Due to the lateral stiffness shortage of the monopile, and in order to improve the applicability of the monopile, a new wind turbine foundation type which is combining the jacket and monopile foundations is proposed. Based on the engineering example of offshore wind farm in Fujian Province, China, the main design parameters are considered. A 5 MW wind turbine foundation finite element model is established as the research object. Compared the results of monopile and the new composite foundation, conclusions are obtained. These findings will provide some reference for the new type foundation design.
In recent years, the Chinese government on the development of clean energy, especially wind power gives a great policy support. China's wind power sector gained momentum due to the government's supportive policies. Sea wind is a permanent source which is inexhaustible and green new energy. Compared with onshore wind power, offshore wind power has many advantages, such as small land occupation, large wind speed, stable wind direction and little influence from surrounding buildings, etc.(Karadeniz et al., 2009) Commonly offshore wind turbine foundation type has gravity foundation, monopile foundation, tripod foundation, jacket foundation, floating foundation, suction caisson foundation and other types.(Westgate and DeJong, 2005)
Monopile foundation as the most simple foundation structure is currently the most used wind turbines foundations for onshore and offshore wind farms.(Zaaijer, 2006) Its main advantages are the simplicity of design and manufacture, easy installation, and low costs.(Achmus et al., 2009) However, with the increase of water depth, the lateral rigidity of monopile becomes insufficient, and it can only be used for water depth within 30 m. Jacket foundation as another foundation for offshore wind turbines is more firm and stable than monopile foundation. It is suitable for 20 ~ 50 m water depth, but the costs for manufacture and installation are higher. (Jonkman et al., 2009)
Ndiwe, Benoit (Lappeenranta University of Technology) | Mvola, Belinga (Lappeenranta University of Technology) | Kah, Paul (Lappeenranta University of Technology) | Bayock, Francois Njock (Lappeenranta University of Technology)
Fusion welding generally requires the use of electrodes. It is, therefore, important to select or design welding electrodes that promote low crack sensitivity and few brittle components. The objective of this study is to analyze the influence of different alloyed elements on microstructure formation at different areas of the weld. The methodology involves a review of the outcomes of scientific papers and experiments conducted in our laboratory. The expected results are benchmark criteria for the design of electrodes for welding dissimilar grades of steel with a higher Mn content and also for the selection of existing electrodes.
The stability of austenite depends on alloying chemical components for steels mechanical properties in temperature less than zero which include steels with a high Mn content. (Keil, Zinke and Pries, 2011) The Mn content tends to lower the ductile−brittle transition temperature. Fe-Mn-Al-C steel grades emerged in the late 1950s for cryogenic use and critical corrosive service environments to replace Ni-Cr steels. However, the new high-Mn steels are based on complex mechanisms and cooling processes. Moreover, the high percentage of manganese in a steel provides high tensile strength and fragility resistance at temperatures below zero, and, therefore, these steel grades can maintain their ductility in very cold environments. Depending on the Mn content, the Mn addition is between 5 and 12% by weight, whereas in steels with a high Mn content, the Mn content is usually between 15 and 30% by weight. (Dahmen, Lindner, Monfort and Petring, 2016) The basic structure of steels can vary from austenitic to ferritic/austenitic.
The addition of manganese provides beneficial properties to filler metals for less than zero temperature environments. However, concerning the prevention of health problems related to Occupational Safety and Health Administration Occupational Exposure Limit for Mn in dust, the European Commission on Occupational Exposure Limits recommends a weighted average over time.
Wang, Rui (Chinese Academy of Sciences, University of Chinese Academy of Sciences) | Wang, Yu (Chinese Academy of Sciences) | Wang, Shuo (Chinese Academy of Sciences) | Tang, Chong (Chinese Academy of Sciences, University of Chinese Academy of Sciences)
This paper addresses the problem of three-dimensional (3-D) way-point tracking for a biomimetic underwater vehicle (BUV) propelled by undulatory fins: RobCutt-II. Based on the specific mechanical design and control system configuration, the RobCutt-II can perform diversified locomotion patterns, especially submerging or surfacing vertically in the water. In order to deal with the challenging problem of motion control of the BUV in underwater operation, a switching control for 3-D way-point tracking is proposed. Moreover, we design an experiment simulating the actual underwater operation process. The RobCutt-II first dives to the desired depth vertically, then swims successively to the way-points along the horizontal plane, and finally keeps depth at the last way-point. Simulation and experimental results demonstrate the feasibility and effectiveness of the proposed switching control.
Natural selection and evolution make biological systems diversify into nearly every possible habitat and preserve remarkable adaptations for locomotion. Mimicking the unique undulatory propulsion mode of cuttlefish, biomimetic underwater vehicles (BUVs) propelled by undulatory fins have many advantages such as higher maneuverability, stronger disturbance rejection, and quieter actuation than conventional underwater vehicles equipped with jets or axial propellers (Neveln, Bai, Snyder, Solberg, Curet, Lynch and MacIver, 2013). Hence these BUVs are expected to be widely used in marine and military fields (Blidberg, 2001).
As growing research on the wave-like propulsion mechanism and hydrodynamics analysis demonstrate a variety of prospective utilities in undersea vehicles, researchers and engineers have developed many kinds of biomimetic robotic prototypes with fin propulsion. Curet et al. designed a robotic knifefish with an undulatory propulsor using 32 servo motors to drive the long-fin (Curet, Patankar, Lauder and MacIver, 2011). Hu et al. developed a robotic undulating model and proposed a control scheme that enabled it to mimic fin-ray undulation kinematics of live fish (Hu, Low, Shen and Xu, 2014). Rahman et al. designed a squid-like underwater robot with two undulating side fins simulating stingrays or cuttlefishes (Rahman, Sugimori, Miki, Yamamoto, Sanada and Toda, 2013). However, although many types of BUVs have been developed, few BUVs have been put into practical applications as far as we know. There are several possible reasons for this fact. In addition to the complex hydrodynamics and the model uncertainty in underwater environment, researchers seldom consider the closed-loop motion control such as way-point tracking for these BUVs, which are of primary importance for most applications.
In this study, a two-dimensional numerical model (NEWFLUME) is applied to investigate the shear stress distributions in the bubbly flow on a sloping beach (1/20) under breaking waves. The numerical model solves the Reynolds equations for mixture flow and employs the volume-of-fluid (VOF) method to track the free surfaces. Turbulence closure is accomplished by the k - ε scheme. An improved air entrainment model is incorporated to consider the effect of bubbles on the mean flow in present model. The bed stress distributions and the effect of bubbles on shear stress are investigated. It is found that the bed stress decreases by the existence of bubbles in the aeration region. The maximum value of time dependent total shear stress is damped approximately 12% and the depth integrated total shear stress shows a 20% decrement.
Sediment transport process is one of the main parameters in the coastal morphology changes, especially under a plunging breaking waves, which will generate high intensity of turbulence and traps a large amount of air into the water. The entrained air quickly bursts into small air bubbles to interact with the surrounding flow and thus enhance air-sea gas transfer. In the meantime, the dispersed bubbles will play an important role in the local turbulence and bed stress behavior which may have signifiant impact on sediment suspension and transportation. Therefore, a better understanding of the bed stress distribution in bubbly flow is very important in investigating sediment transport under plunging breaking waves in the surf zone.
Several studies on bed stress distributions under wave motion have been conducted. Tanaka (1998) investigated the bed stress under non-linear waves and proposed a formula to predict bed load transport near the surf zone region in which the acceleration effects play an important role. Suntoyo and Tanaka (2008) used a boundary layer approach to assess bed stress under solitary waves. The bed stress under wave motions are well predicted in their numerical computations. Liu et al. (2007) investigated the bottom shear stress under solitary waves. They found that the bed stress changed sign during the deceleration phase. Adityawan et al. (2012) used the SCM (shallow water equation) method and investigated the bed stress under unsteady wave run up and run down process. They found that the bed stress on the direction of leaving the shoreline has more impact in the overall process. However, none of the published work (to the authors' knowledge) has investigated the bed stress distribution in the bubbly flow under breaking waves, which is of great importance to flow dynamics in the surf zone as well as the resulting sediment transport. This will be achieved in present work, in which the NEWFLUME model (Lin and Xu, 2006) is used as the basic framework, which solves the Reynolds equations and employs the volume-of-fluid (VOF) method to track free surfaces. An improved air entrainment model is incorporated into the bubble transport equation. The bed shear stress distribution on the sloping beach is investigated. Based on the numerical simulations, the effect of bubbles on the shear stress distributions will be examined and discussed.
With the improvement of Smoothed Particle Hydrodynamic (SPH), it has become one of most vigorous methods for breaking wave simulation. Recently we have developed an improved SPH method for simulating violent flow and two bodies interaction. The main features of this new method include the incompressible SPH method based the pressure Poisson equation, and it also gives a new scheme to deal with moving boundary. In this paper, this improved method will be applied for simulating waves and its impact with single floating body and two floating bodies, and the results are compared with experiment measurements.
The smoothed Particle Hydrodynamics (SPH) method is a meshless, purely Lagrangian technique which was originally developed by Lucy (1977), and Monaghan and Gingold (1977). While the WCSPH scheme has been successfully used for violent free surface flow, the stiff equation of state can result in large unphysical pressure fluctuations. These spurious oscillations in the pressure field can be mitigated by reducing the sound speed and relaxing the system at the same time. As a remedy for large unphysical pressure oscillations, Colagrossi and Landrini (2003) corrected the density calculation by renormalizing the density using Moving Least Square (MLS) density correction. They showed that, the correction improves mass area density consistency and also filters out pressure oscillations. They also found out that the density re-initialization procedure is beneficial with respect to energy conservation when it is used along with artificial viscosity. Molteni and Colagrossi (2009) have proposed a δ -SPH scheme by modifying the SPH equations and adding a proper artificial diffusive into the continuity equation in order to remove the spurious numerical high-frequency oscillations in the pressure field. However, these WCSPH issues need a very small time-step in order to resolve the artificial compressible equation. Another important improvement of pressure noisy is the scheme of weakly compressible SPH based on Riemann solver. Monaghan (1997) showed that the artificial viscosity is analogous to the terms constructed from signal velocities and jumps in variables across characteristics in the Riemann problem. Parchikov and Stanislav (2002) proposed a modified SPH method using a first order approximation of the acoustic Riemann solver, which does not require an artificial viscosity term for dissipation. Guo et al (2012) introduced the re-normalized approximation to the Riemann solver. According to the form of SPH based on Riemann solver, which is not uniform and some numerical techniques are still in open for discussion, different researchers may introduce different expressions, so SPH based on Riemann solver is not considered in this paper.
Park, Jeong-Seon (Hanyang University) | Park, Duhee (Hanyang University) | Kim, Hansup (Hanyang University) | Yoo, Jin-Kwon (Hanyang University) | Jang, Hwa-Sup (Korea Register of Shipping) | Yoon, Se-Woong (Korea Register of Shipping)
Estimation of accumulated lateral displacement and settlement are critical in design of wind turbine foundation. However, there have been few studies exploring the response of bucket foundation to long-term cyclic loading. We perform a series of three-dimensional finite element analyses of bucket foundations installed in sands and clays. An empirical formulation which captures the stiffness degradation observed in cyclic triaxial and simple shear tests is implemented into the finite element analysis in the form of a user subroutine. It is shown that the cyclically accumulated rotation of the bucket foundation increases with the number of cycles and cyclic amplitude. In particular, the magnitude of the moment and the position of the horizontal load are shown to significantly influence the cyclic response. Extensive numerical simulations are required to develop design charts for predicting the accumulated deformation of bucket foundation under long-term cyclic loading.
Offshore wind energy has been widely developed as an alternative source of energy in the last few decades. Various types of bottom-fixed offshore wind turbines have been designed and constructed. The monopile foundation is the most common type, although bucket foundations have also been used (Zhu et al., 2013). The bucket foundation, which is typically installed using pressure difference between inside and outside of structure, has the advantage of simple and economic installation. A large body of literature on bearing capacity of bucket foundation has been published. Model tests, field tests, and numerical simulations have been performed to determine the bearing capacities of bucket foundations under vertical, horizontal, and moment loadings (Bransby and Randolph, 1998; Yun and Bransby, 2007; Hung and Kim, 2012; Hung and Kim, 2014).
If the bucket foundation is used as a wind turbine foundation, fatigue design is a very important issue. The effect of wind and wave cyclic loading to changes of soil properties has to be considered. However, the long-term cyclic response and the estimation of accurate accumulated rotation are the most important issues for design.
Shi, Yi (China University of Petroleum) | Zhu, Hongwu (China University of Petroleum) | Zhang, Jinya (China University of Petroleum) | Yin, Binbin (China University of Petroleum) | Xu, Ruiting (China University of Petroleum) | Zhao, Junlin (China University of Petroleum)
As an important equipment in subsea booster system, helico-axial multiphase pump plays a significant role in subsea oil and gas development. In this study, the variations of condition parameters in each stage of a three-stage helico-axial pump was investigated by using a commercial CFD code. The three-dimensional (3D), steady-state Reynolds-Averaged Navier-Stokes (RANS) equations with standard SST (shear stress transport) turbulence models are solved in ANSYS CFX. The results show that static pressure increases from the inlet to outlet while gas void fraction decreases. Inlet flow angle of shroud in impeller decreases constantly from the first stage to the third stage and the rate of decline tends to be slow. The main parameters effect the inlet flow angle of shroud are inlet volume flow rate, rotational speed of impeller and gas void fraction(GVF). For every stage of the pump, the inlet flow angle of shroud in impeller increases when the inlet volume flow rate rises while the other parameters remain unchanged. And it decreases when the rotational speed of impeller or GVF grows except the first stage of the pump. In order to diminish the effect of variable inlet condition parameters on pump performance, the stage-by-stage design method should be developed. It contains reducing inlet blade angle of impeller which is related to the inlet flow angle of shroud and modifying the import hub ratio, semi-cone angle and other crucial geometry parameters of impeller at the second and third stage of pump.
Multiphase pumps play a significant role in subsea oil and gas development by increasing hydrocarbon production, enhancing oil recovery and improving field economics. The multiphase pump performance in the presence of gas is a function of several condition parameters such as pump inlet pressure, impeller rotational speed, liquid flow rate, fluid properties and the amount of gas. Besides, the geometry parameters of compression unit have an important influence on the pump performance. Several studies have been carried out to investigate the effect of various parameters on two-phase flow performance of the multiphase pump.
The differential settlement of subgrade may accelerate degradation of tracks, lower passenger comfort and increase derailment risk of trains, which is a troublesome to designers and infrastructure mangers. This paper aims at the numerical simulation of interaction between train, track, and subgrade and running properties of trains under the differential settlement of high-speed railway subgrade. A dynamic analysis model for train-ballasted track-subgrade-subsoil system is established by finite/infinite element method, and validated by experimental data and existing results. The comparison shows the method proposed in this paper is effective in simulating the train- ballasted track-subgrade interaction and riding properties of trains subjected to subgrade differential settlement.
The ballasted track, as a traditional track structure, has been widely used in high speed railways due to good elasticity, low cost, easy maintenance, and obvious absorption to noise. However, the differential settlement of subgrade is a troublesome to designers and infrastructure mangers (Kang, 2016), especially for the soft subsoil easy to generate the differential settlement. In 1997, the Swedish National Rail Administration opened a service with the X-2000 highspeed train along their West Coast Line between Göteborg and Malmö. Shortly after starting the service, excessive vibrations of the railway embankment and surrounding soil were detected at several soft soil areas when the train approaches around 200km/h (Madshus and Kaynia, 2000). Questions were raised about the running safety of the trains, degradation of the supporting track and subsoil, distortion of the embankment, fatigue failure in the rails. Actually, many factors can cause the differential settlement of subgrade and subsoil, such as soil condition, train cyclic loading, subgrade subsidence, and so on. However, once the settlement generates, it may accelerate degradation of tracks, lower passenger comfort and increase derailment risk of trains, especially in high-speed railways.
Issues related to track deformation due to the poor performance of the earthworks are difficult to detect and complex to solve because the subgrade and the subsoil are generally inaccessible. Taufan and Louis (2016) described some current empirical ballast settlement models, and evaluated them using experimental data generated using the Southampton Railway Testing Facility. Due to the limitation of empirical model, the model test and the numerical methods have been applied. Zou (2011) designed a 1:1 model test to study the influence of differential subgrade settlement on the ballasted track. Sol-Sanchez and Pirozzolo (2016) studied the mechanical performance of different configurations for the railway track section by a laboratory approach. Tatsuya (2014) proposed an analytical procedure with iterative calculation by linear finite element analysis to estimate the cyclic plastic deformation of railroad ballast under repeated moving-wheel loads. Paixão and Fortunato (2015) used a FEM method to simulate different scenarios in normal railway lines and gave a parametric study. Zhang (2016) proposed a two-dimensional ballasted railway tracks model utilizing the discrete element method with PFC, and analyzed the dynamic characteristics of the concrete sleeper, clustered ballast stones and the silty clay subgrade under irregular vibration levels caused by train passing the track.
Numerical Weather Prediction models (NWPM) such as Weather Research and Forecasting (WRF) are increasingly used for assessment of offshore wind farms. Prior studies have typically focused on either hub height wind speed over the range relevant to power production or on energy yield from exemplar farms. For design optimisation of wind turbines it is also important to establish peak loading conditions and this requires accurate prediction of both wind shear and turbulent characteristics across the swept area. This is evaluated by comparison to published data at locations near Askervein Hill, Scotland and three met-masts located in the North West of the Indian subcontinent. In regions of flat-terrain the wind speed profile was generally in good agreement with met-mast data, to altitudes of 50 m. At higher altitudes there was greater disparity between predicted and measured velocity with over-prediction of 10% and 9.37% at the flat- and hill-top locations over the range 50 to 300 m respectively. This assessment against multiple met-masts within the same region provides confidence in the prediction of hub height velocity but indicates limitations to the use of such models for characterizing the onset profiles of velocity and turbulence to assess loading of large diameter offshore turbines. The paper discusses implications for turbine loading considering conditions at several potential deployment sites across the Gujarat region, India.
The substantial planned increase in offshore wind energy development gives rise to many challenges. One of the main challenges is to assess wind conditions at planned deployment sites to a sufficient accuracy. This study addresses the suitability of the numerical weather prediction model, weather research and forecasting (WRF) for prediction of offshore wind farm design conditions with a particular focus on Northern India. Within India the installed capacity of wind turbines onshore is more than 20 GW. India has a coastline of over 7600 km and preliminary assessment has shown that there is potential for installation of around 250 GW of offshore wind turbines. (Baldock et al., 2015).
Steel catenary riser is a fundamental configuration in ocean engineering since its wake may be rich of flow dynamics due to the flow interference between the upstream and downstream parts when it is aligned with the oncoming flows. In this paper, we numerically investigated a three-dimensional flow past stationary catenary riser with cylindrical cross-section at Re=100. The flow dynamic is governed by the incompressible Navier-Stokes equations and a high-order spectral/hp element method is employed for the solutions of the flow field. The numerical results present that the wake topology behind the catenary riser is strongly influenced by the degree of bending of the catenary. Furthermore, the interference between the convex and concave parts greatly modifies the wake dynamics and correspondingly changes the characteristics of hydrodynamic force coefficients along the span of the structure. Comparing the wake topology under nine cases, there are four main wake types behind catenary riser. A detailed discussion based on the numerical simulations is presented in the full paper.
With the development of deep-ocean engineering, steel catenary risers are widely used in offshore industry. Apart from the industrial application, the geometry of catenary risers is also a fundamental configuration especially when it is aligned with the flow direction since the wake of curved-shaped body may produce considerably complex flow dynamics. The wake patterns of the catenary riser have received less attention at present, except some researches are about the wake topology of curved circular cylinders, which could be regarded as a half part of catenary riser (Miliou et al. (2003, 2007); Vecchi (2008, 2009); Canabes (2010)).
In this investigation, the catenary string equation is given by
where, x is the horizontal coordinate of the computational domain, y is the vertical coordinate of the computational domain, A is catenary coefficient, which determines the bending degree), the catenary riser can be divided into two parts, the convex part (x<0) and the concave part (x>0). For this reason, the investigation of wake topology of catenary risers are related to the flow past two tandem curved cylinders in different column spacing and the flow past yawed cylinders. In 1989, Ohya et al. based on the investigation of Zdravkovich (1997) summarized a lager of researches of drag forces on two circular cylinders in tandem arrangement. They found the drag force is largely dependent on the cylinder spacing and the Reynolds number, and laminar shedding occurred at the critical spacing at law Reynolds numbers. More recently, Carmo et al. (2010) investigated the same problem about the relationship between the Reynolds number, cylinder spacing and transition point. To study the relationship between local centre-to-centre separation and shedding regimes in tandem arrangements, extensive experiments (Zdravkovich, 1977; Igarashi, 1981) and computations (Meneghini, 2001) had been conducted. Those studies indicated that the occurrence of the bi-stable depended on L/D (where L is the non-dimensional of local centre-to-centre separation of two cylinders and D is the non-dimensional of cylinder diameter). Gao et al. (2011) investigated the characteristics of the flow past circular cylinders of different diameter arrangement at a low Reynolds number of 200, and the bi-stable phenomenon were observed for L/D ranging from 1.8 to 3.8. The Bi-stable were also observed by the investigation of flow around two circular cylinders in a side-by-side arrangement at low Reynolds number. Six kinds of wake patterns were identified in the investigation of Sangmo (2003), and they found that each wake pattern strongly depended on the Reynolds number as well as the gap spacing. Carmo et al. (2010) and Bruno et al. (2010) investigated the secondary instabilities in the wake of the flow around two circular cylinders. Three different shedding regimes were identified in these investigations. For very small separations, the shedding regime SG (symmetric in the gap) is observed. As the separation gradually increased, the shedding regime eventually changes to AG (alternating in the gap). Then, for larger separations, WG (wake in the gap) is observed.