Wichtmann, Torsten (Karlsruhe Institute of Technology) | Triantafyllidis, Theodoros (Karlsruhe Institute of Technology) | Chrisopoulos, Stylianos (Karlsruhe Institute of Technology) | Zachert, Hauke (Arcadis Deutschland GmbH)
The paper presents three engineer-oriented models based on the high-cycle accumulation (HCA) model of Niemunis et al. (2005), dedicated to the prediction of long-term deformations of offshore wind power plant (OWPP) foundations caused by wind and wave action. A sublayering model for shallow foundations under vertical cyclic loading and two different approaches (a sublayering model and a stiffness-degradation model) for monopile foundations subjected to horizontal cyclic loading are presented. The results of these models are compared to the solution from 2-D or 3-D finite element simulations with the original HCA model. Furthermore, the prediction is confronted with the prognosis of other engineer-oriented models proposed for OWPP foundations in the literature. Finally, a simplified procedure for the determination of the HCA material constants is briefly explained.
The cyclic loading of offshore wind power plant (OWPP) foundations due to wind and wave action leads to permanent deformations. They may endanger serviceability since the OWPP generators tolerate only a small tilting (0.5.–1.) of the tower. Therefore, an accurate prediction of these long-term deformations is indispensable. The high-cycle accumulation (HCA) model proposed by Niemunis et al. (2005) is a suitable tool for that purpose. It has been validated based on simulations of model tests and full-scale in situ tests (Hartwig, 2010; Zachert, 2015; Zachert et al., 2014, 2015, 2016). Up to now, the HCA model has been primarily applied in finite element (FE) simulations (e.g., Wichtmann et al., 2010b; Zachert et al., 2014, 2015, 2016). Such calculations usually demand a rather laborious 3-D model and experienced knowledge on the field of FE. To facilitate the practical application of the HCA model to OWPP foundations, several simplified engineer-oriented models for different types of foundation structures have recently been developed by the authors based on the HCA equations:
• A sublayering model for the subgrade of shallow foundations under cyclic vertical loading. For example, such loading conditions are relevant for OWPPs founded on three or four separate footings. The calculation procedure using this model is similar to that in a conventional settlement calculation for foundations subjected to static loading, but with the HCA equations predicting the additional cumulative portion of settlement.
The results obtained from a field lateral loading test and the existing p-y curve models were compared to develop a p-y curve model applicable to the basalt at Jeju Island. The results of the comparison demonstrated underestimated values for the initial tangent modulus and the ultimate subgrade reaction from the p-y curve models presented by Carter and Kulhawy (1992) and overestimated values from the p-y curve model suggested by Yang and Liang (2006). Therefore, in this paper, the initial tangent to a p-y curve suggested by Carter (1984) was modified according to the behavior of the basalt at Jeju Island.
Recently, the exploitation of offshore wind turbines worldwide has been gradually increasing on the basis of the prospects for a new infrastructure for the energy industry that is currently unavailable for onshore wind turbines owing to issues associated with noise, the landscape, and the deficiency of necessary sites.
Thus, active research on source technologies to establish offshore wind turbine systems and optimal large complexes is currently in progress in Korea. Among the sites of large complexes for offshore wind turbine systems, offshore Jeju Island was determined to be one of the optimal places for an offshore wind turbine system due to its favorable windy conditions. Therefore, efforts to develop a marine wind power generation system and associated planning activities are concentrated there.
The representative types of pile foundations required for offshore wind turbines are the monopole, jacket, and tripod, from which an optimal type of pile foundation is determined by the consideration of various factors such as the type and characteristics of the seabed, the depth of the water, the tidal current, the waves and winds, and the economy.
The pile foundation is a substructure designed to support vertical and lateral loads for places where it is difficult to install direct foundations to support upper-structure loads due to soft ground or places with high-water levels, and it is typically designed to resist axial loads. However, offshore wind turbines are frequently subjected to large lateral loads induced by the wind load, the current load, and wave loads. Thus, pile foundations applied to offshore wind turbines should be designed to resist lateral loads as well as axial loads.
Bottom-hinged Oscillating Wave Surge Converters (OWSCs) are an efficient way of extracting power from ocean waves. In our previous studies, wave and OWSC interaction has been investigated via computational fluid dynamics (CFD) models. However, these models were highly time-consuming, and significant re-reflection was observed. The present work couples a Boussinesq wave model with a CFD model in order to extend the scope of the applications of the previous models. This model takes advantage of the Boussinesq wave model, which simulates the wave propagation effectively, and the CFD model, which provides the local flow details comprehensively. The model is validated by a comparison of the present results with those obtained with the pure CFD model and the experimental tank testing. The final objective of the present work is to simulate some events experienced and recorded by the full-scale prototype (Oyster 800 developed by Aquamarine Power) incorporating the real bathymetry at the Oyster 800 site.
Waves interacting with a bottom-hinged Oscillating Wave Surge Converter (OWSC) were investigated by the use of the computational fluid dynamics (CFD) method to understand the viscous effects (Wei et al., 2015) and the 2D wave slamming (Wei et al., 2016) on OWSCs. Although the vortex shedding from the edges and the wave overtopping device were properly described in the 3D model, the computational cost was expensive because this model essentially reproduced the experiment numerically; the computational domain included the entire experimental tank. Moreover, it was observed in the 2D experiments and simulations that the reflected wave from the device might be reflected off the paddle. Such a wave will contaminate the incident wave; hence, there is less confidence in the measured impact pressure for the design. In this paper, we use the terminology “re-reflection” to denote such a wave. In order to simulate the 3D slamming on the OWSCs at an acceptable computational cost, an affordable numerical model was developed (Wei and Dias, 2015). A truncated computational domain was used in the model, and the momentum sources were adopted to avoid the re-reflection from the outer boundary. However, the simulations were performed only under simplified conditions, i.e., the input waves were theoretical waves, and the sea bottom was ideally flat.
In the international regulation framework, the energy-efficient operation of ships is becoming standard. In this respect, restrictions on new construction appear to encourage improvement to existing vessels often equipped with outdated technologies. One of the relevant aspects of propulsion plant design and fleet management is the propulsion need to accomplish the design requirements in a wide set of sea states or in conflicting operative conditions (e.g., laden/ballast, sailing/trawling), requiring very different performances. A preliminary assessment of the energy efficiency of the ship system is then crucial for optimizing both the operating costs and the impact on the sea environment. A new efficiency assessment method that includes engine fuel consumption evaluated by ad hoc statistic regressions and ship resistance in calm water and in waves computed by a 3-D boundary element method is proposed. An application to a hard-chine 18 m trawler is proposed as part of a wider decision support system or weather routing algorithm.
Ships are a significant source of air pollutants, such as sulfur oxides (SOx), carbon oxides (COx), and nitrogen oxides (NOx), that have a relevant impact on both some sea ecosystems and populated coastal areas, especially those close to harbors. Although the International Maritime Organization (IMO) introduced the greenhouse gas (GHG) emissions reduction in its agenda in 1995, only in recent years has this seemed to generate constraints on the design of new units (see, for instance, Coraddu et al., 2014). In addition, considering that most pollutants are strongly related to a vessel’s total fuel consumption, optimizing the propulsive efficiency directly reflects as a reduction of exhaust gas emissions. From a designer’s point of view, the need to improve the available methods for efficiency prediction and optimization to achieve better solutions at very preliminary design phases is clear. For example the IMO (2009a, 2009b) introduces technical and economical indexes for emissions regulation, namely, the Energy Efficiency Design Index (EEDI) and the Energy Efficiency Operational Indicator (EEOI). The former is used to assess the design of a vessel, the latter to evaluate the operational profile of a vessel. Despite the relevance of these indexes, some types of ships, such as cruise ships and working boats, are not included in the baseline values provided in the International Convention for the Prevention of Pollution from Ships (IMO, 2011). Moreover, the proposed baseline values do not take into account the environmental conditions in which ships navigate.
Large Deformation Finite Element (LDFE) modelling is conducted to study the bearing capacity of large offshore foundations in limited clay depth. Complementary visualising centrifuge experiments are reported in clay with interbedded sand, correlating with the numerical study. The current squeezing methodology neglects the conical underpart of the spudcan and any possible deformation of the underlying layers and hence does not predict the measured resistance well. An alternate approach overcoming the limitations of the squeezing theory is presented and verified.
Offshore jackup drilling rigs are often supported by a quasicircular or sometimes polygonal foundation with a conical underpart commonly referred to as a spudcan. The jackup generally operates in shallow-to-medium water depth (up to ~150 m). Seabed stratigraphies in medium water depths can be layered, consisting of several alternate layers of sand and clay (Baglioni et al., 1982; Kostelnik et al., 2007; Dutt and Ingram, 1984). Limited knowledge is currently available for foundation installation in more than two-layer soil stratigraphies. This could be due to the inherent difficulties in physically modelling more than two-layer stratigraphies directly in the geotechnical centrifuge due to possible boundary effects (Ullah et al., 2014; Ullah et al., 2016). Some centrifuge tests mimicking several offshore soil deposits were recently reported in Hossain (2014) for soil stratigraphies up to six layers.
In the absence of detailed analytical methods, solutions developed initially for two-layer stratigraphies are recommended for more general multi-layer stratigraphies (ISO, 2012). This extended application requires additional assumptions that are often not realistic and require further investigation. The International Organization of Standardization (ISO) guidelines suggest that the bearing capacity calculation for soft clay over a strong soil layer (such as sand or stiff clay) should proceed first by calculating the soil resistance from the available single-layer solutions, such as those of Skempton (1951) or Houlsby and Martin (2003), until the depth of transition (dt) is reached. (See Fig. 1 where ID is the relative density and φcv is the constant volume friction angle.) dt refers to the depth measured from the top of the sand layer to the transitional point on the load-penetration curve where the transition from a near linear uniform clay-type response occurs.
In offshore steel structures engineers often face the problem of assessing the criticality of existing hot spots to predict the remaining lifetime and thus to develop sound reliability-based inspection programs. One problem with such an approach is that the past fatigue conditions cannot be appropriately modeled, and the degree to which damage has accumulated in hot spot areas cannot be consistently modeled. This paper shows a practical methodology for predicting the remaining fatigue life of hot spots by using a probabilistic fracture mechanics approach and shows how in general the results can be used in reliability-based inspection programs.
At present, the best practice for modeling remaining fatigue life and identifying hot spots in existing offshore steel structures is the traditional S–N approach (Miner’s rule). The uncertainties in the S–N approach for existing structures are significant, and it is often impossible to take into account the stress cycles to which the structure has been subjected in the past, especially in cases where the structure has been strengthened or modified. This results often in very low fatigue life, sometimes even lower than the actual age of the structure. The S–N approach has thus only limited use for identifying measures and for establishing risk- and reliability-based inspection plans for such hot spots in existing structures.
The present paper shows how a probabilistic fracture mechanics approach can be used to analyze the existing hot spots in an offshore steel structure and how reliability-based inspection planning can be established based on these results. The general approach is to postulate cracks of specific length and depth that match the threshold of known inspection techniques, to have cracks located in specific directions at the hot spot locations, and then to predict the crack growth and failure probability of the postulated cracks.
Liu, Xiaoyi (Shanghai Jiao Tong University) | Zhao, Min (Shanghai Jiao Tong University) | Wan, Decheng (Shanghai Jiao Tong University) | Wu, Jianwei (Wuhan Secondary Ship Design and Research Institute)
With the continuous development of the shipbuilding industry and shipping business, hydrodynamic optimization of hull forms has drawn the attention of both academia and industry. This paper reports the details of an efficient, numerical, design optimization tool for hull form for container ships. This tool is composed of three functional modules: hull form deformation, hydrodynamic performance prediction, and optimization. The free-form deformation (FFD) and radial basis function (RBF) methods are employed to modify the ship hull globally and locally, respectively. To reduce the cost of the numerical optimization, which is always a challenging problem, a new potential theory, the Neumann–Michell (NM) theory, and the approximation model are adopted. In addition, the analysis of variance (ANOVA) method is used to represent the influence of each design variable on the objective functions. The high efficiency is illustrated by the optimization for a container ship. Wave resistance coefficients at three design speeds are minimized, and a Pareto front of solutions is obtained. The optimal hulls are verified and analyzed by the NM theory and a Reynolds-averaged Navier–Stokes (RANS)-based computational fluid dynamics (CFD) solver. Numerical results confirm the availability and reliability of the optimization tool described.
In recent decades, with the continuous development of the shipbuilding industry and various shipping businesses, hydrodynamic optimization of hull forms has drawn the attention of both academia and industry. The economic efficiency of container ships, in particular, depends mainly on hydrodynamic performance. To obtain a hull form with the best hydrodynamic performance, design engineers have devised some approaches with different hydrodynamic analysis methods, geometrical modification techniques, and optimization algorithms. However, because of the complexity of ship hydrodynamics and the great number of evaluations of objective functions in optimization, ship hull optimization is quite time consuming. To solve this problem, a combination of a new, efficient hydrodynamic analysis method and an approximation model is adopted as a feasible scheme for ship hull optimization.
This contribution addresses the applicability of an efficient lattice Boltzmann-based single-phase free-surface model for the simulation of wave impact on the side walls of 2-D containers. The computational efficiency of the method is known to allow for very short turnaround times, but wave impact simulations have not been investigated in detail yet. Results for a selected wave impact case are discussed, the convergence behavior in space and time is analyzed, and limitations of the single-phase free-surface model are revealed. The results show that lattice Boltzmann method (LBM)-based single-phase free-surface models are a viable tool for predicting the impact wave behavior, but the quality of the pressure signal is limited, because of the absence of air in the simulations.
The efficient numerical simulation of violent tank sloshing and wave impact is important to many different fields of engineering. Besides the numerical accuracy of the employed solvers, the computational efficiency and the time to solution are of interest as well, as even two-dimensional simulations of tank sloshing require a substantial amount of computational time. In this context, a very efficient numerical methodology based on the lattice Boltzmann method (LBM) is assessed in this paper. The LBM is an alternative to conventional methods on the basis of the Navier–Stokes equations that offers solver-specific advantages in terms of data locality and parallel computing. The LBM usually operates on a finite difference grid, is explicit in time, and requires only next neighbor interaction. It is very suitable for implementation on graphics processing units (GPUs) and other high-performance computing (HPC) hardware. Recently published LB results comprise laminar and turbulent bulk flows, multiphase flows, and free-surface flow applications. For all applications, a comparably high computational performance on both CPU- and GPU-based parallel architectures is reported.
In the scope of this contribution, the applicability of the LBM to tank sloshing and wave impact is analyzed. Emphasis is given to the actual result accuracy, times to solution, and potential problems of the free-surface model. First, a short description of the LBM for bulk flows and the employed LBM free-surface model is given before addressing the violent tank sloshing case. Finally, conclusions are drawn and future perspectives are discussed.
Shimizu, Tsutomu (National Institute of Advanced Industrial Science and Technology) | Yamamoto, Yoshitaka (National Institute of Advanced Industrial Science and Technology) | Tenma, Norio (National Institute of Advanced Industrial Science and Technology)
The local two-phase gas/liquid flow behavior at a high velocity gradient is essential for managing gassy wells. In this study, the methane/water bubbly flows passing through a perforated pipe were characterized in a 10.4-m flow loop in which the pressure was varied up to 5.5 MPa at 291 K. To characterize the two-phase flow behavior at the bore, we obtained the bubble sizes from high-speed photographs and digital image analysis. As the flow velocity and/or pressure increased, the flow patterns shifted from bubbling to jetting, suggesting that the local two-phase flow pattern can control the bubble size in flowlines.
Multiphase flow control in wells and pipelines is crucial in the oil and gas industries. The most important components affecting the production efficiency, cost, and safety of gassy wells are the gas/liquid separators and multiphase flow pumps. For instance, the gas/liquid separators reduce the void fraction at the pump intake, thereby minimizing the pump surging (Hua et al., 2012; Gamboa and Prado, 2011). Phase separation reduces the risk of pipe plugging through the formation of gas hydrates (Shimizu et al., 2017; Joshi et al., 2013; Sakurai et al., 2014). In gas production from offshore natural gas hydrate reservoirs, these devices must handle two-phase flows under variable pressure and void fraction in a natural-gas/seawater mixture, while stably maintaining the bottom-hole pressure below the three-phase equilibrium pressure (Cyranoski, 2013). Optimizing the performance of these devices in such situations is a necessary yet challenging task.
Bubbles formed by breakup and coalescence are of paramount importance in industrial heat and mass transport processes and are typically generated by a gas distributor (Idogawa et al., 1987; Quinn and Finch, 2012; Tsuge and Hibino, 1983) or a rotating impeller (Kracht and Finch, 2009; Minemura et al., 1998; Masui et al., 2011). Hence, bubble formation has been studied extensively for decades. However, few studies have focused on the bubble behavior under a high velocity gradient in pipelines, where bubbles assist the transmission of the gas/liquid flow mixture in practical production fields.
In this paper, the effect of an Offshore Wind Farm (OWF) on the surrounding wave field is numerically investigated in the frequency domain through the use of a Boundary Element Method (BEM) numerical model based on the potential flow theory. The analysis is performed for regular waves of various periods and incident wave directions and for irregular waves with variable peak periods and significant wave heights. Specific cases of regular and irregular waves are compared, revealing the differences between the regular wave model and the real sea states. Through the numerical simulation of the incident wave and the scattering effects caused by the OWF, indications are provided regarding the impact of the OWF on the local wave climate. Finally, the impact of hydrodynamic interaction effects on the forces applied to the offshore wind turbines is examined.
In recent years, the increasing energy demand has led to a growing interest in the efficient exploitation of renewable energy sources. Under this framework, offshore wind energy has become an increasingly attractive option, offering multiple benefits and addressing effectively the well-known obstacles and problems associated with the exploitation of wind energy onshore (Henderson et al., 2003; Breton and Moe, 2009). Consequently, the offshore wind energy sector is continuously growing, and this has resulted in the large-scale commercial deployment of Offshore Wind Farms (OWFs), especially in the coastal and offshore areas of northern Europe (EWEA, 2015). So far, most OWFs operating in Europe have been installed in shallow waters of average depth equal to 22.4 m and at an average distance from the shore equal to 32.9 km (EWEA, 2015). Moreover, the deployed support structures correspond mainly to fixed bottom configurations, i.e., the monopile, tripod, and jacket (EWEA, 2015).
Although OWFs may contribute significantly to the coverage of the increasing energy demands, their installation and operation should be implemented by not only the consideration of economic and engineering factors but also the assessment and prediction of reliably possible negative environmental impacts on the corresponding marine environment (e.g., the undesirable effects on the local wave climate, changes in sediment transport patterns, loss of biodiversity, etc).