Laboratory experiments have been carried out in a piston-driven wave flume to investigate the solitary wave impact on three types of vertical cylindrical objects, including the vertical cylinder, cone and bell-shaped lighthouse structure. First, the quality of the generated waves is examined by comparing their profiles with the theoretical solutions. The range of piston movements that generates good-quality solitary waves is determined. The scattering of solitary waves around geometrically different vertical structures is then focused on. The wave heights around the structures are measured in detail, from which the wave impact force is derived. The results regarding vertical cylinders agree well with the past research outcomes. It shows that the vertical cone structure experiences the largest impact force. Although the bellshaped lighthouse structure incurs slightly larger impact force than the vertical cylinder, it has a much larger and thus more stable base to resist the overturning moment. The findings confirm the good mechanical performance of some of the old lighthouse structures, which have existed for around 200 years.
A solitary wave is described by Goring (1978) as a single “hump” of water that is entirely above still water level with an infinite wavelength. Solitary waves were first identified by Russell (1845) and have been the subject of considerable research ever since. Theoretical and experimental work by Hammack and Segur (1974) showed that from any water displacement above still water level at least one solitary wave shall emerge followed by a number of dispersive waves.
The Boxing Day tsunami of 2004 will go down in history as being one of the worst natural disasters ever recorded as it claimed the lives of more than 150,000 people, left millions homeless and caused billions of dollars’ worth of damage (Briggs et al. 2005). Although the Boxing Day tsunami shall be remembered for the devastating amount of damage it caused, it is important that it is not classed as being a “freak event of nature”. The tsunami itself was fairly typical of other tsunamis that have occurred in the past and are likely to strike again as shown recently by the tsunami that struck the north-east coast of Japan on the 11th March 2011. Solitary waves are not only used to model tsunamis but are also used to model large storm surges that commonly occur as a result of hurricanes.
Hodge, Caitlin Worden (Industrial Doctoral Centre for Offshore Renewable Energy (IDCORE) & Zyba) | Bateman, Will (Industrial Doctoral Centre for Offshore Renewable Energy (IDCORE) & Zyba) | Yuan, Zhiming (University of Exeter) | Thies, Philipp R. (University of Strathclyde) | Bruce, Tom (University of Edinburgh)
The mechanical motion of a wave energy converter (WEC) is converted by the power take-off (PTO) system into electricity, but these two systems are not independent, as they have been treated in WEC modelling. Treating them as such leads to inaccuracies in prediction of power output and reliability, and can erode confidence in numerical modelling tools. This paper presents a methodology for the two-way coupling of high fidelity modelling of WEC hydrodynamics with a more accurate representation of the PTO and investigates the impact of using simplified PTO models. Simplified models represent the full PTO with a single parameter which is in itself difficult to choose and may require a number of iterations. These different methods were used to assess the behaviour of the CCell WEC in a regular wave, with the calculations for mean power varying considerably in different wave conditions and the range of motion consistently under predicted by the simpler models. The coupled model increased the computational requirement for the simulation, however it provided the developer a better understanding of the impact on and utilisation of different hydraulic components.
WECs are designed to convert the energy from waves into a mechanical motion which is then converted into electrical power through the power take-off (PTO) system. Oscillating wave surge converters, such as the CCell device, Fig. 1, have generally evolved as buoyant bottom-hinged flap WECs, which pitch back and forth from sea to shore under the influence of the horizontal motion of waves (Cameron, L, 2010). This pitching motion is transformed into useful energy usually through a hydraulic piston, which draws on the robustness and high power density that hydraulic circuits offer. Similar systems have also commonly been used in heaving buoys (Cargo, C, 2012).
Numerical modelling has become a valuable toolbox for WEC developers as it allows rapid modifications to a WEC design, without the additional manufacture and testing costs, or scaling issues. It can build up a picture of the Mean Annual Energy Production (MAEP) and the load estimates on the device which can aid design decisions and inform the required O&M procedures. However, numerical simulations can be slow to compute without adequate computer power and some simplifications must be introduced for efficiency, especially regarding the modelling of the power take-off system and/or the hydrodynamics.
Ranasinghe, D. P. L. (Lanka Hydraulic Institute Ltd.) | Kumara, I. G. I. K. (Lanka Hydraulic Institute Ltd.) | Caldera, H. P. G. M. (Lanka Hydraulic Institute Ltd.) | Engiliyage, N. L. (Lanka Hydraulic Institute Ltd.)
Directional wave gauges are used to estimate the directional spreading of the generated wave energy spectrum in 3D basin despite the high cost involved in. In this study, five non-directional wave gauges array has been used to capture the directional distribution at desired locations to omit the reflection component in uni-directional waves (175°N and 195°N). The directional wave measurements have been carried out at the nearshore during the calibration to assess wave reflection from the boundaries if any. During the testing, directional gauge system was placed at the offshore to assess the incident wave climate. The spectral analysis has been performed to identify the incident wave component by selecting a narrow direction band of 120°N −240°N out of the entire direction band of 0°N-360°N. The results depicted that 3% of the observed wave heights at −20 m depth contains the reflected waves from the breakwater structure at −14 m depth. Further, this method has been successfully used to generate the required wave condition at the offshore depth (-20 m depth) with the minor deviation of ±3%. Additionally, the results confirmed the appropriateness of the model setup for the basin extent as the generated reflection from the boundary walls was negligible.
Unlike in inland developments, design and construction of coastal structures is still a challenging task due to the complexity and variability of coastal environment. As the behaviour of a coastal zone is completely different from one to another, site specific parameters need to be found prior to the design of coastal structures. Therefore, numerical models play a vital role in obtaining the design parameters. Furthermore, empirical formulae to estimate hydraulic stability and overtopping are useful for the preliminary design of coastal structures. However, physical models are necessary to assess the hydraulic performance of a coastal structure as it reproduces the actual phenomenon in a small scale without any schematization. Apart from that, physical models are the key tools in optimizing the breakwater structures and the layouts which could be utilized for reducing the construction cost by considerable amount. However, there are some disadvantages associated with physical model testing that are related to experimenting in facilities of limited spatial extent which require that phenomena are scaled down from the natural system and which can, by their design, produce laboratory effects not present in nature (IAHR, 2011).
This paper describes a novel approach to early kick detection. A cyber-physical approach is utilized to improve the speed and consistency at which a kick can be identified during drilling operations, thus automating the kick detection process. The proposed methodology combines physics-based modelling with Bayesian mathematics for detecting subtle changes in noisy and uncertain measurements. For early kick detection purposes, the physics model is that of a lumped parameter model that describes the fluid flow in a well and the noisy and uncertain measurements are the data streams from rig sensors available during well construction operations, which are used to fit/correct the results of the model via an extended Kalman filter approach.
A tool was built that is able to consume real-time and archived data, solve the lumped parameter model and Kalman filter implementation, and output the computed results for real-time viewing purposes. Historical datasets were used to demonstrate that the tool is able to detect kicks earlier than conventional methods, while providing an acceptable low false alarm rate. A real-time trial was conducted within a real-time monitoring center to evaluate the performance of the tool as an effective early kick detection technology via a set of key performance indicators and test the developed operational protocols and training materials with monitoring center personnel. The metrics put in place evaluated the tool in three categories: software robustness, data quality and early kick detection technical performance. A test plan was developed with test procedures and a workflow for utilizing the information given by the tool in the context of a real-time monitoring center. No kicks occurred during the construction of the well. However, certain operational events that resemble a kick demonstrated that the tool is able to identify them and trigger an alarm. The overall results of the real-time trial were favorable and potential enhancements to the tool were identified.
The numerical wave flume using first order wavemaker theory has been well established and widely used for a long time. But the existing numerical models based on the first order wave-maker theory will lose accuracy as the nonlinear effects enhance. Because of the different propagation velocities of the spurious harmonic waves and the primary waves, the simulated waves with the first order wave-maker theory have an unstable wave profile. In this paper, a numerical wave flume with piston-type wave maker is established. The comparison of the surface elevation using first order and second order wave-maker theory proves that second order wave-maker theory can make stable wave profile in both temporally and spatially. Harmonic analysis is applied to prove the superiority of second order wave-maker theory.
Piston-type wave-maker has been widely used to generate waves in laboratory flume or basin. Havelock (1929), Svendsen (1985) and Dean & Dalrymple (1991) had well established first order wave-maker theory. Flick & Guza (1980) and Ursell et al. (1960) had verified the first order wave-maker theory by experiments in the laoratory (see also Galvin, 1964; Keating & Webber, 1977). Small-amplitude assumption is the basic assumption of first order wave-maker theory. The small-amplitude waves will decompose into a primary wave and spurious superharmonic wave, which will affect the stability of the wave profile (see Gōda and Kikuya, 1964; Multer and Galvin, 1967; Iwagaki and Sakai, 1970), when the motion of the wave-maker is sinusoidal. In early 1847, Stokes found the superharmonic wave by regular wave in terms of a perturbation series using the wave steepness as the small ordering parameter. But the problem of generated nonlinear wave was gave a solution by Fontanet (1961). He found the spurious superharmonic wave by piston-type wave-maker with sinusoidal motion in Lagrangian coordinates and suggested that it can be restrained using wave paddle control signal with an addition component.
Further, Moubayed & Williams (1994) extended second order wave-maker theory from the regular wave to the bichromatic wave. For the irregular waves, second order wave-maker theory has both sum and difference frequencies in the interaction terms. Longuet-Higgins & Stewart (1962, 1964) deduced the subharmonics generated by wave components interaction under the narrow band assumption. Flick & Guza (1980) pointed out that spurious long wave will be generated by a first order bichromatic control signal. Barthel et al. (1983) used the second order difference frequencies wave paddle control signal to restrain spurious long wave and expended the theory to the flap-type wave-maker. Schaffer (1996) derived second order wave-maker theory including sum frequencies and difference frequencies components without the narrow band assumption. The theory was applied to the piston-type and flap-type wave-makers and was verified by experiments. Schaffer & Steenberg (2003) extended the second order wave-maker theory to multidirectional waves.
Wang, Dongxu (Ocean and civil engineering Dalian Ocean University) | Gui, Jinsong (Ocean and civil engineering Dalian Ocean University) | Sun, Jiawen (Environmental Monitoring Center of the State Oceanic Administration) | Ma, Zhe (Dalian University of Technology)
A new solver based on OpenFOAM is utilized to simulate the process of three-dimensional regular wave impact on a horizontal plate. The wave is generated by a numerical piston wave-maker and absorbed by means of active absorption, which is applied to both the inlet and outlet boundary. The result is contrasted with a similar physical model test. It comes out that the calculated pressure generally matches well with the result of model test, which proves this new solver is correct. And the slamming, splash and overtopping of the wave have been simulated commendably. The result also indicates that this new solver could be used to study the problem of wave-structure interactions.
OpenFOAM (Open Field Operation And Manipulation) is a free and open source finite volume CFD toolbox originally developed at the Imperial College. It comprises of a bundle of libraries and codes to solve complex problems. This library is written in C++ and is object oriented. Its modular structure is an advantage to program new solvers, boundary conditions or applications, allowing not to digging deeply in the source code. For the coastal and offshore engineering, there are two famous solvers named Wave2Foam (2011) and OLAFoam (2015). Both of them are built on the two incompressible phase solver named interFoam and also interDyMFoam in OpenFOAM. For interDyMFoam, ”DyM” is the abbreviation of ”dynamic mesh” which indicates that this solver can call the dynamic mesh in OpenFOAM.
Wave2Foam is released earlier than OLAFoam, and so it owns more users, but the dynamic mesh is not involved in it. Obviously, it cannot simulate the piston wave-maker which is widely used in the laboratory. OLAfoam is first released in 2013 with the name of IHFoam, and its name has changed to OLAFoam in the year of 2016. It could call the dynamic mesh module in OpenFOAM to simulate the piston wave-maker, but it needs lots of discrete data to make the paddle move, which is not convenient, especially when the simulated time is very long. So a more convenient solver based on the dynamic mesh is needed for simulating the piston wave-maker in the flume and tank.
Computational Fluid Dynamics (CFD) could be an inexpensive complement and even an alternative to physical modelling for investigating the interaction of ocean waves with offshore structures. CFD models however cannot be relied on unless they are well validated. We validated the OpenFOAM® CFD toolbox, a publically available open-source model, for modelling the interaction of extreme regular and irregular waves with offshore gravity-based structures. CFD results including water levels, pressures and forces generally compared well with results for a physical model test program previously conducted by the National Research Council Canada (NRC).
For optimal and safe design of offshore structures subjected to waves, accurate estimation of forces and water levels are needed. This estimation is usually obtained either by physical or numerical modelling approaches. Each approach has pros and cons: physical modelling usually involves smaller-than-reality model structures where the wave-structure interaction processes are subject to scale effects, and the study outputs are uncertain as a result. Physical modelling requires test facilities, model fabrication and instrumentation and hence can be expensive. Physical modelling however reveals many details of wave-structure interaction. Numerical modelling on the other hand is a relatively inexpensive approach without the model size limitation. If numerical models are based on correct physics and well validated, they are a reliable complement and perhaps even an alternative to physical modelling.
The literature on the validation of Computational Fluid Dynamics (CFD) models for studying wave-structure interaction is fast expanding and not fully developed. This is particularly true for the application of publically available open-source CFD models. Some of the most relevant recent literature is (Ong et al., 2017; Hu et al., 2016; Palomares, 2015; Chen et al., 2014; Paulsen et al., 2014; Palemón-Arcos et al., 2014; Lambert, 2012; Thanyamanta et al., 2011; Afshar, 2010).
We have previously examined (Babaei et al., 2016) the applicability of the OpenFOAM CFD Toolbox, freely available and open source, for estimating the interaction of extreme regular waves with a four-column offshore gravity-based structure. Therein, it was shown that OpenFOAM results are very similar to results of an equivalent physical model test program conducted previously at the National Research Council Canada (NRC). A summary of the physical model test program is given in the next section.
The design and construction of waterborne craft using "scientific" methods is a relatively recent development in the context of the whole history of humankind, and is by no means universally applied even today. Many traditional craft in current service still rely on the process akin to Darwin's natural selection concept. And the evolutionary process continues from Madagascar outrigger fishing canoes to Bangkok water taxis with "long-tail" propulsion systems, and from Haitian fishing boats with high performance new sails built from poly-tarp material to whaling umiaks in NW Alaska covered with tensioned membrane skins made from walrus hide and equipped with outboard motors. There can be value in studying the design, construction and operational approaches of these craft, which can lead to insights for the modern naval architect. Lessons such as optimizing weight/strength ratios, minimizing resistance, utilizing materials in clever ways, developing repairable structures etc., can all be learned from the study of indigenous craft. The indigenous peoples living above of the tree line on the North American continent exist in an environment where much of what is required to survive and thrive comes from the sea. This paper will specifically describe the development of skin-covered craft for use to support of the lifestyles of the peoples of Arctic and Sub-Arctic North America in Alaska, Canada and Greenland.
In recent years, laboratory testing has become an important part of marine and offshore activities. In this field, there is a growing demand of better understanding of the wave and current interaction and its impact on offshore structures. Complementary to the physical testing, numerical simulations play an equally important role for fine-tuning of the experimental set-ups as well as further detailed investigations. However, the complexity of wave current interactions poses great challenges, not only on laboratory experiments but also on numerical models. For instance, reflected wave and current are unwanted but unavoidable problems in laboratory experiments due to the physical enclose of laboratory-scale basins. Nevertheless, these problems are often overcome in numerical models by using artificial relaxation zones or open boundary conditions which could allow almost ideal boundary conditions. That might, however, lead to discrepancies in experimental and numerical results, especially in prolonged experiments. Aiming to address some of these challenges, the present work focuses on simulations of wave and current in actual laboratory basins taking into account of main physical features. The numerical model would represent the experimental set-up as closely as possible, including wave generation by paddle motions and wave absorption by beach. Simulations of wave and current interactions validated with experimental data will be presented.
Offshore structures such as floating platforms often have to operate in harsh conditions in the oceans. Rigorous but costly laboratory tests are required to ensure the safety of the structures under extreme operational conditions. Laboratory testing of structures under combined wave and current conditions could be carried out in sophisticated ocean basin facility such as MARIN Offshore Basin in The Netherlands (Buchner et al., 1999). The new basin facility, FloWave, in the University of Edinburgh (Ingram et al., 2014) even allows combinations of waves and currents in any direction. Parallel to the advancement of laboratory experiments, numerical simulations are becoming popular with the rapid development of supercomputers and numerical methodologies. Numerical simulations are much cheaper but able to produce full range of data in the studied domain at full scale which is impossible in laboratory experiments. Numerical simulations are, however, subject to many sources of uncertainties due to model simplification and necessary assumptions. Combinations of laboratory experiments and numerical simulations could make use the strengths while complementing the weaknesses of the individual approach. To complement a physical one, the numerical model would reflect closely the real phenomena in the physical basin, including the effects of moving paddles to the wave field, reflection on wave absorbers. A numerical wave-current basin using artificial relaxation zones or open boundary conditions could allow almost ideal boundary conditions to be generated. That might, however, lead to discrepancies in experimental and numerical results, especially in prolonged experiments. We have, therefore, chosen to explicitly model the wave generators as moving solid walls to replicate the physical movement of the wave paddles in laboratory basins. The approach was derived in Grilli (1997) for a two-dimensional potential flow numerical wave tank. By modeling physical movement of wave paddles active absorption, such as those used in FloWave, could be implemented. In addition, physical passive wave absorbers are also explicitly modeled to provide closest wave reflection conditions in the physical basins. The current generation systems are also modeled according to the setup in physical basin to model the flow field accurately.
Runup of a train of three successive solitary waves on a mild slope is investigated experimentally. The modified Goring’s method and the third order solution of solitary wave are used to generate two or three solitary waves by a piston type wave maker of long stroke. It is first reported that the runup amplification coefficients of the following solitary waves are smaller than that of the leading solitary wave for the triple solitary waves while the runup amplification coefficient of the third wave is greater than that of the second wave. The mechanism of the variations of the runup amplification coefficients of all waves in the successive solitary wave train is that the down rush flows of the leading wave causes breaking of the following wave during runup.
Runup of tsunami on a beach is an important subject to evaluate inundation risk on coastal regions. Due to moving waterline on a beach and breaking of waves, it has been one of most challenging problems in nonlinear water wave dynamics associated with coastal engineering. Much work has been done on theoretical analysis, numerical simulation and experimental measurement on runup of a single solitary wave on beaches. Synolakis (1987) developed an analytical solution for nonbreaking waves on a plane beach based on the nonlinear shallow water equation. Li & Raichlen (2001) proposed a nonlinear solution to the classical shallow water equation by using a hodograph transformation and reported an experimental study on runup process of nonbreaking and breaking solitary wave. Fuhrman and Madsen (2008) proposed the reduced surf similarity parameter for solitary waves, the beach slope divided by the offshore wave height to depth ratio, which provides good coherency with experimental breaking and runup data and analytical nonbreaking runup expressions. The full nonlinear and highly dispersive Boussinesq equations were used by Zhao et al.(2012) to investigate the evolution and run-up of solitary waves and N-waves on plane beaches Zhao et al.(2013) carried out numerical simulation of tsunami waves propagating on the continental shelf with an extremely gentle slope. The numerical results show that the N-shape tsunami wave could evolve into a train of periodic waves, undular bores or solitons when it propagates from deep water to shallow water over a long plane beach of gentle slope. Firstly, experiments on runup of two successive solitary waves with same amplitude on a plane beach were carried out by Lo et al. (2013) and Pujala et al. (2015). Xuan et al. (2013) implemented the generation and runp of two successive solitary waves with different wave amplitudes in a wave flume. Even though much work has been done about the overtaking or head-on collision of two solitary waves theoretically, particularly at the multi-solitary wave solution of the KdV equation, there is little work on runup of three or more solitary waves on a beach.