundations.

One of the key challenges in jacket design is optimizing tubular joints in terms of fatigue resistance. As it is not practically possible to include detailed FEM joint models in global jacket models designers are forced to look for alternative methods to obtain realistic joint representations. This is done by calculating influence factors (INF) and stress concentration factors (SCF) to be applied to simplified models of relevant tubular joints in global models in order to achieve a realistic force flow in the structure. One simple and widely used method is to apply parametric formulas, e.g. those suggested by Efthymiou. However, these approximating formulas have a fairly limited validity range. Therefore, on complex joint the most reliable way to determine INF’s is by setting up refined FEM models of relevant joints and then subsequently using the calculated factors in the global model. This strategy is computationally demanding and hence, very time consuming, as a new detailed FEM analysis of the tubular joint must be conducted for each step in the optimization process.

The present paper demonstrates how this time consuming procedure can be avoided by use of artificial neural networks (ANN) trained to estimate INF’s on tubular joints. The neural network is trained on a pre-generated library of detailed FEM joint models and is then able to predict INF’s on joints that are not part of the library — and thereby providing a significant reduction in calculation time during the jacket/joint optimization process. The analysis is conducted on a typical joint on a three legged jacket structure. The joint is located on a jacket leg and has two incoming braces. Such a joint has a finite number of free design variables, e.g. chord diameter/thickness, brace diameter/thickness, brace angle, gap etc. Each of these free variables can be considered as a dimension in the joint design space. Having a sufficient number of FEM joint models in the library the neural network can be trained to recognize and predict underlying patterns in this design space. The method is demonstrated on a limited number of design variables but should easily be extended to cover all variables as the joint library is expanded to include all dimensions.
The Dynamic Behaviour of an ROV in Wave Affected Zones
Authors: Bashir Musa B. , Menon Mahesh , Benson Simon D. , Eastwood Rob , Murphy Alan J. , Cunny Daniel , van Zwanenberg Michael
Published: October 2016

We present a method to directly predict the hydrodynamic response of a Remotely Operated Vehicle (ROV) as it transitions through the wave-affected splash zone during launch or recovery. ROVs are commonly used in offshore operations and for many different purposes. They are usually deployed from a purpose built Launch and Recovery System (LARS) located on the open deck of a surface ship. Current industry practice used for LARS design is to use a pre-defined dynamic amplification factor (DAF) together with high factors of safety. However, this does not properly account for the influence of different sea states and operational profiles. We therefore propose an improved method to directly predict the hydrodynamic response of an ROV as it transitions through the wave-affected splash zone. We apply the approach to a trencher ROV and validate using physical experiments completed in a wave tank. We demonstrate the need to for an accurate definition of the added mass and damping characteristics of the ROV. The method is incorporated into OrcaFlex for direct use within the design and analysis of a LARS system.
Mass, Momentum and Energy Flux in Nonlinear Water Wave Based on HAM Solution
Authors: Lin Zhiliang
Published: October 2016

In this paper, the Homotopy Analysis Method (HAM) is applied to solve the fully nonlinear partial differential equation for the steady propagating periodic gravity wave of finite water depth. The series solution of the wave elevation and the velocity potential function are obtained. And then the velocity and pressure fields are plotted and discussed carefully. In order to overcome the drawback of the integral calculations with complex free surface elevation, the discrete integration and fitting procedure based on high-order Fourier series is developed. Based on the accurate HAM solution and fitting technique, the mass, momentum and energy conservation equations are validated. At last, the corresponding mean fluxes are calculated and the velocities of the mass transport and energy transport are supplied accurately.
Parametric Study on Hub Vortex Reducing Effects of Propeller Boss Cap Fins by Force and Wake Field Measurements in Open Water and Self-Propulsion Conditions
Authors: Seo Jeonghwa , Han Bumwoo , Rhee Shin Hyung , Lee Seung-Jae , Kim Hyeseong , Kim Jeongmin , Kwon Kangkyu , Park Jaehyun
Published: October 2016

Propeller boss cap fins (PBCF) is one of the most popular ESDs in the industry. The present study aims to investigate effects of design variations of PBCFs on the propulsive efficiency and propeller wake field, with special attention on hub vortex dynamics. The wake fields and force on the whole propulsive system were measured by a towed underwater stereoscopic particle image velocimetry (SPIV) system and a propeller open water (POW) test dynamometer, respectively. Design parameters of PBCFs, i.e., the fin surface area and the angle of attack onto the fins, were varied to control fin loading on the PBCF. In the wake field, root vortices generated from the propeller blades were separated by PBCF and did not form a strong hub vortex, which caused pressure drop on the propeller boss cap. The hub vortex reduction practically increased total thrust, as evidenced in the global force measurement results. In PBCF design variations, Total efficiency increased linearly as the pitch angle and fin chord length decreased. The global force measurement results implied that PBCF in light loading separated root vortices efficiently. Hub vortex reduction by PBCF in light loading was also confirmed by the wake field measurement. In the case of low fin height, however, root vortices were not blocked and actually merged to form a hub vortex. Therefore, the primary function of PBCF, i.e., reducing hub vortex, was not effective anymore and the total efficiency decreased. In heavy loading conditions, axial velocity near the center retarded further, causing greater drag and diminishing the total efficiency. The model tests were also conducted in self propulsion condition, to reveal that the new PBCF with reduced loading also improves the energy saving performance when it works in the wake of the ship.
Structural Fatigue Assessment of Offshore Platform Considering the Effect of Nonlinear Drag Force
Authors: Ding Weichen , Pang Liang
Published: October 2016

Fatigue assessment for jacket platforms is an indispensable practical issue. Because of the small-scale leg diameter, these structures are often drag dominated and wave-induced force in these structures can be tackled by using either linear or nonlinear form of Spectral Morison Equation. However, it is really complicated and difficult to incorporate nonlinear form of the Morison Equation to acquire the spectral density of the wave force, which is an important step of fatigue estimation. In this paper, in order to estimate the influence of nonlinear effect in wave force, fatigue assessments containing nonlinear effect for the fixed offshore structure are presented. Firstly, shallow-water jacket model locating at a water depth of 20 m is established and involved in calculation. Besides, for the sake of validating the effectiveness of the nonlinear term, the linear and nonlinear form of wave-induced force spectral densities are calculated by the Morison Equation in frequency domain. Secondly, the fatigue life of the jacket platform is assessed in time domain, where time-history of wave force can be obtained by transforming the linear or nonlinear wave force spectral densities from frequency domain to time domain. After the contrast of acquired fatigue life, the comparative results can indicate that the nonlinear drag force contributes a 14% fatigue damage to the total and the influences of the nonlinear term cannot be ignored for the jacket model.
Experimental and Numerical Study of Steel Lazy Wave Riser Response in Extreme Environment
Authors: Cheng Jingyun , Cao Peimin , Fu Shixiao , Constantinides Yiannis
Published: October 2016

The paper presents numerical simulations and analysis of experimental results obtained from a model scale test on steel lazy wave riser conducted by the DeepStar® consortium. The study focuses on the hydrodynamic response of a steel lazy wave riser under forced oscillatory and random motions in a wave basin. The riser pipe deformations were measured by Fiber Bragg Grating (FBG) strain sensors, as well as force transducers. Both in-plane and out-of-plane signals were recorded. Numerical simulations were performed, focusing on in-plane riser response, and compared with the experiments. The typical hydrodynamic coefficients (added mass, and drag) for the riser design are used in analysis and the calculated riser reponses are compared with measurements. In addition, some coefficients are adjusted to improve the comparison and the results are promising. The test data post-processing approach and modeling of steel lazy wave riser are also discussed in the paper.
Variational Modelling of Wave-Structure Interactions for Offshore Wind Turbines
Authors: Salwa Tomasz , Bokhove Onno , Kelmanson Mark A.
Published: October 2016

We consider the development of a mathematical model of water waves interacting with the mast of an offshore wind turbine. A variational approach is used for which the starting point is an action functional describing a dual system comprising a potential-flow fluid, a solid structure modelled with (linear) elasticity, and the coupling between them. The variational principle is applied and discretized directly using Galerkin finite elements that are continuous in space and dis/continuous in time. We develop a linearized model of the fluid-structure or wave-mast coupling, which is a linearization of the variational principle for the fully coupled nonlinear model. Our numerical results indicate that our variational approach yields a stable numerical discretization of a fully coupled model of water waves and a linear elastic beam. The energy exchange between the subsystems is seen to be in balance, yielding a total energy that shows only small and bounded oscillations whose amplitude tends to zero as the timestep goes to zero.
Progressive Collapse Analysis of a Medium-Rise Circular RC Building Against Blast Loads
Authors: Al-Salloum Yousef , Almusallam Tarek , Ngo Tuan , Elsanadedy Hussein , Abbas Husain , Mendis Priyan
Published: October 2016

This paper investigates the vulnerability of a typical medium-rise circular RC building against progressive collapse as a result of blast generated waves. The building is an eight storied (including one story basement) commercial complex. The likely blast threat scenario was identified by qualitatively assessing the vulnerability of the critical elements of the structure. LS-DYNA was used for the finite element modelling of the structure. The study presents local model analysis of one of its circular columns for which fluid-structure interaction through Alternate Lagrangian Eulerian (ALE) element formulation has been employed. The concrete volume in the columns was modeled using 8-node reduced integration solid hexahedron elements.

The global model analysis was carried out to examine the overall response of the structure due to the failure of one of the critical columns. The building was modeled using beam and shell elements. The 2-node axial beam elements with tension, compression, torsion, and bending capabilities were employed to represent the RC beams and columns, whereas the four node quadrilateral and three node triangular shell elements were used to represent the core wall, floor slabs, retaining walls and facade. The column bases of the building were fixed at the level of raft slab. The results of the study are proposed to be used to control or prevent progressive collapse of RC buildings.
Mathematical and Numerical Modelling of Wave Impact on Wave-Energy Buoys
Authors: Kalogirou A. , Bokhove O.
Published: October 2016

We report on the mathematical and numerical modelling of amplified rogue waves driving a wave-energy device in a contraction. This wave-energy device consists of a floating buoy attached to an AC-induction motor and constrained to move upward only in a contraction, for which we have realised a working scale-model. A coupled Hamiltonian system is derived for the dynamics of water waves and moving wave-energy buoys. This nonlinear model consists of the classical water wave equations for the free surface deviation and velocity potential, coupled to a set of equations describing the dynamics of a wave-energy buoy. As a stepping stone, the model is solved numerically for the case of linear shallow water waves causing the motion of a simple buoy structure with V-shaped cross-sections, using a variational (dis)continuous Galerkin finite element method.
Background for Revision of DNVGL-RP-C203 Fatigue Design of Offshore Steel Structures in 2016
Authors: Lotsberg Inge , Fjeldstad Arne , Ronold Knut O.
Published: October 2016

The recommended practice DNVGL-RP-C203 on fatigue design of offshore steel structures has been revised a number of times since it was first issued in 2001. The 2016 revision includes additional information on a number of items asked for by the industry such as: Residual stresses in tubular sections made from cold forming, S-N data for subsea application, change in validity of S-N curve in seawater with cathodic protection, relation between surface roughness and requirements for roughness for coating, S-N curves and stress concentration factors for fatigue assessment of pipelines, relation between fabrication tolerances and S-N curves in different types of structural details and hot spot stress analysis for rainflow counting.

The background for this is explained in the paper. In addition the paper includes some information on recommendations for fatigue design of single-sided tubular joints that was included in the 2014 revision of the Recommended Practice.
Sandwich Pipe for Long Distance Pipelines: Flow Assurance and Costs
Authors: Estefen Segen F. , Lourenço Marcelo Igor , Feng Junkai , Paz Claudio Moura , Bessa de Lima Dirney
Published: October 2016

The low oil prices have produced severe impact on the offshore industry slowing down new projects. The projects to become economically feasible shall incorporate innovative solutions for subsea equipment. In this context, the use of sandwich pipes for deepwater scenarios can be a useful alternative to conventional long distance pipelines. Sandwich pipe is a new concept composed of two concentric steel pipes separated by and bonded to a cement annulus that provides a combination of high structural strength with thermal insulation. For ultra deepwater scenarios, related to water depths beyond 1,500 m, the single wall steel pipe requires very thick wall and high insulation capacity. In this work, structural configurations for single wall pipe (SW), pipe-in-pipe (PIP) and sandwich pipe (SP) are designed for several deepwater scenarios. Collapse and buckling propagation are considered to overcome the high pressure environment while flow assurance is considered to overcome the low temperature. Simulations on flow assurance and cost evaluation will provide the basis for the comparative studies and recommendation about the appropriate scenarios for sandwich pipe applications. Flow assurance was simulated with OLGA software for the considered scenarios to avoid solid deposition. The costs related to each pipeline technology are finally compared for the established scenarios, and the sandwich pipe was considered to be a promising technology when high collapse strength and good thermal insulation are required.
Suppression of Irregular Frequency in Multi-Body Problem and Free-Surface Singularity Treatment
Authors: Liu Yujie , Falzarano Jeffrey M.
Published: October 2016

Multibody operations are routinely performed in offshore activities. One classical example is the FLNG and LNGC side-by-side offloading case. To understand the phenomenon occurring inside the gap is of growing interest to the offshore industry. One important issue is the existence of the irregular frequency effect. The effect can be confused with the physical resonance. Thus it needs to be removed.

An extensive survey of the previous approaches to the irregular frequency problem has been undertaken. The matrix formulated in the boundary integral equations will become nearly singular for some frequencies. The existence of numerical round-off errors will make the matrix still solvable by a direct solver, however will result in unreasonably large values in some aspects of the solution, namely the irregular frequency effect.

The removal of the irregular effect is important especially for multi-body hydrodynamic analysis in identifying the physical resonances caused by the configuration of floaters. This paper will mainly discuss the lid method on the internal free surface. To reach a higher accuracy, the singularity resulting from the Green function needs special care. Each term in the wave Green function will be evaluated using the corresponding analysis methods. Specifically, an analytical integral method is proposed to treat the log singularity. Finally, results with and without irregular frequency removal will be shown to demonstrate the effectiveness of our proposed method. The validation cases include mini-boxbarge, boxbarge and cylindrical dock, which has apparent irregular frequency effect in their output results.
A Study of Anti-Rolling Tank on Floating Vessel
Authors: Kim Kyung Sung , Lee Byung Hyuk , Kim Moo-Hyun , Park Jong-Chun , Choi Han Suk
Published: October 2016

Active anti-rolling tank (ART) is sophisticated equipment on a floating vessel to reduce roll motion for the slender ship-shape vessel. Three-dimensional panel based diffraction and radiation linear potential program employed to obtain hydrodynamic coefficients of floating vessel. For the ship motion, a BEM (Boundary Element Method)-based ship motion program was used and inner sloshing effects were conducted by a particle-based CFD (Computational Fluid Dynamics) program which is the Moving Particle Semi-implicit (MPS). By using panel program, the hydrodynamic coefficients were obtained in frequency domain, and then were converted into time domain ship motion simulation program. In this procedure, time memory effect was considered by Volterra series expansion. The ship motion program and sloshing program was coupled dynamically; inner tank received displacement, velocity and acceleration data from ship motion program and use them for inner tank motion, while the ship motion program was waiting external forces due to sloshing impact loads and inertia forces/moments from sloshing simulation program. Thus, two programs run simultaneously and allowed real time coupling effects of inner sloshing on vessel motion. By comparing response amplitude operator (RAO) of the vessel without anti-rolling tank, it was shown both values have good agreement. And then comparing between vessels with and without anti-rolling tank, it is shown that the effects of ART changed and shift RAOs. Furthermore, by changing the location of ART, location effects of ART were also investigated.
Deep and Shallow Water Low-Speed Maneuvering Tests: Comparison Between Experimental and Simulation Results
Authors: Ribolla Masetti Felipe , de Mello Pedro Cardozo , Rosetti Guilherme F. , Tannuri Eduardo A.
Published: October 2016

This paper presents small-scale low-speed maneuvering tests with an oceanographic research vessel and the comparison with mathematical model using the real time maneuvering simulator developed by the University of São Paulo (USP). The tests are intended to verify the behavior of the vessel and the mathematical model under transient and low speed tests. The small-scale tests were conducted in deep and shallow waters, with a depth-draft ratio equal to 1.28, in order to verify the simulator ability to represent the vessel maneuverability on both depth conditions. The hydrodynamic coefficients used in the simulator model were obtained by CFD calculations and wind tunnel model tests carried out for this vessel. Standard turning circle and accelerating turn maneuvers were used to compare the experimental and numerical results. A fair agreement was achieved for shallow and deep water. Some differences were observed mainly in the initial phase of the accelerating turn test.
WEC-Sim Phase 1 Validation Testing: Numerical Modeling of Experiments
Authors: Ruehl Kelley , Michelen Carlos , Bosma Bret , Yu Yi-Hsiang
Published: October 2016

The Wave Energy Converter Simulator (WEC-Sim) is an open-source code jointly developed by Sandia National Laboratories and the National Renewable Energy Laboratory. It is used to model wave energy converters subjected to operational and extreme waves. In order for the WEC-Sim code to be beneficial to the wave energy community, code verification and physical model validation is necessary. This paper describes numerical modeling of the wave tank testing for the 1:33-scale experimental testing of the floating oscillating surge wave energy converter. The comparison between WEC-Sim and the Phase 1 experimental data set serves as code validation. This paper is a follow-up to the WEC-Sim paper on experimental testing, and describes the WEC-Sim numerical simulations for the floating oscillating surge wave energy converter.
A Multi-Objective Real-Coded Genetic Algorithm Method for Wave Energy Converter Array Optimization
Authors: Sharp Chris , DuPont Bryony
Published: October 2016

For consumers residing near a coastline, and especially for those living or working in remote coastal areas, ocean energy is a promising source of electricity that has the potential to serve as a primary energy source. Over the last decade, many wave energy converter (WEC) designs have been developed for extracting energy from the ocean waves, and with the progression of these devices’ ocean deployment, the industry is looking ahead to the integration of arrays of devices into the grid. Due to the many factors that can potentially influence the configuration of an array (such as device interaction and system cost) optimal positioning of WECs in an array has yet to be well understood. This paper presents the results of a novel real-coded genetic algorithm created to determine ideal array configurations in a non-discretized space such that both power and cost are included in the objective. Power is calculated such that the wave interactions between devices are considered and cost is calculated using an analytical model derived from Sandia National Laboratory’s Reference Model Project. The resulting layouts are compared against previous array optimization results, using the same constraints as previous work to facilitate algorithm comparison. With the development of an algorithm that dictates device placement in a continuous space so that optimal array configurations are achieved, the results presented in this paper demonstrate progression towards an open-source method that the wave energy industry can use to more efficiently extract energy from the ocean’s vast supply through the creation of array designs that consider the many elements of a WEC array.
A Response Model for Vortex Induced Vibrations in Low KC Number Flows
Authors: Vedeld Knut , Sollund Håvar , Fyrileiv Olav , Nestegård Arne
Published: October 2016

Vortex induced vibrations (VIV) are generally less critical in wave dominated flow conditions than in pure current flows. A steady state response needs time to build up and continuous variation in flow velocity generally reduces the vibration amplitudes. For low Keulegan-Carpenter (KC) flows in-line VIV is generally removed entirely, replaced by forced response at the wave frequency, while cross-flow vibration amplitudes are reduced. For large KC numbers, the wave induced flow behaves similarly to a current and the oscillatory nature of the flow no longer influences the VIV response as much.

Empirical models to predict the influence of waves in VIV design of offshore cylindrical structures are formulated in offshore design codes. For most flow regimes the models are sound and supported by a solid base of empirical test data. There are, however, exceptions — particularly for the low KC number regime, defined here as LKCR. For such flows the oscillating pressure differentials which cause vortex induced vibrations are no longer governed by traditional dimensionless parameters such as the reduced velocity, but instead the oscillating lift is governed by the ratio between the eigen-frequency of the structure and the wave frequency. Particularly, the frequency of the lift force is twice the wave frequency in regular waves. In irregular wave conditions there are necessarily also spectral peaks at both the actual wave frequency and at three times the wave frequency, but the governing spectral density is concentrated at two times the wave frequency.

The present study introduces a novel response model to conservatively assess cross-flow VIV in LKCR.
The Dynamic Behavior of Sandwich Plate With Layered Graded Metallic Honeycomb Cores
Authors: Wang Zhihua , Li Shiqiang , Wang Genwei , Lu Guoyun , Zhao Longmao
Published: October 2016

Sandwich cellular structure, associated with more excellent mechanical properties and physical characteristic, such as, panel ductility and high core compressibility, is widely used in aerospace, transportation, military equipment and civil infrastructure. Due to the special compression properties of functional graded cores, the graded sandwich structure presented more excellent impact resistance than the ungraded sandwich.

This article presents the results of experimental, numerical investigation into the failure mode and dynamic plastic response of layered graded sandwich plate with layered honeycomb cores to air blast loading. The core arrangements (different core density) effects on the structure’s deformation behavior, energy absorption and impact resistance were mainly discussed. Three typical failure modes can be observed, that was, local deformation, global bending and penetration failure. Under specific loading conditions, the graded sandwich had more excellent impact resistant and energy absorption ratio, especially for the density-decreased core arrangement sandwich.
Frequency and Time Domain Methods for Forced Vibration Analysis of an Oscillating Cascade
Authors: Rahmati M. T.
Published: October 2016

Unsteady flow around an oscillating plate cascade has been computationally studied, aimed at examining the predictive ability of a non-linear frequency solution method for hydro-elasticity analysis compared with a standard analytical solution. The comparison of computational and analytical solutions for flow around an oscillating plate configuration demonstrates the capabilities of the frequency domain method compared with the analytical solution in capturing the unsteady flow. It also shows the great advantage of significant CPU time saving by the frequency methods over the nonlinear time method. This approach is based on casting the unsteady flow equations into a set of steady-like equations at a series of phases of a period of unsteadiness. So, One of the advantages of this method compared with other conventional time-linearized frequency domain methods is that any steady flow solution method can be easily used in a straightforward simple method for modelling unsteady perturbations.
Computation of Hydrodynamic Load and Strength Assessment of Highly Skewed Propeller
Authors: Awasare Sachin S. , Joga Ram Kumar , Dhavalikar Sharad S. , Kar Apurba R.
Published: October 2016

Ship Classification societies have rule requirements for assessment of propeller strength in terms of minimum blade thickness, however these rules generally covers only propellers of conventional geometry. In case of nonconventional (e.g. highly skewed) propellers, class societies ask for direct and detailed stress analysis for examination and approval. This paper introduces a procedure/strategy for assessing the strength of highly skewed propeller by determination of hydrodynamic load and consequent structural stress. The hydrodynamic analysis is performed separately using Reynolds Averaged Navier Stokes (RANS) solver STARCCM+ and structural analyses by finite element solver ANSYS. The hydrodynamic pressures as computed by CFD are transferred to the propeller model to calculate the resulting stationary stresses for maximum and minimum blade loading, using a quasi-static approach. The significance of root fillet in propeller design is demonstrated by comparing blade stress results with and without fillet at blade root. The stress concentration as a function of geometry and loading is also represented by the presented examples. The proposed method can assist the designer or reviewer to assess adequacy of strength of solid propellers as well as composite propellers of any geometrical configuration.
Group Maintenance Strategy for FPSO Offloading System Based on Reliability Analysis
Authors: Gao Song , Kang Jichuan
Published: October 2016

Floating Production Storage and Offloading (FPSO) is being widely used in offshore petroleum production. As a key subsystem, the offloading system often fails and causes marine disaster. The aim of this study was to investigate the failure events and failure rate to determine the efficient maintenance strategy, in particular maintenance interval and overhaul cycle. Failure Mode and Effects Analysis (FMEA) method was applied for the reliability analysis of the main failure events and their interrelations. In order to guarantee the accuracy, the analysis is based on four primary subsystem of offloading system: platform safety system, offloading hose system, center control system and auxiliary system. The logic structure of the main failure events was demonstrated by dynamic fault tree analysis (DFTA) in order to obtain the minimal cut sets, mean time between failure (MTBF) and failure probability by numerical analysis method. At last, group maintenances theory is applied to obtain the maintenance interval and overhaul cycle for the FPSO to achieve global optimum and feasibility. The result suggested the overhaul cycle is 4.8 years. After the comparison with actual engineering data, it is proved to be precise and this method is stable to be applied for engineering practice.
Statistical Investigation of the Surface Profile of Rogue Waves in 2D Non-Breaking Seas
Authors: Guo Qiuchen , Alam Mohammad-Reza
Published: October 2016

Rogue waves or freak waves are very large amplitude waves compare to the ambient waves in a given sea state. There are very few recording of such waves and therefore, despite years of research, very little is known about their properties. Here we invoke a statistical approach to find out about the typical shape of these giant waves. We consider different sea states and unidirectional vs crossing seas and study how each environment affects the morphology of oceanic rogue waves.
Reliability Analysis of Ultimate Longitudinal Strength for Ships in Yangtze River
Authors: Deng Le , Guo Ji , Fang Chuang , Zou Lu-yao
Published: October 2016

With Consideration of the uncertainties of materials yield strength, still water bending moment and wave-induced bending moment, the failure equation is established based on reliability analysis method. The reliabilities of 33 ships navigating in Yangtze River are calculated using improved FOSM method, according to which the target reliability is obtained, and the partial safety factors for hull girder ultimate strength, still water bending moment and wave-induced bending moment are given. As a result the formula for probability assessment of ultimate strength is established. Verification of sample ships shows that the formula is accurate and useful.
Aerodynamic Design Optimization of Elliptical-Bladed Savonius-Style Wind Turbine by Numerical Simulations
Authors: Alom Nur , Kolaparthi Satish Chandra , Gadde Sarath Chandra , Saha Ujjwal K.
Published: October 2016

Savonius-style wind turbine (SSWT), a class of vertical-axis wind turbine, appears to be promising for off-shore applications because of its design simplicity, good starting ability, insensitivity to wind direction, relatively low operating speed, low cost and easy installation. Various blade shapes have been used over the years to improve the performance of this class of turbine. In the recent past, an elliptic-bladed profile with sectional cut angle of 50° has shown its potential to harness the wind energy more efficiently. The present study aims to optimize this profile by numerical simulations. In view of this, the elliptical-bladed profiles are tested at different sectional cut angles of θ = 45°, 47.5°, 50° and 55°. The shear stress transport (SST) k-ω turbulence model is used to simulate the flow field, and thereafter, the torque and power coefficients are obtained at the rotating conditions. From 2D simulation, pressure and velocity contours are generated and analyzed. 2D simulations are also carried out for a semi-circular bladed profile in order to have a direct comparison. The numerical study demonstrates an improved flow characteristics, and hence the power coefficient of the elliptical-bladed profile at = 47.5°. Finally, 3D simulation is carried out to visualize and analyze the flow field around the optimum elliptical-bladed rotor at a tip speed ratio of 0.8. The aspect ratio of the rotor for the 3D simulation is kept at 0.7.
The Experiment and Finite Element Analysis of Carbon Fiber Sandwich Beam With Pyramidal Truss Core Structure
Authors: Gu Jiguang , Yang Nana , Guo Zhanyi , Yao Xiongliang
Published: October 2016

A new technology method is adapted to manufacture carbon fiber lattice sandwich beam with pyramidal truss core. The flat crush test experiment is to test the resistance to compression of the carbon fiber sandwich plate with pyramidal truss core. The result shows that after the pressure head contact the specimens adequately, and the stiffness of structure is the maximum. If the load is continuing increase, the pyramidal truss core may be destroyed, and both sides of the carbon fiber panel begin tottering. It emerges permanent deformation on the structures after an uninstall. The three-point bending test of lattice sandwich beam referred to ASTM C393-00 is designed to research the mechanical properties of face sheet and pyramidal truss core of lattice sandwich beam with theoretical analysis. Load-deflection curves of the middle of lattice sandwich beam in long span and in short span tests are retained, which are applied to obtain flexure stiffness of face sheet and shear strength of pyramidal truss core. It is found that span length has some influence on damage modes of lattice sandwich beam with pyramidal truss core. Debonding between face sheet and lattice core occurs when span is larger and core collapse appears when span is smaller. Crack expansion and fracture of resin base also both emerge in these two damage modes and the crack expansion consists of two different types which are crack expansion inside the resin base and crack expansion from the indenter to the support. Contrast with other lattice sandwich beam with similar or different shapes of core in the other references, the mechanical properties of this lattice sandwich beam by this new fabrication have obvious advantage at the same relative density.
Indo-Pacific humpback dolphin occurrence north of Lantau Island, Hong Kong, based on year-round passive acoustic monitoring
Authors: Munger Lisa , Lammers Marc O. , Cifuentes Mattie , Würsig Bernd , Jefferson Thomas A. , Hung Samuel K.
Published: October 2016

Long-term passive acoustic monitoring (PAM) was conducted to study Indo-Pacific humpback dolphins, Sousa chinensis, as part of environmental impact assessments for several major coastal development projects in Hong Kong waters north of Lantau Island. Ecological acoustic recorders obtained 2711 days of recording at 13 sites from December 2012 to December 2014. Humpback dolphin sounds were manually detected on more than half of days with recordings at 12 sites, 8 of which were within proposed reclamation areas. Dolphin detection rates were greatest at Lung Kwu Chau, with other high-occurrence locations northeast of the Hong Kong International Airport and within the Lung Kwu Tan and Siu Ho Wan regions. Dolphin detection rates were greatest in summer and autumn (June–November) and were significantly reduced in spring (March–May) compared to other times of year. Click detection rates were significantly higher at night than during daylight hours. These findings suggest high use of many of the proposed reclamation/development areas by humpback dolphins, particularly at night, and demonstrate the value of long-term PAM for documenting spatial and temporal patterns in dolphin occurrence to help inform management decisions.