Tsuboi, H. (Fudo Tetra Corporation) | Fukada, H. (Fudo Tetra Corporation) | Ootsuka, M. (Fudo Tetra Corporation) | Nitao, H. (Fudo Tetra Corporation) | Isoya, S. (Fudo Tetra Corporation) | Higashi, S. (Fudo Tetra Corporation) | Kusakabe, F. (Fudo Tetra Corporation) | Matsui, T. (Fukui University of Technology)
The jet grout mixing methods are classified as one of deep mixing methods widely used in Japan. The merits of these methods are that the equipment is extremely compact and can therefore operate even at small sites. The authors have developed a new type method which is called the FTJ method, combining blades mixing and jet-grout techniques, using a twin-jet horizontal injection system. In this method, two lines of horizontal jet injection have adopted and enable the higher-speed implementation and the larger diameter with cement-improved soil columns, comparing with the conventional one line of horizontal jet injection. In this paper, they investigate the quality characteristics of this method, based on the monitoring data at some sites, focusing on strengths and diameters of improved soil columns. As the results, factors influencing to the improvement diameter and strength were analyzed, followed by confirming their effect through field data. INTRODUCTION The deep mixing methods have widely been used for ground improvement in Japan including high-pressure injection methods. In these methods the impact of high-pressure jets is used to cut through and break up the ground. The broken-up section is then charged with stabilizing agent, and the loosened soil is mixed with stabilizing agent, followed by forming an improved high-strength body in the ground. High-pressure injection improvement methods are widely used on sites adjacent to existing structures because the equipment is compact and the work can be undertaken in confined locations. The authors have developed a new high-pressure injection method called the FTJ method, which combines mechanized mixing and grout injection techniques, using a twin-jet horizontal injection system. By modifying the conventional single-jet horizontal injection to twin-jet one, this new method makes possible more efficient, larger-diameter implementation. In this paper, the authors investigate the quality characteristics of the FTJ method, based on the monitoring data at some sites, especially focusing on strengths and diameters of improved soil columns.
Impact forces of highly nonlinear waves are one of the important factors in ocean structure design. However, they are difficult to estimate due to the complex interaction behavior. In this study, the numerical program based on N.S. equations is developed to investigate impact forces of steep waves generated by a numerical wave maker. The complex geometry of interaction is modeled by multi-block grid. The numerical simulation has been carried out by varying time step and grid size. The numerical results show much less values than experiment. From the analysis, it is concluded that impact forces vary quite severely regardless of wave breaking. INTRODUCTION Studies of impact forces of freak waves on ocean structures have been introduced by many researchers. Natvig(1994) has pointed out that the ringing of structures can be occurred by nonlinear and un-symmetric freak waves that are impulsive with high frequency components. Impact forces are very difficult to analyze numerically and experimentally because of big magnitude and instantaneous occurrence. The local height function methods are applied in the study to track free surface (Gerrits, J, 2001) Impact forces occur when waves with large incident velocity run against to vertical cylinder. To investigate the influences of incident wave, it is assumed that the waves are in contact with vertical cylinder. It is found that the magnitude of impact forces is function of the squares of incident velocity. THEORIES The non-dimensional continuity and momentum equations for the incompressible viscous flow can be expressed as below where u, v and w are non-dimensionalized by the characteristic velocity Uo, t by the o L/Uo,
The sonic-echo signals of full-scaled drilled shafts embedded into bed rocks were analyzed in terms of the changes of the stiffness ratio between the shafts and the surrounding rocks. Built-in defects simulating the reductions of the cross-section at the soil-rock interface as well as segregation in the middle of the shaft were investigated by the sonic-echo tests. Moreover, the correlations between the reflected signals from the surrounding ground and the initial shear stiffness of the surrounding grounds were obtained from static load tests. INTRODUCTION Sonic-echo test, or impact-echo test is used to evaluate the integrity of drilled shafts. When a stress wave generated by hand-held hammer at the pile head penetrates along the pile and meets different media like defects and embedded ground, it is reflected to the pile head and received by the accelerometer or velocity transducer. The test is also called the low strain test because a small hammer induces a low strain on the pile head upon impact. Hearne, Stokoe and Reese (1981) introduced test procedures and their equipment arrangements consisting of accelerometers on the pile head as well as embedded geophones bonded to rebar cages. Hearne et al (1981) concluded that sonic-echo test is a crude method for verifying pile integrity. O’Neill and Reese (1999) presented some limitations of this test. An upper limit to the depth to which this test with modern equipment is useful is about 20m. They reported that some experts relate the upper depth limit to length-to-diameter ratio and stiffness of the surrounding soil, with a maximum depth-to-diameter ratio of about 30. Wave energy is not likely to be reflected from defects unless the defect is either relatively thick or extends nearly across the entire cross-section of the shaft. Samman and O’Neill (1997) reported an experimental study, in which defects that were about 25 mm thick could not be reliably detected experimentally by this method.
In this study, the dynamic behavior of a moored FPSO subjected to wind, waves and current loadings has been computed over a one year long time period using Direct Simulation Approach (DSA). FPSO displacement and mooring line tensions are compared with measurements leading to reasonably good agreement. The maximum values are extrapolated based on the direct simulation, and compared with the one prescribed for design, thus giving an estimate of the conservatism of the actual design methodology. A general method for deriving simplified Response Surface Models (RSM) is then proposed with aim to study the reliability of the system. INTRODUCTION A structure is traditionally designed by computing its response to extreme environmental conditions defined by a given return period (i.e. 100-year metocean conditions). But it should be kept in mind that the structural response to a 100-year extreme environment will not necessarily have a probability of occurrence of once every 100 years. Thus ideally, the choice of the environmental conditions used for design should also depend on the structural response. In other words, a more rational (and cost effective) design should be based on computing the extreme response associated to a given return period (i.e. 100-year response) as presented by Orsero, Fontaine, Quiniou, ISOPE 2007. However, such a design method requires the knowledge of extreme values and joint probability distributions of the environmental variables (see Nerzic et al., ISOPE 2007), together with the ability to describe accurately and rapidly the structural response. Precisely, it seems that the time has been reached when Response- Based Design is made potentially more efficient thanks to very complex hydrodynamic and structural models with improved computing time. Numerical models are also now better validated by tank tests, even sometimes by in-situ motion & structural measurements (as presented), which provides a better confidence in the calculations.
A frequency domain method to determine the influence of steel catenary risers (SCR) on the first order motions of a semi submersible is described. In this method, risers are represented by damped mass – spring systems in the motion analysis of the floater. This method is applied for a particular test case and validated with time domain coupled analysis. The results of the test case show that the influence of steel catenary risers on the first order motions of a semi submersible can be significant and that the frequency domain method gives very satisfying results, with a relatively short calculation time. INTRODUCTION Deep-water semi submersibles with over fifty steel catenary risers and a total vertical riser load equal to 15 percent of the hull displacement are being built nowadays. Several studies have explored the effect of risers on second order motions, but until now, little effort has been made to study the effect on first order motions. For small numbers of risers, often only the stiffness and pretension of the riser system are taken into account during the wave frequency motion analysis of a semi submersible. However, for a large number of risers it may become significant to include the damping and inertia of the riser system as well. Coupled time domain analysis of the semi submersible and its risers offers the possibility to include these effects, but is very expensive in terms of calculation time and effort. A frequency domain method to deal with the effect of risers would therefore be favourable. This paper describes a numerical study of the effects of steel catenary risers on the first order wave response of a semi submersible. A practical method to include the effect of risers in frequency domain motion analysis is described and compared with time domain coupled analysis.
When gas pipelines go through oceans or rivers, according to KOGAS (Korea Gas Corporation) design criteria, buried pipes are encased in concrete due to heavy water pressure and sudden impacts. Especially, the submarine pipes for HVL (High Volatility Liquid) in the deep-seabed are recommended to be encased in assistant structures. This study includes the behavior of pipelines encased in rectangular concrete which suffer from external pressure according to the depth, as well as the inner pressure generated by fluid inside. Based on classical theory of elasticity, the interface stress for the steel-concrete composite pipeline was defined using results from a FEM analysis. Investigations were done on local deformities of the diameter at each part of the pipelines. After calculation the local deformed diameter by using an existing equation for computing the stress in double walled cylinders, which considers the internal and soil pressure, the rate of diameter change is determined. The results were then compared with the behavior of the pipeline encased in rectangular concrete. Through this study, it was verified that pipelines encased in rectangular concrete are about 50% more effective in stress reduction than pipelines without any complementary structure and almost all the external loads are supported by concrete encasement. This study is an elementary part of the research for a new efficient integrity estimation method of concrete-covered submarine pipelines. With this result, the basic design element needed in designing submarine pipelines with protective structure was found. INTRODUCTION Lifeline systems have fundamental maintenance problems because almost all lifelines are buried underground, or located offshore or in rivers, making access difficult. In addition, damage to a buried lifeline structure can result in the shutdown of the entire system as well as enormous economic and social losses instantly. Currently, stability in design of submarine pipelines is critical issues in the field of offshore and coastal engineering.
Physical parameters characterizing the deformation capacities of high grade line pipes were normally yield ratio, uniform elongation, and strain hardening exponent. The purports to control these parameters were presented in the paper and the demand of line pipes to resist plastic deformation was analyzed. By means of statistics and regression analysis of test data, the relationship between uniform elongation, strain hardening exponent, and yield ratio were developed. INTRODUCTION Physical Significance of Yield Ratio, Uniform Elongation, Strain Hardening Exponent Yield ratio is the ratio between yield strength and ultimate strength. Combined with yield strength, the yield ratio represents the capacity of the material to carry the load from yield initiation to fracture. In a uniaxial tension test, most material can still sustain larger load after initial yielding and show considerable plastic deformation capacity. From the concept of plastic mechanics and material physics, it is the result of strain hardening. If the increase of yield strength caused by strain hardening is greater than the increase of stress caused by the load and the reduction of load-bearing area, localized plastic deformation will not occur and the plastic deformation.will be uniform. However, if the plastic strengthening cannot match with the stress increase, the plastic deformation will be localized. Necking can occur and stress will reach ultimate strength. The plastic deformation can be expressed by elongation δ, including uniform plastic deformation δB and localized plastic deformation δN. The δB represents the total plastic deformation before reaching the ultimate strength and reflects the strain hardening exponent (n) of the material. In the engineering point of view, the strain hardening exponent indicates the materials’ resistance to plastic deformation. It enables the materials to sustain overload and raises the safety margin. In general, the true stress-true strain curve of polycrystal materials after yielding is paracurve.
We numerically analyze the flushing effects and the likelihood of a vertical breakwater consist of immersed water channel and water chamber, originally proposed by Nakamura (1999, 2003, 2005) for the alleviation of reflected waves, as a wave energy extraction measure. As a wave driver, we use the Nervier-Stokes equations and mass balance equation, and the numerical integration of which is carried out based on the smooth particle hydrodynamics with a Gaussian Kernel function. As a water level in front of curtain wall, where an anti-node of standing wave due to partial reflection is located, approaches its lowest level, a unidirectional flow in the water chamber formed by a preceding wave starts to move offshore. Once it exits water chamber, this energetic flow feeds necessary energy into the vortex in front of the water chamber to sustain long enough until next wave comes. Considering the facts that an intensity of the flow absorbed through the immersed water channel is strongly proportional with an extent and strength of the vortex formed on offshore side of front curtain wall and a curved path line of sucked water particles, we can deduce that aforementioned vortex is responsible for the flushing effects of the vertical breakwater consist of immersed water channel and water chamber. It is also shown that net flux through the immersed water channel increases as the mass inflow into a water chamber is getting larger (T=1.4sec, Le =6cm), which also confirm our conclusion. INTRODUCTION Due to the sharp increase in crude oil price and exhaustion of fossil energy such that we have been merely 41 years, 67 years, 192 years away from running out of oil, natural gas, and coal, respectively, the development of alternative, renewable energy is emerged as an urgent task in South Korea, which heavily relies on the imported oil from overseas.
Flaring Shaped Seawall has an excellent checking effect of the wave overtopping due to its deeply curved cross section. On the other hand, an impulsive breaking wave pressure tends to occur on its upper curved face depending on incident wave conditions as well as mound configurations. In this study, a slit type wave dissipating structure, which consists of a row of circular columns, was employed to reduce the impulsive wave pressures on the Flaring Shaped Seawall with maintaining its excellent checking effect of the wave overtopping. A series of hydraulic experiments were carried out to investigate the efficiency of the cylindrical slit wall on reducing the impulsive wave pressures. The wave overtopping rate and the wave reflection coefficient were also measured to confirm the influences of the cylindrical slit on these hydraulic quantities. INTRODUCTION Flaring Shaped Seawall as shown in Fig.1 has an excellent checking effect of the wave overtopping due to its deeply curved cross section (Murakami, et al. 1996). Authors have been improving the seawall cross section to increase its stability against wave actions (Kamikubo, et al. 2000). Depending on a sea bottom configuration or a seawall depth, a rubble mound structure is required under the Flaring Shaped Seawall to keep an appropriate construction space. The mound structure causes a forward tilting of incoming waves on its slope. The waves, that have a steep profile due to mound structures, often bring an impulsive wave pressure, and an appropriate countermeasure has to be applied under certain circumstances to reduce this wave pressure. Kataoka, et al. (1999) proposed a composite wave dissipating works, which consists of both a slit structure inside the curved section and wave dissipating blocks piled up in front of the mound with a low crest elevation, to reduce the impulsive wave pressures acting on the Flaring Shaped Seawall.
The Direct Electrical Heating (DEH) system which has been qualified for flow assurance of subsea pipelines is now in use on several pipelines on the Norwegian Continental Shelf. Electrical heating has proven to be an attractive alternative for preventing plugs (hydrate) and wax deposition compared to the traditional methods that use chemical inhibitors, which are expensive and represents a risk to the environment. For design and rating of the DEH system, the supply current, the power loss (heat generation) in the pipeline and the total system impedance are the governing parameters. They are calculated by a computer program based on finite element (FE) analyses. Electrical and magnetic properties of steel pipe materials are essential data for determining heat generation and efficiency of the DEH system. These data are not available from the pipe manufacturers and have to be determined by measurements. Experience has shown that between individual pipejoints the resistivity is approximately invariant, but the magnetic properties may vary significantly even in pipejoints from the same production batch. A difference in magnetic characteristics of pipejoints results in a variation of the heat generation. Measurements of impedances have been carried out on a large number of pipejoints in a laboratory setup by supplying current directly to the pipejoints. A comparison is then made to FE simulations calculated with both linear and non-linear magnetic steel materials assuming eddy current losses only. FE simulations show good accordance for 13Cr steels (Steel material of 13% chromium content). For carbon steels the calculations deviate from the measured values. Modification of the simulation model is required in order to achieve acceptable agreement for both the system resistance (power loss) and the system reactance when using carbon steels. This will give reliable data for rating the topside and subsea DEH equipment.