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Proceedings Papers

Publisher: Society of Exploration Geophysicists

Paper presented at the 2012 SEG Annual Meeting, November 4–9, 2012

Paper Number: SEG-2012-1030

... we propose a

**double**-**square****root**traveltime approximation for converted waves from a curved interface. This approximation is appropriate for large offsets. In the special case of monotypic waves it gives comparable results with other multi-parameter moveout formulas. Furthermore, we demonstrate that a...
Abstract

SUMMARY Over the past years, multi-parameter stacking has become a standard tool for seismic reflection data processing. Although several traveltime approximations for converted waves may be found in the literature, all of them are designed for a CMP-based observation geometry. In this paper we propose a double-square root traveltime approximation for converted waves from a curved interface. This approximation is appropriate for large offsets. In the special case of monotypic waves it gives comparable results with other multi-parameter moveout formulas. Furthermore, we demonstrate that a CRS-type traveltime approximation for converted waves may be derived from the new approximation. In this context, we also discuss a pragmatic search strategy. The key step of the strategy is the simulation of a zero-offset section by a stack of ?-CMP gathers, which may be considered as a first approximation of common conversion point gathers. For non-converted waves this approach transforms to the well-known pragmatic approach by Müller et al. (1998). The new operator does not require separation of PP and PS wavefields prior to stacking. In our examples, the effective wavefield attributes of converted and monotypic waves obtained by the corresponding type of the operator have comparable values and may be used in joint interpretation.

Proceedings Papers

Publisher: Society of Exploration Geophysicists

Paper presented at the 2013 SEG Annual Meeting, September 22–27, 2013

Paper Number: SEG-2013-1173

... SUMMARY The

**double**-**square**-**root**(DSR)**equation**can be viewed as a Hamilton-Jacobi**equation**describing kinematics of downward data continuation in depth. It describes simultaneous propagation of source and receiver rays which allows computing reflection wave prestack traveltimes (for multiple...
Abstract

SUMMARY The double-square-root (DSR) equation can be viewed as a Hamilton-Jacobi equation describing kinematics of downward data continuation in depth. It describes simultaneous propagation of source and receiver rays which allows computing reflection wave prestack traveltimes (for multiple sources) in a one run thus speeding up solution of the forward problem. Here we give and overview of different alternative forms of the DSR equation which allows stepping in two-way time and subsurface offset instead of depth. Different forms of the DSR equation are suitable for computing different types of waves including reflected, head and diving waves. We develop a WENO-RK numerical scheme for solving all mentioned forms of the DSR equation. Finally the extended exploding reflector concept can be used for computing prestack traveltimes while initiating the numerical solver as if a reflector was exploding in extended imaging space.

Proceedings Papers

Publisher: Society of Exploration Geophysicists

Paper presented at the 2013 SEG Annual Meeting, September 22–27, 2013

Paper Number: SEG-2013-0322

...Exploring imaging capabilities of the extended prestack wave eld Tariq Alkhalifah, Zedong Wu, KAUST, Saudi Arabia, and Sergey Fomel, University of Texas at Austin SUMMARY The dynamic form of the

**double****square**-**root**(DSR)**equation**provides a mechanism to extrapolate prestack wave elds by moving the...
Abstract

Summary The dynamic form of the double square-root (DSR) equation provides a mechanism to extrapolate prestack wavefields by moving the sources and receivers in space with respect to a potential image (scatterer) point. Its classical implementation for imaging often assumes sources and receivers are at the same horizontal surface. Reverse time migration (RTM), as well as common shot migrations in general, through its separate treatment of the sources and receivers, allows for more flexibility in source and receiver configurations. A simple modification to the classical DSR, equation provides such flexibility. Specifically, we define a 7-dimensional prestack wavefield for 3-D media that includes the vertical source and receiver offset. The corresponding dispersion relation can be used to extrapolate such wavefields. However, the cost for such a definition and extrapolation can be prohibitive, considering the high dimensionality of the problem. We reduce the dimensionality by recognizing that the sources and receivers often share the same horizontal plane, and thus, obtain the conventional DSR, formulation. An efficient implementation of DSR in time yields extrapolation speeds that nominally exceed those obtained from reverse time migration. We can also reduce the dimensionality by setting the horizontal offset between the source and receiver at the image point to zero, or using a DSR-like formulation to correct for source-receiver vertical offset or topography.

Proceedings Papers

Publisher: Society of Exploration Geophysicists

Paper presented at the 2013 SEG Annual Meeting, September 22–27, 2013

Paper Number: SEG-2013-0288

..., 2809 2813. de Hoop, M. V., J. H. Le Rousseau, and B. L. Biondi, 2003, Symplectic structure of wave-

**equation**imaging: A path-integral approach based on the**double**-**square**-**root****equation**: Geophysical Journal International, 153, no. 1, 52 74, httpdx.doi.org/101046/j.1365-246X.2003.01877.x. Etgen, J., S...
Abstract

Summary The double-square-root (DSR) relation offers a platform to perform prestack imaging using an extended single wavefield that honors the geometrical configuration between sources, receivers and the image point, or in other words, prestack wavefields. Extrapolating such wavefields in time, nevertheless, is a big challenge because the radicand can be negative, thus reduce to a complex phase velocity, which will make the rank of the mixed domain matrix very high. Using the vertical offset between the sources and receivers, we introduce a method for deriving the DSR formulation, which gives us the opportunity to derive approximations for the mixed domain operator. The method extrapolates prestack wavefields by combining all data into one wave extrapolation procedure, allowing both upgoing and downgoing wavefields since the extrapolation is done in time, and doesn’t have the v(z) assumption in the offset axis of the media. Thus, the imaging condition is imposed by taking the zero-time and zero-offset slice from the multi-dimensional prestack wavefield. Unlike reverse time migration (RTM), no crosscorrelation is needed and we also have access to the subsurface offset information, which is important for migration velocity analysis. Numerical examples show the capability of this approach in dealing with complex velocity models and can provide a better quality image compared to RTM more efficiently.

Proceedings Papers

Publisher: Society of Exploration Geophysicists

Paper presented at the 2012 SEG Annual Meeting, November 4–9, 2012

Paper Number: SEG-2012-0925

... procedures with the survey-sinking or

**double**-**square**-**root**(DSR) formulation of the wave**equation**(Claerbout, 1985; Popovici, 1996; de Hoop et al., 2003) provides a straightforward connection between the image and the data, free of the cross-correlation imag- ing step. A limitation of the DSR formulation is...
Abstract

SUMMARY The double-square-root (DSR) relation offers a platform to perform prestack imaging using an extended single wavefield that honors the geometrical configuration between sources, receivers and the image point, or in other words, prestack wavefields. Extrapolating such wavefields Nevertheless, suffers from limitations chief among them is the singularity associated with horizontally propagating waves. I introduce approximations free of such singularities, and are highly accurate. Specifically, I use Pade expansions with denominators given by a power series that is an order lower than that of the numerator, and thus, introduce a free variable coefficient to balance the series order and normalize the singularity. For the higher order Pade approximation the errors are negligible. Additional simplifications, like recasting the DSR formula as a function of scattering angle, allow for a singularity free form that is useful for constant angle gather imaging. A dynamic form of this DSR formula can be supported by kinematic evaluations of the scattering angle to provide efficient prestack wavefield construction.

Proceedings Papers

Publisher: Society of Exploration Geophysicists

Paper presented at the 2011 SEG Annual Meeting, September 18–23, 2011

Paper Number: SEG-2011-3017

... ABSTRACT Most prestack traveltime relations we tend work with are based on homogeneous (or semi-homogenous, possibly effective) media approximations. This includes the multi-focusing or

**double****square**-**root**(DSR) and the common reflection stack (CRS)**equations**. Using the DSR**equation**, I analyze...
Abstract

ABSTRACT Most prestack traveltime relations we tend work with are based on homogeneous (or semi-homogenous, possibly effective) media approximations. This includes the multi-focusing or double square-root (DSR) and the common reflection stack (CRS) equations. Using the DSR equation, I analyze the associated eikonal form in the general source-receiver domain. Like its wave-equation counterpart, it suffers from a critical singularity for horizontally traveling waves. As a result, I derive expansion based solutions of this eikonal based on polynomial expansions in terms of the reflection and dip angles in a generally inhomogenous background medium. These approximate solutions are free of singularities and can be used to estimate travetimes for small to moderate offsets (or reflection angles) in a generally inhomogeneous medium. A Marmousi example demonstrates the usefulness of the approach.

Proceedings Papers

Budi Priyatna Kantaatmadja, Fadzlin H. Kasim, W. Nur Zainudin, Emad Elsebakhi, Ernest A. Jones Jr, Amita M. Ali

Paper presented at the International Petroleum Technology Conference, March 23–April 1, 2021

Paper Number: IPTC-21436-MS

...) The goal is prediction/classification or understanding which predictors are useful? How and Where is there "interesting" structure? The accuracy for predicting regression (continuous target) is the correlation coefficient (R 2 ), and the expected

**root**mean**squared**error; while the accuracy of the...
Abstract

Predicting permeability in low-medium quality reservoirs (> 10 md to <100mD) is important in brownfields since many of them can still produce hydrocarbons. Developing an approach relating geologic properties to permeability prediction can increase field reserves and extend producing life. The common practice of predicting permeability includes linear regressions of core-porosities vs. core-permeabilities applying different lithofacies. However, these methods discount data scattering around regression-lines. This paper describes an innovative-technique for permeability prediction that combines rock-types, flow-zone-indicator (FZI), and machine-learning techniques (ML). FZI is a reservoir-flow-unit that controls hydraulic fluid-flow and is influenced by pore-geometry resulting from diagenetic-processes. In reservoirs, pore-geometry usually is heterogenous due to mineral-composition, rock-texture, cementation, and compaction. Thus,the commonly used permeability equation of Kozeny-Carman (KC) equation still can be used but it needs to be modified for better connecting FZI to hydraulic-flow-units. The modified KC equation incorporates heterogeneous poregeometry as a non-linear-function of porosity by adding cementation-exponent (m) into the equation, where the original KC equation assumes m is equal to one. The semi-log cross-plot between Reservoir-Quality-Index (RQI) vs. PHIZ*Por (m-1) (or FZI m ) from the modified KC equation can be constructed using rock-type class. The ML approach was applied to predict FZI groups using 4 standard-logs: gamma-ray, resistivity, density, and neutron-porosity. Cross-plots of RQI vs. PHIZ (conventional FZI) can be compared to RQI vs. PHIZ*Por (m-1) (modified FZI model) usingdata from 11cored wells in oil field offshore Malaysia. The modified FZI model shows less data clustering compared to the conventional FZI model, shown by higher R 2 coefficient correlation accuracy. The proposed modified FZI model shows narrower permeability range at low porosity which is a good indication of more accurate hydraulic-flow-unit interpretation. When applying the original and modified FZI models, each lithofacies may occur in more than one hydraulic-flow-unit due to pore-geometry difference within the same lithofacies. Furthermore, the hydraulic-flow-unit generated by the modified FZI model is more sensitive to total porosity when comparing to original FZI model. Each generated hydraulic-flow-unit has better correlation to total porosity and with less scattered permeability at the same porosity. The permeability calculated by modified FZI model was then verified with core permeability showing an excellent overall match. On the ML technique, the "Random Forests" technique will be utilized due to recognized as one of the most recent ML algorithm(s) developed as an innovative technique based on both classifications and regression trees techniques. The Random Forests technique has shown its great accuracy on predictive exactness for these challenge permeability estimations. The prediction quality was benchmark by R 2 value of > 0.9 for all crossplots (porosity, permeability, and water saturation) when comparing to routine core analysis lab measurements.

Journal Articles

Gilson Moura Silva Neto, Ricardo Vasconcellos Soares, Geir Evensen, Alessandra Davolio, Denis José Schiozer

Journal:
SPE Journal

Publisher: Society of Petroleum Engineers (SPE)

*SPE Journal*(2021)

Paper Number: SPE-205029-PA

Published: 10 February 2021

... the ensemble size's

**square****root**because a larger N reduces the sampling errors in the covariance estimation; its drawback is higher computational costs. For a detailed explanation on the threshold estimation, refer to Luo et al. (2018a ) and Luo and Bhakta (2020) . After obtaining the...
Abstract

Summary Time-lapse-seismic-data assimilation has been drawing the reservoir-engineering community's attention over the past few years. One of the advantages of including this kind of data to improve the reservoir-flow models is that it provides complementary information compared with the wells' production data. Ensemble-based methods are some of the standard tools used to calibrate reservoir models using time-lapse seismic data. One of the drawbacks of assimilating time-lapse seismic data involves the large data sets, mainly for large reservoir models. This situation leads to high-dimensional problems that demand significant computational resources to process and store the matrices when using conventional and straightforward methods. Another known issue associated with the ensemble-based methods is the limited ensemble sizes, which cause spurious correlations between the data and the parameters and limit the degrees of freedom. In this work, we propose a data-assimilation scheme using an efficient implementation of the subspace ensemble randomized maximum likelihood (SEnRML) method with local analysis. This method reduces the computational requirements for assimilating large data sets because the number of operations scales linearly with the number of observed data points. Furthermore, by implementing it with local analysis, we reduce the memory requirements at each update step and mitigate the effects of the limited ensemble sizes. We test two local analysis approaches: one distance-based approach and one correlation-based approach. We apply these implementations to two synthetic time-lapse-seismic-data-assimilation cases, one 2D example, and one field-scale application that mimics some of the real-field challenges. We compare the results with reference solutions and with the known ensemble smoother with multiple data assimilation (ES-MDA) using Kalman gain distance-based localization. The results show that our method can efficiently assimilate time-lapse seismic data, leading to updated models that are comparable with other straightforward methods. The correlation-based local analysis approach provided results similar to the distance-based approach, with the advantage that the former can be applied to data and parameters that do not have specific spatial positions.

Journal Articles

Gunnhildur Högnadottir Steinbakk, Lars Holterud Aarsnes, Magne Aldrin, Ole Christian Astrup, Ola Haug, Gaute Storhaug, Erik Vanem

Journal:
Journal of Ship Research

*Journal of Ship Research*64 (03): 266–277.

Paper Number: SNAME-JSR-02190008

Published: 18 September 2020

... by penalized least

**squares**(LS). Here, Fns¼U= ffiffiffiffiffiffiffiffiffiffiffiffiffi g lpps p is the Froude number (g, acceleration of gravity) and**es**is the independent zero-mean Gaussian noise with variance 2s : The variables bs, ds, and tlcs are divided by the ship length lpps to make them...
Abstract

Combined hydrodynamic and structural models are used to simulate structural responses on ship hulls in a seaway for design and risk assessment purposes. From a safety and inspection perspective, there is demand for continuously monitoring the ship hull conditions to estimate the structural utilization in real time. However, setting up the computer models and running the analysis are time consuming and costly, preventing such models from being used operationally. We developed a statistical model that approximates the wave bending moment output from the computer model, by linking the wave bending moment to ship design parameters and environmental information. This statistical model is computationally cheap and much faster to run than a hydrodynamic model, and may thus act as a virtual indicator sensor for structural condition monitoring. The approximated wave bending moment can also be used to compute fatigue damage for a ship as an indicator for crack risk. Although somewhat sensitive to the training dataset, a validation study reveals that the statistical model performs decently well. For fatigue rates, relative errors range from1.4% to 60% for out-of-sample results with weighted least squares, which is deemed acceptable for an indicator model used for screening a fleet of vessels.

Proceedings Papers

Paper presented at the SNAME Maritime Convention, September 29–October 2, 2020

Paper Number: SNAME-SMC-2020-075

...: = , = , (4) The second fundamental form II is computed as follows: = 2 + 2 + 2, (5) = = = (6) where and denote the partial differential

**equations**defined as follows: = 2 2 , = 2 , = 2 2 , (7) where denotes the normal vector at the point obtained by: = , (8) The**roots**of the...
Abstract

We addressed the problem of estimating man-hours to fabricate a curved hull using cold and thermal forming methods and analyzed the current forming methods and identified parameters affecting the forming time. Subsequently, we obtained mathematical formulae that relate geometric parameters (curvedness and area), with a forming parameter (forming time), through experiments and simulations to estimate the forming time when a curved plate is provided as an input. We tested the formulae using examples and analyzed the results to verify their validity. Results show that the derived relations can be used for man-hour prediction of curved hull plates in shipbuilding.

Proceedings Papers

Cenk Temizel, Celal Hakan Canbaz, Ihsan Murat Gok, Shahrzad Roshankhah, Yildiray Palabiyik, Melek Deniz-Paker, Fatma Bahar Hosgor, Hakan Ozyurtkan, Firat Aksahan, Ender Gormez, Suleyman Kaya, Onur Alp Kaya

Publisher: Society of Petroleum Engineers (SPE)

Paper presented at the SPE Latin American and Caribbean Petroleum Engineering Conference, July 27–31, 2020

Paper Number: SPE-198994-MS

... in the upscaled flow model as each type of SRV contributes differently to the overall production. The variation of flow enhancement is apparent in well test analysis as several slopes of pressure versus

**square****root**of time in log-log plot are observed (see Figure 5 ). Suliman et al. correlate the...
Abstract

As major oil and gas companies have been investing in shale oil and gas resources, even though has been part of the oil and gas industry for long time, shale oil and gas has gained its popularity back with increasing oil prices. Oil and gas industry has adapted to the low-cost operations and has started investing in and utilizing the shale oil sources significantly. In this perspective, this study investigates and outlines the latest advances, technologies, potential of shale oil and gas reservoirs as a significant source of energy in the current supply and demand dynamics of oil and gas resources. A comprehensive literature review focusing on the recent developments and findings in the shale oil and gas resources along with the availability and locations are outlined and discussed under the current dynamics of the oil and gas market and resources. Literature review includes a broad spectrum that spans from technical petroleum literature with very comprehensive research using SCOPUS database to other renowned resources including journals and other publications. All gathered information and data are summarized. Not only the facts and information are outlined for the individual type of energy resource but also the relationship between shale oil/gas and other unconventional resources are discussed from a perspective of their roles either as a competing or a complementary source in the industry. In this sense, this study goes beyond only providing raw data or facts about the energy resources but also a thorough publication that provides the oil and gas industry professional with a clear image of the past, present and the expected near future of the shale oil/gas as it stands with respect to other energy resources. Among the few existing studies that shed light on the current status of the oil and gas industry facing the rise of the shale oil are up-to-date and the existing studies within SPE domain focus on facts only lacking the interrelationship between heavy and light oil as a complementary and a competitor but harder-to-recover form of hydrocarbon energy within the era of rise of renewables and other unconventionals. This study closes the gap and serves as an up-to-date reference for industry professionals.

Proceedings Papers

Paper presented at the SNAME Maritime Convention, October 30–November 1, 2019

Paper Number: SNAME-SMC-2019-081

... about the behavior of a system can be made with the minimum use of resources (Telford 2007). Enabling optimization and gaining insight into the behavior of complex engineering systems are major areas of DOE utilization. Although DOE has its

**roots**in physical experiments, physical experimentation is not...
Abstract

A design of experiments approach to set creation is described for implementation of set-based design (SBD). The design points in the experimental design space generated during set creation represent individual design alternatives exhibiting unique characteristics and performance. Exploration and evaluation activities leading to design selection in SBD are based on the feasibility and viability of the design points, so the way they are generated is important. Several Latin hypercube designs are presented and summarized. An application of Latin hypercube sampling is demonstrated as a method of set creation, which produces sets of design alternatives that are depicted as a point cloud with a level of design space coverage that is not possible with traditional curve fit or response surface techniques.

Proceedings Papers

Paper presented at the SNAME Maritime Convention, October 30–November 1, 2019

Paper Number: SNAME-SMC-2019-078

... approximation. It is essentially the

**square****root**of the restoring coefficient divided by the summation of mass of the fluid domain and the added mass of the upper and lower domains during free fluid heaving motion within the moonpool. We intersect the frequency = with the natural frequency model in Eq. 7, in...
Abstract

The effect of moonpool geometry on a ship hull's calm water resistance was investigated numerically using fully viscous CFD simulations with 2D and 3D computational domains. Simulations were performed using a Series 60 hull form with a block coefficient of 0.65 as the base case. The effect of changing the cross-sectional shape of the moonpool recess was examined by comparing the predicted resistance of the original hull with the result of the model fitted with various moonpool configurations. The kinematics of the separated flow within the moonpool and downstream of the underside of the moonpool recess revealed a relationship between the length of the recess and added resistance. Additionally, the impact of varying the recess height was investigated by comparing the free surface elevations between simulations. It was determined that shallower recesses increased the amplitude of the moonpool wave motions and therefore the overall vessel resistance. It was also determined that 3D effects were necessary to adequately capture moonpool hydrodynamics.

Proceedings Papers

Publisher: Society of Exploration Geophysicists

Paper presented at the SEG International Exposition and Annual Meeting, September 15–20, 2019

Paper Number: SEG-2019-3216200

... measured S-wave velocity. Its average prediction accuracy is up to 96% for the complex reservoir, and the

**root**mean**square**error is 8.1. The average prediction accuracy of the Xu-White model and DEM-Gassmann model is only 93% and 94.5%, respectively. In addition, The Xu-White model did not perform well in...
Abstract

ABSTRACT S-wave velocity is an important parameter for pre-stack inversion and reservoir characterization. However, due to various reasons, measured S-wave velocities are usually not available in most wells. Numerous conventional methods have been used to estimate S-wave velocity, such as empirical formulas and rock physical models. These methods also have some challenges to predict S-wave velocity for complex reservoirs, and the accuracy and efficiency of prediction results are limited. In order to solve the problems, Long short-term memory (LSTM) networks, a special kind of recurrent neural networks (RNNs) used in the field of artificial intelligence, were proposed to predict S-wave velocity from conventional well logs for complex reservoirs. The method is based on deep learning technology. LSTM networks can not only memorize previously received inputs to process sequences of data, but also avoid the exploding and vanishing gradient problems of classic RNNs. Well logs are typical sequential data and LSTM networks are well-suited to process the sequential data. In this paper, a LSTM network was established to predict S-wave velocity for lacustrine tight dolomite reservoirs, a kind of complex reservoirs. Compared with the measurements and other two rock physical models, the prediction results proved that the LSTM network can provide more accurate, efficient and robust S-wave velocity for complex reservoirs. Presentation Date: Wednesday, September 18, 2019 Session Start Time: 9:20 AM Presentation Time: 11:00 AM Location: Poster Station 2 Presentation Type: Poster

Proceedings Papers

Publisher: Society of Exploration Geophysicists

Paper presented at the 2000 SEG Annual Meeting, August 6–11, 2000

Paper Number: SEG-2000-0842

... Summary The

**double****square****root**(DSR)**equation**for laterally varying media in midpoint-offset coordinates provides a convenient framework for developing efficient 3-D prestack wave**equation**depth migrations with screen propagators. Common offset pseudo-screen depth migration is a fast and...
Abstract

Summary The double square root (DSR) equation for laterally varying media in midpoint-offset coordinates provides a convenient framework for developing efficient 3-D prestack wave equation depth migrations with screen propagators. Common offset pseudo-screen depth migration is a fast and accurate method for migrating common offset, common azimuth seismic data. The common offset pseudo-screen propagator down-ward contuiens the source and receiver wave fields simultaneously based on the DSR equation.

Journal Articles

Journal:
SPE Drilling & Completion

Publisher: Society of Petroleum Engineers (SPE)

*SPE Drilling & Completion*34 (04): 458–467.

Paper Number: SPE-197041-PA

Published: 19 July 2019

.... 2018c ). Especially in the case of deepwater and ultradeepwater wells, in which retrieving and deploying a BOP take significant time, BOP–related faults and failures are the

**root**cause of almost half of the unplanned downtime on the rig. Failure events caused by the BOP control system, such as control...
Abstract

Summary Presented here is an adaptive model–based approach for the real–time condition and health monitoring of annular blowout–preventer (BOP) functions. The proposed approach fuses a first–principles model with in–field data by adapting the model coefficients to match the data. These adapted coefficients are directly interpreted as the annular preventer health indicators. The performance of the proposed adaptive model was evaluated using an annular–BOP digital twin (i.e., Virtual Annular Preventer) implemented in a simulation environment. The model was then adapted using data collected from six different offshore rigs. The resultant model coefficients indicated that the proposed methodology might detect degradation of annular preventers and, hence, provide drillers and subsea engineers with real–time information about the health of the annular–BOP function.

Journal Articles

Journal:
Journal of Ship Research

*Journal of Ship Research*63 (02): 130–142.

Paper Number: SNAME-JSR-2019-63-2-130

Published: 01 June 2019

...: FnonðtÞ¼ 4FðtÞ gDHl; (11) Then, the average amplitude of the force was computed based on the

**root**mean**square**force Frms given by the following**equation**: Famp ¼ ffiffiffi 2 p Frms ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 T ð T 0 F2nonðtÞ dt vuut ; (12) where...
Abstract

This article presents an investigation on the best practice for modeling of water wave generation and wave-structure interaction using a widely spread two-phase flow solver with a specific interface compression technique in OpenFOAM ® (OpenFOAM Foundation Ltd., London, United Kingdom). A series of numerical experiments were conducted to examine the effects of the employed schemes, mesh resolution, time step resolution, and compression coefficient. Both surface elevation and velocity profile were considered as the criteria for assessment of the quality of the generated waves. The numerical experiments showed that by using a blending scheme between the Crank-Nicolson and Euler scheme, relatively high quality waves were produced, where the spurious current at the interface region was effectively reduced. Meanwhile, it was also recommended to apply a compression coefficient Cα = 1, the Courant number limit Co = .1, and a mesh resolution of 18 cells per wave height. This set of parameters was used to validate the numerical model for two sets of cases for wave forces on half-immersed horizontal cylinders. The results in general agreed well with the experimental data, although the inline forces were slightly but consistently overestimated. 1. Introduction In the past few decades, because of the increase in the computing ability and resources, computational fluid dynamic (CFD) approach has been gaining popularity in the naval hydrodynamic communities. Considering that a large portion of naval hydrodynamic problems are concerned with the free-surface flows, e.g., ocean waves and its interaction with structures, a free-surface model is needed to couple with the Navier-Stokes solver. Eventually, as a result of direct application of the first principles, the solver can model both inertia and viscous dominant flows. Even local wave breaking and violent flows can be properly resolved. Different methods have been proposed to track the interface of free-surface flows. This includes height function method and line segment method, which are the simplest techniques to represent and configure a free surface. However, they have deficiencies when two surfaces intersect or when a surface folds over on itself, which are overcome by the marker and cell method proposed in Harlow and Welch (1965). In their method, massless particles are used to track the free surface in a Lagrangian manner. Alternatively, the level-set (LS) method or volume of fluid (VOF) method can be used to implicitly capture the surface in an Eulerian way. The LS method was introduced in Osher (1988) where the interface is defined as the one on which a level-set function is equal to zero. This LS function is continuous across the interface. Therefore, it is not significantly influenced by the numerical diffusion. However, the originally proposed LS method suffers from mass conservation problems, which has been resolved in, e.g., Olsson and Kreiss (2005) and Olsson et al. (2007).

Proceedings Papers

Paper presented at the The 29th International Ocean and Polar Engineering Conference, June 16–21, 2019

Paper Number: ISOPE-I-19-005

... plotted in Figure 4 at the focus location (x = 4.25). In previous work (Brown et al., 2019), the convergence of the mesh was determined using a single

**root**mean**square**(RMS) parameter. In this work, multi- ple criteria, based on parameters which will be supplied to the CCP-WSI Blind Test Series 3, are...
Abstract

ABSTRACT This work concerns focused wave interactions with floating structures and represents an individual contribution to the CCP-WSI Blind Test Series 3, in which the submitted results are compared against both physical and alternative numerical solutions for varying wave steepness. The present numerical results are obtained using open-source CFD with waves generated via linear superposition of first order wave components, derived from empty tank data. Two geometries are considered and the effect of wave steepness on surge, heave and pitch motion, along with the load in the mooring, is examined. A ‘blind’ estimation of error is presented, thought to be 10% or less in peak motion and mooring loads. INTRODUCTION Two key issues that are limiting the routine use of computational fluid dynamics (CFD), are the uncertainty in the accuracy and the required time to obtain numerical results. The time taken to run a simulation is notoriously large, but this can be decreased through use of a larger computational resource. However, an often-overlooked factor is the man-hours required to set up a case through processes such as mesh design, and this has the potential to be considerably more time-consuming than the simulation time. For industry to benefit from the strengths of CFD models, the efficiency of the set up process needs to be increased, and this could be achieved through increased confidence in prediction by parametrically understanding numerical accuracy and providing standardised, ‘best practice’ implementations. Therefore, the scope of this work is to provide a ‘blind’ estimation of numerical accuracy, based purely on the reproduction of empty tank data. This approach was previously utilised for a case with a fixed structure (Brown et al., 2019), but here the focus is on the interaction of focused wave events with floating structures that represent a simplified wave energy converter (WEC). The presented work represents an individual contribution to the CCP-WSI Blind Test Series 3 (CCP-WSI, 2018), in which the submitted results are compared against both physical and alternative numerical solutions for varying wave steepness. The numerical results reported here are obtained using the open-source CFD software OpenFOAM (version 5.0). Wave generation is achieved via an expression-based boundary condition based on a linear superposition of first order wave components (derived from the empty tank data). Two different geometries are considered: a hemispherical-bottomed buoy; and a cylindrical structure with a moon-pool (Figure 1). For both structures, the effect of wave steepness on the motion of the structure, along with the load in the mooring, is examined.

Journal Articles

Journal:
Journal of Sailing Technology

*Journal of Sailing Technology*3 (12): 1–37.

Paper Number: SNAME-JSBT-2014-01

Published: 12 April 2019

... the suspension. The variation of the two periods with l1 is thus given by the second

**square****root**, leaving the periods to scale with 2l . Differentiating**equation**(19) to derive 1pk T and 2pk T , and then assuming that the uncertainties ± T in both periods are the same, leads to ( ) ( )21p p pk T...
Abstract

Measurements of the inertia parameters (Gregory, 2006) of a keelboat hull using a bifilar suspension (Newman and Searle, 1951) are described. Bifilar yaw moment measurement normally entails accurate measurement of the length l and spacing 2 d of the suspension, and of T y the period of pure yaw oscillation (Miller, 1930). The primary difficulty with a bifilar suspension is avoiding unwanted modes of oscillation, specifically sway when measuring yaw. However, for an athwartships suspension, the sway motion is that of a simple pendulum of period T s and observation of the combined motion allows the yaw gyradius k y ≡ k zz to be determined as k y = (T y /T s )d . Thus only the ratio of the periods and the suspension spacing need to be measured. Measurements of the normal mode periods of the double pendulum motion (Rafat, Wheatland et al., 2009) when the hull is displaced in surge allow for the pitch gyradius k p ≡ k yy and the height l 2 of the center of mass to be determined. The latter can be confirmed by measuring the incline angle of the hull when a weight is suspended from the stern and/or the bow. Repeating yaw measurements with the hull tilted, and then with the bifilar suspension fore and aft to measure the roll gyradius, k r ≡ k xx , allows for the angle ψ of the inertia ellipsoid (Wells 1967) principal x axis to the hull x axis to be calculated. Although the present keelboat measurements were made using ultrasonics (Daedalon, 1991) and photogates (Pasco, 2000), such measurements can now be more easily made using MEMs gyros, such as that in the iPhone (xSensor, 2010). This is illustrated by the measurements on a model keelboat.

Journal Articles

*Petrophysics - The SPWLA Journal of Formation Evaluation and Reservoir Description*60 (02): 208–227.

Paper Number: SPWLA-2019-v60n2t2

Published: 01 April 2019

...

**square****root**, viz., (Archie s Eq. (5) and (Archie s Eq. (6 That is the full explanation of Archie s contribution as he introduced it in 1941. It seems an obvious step given that he has written down Archie Eq. (4) as Rt = R0Sw -n to then substitute Archie Eq. (3) R0 = Rw -m to produce the combined...
Abstract

Prologue The standard model for relating bulk formation resistivity to porosity and water saturation was introduced to the petroleum industry in 1941; it remains the industry standard to this day. The model was discovered empirically by means of graphical analysis. Basically, G.E. Archie discovered that when the logarithm of formation resistivity factor was plotted against the logarithm of porosity the resulting trend could be fitted by a straight line. A similar relationship was discovered connecting the logarithms of resistivity index and water saturation. When these two power laws are combined into a single equation, it can be solved for water saturation (which is not observable from a borehole) in terms of bulk formation resistivity, interstitial brine resistivity, and porosity (all of which can be estimated from observations made in boreholes). This revolutionized log interpretation. There has always been a problem with the model in terms of its "explainability". That is, it cannot be derived in any straightforward way from accepted first principles of physics. It does not contradict any first principle, but neither does it seem to follow ineluctably from them. However, since the model works, most formation evaluators have memorized the relationships that follow from the model and simply "get used to them". That remains the situation to this day. However, there is a path around this obstacle to understanding formation resistivity at a fundamental level, and that way forward is to abandon the resistivity formulation in favor of its reciprocal, conductivity. It is surprising that such a seemingly trivial change could open a new vista into the relationships among formation electrical properties. A conductivity formulation permits the asking of questions about how a formation's conductivity should respond to changes not only in brine conductivity, but also in the fractional amount of brine in a formation, and its geometrical configuration. By answering these questions in an obvious way, and with some analysis of data taken in the laboratory, an intuitively obvious model explaining bulk formation conductivity emerges. The model is not the same as the Archie model. However, when certain parameters are taken to their limiting values, and the model is converted into resistivity space, Archie's power law model is revealed as an approximation to the limiting cases. Thus, from the conductivity formulation, an intuitive understanding of the Archie model emerges. Moreover, the conductivity model can be derived in at least three different ways, each yielding different insights into formation conductivity.

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