Wave–seabed–structure interaction has become one of the main concerns of coastal engineers and researchers, as it may largely affect seabed instability and structure safety. In this study, a series of experiments has been carried out in a wave flume to investigate wave motion and wave-induced pore pressure around a submerged impermeable breakwater as well as in conditions without a breakwater. Pore pressure within the sandy bed and water surface elevation are measured simultaneously in the experiments, which are used to investigate the effects of wave height, wave period, and breakwater structure on the wave–seabed–structure interactions at different water depths. Comparison of the experimental results of two different constraint conditions (around a submerged impermeable breakwater as well as in conditions without a breakwater) shows that the interaction of water waves and a submerged breakwater causes a significant change of wave motion and wave-induced pore pressure within the sandy bed. Meanwhile, it is found that the values of pore pressure along both the wave propagation direction and vertical seabed depth are largely dependent on water depth.
With the rapid development of coastal management strategies in the last few decades, offshore structures such as breakwaters are commonly constructed to protect the coastal environment. Meanwhile, offshore breakwaters are becoming common features of deepwater ports. Like other marine structures, the stability of submerged impermeable breakwaters may be largely affected by the strong interaction with the waves and seabed. Damage to offshore structures includes two general failure modes, namely the structural failure caused by wave forces acting on the structure and foundation failure in the vicinity of the structure (Jeng et al., 2004). Owing to seepage flow and pore pressures within the sandy bed induced by progressive water waves, many breakwaters have reportedly been damaged in past years (Smith and Gordon, 1983; Lundgren et al., 1989).
Lee, Young-Hak (Chungnam National University) | Park, Sung-Yong (Ministry of Public Safety and Security, Ulsan, Korea) | Tokida, Ken-Ichi (Public Works Research Center, Tokyo) | Lee, Dal-Won (Chungnam National University)
Recently, the deterioration of reservoir embankments has progressed severely; thus, a more efficient design and construction method for deteriorated reservoir structures is required. For a fill-type reservoir embankment, a method that employs intervals between the horizontal filter and the embankment has been used and shown to achieve an efficient design. Since the horizontal filter is an important structure that provides stable drainage during an earthquake or concentrated leak, it is necessary to examine any change in the seepage characteristics depending on the filter intervals. In this study, the seepage characteristics of a reservoir embankment were examined according to the filter interval range via 3-D finite element analysis; additionally, potential risks such as those from piping were reviewed relative to the design interval of structurally stable filters. Consequently, results showed that the maximum filter intervals to yield efficient seepage characteristics were within 0–20.5 m for the pore water pressure of the core and the height of the seepage line. The piping evaluation results, as based on the range of the critical hydraulic gradient (Icr : 0.8–1.4), demonstrated that the piping maintained stability within the filter interval range of 0–20.5 m; applying this range, stability was maintained when filters were installed throughout the embankment. However, when the filter interval exceeded 20.5 m, the piping was observed to fluctuate between stable and unstable conditions according to the range of the critical hydraulic gradient. Thus, to ensure structural stability, this study has proposed a decision method for the interval of horizontal filters based on the results of 3-D seepage analysis and the theoretical range of critical hydraulic gradient.
Recently, because of the effects of climate change, concerns for functional degradation are rising for soil reservoirs vulnerable to seepage; moreover, the risk of disaster is expected to increase because risk factors for the collapse of embankment still exist. At present, 99.3% of all agricultural dams and reservoirs in South Korea are fill dams with the cross-sectional horizontal filter zone designed downstream of the embankment according to the relevant specifications (MAFRA, 2002). The interval of horizontal filters is designed to range between 10 and 40 m, and these filters regulate water drainage from the collected water inside the embankment.
Hong, Sa Young (Korea Research Institute of Ships and Ocean Engineering) | Nam, Bo Woo (Korea Research Institute of Ships and Ocean Engineering) | Ha, Yoon-Jin (Korea Research Institute of Ships and Ocean Engineering) | Hong, Seok Won (Korea Research Institute of Ships and Ocean Engineering)
A fully coupled analysis is carried out to evaluate the offshore mating operation for a large heavy topside module on a deck of floating liquefied natural gas (FLNG). Fully coupled hydrodynamic interactions are considered between a crane vessel lifting the heavy topside module and the FLNG. Multiple crane wires and slings are modeled with linear springs, while the leg mating unit (LMU) guide is modeled with bilinear springs. Special attention is paid to the accurate prediction of coupled hydrodynamic coefficients and pendulum dynamics of heavy hanging objects. Simulation results are validated by comparing model test results on key parameters affecting the deck mating operations on-site. Good agreement has been obtained between the simulations and the model tests.
Because of the increasing need of an environmentally friendly fuel supply, liquefied natural gas (LNG) has become one of the emerging resources to replace oil as well as renewable energies such as solar, offshore wind, etc. The recent drastic fall of oil prices requires a variety of innovations for more economic solutions and earlier first oil. In spite of state-of-the-art exploration technology, it is not uncommon to find oil and gas fields that have much larger or smaller reserves than first estimated. Especially in the case of a gas field of much larger reserves, it is necessary to accommodate additional large topside modules on-site. In such a case, more effective and safer installation scenarios should be prepared. The assurance of safety and operability of installation of the large topside module on-site should be checked under combined wind, wave, and current conditions because the large topside module can cause a serious hazard on the existing floating LNG (FLNG) in case of poor installation scenario and operation.
For the performance evaluation of floating crane operations, the time-domain dynamic analysis has been widely used in the design stage to predict the motion response and determine the capacity of the installation equipment and the weather windows. Clauss et al. (2000) presented a comparative study of the operation capabilities of floating cranes. They also reported the nonlinear phenomena of the coupled system of floating structures and swinging load. Ellermann et al. (2002) discussed the nonlinear dynamics of floating cranes. Cha et al. (2010) applied multibody system dynamics to study the dynamic response simulation of heavy cargo suspended by a floating crane. Similarly, Park et al. (2011) presented the dynamic factor analysis based on multibody dynamic simulations for a floating crane and cargo considering an elastic boom. Nam et al. (2015) developed a time-domain analysis program for floating crane vessel systems. They investigated the effect of a heave compensator during a lowering operation of subsea equipment.
We summarize the studies of two dynamical problems arising from harbor engineering. The first is on the long-period oscillations in a harbor forced by short and random incident waves. The second is on the unexpected oscillations of the mobile storm barrier designed for the Venice lagoon. In both problems nonlinearity plays a crucial role in the mathematical analysis.
PART I. LONG-PERIOD HARBOR OSCILLATIONS EXCITED BY RANDOM WIND WAVES
Harbors are designed to protect ships from storm-induced sea waves. Long-period oscillations inside a harbor are hazardous to ship operations in general and to loading and unloading of cargos in particular. They can cause excessive straining and breakage of mooring lines as well as collision of ships against piers, etc. During strong oscillations, the swift current at the entrance hammers the passing of small vessels. Early studies of harbor oscillations were focussed on the linear mechanism of synchronous resonance, which is appropriate for treating the effects of long-period tsunamis (Miles and Munk, 1961; Lee, 1971; Chen and Mei, 1974). However, many harbors in the world are troubled more often by the short incident waves generated by storms. An example is the case of Barbers Point Harbor in Hawaii (Okihiro, 1993). The wave gauge at an inside station recorded a sharp peak at the frequency of f = 0.001 Hz corresponding to the wave period of T = 100 s, which is ten times the typical wind wave period of O(10) s (see Fig. 1).
Hualien Harbor is located on the eastern (Pacific) coast of Taiwan, which is invaded by typhoons several times each year. During Typhoon Longwang on October 2, 2005, a 7000-ton cargo ship broke loose from its dock at the northern end of the basin, drifted southward for 1 km, ran aground outside the harbor, and broke in half. Figure 2 displays the wave height spectra recorded at several stations during Typhoon Tim of 1994. Those spectra at outside stations # 2 and # 5 show high peaks at quite short periods O(15) s. However, at stations #8 and #10 inside the basin, high peaks appear instead at quite long periods O(140) s.
Borisade, Friedemann (University of Stuttgart) | Koch, Christian (University of Stuttgart) | Lemmer, Frank (University of Stuttgart) | Cheng, Po Wen (University of Stuttgart) | Campagnolo, Filippo (Technical University of Munich) | Matha, Denis (Ramboll Wind)
The subject of this study is the verification and the validation of existing numerical codes for floating offshore wind turbine structures using wave tank model tests as part of the INNWIND.EU project. A model of the OC4-DeepCwind semisubmersible platform, together with a Froude-scaled rotor model, is tested in a combined wind-and-wave basin. The simulation environment comprises a multibody approach with hydrodynamic and aerodynamic loads and mooring line forces. The focus of this paper is the validation of the hydrodynamics of a modified model hull shape, which compensates for the excess mass of the nacelle. The results show that the simulation model agrees well with the experiment.
At offshore sites with higher water depths, the use of floating structures is more reasonable than the use of large fixed-bottom structures such as monopiles, tripods, and jackets as described by James and Costa Ros (2015) and Beiter et al. (2016). A floating wind turbine experiences many different loading conditions. Floater motion with six degrees of freedom (6DOF) as well as aerodynamic and hydrodynamic loads have to be considered. At this point, few floating wind turbine prototypes have been built, e.g., the Fukushima FORWARD project, which was started in 2013.
To increase the reliability of wind turbines for floating applications, validated simulation codes are needed to predict the forces on the system structure and their dynamic responses for combined stochastic wave and wind loadings (Müller et al., 2016). Although several verification tests have been done by Robertson et al. (2013), Huijs et al. (2014), and Müller et al. (2014), for example, the validation of coupled simulation of floating wind turbines is still part of current research.
This work is associated with task 4.2 of the INNWIND.EU project as part of its model test campaign at LHEEA, École Centrale de Nantes (ECN), France, in 2014. INNWIND.EU, with its 27 European partners, aims to improve the design of beyond-state-of- the-art 10–20 MW offshore wind turbines, including hardware demonstration. A scaled 10 MW model of the OC4-DeepCwind semisubmersible was built at the University of Stuttgart, together with a Froude-scaled wind turbine with low Reynolds rotor blades, developed by the Politecnico di Milano.
Gong, Yufeng (Huazhong University of Science and Technology) | Zhang, Zhengyi (Huazhong University of Science and Technology) | Liu, Jingxi (Huazhong University of Science and Technology) | Xie, De (Huazhong University of Science and Technology)
Offshore structures in the Bohai Gulf are often under the threat of sea ice. Due to the fact that the ice environments, such as the ice thickness and tensile strength, often change over the years and in the sea areas, it is better to determine a series of ice cases with the occurrence probability to predict the ice force on the structure in the Bohai Gulf. To calculate the ice force on the structure accounting for the uncertainty of the ice environments, such as the ice thickness and tensile strength, a method for determining the ice case in the Bohai Gulf was proposed, and an element was developed to simulate the sea ice as a user modification of the Finite Element Analysis (FEA) commercial software ABAQUS. At the same time, the Winkler foundation and the failure criterion introduced by Reinicke and Remer were implemented in the element to take into account the bending failure of the sea ice and the influence of the buoyancy force. An open and universal framework for determining the ice cases in the Bohai Gulf has been proposed.
With the rapid development of China’s economy, there is a huge need for clean and sustainable energy such as solar and wind power. In recent years, the demand for an offshore wind turbine in the Bohai Gulf located northeast of China has been growing fast. However, the ice conditions in the Bohai Gulf can affect the safety of an offshore wind turbine foundation. To design an offshore wind turbine foundation in the Bohai Gulf, the force caused by the interaction of the ice sheet and structure needs to be assessed. Over the past decades, researchers have found that the ice force on a conical structure is significantly smaller than the force on a cylindrical structure of similar size. At the same time, many theoretical studies have been performed to predict the ice force on conical structures. Bercha and Danys (1975) presented a theoretical expression to estimate the ice forces on cones. In their study, the ice sheet was idealized as a linear elastic plate on an elastic foundation. The failure of the ice sheet was assumed to be governed by a brittle failure based on the maximum tensile stress failure criterion. Ralston (1980) studied the ice forces on cones on the basis of the plastic upper limit theory of a floating plate.
Martínez-Ferrer, Pedro J. (Manchester Metropolitan University) | Qian, Ling (Manchester Metropolitan University) | Causon, Derek M. (Manchester Metropolitan University) | Mingham, Clive G. (Manchester Metropolitan University) | Ma, Zhihua (Manchester Metropolitan University)
This paper presents the numerical investigations of an oscillating wave surge converter (OWSC) operating in extreme sea states leading to slamming. We use the open-source computational fluid dynamics (CFD) library OpenFOAM to carry out the two-dimensional numerical simulations. A preliminary study is done to verify the convergence of our results, while scalability tests confirm the high-performance computing capabilities of OpenFOAM and the possibility of extending this study to large three-dimensional configurations. The OWSC device is simulated with both incompressible and compressible solvers, and the results are compared against previous numerical and experimental results. It is shown that an incompressible solver can capture the dynamics and general behavior of the flap device. Nevertheless, the compressibility effects can be reproduced only with the aid of a compressible solver, which takes into account the density changes in the air and water phases. Those effects produce high-frequency, small oscillations on the seaward side of the flap but do not contribute to further increasing the peak pressure values characteristic of slamming.
The use of renewable energies, such as wind and solar, has experienced a noticeable increase in recent years. However, other sources of renewable energies, such as the one extracted from ocean waves nearshore, still remain largely underexploited at present. Therefore, more experiments and accurate numerical simulations need to be carried out in this area with special focus on structure survival as a consequence of harsh ocean conditions.
This paper focuses on the Oyster oscillating wave surge converter (OWSC) (Whittaker and Folley, 2012), which consists of a flap device hinged on the seabed and driven back and forth by the action of waves. The energy taken from the waves is utilized to pump fresh water into a hydraulic plant inshore, where it is finally converted into electricity. OWSCs obtain their maximum efficiency in nearshore locations of shallow water depths, where they acquire larger motions from the waves. One of the current challenges of the OWSCs is their survivability in extreme sea states, e.g., winter sea storms (Kay, 2014), in which large and infrequent extreme waves may compromise their structural integrity and consequently lead to an increase in their maintenance costs. Furthermore, global warming and climate change are likely to increase the frequency as well as the intensity of storms and hurricanes, which may influence the design of future OWSCs.
Choi, Young-Myung (Korea Research Institute of Ships and Ocean Engineering) | Nam, Bo Woo (Korea Research Institute of Ships and Ocean Engineering) | Hong, Sa Young (Korea Research Institute of Ships and Ocean Engineering) | Kim, Jong Wook (Advanced Technology Institute, Hyundai Heavy Industry)
In this study, a series of model tests were carried out to evaluate the performance of an active heave compensator (AHC) in deepwater installation operations. In the tests, a crane vessel and three different subsea structures were examined during deepwater crane lifting operations with an AHC. To validate the experimental results, time-domain numerical simulations were performed with the same AHC control algorithm. First, free-decay tests were conducted to identify the added mass of the subsea structure and the natural period of the hoisting system. Then, the performance of the AHC was evaluated for regular and irregular wave conditions. The effects of winch capacity and installation water depth on AHC performance are discussed.
To install various subsea equipment or structures in deep water, safe and economic installation methods should be taken into account. Among various installation methods for subsea equipment, the conventional crane-wire installation method has been widely used in real-sea operations. Safe crane lifting operations require checking the crane capacity, rigging design, and structural strength of the lifted object. If the weight of the lifted object is considerable, the coupled dynamics of the crane vessel and the lifted object with crane wire become quite important. In particular, it is well known that vertical oscillation of the lifted object can be a significant factor during deepwater crane installation operations, especially for the landing phase. In addition, the resonant vertical motions of the lifted object can cause large dynamic tension of the hoisting wire during the lifting operation.
In the field of subsea installation, as installation depth and weight of installation objects increase, the needs of the heave compensation system increase. Heave compensators can be categorized into two types. One is the passive heave compensator (PHC), which is a kind of spring-damper system that shifts the resonant frequency of the vertical motion of the hoisting wire system. The PHC is also designed to reduce impacts on offshore cranes by adding damping in the hoisting wire. However, it depends on the types of subsea structure and sea conditions. The other type is the active heave compensator (AHC), which compensates for the vertical motion of a lifted object by using either controlled winches or hydraulic pistons with reference signals. The AHC systems generally use information from a vessel motion reference unit (MRU) to control the payout length of the winch line. The AHC system is rather complex and expensive but it is relatively unaffected by the subsea structure type and sea conditions. Also, the control mode can be changed adaptively for varying installation stages.
It is generally accepted that an estimate of mean power capture for a wave energy converter (WEC) in a given sea state can only be obtained over many hundreds (or thousands) of wave cycles. The difficulty stems from the fact that WECs typically exhibit significant nonlinearities in their responses. A reduction in the number of wave cycles needed to obtain accurate results would allow the use of numerical tools for design optimization tasks that are currently too computationally demanding. In this paper, experimental time traces are analyzed to provide reasonable estimates of relative variations in device performance using short-duration sea states. We examine the suitability of various metrics of surface elevation time traces by comparing corresponding WEC data of interest. The results show that carefully selected wave traces can be used to reliably assess variations in power output due to changes in hydrodynamic design or wave climate. It is also demonstrated how confidence levels increase with running time, so in the future simulations could be run until sufficient accuracy is achieved to choose the best design.
One of the most common methods used in the development of wave energy converters (WECs) is physical experimentation undertaken in a wave tank, which can be both time consuming and expensive.
Oscillating wave surge converters (OWSCs) are designed to be deployed in the near shore region in water depths of approximately 12–15 m and utilize the amplified surge motion of water surface waves in this region to pitch back and forward about a hinge mounted on the seabed. The basic concept is shown in Fig. 1.
One design feature of OWSCs is the shape of the side edges (as shown in Fig. 2). The thickness of the edges of the flap affects energy loss, as a result of viscous effects, and thus power capture (Cameron et al., 2010).
This paper presents a practical interpretation of the hydrodynamic force equation developed based on the velocity potential. It expresses the equation in an original version of the Morison equation as well as in its subsequent numerous extended or modified versions in the industry design practice. It is demonstrated that the hydrodynamic force equation derived from a velocity potential for slender structures can be expressed term-by-term in a form of the original as well as modified versions of the Morison equation as used for floating and flexible structures in motion in waves. From this expression, the validity range of Morison equation is presented and discussed from the point of view of the hydrodynamic force equation. Also further discussed are: (a) the drag term empirically added to the hydrodynamic force equation, similar to the term in the Morison equation; (b) the use of the frequency-dependent, added mass and radiation (or wave) damping accounting for free-surface effect; (c) periodic time-dependency in practice; and (d) member diameter relative to incoming wave length.
Since the late 1940s, oil and gas drilling and production activities have been moving from coastal to offshore areas. The industry faced a new challenge in the design of offshore bottom-fixed pile structures in accounting for current and water waves in shallow depth: wave and current-induced forces on the submerged part of the pile structures.
Since the 1950s, the Morison equation has been applied to the design of numerous jacket platforms, risers, and pipelines with only few failures reported. This indicates that the Morison equation has worked as a safe design tool for the offshore structures and equipment for more than half a century. Even these days, however, there have been occasional debates over the validity of the equation, from some theoretical hydrodynamics points of view, and on the accuracy or applicability of various extended or modified versions of the equation. Expressions of periodic forces in the Morison equation used in the industry have been disputed by some. This prompted the author to make the connection between the Morison equation (Morison et al., 1950) and theoretical hydrodynamic equations (Chung, 1975, 1976).