bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effects of Scale and Configuration of Air Chamber of OWC Type WECs on Air Chamber Characteristics
Authors: Ikoma Tomoki , Hirai Shota , Aida Yasuhiro , Masuda Koichi , Eto Hiroaki 
Published: December 2020

Abstract
This paper describes scale effects and influence of configurations of oscillating water column type wave energy converters from model tests and theoretical calculations. Many researches regarding wave energy converters (WECs) have been conducted. The behavior of an oscillating water column of an OWC type WEC is complicated because of including wave-air-turbine interaction, and thus several issues remain. One of the issues is that influence of difference in scale between small scale experimental models and full scale models is unclear. It is important to understand its characteristics accurately to improve design technologies for such as complicated systems. In this study, we carried out forced oscillation tests using multiple scales and shapes of OWC models in still water, and measured the pressure inside the air chamber and the internal mean water level with a multi-line wave probe. The experimental models used have a box like air chamber or manifold type air chamber, and which scales were 1/1, 1/2 and 1/4.The difference of the two air chambers is an orifice or a duct to be inlet-outlet of air. As a result, the difference in scale and configuration of the air chamber affected the characteristics of the air chamber. In addition, as a result of numerical calculation using the linear potential theory and comparison with experimental results, the experimental results could be reproduced by numerical calculation. Besides, we could discuss the effects and the influences of the air chamber basically.
Large-Scale Tests With Hydraulic and Pneumatic Overpressure for Monopile Decommissioning of Offshore Wind Turbines
Authors: Hinzmann Nils , Gattermann Jörg , Lehn Patrick 
Published: December 2020

Abstract
The complete decommissioning of an offshore wind farm can be considered as a highly complex and hazardous approach. An unknown number of variables and unforeseen circumstances are involved in the decommissioning process.
On the one hand the decommissioning of the top structure, such as blades, turbine and mast, can be handled relatively risk free by reversing the installation steps. More focus needs to be given to the recycling method and logistic. The foundation decommissioning on the other hand is a much more challenging procedure. Originally designed for high axial and lateral loads, the foundations are generally oversized concerning the loading capacity. With a diameter up to nine meter, an embedment of about 40 meter and a set up effect over 25 years, the necessary force to pull the pile out of the seabed can be assumed, if at all determinable, to be enormous.
Different methods and techniques for a complete removal of offshore pile foundation are currently investigated within the project DeCoMP. Vibratory extraction and jet drilling aim for a reduction of the pile skin friction by creating a layer of less density between the pile shaft and pending soil.
In a different approach the seabed is used as an abutment and a pressing force is applied by creating an overpressure inside the pile. The results of pilot test, presented in this paper, show the capability of overpressure pile decommissioning. On this basis scaled test with an extensive measurement concept will be carried out in 2020.
A Numerical Study on the Evolution of Random Seas With the Occurrence of Rogue Waves
Authors: Zhou Zhuowei , Zhang Ningchuan , Huang Guoxing 
Published: December 2020

Abstract
Numerous numerical and experimental investigations show that rogue waves present a much larger probability of occurrence than expected from the linear random wave model, i.e., Gaussian distributed waves. The deviation from normal statistical events excites a continuous concern about rogue-wave research. In this study, rogue waves under random wave seas are addressed within the framework of the horizontal 1-D fully nonlinear Euler equations. The JONSWAP wave spectra with a different set of random phases are selected as the initial state of the recurrences of incoming wave trains. Different values of spectrum parameters (i.e., enhancement factor γ and significant wave height Hs) for JONSWAP spectra are chosen in order to reproduce different random sea states with different BFI values. The results of the numerical method using in this study are compared with classical experimental studies of rogue waves and show good agreements. Nonlinear wave interactions and the evolution of simulated waves are investigated in order to study the emergence of rogue waves. Statistics analysis is applied to the simulating results to find the deviations with normal distributions. Numerical results reveal that the initial unstable waves need some space to evolve, i.e., around 20 wavelengths, and will keep in an energetic state for the formation of rogue waves.
Feasibility of Using Hindcast Data for Fatigue Assessment of Permanently Moored Offshore Units in West-Africa
Authors: Hageman Remco , Aalberts Pieter , L’Hostis Didier , Ledoux Alain 
Published: December 2020

Abstract
To register the wave loads acting on offshore floating structures, wave buoys have been the generally accepted as the most accurate method. However, over the last couple of years, hindcast data has become increasingly accurate. The availability of this data allows assessment of the response of offshore floating structure without the need of local wave buoys.
The accuracy when using hindcast data to assess fatigue accumulation on FPSO hulls is assessed in this paper. For this analysis, calculations have been executed for multiple spread-moored production units in the West-Africa region. The purposely deployed wave buoys provide a full wave spectrum including multiple wave components and directional variation of the wave energy. This data is used to derive statistical data description of the sea state.
In this paper, we consider the use of WaveWatch III, ERA-5 and Copernicus hindcast models. These are all models which provide sea state descriptions on a global scale. The statistical data from the hindcast models is compared against the reference data provided by the wave buoys. Overall, the three models provide decent results in this mild environment. However, it is found that Copernicus provides more accurate estimates for the larger wave heights.
Fatigue analyses have been executed. The full spectral data provided by the wave buoys is used to conduct a spectral fatigue assessment. The hindcast models generally provide more limited details of the sea state, information such as spectral shapes and directional spreading may be missing. Understanding the influence of these missing characteristics is vital for reliable long term fatigue assessment. Multiple fatigue analyses have been executed to examine the sensitivity to these missing characteristics.
It has been shown that the multi-modality of the sea state and spectral shape are the most important parameters driving the deviation between the fatigue assessment based on hindcast data and wave buoy data. These differences can accumulate to a factor of 2 on lifetime consumption. The influence of the accuracy of the statistical parameters provided by the hindcast models and wave spreading is considerably less with a typical contribution of around 30% on lifetime consumption.
Experimental Study on the Effects of Mooring System on Air Gap Response of Semi-Submersible Platform
Authors: Li Xu , Xiao Longfei , Wei Handi , Liu Mingyue 
Published: December 2020

Abstract
The air gap response is crucial for the safe design and operation of large-volume floating platforms such as semi-submersible and tension leg platforms. It is a complex task to perform numerical simulation on the air gap response considering the wave free surface elevation and the motions of the floating vessel. Therefore, the prediction of air gap response still relies heavily on model tests. This paper attempts to investigate the effects of the mooring system, especially the effects of the length of mooring lines, on the air gap response of semi-submersible platform based on model tests results. The scaled model of the semi-submersible platform is supported by a symmetric mooring system composed of 8 mooring lines. A set of model tests with different length of mooring lines was performed in the State Key Laboratory of Ocean Engineering basin at Shanghai Jiao Tong University, and the air gap responses of 15 locations were measured using wave probes. The results indicate that the mooring system plays an important role in the air gap response of semi-submersible platform.
Sensitivity Analysis of an ANN-Based System for Detection of Mooring Line Failure
Authors: Sidarta Djoni E. , Tcherniguin Nicolas , Bouchard Philippe , Lim Ho-Joon 
Published: December 2020

Abstract
Monitoring the integrity of mooring lines on floating offshore platforms is one of the key factors in ensuring safe and productive offshore operations. Sensors, such as inclinometers, compressive load cells or strain sensors, can be used to monitor the inclination angles or tensions on mooring lines. An alternative method using only dry monitoring systems, such as DGPS (Differential Global Positioning System), Gyrocompass and/or IMU (Inertial Measurement / Motion Unit), can also be used to monitor the integrity of mooring lines. This method uses the measured motions and positions of a vessel without any information on the environmental conditions to detect mooring line failure. The detection of mooring line failure is based on detecting shifts in low-frequency periods and mean yaw angles as a function of vessel position, mass and added mass. The proposed method utilizes Artificial Neural Network (ANN) to recognize and classify patterns.
The training of an ANN model requires examples/data associated with intact mooring lines and broken mooring line(s). Examples/data of broken mooring line(s) are practically available only from numerical simulations. Therefore, it is important to address these two key topics: (1) Is the real behavior of the floating offshore platform sufficiently aligned with numerical simulations? and (2) The effect of the accuracy of monitoring equipment on the performance of an ANN-based system.
The first topic is reviewed briefly with its possible solution including some sensitivity tests, and this paper focuses on addressing the second topic. A system architecture is discussed in this paper along with the accuracy of the monitoring equipment. As an example, an ANN model has been trained to detect a broken mooring line of a spread-moored FPSO. This ANN model has been tested on its performance in dealing with a range of possible errors associated with the monitoring equipment. Furthermore, the tests have been carried out for a combination of variables that are not included in the ANN training, such as: vessel draft (mass), sea state conditions and directions. This paper presents the results of the tests for various variable sensitivities, which cover vessel positions, mean yaw angles and vessel drafts. These are essentially testing the tolerance of a trained ANN model against error or noise in the data. The results show that a trained ANN model can be error/noise tolerant.
Experimental Study the Performance of a Compact Mooring System for a Dual Chamber Floating Oscillating Water Column Device
Authors: Xu Sheng , Rezanejad K. , Wang Shan , Gadelho J. F. M. , Guedes Soares C. 
Published: December 2020

Abstract
A compact mooring system concept is proposed. This novel mooring is composed of submerged buoy and three segments of nylon ropes, which is suitable for the large wave energy converters and wave energy converter array due to its high flexibility and small mooring radius. The performance of this mooring concept was studied experimentally when it was moored to an oscillating water column. The damping of the oscillating water column was modelled by an orifice on top of the chamber. Both regular and irregular head sea wave tests were conducted. In order to study the influence of wave height on system dynamics, two series of regular wave tests with same periods but different wave heights were conducted. An optical tracking system was installed to capture six degrees of freedom motion responses of oscillating water column. The air pressure in the chamber was measured by the air pressure sensor. Two load cells were installed on the top of mooring lines to measure mooring tension time series. Besides, the wave surface elevations inside the chamber were measured by the wave gauges. According to the experimental results, the six degrees of freedom motion responses of floating wave energy converter and mooring tensions are analyzed. Besides, the energy conversion efficiency is evaluated based on the measured data.
Real-Time Prediction of Reliability of Dynamic Positioning Sub-Systems for Computation of Dynamic Positioning Reliability Index (DP-RI) Using Long Short Term Memory (LSTM)
Authors: Fernandez Charles , Bhushan Kumar Shashi , Lok Woo Wai , Norman Rosemary , Kr. Dev Arun 
Published: December 2020

Abstract
In this study, a framework using Long Short Term Memory (LSTM) for prediction of reliability of Dynamic Positioning (DP) sub-systems for computation of Dynamic Positioning Reliability Index (DP-RI) has been proposed. The DP System is complex with significant levels of integration between many sub-systems such as the Reference System, DP Control System, Thruster / Propulsion System, Power System, Electrical System and the Environment System to perform diverse control functions. The proposed framework includes a mathematical computation approach to compute reliability of DP sub-systems and a data driven approach to predict the reliability at a sub-system level for evaluation of model performance and accuracy. The framework results demonstrate excellent performance under a wide range of data availability and guaranteed lower computational burden for real-time non-linear optimization.
There are three main components of the proposed architecture for the mathematical formulation of the DP sub-systems based on individual sensor arrangements within the sub-system, computation of reliability of sub-systems and optimized LSTM deep learning algorithm for prediction of its reliability. Firstly, the mathematical formulation for the reliability of sub-systems is determined based on the series/parallel arrangement of the sensors of each individual equipment item within the sub-systems. Secondly, the computation of the reliability of sub-systems is achieved through an integrated approach during complex operation of the vessel. Thirdly, the novel optimized LSTM network is constructed to predict the reliability of the subsystems while minimizing integral errors in the algorithm.
In this paper, numerical simulations are set-up using a state-of-the-art advisory decision-making tool with mock-up and real-world data to give insights into the model performance and validate it against the existing risk assessment methodologies. Furthermore, we have analyzed the efficiency and stability of the proposed model against various levels of data availability. In conclusion the prediction accuracy of the proposed model is scalable and higher when compared with other model results.
Stress Concentration Factors Calculation: Analytical and Numerical Approaches for Welded Tubular Joints
Authors: Paruolo Nathalia , Mello Thalita , Teixeira Paula , Pérez Marco 
Published: December 2020

Abstract
In the oil and gas industry, fixed platforms are commonly applied in shallow water production. In-place environmental conditions generates cyclic loads on the structure that might lead to structural degradation due to fatigue damage. Fatigue is one of the most common failure modes of offshore structures and is typically estimated when dimensioning of the structure during design phase. However, in times when life extension of existing offshore structures is being a topic in high demand by industry, mature fields may represent an interesting investment, especially for small companies.
Concerning fixed platforms, composed mainly by welded tubular joints, the assessment of hot spot stresses is considered to predict structure fatigue. The estimation of welded joint hot spot stresses is based on the stress concentration factors (SCFs), which are given by parametric formulae, finite element analysis (FEA) or experimental tests. Parametric formulae may be defined as a fast and low-cost method, meanwhile finite elements analysis may be time consuming and experimental tests associated with higher costs. Given these different characteristics, each method is applied according to the study case, which will rely on the joint geometry and associated loads.
Considering simple joint geometries several sets of parametric equations found in the literature may be applied. On the other hand, the SCFs calculation of non-studied yet complex joints consider known formulae adapted according to the under load joint behavior and geometry. Previous analysis shows that this adaptation may furnish different results compared to those obtained by FEA. Furthermore, it is observed that even for simple joints the results derived from the different methods may differ.
Given their importance for the oil and gas industry, since they are the basis for the assessment of the fatigue life of welded tubular joints which may impact on additional costs related to maintenance and inspection campaigns, the estimation of SCFs must be the most accurate as possible. Therefore, this paper intends to investigate the differences between results derived from parametric formulae and different FEA studies.
Local Study of Jet of a Fluid Sloshing Inside a Rolling Tank
Authors: Tsao Wen-Huai , Kinnas Spyros A. 
Published: December 2020

Abstract
Sloshing is an important topic for the integrity of LNG tanks and the overall stability of the vessel. In the past, the interaction between the free surface and its substructure have been studied, especially where high-speed fluid jets impinge on the interior surface of the tank, and sometimes cause damage. In this paper, a boundary element method (BEM) with a fourth-order Runge-Kutta scheme is used to study the local phenomenon of nonlinear free-surface motion in a two-dimensional tank subject to roll motions. As the external excitations are nearly resonant with the fluid inside the tank, large free-surface deformations usually take place. The dynamic responses including the fluid velocity and pressure will grow drastically as the fluid is slamming on the walls. If no adequate conditions are applied, it is difficult to capture the peak physical quantities associated with strong nonlinear waves by most conventional Eulerian-Lagrangian methods. The numerical error will be accumulated and enlarged in just a few computational steps, which will eventually lead to unstable solutions. This paper will study the local phenomenon of a fast-moving jet forming on the wall of the tank, and its effect on the numerical stability and accuracy of the method overall. Some special numerical treatments are carried out for the local phenomenon approximation. The conservation of fluid mass is employed to obtain a reasonable geometry of the jet and its velocity along the wall. This provides a new set of rational boundary conditions applied on the walls, rather than using the artificial damping effect of other researchers. Results from the present method are compared with those from the volume of fluid (VOF) method implemented in ANSYS Fluent. The local free-surface shape in the vicinity of the jet and some local and global flow field patterns, including velocity and pressure, will be compared with and verified with experimental observations and measurements.
Accelerated Hydrodynamic Analysis for Spar Buoys With Second-Order Wave Excitation
Authors: Pegalajar-Jurado Antonio , Bredmose Henrik 
Published: December 2020

Abstract
The simplified numerical models commonly employed for the pre-design of floaters for offshore wind only include linear wave loads, due to the higher computational effort required by second-order methods. Second-order hydrodynamics, on the other hand, need to be considered from an early stage, since they cause resonance of the moored structure. In the present study, we introduce a new method to include second-order inviscid hydrodynamic loads at a computational cost similar to linear loads. We compare the accelerated method to standard second-order diffraction theory and to second-order Rainey forcing with Sharma & Dean wave kinematics. The comparison, based on the loads and response of a spar floating wind turbine in surge and pitch, is carried out for three different sea states. We find that a good prediction of the second-order resonant response can be obtained with the accelerated method for medium and severe sea states, while the match is not as good for the mild sea state. The accelerated method is between 400 and 850 times faster than commonly used second-order approaches, for an 1-hour realization of a given sea state. This speed up allows the application of the load model in the floater pre-design, where efficient numerical models are the key to achieve optimal designs and the consequent reduction in the cost of the floater.
Using Kriging Surrogate Models to Predict the Vibration Responses of a Submerged Riser
Authors: Damasceno Marcelo , Neto Hélio Ribeiro , Costa Tatiane , Cavalini Júnior Aldemir , Aguiar Ludimar , Martins Marcos 
Published: December 2020

Abstract
Fluid-structure interaction modeling tools based on computational fluid dynamics (CFD) produce interesting results that can be used in the design of submerged structures. However, the computational cost of simulations associated with the design of submerged offshore structures is high. There are no high-performance platforms devoted to the analysis and optimization of these structures using CFD techniques. In this context, this work aims to present a computational tool dedicated to the construction of Kriging surrogate models in order to represent the time domain force responses of submerged risers. The force responses obtained from high-cost computational simulations are used as outputs for training and validated the surrogate models. In this case, different excitations are applied in the riser aiming at evaluating the representativeness of the obtained Kriging surrogate model. A similar investigation is performed by changing the number of samples and the total time used for training purposes. The present methodology can be used to perform the dynamic analysis in different submerged structures with a low computational cost. Instead of solving the motion equation associated with the fluid-structure system, a Kriging surrogate model is used. A significant reduction in computational time is expected, which allows the realization of different analyses and optimization procedures in a fast and efficient manner for the design of this type of structure.
Comparison of Two Versions of the MNLS With the Full Water Wave Equations
Authors: Tang Tianning , Li Ye , Bingham Harry B. , Adcock Thomas A. A. 
Published: December 2020

Abstract
Versions of the non-linear Schrödinger equation are frequently used for modelling the non-linear propagation of water waves. In this paper, we compare two models against the results of fully non-linear numerical simulations. We consider uni-directional versions of the non-linear Schrödinger equation of Dysthe et al. with the hybrid model of Trulsen et al. The model of Trulsen et al. is shown to have clear advantages in all situations considered including modelling wave crest statistics for highly non-linear cases. However, for very broad bandwidths this model does start to break down, presumably due to the inherent limitation of the envelope representation of water waves. This in turn leads to a small, non-physical, leakage of energy in nonlinear simulations, although, this leakage is much smaller than for the version with 5th order linear dispersion relationship.