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.

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.

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.

An Evaluation of Wave Impact Indicators

2009 ◽  
Author(s):  
Zhigang Tian
Keyword(s):  
Wave Breaking ◽  
Offshore Structures ◽  
Phase Method ◽  
Impact Loads ◽  
Wave Steepness ◽  
Wave Load ◽  
Wave Impact ◽  
Large Wave ◽  
Impact Indicators ◽  
Local Wave

Wave impact on offshore structures has been the focus of several studies, due to its significant effect on offshore operations. We evaluate several parameters (wave impact indicators) which can be adopted to indicate the possibility of wave impact on offshore structures due to extreme waves. The indicators can be estimated quickly with given sea states, and thus may provide useful information to offshore structure designers at early design phases. Definitions of three wave impact indicators are presented and discussed. The first indicator, Ψ, is proposed by Stansberg (2008). The second one considered is a wave breaking parameter, μ, originally presented by Song and Banner (2002) in their construction of a wave breaking criterion. Finally, we propose a more generalized impact indicator, βn. The subscript n indicates its dependence on local wave steepness. Our study demonstrates that the three indicators are analytically related. To evaluate these indicators numerically, 2nd order random surface waves are generated with random phase method and Two-Dimensional Fast Fourier Transform (2D FFT). Hilbert analysis of the wave signal reveals that all indicators are able to identify steep and energetic waves that may potentially cause large wave impact loads. Further numerical study demonstrates that the quantitative correlation of wave impact loads to μ is less promising than that to Ψ and βn; while βn provides the best relationship to both local wave impact load and global wave load with its dependence on local wave steepness adjusted (i.e. adjusting n). The correlation is independent of sea states. Estimations and recommendations for thresholds of the two impact indicators (i.e. Ψ and βn with n = 1) are made based on model test results. With proper estimation of the thresholds, both indicators can be applied to predict wave impact and wave impact probability in given sea states.

Investigation and Prediction of Wave Impact Loads on Ship Appendage Shapes

2007 ◽  
Author(s):  
Anne M. Fullerton ◽  
Thomas C. Fu ◽  
David E. Hess
Keyword(s):  
Wave Height ◽  
Impact Force ◽  
Design Guidelines ◽  
Impact Loads ◽  
Wave Steepness ◽  
Wave Impact ◽  
Ship Structures ◽  
Impact Forces ◽  
Hull Structure ◽  
Horizontal Wave

Navy fleet problems with damage to hatches and other appendages after operation in high sea states suggest that wave impact loads may be greater than the current design guidelines of 1000 pounds per square foot (48 kilopascal) (Ship Specification Section 100, General Requirements for Hull Structure and Guidance Manual for Temporary Alterations, NAVSEA S9070-AA-MME-010/SSN, SSBN). These large impact forces not only cause damage to ships and ship structures, they can also endanger the ship’s crew. To design robust marine structures, accurate estimates of all encountered loads are necessary, including the wave impact forces, which are complex and involve wave breaking, making them difficult to estimate numerically. An experiment to investigate wave impact loads was performed at the Naval Surface Warfare Center, Carderock Division in 2005. During this experiment, the horizontal and vertical loads of regular, non-breaking waves on a 12 inch (0.305 m) square plate and a 19.75 inch (0.5 m) diameter horizontal cylinder were measured while varying incident wave height, wavelength, wave steepness, plate angle and immersion level of the plate and cylinder. Wave heights of up to 1.5 feet (0.46 m) were tested, with wavelenghs of up to 30 feet (9.1 m). In all cases, the horizontal wave impact force increased with wave steepness. For some angles, the horizontal wave impact force increased with greater submergence. A feed-forward neural network (FFNN) developed by Applied Simulation Technologies was used to predict the horizontal forces measured during the experiment based on the values of wave height, wavelength, wave steepness, plate angle and immersion level of the plate and cyclinder. A FFNN is a computational method used to develop nonlinear equation systems that use input variables to predict output variables. Predictions of forces from the FFNN compare well with the experimental data, and may be useful in future design of ships and ship structures.

Transducer Qualification for Wave Impact Load Measurements Using Wedge Drop Tests

2015 ◽  
Author(s):  
Zhenjia (Jerry) Huang ◽  
Robert Oberlies ◽  
Don Spencer ◽  
Jang Kim
Keyword(s):  
Impact Load ◽  
Research Work ◽  
Offshore Structures ◽  
Model Tests ◽  
Impact Loads ◽  
Dynamic Impact ◽  
Wave Impact ◽  
Impact Forces ◽  
Industry Practice ◽  
Drop Tests

For the design of offshore structures in harsh wave environments, it is essential to accurately determine the wave impact loads on the structure. To date, robust numerical prediction methods / algorithms for determining wave impact forces on offshore structures do not exist. Model testing continues to be the industry practice for determining wave impact forces on offshore structures. Accurate measurements of wave impact loads in model tests have been challenging for several decades. Transducers require the ability to capture the short duration, dynamic nature and high magnitude of impact loads. In order to qualify transducers for these types of measurements, we need to develop a way to physically impose dynamic impact loads on the transducers and to establish benchmark values that can be used to check the effectiveness of their measurements. In this paper, we present our recent research work on transducer qualification for wave impact load measurements, including their development, numerical analysis and wedge drop model tests. Our findings show that wedge drop tests can be used to impose dynamic impact loads for transducer qualification, and that the Wagner solution and / or validated computational fluid dynamics (CFD) simulations that include the effects of viscosity, compressibility and hydroelasticity can provide the appropriate benchmarking values. Numerical simulation results, model test measurements and findings on transducer qualification are presented and discussed in the paper.

Experimental Evaluation of Wave Impact Loads on Semi-Submersible Structure According to Trim Angle

2019 ◽  
Author(s):  
Min-Guk Seo ◽  
Yoon-Jin Ha ◽  
Nam-Woo Kim ◽  
Bo Woo Nam ◽  
Kang-Su Lee
Keyword(s):  
Experimental Test ◽  
Experimental Evaluation ◽  
Impact Force ◽  
Impact Load ◽  
Impact Loads ◽  
Test Model ◽  
Wave Impact ◽  
Wave Flume ◽  
Wave Elevation ◽  
Series Of Experiments

Abstract This study considers the wave impact loads on the semi-submersible structure. To evaluate wave impact loads on the semi-submersible structure, a series of experiments are conducted in a 2D wave flume. In the experimental test, the semi-submersible half model is used, and 11 uniaxial force sensors are installed in deck side, column side, and deck bottom. Wave probes are, also, attached in the test model to measure the relative wave elevation. To generate horizontal and bottom wave impact on the test model, focusing wave is applied. The test model is fixed without any motion during each test, while the trim angle of the test model is changed to examine the effect of trim angle on wave impact load. Through this, the characteristics of the wave impact force at each position were investigated.

Numerical Simulation and Analysis of Phase-Focused Breaking and Non-Breaking Wave Impact on a Fixed Offshore Platform Deck

2020 ◽  
Vol 142 (5) ◽  
Author(s):  
Rameeza Moideen ◽  
Manasa Ranjan Behera ◽  
Arun Kamath ◽  
Hans Bihs
Keyword(s):  
Open Source Software ◽  
Impact Force ◽  
Offshore Structures ◽  
Breaking Wave ◽  
Wave Impact ◽  
Large Wave ◽  
Air Water Interface ◽  
Wave Heights ◽  
Fixed Offshore Platform ◽  
Vertical Impact

Abstract Extreme wave impact due to tsunamis and storm surge create large wave heights that cause destruction to coastal and offshore structures. Focused waves have been used to represent such extreme waves, and in the present study, its impact on offshore deck has been studied numerically. Numerical modeling has been carried out using open-source software reef3d, with the level set method to capture the air–water interface. Focused waves are generated by phase focusing a group of waves at a particular position and time. The nonlinearity of focused waves and its effect on the vertical impact force has been quantified for different airgaps and increasing wave heights. The wave steepness was increased to initiate phase-focused breaking in the numerical wave tank, which was then validated with the experimental results. This breaking-focused wave impact on offshore deck is then studied at different breaking locations. The results for different positionings of the deck with respect to breaker location show that the maximum horizontal impact force on the deck occurs when the plunging crest hits the deck side.

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.

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