bluuebunny/crossref_metadata_2025 huggingface dataset:
Risk Acceptance Criteria for Temporary Phases
Authors: Vinnem J. E.
Published: February 2008

The full life cycle approach to risk acceptance is proposed as the basis for establishing of acceptance criteria for the temporary phases, i.e., onshore and offshore construction as well as installation. The background for such criteria is discussed, as well as evaluations that may be used to formulate risk acceptance criteria in the construction and installation phases. The criteria are focused on personnel safety. Environmental spill protection and protection of assets are also addressed, but less extensively. FAR or AIR values are used to express tolerable personnel safety, whereas the acceptability of exposure of the investment is expressed in terms of acceptable probabilities of accidents with significant effect on the project time schedule, i.e., the possibility to delay significantly the start-up of the production phase. The approach is based on an ALARP principle.
On Irregular, Nonlinear Waves in a Spread Sea
Authors: Jonathan P. , Taylor P. H.
Published: February 2008

Optimal design and reassessment of offshore structures requires a good understanding of the ocean environment. The motion of the sea surface can be viewed as a three-dimensional, nonlinear stochastic process in time. In order to characterize the wave environment adequately, we need to model its random, nonlinear, and spread nature. In this paper, we address: • the expected shape of a wave near a crest or trough, • the expected shape of the ocean surface at one point, given a crest at a different point, • an efficient method to incorporate nonlinear effects within linear wave simulations, • the magnitude of wave nonlinearity as a function of wave amplitude. Detailed comparison of theory and full-scale offshore measurements at the Shell Expro Tern platform show good agreement.
Fatigue Reliability of TLP Tether Systems
Authors: Hovde G. O. , Moan T.
Published: February 2008

The tension leg platform (TLP) concept is considered to be economically competitive with other offshore production concepts in deepwater locations. The critical part of this concept regarding structural failure is the tether system, in which fatigue is an important failure mode. The present paper presents a computational efficient and general probabilistic procedure for estimating the fatigue reliability of the TLP tether system. Two crack growth rate models of single crack sites are applied. Among uncertainties accounted are those in the long-term stochastic load process, the fatigue strength, and the initial crack sizes.
Prediction of Viscous Ship Roll Damping by Unsteady Navier-Stokes Techniques
Authors: Korpus R. A. , Falzarano J. M.
Published: February 2008

This paper describes a numerical technique for analyzing the viscous unsteady flow around oscillating ship hulls. The technique is based on a general Reynolds-averaged Navier-Stokes (RANS) capability, and is intended to generate viscous roll moment data for the incorporation of real-flow effects into potential flow ship motions programs. The approach utilizes the finite analytic technique for discretizing the unsteady RANS equations, and a variety of advanced turbulence models for closure. The calculations presented herein focus on viscous and vortical effects without free-surface, and utilize k-epsilon turbulence modeling. Series variations are presented to study the effects of frequency, amplitude, Reynolds number, and the presence of bilge keels. Moment component breakdown studies are performed in each case to isolate the effects of viscosity, vorticity, and potential flow pressures.
Code Development for Ship Structures—A Demonstration
Authors: Mansour A. E. , Wirsching P. H. , Ayyub B. , White G.
Published: February 2008

A demonstration summary of a reliability-based structural design code for ships is presented for two ship types: a cruiser and a tanker. One reason for the development of such a code is to provide specifications which produce ship structure having a weight savings and/or improvement in reliability relative to structure designed by traditional methods. Another reason is to provide uniform safety margin for ships within each type. For both ship types, code requirements cover four failure modes: hull girder bulkling, unstiffened plate yielding and buckling, stiffened plate buckling, and fatigue of critical detail. Both serviceability and ultimate limit states are considered. Because of limitation on the length, only hull girder modes are presented in this paper. Code requirements for other modes will be presented in future publication. A specific provision of the code will be safety check expression, which, for example, for three bending moments (still water Ms, wave Mw, and dynamic Md), and strength Mu, might have the form, following the partial safety factor format: γsMs+γwMw+γdMd≤φMu γs, γw, γd, and φ are the partial safety factors. The design variables (M’s) are to be taken at their nominal values, typically values in the safe side of the respective distributions. Other safety check expressions for hull girder failure that include load combination factors, as well as consequence of failure factors, are considered. This paper provides a summary of safety check expressions for the hull girder modes.
Heat-Affected Zone Toughness of a TMCP Steel Designed for Low-Temperature Applications
Authors: Gianetto J. A. , Braid J. E. M. , Bowker J. T. , Tyson W. R.
Published: February 2008

The objective of this investigation was to provide a detailed evaluation of the heat-affected zone (HAZ) toughness of a high-strength TMCP steel designed for low-temperature applications. The results from both Charpy-vee notch (CVN) and cracktip-opening displacement (CTOD) tests conducted on two straight-walled narrow groove welds, produced at energy inputs of 1.5 and 3.0 kJ/mm, show that significantly lower toughness was exhibited by the grain-coarsened HAZ (GCHAZ) compared with the intercritical HAZ (ICHAZ) region. This is explained based on the overall GCHAZ microstructure, and the initiation mechanism which caused failure. For the particular TMCP steel investigated in this study very good ICHAZ toughness properties were recorded using both HAZ Charpy and CTOD tests. In general, this was attributable to the low hardness, relatively fine ferrite microstructure, and the formation of secondary microphases that were not overly detrimental to the toughness. The lower-bound GCHAZ CTOD results obtained for both welds (KA W-L and KA W-H) did not meet the targeted requirement of δ = 0.07 mm at −50°C. It was found in both welds that low CTOD toughness was associated with the initiation of fracture from nonmetallic inclusions, which were complex oxides containing Ce, La, and S. The sites were located in the subcritical GCHAZ (SCGCHAZ) region in the case of the 1.5 kJ/mm weld and in the GCHAZ for the 3.0 kJ/mm weld. Some variation in CVN toughness was observed at different through-thickness locations. Toughness was lowest for the GCHAZ of the weld deposited at 3.0 kJ/mm and was related to the proportion of GCHAZ being sampled, which was ~55 percent for the bottom compared to 25–30 percent for that of the top location. Recommendations are proposed on the preferred practices and criteria that should be used in establishing guidelines and specifications for evaluating the HAZ toughness of candidate steels for construction of Arctic class ships.
Non-Gaussian Extreme Waves in the Central North Sea
Authors: Skourup J. , Hansen N.-E. O. , Andreasen K. K.
Published: February 2008

The area of the Central North Sea is notorious for the occurrence of very high waves in certain wave trains. The short-term distribution of these wave trains includes waves which are far steeper than predicted by the Rayleigh distribution. Such waves are often termed “extreme waves” or “freak waves.” An analysis of the extreme statistical properties of these waves has been made. The analysis is based on more than 12 yr of wave records from the Mærsk Olie og Gas AS operated Gorm Field which is located in the Danish sector of the Central North Sea. From the wave recordings more than 400 freak wave candidates were found. The ratio between the extreme crest height and the significant wave height (20-min value) has been found to be about 1.8, and the ratio between extreme crest height and extreme wave height has been found to be 0.69. The latter ratio is clearly outside the range of Gaussian waves, and it is higher than the maximum value for steep nonlinear long-crested waves, thus indicating that freak waves are not of a permanent form, and probably of short-crested nature. The extreme statistical distribution is represented by a Weibull distribution with an upper bound, where the upper bound is the value for a depth-limited breaking wave. Based on the measured data, a procedure for determining the freak wave crest height with a given return period is proposed. A sensitivity analysis of the extreme value of the crest height is also made.
Dynamic Analysis for the Design of CALM System in Shallow and Deep
Waters
Authors: Hwang Y.-L.
Published: February 2008

This paper presents a time domain analysis approach to evaluate the dynamic behavior of the catenary anchor leg mooring (CALM) system under the maximum operational condition when a tanker is moored to the terminal, and in the survival condition when the terminal is not occupied by a tanker. An analytical model, integrating tanker, hawser, buoy, and mooring lines, is developed to dynamically predict the extreme mooring loads and buoy orbital motions, when responding to the effect of wind, current, wave frequency, and wave drift response. Numerical results describing the dynamic behaviors of the CALM system in both shallow and deepwater situations are presented and discussed. The importance of the line dynamics and hawser coupled buoy-tanker dynamics is demonstrated by comparing the present dynamic analysis with catenary calculation approach. Results of the analysis are compared with model test data to validate the mathematical model presented.
Use of Numerical Wind-Wave Models for Assessment of the Offshore Wave
Energy Resource
Authors: Pontes M. T. , Barstow S. , Bertotti L. , Cavaleri L. , Oliveira-Pires H.
Published: February 2008

In the last two decades the performance of numerical wind-wave models has improved considerably. Several models have been routinely producing good quality wave estimates globally since the mid-1980s. The verifications of wind-wave models have mainly focused on the evaluation of the error of the significant wave height Hs estimates. However, for wave energy purposes, the main parameters to be assessed are the wave power Pw and the mean (energy) period Te. Since Pw is proportional to Hs2 Tc, its expected error is much larger than for the single-wave parameters. This paper summarizes the intercomparison of two wind-wave models against buoy data in the North Atlantic and the Mediterranean Sea to select the most suitable one for the construction of an Atlas of the wave energy resource in European waters. A full verification in the two basins of the selected model—the WAM model implemented in the routine operation of the European Centre for Medium-Range Weather Forecasts—was then performed against buoy and satellite altimeter data. It was found that the WAM model accuracy is very good for offshore locations in the North Atlantic; but for the Mediterranean Sea the results are much less accurate, probably due to a lower quality of the input wind fields.
Efficient Calculation of Hydrodynamic Properties of OWC-Type Devices
Authors: Evans D. V. , Porter R.
Published: February 2008

A simple model of an OWC-type wave-energy device is used to illustrate a powerful accurate method for computing the hydrodynamic coefficients when sharp edges are present. The device consists simply of a vertical partially immersed circular cylinder open at either end, with power being extracted by constricting the flow of the air trapped in the cylinder above the internal free surface. The method involves the use of the theory of pressure distributions for OWC devices and the hydrodynamical coefficients are computed using an accurate Galerkin method which preserves the reciprocity relations.
Application of Large Eddy Simulation to Flow Past a Circular Cylinder
Authors: Lu X. , Dalton C. , Zhang J.
Published: February 2008

A steady approach flow around a circular cylinder is investigated by using a large eddy simulation (LES) with the Smagorinsky subgrid-scale model. A second-order accurate in time fractional-step method and a combined finite-difference/spectral approximation are employed to solve the filtered three-dimensional incompressible Navier-Stokes equations. To demonstrate the viability and accuracy of the method, we present results at Reynolds numbers of 100, 3 × 103, 2 × 104, and 4.42 × 104. At Re = 100, the physical flow is two-dimensional and the calculation is done without use of the LES method. For the higher values of Re, the flow in the wake is three-dimensional and turbulent and the LES method is necessary to describe the flow accurately. Calculated values of lift and drag coefficients and Strouhal number are in good agreement with the experimentally determined values at all of the Reynolds numbers for which calculation was done.
Application of a Hilbert Transform-Based Algorithm for Parameter Estimation of a Nonlinear Ocean System Roll Model
Authors: Gottlieb O. , Feldman M.
Published: February 2008

We combine an averaging procedure with a Hilbert transform-based algorithm for parameter estimation of a nonlinear ocean system roll model. System backbone curves obtained from data are compared to those obtained analytically and are found to be accurate. Sensitivity of the results is tested by introducing random noise to a nonlinear model describing roll response of a small fishing boat. An example field calibration test of a small semisubmersible exhibiting nonlinear damping is also considered.
Reliability-Based Tubing and Casing Design: Principles and Approach
Authors: Maes M. A. , Gulati K. C. , Brand P. R. , Lewis D. B. , McKenna D. L. , Johnson R. C.
Published: February 2008

This paper describes the development of an LRFD format reliability-based design of tubulars used in oil and gas wells. The approach is based on the principles of limit state design. The paper identifies the applicable limit states of pipe performance, discusses stochastic modeling of the load and resistance variables, and describes calibration of the design check equations. The calibration considers various combinations of hardware configurations, loadings, and tubular application zones with their respective risk levels. Application of the design principles is illustrated with an example problem.
Ultimate Compression Strength and Probabilistic Analysis of Stiffened Plates
Authors: Pu Y. , Das P. K. , Faulkner D.
Published: February 2008

In the paper, a formulation for predicting the ultimate strength of a stiffened plate is proposed by incorporating Guedes Soares’s formula, which gives the best prediction for unstiffened plates according to the calibration carried out recently by the authors, into Faulkner’s method (Faulkner et al., 1973). The proposed algorithm is then calibrated by using a considerable amount of experimental and numerical data. It is observed that: (a) The proposed method shows better prediction than Faulkner’s original method if only the experimental data (63 samples) are included in the calibration, the bias and COV of the model uncertainty of the proposed method are 0.992 and 0.099, respectively, while they are 1.039 and 0.143 for Faulkner’s original method, and the skewness of the proposed method is small (only −0.105 slope, which is defined as the slope of the regressed straight line on the plot of model uncertainty against predicted value). (b) On the whole, including experimental and numerical data, the results of the proposed method demonstrate more or less the same accuracy as that of the original Faulkner method with better bias and skewness, but slightly larger scatters than the original Faulkner method. In addition, the reliability analyses of stiffened plates are carried out by using advanced first-order second-moment method (AFOSM), the second-order reliability method (SORM), and Monte Carlo simulation to investigate the accuracy of the first and second-order methods. It is found that the difference between the two methods is so small that the values obtained from AFOSM are acceptable in practice, considering the nominal nature of the reliability index.
A New Nonslender Ringing Load Approach Verified Against Experiments
Authors: Krokstad J. R. , Stansberg C. T. , Nestega˚rd A. , Marthinsen T.
Published: February 2008

New results from the most recent work within the Norwegian Joint Industry Project (JIP) “Higher Order Wave Load Effects on Large Volume Structures” are presented. A nonslender theoretical model is validated from experiments for two fixed, vertical cylinders with different diameter/peak wavelength ratios. A combination of complete diffraction first-order simulations, sum and difference frequency second-order simulations, and third-order FNV (Faltinsen, Newman, and Vinje, nonlinear long wave model) is implemented in order to develop a simplified and robust ringing load model for a large range of cylinder diameter/peak wavelength ratios. Results from the full diffraction second-order analysis show a significant reduction of second-order loads compared to pure FNV in the wavelength range relevant for ringing loads. The results show improved correspondence with high-frequency experimental loads compared with the unmodified FNV. Results for different cylinder peak wavelength ratios are presented, including validation against experiments. In addition, a few simplified response simulations are carried out demonstrating significant improvements with the modified FNV model.
Fatigue Reliability of Maintained Welded Joints in the Side Shell of Tankers
Authors: Garbatov Y. , Guedes Soares C.
Published: February 2008

A formulation is presented for the assessment of the reliability of a local ship hull structure regarding fatigue failure. The potential cracks are considered to occur in the side shell, in the connections between longitudinal stiffeners and transverse web frame. The analysis is performed for T-shaped stiffeners. The model accounts for the crack growth process applying linear elastic fracture mechanics. The long-term stress range acting on the elements is defined as a function of the local transverse pressure of internal cargo and outside water, combined with the stresses resulting from the longitudinal bending of the hull. The global hull loading is a combination of horizontal and vertical bending moments. The present paper analyzes the importance of including the transverse pressure loading for the process of the crack growth and also the distribution of the net section area of midship and location of stiffeners. The formulation has been applied to a tanker. The fatigue reliability is predicted by a time variant formulation and the effects of maintenance actions in updating the reliability assessment are shown.
The Dynamics of Falling Marine Chain: A Continuum Model
Authors: Webster W. C.
Published: February 2008

This paper investigates the dynamics of a length of chain that is attached to the ocean floor, initially oriented vertically and the top end allowed to fall. A series of simulations of the falling was performed using a continuum model of the chain. It was found that, although initially the chain does fall straight down, when its speed approaches terminal velocity in this direction, it develops instabilities which cause severe motions in the chain, particularly at the top end of the chain. It was found that these motions are sensitive to the longitudinal drag coefficient, Cd,L. A statistical analysis of many different simulations of the falling process indicates that the whipping of the top end of the chain may be more dangerous to neighboring platforms or equipment than the impact of the chain on the bottom.
Effects of Appendages and Small Currents on the Hydrodynamic Heave Damping of TLP Columns
Authors: Thiagarajan K. P. , Troesch A. W.
Published: February 2008

A previous paper by the authors reported on the estimation of resonant heave (springing) damping of tension leg platforms (TLPs) and a method of scaling for full-scale prototypes. The present paper is a sequel to this work, and examines the effect of adding an appendage in the form of a disk to TLP columns, and the influence of a small uniform current. Model tests conducted on a cylinder + disk in heave show that the heave damping induced by the disk is linear with the amplitude of oscillation. The disk is found to increase the form drag coefficient twofold, in accordance with published results based on isolated edge theory. The effects of a small uniform current were also examined during the model tests. Results show an increase in heave damping ratio that is linear with the current velocity. In the presence of a disk, the damping induced by the current is doubled as well. Scaling laws are proposed in this paper that enable extrapolation of heave damping due to appendages and small currents to full scale. An example calculation shows that for a full-scale TLP column, the heave damping is increased by about 300 percent due to addition of the disk, and by 87 percent due to a small current. The combination of the disk and the current increases the heave damping of the column by a factor of six.
Trapping of Leaked Oil With Tandem Oil Fences With Lagrangian Analysis of Oil Droplet Motion
Authors: Lee C. M. , Kang K. H. , Cho N. S.
Published: February 2008

The effectiveness of two oil fences deployed in tandem to maximize the containment of oil is investigated. To assess the effectiveness of the tandem fences, the viscous flow field around the fences in tandem are analyzed numerically. Then, the trajectories of oil droplets which escaped beneath the fore fence are computed applying the Lagrangian particle-tracking method, to check under what conditions the droplet can be contained between the tandem fences. The validity of the calculated trajectories is checked experimentally by using spherical beads made of paraffin and droplets of kerosene, and the model fence of draft of 4 cm. The numerically predicted trajectories of the droplets show fairly good agreement with the experimental results. The method is applied to predict the motion of the weathered oil. It is shown, numerically, that most of the leaked oil can be trapped between tandem fences, when the distance between the fences is about 10 times the draft of the fore fence.
On Markov Theory and Wave-Crest Statistics
Authors: Dawson T. H.
Published: February 2008

The Longuet-Higgins connection between Markov description of runs of high waves and the truncated frequency spectrum of a given sea is developed in a simplified, experimentally based manner using combined results from envelope and Markov theories. A high-frequency truncation limit is proposed as 2ωp – 0.5ωo, where ωp denotes peak spectral frequency and ωo denotes mean frequency. Results are shown to apply to measurements from laboratory and computer simulations of standard Bretschneider and Jonswap seas.
Problems and Solution Strategies in Reliability Updating
Authors: Sindel R. , Rackwitz R.
Published: February 2008

Reliability updating by inspections has become widely used in the offshore industry, especially within the framework of FORM/SORM. However, certain algorithmic difficulties can occur. This is especially true for reliability updating in the presence of structural fatigue (crack propagation and instability), but the problem appears to exist whenever conditional probabilities are to be calculated. New and widely successful strategies to select appropriate starting points for the β-point search as well as strategies to avoid physically or mathematically inadmissible domains are proposed. They are illustrated at a typical example in which it is also demonstrated that the simpler crude FORM approaches are inadequate.
Coupled Sea Ice-Ocean Model of the Arctic Ocean
Authors: Polyakov I. V. , Kulakov I. Yu. , Kolesov S. A. , Dmitriev N. Eu. , Pritchard R. S. , Driver D. , Naumov A. K.
Published: February 2008

A fully prognostic coupled ice-ocean model is described. The ice model is based on the elastic-plastic constitutive law with ice mass and compactness described by distribution functions. The ice thermodynamics model is applied individually to each ice thickness category. Advection of the ice partial mass and concentrations is parameterized by a fourth-order algorithm that conserves monotonicity of the solution. The ocean is described as a three-dimensional time-dependent baroclinic model with free surface. The coupled model is applied to establish the Arctic Ocean seasonal climatology using fully prognostic models for ice and ocean. Results reflect the importance of the ice melting/freezing in the formation of the thermohaline structure of the upper ocean layer.
Open-Close Loop Control of ALP With Improved Performance Function
Authors: Suneja B. P. , Datta T. K.
Published: February 2008

A linear open-close loop control strategy is presented for controlling the response of the offshore articulated leg platform (ALP) in which the performance function involves structural acceleration in addition to the usual state variables, i.e., angular displacement and velocity of the tower, and the sea surface elevation and its variables. The weightage of the structural acceleration in the performance function is regulated by incorporating an acceleration weighting factor. The performance function is minimized to obtain the control law using the well-known Ricatti equation. The control law thus achieved is implemented to control the peak responses of a 180.5-m tall ALP under a random sea state, using an iterative frequency domain technique to consider all the nonlinearities present in the system. The efficiency of the proposed strategy is investigated through an extensive parametric study.
A Study on the Degradation of Coating and Corrosion of Ship’s Hull Based on the Probabilistic Approach
Authors: Yamamoto N. , Ikegami K.
Published: February 2008

In this paper, phenomena of general corrosion are assumed to be the results of three sequential processes: degradation of paint coatings, generation of pitting point, and progress of pitting point. A consistent corrosion model is proposed which can evaluate the generation and progress of corrosion quantitatively by introducing appropriate simple probabilistic models for each process. This probabilistic corrosion model can be identified by analyzing existing data collected from plate thickness measurements. Applicability of this model is verified by comparing the estimated behavior of corrosion progress and dispersion with those from actual data.
Reliability-Based Screening of Offshore Platforms
Authors: Bea R. G. , Mortazavi M. M.
Published: February 2008

This paper describes development of a reliability-based screening procedure for platform assessments and requalifications. The objective of this procedure is to identify the critical structures and structure components that need to undergo a detailed assessment so that limited resources can be efficiently used. The reliability-based screening procedure helps identify potential failure modes and weak links in the structure, and provides information on the effects of load reduction and strengthening measures on the reliability of the platform. The reliability analysis screening procedure is based on a first-order second-moment (FOSM) formulation. A study is summarized of the implications of the simplified FOSM method. In the case of an eight-leg drilling and production platform located in the Gulf of Mexico, the results from FOSM reliability analysis are compared with those from first and second-order reliability methods (FORM and SORM). There is excellent agreement between the FOSM and FORM/SORM results.
Reliability-Based Design of Externally Pressurized Vessels
Authors: Morandi A. C. , Das P. K. , Faulkner D.
Published: February 2008

A reliability-based Level I design procedure is proposed for ring-stiffened cylindrical shells under external pressure. The main collapse modes and the present safety factor approach are reviewed. The major aspects involved in code development, such as the statistical properties of the basic variables, reliability methods, sensitivity studies, code format, target reliability, partial safety factor optimization, and comparison with the present practice are described. Safety margins for design are proposed which depend on the design pressure, maximum expected overdive, and shell slenderness. Some suggestions for future work are also given.
Nonlinear Diffraction of Waves by a Submerged Shelf in Shallow Water
Authors: Ertekin R. C. , Becker J. M.
Published: February 2008

The diffraction of water waves by submerged obstacles in shallow water generally requires the use of a nonlinear theory since both dispersive and nonlinear effects are important. In this work, wave diffraction is studied in a numerical wave tank using the Level I Green-Naghdi (GN) equations. Cnoidal waves are generated numerically by a wave maker situated at one end of a two-dimensional numerical wave tank. At the downwave end of the tank, an open-boundary condition is implemented to simulate a wave-absorbing beach, and thus to reduce reflections. The GN equations are solved in the time-domain by employing a finite-difference method. The numerical method is applied to diffraction of cnoidal waves by a submerged shelf, or a sand bar, of considerable height relative to water depth. The predicted results are compared with the available experimental data which indicate the importance of nonlinearity for the shallow-water conditions.
Some Observations on the Heave Behavior of Spar Platforms
Authors: Fischer F. J. , Gopalkrishnan R.
Published: February 2008

The spar platform is receiving considerable attention from the offshore petroleum industry, especially following the successful introduction of the first production spar in the Gulf of Mexico by Oryx and CNG in 1996. This paper presents a collection of observations regarding important heave characteristics of spar platforms that have been gleaned from wave-basin model tests, numerical simulations, and comparisons of the two. These observations are pertinent to the heave performance of “classic” (uniform-cylinder) spars. Excessive spar heave could be problematic for production and export risers. Spar-system characteristics affecting heave include draft, heave natural period, heave-damping augmentation, and presence/absence of a vertical tether/tendon. Tether requirements for shallow-draft spars have been found to be much less onerous than once thought; steel tethers yielding a heave natural period of 12 s have been found to be satisfactory. Heave-damping augmentation using multiple “plates” or “disks,” following the work of Troesch at the University of Michigan, has also been found to be quite attractive.
Application of a Two-Fluid Finite Volume Method to Ship Slamming
Authors: Sames P. C. , Schellin T. E. , Muzaferija S. , Peric M.
Published: February 2008

A newly developed finite volume method was applied to ship slamming. The computational method accounts for arbitrary free surface deformations and uses unstructured grids for the discretization of the domain. A linear panel method was used to predict motions of a modern 2400 TEU container ship. Resulting relative velocities at the ship’s Keel were used to estimate the maximum vertical re-entry velocities at the bow in North Atlantic wave conditions. Water entry of three bow in North Atlantic wave conditions. Water entry of three bow sections was numerically simulated to determine pressures at the bow flare. Prescribed vertical velocity histories significantly affected the determination of realistic pressure levels.
Analysis for Wrinkling Behavior of Girth-Welded Line Pipe
Authors: Souza L. T. , Murray D. W.
Published: February 2008

The paper presents results for finite-element analysis of full-sized girth-welded specimens of line pipe and compares these results with the behavior exhibited by test specimens subjected to constant axial force, internal pressure, and monotonically increasing curvatures. Recommendations for the “best” type of analytical finite element model are given. Comparisons between the behavior predicted analytically and the observed behavior of the experimental test specimens are made. The mechanism of wrinkling is explained and the evolution of the deformed configurations for different wrinkling modes is examined. It is concluded that the analytical tools now available are sufficiently reliable to predict the behavior of pipe in a manner that was not previously possible and that this should create a new era for the design and assessment of pipelines if the technology is properly exploited by industry.
Reliability of Structures Containing Short Cracks
Authors: Lanning D. , Shen M.-H. H.
Published: February 2008

The reliability of structures containing short cracks is investigated. Short fatigue cracks are generally acknowledged to be small cracks propagating at rates that are faster or anomalous when compared to standard fracture analysis predictions developed for longer cracks. The extent of the short crack behavior tends to depend on mechanism such as the existence of notch plasticity surrounding the short crack. Techniques are presented for calculating the probability that short surface cracks will propagate without arresting to a size where the growth can then be analyzed using linear elastic fracture mechanics concepts. It is also shown how crack closure measurements can be incorporated into the fatigue reliability analysis to assist life predictions for short cracks.
A Method for Reliability-Based Sensitivity Analysis of Ship’s Hull Structures Using Combined Plate and Frame Structure Models
Authors: Okada H. , Masaoka K. , Murotsu Y. , Hibi S. , Kiyokawa W.
Published: February 2008

This paper is concerned with reliability-based sensitivity analysis of ship’s hull structures, which are modeled by using combined spatial plate and frame elements, based on the ultimate strength analysis. At first, a newly proposed system using combined plate and beam elements is presented to expand the applicability of the structural reliability-based sensitivity analysis combined with collapse mode analysis. Next, the proposed system is demonstrated to be applicable through a numerical example applied to a simple box girder with longitudinal stiffeners. Finally, features of the structural reliability and sensitivity factors of a high-speed container ship based on collapse mode analysis are investigated by applying the newly developed system.
Estimating Bimodal Frequency-Direction Spectra From Surface Buoy Data Recorded During Tropical Cyclones
Authors: Ewans K. C. , van der Vlugt T.
Published: February 2008

Techniques for computing wave frequency-direction spectra are evaluated using wave buoy data recorded on the North West Shelf of Australia. Frequency-direction spectra are computed by each of four techniques and compared with the wave directionality expected at the measurement location, based on the meteorological conditions prevailing at the time. Particular emphasis is placed on the wave conditions occurring during tropical cyclones. Such sea states show strong directional bimodality, consisting largely of a cyclone-generated swell component and a local wind sea. The cyclone swell can be expected to have a narrow beam width when the cyclone is several hundred kilometers from the location. It is found that the bimodality is well indicated by a unimodal/bimodality parameter; and of the four techniques for computing the frequency-direction spectrum, the maximum entropy method is best at reproducing the directional bimodality while maintaining the narrow directional spreading in the cyclone swell.
Coupled Dynamic Analysis of a Moored Spar in Random Waves and Currents (Time-Domain Versus Frequency-Domain Analysis)
Authors: Ran Z. , Kim M. H. , Zheng W.
Published: February 2008

Nonlinear coupled responses of a moored spar in random waves with and without colinear currents are investigated in both time and frequency domains. The first and second-order wave forces, added mass and radiation damping, and wave drift damping are calculated from a hydrodynamics software package called WINTCOL. The total wave force time series (or spectra) are then generated in the time (or frequency) domain based on a two-term Volterra series model. The mooring dynamics are solved using the software package WINPOST, which is based on a generalized-coordinate-based finite element method. The mooring lines are attached to the platform through linear and rotational springs and dampers so that various boundary conditions can be modeled using proper spring and damping values. In the time-domain analysis, the nonlinear drag forces on the hull and mooring lines are applied at the instantaneous position. In the frequency-domain analysis, nonlinear drag forces are stochastically linearized, and solutions are obtained by an iterative procedure. The time-domain results are systematically compared with the frequency-domain results.
Assessment of Vortex-Induced Vibrations on Deepwater Risers by Considering Fluid-Structure Interaction
Authors: Herfjord K. , Drange S. O. , Kvamsdal T.
Published: February 2008

A methodology for computing vortex-induced vibrations (VIV) on risers is presented. It is based on computation of the flow by a CFD program, structural dynamics by a nonlinear structural (CSD) code, and a coupling between them. The CFD computations are performed in 2-D at a number of sections along the riser. The load is imposed on the riser in a strip theory manner. The coupling between the CFD planes takes place through the response of the riser. The local deformation of the riser is taken into account by the CFD program, thus completing a fluid-structure interaction loop each time step. The methodology is validated by comparing results from simulations with results from model tests.
Nonlinear Dynamic Response of a Tension Leg Platform
Authors: Bar-Avi P.
Published: February 2008

Of the classes of offshore structures, the tension leg platform (TLP) is particularly well suited for deepwater operation. The structure investigated in this paper is assumed to consist of a flexible cable attached to a buoyant deck at the top. The cable is modeled as a beamlike continuous system subjected to wave, current, and wind forces. The derivation of the nonlinear equations of motion include nonlinearities due to geometry as well as due to wave forces. The equations of motion are solved and the TLP’s response to various environmental conditions and other physical parameters is evaluated.
Prediction of Surface Wave Elevation Based on Pressure Measurements
Authors: Meza E. , Zhang J. , Seymour R. J.
Published: February 2008

A deterministic method for predicting wave elevation based on pressure measurements is developed. The method is based on the hybrid wave model (HWM), which employs both conventional and phase modulation methods for modeling wave-wave interactions in an irregular wave train. The predicted wave elevation using the HWM based on the pressure measurement of a steep transient wave train is in excellent agreement with the corresponding elevation measurement, while that using linear wave theory (LWT) has relatively large discrepancies.
Environmental Load Factors for Offshore Structures
Authors: Hong H. P. , Nessim M. A. , Jordaan I. J.
Published: February 2008

An analysis of the impact of model uncertainties on the design factors for environmental loads on offshore structures was carried out. Considering uncertainties in environmental processes, the load effect models and the member resistance, an approach was developed that gives explicit consideration to model uncertainty in codified design. For frequent environmental load effects, a two-factor approach was developed that defines the overall load factor as the product of two components: a basic factor accounting for uncertainty in the environmental process and a separate factor accounting for model uncertainty. The overall load factor is to be applied to the specified load, which is defined as the load corresponding to the environmental process value associated with a specified return period. This load can be calculated from the environmental process without considering model uncertainty. The model uncertainty factor was defined as a linear function of the mean and the standard deviation of the model uncertainty parameter. It can be estimated based on a specific model and reliability analysis. This two-factor approach has two advantages: (a) it allows for reductions in the load factor if conservative or more accurate models are used; and (b) it eliminates the need for the designer to consider model uncertainty in estimating the specified load. The approach was used to develop a set of load factors for environmental loads on offshore structures. These factors were calibrated to produce reliability levels consistent with those implied by the load factors in CSA-S471.
Wave Kinematics Computed With the Nonlinear Schro¨dinger Method for Deep Water
Authors: Trulsen K.
Published: February 2008

The nonlinear Schro¨dinger method for water wave kinematics under two-dimensional irregular deepwater gravity waves is developed. Its application is illustrated for computation of the velocity and acceleration fields from the time-series of the surface displacement measured at a fixed horizontal position. The method is based on the assumption that the waves have small steepness and limited bandwidth.
Manufacturing Process Modeling and Optimization Based on Multi-Layer Perceptron Network
Authors: Liao T. Warren , Chen L. J.
Published: February 2008

It has been shown that a manufacturing process can be modeled (learned) using Multi-Layer Perceptron (MLP) neural network and then optimized directly using the learned network. This paper extends the previous work by examining several different MLP training algorithms for manufacturing process modeling and three methods for process optimization. The transformation method is used to convert a constrained objective function into an unconstrained one, which is then used as the error function in the process optimization stage. The simulation results indicate that: (i) the conjugate gradient algorithms with backtracking line search outperform the standard BP algorithm in convergence speed; (ii) the neural network approaches could yield more accurate process models than the regression method; (iii) the BP with simulated annealing method is the most reliable optimization method to generate the best optimal solution, and (iv) process optimization directly performed on the neural network is possible but cannot be especially automated totally, especially when the process concerned is a mixed integer problem.
Analysis of High Speed Axially Symmetric Cutting for Stripping Peripheral Coating
Authors: Keer L. M. , Xu Y. , Luk V. K.
Published: February 2008

A theory of axially symmetric cutting is used to estimate the resistance to material removal of a composite cylinder undergoing stripping of a peripheral coating. The removal of materials from the surface of the cylindrical workpiece is analyzed using the minimum energy theory of Merchant. It is assumed that the shear surface is oriented at an angle π, such that the total work done at the shear surface by relative sliding and at the rake face due to friction has a minimum value. When the material conditions and the depth of cut are the same for cuttings in the axially symmetric and orthogonal modes, chips removed in axially symmetric cutting will have an additional energy loss due to the tensile hoop stress. The resistance due to the axially symmetric cutting is therefore larger than that arising from orthogonal cutting. The predicted resistance of axially symmetric cutting are in favorable agreement with data of dynamic stripping tests performed at Sandia National Laboratories.
Chip Formation Model of Cutting Fiber-Reinforced Plastics Perpendicular to Fiber Axis
Authors: Pwu H. Y. , Hocheng H.
Published: February 2008

The anisotropy of fiber-reinforced plastics heavily influences the chip formation and cut quality during machining. Fiber arrangement is found more dominant than the often stressed cutting conditions. The present paper studies the chip formation and constructs an original model for the case of cutting perpendicular to the unidirectional fiber axis. Bending failure controls the chip formation in this process. Beam theory and laminate mechanics are used to establish the correlation between cutting force, chip length and thickness. The proposed model can explain the major trend of the experimental results. Chip separation in thickness due to intrinsic bonding defects and overbent chips due to inhomogeneous local material strength are also found. They are understood based on the current model.
Prediction of Chip Flow Direction, Cutting Forces and Surface Roughness in Finish Turning
Authors: Arsecularatne J. A. , Fowle R. F. , Mathew P.
Published: February 2008

A method is described for calculating the chip flow direction in terms of the tool cutting edge geometry and cutting conditions. By defining an equivalent cutting edge based on the chip flow direction it is shown how cutting forces can be predicted under finishing conditions when the work material’s flow stress and thermal properties are known. A comparison between predicted and experimental results obtained for a range of tool geometries and cutting conditions shows good agreement. Surface roughness Ra values measured in the tests are compared with the theoretical values determined from the nose radius and feed.
Two-Dimensional Transient Thermal Behavior of Work Rolls in Rolling Process
Authors: Guo Remn-Min
Published: February 2008

In the rolling process, deformation and friction energy generated in the roll bite transfer to work rolls. Work rolls are cooled by the cooling medium in both entry and exit sides of the mill. The transient cooling behavior of the roll affects the temperature distribution and the thermal profile. This article proposes a semi-analytical solution to solve the temperature field of the work roll which is subjected to various cooling and heating boundary conditions during a rolling campaign. Laplace and inverse transforms are applied to pursue the unsteady solution for one particular boundary condition. The complete solution for various boundary conditions is superimposed by related individual solutions using Duhamel’s rule. Case studies as demonstrated in this paper show versatility of the developed model.
Modeling the Chip-Work Contact Force for Chip Breaking in Orthogonal Machining With a Flat-Faced Tool
Authors: Ganapathy B. K. , Jawahir I. S.
Published: February 2008

The present tendency towards increased automation of metal cutting operations has resulted in a need to develop a model for the chip breaking process. Conventional cutting force models do not have any provision for the study of chip breaking since they assume a continuous mode of chip formation, where the contact action of the free-end of the chip is ignored in all analyses. The new cutting force model proposed in this work incorporates the contact force developed due to the free-end of the chip touching the workpiece, and is applicable to the study of two-dimensional chip breaking in orthogonal machining. Orthogonal cutting tests were performed to obtain two-dimensional chip breaking. The experimentally measured cutting forces show a good correlation with the estimated cutting forces using the model. Results show that the forces acting on the chip vary within a chip breaking cycle and help identify the chip breaking event.