Publications
7 results found
Huo C, Swan C, Karmpadakis I, et al., 2023, An efficient method of defining the tail of a crest height distribution, Ocean Engineering, Vol: 289, ISSN: 0029-8018
The exceedance probability of wave crest elevation is a critical environmental input for the design/re-assessment of marine structures. With attention often focused on structural reliability, and in some cases survivability, the largest wave crests arising at the smallest exceedance probabilities, said to be located in the tail of a distribution, are of primary interest. This paper explains why present design practice may be non-conservative in the most extreme seas and outlines a new method by which the tail of the distribution can be defined using a relatively small number of deterministic wave events. This avoids the need to explore the entire distribution using very long (and expensive) random wave simulations. The new approach allows both an extension of the distribution to smaller exceedance probabilities and a concentration on the largest most design relevant crest heights. Having demonstrated the success of the proposed method by comparisons to laboratory data, the analysis is extended to include the effective prediction of the associated confidence intervals (CIs). With the highest waves subject to the largest statistical uncertainty, the paper explores the nonlinear changes in CI, demonstrates that these can also be accurately and efficiently defined, and explains how CI may be reduced. The focus of the paper lies in improved design calculations, based upon the nonlinear dynamics of extreme waves in realistic seas.
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
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.
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.
Pye J, Abbasi E, Arjomandi M, et al., 2019, Towards testing of a second-generation bladed receiver, 24th International Conference on Concentrating Solar Power and Chemical Energy Systems (SolarPACES), Publisher: American Institute of Physics, ISSN: 0094-243X
A bladed receiver design concept is presented which offers a >2% increase in overall receiver efficiency after considering spillage, reflection, emission and convection losses, based on an integrated optical-thermal model, for a design where the working fluid is conventional molten salt operating in the standard 290–565°C temperature range. A novel testing methodology is described, using air and water to test the receiver when molten salt facilities are not available. Technoeconomic analysis shows that the receiver could achieve a 4 AUD/MWhe saving in levelised cost of energy, but only if the bladed receiver design can be implemented at no additional cost.
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.