bluuebunny/crossref_metadata_2025_split huggingface dataset:
Neural Network-Based Method for Structural Damage and Scour Estimation Using Modal Parameters and Dynamic Responses
Authors: Wang Shuqing , Jiang Yufeng 
Published: December 2020

Abstract
Wind energy is the most promising clean, renewable energies to the power industry in the world. More and more wind turbine structures equipped with the larger capacity, taller towers, and longer blades were installed at the offshore/onshore wind farms. But these structures face many harsh environmental conditions, and structural damage and foundation scour are continuously accumulated. It could alter the modal parameter and dynamic response and further reduce the safety of structures. It is a significant challenge on how to accurately estimate the structural states if there is structural damage or foundation scour.
For addressing these limitations, a One Dimensional Convolutional Neural Network (1D CNN) method is developed to estimate the structural state. After the Fast Fourier Transform of the acceleration signals, these frequency responses are used as the input to train the 1D CNN, while these states are estimated as the output. A simplified spring-beam model is introduced to simulate the pile-soil interaction, and the effects of the damage and scour on natural frequencies are investigated and compared. The effectiveness and robustness of the proposed 1D CNN method have been numerically investigated by several scenarios associated with the wind turbine structure. Results demonstrate that the 1D CNN method can accurately estimate the structural states, even under a noisy environment. Further, the 1D CNN method can identify the location of damage and scour depth with very high accuracy. This approach may be useful in the on-site structural health monitoring in the wind turbine structure.
Relation Between Cyclone Evolution and Fetch Associated With Extreme Wave Events in the South Atlantic Ocean
Authors: Gramcianinov C. B. , Campos R. M. , de Camargo R. , Guedes Soares C. 
Published: December 2020

Abstract
Extreme wave generation in middle and high latitudes is mainly associated with extratropical cyclones. The wave generation process depends on the size and orientation of the fetch associated with each cyclone, which is usually not taken into account in traditional statistical approaches. A better understanding of the combined effect of the fetch orientation, displacement, and position within the extratropical cyclone can contribute with more accurate wave hindcasts and forecasts, which are crucial to marine operations. The main goal of this work is to investigate the fetch patterns and configurations associated with extratropical cyclones that promote extreme wave events in the western portion of the South Atlantic Ocean. Cyclones are tracked using an objective algorithm and linked to winter extreme Hs events in 10 years of ERA5. The results show the occurrence of 11.4 ± 2.8 storms per winter associated with extreme waves within the domain. Among these extreme events, the maximum and mean Hs was 10.3m and 6.1m respectively. The analysis of the fetch evolution during the lifecycle of the cyclones associated with the 10 most extreme events showed that the surface winds present its maximum usually 12h to 24h before the maximum Hs. The spatial pattern of the most severe events shows the development of a large fetch along the continental shelf, usually within the cold sector of the cyclone.
Numerical Model of a Vertical-Axis Cross-Flow Tidal Turbine
Authors: Zhao Ruiwen , Creech Angus C. W. , Borthwick Alistair G. L. , Nishino Takafumi , Venugopal Vengatesan 
Published: December 2020

Abstract
An array of close-packed contra-rotating cross-flow vertical-axis tidal rotors, a concept developed to maximize the fraction of flow passage swept, has potential advantages for hydrokinetic power generation. To predict the commercial feasibility of such rotors in large-scale application, a numerical model of a vertical-axis turbine (VAT) with a torque-controlled system is developed using an actuator line model (ALM). The open-source OpenFOAM computational fluid dynamics (CFD) code is first coupled with this ALM model, and efficiently parallelized to examine the characteristics of turbulent flow behind a vertical axis tidal turbine. The numerical model is validated against previous experimental measurements from a 1:6 scale physical model of a three-bladed reference vertical axis tidal turbine at the University of New Hampshire (UNH-RM2). Satisfactory overall agreement is obtained between numerical predictions and measured data on performance and near-wake characteristics, validating the numerical model. Details of the model setup and discussions on its output/results are included in the paper.
Application of Structural Monitoring Data for Fatigue Life Predictions of Monopile-Supported Offshore Wind Turbines
Authors: Petrovska Elize , Le Dreff Jean-Baptiste , Oterkus Selda , Thies Philipp , McCarthy Edward 
Published: December 2020

Abstract
Support structure fatigue is a key component in determining the structural lifetime of an offshore wind turbine (OWT). As the currently installed assets age, turbine operators are exploring options for lifetime extension to potentially increase the long-term financial return. Strain monitoring at critical points on a turbine is commonly performed to improve understanding of structural integrity and ultimately reassess its remaining useful life. Reliable application of the findings of a time-limited structural monitoring programme in predicting structural response over a turbine’s lifetime requires a good understanding of the representativeness of the dataset.
Uncertainties arise in fatigue damage estimations due to the stochastic nature of the environmental loading. Statistical treatment of the environmental loads and the corresponding structural response is made by defining measured load cases (MLCs), within which the turbine operational state and associated range of environmental parameters are specified. For OWTs, this leads to a multi-dimensional problem, as both wind and wave parameters need to be accounted for. The complexity of the analysis is thus increased, requiring identification of the critical external and operational parameters that influence overall fatigue. The associated statistical uncertainty can then be estimated by considering repeated measurements throughout the monitoring period.
The work presented in this paper investigates the application of statistical resampling techniques in evaluating the uncertainty in total measured fatigue damage experienced by an offshore wind turbine. Direct fatigue computation over the given measured dataset has been contrasted with statistical approaches applying probability distributions of MLCs to give indication of the influence of the key environmental parameters. Strain monitoring data from a 2.3MW OWT was utilised in conjunction with the corresponding operational and environmental measurements.
The methods and outcomes of this study can be used to improve the remaining fatigue life prediction of installed turbine foundations, by assessing the representativeness of strain measurements. As structural design uses industry defined safety margins, comparison of design predictions against operational measurement data will allow verifying that these safety margins are not exceeded, within the bounds of the given uncertainties. Finally, an understanding of data uncertainties will allow estimates to be made regarding the reliability of the consequent fatigue lifetime reassessment or of the numerical model validation procedures. Such information is useful to wind turbine operators as it provides the first step towards data-driven lifetime extension and informs on measurement campaign utilisation.
Low-Frequency Oscillations Within the Hambantota Port During the Southwest Monsoon, 2019
Authors: Zheng Zhenjun , Ma Xiaozhou , Huang Xuezhi , Dong Yujin , Dong Guohai 
Published: December 2020

Abstract
Long waves with periods greater than tens of seconds propagating into a harbor may be trapped and significantly amplified, thereby resulting in detrimental effects on port operations. The water surface elevation in the Hambantota Port, Sri Lanka, was measured to investigate the low-frequency oscillations and their forcing mechanisms. Results show that the port is protected well from short waves with periods less than 30 s; however, the protection against long waves with periods larger than 30 s is inadequate. The spectral analyses identified four dominant periods within the low-frequency wave range. Modal analysis based on the extended mild-slope equation shows that the measured spectrum density for some dominant periods is low because the measurement point is close to the corresponding modal lines. Correlation analysis shows that low-frequency oscillations inside the Hambantota Port are excited directly by the low-frequency waves contained within the incident waves. The low-frequency waves outside the Hambantota Port are generated from the higher-frequency gravity waves (swell and wind waves) due to nonlinear interactions. Empirical formula is adopted to estimate the low-frequency wave height outside the Hambantota Port.
Validation of Predictions of Wire Stress of Flexible Pipe With Damaged Tensile Armor Wires Under Combined Tension and Bending
Authors: Doynov Krassimir , Rombado Gabriel , Cooke Nathan , Majed Arya 
Published: December 2020

Abstract
The nonlinear kinematic response of a damaged 2.5” flexible pipe under combined tensile and bending cyclic loads is simulated and compared to experimental results. High fidelity finite element model substructures are constructed for intact and broken outer and inner armor wire configurations and assembled in a nonlinear dynamic substructuring (NDS) framework to efficiently simulate the full-scale test configurations. Overall, 12 analysis configurations involving all intact wires, up to 4 broken outer wires, and 2 and 4 broken inner wires combined with 4 broken outer wires are constructed. Each analysis configuration is first preloaded axially and then subject to multiple cycles of (i) pure tension and (ii) combined tension and bending. For each case, tensile armor wire strains are extracted from the simulations and compared to strain measurements from the test. For all cases, numerical predictions and test measurements agree well accurately capturing the redistribution of strains into the adjacent intact wires which result in stress concentration factors.
This comprehensive demonstration of accurate capture of flexible pipe damaged wire kinematics by high fidelity finite element models and nonlinear simulations has direct applications to flexible pipe integrity management and remnant life assessments. Given that the NDS framework allows highly efficient computation, it is now feasible to execute real-time irregular wave local fatigue simulations with finite element models that include damaged wire data from physical inspections to more accurately predict remnant life.
A Study on the Thermal Deformation of Fillet Joint Friction Stir Welding
Authors: Kang Sungwook , Kim Jaewoong , Kim Donghyun , Lee Kwangjin , Jung Yoonchul 
Published: December 2020

Abstract
In this study, experiments and simulations were performed for fillet joint friction stir welding according to tool shape and welding conditions. Conventional butt friction stir welding has good weldability because heat is generated by friction with the bottom of the tool shoulder. However, in the case of fillet friction stir welding, the frictional heat is not sufficiently generated at the bottom of the tool shoulder due to the shape of the tool and the shape of the joint. Therefore, it is important to sufficiently generate frictional heat by slowing the welding speed as compared to butt welding. In this study, experiments and simulations were carried out on an aluminum battery housing made by friction stir welding an extruded material with a fillet joint. The temperature of the structure was measured using thermocouple during welding, and the heat source was calculated through correlation analysis. Thermal elasto-plastic analysis of the structure was carried out using the calculated heat source and geometric boundary conditions. It is confirmed that the experimental results and the simulation results are well matched. Based on the results of the study, the deformation of the structure can be calculated through simulation even if the tool shape and welding process conditions change.
Effect of the Concentration of Viscosifier on Desorption Kinetics of Methane From Synthetic Based Fluids
Authors: Ojedeji Damilola , Chen Yuanhang 
Published: December 2020

Abstract
Synthetic-oil based drilling mud is currently the most commonly used type of drilling fluid for offshore drilling in the Gulf of Mexico, due to the environmental regulation in the area, as well as the numerous operational benefits they provide. However, early kick detection and well control decision-making are more challenging due to the solubility of formation gas in synthetic-based fluids. This partially contributes to the poor understanding of the mass transfer kinetics of formation gas in and out of synthetic fluids during these well control events. The objective of this work was to better understand the mass transfer of gas from a solution by evaluating the influence of viscosifier concentration on the desorption kinetics of methane from pure internal olefin and internal olefin-viscosifier mixture. The desorption coefficients were determined from a custom-built mass transfer apparatus. Different suspentone concentrations ranging from 0 to 5wt% by volume of liquid were used to investigate the influence of viscosifier concentration on the desorption coefficient. It was observed that the presence of suspension agents in the liquid phase decreased the mass transfer coefficient. This decrease could be due to an increase in the resistance to the flow of gas bubbles evolving from the liquid phase.
S-N Curve Model for Assessing Cumulative Fatigue Damage of Deep-Water Composite Riser
Authors: Singh Manander , Ahmad Suhail , Jain A. K. 
Published: December 2020

Abstract
Random stress produces fatigue damage due to fluctuation in the presence of high bending stresses gets enhanced and leads to the primary failure of the production composite riser. The main problems with the fatigue deterministic model are related to the fact that not all waves have the same period. The second problem is due to the assumption that all waves are regular and do not into account the stochastic nature of the marine environment. The wave data is provided on a statistical basis. Work attributed to damage related stress distribution and interaction of composite along with integral difficulties in fatigue is challenging task. Resulting flexural stress history is random in nature. The purpose of this study is to present a methodology for assessing cumulative fatigue damage using MATLAB of the deep-water composite riser. The simulation of 12 random sea states is examined for the purpose of estimation of accumulated damage. Using the recommended method by DNV-OS-C501, fatigue life of composite riser is determined for different conditions. Damage induced by stress signals of varying amplitudes is calculated using Palmgren-Miner’s rule. Total fatigue life for stress history is estimated by the summation of damage fractions. Estimation of fatigue damage over riser’s entire service life is done taking into consideration the entire expected sea area. Extreme flexural stress random fluctuations are estimated with and without currents.
Fighting Invisible Kicks: Study of Time Dependent Vaporization of Methane Gas Dissolved in Base Oil
Authors: Nilsen Marius Staahl , Hovda Sigve 
Published: December 2020

Abstract
Understanding the interaction between the drilling fluid and the natural gas from a gas kick may be of great importance when predicting how a well control incident evolves during drilling operations. This is especially true for oil based mud, which has the ability to dissolve large quantities of gas under high pressure, thus potentially hide any volumetric impact of a gas kick. When the pressure of the dissolved gas decreases below the bubble pressure, free gas will start to emerge. Dangerous situations can occur if the bubble point pressure is low and located close to the surface. This may result in a rapid volumetric expansion of the free gas, as it emerges from solution, thus little to no time to react and initiate proper well control procedures.
Most conventional well control simulators that takes gas solubility into consideration assumes an instantaneous vaporization of gas as the vapour-liquid phase equilibria changes. However, this assumption might not always be realistic. It may take some time before a new equilibrium is reached when the conditions are changed. This will thus affecting the rate of gas liberation from the liquid.
To better understand this complex issue, an analytical expression for the transition rate of dissolved gas to free gas in a supersaturated liquid has been derived for low pressure systems. The analytical model is strongly dependent on the solubility coefficient, Kh, and the transition rate factor, γ, and follows an exponential curve. In this expression, Kh is a measure of how much the liquid is supersaturated at any given time and controls how much gas that will be liberated. γ determines how fast the system will reach a new equilibrium, i.e. how fast the gas will be liberated based on the size of the supersaturation. Both Kh and γ are thought to be values given for a specific gas-liquid combination.
In order to verify the analytical expression, experimental testing has been conducted. The experiment is carried out by pressurizing a tank partly filled with the base oil Exxsol D60 by feeding it with methane gas. Some of the gas will dissolve into the liquid. The rest will flow to the top as free gas and pressurize the tank. By quickly removing some of the free gas, thus depressurize the tank, the liquid will instantaneously become supersaturated, hence triggering liberation of free gas from the solution until a new equilibrium is established. By measuring the tank pressure throughout the degassing phase, values for Kh and γ can be estimated and compared to the analytical model.
Design and Fabrication of a Fully Elastic Ship Model
Authors: Chen Yingying , Zhang Shanli , Chen Wei Kean , Magee Allan 
Published: December 2020

Abstract
Ship structures are subjected to various deteriorating mechanisms throughout their service life. Continuous awareness of the vessel’s structural health is a critical aspect of the overall situational awareness. Model tests are often used to validate software which predicts the vessels’ hydrodynamic loading and dynamic structural response. Generally, two different methods can be used to model the flexibility of the ship. The most common method is to sub-divide the hull into a number of rigid segments that are interconnected by a flexible backbone beam. The elasticity of the model is represented by the elastic beam to which rigid segments are connected. However, the segmented model limits the measurements to prescribed locations between segments. The other method is to fabricate the model using a continuous elastic material. In this paper, a new method for fabricating a fully elastic model is introduced as part of a structural health monitoring system. Since the model is built from continuous materials with known elastic properties, it can be instrumented to measure strain at a larger number of locations. A suitable material for construction of the elastic model has been identified. Material tests are conducted to better understand the static and dynamic behavior of the elastic material. The material shows linear stress-strain relationship and stable mechanical properties within the loading range. Due to the low elastic modulus of the material, the strain gauge stiffening effect is obvious and has been taken into account in the calibration process. Using the elastic material, a fully elastic model of the S175 midship section is designed. As a first step trial, the middle part of the model representing a three-cargo hold is manufactured. Static bending tests are conducted to examine the elastic characteristics of the fabricated model. Wave experiments are carried out. The results from these experiments are compared to numerical simulations.
Loads on a Point-Absorber Wave Energy Converter in Regular and Focused Extreme Wave Events
Authors: Katsidoniotaki Eirini , Ransley Edward , Brown Scott , Palm Johannes , Engström Jens , Göteman Malin 
Published: December 2020

Abstract
Accurate modeling and prediction of extreme loads for survivability is of crucial importance if wave energy is to become commercially viable. The fundamental differences in scale and dynamics from traditional offshore structures, as well as the fact that wave energy has not converged around one or a few technologies, implies that it is still an open question how the extreme loads should be modeled. In recent years, several methods to model wave energy converters in extreme waves have been developed, but it is not yet clear how the different methods compare. The purpose of this work is the comparison of two widely used approaches when studying the response of a point-absorber wave energy converter in extreme waves, using the open-source CFD software OpenFOAM. The equivalent design-waves are generated both as equivalent regular waves and as focused waves defined using NewWave theory. Our results show that the different extreme wave modeling methods produce different dynamics and extreme forces acting on the system. It is concluded that for the investigation of point-absorber response in extreme wave conditions, the wave train dynamics and the motion history of the buoy are of high importance for the resulting buoy response and mooring forces.
Experimental Assessment of the IFPEN Solution to the WEC Control Competition
Authors: Tona Paolino , Sabiron Guillaume , Nguyen Hoai-Nam , Mérigaud Alexis , Ngo Caroline 
Published: December 2020

Abstract
This paper describes the experimental assessment of the solution developed by IFP Energies nouvelles (IFPEN) for the WEC Control Competition (WECCCOMP), a benchmark devised to compare energy-maximizing controllers for wave energy converters (WECs). For the first round of the competition, carried out in simulation with WEC-Sim, IFPEN had submitted an MPC-based solution, which eventually scored best among the contestants, with respect to the energy-related criterion defined by the organizers and computed on a set of six irregular waves. For the second round of the competition, the performance of this MPC solution has been assessed in a tank test situation, on the same Wavestar-like scale device that had been simulated in WEC-Sim.
The paper first recalls the features of the control solution implemented for the competition, an offline-tuned weighted-QP MPC algorithm using short-term prediction from present and past wave excitation force estimates. Then, the major steps of the test plan are described and control design choices are explained.
Finally, the experimental results in closed loop are presented. Despite noise and an imperfectly controlled PTO actuator, IFPEN’s MPC solution proves efficient and robust: its experimental performance in terms of harvested electrical energy is in accordance with the nominal results obtained in simulation using the linear design model, and this, for different realizations of each sea state. This performance has allowed the IFPEN team to win the competition also at the experimental stage.