Currently, the development of renewable energy has become a trend with the increasing demand for energy. Wind energy, as a renewable source of energy, is also getting more attention. Increasing effort is devoted to developing floating offshore wind turbines in deep water. In this paper, a V-shaped semisubmersible floating wind turbine was adopted to investigate the dynamic response of the system. Numerical simulations are conducted using aero-hydro coupled analysis in a time domain. The performance of the V-shaped semisubmersible floating wind turbine with respect to global platform motion, mooring line tensions and tower base moment is evaluated in this study. It turns out that the V-shaped semisubmersible offshore wind turbine is a promising concept that provides a good practice for the application of wind energy in deep water in the future.
Currently, due to energy deficiency, many countries are devoted to developing renewable energy to meet energy demands. According to the Chinese 13th renewable energy development five year plan, by 2020, total electric from renewable energy will grow to up to 27% of the total electricity generated(NDRC, 2017). Wind energy, one of the promising renewable energies, has attracted more and more attention because of its low environmental pollution. Compared with onshore wind energy, offshore wind energy has better wind condition, unlimited sites and negligible environmental impact. Especially in China, the area with rich onshore wind resource is far from the energy consumption center, which is located near the eastern coastline (Li et al., 2012). A number of studies have been carried out for offshore wind turbine analysis (Jiang et al., 2015; Shi et al., 2016; Shi et al., 2014). The bottom-fixed wind turbine is not suitable for deep water due to increase in cost (Shi, 2015). Therefore, the floating offshore wind turbine (FOWT) is becoming one of the promising solutions.
According to the offshore oil and gas industry, several different foundations are suitable for FOWT: spar buoy, tension leg platform (TLP), semi-submersible platform and barge. In particular, the semisubmersible platform, compared with spar buoy and TLP, has more feasibility in various water depth, seabed conditions and low installation costs due to the simpler installation (it is fully constructed onshore). The semi-submersible platform can also avoid the main energy range of the waves because of its relatively large natural period. The OC4 semi-submersible offshore wind turbine was simulated by Bayati (2014) to focus on the impact of second-order hydrodynamics on semi-submersible platforms. Moreover, the second-order hydrodynamic force can stimulate the oscillation of the platform and further cause fatigue damage to the structure. How the mooring systems influence the motion of the FOWT (Masciola and Robertson, 2013) determined by using coupled and uncoupled model on DeepCwind semi-submersible FOWT. Luan et al. (2016) employed a braceless semi-submersible platform to establish a numerical model and performed extreme sea states analysis on a braceless semi-submersible platform. The results showed that the platform has good stability under extreme sea and is a good design concept. A 5 WM wind turbine was employed by Kim et al. (2017), and WindFloat and OC4 floating platform were carried out to focus on the motion of FOWT and evaluating the mooring system force by using FAST (Jonkman, 2005) code.
ABSTRACT: A monitoring program employed to measure the support performance of spiles in a recent Canadian tunnel project is presented. Of novelty in this research is the use of a distributed optical strain sensing technique to measure the load distribution of individual spile support members at a spatial resolution of 0.65mm. The distinct challenges and technique developed to instrument the 6-meter long, self-drilling spile members specified by the design are presented. Additionally, the considerations to effectively monitor the instrumented spiles over a 15-meter section of the tunnel are discussed. Selected load profiles are presented for instrumented spiles over one month of monitoring and approximately 36 meters of tunnel advance. Significant correlations between the measured load of each spile using the optical fiber sensing technology and the qualitative assessments made from within the tunnel were found and highlight the necessity of the support installation regulations implemented at the project.
The management of increased transportation needs in growing metropolises has led to the development of many new subterranean infrastructure projects to support rail and road transit. In many cases, this situates shallow urban tunnel construction projects within proximity to existing infrastructure. The construction of the Valley Line LRT project in Edmonton, AB is an example of such a project where the disturbance to existing buildings, roadways, and utilities must be minimized during tunnel excavation. A major aspect of this project, therefore, involves restricting excavation induced displacements to predefined levels by implementing ground control measures such as pre-support. Pre-support and/or face- support techniques include, but are not limited to: face bolting, ground freezing, pre-vaults, and umbrella arch techniques. Of interest in this research effort is the spile support member, which falls under the collection of umbrella arch techniques. According to Oke et al. (2014), the umbrella arch is a temporary support system forming a structural umbrella around the excavation from the insertion of longitudinal support members installed from within the tunnel, above and around the crown of the tunnel face. Installed prior to the first pass of the excavation, the umbrella arch provides support to the ground ahead of and at the working facing, as well as the unsupported span immediately behind the working face (i.e., within the tunnel). The latter is a distinguishing feature of the umbrella arch in comparison to other presupport techniques such as face bolting, which may be used in combination with the umbrella arch.
ABSTRACT: Rock salt exhibits a strain and time dependent response under deviatoric stresses. The practical implications of this creeping behavior have been studied extensively for the design of underground excavations. Advanced constitutive models combined with modern computing capabilities can be used for the analysis of rock salt behavior under complex stress regimes. The implementation and application of state of the art constitutive models to practical situations, such as those encountered in conventional underground mining, is progressively emerging. The following presents an investigation on the application of a strain hardening model to represent unified transient and steady state creep at the Morton Salt Weeks Island mine (Louisiana). A twodimensional modelling simulation is presented to illustrate how traditional convergence measurements at the mine can be used to capture the effect of panel sequencing on creep behavior, and how to introduce the effect of the transient phase on the long term steady state behavior.
Polycrystalline halite, the mineral of rock salt, presents unique mechanical features from the response to natural stress fields and around man-made excavations. Rock sallt exhibits a deviatoric stress induced response, commonly referred to as creep, which results in inelastic deformation under sustained loading. Various investigations have indicated that rock salt exhibits a ductile behavior below its brittle (damage initiation) threshold which results in inelastic deformation (without failure).
The ductile behavior of rock salt is particularly important when considering long term conditions around underground excavations. Numerous studies has been devoted to better understand this long term behavior and overall integrity of underground salt caverns (e.g. Vouille et al., 1993; Souley et al., 2008; Bérest et al, 2013; Warren, 2017). Salt caverns (and other openings in rock salt) have generated an interest regarding oil and gas storage and the long term disposal of radioactive waste due to the very low permeability of halite and its intrinsic healing capabilities (Bérest et al., 1996; Du et al., 2012; Moghadam et al, 2013). For instance, the waste isolation pilot plant (WIPP), located in New Mexico, USA, has generated extensive research results that have led to the development of advanced constitutive modelling concepts (e.g. Munson and Dawson, 1979; Munson, 1997), and sampling and characterization techniques (Brush, 1990).
ABSTRACT: Spatially broad surface deformation was detected over the Leeville underground mining complex, which includes dewatered stoping operations of Carlin-style hydrothermal gold deposits in northeast Nevada. Geologic, groundwater, and extraction data were reviewed to form hypotheses of the causes and controls of the deformation, which were poorly understood. It was hypothesized that the hydrothermally altered materials hosting the orebodies form controls on ground behavior because they affect the mechanical properties of the rock mass. To investigate this hypothesis, 3-D material domains of altered and unaltered materials were needed. Because no dataset on alteration intensity was available covering the entire project area, multiple datasets were utilized, including data mining of the drill dataset to produce consistent alteration intensity rankings, density measurements, density interpreted from gravity surveys, and geologic interpretations. These datasets were used to delineate regions of the rock mass with styles of alteration that result in weaker and less stiff materials, and these 3-D geometries were subsequently used to designate material domains in a numerical model. This case study demonstrates a methodology for amalgamating multiple datasets to increase data coverage and improve confidence in the spatial modeling of geomechanical domains. The methods developed here would likely have application at nearby sites and in areas with similar geologic conditions for numerical model geometry-building.
Surface deformation can be produced by both underground mining or groundwater extraction, and subsurface geology can form controls on its expression in multiple ways. The mechanical properties of intact rock are generally a function of lithology and subsequent alteration processes, and consequently these factors can influence ground behavior when a rock mass is subjected to mining or groundwater pumping.
A broad, irregular shaped deformation pattern was identified by InSAR methods in the region overlying the Leeville underground mining complex, which includes extraction of multiple Carlin-style sediment-hosted gold deposits using longhole stoping with backfill and cut and fill mining methods. Cumulative InSAR displacements measured between 2004-2010 are shown in Fig. 1.
ABSTRACT: In this study, a step-by-step methodology on how to simulate sparsely to moderately fractured, highly interlocked rockmasses is demonstrated, depicting the necessary steps from the discontinuity data collection stage to the rockmass scale numerical modelling. For this purpose, the first step involves the use of LiDAR to collect field joint data in order to determine the geometrical parameters of discontinuities (orientation, trace lengths, density, intensity, persistence etc.) for tunnel excavations. Next, geometrical modelling of the rockmass using discrete fracture networks (DFNs) is conducted in order to create a realistic geometrical representation of the discontinuity network based on the field observations or the LiDAR acquired data. The DFN is finally integrated into the Irazu and UDEC codes utilizing a hybrid finite-discrete element method (FDEM) and the distinct element method (DEM) respectively. Highlighted within the paper is the innovative use of these numerical tools and methods in order to simulate excavations within rockmasses in a practical and realistic manner. Results of the present study show the significance of joints in hard rocks under high stresses and the need to be included explicitly in the simulation.
Various factors including the nature of the intact rock material, the presence (or not) of natural discontinuities, the joint network geometry, and the specifications and requirements of a project (near ground surface or deep excavation, slope stability etc.) affect the material behaviour and its response during the construction process.
Analytical solutions, semi-empirical and empirical approaches, classification systems, and continuum numerical techniques have been widely utilized in assessing and evaluating the behaviour of rockmasses under various conditions in order to assist in the geotechnical and geological engineering design. The aforementioned techniques have proven their value in the rock mechanics realm when they are applied correctly. However, analytical, semi-empirical and continuum numerical approaches are subjected to specific limitations under extreme anisotropic stress or joint conditions within massive, sparsely and/or moderately fractured rockmasses. Within such materials, the presence of nonpersistent discontinuities and rock bridges govern the material behaviour and as such, they need to be taken into account explicitly in the modelling and design process.
ABSTRACT: During deep tunneling or mining infrastructure development, the assumed stress state has significant implications on geomechanical design. Remote measurement of the three-dimensional stress state at depth has proven to be a significant challenge and is often assumed using historic tests or implied from the regional tectonic setting. To date, borehole breakout analysis has provided some assistance for the orientation of the stress field, although very little has been done to correlate breakout with stress magnitude. This paper proposes a methodology of predicting stress from acoustic televiewer surveys of breakout geometry. Data from two boreholes at KGHM's Victoria Project in Sudbury, Canada are used to demonstrate the methodology. From material testing and breakout shape characterization, a continuum based, numerical back analysis of breakout was done through the creation of a generic database of stress dependent numerical models. When compared with the in situ breakout profiles, stress magnitudes were estimated as a function of depth along each hole. Results of this analysis relate well in terms of both magnitude and orientation of stress, as compared to other measurements within the Sudbury mining district and regionally throughout the Canadian Shield.
In the field of geomechanics, one of the most important considerations during design is the state of stress that exists at the location of a project. Despite this, stress is often the most poorly understood site characteristic, given the current challenges in accurately measuring it. This stems from the fact that stress can't be directly measured, but must be inferred by disturbing the rockmass and recording its response. Although some methods do exist for the prediction of in situ stress, this only provides a point estimate and is often plagued with uncertain results and practical limitations in the field. This is of particular importance as mining companies look to develop deeper and more technically demanding deposits. In these high-risk settings, being able to predict the magnitude of overbreak or ore dilution has direct ramifications throughout a mine's life, from pre-feasibility mine design studies all the way through to site reclamation.
Shi, Wei (Dalian University of Technology) | Tan, Xiang (Nanyang Technological University) | Zhou, Li (Jiangsu University of Science and Technology) | Ning, Dezhi (Dalian University of Technology) | Karimirad, Madjid (Queen's University)
The ice loading process has a clear stochastic nature due to variations in the ice conditions and in the ice-structure interaction processes of offshore wind turbine. In this paper, a numerical method was applied to simulate a monopile fixed-bottom and a spar-type floating wind turbine in either uniform or randomly varying ice conditions, where the thickness of the ice encountered by the spar were assumed to be constant or randomly generated. A theoretical distribution of the ice thickness based on the existing measurements reported in various literatures was formulated to investigate the response characteristics of the monopile wind turbine and spar wind turbine in such ice conditions. The effect of the coupling between the ice-induced and aerodynamic loads and responses for both operational and parked conditions of the rotor was studied. Moreover, the dynamic response of wind turbine in randomly varying ice was compared and verified with that of the wind turbine in constant ice.
So far, more than 80% of the energy all over the world comes from fossil fuels. Excessive and improper use of fossil fuels has caused climate change and threatened human security and development. The Paris Agreement, which entered into force on 4 November, 2016, is a major step forward in the fight against global warming. Due to severe smog, forty Chinese cities reel under heavy air pollution. Air pollution becomes one of the key words in China in 2016 (PTI, 2016). Renewable energies play an important role for reducing greenhouse gas emissions, and thus in mitigating climate change. Offshore wind energy is recognized as one of the world's fastest growing renewable energy resources. By the end of 2015, totally 12,107 MW of offshore wind energy was installed around the world according to Global Wind Energy Council (GWEC) report (Fried, 2016). In Europe, 3230 turbines are now installed and grid-connected, making a cumulative total of 11,027 MW (Ho, 2016). However, governments outside of Europe have set ambitious targets for offshore wind and development is starting to take off in China, Japan, South Korea and the US. The 1.2 GW of capacity installed in Asia as of the end of 2015 was located China and mainly in Japan.
The present study is devoted to the flow peoperties of a vortex shedding system behind a bluff body that has a crescent cross-section, facing downstream of the flow. The investigation is conducted numerically using a lattice Boltzmann method. As a benchmark validation, the studied system was compared to the system of vortices formed behind the classical case of a circular cylinder for the same flow configuration and dynamic conditions, while varying the Reynolds number. The flow simulations have revealed that the crescent body affected substantially the shedding mechanism with remarkable differences when compared to the classical cylindrical case. There was a clear decrease in the shedding frequency with the crescent body that is beneficial when using this system as a metering tool.
The vortex shedding flow system had, and still has, a lot of interest from researchers in fluid dynamics due to its many applications for flow metering purposes, for studies of vortical flows or as a benchmark case for numerical methods (Tuann and Olson, 1978; Loc, 1980; Coutanceau & Defaye, 1991; Lakshmipathy, 2004; Reich et al., 2005). The Von-Karman vortex system, known as vortex shedding, occurs when a flow passes around a bluff body, classically cylindrical. To investigate different aspects of the vortex shedding, bluff bodies with different shapes were studied both experimentally and numerically.
The flow behind a circular cylinder is classicaly supposed symmetric at low Reynolds numbers in the range of 70 to 100. When Reynolds number increases, the flow begins separating just at the downstream edge of the cylinder and forms an alternating system of vortices with a constant shedding frequency. This phenomenon has been used as means to measure flow rates with a high accuracy due to its low cost and low maintenance, and being not sensitive to physical properties of the fluid flows measured. Therefore, this metering method has been used in several industries to measure flow rates of liquids, gases and steam flows over a large interval of Reynolds numbers. From the fundamental point of view, this flow system was investigated numerically by many authors. One of first authors was Payne (1958) who studied this flow system for Reynolds numbers below 100 and characterized in details the shedding system in this flow rates range.
ABSTRACT: Understanding the behavior of a rock block after detachment is essential to effectively control rockfall hazards and implement protection measures. Rigid body rockfall models allow a more realistic interpretation of rockfall events by representing rock with a real size and geometrical form. This study presents the analysis of potential rockfalls that threaten the Sumela Monastery, Turkey, which is one of the important historical and touristic places. The structure of the Monastery is located in a rocky steep cliff, surrounded by a highly fractured surface of the cliff from top. The structure has been subjected to several damages due to rock detachments from the cliff surface in the last couple of years and is considerably threatened by potential rock blocks that can fall. Field studies and discontinuity surveys are performed to determine the potential rockfall source areas. The geomechanical properties of the rock materials are determined from laboratory tests. A LIDAR measurements as well as aerial photos are used to prepare slope profiles. According to previous rockfall evidences, back analyses are performed to identify the slope surface characteristics like restitution coefficients. For each profile, different rock size and shapes are simulated and corresponding runout distance, bounce height, kinetic energy and translational velocity are recorded. It is found that, since the structure is located in an almost vertical cliff with considerable height, in all cases, the detached blocks hit the structure with large peak kinetic energy values.
Rockfalls are spontaneous rapid phenomena with high-energy bearing features in hilly regions, which cause significant threats to the environment as well as human lives and property. A rockfall occurs when rock boulders detach from their original locations following four basic types of motions: free-falling, bouncing, rolling and sliding. Rockfall kinematics and dynamics mainly depend on the topography, block weight and geometry, mechanical properties of the slope forming material such as friction angle, roughness, restitution characteristics and rolling resistances (Azzoni et al. 1995, Dorren & Seijmonsbergen 2003). When a rock-fall event reveals a threat to people or structures, it is essential to describe the trajectory of the falling rock along a slope in order to design and implement protection measures or to prevent hazard by an appropriate land use planning. The accuracy in the estimation of the trajectories and motion of the potential falling rocks forms the basis of a safe design and verifies the protective measures.