102 results found
Zve ES, Swan C, Hughes GO, 2023, Crest-height statistics in finite water depth. Part 1: The role of the nonlinear interactions in uni-directional seas, OCEAN ENGINEERING, Vol: 289, ISSN: 0029-8018
Ma L, Swan C, 2023, Wave-in-deck loads: an assessment of present design practice given recent improvements in the description of extreme waves and the nature of the applied loads, Ocean Engineering, Vol: 285, Pages: 1-19, ISSN: 0029-8018
This paper contributes to the on-going discussion of how best to calculate the reliability of a fixed offshores structure. This discussion has been driven, in large part, by improved physical understanding of waves arising in realistic design sea-states, with a growing appreciation that many ‘design wave events’ will be breaking, irrespective of water depth. As such, it is increasingly acknowledged that some aspects of present design practice are non-conservative. In re-assessing older structures, the accurate calculation of horizontal wave-in-deck (WID) loads is often the most important and least tractable part. This paper explains the underlying reason, highlights the wider implications for identifying an appropriate design point, and raises fundamental questions in the assumptions underpinning present practice. Specifically, a large laboratory data base of WID events is used to assess the success of available models. These comparisons confirm that recommended practice, including recent updates, consistently under-predict the maximum WID loads on which reliability calculations should be based. In contrast the recently developed Lagrangian Momentum Absorption (LMA) model (Ma and Swan, 2020b), a simple but complete load model that combines fully-nonlinear wave inputs and the openness/porosity of a structure, provides highly accurate predictions. This is achieved without empirical coefficients/calibrations and therefore ideally suited to design/re-assessment applications.
Ma L, Swan C, 2023, An experimental study of wave-in-deck loading and its dependence on the properties of the topside structure, Marine Structures, Vol: 88, Pages: 1-24, ISSN: 0951-8339
This paper concerns the largest and arguably the most threatening wave loading component experienced by a broad range of offshore structures. It arises when an incident wave crest exceeds the elevation of the underside of the deck structure, leading to direct wave-in-deck (WID) loading. The extent of this loading may be limited to the partial submergence of some of the lowermost deck beams, or could involve the large-scale inundation of the entire deck area. Either way, very large loads can arise which must be taken into account when assessing the reliability of the structure. In an earlier contribution Ma and Swan (2020) provided an extensive laboratory study exploring the variation of these loads with the properties of the incident wave. The present paper describes a second stage of this experimental study in which the variation of the WID loads with the properties of the topside structure is addressed. Specifically, it considers the porosity, position and orientation of the topside relative to the incident wave conditions, and seeks to explore both the variations in the maximum load and the loading time–history resulting from these changes.Given the highly transitory nature of a WID loading event, coupled with the fact that the problem is governed by flow conditions at, or very close to, the instantaneous water surface, the loading process is driven by an exchange of momentum from the wave crest to the topside structure. A recently developed WID load model, based on exactly these arguments (Ma and Swan 2020), is used alongside the laboratory data to provide a break-down of the load into its component parts. This provides an enhanced physical understanding of the resulting load time–history. The first part of the study is based upon an idealised generic topside structure, allowing a systematic variation in key parameters, particularly porosity. The second part addresses a realistic topside structure demonstrating the practical relevance of earlier work
Karmpadakis I, Swan C, 2022, A new crest height distribution for nonlinear and breaking waves in varying water depths, Ocean Engineering, Vol: 266, Pages: 1-17, ISSN: 0029-8018
The statistical distribution of zero-crossing crest heights represents a critical design input for a wide range of engineering applications. The present paper describes the development and validation of a new crest height model, suitable for application across a broad range of water depths. The purpose of this model is two-fold: first, to describe the amplifications of the largest crest heights arising due to nonlinear interactions beyond a second-order of wave steepness, and second, to incorporate the dissipative effects of wave breaking. Although these two effects act counter to each other, there is substantial evidence to suggest departures from existing models based upon weakly nonlinear second-order theory; the latter corresponding to current design practice. The proposed model has been developed on the basis of a significant collection of experimental results and a small subset of field measurements. It incorporates effects arising at different orders of nonlinearity as well as wave breaking in a compact formulation and covers a wide range of met-ocean conditions. Importantly, the new model has been independently validated against a very extensive database of experimental and field measurements. Taken together, these include effective water depths ranging from shallow water (
Ma L, Swan C, 2020, The effective prediction of wave-in-deck loads, Journal of Fluids and Structures, Vol: 95, ISSN: 0889-9746
The present paper concerns the extreme wave loads acting on an offshore structure; specifically the wave-in-deck loading component that arises when the height of an incident wave crest exceeds the elevation of the topside structure. In this case wave inundation occurs, the resulting loads on the topside structure represent a significant part of the total wave load. A new model for the effective prediction of this important loading component is presented. This is based upon the conservation of momentum, is formulated in a Lagrangian frame of reference, can incorporate any incident wave form, and takes due account of the porosity (or openness) of the topside structure. Comparisons between the model predictions and wide-ranging laboratory observations are shown to be in good agreement; the latter based upon deterministic focused wave events that are known to be representative of the largest waves arising in realistic sea-states. In addition, comparisons are also made with independent cfd calculations. Taken together, the proposed model is shown to accommodate changes in the spectral shape, the spectral peak period, the incident crest elevation (and hence the level of inundation), the directional spread of the incident waves, and the porosity of the topside structure. Importantly, this agreement applies to both non-breaking and breaking waves, involves no empirical calibration, and can be achieved with limited computational resources. As such, the model is ideally suited to design/re-assessment calculations in which the reliability of any offshore structure must be based upon a rigorous assessment of the long-term distribution of the total wave loads, including any wave-in-deck loading component.
Karmpadakis I, Swan C, 2020, On the average shape of the largest waves in finite water depths, Journal of Physical Oceanography, Vol: 50, Pages: 1023-1043, ISSN: 0022-3670
This paper investigates the average shape of the largest waves arising in finite water depths. Specifically, the largest waves recorded in time-histories of the water surface elevation at a single point have been examined. These are compared to commonly applied theories in engineering and oceanographic practice. To achieve this both field observations and a new set of laboratory measurements are considered. The latter concern long random simulations of directionally spread sea-states generated using realistic JONSWAP frequency spectra. It is shown that approximations related to the linear theory of Quasi-Determinism (QD) cannot describe some key characteristics of the largest waves. While second-order corrections to the QD predictions provide an improvement, key effects arising in very steep or shallow water sea-states are not captured. While studies involving idealised wave groups have demonstrated significant changes arising as a result of higher-order nonlinear wave-wave interactions, these have not been observed in random sea-states.The present paper addresses this discrepancy by decomposing random wave measurements into separate populations of breaking and non-breaking waves. The characteristics of average wave shapes in the two populations are examined and their key differences discussed. These explain the mismatch between findings in earlier random and deterministic wave studies.
Karmpadakis I, Swan C, Christou M, 2020, Assessment of wave height distributions using an extensive field database, Coastal Engineering, Vol: 157, Pages: 1-15, ISSN: 0378-3839
The present paper investigates the short-term statistical distribution of wave heights. Specifically, some of the most commonly applied wave height distributions are assessed using field measurements. The latter comprise of wave radar observations from 10 different locations in the North Sea and cover water depths between 7.7 m and 45 m. In total, the field database includes more than 200 million waves, making it one of the largest of its kind in this water depth regime. In using these data, the accuracy of existing wave height distributions has been examined and guidance is provided concerning the best performing models and their domain of applicability. Additionally, insights concerning the influence of key met-ocean parameters are also provided. Taken together, the results in this paper present an overview of the statistical behaviour of wave heights in finite water depths, as observed in the field.
Ma L, Swan C, 2020, An experimental study of wave-in-deck loading and its dependence on the properties of the incident waves, Journal of Fluids and Structures, Vol: 92, Pages: 1-21, ISSN: 0889-9746
Recent advances in the description of extreme ocean waves have led to the definition of more severe design conditions. These changes include increases in the sea-state severity for a given return period, the nonlinear amplification of crest elevations beyond second-order and, perhaps most importantly, the occurrence of wave breaking in both intermediate and deeper waters. These developments raise important questions as to whether present design practice, commonly based upon simplified regular wave theories, provides a realistic estimate of the maximum design loads on fixed offshore structures. This is especially relevant if the applied wave load involves the loss of an effective air-gap and, the occurrence of wave-in-deck (wid) loading; the latter believed to be the most common cause of failure in severe seas.To address these issues, an extensive laboratory study of wid loading has been undertaken. This paper presents the first part of the findings from this study; the aim being to provide an improved physical understanding of wid loading in a wide range of incident wave conditions. The study shows that the applied loads are critically dependent upon both the wave shape and the water particle kinematics arising at the highest elevations within the wave crest; both properties being strongly influenced by the occurrence of wave breaking, particularly wave over-turning. Indeed, the occurrence of wave breaking leads to markedly different load time-histories with important consequences for both the maximum applied load and the onset of a dynamic excitation. The results presented herein provide important guidance as to the effective modelling of these critical loading events.
Karmpadakis I, Swan C, Christou M, 2019, Laboratory investigation of crest height statistics in intermediate water depths, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 475, Pages: 1-24, ISSN: 1364-5021
This paper concerns the statistical distribution of the crest heights associated with surface waves in intermediate water depths. The results of a new laboratory study are presented in which data generated in different experimental facilities are used to establish departures from commonly applied statistical distributions. Specifically, the effects of varying sea-state steepness, effective water depth and directional spread are investigated. Following an extensive validation of the experimental data, including direct comparisons to available field data, it is shown that the nonlinear amplification of crest heights above second-order theory observed in steep deep water sea states is equally appropriate to intermediate water depths. These nonlinear amplifications increase with the sea-state steepness and reduce with the directional spread. While the latter effect is undoubtedly important, the present data confirm that significant amplifications above second order (5–10%) are observed for realistic directional spreads. This is consistent with available field data. With further increases in the sea-state steepness, the dissipative effects of wave breaking act to reduce these nonlinear amplifications. While the competing mechanisms of nonlinear amplification and wave breaking are relevant to a full range of water depths, the relative importance of wave breaking increases as the effective water depth reduces.
Latheef M, Swan C, Spinneken J, 2017, A laboratory study of nonlinear changes in the directionality of extreme seas, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 473, ISSN: 1364-5021
This paper concerns the description of surface water waves, specifically nonlinear changes in the directionality. Supporting calculations are provided to establish the best method of directional wave generation, the preferred method of directional analysis and the inputs on which such a method should be based. These calculations show that a random directional method, in which the phasing, amplitude and direction of propagation of individual wave components are chosen randomly, has benefits in achieving the required ergodicity. In terms of analysis procedures, the extended maximum entropy principle, with inputs based upon vector quantities, produces the best description of directionality. With laboratory data describing the water surface elevation and the two horizontal velocity components at a single point, several steep sea states are considered. The results confirm that, as the steepness of a sea state increases, the overall directionality of the sea state reduces. More importantly, it is also shown that the largest waves become less spread or more unidirectional than the sea state as a whole. This provides an important link to earlier descriptions of deterministic wave groups produced by frequency focusing, helps to explain recent field observations and has important practical implications for the design of marine structures and vessels.
Rodriguez M, Spinneken J, Swan C, 2016, Nonlinear loading of a two-dimensional heaving box, Journal of Fluids and Structures, Vol: 60, Pages: 80-96, ISSN: 1095-8622
A numerical investigation is presented addressing the nonlinear heave response of a rectangular box. The work specifically concerns the importance of the relative body dimensions, expressed through the product of the half-beam b and the wavenumber k . When subjected to moderately steep incident waves, the second-harmonic content of the heave motion is found to be as large as 25% of the first-harmonic content. In considering the extent of this second-harmonic motion, three regimes may be defined: (i) the long wave regime, where kb≤0.4, (ii) the intermediate regime, where 0.4<kb<1.0 and (iii) the diffraction or short wave regime, where kb≥1.0. Expressed in terms of the wavelength λ=2π/k, these regimes correspond to (i) b≤0.06λ, (ii) 0.06λ<b<0.16λ and (iii) b≥0.16λ. The second-harmonic motion content is found to be particularly pronounced in regimes (i) and (iii). Perhaps surprisingly, this second-harmonic content is also found to be practically non-existent for some intermediate cases lying within regime (ii). Three sources of nonlinearity are shown to be particularly important. First, the interaction between the first-order incident waves and the first-order scattered waves is key to the nonlinear loading in regime (iii). Second, the generation of freely propagating second-harmonic radiated waves due to the body motion is important in (i). Third, the local standing wave field associated with the radiation problem is found to contribute to the loading in regime (iii). In addition, the location of the body resonance also plays a critical role in defining the extent of the second-harmonic motion content. The focus of the present work lies on a clear physical interpretation of the sources of these nonlinear loads, coupled with an analysis of the body dynamics.
Peric M, Swan C, 2015, An experimental study of the wave excitation in the gap between two closely spaced bodies, with implications for LNG offloading, APPLIED OCEAN RESEARCH, Vol: 51, Pages: 320-330, ISSN: 0141-1187
Swan C, Sheikh R, 2015, The interaction between steep waves and a surface-piercing column, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 373, ISSN: 1364-503X
Experimental observations are presented of a single surface-piercing column subject to a wide range of surface gravity waves. With the column diameter, D, chosen such that the flow lies within the drag-inertia regime, two types of high-frequency wave scattering are identified. The first is driven by the run-up and wash-down on the surface of the column in the vicinity of the upstream and downstream stagnation points. The second concerns the circulation of fluid around the column, leading to the scattering of a pair of non-concentric wavefronts. The phasing of the wave cycle at which this second mode evolves is dependent upon the time taken for fluid to move around the column. This introduces an additional time-scale, explaining why existing diffraction solutions, based upon a harmonic analysis of the incident waves, cannot describe this scattered component. The interaction between the scattered waves and the next (steep) incident wave can produce a large amplification of the scattered waves, particularly the second type. Evidence is provided to show that these interactions can produce highly localized free-surface effects, including vertical jetting, with important implications for the setting of deck elevations, the occurrence of wave slamming and the development of large run-up velocities.
Hennig J, Scharnke J, Swan C, et al., 2015, EFFECT OF SHORT-CRESTEDNESS ON EXTREME WAVE IMPACT - A SUMMARY OF FINDINGS FROM THE JOINT INDUSTRY PROJECT "SHORTCREST", 34th ASME International Conference on Ocean, Offshore and Arctic Engineering (OMAE2015), Publisher: AMER SOC MECHANICAL ENGINEERS
Spinneken J, Christou M, Swan C, 2014, Force-controlled absorption in a fully-nonlinear numerical wave tank, Journal of Computational Physics, Vol: 272, Pages: 127-148, ISSN: 0021-9991
An active control methodology for the absorption of water waves in a numerical wave tank is introduced. This methodology is based upon a force-feedback technique which has previously been shown to be very effective in physical wave tanks. Unlike other methods, an a-priori knowledge of the wave conditions in the tank is not required; the absorption controller being designed to automatically respond to a wide range of wave conditions. In comparison to numerical sponge layers, effective wave absorption is achieved on the boundary, thereby minimising the spatial extent of the numerical wave tank. In contrast to the imposition of radiation conditions, the scheme is inherently capable of absorbing irregular waves. Most importantly, simultaneous generation and absorption can be achieved. This is an important advance when considering inclusion of reflective bodies within the numerical wave tank.In designing the absorption controller, an infinite impulse response filter is adopted, thereby eliminating the problem of non-causality in the controller optimisation. Two alternative controllers are considered, both implemented in a fully-nonlinear wave tank based on a multiple-flux boundary element scheme. To simplify the problem under consideration, the present analysis is limited to water waves propagating in a two-dimensional domain.The paper presents an extensive numerical validation which demonstrates the success of the method for a wide range of wave conditions including regular, focused and random waves. The numerical investigation also highlights some of the limitations of the method, particularly in simultaneously generating and absorbing large amplitude or highly-nonlinear waves. The findings of the present numerical study are directly applicable to related fields where optimum absorption is sought; these include physical wavemaking, wave power absorption and a wide range of numerical wave tank schemes.
Haley JF, Swan C, Gibson R, 2014, AN EXPERIMENTAL INVESTIGATION OF WAVE IMPACT LOADS ON A SLENDER HORIZONTAL CYLINDER, 33rd ASME International Conference on Ocean, Offshore and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS
Latheef M, Swan C, 2013, A laboratory study of wave crest statistics and the role of directional spreading, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 469, ISSN: 1364-5021
Katsardi V, de Lutio L, Swan C, 2013, An experimental study of large waves in intermediate and shallow water depths. Part I: Wave height and crest height statistics, COASTAL ENGINEERING, Vol: 73, Pages: 43-57, ISSN: 0378-3839
Spinneken J, Swan C, 2012, The operation of a 3D wave basin in force control, Ocean Engineering, Vol: 55, Pages: 88-100
The present study concerns the generation and absorption of directional waves in a facility where the generation and absorption mechanism is based on force-feedback control. Many laboratories worldwide are now equipped with such technology, and the world’s largest wave basin (the David Taylor Basin at the Naval Surface Warfare Centre, Carderock, USA) is currently being refurbished utilising force-driven wavemakers. Traditionally, the wave generation in such facilities was based on an empirical transfer function. In contrast, an entirely analytic approach is presented herein. A theoretical transfer function, enabling fully deterministic wave generation in force-controlled waves basins, is derived for the first time. The theory is applicable to both flap- and piston-type wave machines. Even though the present study focuses on flap-type geometries, the results are readily adoptedto the piston case. To demonstrate the successful application of the novel transfer function, a substantial experimental study is presented. As part of the experimental work, a direct comparison to an empirical transfer function is made and the benefits of either approach are discussed. Overall, very good agreement betweenthe expected wave field and the experimental data is shown. However, some departures remain, particularly for highly directional waves and broad-banded spectra.
Bateman WJD, Katsardi V, Swan C, 2012, Extreme ocean waves. Part I. The practical application of fully nonlinear wave modelling, APPLIED OCEAN RESEARCH, Vol: 34, Pages: 209-224, ISSN: 0141-1187
Spinneken J, Swan C, 2011, Theoretical Transfer Function for Force-controlled Wave Machines, INTERNATIONAL JOURNAL OF OFFSHORE AND POLAR ENGINEERING, Vol: 21, Pages: 169-177, ISSN: 1053-5381
Katsardi V, Swan C, 2011, The evolution of large non-breaking waves in intermediate and shallow water. I. Numerical calculations of uni-directional seas, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 467, Pages: 778-805, ISSN: 1364-5021
Katsardi V, Swan C, 2011, AN EXPERIMENTAL STUDY OF SHALLOW WATER WAVE STATISTICS ON MILD BED SLOPES, 30th International Conference on Ocean, Offshore and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 711-719
Latheef M, Swan C, 2011, WAVE STATISTICS IN NONLINEAR SEA STATES, 30th International Conference on Ocean, Offshore and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 761-769
Spinneken J, Swan C, 2010, Theoretical Transfer Function of Force-controlled Wave Machines, 20th International Offshore and Polar Engineering Conference, Pages: 409-417
Roos J, Swan C, Haver S, 2010, WAVE IMPACTS ON THE COLUMN OF A GRAVITY BASED STRUCTURE, 29th ASME International Conference on Ocean, Offshore and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 365-373
Spinneken J, Swan C, 2009, Wave generation and absorption using force-controlled wave machines, Pages: 397-405, ISSN: 1098-6189
Recent research on wave generation utilizing absorbing force-controlled machines has shown three key factors: (i) a theoretical transfer function between demand signal and surface elevation can be derived; (ii) high absorption efficiency can be achieved over a wide range of frequencies and (iii) little second-order spurious content is introduced when driven with a first-order demand signal. Thus far, both theoretical and experimental work has been limited to the generation of regular wave trains and hinged wave board geometries. The present study extends this work to the generation of irregular waves considering both flap-and piston-type wave machines. Whilst limiting the discussion to the effects arising at first-order, specifically addressing the theoretical transfer function and absorption efficiency, an enhanced understanding of the machine's controller is sought. The implemented absorption strategy is based on controlling the complex relationship between applied force and wave board velocity. The properties of the optimum a-causal controller for this particular strategy are investigated and a causal, hence practical, approximation is derived. A causal controller based on infinite impulse response filters (IIR) and direct optimization in the frequency domain is considered. The relative merits of this approach are compared to methods based on an approximation in the time domain. Experimental evidence is also provided to substantiate the IIR modelling approach. The results are directly relevant to the operation of many installed force-controlled wave machines, provide guidance as to the effective operation of others, contribute to the wave power debate and could be incorporated within advanced numerical wave tanks to provide simultaneous generation and absorption. Copyright © 2009 by The International Society of Offshore and Polar Engineers (ISOPE).
Hague CH, Swan C, 2009, A multiple flux boundary element method applied to the description of surface water waves, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 228, Pages: 5111-5128, ISSN: 0021-9991
Christou M, Hague CH, Swan C, 2009, The reflection of nonlinear irregular surface water waves, ENGINEERING ANALYSIS WITH BOUNDARY ELEMENTS, Vol: 33, Pages: 644-653, ISSN: 0955-7997
Roos J, Swan C, Haver S, et al., 2009, AN EXPERIMENTAL INVESTIGATION OF WAVE IMPACTS ON THE DECK OF A GRAVITY BASED STRUCTURE, 28th International Conference on Ocean, Offshore and Arctic Engineering (OMAE), Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 405-414
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