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.

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.

Wave Impact Loads on the Bottom of Flat Decks

2021 ◽  
Author(s):  
Daniel de Oliveira Costa ◽  
Julia Araújo Perim ◽  
Bruno Guedes Camargo ◽  
Joel Sena Sales Junior ◽  
Antonio Carlos Fernandes ◽  
...  
Keyword(s):  
Scale Effects ◽  
Offshore Structures ◽  
Model Tests ◽  
Analytical Models ◽  
Navier Stokes ◽  
Impact Loads ◽  
Wave Impact ◽  
Wave Channel ◽  
Waves And Currents ◽  
The Impact

Abstract Slamming events due to wave impact on the underside of decks might lead to severe and potentially harmful local and/or global loads in offshore structures. The strong nonlinearities during the impact require a robust method for accessing the loads and hinder the use of analytical models. The use of computation fluid dynamics (CFD) is an interesting alternative to estimate the impact loads, but validation through experimental data is still essential. The present work focuses on a flat-bottomed model fixed over the mean free surface level submitted to regular incoming waves. The proposal is to reproduce previous studies through CFD and model tests in a different reduced scale to provide extra validation and to identify possible non-potential scale effects such as air compressibility. Numerical simulations are performed in both experiments’ scales. The numerical analysis is performed with a marine dedicated flow solver, FINE™/Marine from NUMECA, which features an unsteady Reynolds-averaged Navier-Stokes (URANS) solver and a finite volume method to build spatial discretization. The multiphase flow is represented through the Volume of Fluid (VOF) method for incompressible and nonmiscible fluids. The new model tests were performed at the wave channel of the Laboratory of Waves and Currents (LOC/COPPE – UFRJ), at the Federal University of Rio de Janeiro.

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.

Wave Impact Experiment of a GBS Model in Large Waves

2017 ◽  
Author(s):  
Zhenjia (Jerry) Huang ◽  
Don Spencer ◽  
Robert Oberlies ◽  
Gracie Watts ◽  
Wenting Xiao
Keyword(s):  
Structural Model ◽  
Probability Distributions ◽  
Offshore Structures ◽  
Model Tests ◽  
Wave Crest ◽  
Maximum Pressure ◽  
Impact Loads ◽  
Wave Impact ◽  
Wave Basin ◽  
The Impact

For the design of offshore structures in harsh wave environments, model testing continues to be the recommended industry practice for determining wave impact forces on offshore structures. Accurate measurements of wave impacts in model tests have been a challenge for several decades. Transducers are required to accurately capture the short duration, high magnitude, and dynamic nature of impact loads. The structural model, transducers, and the transducer mountings need to be designed such that mechanical vibrations in the integrated transducer-mounting-structural model system do not contaminate the wave impact measurements. In this work, the dynamic oscillations in the measurements were controlled through the design and fabrication of transducers, their mounting and the GBS model. Wave crest probability distributions were developed that included fully nonlinear effects. These distributions were used as a benchmark to qualify the waves in the wave calibration tests. The highly stochastic nature of impact loads makes it challenging to obtain converged probability distributions of the maximum impact loads (i.e. forces or pressures) from model tests. To increase the confidence in the statistical values of wave impact loads, a large number of realizations were used for a given sea state. Variability of the maximum pressure due to wave basin effects (such as wait-time between tests) was examined with fifteen repeat tests using the same wave maker control signal. These tests provided insights into the random behavior of the impact loads.

scholarly journals Structural Integrity of Fixed Offshore Platforms by Incorporating Wave-in-Deck

2021 ◽  
Vol 9 (9) ◽  
pp. 1027
Author(s):  
Nurul Uyun Azman ◽  
Mohd Khairi Abu Husain ◽  
Noor Irza Mohd Zaki ◽  
Ezanizam Mat Soom ◽  
Nurul Azizah Mukhlas ◽  
...  
Keyword(s):  
Structural Integrity ◽  
Limit State ◽  
Pushover Analysis ◽  
Offshore Structures ◽  
State Equation ◽  
Air Gap ◽  
Wave Crest ◽  
Offshore Platforms ◽  
Offshore Structure ◽  
The Impact

The structural integrity of offshore platforms is affected by degradation issues such as subsidence. Subsidence involves large settlement areas, and it is one of the phenomena that may be experienced by offshore platforms throughout their lives. Compaction of the reservoir is caused by pressure reduction, which results in vertical movement of soils from the reservoir to the mud line. The impact of subsidence on platforms will lead to a gradually reduced wave crest to deck air gap (insufficient air gap) and cause wave-in-deck. The wave-in-deck load can cause significant damage to deck structures, and it may cause the collapse of the entire platform. This study aims to investigate the impact of wave-in-deck load on structure response for fixed offshore structure. The conventional run of pushover analysis only considers the 100-year design crest height for the ultimate collapse. The wave height at collapse is calculated using a limit state equation for the probabilistic model that may give a different result. It is crucial to ensure that the reserve strength ratio (RSR) is not overly estimated, hence giving a false impression of the value. This study is performed to quantify the wave-in-deck load effects based on the revised RSR. As part of the analysis, the Ultimate Strength for Offshore Structures (USFOS) software and wave-in-deck calculation recommended by the International Organization for Standardization (ISO) as practised in the industry is adopted to complete the study. As expected, the new revised RSR with the inclusion of wave-in-deck load is lower and, hence, increases the probability of failure (POF) of the platform. The accuracy and effectiveness of this method will assist the industry, especially operators, for decision making and, more specifically, in outlining the action items as part of their business risk management.

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.

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 ON THE IMPORTANCE OF CONSIDERING MULTIPLE SEASTATE REALIZATIONS WHEN ESTIMATING DESIGN LOADS IN MULTI-DIRECTIONAL SHALLOW-WATER WAVES

2020 ◽  
pp. 46
Author(s):  
Andrew Cornett ◽  
Scott Baker
Keyword(s):  
Shallow Water ◽  
Water Waves ◽  
Offshore Structures ◽  
Scale Model ◽  
Offshore Structure ◽  
Wave Condition ◽  
Directional Waves ◽  
Wave Heights ◽  
Design Loads ◽  
Peak Pressures

The objectives of this work are to close some of the knowledge gaps facing designers tasked with designing new offshore structures or upgrading older structures located in shallow waters and exposed to energetic multi-directional waves generated by passing hurricanes or cyclones. This will be accomplished by first investigating and characterizing the natural variability of the maximum wave heights and crest elevations found in multiple 2-hour long realizations of several short-crested shallow-water near-breaking seastates. Following this, the variability and repeatability of peak pressures and peak loads exerted on a 1/35 scale model of a gravity-based offshore structure are explored. The analysis focuses on establishing extreme value distributions for each realization, quantifying their variability, and exploring how the variability is diminished when results from multiple seastate realizations and repeated tests are combined. The importance of considering multiple realizations of a design wave condition when estimating peak values for use in design is investigated and highlighted.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/16bCsMd0OMc

Practical Structural Assessment of Offshore Structures for Wave Slap

2012 ◽  
Author(s):  
Joong Soo Moon ◽  
Tae Hyun Park ◽  
Woo Seung Sim ◽  
Hyun Soo Shin
Keyword(s):  
Static Pressure ◽  
Impact Load ◽  
Offshore Structures ◽  
Model Tests ◽  
Design Stage ◽  
Load Prediction ◽  
Structural Assessment ◽  
Pressure Impulse ◽  
The Impact ◽  
Design Pressure

By the combination of theoretical and empirical approach, the methodology for practical structural assessment of offshore structures for wave slap is proposed. It is developed for engineers in the sense that the precise design pressure is easily obtainable and quickly applicable in early and detail design stage. For impact load prediction, the Pressure-Impulse theory that was well developed and validated in coastal engineering field is applied. The impact pressures are classified into three types (traditional, sharp, and immersed slap) according to model tests and BP Schiehallion FPSO’s bow monitoring. The time histories of impact pressures for the classified impact types are generated with the pressure impulse predicted by the Pressure-Impulse theory. Nonlinear transient structural analyses are performed using the time series of impact pressures to obtain equivalent static pressure factors. Finally, the design pressure is determined by multiplying the maximum peak pressure by the equivalent static pressure factor. The results are validated through the comparison with model tests and dedicated reports.

Numerical Study of Phase Change Influence on Wave Impact Loads in LNG Tanks on Floating Structures

2018 ◽  
Author(s):  
Matthieu Ancellin ◽  
Laurent Brosset ◽  
Jean-Michel Ghidaglia
Keyword(s):  
Natural Gas ◽  
Phase Change ◽  
Numerical Study ◽  
Model Tests ◽  
Impact Loads ◽  
Wave Impact ◽  
Floating Structures ◽  
Physical Phenomena ◽  
The Impact ◽  
Phase Phase

Understanding the physics of sloshing wave impacts is necessary for the improvement of sloshing assessment methodology based on sloshing model tests, for LNG membrane tanks on floating structures. The phase change between natural gas and liquefied natural gas is one of the physical phenomena involved during a LNG wave impact but is not taken into account during sloshing model tests. In this paper, some recent numerical and analytical works on the influence of phase change are summarized and discussed. For the impact of an ideally shaped wave, phase change influences two different steps of the impact in different ways: during the gas escape phase, phase change leads to a higher impact velocity; for entrapped gas pockets, phase change causes a reduction of the pressure in the gas pocket. However, this influence is quantitatively small. The generalization to more realistic wave shapes (including e.g. liquid aeration) should be the focus of future works.

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