bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear Generation of Narrow-Banded Wave Trains
Authors: Khait Anatoliy , Shemer Lev
Published: November 2019

Analytic method for nonlinear wave generation by a wavemaker that is somewhat different from the nonlinear theory of Schäffer is proposed. The method that is based on the Nonlinear Schrödinger (NLS) equation and the nonlinear boundary condition at the wavemaker is free of 2nd order limitation inherent to the existing wavemaker theories. Advantages offered by the NLS model allowed simplification of the expressions for determination of the wavemaker driving signal and thus made them easily applicable in practice. The nonlinear correction to the wavemaker driving signal is calculated from the complex surface elevation envelope obtained as a solution of the NLS equation at the prescribed location in the wave flume. The domain of applicability of the generation method was determined on the basis of numerous experiments in the wave flume. A very good generation of the required wave train shape was obtained for sufficiently narrow-banded wave trains.
Identifying Higher-Order Interactions in Wave Time-Series
Authors: Ewans Kevin , Christou Marios , Ilic Suzana , Jonathan Philip
Published: November 2019

Reliable design and reanalysis of coastal and offshore structures requires, amongst other things, characterisation of extreme crest elevation corresponding to long return periods, and of the evolution of a wave in space and time conditional on an extreme crest.

Extreme crests typically correspond to focussed wave events enhanced by wave-wave interactions of different orders. Higher-order spectral analysis can be used to identify wave-wave interactions in time-series of water surface elevation.

The bispectrum and its normalised form (the bicoherence) have been reported by numerous authors as a means to characterise three-wave interactions in laboratory, field and simulation experiments. The bispectrum corresponds to a frequency-domain representation of the third order cumulant of the time-series, and can be thought of as an extension of the power spectrum (itself the frequency-domain representation of the second order cumulant). The power spectrum and bispectrum can both be expressed in terms of the Fourier transforms of the original time-series. The Fast Fourier transform (FFT) therefore provides an efficient means of estimation. However, there are a number of important practical considerations to ensuring reasonable estimation.

To detect four-wave interactions, we need to consider the trispectrum and its normalised form (the tricoherence). The trispectrum corresponds to a frequency-domain (Fourier) representation of the fourth-order cumulant of the time-series. Four-wave interactions between Fourier components can involve interactions of the type where f1 + f2 + f3 = f4 and where f1 + f2 = f3 + f4, resulting in two definitions of the trispectrum, depending on which of the two interactions is of interest. We consider both definitions in this paper. Both definitions can be estimated using the FFT, but it’s estimation is considerably more challenging than estimation of the bispectrum. Again, there are important practicalities to bear in mind.

In this work, we consider the key practical steps required to correctly estimate the trispectrum and tricoherence. We demonstrate the usefulness of the trispectrum and tricoherence for identifying wave-wave interactions in synthetic (based on combinations of sinusoids and on the HOS model) and measured wave time-series.
Study on the Generated Power Changes by the Relation Between an Arrangement of an Array of Point Absorber Type WECs and an Incident Wave Angle
Authors: Murai Motohiko , Li Qiao , Funada Junki
Published: November 2019

Using marine renewable energy can contribute providing the energy instead of using fossil fuel. There are some types of marine renewable energy. Wave energy is one of these and no one doubts its vast potential once they glance the sea. The object of this paper is an array of Point Absorber Wave Energy Converters (PA-WECs). In the array, complex hydrodynamic interactions among the array will occur. The interaction influences the wave power density among the array both increase and decrease. When we consider the maximization of generated electric power of the array, the suitable choice of the control force of the generator taken into a positive effect of the hydrodynamic interactions is the key. In the past research, as for a detection of the suitable control force of a single PA-WEC it has been shown the analytical solution within linear system. In this paper, we show the numerical solution of the control force parameter for maximizing the generated electric power of the array taking the hydrodynamic interaction among the array both in diffraction and radiation problem into account. Then we carried out the numerical examination and discussed the good arrangement of an array in waves based on the proposed method and an effect of the choice of the best and better control force.
Evaluation Adaptability of Nano Wetting Fluid for Releasing Tight Sandstone Gas Reservoir Water Locking Effect
Authors: Yao Erdong , Wang Jie , Xue Yanpeng , Zhou Fujian , Zhang Le , Li Yafei
Published: November 2019

At present, the most effective means of releasing the “water blocking damage” in the near-well zone is to change the wettability of the rock surface from liquid wetting to gas wetting. In this study, a novel nano wetting fluid was used to change the sandstone surface wettability aqueous phase from wetting phase to non-wetting phase. The adaptability of 0.5 wt% nano wetting fluid at different aging times, pH, salinity type, salinity and temperature was evaluated. The results show that 0.5 wt% nano wetting fluid has better adaptability under different conditions. The aqueous phase of the core surface can be changed from wetting phase to non-wetting phase; after adding nano wetting fluid, the drainage rate increased by 19.09%; at the same time, SEM characterization experiments showed that nano wetting fluid could achieve hydrophobic effect by changing the surface roughness of illite and chlorite.
Numerical Hull Resistance and Hydrodynamic Characteristics of an Independently Rotating Multi-Hull Vessel
Authors: Srinivasamurthy Sharath , Sakamoto Hiroshi , Nishikawa Tatsuo , Nihei Yasunori
Published: November 2019

In this study, an attempt is made to understand the hydrodynamic characteristics of an automated multi-hull vessel named as Quadmaran. Firstly, a single hull of the Quadmaran vessel is considered and numerical CFD simulation is performed using OpenFOAM. Hull resistance predictions are compared with the experimental findings and numerical simulation is validated. After validating the simulation scheme, further CFD simulations are performed on two hulls with varying distance between them to understand the hydrodynamic characteristics of different configurations. Pressure distribution and wave patterns around the hulls are also summarized and discussed in the paper. Further, efforts are made to optimize the distance between the hulls for reducing hull resistance. A basic framework is suggested to carefully consider the hydrodynamic effects for multi-hull vessels.
Real-Time Measurement of Eccentricity in Primary Cementing of Oil and Gas Wells
Authors: Maleki Amir , Frigaard Ian
Published: November 2019

We propose a new method to measure eccentricity of the annulus in real-time during primary cementing. Our methodology is based on seeding the displacing fluids with neutrally buoyant particles. The process of seeding can happen by a specially designed bottom plug that releases a particle every few seconds. Particles at different azimuthal position experience a different axial velocity, because of the eccentricity of annulus. We infer the local velocity of the fluids by tracking the trajectories of particles and correlate it to the local eccentricity of the annulus. We show that this methodology allows us to find an axial profile of eccentricity along the well, and helps to identify geometrical irregularities, such as washouts, along the annulus.
Impacts of Educational Outreach on Envisioning Marine Cities of the Future Development Project
Authors: Yoshida Ikuo
Published: November 2019

Given today’s changing global environment and the need for people to be able to lead prosperous lives in the future, we will need technical experts who can think flexibly and respond to change, and we will also need to provide opportunities for children to awaken and develop their interest in relevant technical areas. I have used lectures and seminars on the Marine Cities of the Future Development Project [1][2], a project in which I am deeply engaged, as opportunities to verify the impact of educational outreach on students. In these lectures, I emphasize aspects such as “pushing the boundaries” and “realizing goals through reliable technology,” and tailor the content to stimulate students’ career aspirations in STEM fields. Requests from educators for further lectures aimed at cultivating students’ career aspirations and students’ heightened interest in proposals of new technology as revealed by post-lecture surveys, confirm that these efforts have had the intended effect.
Development of YS 500MPa Class Thick Steel Plate With Weld Joint CTOD Property for Offshore Structures
Authors: Terazawa Yusuke , Ichimiya Katsuyuki , Ueda Keiji , Igi Satoshi , Suwa Minoru , Tanaka Toshitaka , Tsuji Akiyoshi
Published: November 2019

Recently, the installation areas of offshore structures has expanded into deep sea areas, and as a result, strength and plate thickness of the steel plate required for the offshore structures have become higher. And the main property of offshore structure steel is the crack tip opening displacement (CTOD) property of multi-pass weld joint. Thus, the developed steel was designed to improve HAZ toughness of multi-pass weld joint. Firstly, in order to achieve both high strength of the base metal and good HAZ toughness, the TMCP technology was applied, and Ceq. was minimized. And to obtain fine HAZ microstructure, fine TiN and Ca(O,S) particles having pinning effect and nucleation site effect were used. Finally, to decrease the formation of brittle martensite-austenite constituent (M-A) in HAZ of multi-pass weld joint, Si content was decreased. By using above technologies, the YS 500MPa class thick steel plate with good weld joint CTOD property have been developed.
Experimental and Numerical Study on Dynamic Response of FSRU-LNGC Side-by-Side Mooring System
Authors: Yue Jingxia , Kang Weili , Mao Wengang , Chen Pengfei , Wang Xi
Published: November 2019

Floating Storage and Regasification Unit (FSRU) becomes one of the most popular equipment in the industry for providing clean energy due to its technical, economic and environmental features. Under the combined loads from wind, wave and current, it is difficult for the prediction of the dynamic response for such FSRU-LNGC (Liquified Natural Gas Carrier) side-by-side mooring system, because of the complicated hydrodynamic interaction between the two floating bodies. In this paper, a non-dimensional damping parameter of the two floating bodies is obtained from a scaled model test. Then the numerical analysis is carried out based on the test results, and the damping lid method is applied to simulate the hydrodynamic interference between floating bodies. The dynamic response of the side-by-side mooring system including six degrees of freedom motion, cable tension and fender force are provided and analyzed. According to the comparisons between numerical results and the test results, it is shown that the proposed coupled analysis model is reliable, and the numerical analysis can properly describe the dynamic response of the multi-floating mooring system in the marine environment. Moreover, the non-dimensional damping parameter which is used in numerical analysis can act as a good reference to the dynamic response analysis of similar multi-floating mooring systems.
Study of Wave Added Resistance and Motions of KCS in Waves With Different Wave Lengths
Authors: Guo Hao , Wan Decheng
Published: November 2019

Estimating added resistance and motions of a ship in waves are essential to predict fuel consumption and speed loss. The added resistance and motions of the 3600 TEU KRISO container ship (KCS) in regular head waves under different wavelengths are investigated using Reynolds-Averaged Navier-Stokes (RANS) method. Volume of Fluid (VOF) method is applied to capture the free surface. The in-house computational fluid dynamics solver, naoe-FOAM-SJTU, is used to compute the added resistance and motions of KCS in regular head waves. Firstly, the first-order Stokes waves in deep water are adopted and generated in naoe-FOAM-SJTU as a numerical wave tank. Secondly, it is presented that the KCS with a Froude number of 0.261 advances in these waves. Regular wave conditions with a wide range of wavelength (0.65 < λ/L < 1.95) are considered. The variations of resistance, pitch and heave show good agreement with experimental results.

To investigate nonlinear behaviors of predicted results, Fast Fourier Transform (FFT) is applied to analyze the results of resistance, heave and pitch with in head wave (μ = 180°). KCS with and without motions is also compared to investigate the relationship between components of resistance and wavelengths. The results of added resistances show that the added resistance of KCS is mainly induced by ship diffraction in short waves. The wave diffraction is not affected by wave frequency. In addition, CFD can accurately calculate the problem on added resistance and ship motions.
ECAs and Lateral Buckling
Authors: Cosham Andrew , Macdonald Kenneth A. , MacRae Ian , Carr Malcolm
Published: November 2019

An engineering critical assessment (ECA) is commonly conducted during the design of an offshore pipeline in order to determine the tolerable size of flaws in the girth welds. API 579-1/ASME FFS-1 2016 and BS 7910:2013+A1:2015 Incorporating Corrigenda Nos. 1 and 2 give guidance on conducting fitness-for-service assessments of cracks and crack-like flaws. DNVGL-RP-F108, 2017 Assessment of flaws in pipeline and riser girth welds describes a methodology to satisfy the fracture and fatigue limit states in DNVGL-ST-F101, 2017 based on Option 2 with ductile tearing in BS 7910:2013. It requires that the stress-strain curve used in a strain-based fracture mechanics analysis should represent a high yield strength combined with low strain-hardening properties (a characteristic high stress-strain curve with low strain hardening), and that used in a stress-based fracture mechanics assessment should represent a low yield strength.

A pipeline operating at high temperatures and/or high pressures is subject to high compressive axial forces. The pipeline might then relieve these forces by buckling. A design that incorporates controlled lateral buckling is an efficient solution to the problem of high compressive axial stresses. Lateral buckling does, however, give rise to relatively high tensile axial strains (possibly exceeding 0.4 percent) in the pipe wall, and, relatively high fatigue loading associated with movement of the buckle under start-up and shut-down cycles.

The calculated tensile axial strain in the pipe wall in a lateral buckle depends on the assumed stress-strain curve. It tends to be higher if a low yield strength combined with low strain-hardening properties is assumed. There is then an apparent inconsistency between the two sets of assumptions.

A deterministic assessment of a circumferentially-orientated, internal surface crack-like flaw in a girth weld in a lateral buckle is used to investigate the significance of this apparent inconsistency.
Development of a Model Predictive Controller for the Wave Energy Converter Control Competition
Authors: Ling Bradley A.
Published: November 2019

The University of Maynooth is hosting a competition to develop a control strategy for a heaving point absorber wave energy converter (WEC). A linear model predictive control (MPC) design for the competition is presented. The state space model used in the MPC was derived numerically from the provided WEC-Sim model using linear system identification methods. A Kalman filter was used as the estimator, while also serving as an unknown input estimator to provide estimates of the excitation force on the WEC. The required excitation force predictions were made using an autoregressive linear prediction model. The inputs to the prediction model included estimated wave excitation forces and measured water surface elevation values from an up-field wave probe. Simulation results of the final control system design are also presented for each of the six wave cases specified by the competition organizers.
The Evolutionary Geometric Physics of Vortex-Induced Vibrations
Authors: Zueck Robert F.
Published: November 2019

Induced vibrations are multi-dimensional oscillations in a marine riser, suspended wire or other slender structures, whereby the maximum amplitude of deflection is generally perpendicular to the sustained action. In previous OMAE papers, we have shown how sustained actions evolve physical changes in slender structures. Using nonlinear physics-based simulations, we showed how these structural changes fundamentally determine the nature of the vibrations that any sustained action (including flowing fluid around the structure) can induce. In this paper, we step back to focus on a classical laboratory experiment, whereby the structure has been constrained to function as a simple mass-on-spring oscillator. In this unchanging structure, we show how the geometric physics of the fluid drag load induce Vortex-Induced Vibrations (VIV). We show how these vibrations naturally grow in time to maximum amplitude.
Time Domain Simulations of Ship Maneuvering and Roll Motion in Regular Waves Based on a Hybrid Method
Authors: Ma Chengqian , Ma Ning , Gu Xiechong
Published: November 2019

Ship maneuvering performance and rolling in waves under complicated environmental conditions are of significant importance for safety and economic reasons. The existing methods for predicting the maneuvering in adverse sea conditions can be categorized into unified two-time scale model, hybrid approach and CFD method. However, traditional potential methods rely tightly on ship viscous force data from test results, and CFD methods of free running ship require large computational resources consumption. In this paper, a 4-DOF (surge, sway, yaw and roll) model based on MMG method considering the wave effect is established to predict the trajectory and rolling motion with better time efficiency.

The 1st order wave force and mean 2nd order drift force in this time-domain model are calculated by the 3D panel method and Cummins impose response function. Instead of model experiments, the hydrodynamic derivatives in the maneuvering model can be calculated by RANS-based numerical simulations of the Planar Motion Mechanism (PMM) test in calm water. Verification for grid convergence is also conducted according to state-of-the-art study. The predicted turning trajectory and rolling angle of the S175 containership in regular waves using CFD results show better agreement with experiment data than empirical formula results. Furthermore, it has been demonstrated that this model is also capable of predicting the ship motion in regular waves with practical accuracy. And the effects of the wave frequency, wave height are investigated consequently base on numerical simulation results.
On Disaster Risk Reduction in Norwegian Oil and Gas Industry Through Life-Cycle Perspective
Authors: Ibrion Michaela , Paltrinieri Nicola , Nejad Amir R.
Published: November 2019

This paper presents the risk reduction in Norwegian oil & gas industry over the time (1975–2016) through a life cycle perspective analysis with the aim to identify the critical stage(s) both in terms of accident occurrence and cause of the accident. Fifteen accidents, major accidents and disasters for example Ecofisk 2/4 Alpha 1975, Alexander L. Kielland 1980, Songa Endurance 2016 were studied. Cases from outside of the Norwegian offshore field — the Piper Alpha 1988, the Bourbon Dolphin 2007, and the Deep Water Horizon 2010 — were also considered as comparison. For each accident and through the life cycle analysis, the occurrence stage of the accident and its main technical causes were identified and compared. It was found that a high risk is concentrated in the Operation (In-Service) stage and associated Marine Operations. Furthermore, it was observed that a high number of accidents in oil and gas industry are associated with mobile structures. All the investigated accidents have acted as powerful reminders to the oil and gas industry that a continuous improvement of risk management and reduction of uncertainty are of paramount importance in order to ensure safe operations and risk reduction for accidents, major accidents and disasters. However, a reactive learning from major accidents and disasters needs to be supported by a proactive learning and development of a dynamic risk culture in the oil and gas industry.
Use of 3D Scan of Weld Joint in Finite Element Analysis and Stochastic Analysis of Hot-Spot Stresses in Tubular Joint for Fatigue Life Estimation
Authors: Larsen Mikkel L. , Arora Vikas , Lützen Marie , Pedersen Ronnie R. , Putnam Eric
Published: November 2019

Several methods for modelling and finite element analysis of tubular welded joints are described in various design codes. These codes provide specific recommendations for modelling of the welded joints, using simple weld geometries. In this paper, experimental hot-spot strain range results from a full-scale automatically welded K-node test are compared to corresponding finite element models. As part of investigating the automatically welded K-joint, 3D scans of the weld surfaces have been made. These scans are included in the FE models to determine the accuracy of the FE models. The results are compared to an FE model with a simple weld geometry based on common offshore design codes and a model without any modelled weld. The results show that the FE model with 3D scanned welds is more accurate than the two simple FE models. As the weld toe location of the 3D scanned weld is difficult to locate precisely in the FE model and as misplacement of strain gauges are possible, stochastic finite element modelling is performed to analyse the resulting probabilistic hot-spot stresses. The results show large standard deviations, showing the necessity to evaluate the hot-spot stress method when using 3D scanned welds.
Study on the Influence of Mesh Grouping on Numerical Simulation Results of Fish Cages
Authors: Huang Liuyi , Li Yuyan , Ni Yi , Cheng Hui , Wang Xinxin , Wang Gang , Zhao Fenfang
Published: November 2019

The Morison Model is widely applied in the numerical simulation for the hydrodynamics assessment of fish cage. In the Morison model, hydrodynamic forces are calculated based on twines. To reduce the computational time, mesh grouping methods (replacement of multiple meshes with less number of equivalent meshes) have been implemented widely in the calculation. However, as the hydrodynamic equation for a mesh is quite different under wave and current, it is not appropriate to use the classical mesh grouping. On the basis of basic hydrodynamic equations of the meshes with current and wave, this paper carried out the study about theoretical analysis of two methods (the equivalent area method and the equivalent volume method) of the mesh grouping, using main parameters of nets such as the diameter, solidity ratio and elastic modulus related to mesh grouping. A single-point mooring cage, of which tension and displacement could be calculated with finite element method, was selected as a case to carry out the verification of two mesh grouping methods. Flume model experiment was used to validate the accuracy of mesh grouping methods. The results indicated that, the equivalent area method has a higher accuracy in the pure current condition, while the equivalent volume method was more accurate in combined waves and current. The accuracy of results using mesh grouping to analyze hydrodynamics of nets within a certain range of grouping times could be insured with an improved calculation speed. This paper can provide practical advice on the mesh grouping process in the numerical simulation of fish cages and fishing gear.
Dented Externally-Pressurised Pipes Subjected to Cyclic Axial Loading
Authors: Chatziioannou Konstantinos , Huang Yuner , Karamanos Spyros A.
Published: November 2019

The present paper describes a numerical investigation of the mechanical response of externally-pressurized dented stainless-steel pipes, subjected to reverse cyclic axial loading. This is the first part of a large-scale project, between The University of Edinburgh and Tianjin University, and is motivated by the mechanical response of offshore pipelines, which are often subjected to cyclic loading during installation or operation. Under those cyclic loading conditions, the pipe may collapse because of accumulation of plastic deformations at the dent area.

The paper describes a numerical simulation of the above physical problem, in an attempt to support experiments on 50mm-diameter stainless steel pipes, which are being performed at the laboratory facilities of Tianjin University. Pipe segments are subjected to reverse cyclic axial loading (tension and compression), in the presence of external pressure. Prior to the application of external pressure and axial load, the pipes are locally dented, in the form of “smooth dent” or “local ovalization”, so that collapse initiates at the dent area. The numerical simulation is aimed at examining some aspects of pipeline behavior to support and complement the experimental observations. The simulation is conducted using rigorous finite element tools, which account for large displacement and nonlinear material. Towards this purpose, an advanced material model is employed, capable of describing the phenomenological aspects of material response under cyclic loading, such as the accumulation of plastic strain and ratcheting.

In the first part of the analysis, the local ovalization (denting) process is simulated. Subsequently, the pipes are subjected to uniform constant external pressure and, keeping the pressure level constant, monotonic or cyclic axial loading is applied until collapse. The numerical results are aimed at identifying the interrelation between the magnitude of the applied loading and the number of loading cycles to failure. The results are presented in diagrams of axial displacement, ovalization and local strain versus the corresponding number of cycles to failure, for specific levels of external pressure.
A Study on Improvement of Fatigue Performance by Shot Blasted Surface Treatment
Authors: Fujii Toshihiro , Gotoh Koji , Deguchi Junichi , Murakami Koji
Published: November 2019

Recently, the strength of the hull structure including fatigue strength has seen significant improvement because of application of IACS CSR (Common Structure rule), developed by IACS. Various methods which can increase the fatigue strength at welded joints are suggested, since the damage morphology on the recently welded structures was observed to be primarily fatigue failure. In Shipbuilding industry, smoothening of the welding bead toe by grinding is the method that is customarily adopted in order to increase fatigue strength. And recently, peening treatment at the welding bead has also been adopted. The peening treatment can reduce stress concentration by smoothening of the welding bead toe, and a compressive residual stress field generated by the impact of peening.

In case of generating compressive residual stress by impact, the same effect could be expected by shot blasting, which is the steel surface treatment before painting.

In this study, the condition of the residual stress generated at welding toe by shot blasting was investigated, and then from results of the fatigue test, the increase in fatigue strength due to residual stress is verified.
Optimization of a Horizontal Axis Tidal Current Turbine by Multi-Objective Optimization
Authors: Nagataki Takumi , Kurokawa Ko , Yamada Reiko , Sakaguchi Daisaku , Kyozuka Yusaku
Published: November 2019

A global search optimization system is applied to the design of a horizontal axis tidal current turbine with shroud. 11 design parameters of the turbine blade and 4 design parameters of the shroud casing are considered for the optimization using a genetic algorithm. In order to reduce the simulation cost, a neural network is applied as the meta-model of the RANS (Reynolds-averaged Navier–Stokes) equation solver. Multi-objectives of the power coefficient at different tip speed ratios are applied to cover a wide operating range of the turbine. The CFD (Computational fluid dynamics) for optimization is validated experimentally for the case of a baseline design, and an optimum design is proposed. In this paper, a static structural analysis has been performed, and its robustness is confirmed under several operating conditions. Furthermore, internal flow of the optimized turbine is discussed in detail. It is found that the optimized blade generates a swirling flow and suppresses flow separation at the diffuser wall. The wide angle of the diffuser successfully achieves a high pressure recovery ratio and results in a high level of suction at the inlet of the turbine. It is found that the high-performance tidal turbine is possible to design if both the blade and the shroud diffuser are optimized at the same time.
Application of the Spectral Nudging on Global Tides Towards a Global Total Water Level Prediction System
Authors: Kodaira Tsubasa , Bernier Natacha , Thompson Keith R.
Published: November 2019

With the long-term goal of developing an ensemble forecast system for coastal flooding, we are developing a dynamically-based, numerical model of the global ocean. The model is based on the NEMO framework and has been used to predict global tides and surges in previous studies. This study focuses on the optimization of the joint prediction of both tides and surges, the two main components of total water level that cause coastal flooding. To improve the predictions of the tide we use a modified form of “spectral nudging”. We show this leads to significant improvements in the prediction of the M2 tide in the open ocean, and also in the shallow regions closer to shore where the model is not nudged. The median value of the vector difference of the tidal amplitude based on sea level observations and a data-assimilative model, and the predictions of our ocean model, is reduced from 11.2 cm to 2.66 cm by the nudging. The improvement deteriorates significantly however if additional tidal constituents are included in the model (most notably S2). This is explained in terms of spectral leakage between tidal bands associated with the nudging methodology and a straightforward solution is proposed.
Three-Dimensional Numerical Analysis of Horizontal and Vertical Coalescence of Bubbles at Two Submerged Horizontal Orifices on the Wall
Authors: Li Z. P. , Sun L. Q. , Yao X. L. , Piao Y.
Published: November 2019

In the process of bubbling from two submerged adjacent orifices, bubbles coalescence becomes inevitable. But the study of the evolution and interaction of bubbles from submerged orifices is little, especially numerical simulation. In this paper, combined with mesh smoothing technique, mesh subdivision technique and the technique of axisymmetric coalescence and 3D coalescence, a three-dimensional model of bubbles coalescence at two submerged adjacent orifices on the wall is established by the boundary element method. Then, numerical simulations were carried out for horizontal and vertical coalescence before detachment. Finally, by changing the ventilation rate and the Froude number, the effects of different ventilation rates and buoyancy on the process of bubbles coalescence at two adjacent orifices were investigated. The results show that for horizontal coalescence, the effect of ventilation rate is more pronounced than buoyancy. As the ventilation rate increases or the influence of buoyancy is decreased, the amplitude of internal pressure fluctuation of the bubble decreases and the coalescence time decreases. For vertical coalescence, the effect of buoyancy is more pronounced than ventilation rate. With the influence of buoyancy is decreased, the vertical coalescence time is increased, the internal pressure of the bubble is decreased. The influence of ventilation rate is similar to that of horizontal coalescence.
Analysing Dependent Failures in a Bayesian Belief Network
Authors: Fam Mei Ling , Konovessis Dimitrios , He Xuhong , Ong Lin Seng , Tan Hoon Kiang
Published: November 2019

Fault trees (FT) and event trees (ET) have been used thoroughly in risk analysis and there have been a few published articles outlining how to map FTs and ETs to Bayesian Belief Networks (BBN). There have been documented benefits of a BBN being able to consider Common Cause Failures (CCF) and conditional dependencies. With modelling CCFs in a BBN, there is a possibility to increase the level of analysis of a CCF by breaking down the analysis to the respective CCF Categories, such as Environment, Maintenance or Design. This allows a better understanding of the contributing events given a defined accident scenario. Also, in the decommissioning industry, there is no established database yet for CCF of components, as decommissioning projects are sparse and spread out across different operating conditions. Hence it may be practical to adjust generic CCFs to obtain facility-specific parameters for common cause failures. The paper thus highlights how to express CCFs with a Beta-Factor Model in a BBN and by extension, undertake an extended level of analysis according to CCF categories and adjust generic database common cause factors to a facility-specific factor based on a checklist. The technique is applied to a risk analysis of a well plugging and abandonment event.
Wave Response of a Novel Breakwater Concept With Oscillating Water Columns
Authors: Han Mengmeng , Wang Chien Ming , Duan Wenhui
Published: November 2019

A novel concept of a mega floating breakwater-windbreak is proposed for reducing both wave height and wind speed on its leeward side. L-shaped internal channels are installed inside the breakwater hull that surface from the floating breakwater as tubes that function like “trees” to break the wind. The channel openings are on the vertical walls at the upstream side of the breakwater, so that water is allowed to oscillate inside the channels and create extra viscous damping. Numerical studies using both linear boundary element method and CFD were carried out to determine the wave transmissibility and wave force on the breakwater with and without the internal channels. The wave condition in the Gold Coast, Australia was selected as an environmental input. Operating wave condition and extreme wave condition were separately investigated. Scaled dimensions were used to study the breakwater behavior in a wave flume. The results show that when water oscillates inside the channels, the transmitted wave height and lateral wave force acting on the breakwater will be reduced. In addition, since the viscous effect influences waves of all lengths, the internal channels improves the breakwater performance in blocking long waves.
Fundamental Study on Development on Numerical Method for Evaluation of Wave Power Generating Systems With Pendulum Type by the Particle Method
Authors: Murata Kazuki , Rheem Chang-kyu , Ikoma Tomoki
Published: November 2019

In this paper, we report the proposal and development of the numerical model. The model applies to two and three dimensional problems. We introduce the model to the MPS method which is a kind of particle method, and discuss applicability of the model on numerical test of aerial physical pendulum in free torque load; the present model was compared with the theoretical solution by the 4th order Runge-Kutta method. As a result, we confirmed the good reproducibility of the model. And we can say that the model is suitable for analyzing the problem of physical pendulum.

Secondary, we examine the practicability of this model in the regular wave from the calculation results. The tests are examined by a two-dimensional problem in order to reduce calculation cost. In addition, we conducted fluid-structure interaction analysis using the present model and confirmed the applicability of its model to fluid field analysis.
Research on the Improved Body-Force Method Based on Viscous Flow
Authors: Li Zhiheng , Yu Jiawei , Feng Dakui , Jiang Kaijun , Zhou Yujie
Published: November 2019

The virtual propeller model can achieve the rapid numerical prediction of the ship self-propulsion performance through viscous flow, which used the improved body-force method. The two-dimensional lift coefficient CL and the drag coefficient CD are very important parameters in this method, which are generally obtained by the potential flow methods and cannot incorporate viscous effects. This study will perform a fully nonlinear unsteady RANS (Reynolds Average Navier-Stokes) simulation to get the KP505 open-water characteristics and then divide its blade into several parts to get the lift coefficient CL and the drag coefficient CD on each one. Then fitting by multivariate regression method, the relationship between CL, CD and propeller parameters is obtained. The Unsteady Blade Element Theory (UBET) is coupled with RANS in house CFD code HUST-Ship (Hydrodynamic Unsteady Simulation Technology for Ship) to calculate the flow around the propeller. RANS equations are solved by the finite difference method and PISO arithmetic. have been made using structured grid with overset technology. The results show that comparing with the EFD data, the maximum differences of the result of the improved body-force method are 4.32% and 2.7% for the thrust coefficient and the torque coefficient respectively near the propeller operating point.
Numerical Simulations of OC3 Spar and OC4 Semi-Submersible Type Platforms Under Extreme Conditions in the East Sea, Korea
Authors: Shin Hyunkyoung , Yu Youngjae , Pham Thanh Dam , Kim Junbae , Kumar Rupesh
Published: November 2019

Since the Paris Conference of the parties in 2015, interest in renewable energy around the world is higher than ever. Korea also has plans to increase the proportion of renewable energy to 20% by 2030 through the renewable energy 3020 policy. Of these, 16.5GW is filled with wind power, the installation area is expanding from land to sea. Among them, some of big plans are using floating offshore wind turbines based on the marine environments in Korea. In this study, numerical simulations of the NREL 5MW wind turbine were performed using NREL FAST V.8. A comparison was made between two types of floaters, spar and semi-submersible, installed 58km off the Ulsan Coast with 150m water depth in the East Sea, Korea. The environmental data were obtained from the Meteorological Administration’s measured data and NASA’s reanalysis data, MERRA-2. Design Load Cases were selected by referring to IEC 61400-3. Maximum moments at both blade root and tower base, six-degrees of freedom motions and three mooring line tensions were compared.
Computational Fatigue Assessment of Chains Working in Twisted Conditions
Authors: Martinez Perez Imanol , Gabrielsen Øystein
Published: November 2019

Chains are extensively used in the Oil and Gas industry for mooring vessels or floating platforms, both for temporary and long-term moorings. Moreover, recently they found a new application in the mooring of floating wind turbines. The integrity of mooring systems is of great importance for both industries. Failure Records [1]–[2] indicate that fatigue of mooring chains is one of the main causes of failure, and therefore their lifetime prediction represents an important challenge for industry. Chains are intended to work under Tension Loading, however, anomalous loading modes such as Out-of-Plane Bending (OPB) or Twisting can appear. For example, OPB caught the attention of engineers after the Girassol incident; where it was identified as the main cause of failure of several chain links eight months after the installation [3]. Since this failure, OPB has attracted the attention of several research programs and multiple publications can be found in the literature (For example, [4]–[5]). Twist may occur during the installation of a chain or during service due to torque generated in the near-by elements of the mooring (for example wire cables). This paper presents a computational fatigue assessment of mooring chains working in twisted conditions. It is based on a two steps analysis: a mechanical and a fatigue analysis. The mechanical analysis accounts for the initial residual stress state of the chain after manufacturing and subsequent proof loading. The result is the stabilized cycle of the multiaxial stress field, which is obtained through an elastic-plastic three-dimensional Finite Element Analysis (FEA). Then, the fatigue analysis is performed to predict the failure location and determine the lifetime. The predicted failure location has shown good agreement with fatigue cracks found in recovered chain links in the North Sea after more than 15 years of service.
The Influence of Tidal Unsteadiness on a Tidal Turbine Blade Flow-Induced Vibration
Authors: Arini Nu Rhahida , Turnock Stephen R. , Tan Mingyi
Published: November 2019

The influence of unsteady tidal flow on the flow-induced vibration of a vertical axis tidal turbine blade is investigated numerically in this paper. A 2D CFD model is developed to simulate the blade flow-induced vibration in OpenFoam. The vibration is caused by dynamic loading from the unsteady tide. It is recognized that the unsteady tidal current mainly comes from the changes in tidal velocity magnitude and angle of attack experienced by a tidal turbine blade as it rotates. This paper studies numerically how velocity magnitude and initial angle of attack influence tidal turbine blade vibrations and the effects of the velocity and angle of attack are evaluated separately where the unsteadiness parameters are varied around a set of environmental condition. The vibration is examined through time histories of blade displacement, pressure distribution on the blade surface and the tidal current regime. The blade is assumed to have pitch and heave responses thus the vibration is in the form of transverse and torsional vibrations. The results show that increasing tidal velocity magnitude strengthens the torsional vibration. The increase of angle of attack is likely to generate chaotic motions and enhance both transverse and torsional vibrations.
Numerical Study of a Single-Point Mooring Gravity Fish Cage With Different Deformation-Suppression Methods
Authors: Cheng Hui , Aarsæther Karl Gunnar , Li Lin , Ong Muk Chen
Published: November 2019

Compared to the multi-point mooring fish cage, the single point mooring (SPM) fish cage can spread out the accumulation of organic matter to prevent the local environment from being overwhelmed, and reduce the manufacturing cost at the same time. Thus, it has attracted many attentions recently. In this research, different deformation-suppression methods are applied to the SPM system with a typical Norwegian fish cage aiming to reduce the cultivation volume loss. A well-validated software, FhSim, is used to conduct the full-scale numerical study. The effects of the three deformation-suppression methods (i.e. (i) adding the lower bridles, (ii) adding the frontal rigid frame, (iii) adding the trawl door) are analyzed under pure current and combined wave-current conditions. The results indicate that all the three methods can reduce the cultivation volume loss at least by 32% compared to the one with the original SPM system when the current velocity is larger than 0.5 m/s. In addition, moving the conjunction point close to the bottom ring can bring positive effect in the cultivation volume maintaining with an unnoticeable increase in the tension force. This study will provide practical advice and useful guides for the SPM fish cage design.
The Impact of Modelling Air Compressibility in the Selection of Optimal OWC Design Parameters in Site Specific Wave Conditions
Authors: Simonetti Irene , Cappietti Lorenzo
Published: November 2019

The importance of properly modelling the effects of air compressibility in the selection of the optimal design parameters for an Oscillating Water Column wave energy converter is investigated. For this purpose, a wide dataset of capture width ratios, obtained from both experimental tests and Computational Fluid Dynamic simulations, is used to formulate an empirical model able to predict the performance of the device as a function of its basic design parameters (chamber width and draught, turbine damping) and of the wave conditions (wave period, wave height). A multiple non-linear regression approach is used to determine the model numerical coefficients. The data used to formulate the model include the effects of air compressibility. The impact of considering such effects on the selection of the optimal geometry of the device is evaluated and discussed by means of the model application for the optimization of a device to be installed in a site located in the Mediterranean Sea (in front of the coast of Tuscany, Italy).
World First Fatigue S-N Curve for Bonded Reinforcements for FPSO Application
Authors: Abbad El Andaloussi Hamza , Mouton Luc , Sayed Ahmad Firas , Mahérault-Mougin Stéphanie , Paboeuf Stéphane , Errotabehere Xabier
Published: November 2019

Steel is the most common construction material in the Oil & Gas industry, and it begins to rust the day it is cast. This observation raises several challenges for the asset integrity and life extension of all offshore units. Therefore, for the ageing FPSOs (>10 years), frequent and costly structural maintenance operations in hazardous environment may be required to repair their hull. While a standard ship can go to dry-dock for “crop and renew” maintenance operations through standard hot works techniques, a permanently moored asset has to be maintained in situ. The challenge is to perform these offshore structural maintenance operations with no production disruption while maximizing safety even for the most stressed areas such as a mid-ship deck plating.

COLDSHIELD, a co-development between COLD PAD, TOTAL and IFP Energies nouvelles, has been set up to meet this very challenge. This innovative alternative to “crop and renew” is covered by approval certificate from Class. COLDSHIELD stops the corrosion and restores hull structural strength. Two industrial applications have been performed so far. Among the steps to be taken to demonstrate that such repair can be considered as permanent, it was mandatory to characterize the fatigue behavior of this reinforcement solution.

While the fatigue behavior of steel has been an important research topic since the early eighties of the last century, its understanding for bonded composite materials is still at its infancy. Indeed, for the time being there is — to our knowledge — no S-N curve for structural bonded reinforcements. Full scale coupon specimen fatigue tests of the structural bonded reinforcements were conducted by COLD PAD, third-partied by Bureau Veritas and in collaboration with two laboratories (private and public) in order to study the adhesive fatigue. A statistical analysis was performed according to international standards. The resulting S-N curve is fit for industrial fatigue design. It demonstrates a comfortable fatigue strength (S-N curve presenting a slope of 9).

This paper presents the results of a fatigue life assessment campaign of COLDSHIELD including the experimental setup, the fatigue test results, and the numerical analyses. It details the reason why the design is compatible with a stress-based approach. It also explains the conclusions that can be derived in terms of fatigue life for a deck repair campaign.