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.

Resonant Vibrations of Riser Guide Tubes Due to Wave Impact

2004 ◽  
Author(s):  
Arne Nestega˚rd ◽  
Arve Johan Kalleklev ◽  
Kjell Hagatun ◽  
Yu Lin Wu ◽  
Sverre Haver ◽  
...  
Keyword(s):  
High Frequency ◽  
Rapid Change ◽  
Ocean Basin ◽  
Model Tests ◽  
Slender Body ◽  
Irregular Waves ◽  
Resonant Vibrations ◽  
Wave Impact ◽  
Numerical Predictions ◽  
Steep Waves

The Kristin platform is a catenary moored semi-submersible production vessel (SSPV) intended for production of gas at the Kristin field at Haltenbanken. Kristin has 24 riser guide tubes for tie in of flexible risers, umbilicals and electric cables to the riser balcony. The riser guide tubes (RGT) provide the necessary guiding, support and protection for risers and cables. The guide tubes run vertically from the deck and through the extended east pontoon. The guide tubes are welded to the pontoon and horizontally supported at the underside of the balcony deck. During model tests of the Kristin platform performed in the Ocean Basin laboratory at Marintek, high frequency in-line vibrations of the RGTs were observed during passage of steep waves. The resonance period for the individual RGTs is 0.3 sec. To mitigate the vibration problem, a vibration suppression arrangement of stiff rods was introduced between the guide tubes. Model tests were performed with respect to extreme- and fatigue loads in regular and irregular waves, with and without the suppression arrangement. The model included the floating framework representing the hull and the 24 RGTs with correct diameter and resonance period. The model was suspended in a horizontal mooring system, giving resonance periods in surge and sway close to the prototype platform. A load-response model for the interaction between large steep waves and vertical flexible cylinders has been developed. A slender body load model derived from Morison’s equation is shown to be able to excite the resonant vibrations. The dominant part of the loading comes from the rapid change of added mass momentum, giving rise to an additional slamming term in the load formulation. The structural response is calculated from a recognized non-linear slender body response program. Numerical simulations have been carried out and compared with model tests for both regular and irregular waves. The numerical predictions confirm the effect observed in the model tests; i.e. connecting the tubes generally leads to a reduction of the high frequency response amplitudes.

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.

The Interaction Between Steep Waves and a Vertical, Surface-Piercing Column

2005 ◽  
Vol 127 (1) ◽  
pp. 31-38 ◽  
Author(s):  
Rizwan Sheikh ◽  
Chris Swan
Keyword(s):  
High Frequency ◽  
Impact Damage ◽  
Wave Interaction ◽  
Scattered Wave ◽  
Short Wave ◽  
Vertical Surface ◽  
Wave Impact ◽  
Single Column ◽  
High Frequency Waves ◽  
Incident Waves

This paper describes new laboratory observations concerning the interaction between a series of steep incident waves and a vertical, surface-piercing, column. The motivation for the study arose as a result of wave impact damage sustained to the undersides of several concrete gravity-based structures in the northern North Sea. Earlier work, [Swan et al. Appl. Ocean. Res. 19, pp. 309–327 (1997)], demonstrated that in the case of multiple column structures, the individual diameters of which lie outside the typical (linear) diffraction regime, there exists a new and previously unexpected mechanism leading to the scattering of high-frequency waves. Although the implications of this effect was carefully documented, not least because it explained the occurrence of wave impacts in relatively moderate seas, its physical origins remained unclear. In particular, it was uncertain whether this type of scattering would be observed in the case of a single column, or whether it results from the transmission of wave modes trapped between the legs of a multiple column structure. In the case of a single column, if the diameter, D, is such that the flow lies within the drag-inertia regime, D/λ<0.2, where λ is the corresponding wavelength, linear diffraction theory suggests there will be little or no scattered wave energy. The present laboratory observations demonstrate that this is not, in fact, the case. If the incident waves are steep, a strong and apparently localized interaction is clearly observed at the water surface. This, in turn, leads to the scattering of high-frequency waves. Although these waves are relatively small in amplitude, their subsequent interaction with other steep incident waves takes the form of a classic long-wave short-wave interaction and can produce a significant increase in the maximum crest elevation relative to those recorded in the absence of the structure. The present paper will demonstrate that the scattering of these high-frequency waves, and their subsequent nonlinear interaction with other incident waves, has significant implications for the specification of an effective air-gap and hence for the setting of deck elevations.

The Interaction Between Steep Waves and a Vertical, Surface-Piercing Column

2003 ◽  
Author(s):  
Rizwan Sheikh ◽  
Chris Swan
Keyword(s):  
High Frequency ◽  
Impact Damage ◽  
Wave Interaction ◽  
Scattered Wave ◽  
Short Wave ◽  
Vertical Surface ◽  
Wave Impact ◽  
Single Column ◽  
High Frequency Waves ◽  
Incident Waves

The paper describes new laboratory observations concerning the interaction between a series of steep incident waves and a vertical, surface-piercing, column. The motivation for the study arose as a result of wave impact damage sustained to the undersides of several concrete gravity-based structures in the northern North Sea. Earlier work, Swan et al. [1], demonstrated that in the case of multiple column structures, the individual diameters of which lie outside the typical (linear) diffraction regime, there exists a new and previously unexpected mechanism leading to the scattering of high-frequency waves. Although the implications of this effect was carefully documented, not least because it explained the occurrence of wave impacts in relatively moderate seas, its physical origins remained unclear. In particular, it was uncertain whether this type of scattering would be observed in the case of a single column, or whether it results from the transmission of wave modes trapped between the legs of a multiple column structure. In the case of a single column, if the diameter, D, is such that the flow lies within the drag-inertia regime, D/λ &lt; 0.2, where λ is the corresponding wavelength, linear diffraction theory suggests there will be little or no scattered wave energy. The present laboratory observations demonstrate that this is not, in fact, the case. If the incident waves are steep, a strong and apparently localised interaction is clearly observed at the water surface. This, in turn, leads to the scattering of high-frequency waves. Although these waves are relatively small in amplitude, their subsequent interaction with other steep incident waves takes the form of a classic long-wave short-wave interaction and can produce a significant increase in the maximum crest elevation relative to those recorded in the absence of the structure. The present paper will demonstrate that the scattering of these high-frequency waves, and their subsequent nonlinear interaction with other incident waves, has significant implications for the specification of an effective air-gap and hence for the setting of deck elevations.

A Numerical Investigation of Steep Irregular Wave Properties With a Mixed-Eulerian Lagrangian HOS Method

2020 ◽  
Author(s):  
Sébastien Fouques ◽  
Csaba Pákozdi
Keyword(s):  
Computational Cost ◽  
Perturbation Expansion ◽  
Nonlinear Effects ◽  
Irregular Waves ◽  
Extreme Waves ◽  
Wave Impact ◽  
Irregular Wave ◽  
Highly Nonlinear ◽  
Steep Waves ◽  
Steep Wave

Abstract The design of structures at sea requires knowledge on how large and steep waves can be. Although extreme waves are very rare, their consequences in terms of structural loads, such as wave impact or ringing, are critical. However, modelling the physical properties of steep waves along with their probability of occurrence in given sea states has remained a challenge. On the one hand, standard linear and weakly nonlinear wave theories are computationally efficient, but since they assume that the steepness parameter is small, they are unable to capture extreme waves. On the other hand, recent simulation methods based on CFD or fully nonlinear potential solvers are able to capture the physics of steep waves up to the onset on breaking, but their large computational cost makes it difficult to investigate rare events. Between these two extremes, the High-Order Spectral (HOS) method, which solves surface equations, is both efficient and able to capture highly nonlinear effects. It may then represent a good compromise for long simulations of steep waves. Unfortunately, it is based on a perturbation expansion where the small parameter is the wave steepness, and consequently, simulations tend to become unstable when steep wave events occur. In this work, we investigate the properties of irregular waves simulated with a modified HOS method, in which the sea surface is described with a Lagrangian representation, i.e. by computing the position and the velocity potential of individual surface particles. By doing so, nonlinear properties of the surface elevation are simply captured by the modulation of the horizontal and vertical particle motion. The same steep wave is then described more linearly with a Lagrangian representation, which reduces the instabilities of the HOS method. The paper focuses on bi-chromatic waves and irregular waves simulated from a JONSWAP spectrum. We compare simulations performed with the standard HOS and the modified Lagrangian methods for various HOS-orders.

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.

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.

An Assessment of Load and Response for Horizontal Slamming Loads From Model Scale Experiments

2018 ◽  
Author(s):  
Martin Storheim ◽  
Gunnar Lian
Keyword(s):  
Structural Response ◽  
Breaking Waves ◽  
Offshore Structures ◽  
Load Level ◽  
Material Behavior ◽  
Wave Impact ◽  
Large Variability ◽  
Slamming Load ◽  
Reasonable Approach ◽  
Wave Slamming

Steep breaking waves can result in high impact loads on offshore structures, and several model test campaigns have been conducted to assess the effect of horizontal wave slamming. High loads have been measured, and they can be challenging to withstand without significant deformation. For wave slamming problems it is common to estimate the characteristic slamming load and assume that this will give an equivalent characteristic response. One challenge related to the slamming load is that it has a large variability in load level, the duration of the load and the shape of the overall load pulse. This variability can have a large impact on the estimated response to the characteristic load, causing a similar or larger variability in response. Due to the sensitivity to the structural response, it may be difficult to interpret large amounts of such data to arrive at a relevant design load without making overly conservative assumptions. This paper investigates the sensitivity of the structural response to assumptions made in the material modelling and how the short term variability is affected if we instead of load use response indicators such as plastic strain and max deformation to arrive at a characteristic load. For this purpose, a simplified dynamic response model is created, and the recorded wave impact events can then be evaluated based on the predicted structural response from the simplified model. It was found that the structural response is sensitive to the structural configuration. The assumed material behavior and hydro-elastoplastic effects were identified to greatly affect the structural response. A reasonable approach to arrive at the q-annual response seems to be to first estimate the q-annual extreme slamming load, and then run the structural analysis on several of the measured slamming time series with the estimated q-annual extreme pressure.

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