bluuebunny/crossref_metadata_2025 huggingface dataset:
Multi-Scale Finite Element Simulation of Progressive Damage in Composite Structures
Authors: Yu Qing , Wu Jer-Fang
Published: October 2008

A methodology for analyzing progressive damage accumulation on multiple spatial scales (micro- and macro-scale) in composite materials is presented in this paper. Idealization (homogenization) of heterogeneous media and evolution of damage on micro- and macro-scales are considered simultaneously at each incremental analysis step. The classical mathematical homogenization theory is extended to account for damage effects on distinct spatial scales through the introduction of an asymptotic expansion of damage parameter (or damage tensor in general). Local solutions on micro-scale provide the homogenized material properties that a global structure behaves on the macro-scales. The responses in the local fields, i.e. microscopic phases, can be reconstructed through the scale linking relations along with the global responses as input. The application of this multi-scale simulation method to composite patch repair for offshore structures is demonstrated by numerical examples.
Response of a Porous Seabed Under Random Wave Loading
Authors: Liu Haijiang , Jeng Dong-S.
Published: October 2008

The evaluation of the wave-induced soil response is particularly important for many coastal engineering installations such as offshore pipelines, platforms and breakwaters. Most previous investigations have been limited to the linear regular wave loading, even though the real situation is under random waves. In this study, we propose a semi-analytical solution for the random wave-induced pore pressure and effective stresses in marine sediments. Based on the new analytical solutions, different soil responses under the random wave loading are investigated and compared with the corresponding results under the linear regular waves. Numerical examples demonstrate the significant difference on wave-induced seabed response between these two wave loadings due to the irregularity introduced by the random waves. Finally, the influence of several soil parameters on the soil response under random wave loading is also examined.
Nonlinear Dynamic Response and Buckling of Damaged Composite Pipes Under Radial Impact
Authors: Tang Wenyong , Wang Tianlin , Zhang Shengkun
Published: October 2008

In this paper, the nonlinear dynamic response and buckling of damaged composite pipes under radial impact is investigated. A model involving initial geometric deformation, delamination and sub-layer matrix damage is set up for theoretical analysis. Based on the first order shear deformation theory, the nonlinear dynamic equations of the composite pipe considering transverse shear deformation and initial geometric imperfections are obtained by Hamilton’s theory and solved by a semi-analytical finite difference method. The effects of damage on the dynamic response and buckling of composite pipes are discussed.
A Dynamic Model for Ice-Induced Vibration of Structures
Authors: Huang Guojun , Liu Pengfei
Published: October 2008

A dynamic model for the ice-induced vibration (IIV) of structures is developed in the present study. Ice properties have been taken into account, such as the discrete failure, the dependence of the crushing strength on the ice velocity and the randomness of the ice failure. The most important prediction of the model is to capture the resonant frequency lock-in, which is analogue to that in the vortex-induced vibration (VIV). Based on the model, the mechanism of resonant IIV is discussed. It is found that the dependence of the ice crushing strength on the ice velocity plays an important role in the resonant frequency lock-in of IIV. In addition, an intermittent stochastic resonant vibration is simulated from the model. These predictions are supported by the laboratory and field observations reported. The present model is more productive than the previous models of IIV.
Bayesian Estimation of Occurrence Probability of Cracks in Offshore Platforms
Authors: Li D. Q.
Published: October 2008

To ensure safety of offshore platforms, non-destructive inspections are often conducted after the installation of offshore platforms. It is often impractical to inspect 100 percent of tubular joints of offshore platforms due to high costs and possible delays. Accordingly, the presence of cracks may not be fully detected as only a limited number of tubular joints can be inspected. This paper proposes a procedure to predict the occurrence probability of crack in tubular joints of offshore platforms based on ultrasonic inspections supplemented with engineers’ experiences. This procedure is developed in the Bayesian framework. To conduct the updating and estimating of the occurrence probability of cracks, formulae for calculating the updated probability distribution and Bayesian estimator of occurrence probability are derived. Then, the occurrence probability of crack is updated based on results from ultrasonic inspections. Results from numerical examples indicate that a reasonable estimation of the occurrence probability of crack can be obtained using the proposed procedure. Both the inspection results from the ultrasonic inspection and the crack size have a significant influence on the estimated occurrence probability of crack. The use of the available information from engineers’ experiences or similar projects can lead to a more accurate estimation of the occurrence probability of crack. The quality of offshore platforms will be overestimated if the uncertainty in crack size is not accounted for.
Interaction of Load Analysis and Structural Design of Offshore Wind Turbines
Authors: Argyriadis Kimon , Klose Marcus
Published: October 2008

The paper presents the necessary considerations for the determination of design loads for offshore wind turbines (OWTs) and OWT support structures. The design of OWTs strongly depends on the environmental conditions such as wind and wave data, ambient temperatures and soil conditions. Load analyses for OWTs are generally done using sophisticated simulation tools which consider the turbulent wind conditions as well as the structural dynamics and the operational behavior of the turbine. Compared to wind turbines onshore, wave loads are an additional element that needs to be considered. Experience shows that the natural frequencies of the support structure highly influence the design loads on the system due to excitation by the rotor. Therefore, the load analysis in combination with optimization of the support structure is an iterative process. A close cooperation between the designing engineers of turbine and support structure is definitely required. The calculation procedure will be presented as well as results from a sample calculation for a typical configuration.
Experimental Study of Air Gap Response and Wave Impact Forces of a Semi-Submersible Drilling Unit
Authors: Kazemi Saeid , Incecik Atilla
Published: October 2008

An experimental study for predicting the air gap and potential deck impact of a floating offshore structure is the main topic of this research. Numerical modeling for air gap prediction is particularly complicated in the case of floating offshore structures because of their large volume, and the resulting effects of wave diffraction and radiation. Therefore, for new floating platforms, the model tests are often performed as part of their design process. This paper summarizes physical model tests conducted on a semi-submersible model, representing a 1-to-100 scale model of a GVA4000 class, “IRAN-ALBORZ”, the largest semi-submersible platform in the Caspian Sea, under construction in North of Iran, to evaluate the platform’s air gap at different locations of its deck and also measure the impact forces in case of having negative air gap. The model was tested in regular waves in the wave tank of Newcastle University. The paper discusses the experimental setup, test conditions, and the resulting measurements of the air gap and the wave impact forces by using eight wave probes and three load cells located at different points of the lower deck of the platform.
Fatigue Capacity of Fillet Welded Connections Subjected to Axial and Shear Loading
Authors: Lotsberg Inge
Published: October 2008

The status on current design recommendations concerning the fatigue capacity of fillet welds was presented by Maddox (2002), based on a literature survey. In order to examine the validity of the recommendations and to supplement the fatigue test data base, a test matrix with 33 specimens was developed. This included 8 simple fillet welded cruciform joints that were subjected to axial loading and 25 fillet welded tubular specimens that were subjected to axial load and/or torsion for simulation of a combined stress condition in the fillet weld. The data obtained from these fatigue tests are presented in this paper. The test data are also compared with design guidance from IIW (1996), Eurocode 3 (1993) and DNV-RP-C203 (2005).
Innovations in Fishing Gear Design Aiming at the Sustainable Use of Commercial Fish Stocks
Authors: Dahm Erdmann
Published: October 2008

The present status of some of the fish stocks in North Sea and Baltic shows them to be outside safe biological limits. Reasons for this lie partly in hydrographic and ecological changes not in favour for a constant supply of fresh recruits to the fishery every year. On the other hand, fishery has its share in the observed downward trend by growth overfishing and reducing the size and number of possible spawners. The only mean available for the fishery management to restore a fish stock composed of several year classes is to apply technical measures as e.g.closures in space and time or fishing gear regulations. The breakdown of the historical measure “mesh size regulation in diamond mesh trawls” due to technological progress has created a wave of worldwide research. It has focused on trawl codends where the meshes are held artificially in their most open shape or by introducing into the trawl metal structures able to separate small from big fish or fish from crayfish. A parallel development of a scientifically objective mesh measuring instrument will help to enforce the new regulations. However, longer use of some of the new designs has revealed some deficiencies. Other innovative designs or the concept of certified codends will hopefully ensure the permanently better selectivity of contemporary trawl codends.
Wave Transformation and Soil Response Due to Submerged Permeable Breakwater
Authors: Tsai Ching-Piao , Chen Hong-Bin , Jeng Dong-Sheng , Chen Kuan-Hong
Published: October 2008

This study reports the experimental results of the wave transformation and the wave-induced soil response when the waves pass through the submerged permeable breakwater. The model of the submerged breakwater was built on a horizontal sandy bottom. The experimental results of the spectrum of the wave transformation and the wave-induced pore-pressure are first analyzed in this paper. It is found that the wave spectrum is similar to the condition of the impermeable bottom that the higher harmonic mode appears when the waves pass over the submerged structure. However, the higher harmonic mode is not found in the spectrum of the wave-induced pore pressure, showing that the nonlinearity of the pore pressure is damped by the porous bed. The influences of the geometry of the submerged breakwater to the transformation of the wave height and the pore-pressure are also investigated. Based on the experimental results, the regression formulas for the coefficients of the wave reflection, the wave transmission and the wave energy dissipation are obtained in the paper.
Benefits of Risk Based Inspection Planning for Offshore Structures
Authors: Straub Daniel , Goyet Jean , So̸rensen John D. , Faber Michael H.
Published: October 2008

The economical benefits of applying risk-based inspection planning (RBI) for offshore structures subject to fatigue are evaluated based on experiences from past industrial projects. To this end, the factors influencing the cost of inspection, repair and failure of structures are discussed and realistic values of these costs are presented. These are then applied to assess the expected costs from different inspection strategies, both risk-based strategies as well as inspection strategies with fixed inspection intervals for all potentially critical elements. By comparing these expected costs, the financial benefit of RBI is assessed.
B-Splines Geometric Modeling for Dynamic Analysis Behaviour in Offshore Systems
Authors: de Menezes Fa´bio G. T. , Dutra Martins Prota´sio
Published: October 2008

This work reports a study of B-Spline curves and surfaces applied to the geometric definition of hulls of ships and oil drilling and production platforms. The research aims at defining mathematically the floating body surface in suitable formats for the analysis of functional behaviour of the design object with sophisticated methods and tools. The WAMIT system was chosen as a reference in the research due to its reliability as a professional tool for hydrodynamic behaviour of floating systems in practice. The B-Spline model is input to the WAMIT system in the required format for the analysis of hull motion response to waves. The quality of the results obtained with B-Splines modeling was compared the ones obtained with flat panels. B-Splines have shown to be an effective approach, more efficient in computing terms when compared with the flat panels approach and suitable to optimization scripts. It revealed itself as a more adequate procedure to the design work as it simplifies the hull form mathematical definition of floating systems.
Droplet Number Spectra in the Jet Atomization Regime
Authors: Nihous Ge´rard C. , Masutani Stephen M.
Published: October 2008

A previous model [1] to determine the number spectra of droplets generated by the injection of a fluid into another fluid is extended to cases of instantaneous disruption (near-zero jet breakup time). Under this scenario, droplets immediately form at the injection nozzle from unstable antisymmetric jet-surface waves. Resulting droplet characteristics only depend on the initial jet-surface disturbance. Full atomization is attained as the jet fluid is consumed by a series of successive droplet formation events that peel away the jet surface. The model is applied when the initial jet-surface disturbance is represented as broad-band white noise: each peel is characterized by a selected wave amplitude and high-cutoff wavenumber. Very good agreement was observed between model results and experimental droplet number spectra [2] obtained in the atomization regime.
A Simple Estimation Method of the Stress Distribution Normal to Cross Section at Weld Toe in Non-Load Carrying Welded Joints
Authors: Gotoh Koji , Nagata Yukinobu , Toyosada Masahiro
Published: October 2008

Many fatigue damages are occurred in the welded built-up structures designed by the hot spot stress methodology, especially near a boxing fillet weld toe. These fatigue cracks usually initiate from the toe and propagate to the plate thickness direction. Although fatigue life is affected by the stress gradient working over crack propagation path, the effect of stress gradient in cross section is not considered in the hot spot stress methodology. Then, many attempts based on fracture mechanics for the improvement of fatigue life estimation are proposed. Whereas stress distributions along the fatigue crack path must be given in order to apply the methods based on fracture mechanics for the precise fatigue life prediction, no stress distribution along the path considering the stress concentration caused by weld toe shape is obtained in practical structural design stages because the shell elements are used in finite element analyses in the design stages. A simple estimation method of the stress distribution normal to cross section at weld toe in non-load carrying welded joints is proposed in this paper. Calculation results of finite element analysis with shell elements and geometrical conditions (radius and flank angle of fillet weld toe and plate thickness) are used as input data for the estimation. The validity of this method is confirmed by comparing estimation results with ones by finite element analysis with solid elements.
On the Quantification of Robustness of Structures
Authors: Faber Michael H. , Maes Marc A. , Straub Daniel , Baker Jack
Published: October 2008

The paper first reviews different interpretations of robustness. On this basis objectives facilitating the quantification of robustness of engineering systems are formulated. Thereafter a generic framework for risk assessments of engineering systems is presented in which robustness is related to the ability of the system to sustain damages. This framework is then applied to quantify robustness of structural systems and to develop a robustness index facilitating a consistent ranking of structures according to their robustness. The proposed approach to the assessment of robustness principally takes into account the effect of redundancy, ductility, damage and failure consequences as well as strategies for condition control and intervention during the service life of structures. Finally, a simple example illustrates the use of the framework for the assessment of the robustness of a jacket steel structure subject to fatigue damage. The example shows that presently used indicators for the robustness of jacket type steel structures such as the RIF only capture part of the picture and illustrates the merits of a risk based framework for robustness assessments.
Trajectory and Attitude Control of an Underwater Towed Vehicle
Authors: Wu Jiaming , Jin Xuefeng
Published: October 2008

A new concept of control technique to perform operation of trajectory maneuvering to a controllable underwater towed vehicle moving in a designated path with a required attitude is presented. A trajectory and attitude control technique for the towed vehicle is proposed in order to accomplish the vehicle’s trajectory and attitude manipulations. This technique is based on a fuzzy algorithm. The towed vehicle in the research consists of a cylindrical main body equipped with several active horizontal and vertical control surfaces. Numerical simulation on the hydrodynamic and control behavior of the towed vehicle under this control manipulation is conducted based on a fully 3-D hydrodynamic model of an underwater towed vehicle. In the model the governing equation of the towed cable is based on the Ablow and Schechter method. The six-degrees-of-freedom equations of motion for an underwater vehicle simulation proposed by Gertler and Hagen are adopted to estimate the hydrodynamic performance of the towed vehicle. In numerical simulation the deflections of vehicle’s control surfaces are governed by the proposed fuzzy controller to manipulate the vehicle traveling along a 3-D stipulated trajectory configuration and required attitude. The values of the deflections are taken as input parameters for the hydrodynamic model at every time step. The performance of the towed vehicle under different designated trajectory and attitude control manipulations can then be investigated with the hydrodynamic model.
Numerical Simulation of the Flow Around Free-Surface Piercing Bodies in Waves by an Overlapping Grids Approach
Authors: Muscari Roberto , Di Mascio Andrea , Broglia Riccardo
Published: October 2008

The topic of this work is the numerical simulation of a problem of interest in the field of offshore technology, namely the inviscid flow around four free–surface piercing cylinders in waves. Three different angles of the incoming wave front are considered in order to investigate the variation of the resulting global and local loads on the four bodies. The rotation of the cylinders configuration with respect to the wave front has been made easy and efficient by the use of a chimera method for the generation of the computational mesh. The free–surface has been simulated by an one–phase level set approach.
Calibration of Safety Factors for DNV RP-F105 Free Spanning Pipelines
Authors: Sigurdsson Gudfinnur , Mo̸rk Kim , Fyrileiv Olav
Published: October 2008

Free spans often become a significant challenge in pipeline design and operation due to uneven seabed or seabed scouring effects. The trend towards deeper waters, harsher environment and installation of pipelines at very uneven seabed often implies a high number of free spans. High costs related to span intervention puts focus on minimizing these costs and still ensure integrity of the pipeline with respect to vortex induced vibrations (VIV) and associated fatigue damage. On the other hand the potential costs related to fatigue failure of a pipeline (recovery costs and economical loss) are enormous. Therefore it is essential to ensure that the probability of failure for free spans is within acceptable limits, e.g. as required by DNV-OS-F101 “Submarine Pipeline Systems”. This paper describes the structural reliability analysis performed to obtain the safety factors for free span fatigue design. Accumulation of fatigue damage due to VIV of free spans is associated with various sources of uncertainty. The important stochastic parameters are described, and the basis for the uncertainty modeling given. The calibration scope defined from a set of different pipeline cases, span scenarios, and environmental conditions is presented from which calibration results and sensitivities will be discussed.
Results From Model Testing of Ice Protection Piles in Shallow Water
Authors: Gu¨rtner Arne , Berger Joachim
Published: October 2008

The development of oil and gas fields in shallow icy waters, for instance in the Northern Caspian Sea, have increased the awareness of protecting offshore structures by means of ice barriers from the impacts of drifting ice. Protection could be provided by Ice Protection Piles (IPPs), installed in close vicinity to the offshore structure to be protected. Piles then take the main loads from the drifting ice by pre-fracturing the advancing ice sheet. Hence, the partly shielded offshore structure could be designed according to significant lower global design ice loads. In this regard, various configurations of pile arrangements have been model tested during the MATRA-OSE research project in the Ice Model Test Basin of the Hamburg Sip Model Basin (HSVA). The main objective was to analyse the behaviour of ice interactions with the protection piles together with the establishment of design ice loads on an individual pile within the pile arrangement. The pile to pile distances within each arrangement were varied from 2 to 8 times the pile diameter for both, vertical and inclined (30° to the horizontal) pile arrangements. Two test runs with 0.1 m and 0.5 m thick ice (full scale values) were conducted respectively. The full scale water depth was 4 m. Based on the model test observations, it was found that the rubble generation increases with decreasing pile to pile distances. Inclined piles were capable to produce more rubble than vertical piles and considerable lower ice loads were measured on inclined arrangements compared to vertical arrangements. As initial rubble has formed in front of the arrangements, the rubble effect accelerated considerable. Subsequent to the build-up of rubble accumulations, no effect of the pile inclination on the exerted ice loads could be observed. If piles are used as ice barriers, the distance between the piles should be less than 4D for inclined piles and 6D for vertical piles to allow sufficient rubble generation. Larger distances only generated significant ice rubble after initial grounding of the ice had occurred.
Research on Applicability of New Materials to Marine Structures in Tropical Climates: Durability Assessment of New Materials
Authors: Tomosawa Fuminori , Tsujikawa Shigeo , Ono Tadashi , Yonemaru Keisuke , Takizawa Shozo , Kugai Yasuo , Minemura Norishige
Published: October 2008

New structural members that are light and durable are anticipated to reduce the running and maintenance costs of structures exposed to harsh marine environments, such as offshore oil production facilities, thereby reducing their lifecycle cost. This study aims to investigate the applicability of new materials to marine structures, focusing on their durability. To this end, a 5-year exposure test has been conducted beginning in 1999 on 3 types of specimens (for corrosion observation, tension testing, and joint strength testing) made of 21 selected materials (6 nonferrous metals, 8 steels, 4 composite materials, and 3 rope materials). The specimens have been exposed at 3 sites: Okinotori-shima and Miyako-jima, corrosive environments with high temperature and humidity, and a thermo-hygrostatic room in a laboratory. Having completed the natural exposure tests in 2004, the authors conducted strength tests and observation thereafter toward 2005. This paper summarizes the results of such tests and observation conducted so far, while making a final assessment of each material, and refers to the tasks ahead based on these results. No marked differences were observed between the results of exposure at Okinotori-shima and Miyako-jima. Both islands were therefore found to be similar environments in terms of corrosion. The corrosion states of specimens were classified into five Categories: I (no corrosion), II (slight corrosion), III (light corrosion), IV (corrosion with strength loss), and V (corrosion with significant strength loss). Only reference specimens of ordinary steel were designated as Category V. Those designated as Category IV included textile ropes and Ni steel and coated steel panels with damaged coating film. Most other new materials were designated as Categories III or less with no strength losses. Materials designated as Categories I to III were subjected to surface observation using optical and electron microscopes and element distribution analysis over a cross section using an electron beam probe microanalyzer. The progress of corrosion in each material over the five years has thus been elucidated. Within the range of the 5-year exposure test, most of the selected new materials pose no problems in regard to durability. However, marine structures are more vulnerable to alternate stresses than general structural members on land because of constant waves and pulsating gales on the sea. The authors intend to investigate the durability of structural members under continued or cyclic stress as a subject for the future.
The Hilbert-Huang Spectral Analysis Method Applied to VIV
Authors: Pesce Celso P. , Fujarra Andre´ L. C. , Kubota Leonardo K.
Published: October 2008

Vortex-Induced Vibration (VIV) is a highly nonlinear dynamic phenomenon. Usual spectral analysis methods rely on the hypotheses of linear and stationary dynamics. A new method envisaged to treat nonlinear and non-stationary signals was presented by Huang et al. [1] : The empirical mode decomposition and the Hilbert spectrum for nonlinear and non-stationary time series analysis. This technique, called thereafter the Hilbert-Huang transform (or spectral analysis) method, is here applied to VIV phenomena, aiming at disclosing some hidden dynamic characteristics, such as the time-modulation and jumps of multi-branched response frequencies and their related energy spectra.
Simulation of Riser VIV Using Fully Three Dimensional CFD Simulations
Authors: Holmes Samuel , Oakley Owen H. , Constantinides Yiannis
Published: October 2008

Fully three dimensional computational fluid dynamics (CFD) solutions are combined with structural models of a tensioned riser to predict riser vortex induced motion. The use of three dimensional CFD solutions overcomes many of the shortcomings of combining a series of strip or two dimensional simulations to calculate the fluid forces on the riser. Three dimensional vortex structures are treated correctly and straked risers and variations in angle of attack can be studied directly. The proposed method uses finite element methods that are tolerant of sparse meshes and high element aspect ratios. This allows economical solutions of large fluid domains while retaining the important features of the large fluid vortex structures which drive risers. Long risers can be treated with readily available computers and examples of simulations of riser with L/D over 1400 are given and compared with previously published experimental data. These examples are used to illustrate several points regarding the effects of the treatment of the riser structure as well as the efficacy of rotating frame or pinned riser experiments used to simulate sheared currents. The method can also be extended to sheared currents whose heading varies with depth.
Buried Pipelines Subjected to Transverse Ground Movement: Comparison Between Full-Scale Testing and Numerical Modeling
Authors: Karimian Hamid , Wijewickreme Dharma , Honegger Doug
Published: October 2008

A research program has been undertaken to study the behaviour of buried steel pipelines subject to lateral horizontal ground movements, and to provide appropriate data to calibrate and validate numerical model(s). A large sand chamber (2.5 m W × 3.8 m L × 2.5 m H) available at the University of British Columbia was employed to conduct full-scale lateral pullout tests on steel pipelines, with different diameters and buried in sand simulating different overburden ratios. Numerical analyses were performed using finite-difference-method-based software with the soil response simulated using Mohr-Coulomb and hyperbolic elastic constitutive models. The input parameters for the initial computer modeling were based only on element testing results. The numerical predictions, using the two soil constitutive models, are compared with the results of lateral pullout tests. The numerical model, after validation with full-scale test results can be used to predict soil loads on pipe for different overburden ratios, pipe sizes and soil properties.
Pipeline-Trawl Interaction: Effect of Trawl Clump Weights
Authors: Fyrileiv Olav , Askheim Dag O̸. , Verley Richard , Rolsdorph Hanne
Published: October 2008

This paper deals with clump weights, a new type of component in bottom trawl gear that needs to be considered in design and reassessment of pipelines. In recent years it has become popular to use two trawl bags, a so-called twin trawl, to increase the swept area and, thus, the efficiency of the trawler. In addition to the trawl boards at each side of the trawl bags, a heavy weight, a clump weight, is used between the bags to keep them close to the seabed. Clump weights can have a mass up to 9 tonnes, and the largest ones are shaped as spheres or cylinders inside a steel frame. Tests performed have revealed that the loads on pipelines from clump weights may exceed the corresponding loads from trawl boards. Thus, interaction with clump weights may govern the design and qualification testing of coating, and the design related to global pipeline response. This paper discusses loads from clump weights. Updated design approaches including pull-over load estimates are presented. The background in terms of small scale test results is also shown for some cases. The conclusion is that clump weights may govern the trawl design of pipelines, especially for trawl gear impact and pull-over.
Wave-Current Loading on a Vertical Slender Cylinder by Two Different Numerical Models
Authors: Zaman M. H. , Baddour R. E.
Published: October 2008

The study of the effects resulting from the interaction of a combined wave-current field with any ocean structure is important for the design and performance evaluation of that structure. The prudent computation of forces exerted by waves and currents is an essential task in the study of the stability of an offshore structure. A study on the loading of an oblique wave and a current field on a fixed vertical slender cylinder in a 3D flow frame is illustrated in Zaman and Baddour (2004). The three dimensional expressions describing the characteristics of the combined wave-current field in terms of mass, momentum and energy flux conservation equations are formulated. The parameters before the interaction of the oblique wave-free uniform current and current-free wave are used to formulate the kinematics of the flow field. These expressions are also employed to formulate and calculate the loads imparted by the wave-current fluid flow on a bottom mounted slender vertical cylinder. In this work a 2D version of the above 3D model called here Model-I has been used for the numerical computations presented in this paper. The second model denoted model-II in the present paper is based on Euler equations. This model is formulated through the vertical integration of the continuity equation and the equations of motions, Zaman et al (1997). A semi-implicit numerical technique is employed for the numerical solution. In the present paper comparisons are made between the results obtained from the 2D version of the above models in finite depth. Both models are then compared with some relevant experimental data. Morison et al equation (1950) is deployed for the load computations in all cases.
Fatigue Crack Growth Assessment of Welded Joints in Ships Structures: A Reliability-Based Sensitivity Study
Authors: Rizzo Cesare Mario , Ayala-Uraga Efren
Published: October 2008

Fatigue life calculations have been recently introduced as explicit checks in classification societies rules for shipbuilding. Such checks are based on the classic S-N approach, which cannot be easily applied to the condition assessment of structural details. Fracture mechanics approach allows describing the structural degradation of ageing structures and the reliability framework accounts for the inherent probabilistic aspects of fatigue. This approach gains relevance as many ship tankers are being converted to operate as offshore ships. This paper carries out a sensitivity analysis of the reliability model for fatigue life calculations of typical ship’s welded joints based on fracture mechanics, aiming to select the most influencing parameters, before and after inspection updating.
Characteristics of Current Generation System in Deepwater Offshore Basin
Authors: Haining Lu , Jianmin Yang , Tao Peng
Published: October 2008

The current generation system in deepwater offshore basin is important for the correct modeling of ocean environment. It is generally considered to be a challenge to obtain uniform and stable current flow in the basin. As technical assurance numerical and experimental studies are performed to investigate the characteristics of the deepwater current generation in the basin. Reynolds-Averaged Navier-Stokes (RANS) equations and the standard k-ε turbulence model are adopted to simulate the current generation system numerically. In addition a 1:10 scaled model test is also performed. In both numerical and experimental studies horizontal and vertical current velocity profiles, turbulence levels and pressure losses during the current recirculation etc. are studied. It is concluded that the perforated walls are key components of the current generation system. In addition various vertical current velocity profiles can be realized in the basin.
Laboratory Simulation and Analysis of Wave Groups
Authors: Balaji R. , Sannasiraj S. A. , Sundar V.
Published: October 2008

The coastal and offshore structures are some times exposed to group of waves with successive higher wave elevations exceeding the significant wave height, which is considered to be vulnerable for the stability of the structures. Hence, the knowledge on the existence and frequency of occurrence of ocean wave groups at a particular region of interest is important for the design of the ocean structures. In the present study, the wave groups were simulated theoretically and the same was generated in the laboratory wave flume. The measured wave elevations were analysed through statistical, spectral and wavelet approaches to detect the existence of the groupiness.
Overview of Fluid-Structure Coupling in ANSYS-CFX
Authors: Menter Florian , Sharkey Patrick , Yakubov Sergey , Kuntz M.
Published: October 2008

The paper provides information of fluid-structure interaction (FSI) capabilities in the ANSYS multi-physics software environment for off-shore riser applications. Simulation examples are a simple verification of the FSI code by a bending beam in a flow channel, vortex induced vibrations (VIV) for a cylinder in cross flow and the flow around an experimental riser geometry.
Methods for Collapse Pressure Prediction of UOE Linepipe
Authors: Liessem Andreas , Marewski Ulrich , Groß-Weege Johannes , Knauf Gerhard
Published: October 2008

Line pipe intended for deep water applications has to be designed predominantly with regard to external pressure in order to avoid plastic collapse. As a consequence of cold forming during UOE pipe manufacture and the subsequent application of anticorrosion coating, the characteristic stress strain behavior has to be taken into account for a reliable prediction of the collapse pressure. Verification of collapse resistance of large diameter pipes against external pressure requires adequate and reliable component testing using a sufficient number of pipe samples. These samples have to be subjected to test conditions, which closely simulate the situation in service. As the test results may depend significantly on its boundary conditions, the results needs to be thoroughly analysed and compared with existing prediction methods. It is for these reasons that such full-scale testing is time-consuming and costly. The work presented in this paper aims at clarifying and quantifying the effect of existing test boundary conditions on the results of collapse tests (collapse pressures). Correlations will be established between material properties found in laboratory tests and associated component behavior. In this context it had been necessary to develop an accurate and reproducible compression test method. The actual collapse pressures and those predicted using current available equations are compared and verified by Finite Element calculations. The paper concludes with a discussion of the major findings and with a brief outlook to future research issues.
Numerical Investigation of the Unsteady Flow at High Reynolds Number Over a Marine Riser With Helical Strakes
Authors: Pinto Antonio , Broglia Riccardo , Di Mascio Andrea , Campana Emilio F. , Rocco Pierpaolo
Published: October 2008

Prediction of Vortex-Induced Vibrations (VIV) is one of the main topics in the design of deepwater risers. The understanding and modelling of the complex fluid-structure interaction requires advanced analysis techniques coupling, in a correct manner, both structural and fluid dynamics aspects. This study aims to develop, optimise and calibrate a numerical code to provide reliable results within a reasonable analysis timeframe and without, or very limited, need of experimental verification. For this purpose, the unsteady Reynolds Average Navier-Stokes (RANS) code χnavis is applied to solve a typical riser VIV problem and compute the three-dimensional riser-fluid dynamics interaction. During a preliminary analysis phase, the two-dimensional (2-D) flow past (i) a bare circular cylinder and (ii) a straked riser at high Reynolds numbers is simulated (different incidences flow/strake vanes are analysed). Numerical results are validated and calibrated against published test data. The core analysis phase is then focused on the numerical investigation of the unsteady flow over a three-dimensional (3-D) helical strake. In this phase, the three-dimensional flow field, turbulent structures and response frequency patterns are analysed. Spectral analysis of data is performed to identify carrier frequencies deemed to be critical due to the induced vibration of the whole structure, and helical strakes efficiency in reducing the riser vibrations is also addressed. Finally, comparison between numerical and experimental results shows that the complexity of a three-dimensional model is indeed compensated by a significantly improved accuracy of the obtained results.
Numerical Simulation of Nonlinear Wave Propagation and Breaking Over Constant-Slope Bottom
Authors: Dimakopoulos Aggelos S. , Dimas Athanassios A.
Published: October 2008

The numerical simulation of the two-dimensional free-surface flow resulting from the propagation of nonlinear gravity waves over constant-slope bottom is presented. The simulation is based on the numerical solution of the Euler equations subject to the fully nonlinear free-surface boundary conditions and the appropriate bottom, inflow and outflow conditions using a hybrid finite-differences and spectral-method scheme. Wave breaking is accounted for by a surface roller model. The formulation includes a boundary-fitted transformation and is suitable for future extension to incorporate large-eddy and large-wave simulation terms. Results are presented for the simulation of the free-surface flow over two different bottom topographies, with constant slope values of 1:10 and 1:50, and three different inflow wave heights. Over the bottom slope, waves of small wave heights are modified according to linear theory. For nonlinear waves, wavelengths are becoming shorter, the free surface elevation deviates from its initial sinusoidal shape and wave heights increase with decreasing depth. Breaking is observed for the cases with the larger initial wave height and the smaller outflow depth.
Numerical Simulation of Vortex Induced Vibration Using the K-ε Model
Authors: Wanderley Juan B. V. , Souza Gisele H. B. , Levi Carlos
Published: October 2008

Numerical simulations of Vortex Induced Vibration have been failing to duplicate accurately experimental data mostly due to the complexity of the physics involved in the real problem. Therefore, a careful and comprehensive investigation on CFD algorithms is still required to indicate the most suitable numerical scheme to handle such a complicate problem. Grid generation, boundary condition implementation, and coupling between the fluid flow governing equations and body motion equation are known to have strong influence on the qualities of the numerical results. This work presents results obtained from a long-term investigation featuring different CFD methods. The investigations enabled the selection of a very effective algorithm that showed an outstanding agreement between experiment and numerical simulation of the VIV phenomenon. Good agreement is obtained in the entire range of reduced velocity covered by the experimental investigations. The successful algorithm discussed here applies the Beam and Warming implicit scheme to solve the two-dimensional slightly compressible Navier–Stokes equations with the K-ε turbulence model to simulate the turbulent flow at the wake of the cylinder.
Collapse Arrestors for Deepwater Pipelines: Finite Element Models and Experimental Validations for Different Cross-Over Mechanisms
Authors: Toscano Rita G. , Mantovano Luciano O. , Amenta Pablo , Chareau Roberto , Johnson Daniel , Assanelli Andrea , Dvorkin Eduardo N.
Published: October 2008

Using finite element models it is possible to determine the cross-over external pressure of different pipeline arrestor designs. In this paper these finite element models are discussed and validated by comparing their results with experimental determinations. The flipping and flattening cross-over mechanisms are considered in the experimental validation of the numerical models.
Dynamic Analysis of a Flexible Hanging Riser in the Time and Frequency Domain
Authors: Low Ying Min , Langley Robin S.
Published: October 2008

This paper outlines the dynamic analysis of flexible risers in the time and frequency domain using lumped mass discretization, where tension and bending are modeled with extensional and rotational springs respectively. For the time domain analysis, integration is carried out using the Wilson-theta implicit scheme, which allows the use of relatively large time steps without compromising stability. This increases computational efficiency and automatically filters the high frequency axial responses. The time domain code is validated with the commercial software Orcaflex, which employs an explicit scheme, and results are found to match for the same number of elements. The relative merits of implicit and explicit integration schemes are discussed. For the frequency domain analysis, the added mass, damping, axial/bending stiffness matrices are formulated in global coordinates. The nonlinear drag force is linearized iteratively for both regular and random waves. The range of accuracy for the linearized frequency domain simulations is assessed by methodical comparisons with the nonlinear time domain results for varying loading amplitudes. One problem encountered during the early development of an analytical tool is the lack of published results for validation, especially where access to commercial packages and test facilities is unavailable or limited. Hence, the simulation results presented herein are for a flexible hanging riser with simple boundary conditions and load cases to facilitate benchmarking.
A Comparison of Time Domain and Frequency Domain Approaches for the Fully Coupled Analysis of Deepwater Floating Systems
Authors: Low Ying Min , Langley Robin S.
Published: October 2008

The recognition of the need for a fully coupled analysis of deepwater floating production systems has led to the research and development of several coupled analysis tools in recent years. Barring a handful of exceptions, these tools and available commercial packages are invariably in the time domain. This has resulted in a much better understanding and confidence in time domain coupled analysis, but less so for the frequency domain approach. In this paper, the viability of frequency domain coupled analysis is explored by performing a systematic comparison of time and frequency domain methods using computer programs developed in-house. In both methods, a global coordinate system is employed where the vessel is modeled with six degrees-of-freedom, while the mooring lines and risers are discretized as lumped masses connected by extensional and rotational springs. Coupling between the vessel and the mooring lines is achieved by stiff springs, and the influence of inertia and damping from the lines are directly accounted for without the need for prior assumptions. First and second order wave forces generated from a random environment are applied on the vessel, as well as drag and inertia loading on the lines. For the time domain simulation, the Wilson-theta implicit integration scheme is employed to permit the use of relatively large time steps. The frequency domain analysis is highly efficient despite being formulated in global coordinates, owing to the banded characteristics of the mass, damping and stiffness matrices. The nonlinear drag forces are stochastically linearized iteratively. As both the time and frequency domain models of the coupled system are identical, a consistent assessment of the error induced by stochastic linearization can be made.