A CFD Study of Focused Extreme Wave Impact on Decks of Offshore Structures

2014 ◽  
Author(s):  
Xin Lu ◽  
Pankaj Kumar ◽  
Anand Bahuguni ◽  
Yanling Wu
Keyword(s):  
Real World ◽  
Offshore Structures ◽  
Air Gap ◽  
Irregular Waves ◽  
Linear Wave ◽  
Extreme Wave ◽  
Wave Impact ◽  
Loading Test ◽  
Wide Range ◽  
Wave Maker

The design of offshore structures for extreme/abnormal waves assumes that there is sufficient air gap such that waves will not hit the platform deck. Due to inaccuracies in the predictions of extreme wave crests in addition to settlement or sea-level increases, the required air gap between the crest of the extreme wave and the deck is often inadequate in existing platforms and therefore wave-in-deck loads need to be considered when assessing the integrity of such platforms. The problem of wave-in-deck loading involves very complex physics and demands intensive study. In the Computational Fluid Mechanics (CFD) approach, two critical issues must be addressed, namely the efficient, realistic numerical wave maker and the accurate free surface capturing methodology. Most reported CFD research on wave-in-deck loads consider regular waves only, for instance the Stokes fifth-order waves. They are, however, recognized by designers as approximate approaches since “real world” sea states consist of random irregular waves. In our work, we report a recently developed focused extreme wave maker based on the NewWave theory. This model can better approximate the “real world” conditions, and is more efficient than conventional random wave makers. It is able to efficiently generate targeted waves at a prescribed time and location. The work is implemented and integrated with OpenFOAM, an open source platform that receives more and more attention in a wide range of industrial applications. We will describe the developed numerical method of predicting highly non-linear wave-in-deck loads in the time domain. The model’s capability is firstly demonstrated against 3D model testing experiments on a fixed block with various deck orientations under random waves. A detailed loading analysis is conducted and compared with available numerical and measurement data. It is then applied to an extreme wave loading test on a selected bridge with multiple under-deck girders. The waves are focused extreme irregular waves derived from NewWave theory and JONSWAP spectra.

scholarly journals EXTREME WAVE PRESSURES AND LOADS ON A PILE-SUPPORTED WHARF DECK - INFLUENCES OF AIR GAP AND WAVE DIRECTION

2018 ◽  
pp. 5
Author(s):  
Andrew Cornett
Keyword(s):  
Offshore Structures ◽  
Model Tests ◽  
Scale Model ◽  
Extreme Waves ◽  
Wave Direction ◽  
Coastal Structures ◽  
Extreme Wave ◽  
Wave Impact ◽  
The Past ◽  
Water Depths

Many deck-on-pile structures are located in shallow water depths at elevations low enough to be inundated by large waves during intense storms or tsunami. Many researchers have studied wave-in-deck loads over the past decade using a variety of theoretical, experimental, and numerical methods. Wave-in-deck loads on various pile supported coastal structures such as jetties, piers, wharves and bridges have been studied by Tirindelli et al. (2003), Cuomo et al. (2007, 2009), Murali et al. (2009), and Meng et al. (2010). All these authors analyzed data from scale model tests to investigate the pressures and loads on beam and deck elements subject to wave impact under various conditions. Wavein- deck loads on fixed offshore structures have been studied by Murray et al. (1997), Finnigan et al. (1997), Bea et al. (1999, 2001), Baarholm et al. (2004, 2009), and Raaij et al. (2007). These authors have studied both simplified and realistic deck structures using a mixture of theoretical analysis and model tests. Other researchers, including Kendon et al. (2010), Schellin et al. (2009), Lande et al. (2011) and Wemmenhove et al. (2011) have demonstrated that various CFD methods can be used to simulate the interaction of extreme waves with both simple and more realistic deck structures, and predict wave-in-deck pressures and loads.

scholarly journals CFD Modeling of Regular and Irregular Waves Generated by Flap Type Wave Maker

2021 ◽  
Vol 85 (2) ◽  
pp. 128-144
Author(s):  
Ali Shehab Shams Eldeen ◽  
Ahmed M. R. El-Baz ◽  
Abdalla Mostafa Elmarhomy
Keyword(s):  
Numerical Simulation ◽  
Simulation Method ◽  
Irregular Waves ◽  
Cfd Modeling ◽  
Linear Wave ◽  
Ocean Engineering ◽  
Navier Stokes Equation ◽  
Type Wave ◽  
Wave Maker ◽  
Numerical Beach

The improvement of wave generation in numerical tanks represents the key factor in ocean engineering development to save time and effort in research concerned with wave energy conversion. For this purpose, this paper introduces a numerical simulation method to generate both regular and irregular waves using Flap-Type wave maker. A 2D numerical wave tank model is constructed with a numerical beach technique, the independence of the numerical beach slope is tested to reduce the wave reflections. The different governing parameters of the Flap type wave maker were studied such as periodic time dependency and length of the flap stroke. The linear wave generated was validated against the wave maker theory WMT, the numerical results agreed with WMT. The Pierson-Moskowitz model is used to generate irregular waves with different frequencies and amplitudes. The numerical model succeeded to generate irregular waves which was validated against published experimental data and with Pierson-Moskowitz spectrum model using Fourier expansion theory in the frequency domain. Useful results are presented in this paper based on the numerical simulation to understand the characteristics of the waves. This paper produces a full guide to generate both regular and irregular waves numerically using ANSYS-CFX approach to solve the 2D Unsteady Reynolds Averaged Navier-Stokes Equation (URANS).

High Frequency Loading and Response of Offshore Structures in Steep Waves

2011 ◽  
Author(s):  
Thomas B. Johannessen
Keyword(s):  
High Frequency ◽  
Offshore Structures ◽  
Irregular Waves ◽  
Spectral Peak ◽  
Good Representation ◽  
Surface Zone ◽  
Resonant Response ◽  
Wave Impact ◽  
Steep Waves ◽  
Incident Waves

Offshore structures such as the TLP or the GBS have natural frequencies which are much higher than the frequencies of the incident waves in the survival conditions. Nevertheless, many offshore structures experience significant resonant response of modes with periods in the range of 2s to 5s, particularly in steep waves. In particular the ringing response of offshore structures characterised by sudden, large and isolated resonant response packets, has been a concern for many years. The loads which give rise to these events are difficult to describe both because they are small in magnitude relative to the load level close to the wave spectral peak and also because they are nonlinear in nature. In the present paper, available theoretical methods for high frequency loading is employed for irregular waves and compared with model tests. The methods which are used in the present are first and second order diffraction methods as well as a third order loading model for slender cylinders applied to irregular waves with continuous wave spectra. The results are compared with measurements of tether response and overturning moments on a TLP and a GBS respectively. Provided that the incident waves are treated carefully and care is taken in treating the high frequency tail of the incident wave, it is found that methods which are presently available give a good representation of the resonant response for the GBS structure. The GBS structure has a relatively low natural frequency and a mode shape which is excited easily by horizontal loading in the surface zone. In contrast, weakly nonlinear theory does not capture the high frequency loading on a TLP which has resonant frequencies at more than five times the spectral peak in the survival seastates. For this case it is found that wave impact with both the columns and the deck gives significant contributions to the resonant tether response. This is the case even if no significant horizontal deck impact is observed and highlights the need for a reliable deck impact load model.

Experimental Study of Air Gap Response and Wave Impact Forces of a Semi-Submersible Drilling Unit

2006 ◽  
Author(s):  
Saeid Kazemi ◽  
Atilla Incecik
Keyword(s):  
Experimental Study ◽  
Offshore Structures ◽  
Model Tests ◽  
Air Gap ◽  
Scale Model ◽  
Offshore Structure ◽  
The Caspian Sea ◽  
Wave Impact ◽  
Impact Forces ◽  
The Impact

An experimental study for predicting the air gap and potential deck impact of a floating offshore structure is the main topic of this research. Numerical modeling for air gap prediction is particularly complicated in the case of floating offshore structures because of their large volume, and the resulting effects of wave diffraction and radiation. Therefore, for new floating platforms, the model tests are often performed as part of their design process. This paper summarizes physical model tests conducted on a semi-submersible model, representing a 1-to-100 scale model of a GVA4000 class, “IRAN-ALBORZ”, the largest semi-submersible platform in the Caspian Sea, under construction in North of Iran, to evaluate the platform’s air gap at different locations of its deck and also measure the impact forces in case of having negative air gap. The model was tested in regular waves in the wave tank of Newcastle University. The paper discusses the experimental setup, test conditions, and the resulting measurements of the air gap and the wave impact forces by using eight wave probes and three load cells located at different points of the lower deck of the platform.

Non-Linear Air Gap Analyses of a Semi-Submersible Compared With Linear Analyses and Model Tests

2014 ◽  
Author(s):  
Jørgen Kvaleid ◽  
Volkert Oosterlaak ◽  
Tor Kvillum
Keyword(s):  
Simulation Model ◽  
Power Spectra ◽  
Model Tests ◽  
Air Gap ◽  
Irregular Waves ◽  
Wave Impact ◽  
Time Step ◽  
Extreme Response ◽  
Non Linear ◽  
Local Wave

For semi-submersible units, the magnitude of air gap or local wave impact in the survival condition is a key design driver. Linear analyses are widely used in the industry to predict survival air gap for semi-subs. Large relative motions, leading to large changes in shape of the submerged hull and large changes in water plane area make this approach questionable. In this paper, the GG5000 [1], a twin pontoon four legged semi-sub is considered. Both linear analyses and model tests had been performed, but the results were diverging. It was decided to investigate further, using non-linear hydrodynamic analyses. Initially, the model test setup is reproduced in the numerical model. The simulation model is verified for both response power spectra and extreme response distributions. In the non-linear simulations, the wetted surface of the hull is updated for each time step. Both excitation and restoring forces are based on the instantaneous wetted surface. This proves essential for the prediction of large motions. Later, the verified simulation model is run with realistic full scale setup including elastic catenary moorings with coupled cable dynamics, thruster assist, irregular waves and irregular wind. Highly non-linear effects proven to be vital to accurate air gap prediction are investigated and their representation in the non-linear analyses is validated against model tests.

Qualification Criteria and the Verification of Numerical Waves: Part 2: CFD-Based Numerical Wave Tank

2021 ◽  
Author(s):  
Benjamin Bouscasse ◽  
Andrea Califano ◽  
Young Myung Choi ◽  
Xu Haihua ◽  
Jang Whan Kim ◽  
...  
Keyword(s):  
Wave Spectrum ◽  
Vertical Direction ◽  
Offshore Structures ◽  
Irregular Waves ◽  
Numerical Wave Tank ◽  
Regular Waves ◽  
Wave Impact ◽  
Wave Tank ◽  
Modeling Practices ◽  
Numerical Wave

Abstract There is increasing interest in numerical wave simulations as a tool to design offshore structures, especially for the prediction of stochastic nonlinear wave loads like those related to air-gap and wave impact. Though the simulations cannot replace all experiments, they are now competitive on some topics such as the computations of wind and current coefficients. To proceed further it is necessary to improve the procedure to account for another complex environmental factor, wave motion. This paper addresses an industrial collaboration to develop modeling practices and qualification criteria of CFD-based numerical wave tank for offshore applications. As a part of the effort to develop reliable numerical wave modeling practices in the framework of the “Reproducible Offshore CFD JIP”, qualification criteria are formulated for the wave solutions generated from either potential-flow based codes in Part 1 of this work. Part 2 presents first a set of solutions for forcing the qualified waves obtained with the potential codes in the CFD domain. Those solutions follow a set of coupling protocols previously proposed in the JIP framework. Two potential codes and two CFD solvers are combined, so that four possible methods of generating waves and modalities are described. Two different potential models are considered, one using the higher order spectral method for numerical wave tank (HOS-NWT), and another using the finite-element method in the horizontal direction and a modal expansion after a sigma transform in the vertical direction (solver is called TPNWT). Both are equipped with a breaking model to generate extreme sea states. The two CFD solvers tested are Simcenter STAR-CCM+ and OpenFOAM. Simulation setups are proposed for both software. Simulation results from eight academic or industrial partners are presented for two sets of 2D test cases in deep water, one with regular waves and one with irregular waves, both with one very steep condition (ratio of wave height over wavelength of 10% for regular waves and 1000 year return period for Gulf of Mexico for irregular waves). The irregular waves are simulated for 10 sets of 3 hours to apply a stochastic approach to verify the quality of the waves generated in the numerical domain. Attention is given to the wave spectrum and the ensemble probability of the crest distribution, both obtained from the wave elevation at the center of the domain.

Theoretical and Experimental Analysis of Air Gap Response and Wave-on-Deck Impact of Floating Offshore Structures

2007 ◽  
Author(s):  
Saeid Kazemi ◽  
Atilla Incecik
Keyword(s):  
Experimental Analysis ◽  
Impact Force ◽  
Offshore Structures ◽  
Air Gap ◽  
Wave Impact ◽  
Impact Prediction ◽  
Impact Forces ◽  
Wave Elevation ◽  
Local Wave ◽  
Air Gap Responses

A comparative study between the theoretical and experimental analysis of air gap response and potential wave-on-deck impact forces of floating offshore structures is the main topic of this study. Both motion of the platform and the local wave elevation are important in air gap responses and wave impact forces. So, accurate and efficient computational analysis of wave induced loads and resulting platform’s responses and wave elevation is important in the prediction of air gap and evaluation of possible wave impact force. Numerical modelling for air gap and wave impact prediction is particularly complicated in the case of floating offshore structures because of their large volume, and the resulting effects of wave diffraction and radiation. Therefore, for new floating platforms, the model tests are often performed as part of their design process. The overall aim of this study is to introduce a simplified numerical method with sufficient accuracy suitable for preliminary design stages of a floating offshore platform to predict the air gap response using hybrid method and to evaluate the vertical wave impact force using Wagner-based method. The results obtained from the proposed method have been compared with those obtained from the experiments carried out in the wave tank of the Newcastle University.

Effect of Short-Crestedness on Extreme Wave Impact: A Summary of Findings From the Joint Industry Project “ShorTCresT”

2015 ◽  
Author(s):  
Janou Hennig ◽  
Jule Scharnke ◽  
Chris Swan ◽  
Øistein Hagen ◽  
Kevin Ewans ◽  
...  
Keyword(s):  
Wave Breaking ◽  
Good Description ◽  
Spectral Characteristics ◽  
Offshore Structures ◽  
Wave Crest ◽  
Extreme Waves ◽  
Extreme Wave ◽  
Wave Impact ◽  
Directional Model

Long-crested waves are typically used in the design of offshore structures. However, the corresponding statistics, kinematics and loading are significantly different in short-crested waves and up to date, there is no state-of-the-art methodology to apply short-crested models instead. The objective of the “ShortCresT” Joint Industry Project was to take into account short-crestedness in the design of offshore structures against extreme waves based on a good description of their spectral characteristics, statistics, kinematics, breaking and loading and to deliver (empirical) design recommendations and methods. This paper gives an overview of the findings of ShorTCresT regarding wave crest and height distributions, a comparison of basin and field data, the role of wave breaking, the most realistic directional model, hindcast models as well as the related platform loading.

scholarly journals Goal-driven active learning

2021 ◽  
Vol 35 (2) ◽  
Author(s):  
Nicolas Bougie ◽  
Ryutaro Ichise
Keyword(s):  
Decision Making ◽  
Reinforcement Learning ◽  
Learning Process ◽  
Real World ◽  
Imitation Learning ◽  
Learning Approaches ◽  
Wide Range ◽  
Fixed Set ◽  
Complex Decision Making ◽  
Complex Decision

AbstractDeep reinforcement learning methods have achieved significant successes in complex decision-making problems. In fact, they traditionally rely on well-designed extrinsic rewards, which limits their applicability to many real-world tasks where rewards are naturally sparse. While cloning behaviors provided by an expert is a promising approach to the exploration problem, learning from a fixed set of demonstrations may be impracticable due to lack of state coverage or distribution mismatch—when the learner’s goal deviates from the demonstrated behaviors. Besides, we are interested in learning how to reach a wide range of goals from the same set of demonstrations. In this work we propose a novel goal-conditioned method that leverages very small sets of goal-driven demonstrations to massively accelerate the learning process. Crucially, we introduce the concept of active goal-driven demonstrations to query the demonstrator only in hard-to-learn and uncertain regions of the state space. We further present a strategy for prioritizing sampling of goals where the disagreement between the expert and the policy is maximized. We evaluate our method on a variety of benchmark environments from the Mujoco domain. Experimental results show that our method outperforms prior imitation learning approaches in most of the tasks in terms of exploration efficiency and average scores.

scholarly journals Classification of unlabeled online media

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Sakthi Kumar Arul Prakash ◽  
Conrad Tucker
Keyword(s):  
Social Media ◽  
Real World ◽  
Graphical Model ◽  
Ground Truth ◽  
Classification Problem ◽  
Machine Learning Algorithms ◽  
Social Media Networks ◽  
Online Social Media ◽  
Wide Range

AbstractThis work investigates the ability to classify misinformation in online social media networks in a manner that avoids the need for ground truth labels. Rather than approach the classification problem as a task for humans or machine learning algorithms, this work leverages user–user and user–media (i.e.,media likes) interactions to infer the type of information (fake vs. authentic) being spread, without needing to know the actual details of the information itself. To study the inception and evolution of user–user and user–media interactions over time, we create an experimental platform that mimics the functionality of real-world social media networks. We develop a graphical model that considers the evolution of this network topology to model the uncertainty (entropy) propagation when fake and authentic media disseminates across the network. The creation of a real-world social media network enables a wide range of hypotheses to be tested pertaining to users, their interactions with other users, and with media content. The discovery that the entropy of user–user and user–media interactions approximate fake and authentic media likes, enables us to classify fake media in an unsupervised learning manner.

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