In seismic full waveform inversion, regularization or preconditioning have been exploited to stabilize the inverse problem. Isotropic smoothing is one of the easiest way to implement regularization in full waveform inversion. However, the isotropic smoothing has a drawback that it blurs layer interfaces such as faults, sedimentary layers and salt boundaries. Consequently, the application of a structure-oriented regularization based on geological information is required to recover accurate subsurface structures. In this paper, we propose an adaptive structureoriented preconditioning using diffusion tensor and weighted semblance. Our preconditioning algorithm is to apply anisotropic smoothing based on structural information obtained from migration image. By using weighted semblance, structure-oriented smoothing is much more effectively conducted. We apply the proposed structure-oriented preconditioning to the full waveform inversion of the Marmousi and SEG/EAGE salt models. The structure-oriented smoothing filter is periodically and adaptively updated as the inversion progresses. The inversion result shows that the layer interfaces are faithfully preserved and sharpened.
Presentation Date: Monday, October 15, 2018
Start Time: 1:50:00 PM
Location: 207C (Anaheim Convention Center)
Presentation Type: Oral
The analysis of the propulsion performance considering the ship motion in waves is an important factor for the efficient operation of a ship. The interaction between the propeller and the free surface due to the ship motion in waves has a great influence on the propulsion performance. However, most of recent researches about the hydrodynamic performance of ships in waves focus on the added resistance, and the experimental or numerical data on the propulsion performance considering the ship motion in waves is very rare. In this paper, numerical investigation on the nominal wake in regular head waves is performed for a KVLCC2 model ship at a full-load condition. Phase-averaged and instantaneous wake fields during one period are compared with the experimental data measured by SPIV showing good agreement. And the effect of the ship motion on the characteristics of wake field and the propulsion performance are investigated varying the wave length.
IMO(International Maritime Organization) has established the EEDI (Energy Efficiency Design Index) standard in 2013 and has been strengthening regulations on air pollutants that lower the pollutant emission standards such as SOx in ship fuel from 2020. As such regulations have been strengthened, there has been an increased interest in improving the operational efficiency of ships for fuel saving. As a result, studies on the performance of ships in waves similar to actual marine environments have been concentrated.
Previous studies on resistance and propulsion performance of ships in waves have been performed mainly in terms of added resistance. However, the EEDI formula for improving the operational efficiency of a ship includes weather factors(fw). The weather factor is determined by propeller propulsion performance as well as added resistance (Jung et al., 2017). Therefore, it is essential to analyze the flow phenomenon on the propeller plane in terms of propulsion performance for a successful prediction of the speed performance of a ship operating in waves. In case of ship operating in waves, the variation of dynamic position of the ship by vertical motion is predicted and, especially, the propeller submerged depth changes according to the behavior of the stern part. According to the change of the propeller submergence depth, the interaction between the propeller and free surface can causes some risks, such as air ventilation and surface piercing running (Paik, 2017). Therefore, it is necessary to clarify the flow phenomenon on propeller plane considering the vertical motion of the ship in order that analyze accurately the propulsion performance of the ship.
Usually, the lifetimes of geogrid are assessed as the long-term creep behavior which causes shape deformation and collapse of the slopes and embankments. During an earthquake, the structure is subjected to additional loads, which may influence the creep characteristics of the reinforcement. The SIM (stepped isothermal method) test provides an opportunity to study the effect of simulated seismic events or the influence of other additional loads, occurring at different intervals of the life of the structure, on the long-term strength of geosynthetic reinforcement. In this paper, two simulated seismic event related to SIM tests were performed, one with a simulated seismic event at 23°C step, the other tests carried out after 79°C step. Creep strain decreased after seismic event cause of recovery force, then strain increased again. After same conditions of seismic event in different times were applied, strain finally overlapped.
For assessing the long-term tensile deformation of geogrid, 10% creep strain has been used as critical value but, there is no basic theory or empirical data to 10% creep strain (Cho, Lee, Cazzuffi, Jeon 2006; Farrag, Shirazi, 1997;Farrag 1998). In real 10% is relatively big one that of allowable long-term strain in reinforced earth wall. Another criteria for creep related properties of geogrid, is creep rupture strength (Allen, 2005; Koo, Kim 2005). Creep rupture in geogrid shows brittle tendency because of rapid loading rate in test procedure. Besides each improper aspect, creep factors for long-term allowable strength from each criterion are different each other. Also these 2 characteristics never are able to explaining the long-term deformation of geogrid. So it is required that the replacement method to explain the long-term deformation (Jones, Clarke 2007). The isochronous creep curve was used to define the relation between creep strain and allowable strength. In the isochronous curve at given time, we can read the allowable strength at allowable creep strain. The allowable strain gets from specification by directors or manufacturers. The allowable creep strain can be various according to its facing batter, facing type and critical aspect. Otherwise, the required service lifetime of geogrid used for reinforcement of soil structure varies according to the sensitivity of the environmental conditions (Den Hoedt, 1986; Hsieh, Wu, Lin, Hsieh, 2000). This service lifetime implies that the functional engineering properties of the geogrid should remain within acceptable limits during the required service life. Usually, the lifetime of geogrid are assessed as the long-term creep behavior which causes shape deformation and collapse of the soil structure (Tatsuoka, Kongkitkul, 2007). In this study, the creep behavior of geogrid was evaluated by the stepped isothermal method (SIM). For the engineering design perspective, the creep reduction factor was determined from the creep rupture and limited strain. As an accelerated creep test, the SIM test provides an opportunity to study the effect of simulated seismic events or the influence of other additional loads, occurring at different intervals of the life of the soil structure, on the long-term strength of geogrid. Two simulated seismic event related to the SIM tests were performed, one with a simulated seismic event at 23°C step, the other tests carried out after 79°C step. The reason for varying the time of application of the simulated seismic load was to study the effect of the timing of real-life earthquakes. The second was to quantitatively calculate creep reduction factor considering seismic event and to reflect this in the design property.
Kim, Gvan Dek (Seoul National University) | Choi, Jaeho (Seoul National University) | Lee, Kyungbook (Korea Institute of Geoscience and Mineral Resources) | Shin, Hyundon (Inha University) | Choe, Jonggeun (Seoul National University)
In this paper, the selective use of measurement data using Ensemble Smoother is suggested in order to improve its performances by reducing the possibility of misuse of observed data. Key idea is that observed data are selected out on the basis of water breakthrough for better reservoir characterization. We use oil production rates before water breakthrough and water cut rates after water breakthrough for each well because ES cannot interpret the physical characteristic of water breakthrough properly. The consequence is that the proposed method gives us the best reservoir characterization of results with clear channel patterns and connectivity.
Reservoir characterization is one of the most important things for decision making in petroleum engineering. The way to make reliable and proper reservoir models is using static and dynamic data. Prior reservoir models made by using static data only have high geological uncertainties. In order to reduce these uncertainties, history matching is applied to integrate dynamic data, but the uncertainty range might be still high due to modelling error, limited data, or measurement error. Therefore, uncertainty quantification is vital for future performance. To improve performance estimation, many ensemble members are very often utilized to various reservoir characterization methods. The process is called ensemble-based reservoir characterization.
Evensen (1994) offered Ensemble Kalman filter to ocean dynamics. In case of reservoir engineering, EnKF was introduced by Naevdal et al. (2002). EnKF has many advantages such as uncertainty quantification in predicted productions, real-time updating of observed data, easy coupling with any forward simulator, and flexibility for types of model parameters and observed data.
However, EnKF has two critical limitations: overshooting and filter divergence (Aanonsen et al., 2009; Jeong et al., 2010; Oliver and Chen, 2011). These problems occur when model parameters do not follow Gaussian distribution or initial ensembles are not reliable and quite different from the true model. The importance of overcoming EnKF demerits was described by many researchers. However, most of the proposed methods still take high simulation time and have the restart option of a forward simulator due to recursive update of EnKF. Van Leeuwen and Evensen (1996) applied Ensemble Smoother (ES) for meteorology and compared EnKF with ES for history matching. Skjervheim et al. (2011) first proposed ES to reservoir characterization. They suggested that ES showed quite reliable results compare to EnKF. ES is very fast and simple because it assimilates all dynamic data at once, simultaneously. Also it is easier than EnKF for coupling with any reservoir simulator since it doesn't need restart option. However, it is still unstable and exposed to the possible overshooting and filter divergence.
While operating on the Arctic route, ships may face various issues. The ice environment, such as level ice, pre-sawn, pack ice, ice ridge and brash ice, is one of the sources of those issues. Prediction of ship resistance in brash ice is very important for safe operation. There are three ways to estimate the ice resistance: using a mathematical model, numerical simulation, and using empirical formula. In this paper, empirical formulas are used. The main aim of the study is to develop a computer program (I-RES) for prediction of attainable speed in brash ice and for ice resistance estimation. To achieve this goal, first, the brash ice environmental characteristics were analyzed. The results of I-RES were evaluated by comparing with the model test results of brash ice. The accuracy of I-RES calculations was found to be around 5%.
As global warming reduces Arctic sea ice, Russia's Arctic resource development is taking place in earnest. In recent years, Russia has successfully built up Yamal LNG vessels. Interest in the Arctic route has been increasing as a result of the use of the Arctic Sea as a means of shipping and transportation, which saves time and money compared to the existing Suez Canal. Ships operating on the Arctic route are exposed to various ice conditions such as collision with ice and friction. For this reason, it is important to determine the engine power at the initial stage of the ship design, because the ship operating at the Arctic route has a larger hull resistance. For this purpose, research is being conducted in various ways including analytical methods and model tests. The Arctic sea routes have various types of sea ice such as brash ice, which is formed by overlapping small ice, level ice which is frozen flat in a large area, pack ice where ice pieces of different sizes float, ice ridge which is formed by overlapping ice and flat ice. Since ice resistance has a very different value depending on the type of ice, it is important to establish a method for estimating the ice resistance accordingly. The method of estimating the ice resistance includes a mathematical model, a method using a simulation, and use of empirical formulas. The method of using a mathematical model and the method using simulation has an advantage that relatively accurate results can be obtained and the result analysis is also easy. However, these methods are time-consuming to define the shape and characteristics of ships and ice. Therefore, in this study, an empirical formula that can estimate ice resistance in a short time (Kim et al., 2015) was used to estimate ice resistance. The purpose of this study is to expand the application range of ice resistance and to adopt the safe speed estimation program for the brash ice, which was first developed for the level ice. The process of determination of the attainable speed from the estimated ice resistance and calculated engine power was summarized. Engine power was determined from the characteristic curve derived from the relationship between the engine and the propeller. To verify the accuracy and validity of the results of previous studies, we compared the model test results in level ice, pre-sawn, pack ice environment with the I-RES program results. The ice resistance estimation of brash ice developed ice resistance estimation algorithm through the environmental characteristics analysis. Also, the velocity estimation algorithm of brash ice using empirical equation is verified by comparing with the model test. The I-RES program, which has been supplemented with the proven algorithm, can be used to determine the maximum engine power of a ship working at the Arctic route.
There exists high uncertainty in modelling an interbedded shale barrier in heterogeneous oil sands reservoir, which causes a critical impact on SAGD performance. At present, despite the rise of research on shale barrier issues, few studies have attempted to address the prediction of shale barrier size except for 4D seismic data interpretation. In a shale interbedded oil sands reservoir, the SAGD production data such as the oil rate and steam-oil ratio (SOR) increase up to a peak point and decreases down to a trough point again, and the points are named inflection points (IP).
In this study, proxy models were developed for predicting the shale barrier size using reservoir parameters and the IP of SAGD production data. In addition, a field application was carried out for verification of the developed proxy models using the data from Suncor's Firebag SAGD project, Alberta, Canada.
The results of the screening analysis showed that the key parameters affecting IP of the SAGD production data were the reservoir thickness, horizontal permeability, vertical-horizontal permeability ratio, oil saturation, shale width (WD), shale length (LN), and vertical location of the shale (VL). The key parameters were selected and multiple simulation cases were generated using a latin hypercube sampling. As a result, the applicable proxy models for predicting WD, LN, and VL were developed by statistical analysis of the reservoir parameters, shale size and location, and IP. The statistical results of the proxy models show that the adjusted R-squared and predicted R-squared are high.
As a verification procedure, 3-D simulations were performed to reconstruct the predicted WD, LN, and VL, and the oil production profiles of the field well pairs were compared with the simulation results of the reconstructed shale barrier.
The oil production profiles show the similar trend between the simulation results of the reconstructed model and the field SAGD production data. Therefore, the developed proxy models can predict the shale barrier size properly, and may be useful for the field application.
Wave-equation-based refraction traveltime tomography estimates near-surface velocity more accurately than ray-based method does in that the former does not suffer from shadow zone problem. In this study, we address that the wave-equation-based refraction traveltime tomography is applicable to velocity model with irregular topography. In order to describe the irregular topography, we use the finite-element method based on unstructured mesh. The wave equation is solved in the frequency domain to produce strongly damped wavefields. Then, traveltimes can be obtained from the calculated damped wavefields. The proposed traveltime calculation algorithm is robust even for unstructured mesh thank to the damped wavefields. Hence, the refraction traveltime tomography is easily implemented. We use the nonlinear conjugate gradient method for the inversion and precondition the gradient direction by the diagonal elements of approximate Hessian for better convergence. Also, smoothing regularization is added to overcome the ill-posedness of the tomography problem. Through a synthetic data example, it is shown that the first-arrival traveltimes obtained from the true and inverted velocity models are nearly identical. We, also, assessed the applicability of our tomography method for field data. The proposed algorithm is expected to be applicable to static correction which is necessary for land seismic data processing.
Presentation Date: Thursday, September 28, 2017
Start Time: 10:35 AM
Presentation Type: ORAL
Water inflow into the pit is a crucial problem in open pit mine operation when it is constructed below the water table. In this work, inflow rate into the pit mine is simulated by using MODFLOW codes. The model is assumed to be an equivalent porous medium and created in step excavation method. The result from the simulated model is verified by comparing with the analytical solution (Thiem-Dupuit assumption). During model creation, it is observed that Grid size plays an important role in inflow prediction. Model started with very finer size of grid and gradually increased the size to determine the effect of grid size on inflow rate. With the larger grid size, MODFLOW can predict inflow rate much better than the smaller one. At the same time, model boundary up to the ROI (Radius of Influence) gives better results. The inflow rate achieved by the simulated model is very close to the analytical solution (Error rate obtained approximately 0.3%). Several precautions are made to achieve a highly efficient result. Sensitivity analysis is also done for the simulated model with factorial design. A full factorial design is made by considering head, conductivity, drawdown, pit length and grid size as influencing factors. Among all these five factors, conductivity acts as the most significant factor, responsible to influence the inflow rate most.
Computer simulation model increases the ability of understanding hydrogeology of an open-pit mine. It also aids fast analyzing various geological properties of underground and open-pit mine. Model helps to reconstruct known groundwater head distributions and flow rates. Model identifies hydrogeologic parameters, which can influence groundwater flow path and helps to predict the future flow paths. When simulating hydraulic flow into an open pit mine in relatively flat landed region, water is supposed to flow into the pit from all directions. To create this situation, a higher head value is assigned at the boundary and a relatively smaller head value is assigned at the pit seepage face to make the flow towards the pit maintaining a standard linear gradient. Fig. 1 represents the 2D mesh of the model area and AA' line passes through the region.
Flow stress beyond onset of necking is necessary for simulations of ship-to-ship collisions, ship-to-rock strandings, and hydrocarbon explosions of oil and gas platforms. But it is known that flow stress beyond necking is no more obeying power law of plastic flow because uniaxial stress state at necked section is changed to triaxial one and flow stress becomes nonlinear in logarithmic scales of flows stress and plastic strain. In order to experimentally verify flow stress versus plastic strain in large plastic strain regime, average true stress are obtained from a series of tensile tests for round bar specimens. Logarithmic plastic strain data are also obtained by measuring reduction of cross section using DICAs (digital image correlation analyses). The constructed average flow stress data versus logarithmic plastic data, which are fully experimental ones, are compared with estimated flow stress based on three constitutive equations (Hollomon, Ludwik, and Swift). It is proved that Bridgman stress correction reduces the experimental average flow stress data, and thus it is between experimental one and estimated one by Hollomon. Parametric study by changing hardening exponents, a best fit value of the hardening exponent with experimental ones is presented. The correction formula is proposed based on best fit value of the hardening exponent with experimental ones. Predicted flow stress using the new formula well forecasts other experimental results on notched specimen tensile tests by Choung (2011; 2013).
The stress versus strain curve for material is necessarily required in order to carry out numerical analyses for large strain and fracture problems. Tensile tests are commonly used for obtaining various material property such as initial yield strength, ultimate strength, plastic hardening exponent, strength coefficient, etc. Uniform true stress versus strain curves estimated from load-elongation obtained tensile test and extensometer is valid until uniform deformation, it cannot use after necking. Flow stress beyond necking has been calculated using various power law. But it is known that the stress in large strain is no more obeying power law because minimum cross section of specimen experience triaxial stress after necking and flow stress is nonlinear in logarithmic scales. The true stress after necking cannot be obtained directly from tensile test due to triaxial stress, Bridgman equation is widely used for correction of true stress-logarithmic true strain curve with round cross section.
In order to obtain the true stress versus strain curves, specimens with circular and rectangular cross section are commonly used. Mirone (2004) presented a new material independent solution of the necking problem based on experimental and numerical observations for round bar specimens. Choung et al., (2011) carried out simulation for round specimens with various radii, compared simulation result with tests one. Zhu et al. (2015) proposed evolution method of true stress-strain curves in plastic zone based on 3D-Digital image correlation (3D-DIC) measurement system and electronic universal testing machine, two uniaxial tension tests for carbon steel round specimens. In addition, many researchers use round specimens for tensile test (Kamaya and Kawakubo, 2011; Joun et al., 2008).
Capsize and sinking of a coastal car ferry has occurred in a Korean offshore area and has caused hundreds of human casualties. The rapid turn and improper cargo loading are inferred as the main reasons of the accident. It has motivated to develop a new system of cargo securing with improved safety of Korean coastal ferries. This paper provides a new approach regarding cargo securing safety assessment which is purely based on force equilibrium conditions, because IMO CSS is suitable for the ocean-going vessels. The mathematical formulations are presented for the new approach. This paper also introduces a newly developed safety assessment system based on the new approach. Most outstanding features are that it can utilize acceleration data produced from hydrodynamic motion analyses or assumed maximum extents of ship motion components and that securing safety assessment is simultaneously possible for unlimited number of cargoes with finite number of lashings.
A sinking accident of a Korean coastal car ferry has occurred in 2014. It has motivated to review importance of ship stability with cargo securing. It is announced that general cargoes and cars loaded inside of the ship did not use lashing equipment as well as extreme heel has been assumed one of the main reasons of rushing cargoes that were not secured.
Unlike Car ferries, it is well defined that lashing force of containers for full container ships in Rules of Class (GL, 2013). Hwang et al. (2004) studied for fixing kinds and size of lashing equipment for containers, Shin & Hwang (2014) performed optimization of container stowage in container ships.
On the other hand, it is not many that lashing for a variety of cars including cars, trucks, buses, etc. Turnbull & Dawson (1997) suggested mathematical models for calculating lashing force for trailers loaded on car ferries. DNV developed cargo securing evaluation system based on IMO (international maritime organization) CSS (code of safe practice for cargo stowage and securing) (IMO, 2011).
The Force acting on a shipped cargo in a sailing ship is classified static force with dynamic force. It is known that rolling caused the most inertial force one of the dynamic inertial force based on ship motion pitching, heaving as well. IMO CSS (2011) suggests a procedure for verifying lashing equipment safety. However, it is not reasonable for car ferries sailing in Korean coastal area since it does accept inertial force based on sailing in ocean going. Coastal car ferries experience less acceleration than ocean going ships.