bluuebunny/crossref_metadata_2025 huggingface dataset:
Whistling of Pipes With Narrow Corrugations: Scale Model Tests and Consequences for Carcass Design
Authors: Golliard Joachim , Belfroid Stefan , Bendiksen Erik , Frimodt Casper
Published: November 2013

Pipes for gas production and transport with a corrugated inner surface, as used in flexible pipes, can be subject to Flow-Induced Pulsations when the flow velocity is larger than a certain velocity. This onset velocity is dependent on the geometry of the corrugations, the operational conditions and the geometry of the topside and subsea piping.

In this paper, small-scale tests performed on corrugated tubes are reported. The tested geometries include both “classical” profiles, similar to the inner profile of agraff flexible risers, and profiles with less typical variations, such as narrower and/or deeper cavities, or irregular pitch. These tests were performed in order to evaluate the validity of a prediction model developed earlier for the onset of pulsations, for corrugated pipes with these kinds of atypical variations, which are found on a new type of carcass designs. The mechanism of Flow-Induced Pulsations in corrugated pipes is discussed, as well as the principle of the prediction model. The experimental results show that the validity of the model remains reasonable in most cases, except when the cavities are very narrow. In this case, the model becomes overly conservative. This limitation can be attributed to the fact that, for very narrow cavities, the cavity opening becomes too small compared to the boundary-layer momentum thickness, effectively destroying any instability of the shear layer. Furthermore, the shift towards higher frequencies of the acoustic source term due to narrower cavities, and the possible coupling with higher acoustic modes, is considered.

The results of the analysis are used to evaluate the onset velocity and whistling behavior of a newly developed carcass design of flexible risers. A previous analysis has indicated that the particular geometry profile of the new design improves the whistling behavior by pushing the onset velocity outside the typical operational envelope of flexible risers. The analysis confirms that the new design will be less prone to whistling than flexible risers with classical agraff carcasses.
Fatigue, Creep and Electrical Performance of Subsea Power Cable
Authors: Buitrago Jaime , Swearingen Sean F. , Ahmad Sana , Popelar Carl F.
Published: November 2013

Subsea processing is a key aspect of ExxonMobil’s subsea technology qualification efforts. Consequently, the reliable delivery of high voltage power to subsea equipment offshore, such as multi-phase pumps, is essential. The base case involves power delivery up to 30 kV via copper cables embedded in umbilicals to a water depth of 1,500m over a design life of 30 years. Since umbilicals are often suspended from a floating facility in deepwater, the power cables are subjected to static load due to the umbilical weight and to cyclic loads in response to the floating facility motions. Therefore, sufficient mechanical resistance to preclude electrical failure is needed to achieve safe and cost-effective designs.

To that end, an experimental program was completed aimed at developing a qualification methodology and collecting data on the effect of static and cyclic loads on the mechanical and electrical performance of copper power cables. This paper presents the experimental approaches and results obtained from creep, fatigue, and electrical tests performed on 18/30kV, 95mm2 copper conductor cable specimens.

Specifically, the program included (a) five monotonic tension tests to failure to characterize the cable, (b) 11 creep tests to develop a load-time performance curve, and (c) 59 fatigue tests under pulsating tension, of which 50 used short-cable samples to evaluate the effect of mean load and nine used long samples for electrical testing following fatigue. A fatigue curve is proposed that includes the effect of mean load and temperature. However, fatigue results can be sensitive to the cable material and mechanical characteristics of a particular cable.
Advances in Nonlinear Waves With Emphasis on Aspects for Ship Design and Wave Forensics
Authors: Osborne Alfred R.
Published: November 2013

Prof. D. Faulkner emphasized the importance of the study of extreme/rogue waves when he noted that the use of sine waves for computing pressures in the design of ships was no longer tenable, primarily because of the large number of cases where extreme structural damage has been encountered due to highly nonlinear large waves. This perspective resulted in the creation of the European program MaxWave and the subsequent program Extreme Seas soon followed. Recently my own studies of nonlinear effects in water waves at Nonlinear Waves Research Corporation (NWRC) have resulted in a number of successes with regard to the fundamental physical understanding of rogue waves. These studies enlarge our ability to understand the requisite impact of extreme waves on the design of ships. Some of these advances are:

(1) The determination of analytical techniques for describing rogue wave packets in two dimensions for random sea states which are directionally spread.

(2) The description of wave overturning and breaking in directional sea states with the Type II (lateral) instability.

(3) The development of hyperfast computer models for the deterministic simulation of directional sea states.

(4) The development of a fast approach for computing the full Boltzmann integral (FBI) for the nonlinear wave/wave interactions in wind/wave models.

(5) The identification of the actual physical location in the power spectrum for the nonlinear Fourier rogue wave components.

(6) The development of nonlinear Fourier techniques for analyzing times series of ocean waves for the presence of rogue wave states.

(7) The development of fully nonlinear directional spectra (in terms of frequency and direction) from arrays of instruments.

(8) The development of hindcasting and predicting capability for the assessment of the onset of a rogue sea.

I also discuss a number of future developments now underway at NWRC.
Wake-Induced Transverse Vibration of Two Interfering Cylinders in Tandem Arrangement: Modelling and Analysis
Authors: Armin Milad , Srinil Narakorn
Published: November 2013

A considerable number of numerical and experimental studies have been performed on the problem of vortex induced vibration (VIV) of an isolated circular cylinder. A very few studies have considered a practical situation where cylinders are deployed in clusters. This study presents a mathematical fluid-structure interaction modelling and analysis of two flexibly-mounted circular cylinders arranged in tandem and subject to fluid cross flows. The hydrodynamic lift forces and their time variations are approximated by two different semi-empirical wake oscillator models based on the van der Pol and Rayleigh equations. These nonlinear wake oscillators are coupled with linear structural oscillators through the acceleration and velocity coupling terms, respectively. A direct numerical time integration approach is used to predict the response amplitude behaviors and parametrically investigate the vortex- and wake-induced vibration transverse response of the two interfering upstream and downstream cylinders. Some empirical coefficients are calibrated against available, although very limited, computational fluid dynamics results. Preliminary parametric studies are conducted with the case of varying reduced flow velocity, and some insightful aspects on the effect of mass and damping ratio are highlighted. Depending on system parameters, numerical prediction results based on the van der Pol and Rayleigh equations are compared, and a combination of the two wake oscillators is suggested as a new model for predicting the vortex and wake-induced of the two interfering cylinders.
Connectors for Production Risers
Authors: Figoni Marco , Chand Sudheer
Published: November 2013

Increased interest in deepwater and ultra-deepwater field development is leading the industry to revisit the standard design and testing methods of production risers. The main challenges involved in a deepwater project are the riser sizing, management of external pressure at maximum water depth with acceptable weight to keep low stress at riser-vessel interface, the riser/vessel dynamic interaction and riser installation. The most critical areas on the riser system are the top section, where most of the damage occurs, and the touch-down zone. While the second area can be addressed by effective riser design, the connection to the production facility is always required and it must be sufficiently strong to provide an appropriate fluid containment. A connector failure can lead to highly flammable fluid leakage, putting in danger the entire production vessel and the surrounding environment. Technological advancements are leading to new equipment layouts with more efficient materials selection. The top section and connector can be subjected to different kinds of loads and environment induced degradation.

Applying the actual standards is sometimes difficult due to the fact that they generally are too conservative. With the objective of closing this gap, this paper provides an overview of the issues regarding different types of connectors in different working environments. The criteria for achieving an optimum connection selection depends on the type of riser utilized, riser configuration, metocean conditions, support vessel type, properties of fluid/chemicals transported inside the riser, life expectancy, maintenance and accessibility. Three main kinds of connection seals were analyzed, namely pure metallurgical (i.e. welding), pure mechanical (i.e. flange, threaded connections) and a combination thereof for line-end interface (i.e. metal-to-composite). All of the seals must provide retention of internal fluids and exclusion of external fluids and impurities. In an offshore environment, both of these characteristics must be achieved through the use of at least one metal-to-metal contact between mating interfaces and one static seal (i.e. metal rings).
Comparative Evaluation of the Cloud Dispersion of a Liquefied Natural Gas Leakage Using the UDM and a CFD Model
Authors: MirallesSchleder Adriana , Martins Marcelo Ramos
Published: November 2013

Large reserves of natural gas exist worldwide, particularly in areas in which there is no market or where the resources exceed the demand; this natural gas is liquefied for shipping to areas where there is a demand. As the liquefied natural gas is a flammable substance, a leakage in this process may cause undesired events like fires and explosions.

The consequence analysis is used to define the extent and nature of effects caused by undesired events and thus to quantify the damage caused by such events. Specific models are used to analyze the spills or jets of gas and liquid, gas dispersion, explosions and fires. The central step in the analysis of consequences is to determine the concentration of the vapor cloud of hazardous substances released into the atmosphere, in space and time. Gaussian and integral tools are extensively used in risk analysis, mainly to develop analysis about gas flow over flat terrain, providing fast dispersion estimations. Recently, with the computational advances, computational fluid dynamics (CFD) tools are used to short and medium range gas dispersion scenarios. However the advantages and disadvantages of each approach according with the scenario evaluated have not been widely discussed.

This paper evaluates, using a CFD model, the cloud dispersion of a LNG leakage. Then the results are compared with the results previously obtained by UDM (Unified Dispersion Model) and some advantages and disadvantages of each model are discussed. This study contributes to the decision making about the choice of most appropriated model to evaluate the consequences analysis.
Validation Methodology of Crack Growth Measurement Using Potential Drop Method on SENB Specimens
Authors: Antunes Marcio Ribeiro , Renck Tiago , Kwietniewski Carlos Fortis , Diehl Bruno , Abreu Eliakin , Clarke Thomas , Fonseca José , Strohaecker Telmo R. , Santos Fabrício P. , Joia Carlos J. B.
Published: November 2013

The Brazilian pre-salt oil and gas discoveries brought technical challenges as impressive as the reserves themselves. Besides the concerns with exploration, the oil contamination with CO2, H2S and chloride enriched seawater combined with critical cyclic loads due to the relative movement of the production vessel and high water depths imposes an environment chemically and mechanically aggressive. Suitable materials to work on such harsh conditions are few and one should consider the use of special materials, such as supermartensitic and superduplex stainless steels. Although the corrosion and mechanical properties of these materials are well established, still additional crack growth data in specific environments should be provide to the subsea equipment designer. Indeed, due to the combination of cyclic loading and corrosive ambient the corrosion fatigue phenomenon is a major concern. In order to evaluate the effect of oil contaminants on the corrosion fatigue resistance of candidate materials, one should provide methods for crack growth measurement other than the use of crack gauges since those can not be used in chemically aggressive solutions. The present work aims to validate the potential drop crack growth measurement method comparing the results obtained by this technique with those produced by crack gauges on SEN(B) (Single Edge Notch Bending) specimens in air. This validation effort is essential because the ASTM E647 standard only consider the use of C(T) (Compact Tension) specimens which actually does not represents the real cracks propagation path in crucial subsea equipment, such as risers, drill pipes et cetera, that is through the wall thickness. The results produced by the two tested methodologies have an excellent agreement which makes reliable the use of the potential drop method as an alternative to monitor and measure crack growth in corrosive media.
Methodology to Verify Residual Strength in Damaged Pontoon’s Vertical Webs Using FEM Buckling and Nonlinear Analysis
Authors: Ricardo Luiz Carlos H. , de Avellar Torres Fernando , Ruiz João Franciso B.
Published: November 2013

This paper introduces a methodology that can be used to verify damaged vertical webs from Semi-Submersible, Mobile Offshore Drilling Unit (MODU) as well as others offshore structures by finite element method. This paper presents an example applied to a vertical web from a triangular MODU, which consists of a triangular deck, three columns, and other supporting structure. It was subjected to damage to one of the three pontoons. The methodology isolates one of the damaged vertical webs and applies the boundary condition on the adjacent structure where the web is located. The web was modeled, using FEM in the both original and damaged condition based on information gathered from a 3D scanning system. The structure was evaluated using the American Bureau Shipping (ABS) Rules and Guides, [1] as design criteria. Based on the results of the analysis, the damaged member slightly exceeds the allowable limits. Reinforcements of the damaged areas were implemented on the damaged panels in order to reduce the stresses acceptable levels.
Modeling Pozzolanic Systems for Subsurface Cementitious Systems
Authors: Koenders Eduardus , Rocha Camila Aparecida Abelha , Toledo Filho Romildo Dias , Ukrainczyk Neven
Published: November 2013

The secondary pozzolanic reaction mechanism has been modeled explicitly in the Delft hydration model Hymostruc. The model calculates the progress of the hydration process as a function of the particle size distribution, the water cement ratio, the temperature and the cement and pozzolanic chemistry. The consumption of portlandite due to the activation of the pozzolanic materials is shown in detail. The numerical results are validated by an experimental testing plan on G-cement and 8% of silica fume and a water to cementitious ratio of 0.44. The simulated development of portlandite and degree of hydration and the experimental results are in good agreement.
Dynamic Positioning in Ice: Comparison of Control Laws in Open Water and Ice
Authors: Kerkeni Sofien , Dal Santo Xavier , Metrikin Ivan
Published: November 2013

Dynamic Positioning (DP) systems are intensely used in a large range of ship operations nowadays. The growing interest of Arctic exploration and exploitation may introduce a new application area for those systems. The very few full scale DP operations in the Arctic have demonstrated the need for improvements in DP systems for ice-covered waters. External forces due to the ice environment are very different from open water forces and especially the dynamic component of the loads is much higher. This paper firstly reviews DP in open water and spotlights the needs for adaptations to ice-covered regions. The architecture of a controller answering ice requirements is then presented. This system has been successfully tested at the large ice tank of the Hamburg Ship Model Basin (HSVA) in 2012 within the European R&D project DYPIC [1]. The designs of open water and ice control laws are then compared in two simulation frameworks. The first framework involves only the current, wind and waves, while the second framework deals with the ice conditions. The numerical ice simulator, utilized in this paper, is a novel high-fidelity modelling tool developed by the Norwegian University of Science and Technology (NTNU).
Midscale Qualification Testing of End Fittings for High Temperature Flexible Pipes
Authors: Melve Bjørn , Øren Einar
Published: November 2013

The qualification of high temperature flexible risers was a critical element in the field development of the Norne and Åsgard fields. At a later stage this also included the Kristin field. The high temperature leads to the selection of PVDF as the liner (pressure sheath) material. Due to numerous leaks from pressure sheath sliding in the end fittings of risers with three layer PVDF liners in the middle of 90’s, it was necessary to requalify the end fitting design. An extensive test 10 year program was initiated where mid-scale riser samples were cycled between the minimum and maximum operating temperatures. In the 10 year long test program were some test pipes were subjected to more than 1000 cycles without pressure sheath sliding or a failure. The modified end fittings were qualified for service.
Dynamic Excitation of Monopiles by Steep and Breaking Waves: Experimental and Numerical Study
Authors: Bredmose Henrik , Slabiak Peter , Sahlberg-Nielsen Lasse , Schlütter Flemming
Published: November 2013

An experiment with a flexible pile subjected to steep and breaking irregular waves has been conducted. The pile was constructed to represent a monopile wind turbine at scale 1:80. Two point masses were mounted on the pile to achieve the right scaled values for the first and second natural frequency. Emphasis is given to the observed impulsive excitation of the natural modes by steep and breaking waves. Additionally, springing and ringing-type continuous forcing of the first natural mode is seen for the moderately steep waves.

The experiments were carried out at three depths and with two wave climates. The measured data for structural acceleration is analysed with respect to individual wave parameters. It is found that the largest accelerations occur for breaking waves.

The measured wave field and structural response are reproduced numerically with a fully nonlinear potential flow solver for the undisturbed wave kinematics, combined with a finite element model with Morison-based forcing. A good overall reproduction of the wave field and structural response is achieved for two selected episodes. For some of the waves, however, the numerical response magnitude does not match the observed excitations. Ongoing work is therefore an investigation of breaking wave load models and their implementation into the present numerical frame work.
Cementitious Sealing Material: 3D Digital Image Based Characterization of Pore Size Distribution and Modeling of Transport Properties
Authors: Ukrainczyk Neven , Koenders Eduardus A. B. , van Breugel Klaas
Published: November 2013

Exhausted oil and gas reservoirs are one of the most potential storage facilities to sequestrate the worlds CO2. These reservoirs are sealed with cementitious materials, that should have a long time performance. Therefore, this paper emphasizes the characterization of the evolving capillary pore network and transport properties of the cementitious microstructure used to seal the wellbore. The Hymostruc numerical model is employed to simulate the development of an evolving virtual microstructure of cementitious materials. The simulated 3D microstructures were then digitized to form a matrix of cubic voxels. The pore-size distributions of the obtained virtual microstructures were calculated using a combination of three-dimensional digital image processing algorithms: 1) distance transform and 2) medial axis thinning algorithm to obtain a 3D skeleton of the pore structure. Transport properties of the simulated microstructures are analyzed employing a finite difference 3D transport model. The modeling results are compared with available literature results.
An Investigation of the Tolerance of Riser to Corrosion Pitting
Authors: Urthaler Yetzirah , Cerkovnik Mark , Yin Fengjie , Saldana David , Gordon Robin
Published: November 2013

When risers are designed it is common for corrosion to be accounted for by including a corrosion allowance in the wall thickness [3]. However, when designing risers which are subject to fatigue loading from various sources, simply allowing extra thickness in the wall is inadequate to protect against the accelerated fatigue crack growth driven by corrosion.

This paper illustrates a methodology for assessing the fitness for service of a steel catenary riser with various levels of pitting corrosion. The methodology uses FEA tools, as well as classical fracture mechanics, to predict the rates of crack growth and arrive at predictions of life. Once corrosion begins and pits form, the structure may experience an increase in crack growth rate, caused by the influence of the chemistry of the produced fluid on the steel and by the stress effects of the pit geometry. Further complications arise if extreme storms cause riser stresses to exceed yield, which then requires the use of strain based methodology.

The results of the illustrative study demonstrate that riser designs should account for the potential of accelerated crack growth where there is a potential for pitting corrosion, and that by only adding a corrosion allowance to account for loss of burst capacity, an inadequate design can easily result.
Using a Dacon Scoop to Recover a Loaded Liferaft at Sea
Authors: Brown Robert , Evely Kerri-Ann , Small Graham , MacKinnon Scott
Published: November 2013

Service and supply vessels that perform standby duties in the offshore sector are equipped with a variety of resources with which to perform rescue at sea. For light to moderate sea conditions, techniques currently understood and practiced tend to involve using a fast rescue craft (FRC) to rescue survivors and subsequently transfer them to a safe haven (often a standby vessel). Rescuing evacuees in higher, more challenging sea states, however, is difficult given that it can be unsafe to launch and recover FRCs in such conditions. For these cases, many standby vessels in Eastern Canada and the North Sea are equipped with a device called a Dacon Scoop. The Dacon Scoop is a 6 to 8m semi-rigid net that is deployed directly from the side of the standby vessel and designed to rescue people directly from the water. While not a conventional use, it has been suggested that the scoop could also be used to recover small rescue craft (life rafts, lifeboats and fast rescue craft) at sea. Although life rafts provide occupants with some protection from the elements, there are still issues that can make it less than desirable to remain inside for extended periods of time if an effective means of system recovery is available. This paper presents the findings of research carried-out to determine if it is safe to use a Dacon Scoop to recover a loaded liferaft. The research trials made use of a loaded inflatable 25 person davit launched life raft and two different standby vessels 75m in length. A total of 34 tests were carried out over three days in significant wave heights up to 3.7m. Recommendations are given in the context of a human factor assessment related to the procedures, equipment and field trials experiences.
An Approximate Calculation Approach for Ultimate Strength of Ship Hull Girder
Authors: Li Chenfeng , Ren Huilong , Zhao Zhongqiu , Zhao Xiaodong , Zeng Ji , Xu Donghao
Published: November 2013

The assessment of the ultimate strength of a ship structure is very important not only for the initial design but also for the operation, maintenance, and repair of the structure. There exist two typical methods to assess the ultimate strength of stiffened panel and hull girder. One is the approximate calculation method and the other is the progressive collapse analysis. The approximate calculation method is based on ship beam-theory mainly, and the ultimate moment was calculated by assuming stress distribution of the whole section of the ship girders, which can not effectively consider the structural post-buckling behaviour. Based on progressive collapse mechanism and hull’s beam theory, A approximate calculation approach was proposed. By calculating the ultimate strength of stiffened panels in the distal end hull section, the deflections were obtained when the different stiffened panel reaching in the limit state, and then the stress state and bending moment of section were determined with different deflections, the hull’s ultimate moment value is determined by searching and comparing finally. The numerical examples show that the presented method was reasonable and effective compared with traditional direct calculation method and progressive collapse analysis. The method given hereof followed the progressive collapse mechanism through inversion searching in some degree, and the structural post-buckling behaviour was effectively considering.
Numerical Analysis of Offshore Pile Structure for Tidal Current Devise Using FSI Method
Authors: Jo Chul-hee , Lee Kang-hee , Rho Yu-ho , Kim Do-youb
Published: November 2013

Recently, large scale tidal devices have been deployed with a maximum rotor diameter of 20m. These devices impose significant loading on supporting structures. The supporting structure for tidal current power device is under dynamic loadings caused by environmental loadings. Not only the environmental loadings but also the rotating turbine creates dynamic loading as well. The rotating turbine is obviously and continuously deformed for various incoming flow velocities. In many cases, a pile fixed foundation is used to secure the structure. In this study, the commonly used pile fixed type is applied with three blade turbine. A numerical analysis of the hydro-forces from a rotating tidal current turbine to a tower was conducted to determine the deformation distribution along the pile tower. The FSI analysis technique is used in the study.
A Simplified Procedure to Assess the Strength of a Ship Impacting a Lock Mitered Gate
Authors: Buldgen Loïc , Le Sourne Hervé , Rigo Philippe
Published: November 2013

This paper presents a simplified procedure allowing for a rapid prediction of the strength of a lock mitered gate submitted to a ship impact. In this article, the force opposed to the penetration of the vessel is derived by supposing that the bow is perfectly rigid, so the total initial kinetic energy has to be entirely transformed through an internal dissipation. For a given penetration of the striking vessel, an analytical procedure is followed in order to estimate the amount of energy dissipated by local and global deformations of the impacted structure. An equivalent quasi-static force is then derived. A comparison is made with a finite elements simulation in order to test the analytical procedure.
Parameter Identification of Ship Manoeuvring Model Based on Particle Swarm Optimization and Support Vector Machines
Authors: Luo Weilin , Guedes Soares Carlos , Zou Zaojian
Published: November 2013

The method of system based manoeuvring simulation provides an effective way to predict ship manoeuvrability. Accurate determination of the hydrodynamic derivatives in the mathematical model of ship manoeuvring motion is vital to the prediction accuracy. A support vector machines (SVM) based approach is proposed in this paper. By analyzing the data from free-running model tests of KVLCC2 ship, the hydrodynamic derivatives in an Abkowitz model are identified. To diminish the parameter drift in the identification, a difference method is adopted to reconstruct the sample for identification. To obtain the optimized structural parameters in SVM, particle swarm optimization (PSO) method is incorporated into SVM. Predictions of manoeuvring motion are presented based on the regression model. Comparisons between the predicted results and the test results demonstrate the validity of the proposed methods.
Effect of Hydrodynamic Coefficients on a Typical MWA Model
Authors: Huang Guoxing , Zhang Ningchuan
Published: November 2013

In Lazy S system, the riser is similar to free-hanging catenary but has support provided at about Mid-Water by an arch structure. One of the main functions for the introduction of a Mid-Water Arch (MWA) into a riser system is to provide a filter to stop the direct transfer of dynamic tension amplitude to the Touch-down Point (TDP) on seabed that occurs with the free-hanging configuration. By using the Morison equation to calculate the wave loads on a MWA, the difficulty is to determine correct hydrodynamic coefficients. Analysis was performed by using Orcaflex to investigate the effect of hydrodynamic coefficients on the MWA loadings. Dynamic analysis was run under Airy wave with different drag force and added mass coefficients. The effect of drag moment of area was considered as well. Base on the comparison among the simulation results, some conclusions and discussion were presented.
Scaling of the Hydroelastic Response of Flexible Lifting Bodies in Transitional and Turbulent Flows
Authors: Ducoin Antoine , Young Yin Lu
Published: November 2013

The objective of this research is to derive and validate scaling relationships for flexible lifting bodies in transitional and turbulent flows. The motivation is to help the design and interpretation of reduced-scale experimental studies of flexible hydrofoils, with focus on the influence of viscous effects on the hydroelastic response. The numerical method is based on a previous validated viscous FSI solver presented in [1]. It is based on the coupling between a commercial Computational Fluid Dynamics (CFD) solver, CFX, and a simple two-degrees-of-freedom (2-DOF) system that simulates the free tip section displacement of a cantilevered, rectangular hydrofoil. To validate the scaling relations, sample numerical results are shown for three geometrically similar models: full scale, 1/2 scale and 1/10 scale. On the fluid side, although the effects of gravity and compressibility are assumed to be negligible, three different methods of scaling the velocity are considered: Reynolds scaling, Froude scaling, and Mach scaling. The three scaling methods produce different velocity scales when the fluid properties and gravitational constant are the same between the model and prototype, which will lead to different scaling for the material properties. The results suggest that by applying Mach scaling (which does not mean the flow is compressible, but simply requires the relative inflow velocity and fluid properties to be the same between the model and the prototype) and Re ≥ 2 × 106, the same material as the full scale could be used, which will lead to similar stress distributions, in addition to similar strains, and hence similar hydroelastic response and failure mechanisms. However, if Re ≤ 2 × 106 and Mach scale is used, a viscous correction is required to properly extrapolate the experimental results to full-scale.
Assessment of Brittle Fracture for Single Edge Notched Bend Specimens With Different Machined-Notch Depth
Authors: Iwashita Tsutomu , Kobayashi Ryohei , Azuma Koji
Published: November 2013

Current defect assessment procedures based on fracture mechanics usually assume flaws to be infinitely sharp. While this assumption may be appropriate for fatigue cracks, it can be an over-conservative assumption and give a large safety factor for non-sharp flaws such as porosity or weld undercut because of the loss of plastic constraint. Fracture toughness tests are performed for four types of single-edge notched bend steel specimens with different degrees of plastic constraint caused by a machined-notch and a shallow notch. This paper shows the ability of the Weibull stress approach to predict brittle fracture under both high and low plastic constraint conditions. The Weibull stress approach predicts the effects accurately.
Hydrodynamics of Multiple Vertical Axisymmetric OWC’s Devices Restrained in Waves
Authors: Konispoliatis Dimitrios N. , Mavrakos Spyros A.
Published: November 2013

An increased interest in the use of Oscillating Water Column (OWC) devices in recent years has led to the consideration of a number of differentiations in geometry (wall thickness, draught, shape of the chamber), topography installation (on-, near-, off-shore), moving conditions (restrained, freely floating bodies) and number of used devices in order to extract maximum wave energy. Many considerable efforts and advances have been made to wave power absorption by isolated devices; however, much less attention has been paid to the influence of neighbouring OWC structures on wave loading and wave energy extraction. This paper deals with the linearised hydrodynamic interaction problem of regular, small amplitude, surface gravity waves and a stationary group of vertical axisymmetric OWC devices.
Non Linearity of Extreme Vertical Bending Moment: Comparison of Design Wave Approaches and Short Term Approaches
Authors: de Hauteclocque Guillaume , Derbanne Quentin , Mienahou Therard
Published: November 2013

The evaluation of extreme bending moment corresponding to a 25 years return period requires very long simulations on a large number of sea states. This long term analysis is easy to do with a linear model of the ship response, but is impractical when using a time consuming model including non linear loads (Froude-Krylov and hydrostatic non linear forces). In that case some simplified methods need to be applied. These methods are often based on equivalent Design Sea States (DSS) or Equivalent Design Waves (EDW).

The different methods are applied to large database of ships, containing 19 tankers, 29 bulk carriers, and 9 container-ships, in full load or ballast condition. Linear hydrodynamic calculations are carried out to define the extreme linear vertical bending moment, and to define the DSS and the EDW. Then non linear time domain sea-keeping computations are performed, taking into account Froude-Krylov hydrostatic pressure correction and large motions. The difference between linear and non linear hogging and sagging extremes are computed for each type of DSS and EDW.
Analysis of Thin Walls Oscillating Water Columns (OWC) by Potential Methods and Model Testing
Authors: Guimaraes Helio Bailly , Fernandes Antonio Carlos , Machado Bruno Nogueira
Published: November 2013

The present work is dedicated to the behavior of an internal oscillating water column (OWC) under the external incidence of waves. The oscillation of the OWC is a subject of strong research interest since it can be applied to clean energy generation and also offshore exploitation. It is shown that the viscous effects are important so that the linear wave theory based on potential theory should be corrected. The paper discusses methods to achieve good results using panel method within potential theory and shows how to introduce the necessary correction. Both low-order and higher-order method are considered. The thin walls geometry of the specific OWC is challenging. The theoretical results were compared with experimental results using regular wave experiments and decaying tests with reduced scale models. The later was essential to get the viscous effects of the OWC.
Study on Numerical Simulation of Jacket Launching Progress
Authors: Xiong Lingzhi , Yang Jianmin , Li Xin , Xu Xin
Published: November 2013

Jacket type structures are widely used in shallow waters. The size of a jacket is determined by the dimensions of the deck and piles, as well as the environment loads. The jackets are usually transported with barges to the operation water after they have been fabricated in the shipyard, then the jackets slip into the water. Launch operation is the most hazardous stage during the installation of a jacket. The main purpose of this study is to evaluate the physical modeling test results for the launch, and compared to those of numerical simulation. It is found that there are some differences between the numerical simulations and physical model tests. The scale effect cannot be ignored in the prediction of the jacket launching process. The model test is based on the Froude similarity law, and the effect of Reynolds number is neglected. The drag coefficient depending on the Reynolds number plays a crucial role in determining the launch process. With different Reynolds number, the structure has different hydrodynamic force that results to different pitch angle, heave, launch time, acceleration and so on. Thus, a scale effect emerges when we convert the results of a wave basin model test to the prototype scale. This study also includes a sensitivity analysis about some key parameters in the launch process, such as initial barge trim and draft, frictions between barge and jacket.
Nano-Scale Characterisation of Shale Caprock/Acidic Brine Interaction
Authors: Olabode Abiola , Radonjic Mileva
Published: November 2013

Shale-caprock integrity is critical in ensuring that subsurface injection and storage of anthropogenic carbon dioxide (CO2) is permanent. The interaction of clay-rich rock with aqueous CO2 under dynamic conditions requires characterization at the nano-scale due to the low-reactivity of clay minerals. Geochemical mineral-fluid interaction can impact properties of shale rocks primarily through changes in pore geometry/connectivity. Several simulation results had predicted that influx-triggered mineral dissolution/precipitation reactions within clay-rich rock can continuously diminish micro-fracture networks, while pressure and effective-stress evolution initially increase then gradually constrict them. However, most of these studies have not investigated changes under experimental conditions and applying experimental techniques capable of detecting changes at nano-scale, which are ultimately representing pathways for molecular diffusion of fluids through porous media.

The experimental work reported in this paper applied specific analytical techniques in investigating changes in surface/near-surface properties of crushed shale rocks after exposure (by flooding) to CO2-brine for a time frame ranging between 30 days to 92 days at elevated pressure and fractional flow rate. Initial capillary entry parameters for the shale were estimated from digitally acquired pressure data evolution. Flooding of the shale samples with CO2-brine was followed by Nano-scale measurement of changes in internal specific surface area, pore volume and linear/cumulative pore size distribution. The BET Technique showed that changes in the shale caprock occurred due to geochemical interaction with aqueous CO2 will impact petrophysical properties of the rock. The intrinsically low permeability in shale may be altered by changes in surface properties as the effective permeability of any porous medium is largely a function of its global pore geometry. Diffusive transport of CO2 as well as carbon accounting could be significantly affected over the long term. The estimation of dimensionless quantities such as Peclet (Pe) and Peclet-Damkohler (PeDa) Numbers that are associated with geochemical reactivity of rocks and acidic fluid transport through porous media gave insight into the impact of diffusion and reaction rate on shale caprock.
Finite Element Contact/Impact Modeling Capability of Complex Surface Effect Ship Dynamics: Preliminary Assessment
Authors: Challa Ravi , Newborn David , Yim Solomon
Published: November 2013

Experimental tests were conducted on a large scale captive Surface Effect Ship (SES) specimen at the US Navy’s Large Cavitation Channel (LCC) to primarily determine the mechanics of bow finger seal motions. In this paper, preliminary qualitative comparisons between the experimental and numerical simulations comprised of test dependent variables such as spatially distributed channel water elevation, channel water speed and air-cushion pressure are presented.

A surface effect ship is an air-cushion supported vessel with two side hulls and fore and aft flexible seals. The side hulls of the SES along with the wet deck, seals and the fluid-water surface constitute the boundary of the air-cushion. An evaluation of the predictive capabilities of the existing technology of a state-of-the-art nonlinear multi-physics finite element code (LS-DYNA) with its arbitrary Lagrangian Eulerian (ALE) technique in modeling the coupled fluid-structure interaction (FSI) behavior of the boundary of a SES with the water surface is studied.

The predictive capability of the code to model a single stern seal deployment, a single seal impacting a rigid flat surface, multiple seals impacting a water free surface and the centerline profile of multiple stern seal deployment between two rigid surfaces were verified. Simple numerical models for a five-finger bow seal with partial submergence in uniform current have been developed. Additional features including the mechanism to pressurize the air cushion chamber were included. Preliminary finite element (FE) simulation results show that the robust contact and impact algorithm is shown to capture the physics of the complex flexible body FSI problem well. In view of the good qualitative comparison between the experimental and numerical simulations, the ALE feature is being employed for studying the complex flexible structural mechanics including bow and stern seal motions interacting with air cushion and free-surface hydrodynamics.
A Meshless Boundary Element Method for Simulating Slamming in Context of Generalized Wagner
Authors: Helmers Jens B. , Skeie Geir
Published: November 2013

A boundary element method designed for solving the symmetric Generalized Wagner formulation is presented. The flow field is parameterized with analytical functions and can describe the kinematics at any free surface or body location using a small set of parameters obtained from a collocation scheme. The method is fast and robust for all deadrise angles, even for flat plate impacts where classical boundary element methods usually fails. The method is easy to implement and is easy to apply. Given a smooth body contour the only additional input is the requested accuracy. There is no mesh involved. When solving the temporal problem we exploit the analytical distribution of free surface velocities and apply an integral equation formalism consistent with the Wagner formulation. The output of the spatial and temporal scheme is a set of functions and parameters suitable for fast computation of the complete kinematics for any impact trajectory given the position of the keel and the body velocity. The method is developed to be combined with seakeeping programs for statistical impact and whipping assessment.
On Sampling Between Data of Significant Wave Height for Long-Term Analysis With Equivalent Triangular Storm Model
Authors: Arena Felice , Laface Valentina
Published: November 2013

This work proposes an analysis of storms in Pacific and Atlantic Ocean, which is carried out by applying the Boccotti’s Equivalent Triangular Storm (ETS) model. The ETS model represents any actual storm by means of two parameters. The former gives the storm intensity, which is equal to the maximum significant wave height during the actual storm; the latter represents the storm duration and it is such that the maximum expected wave height is the same in the actual storm and in the equivalent triangular storm. Data from buoys of the NOAA-NDBC (National Data Buoy Center, USA) are used in the applications, by considering different sampling Δt between two consecutive records, which varies between 1 and 6 hours. The sensitivity of the ETS model with the variation of Δt is investigated for the long-term modeling of severe storms. The results show that the structure of storms is strongly modified as Δt increases: both the intensity and the duration may change significantly.

The effects of this results for long term statistics are investigated by means of the return period R(Hs > h) of a storm in which the maximum significant wave height exceeds the threshold h, which is evaluated by using data with different sampling Δt between two consecutive records. Finally for different values of the return period R, the return value of significant wave height and the mean persistence Dm(h), giving the mean time during which the significant wave height is greater than fixed threshold (in the storms where the threshold is exceeded), are calculated.
Response Sensitivity to Swell Spectra Off West Africa
Authors: Ewans Kevin , Forristall George Z. , Prevosto Marc , Olagnon Michel
Published: November 2013

Both hindcast and measured wave data were used to establish various descriptions of the swell spectra off West Africa in the West Africa Swell JIP (WASP). The effect on system responses of the various data types and swell spectral descriptions has been estimated.

The response of single degree of freedom oscillators to wave spectra can be used to produce maximum responses as a function of natural period and damping, analogous to the approach used in earthquake engineering. The results can be used to gain insight into the sensitivity of floating systems to the swell characteristics of the location and also to the differences between hindcast and measured data sets. Extreme values of system response can be calculated and used to estimate design spectra, by scaling the measured or hindcast spectrum that produced the largest response.

The sensitivity of the responses of floating systems to various simplifications of the hindcast spectra was evaluated by calculating the responses of an FPSO and CALM buoy for a location off Nigeria and southern Angola. It was found that there is little loss of accuracy if the hindcast spectra are decomposed into a maximum of two partitions, one of which may be a wind-sea, and the swell partitions are described with a lognormal function, the wind-sea partition with a JONSWAP-Glenn frequency function, and the directional spreading with a wrapped normal function, together with parametric forms for the widths of the lognormal and wrapped normal functions. As a consequence it is concluded that the parametric forms for the swell frequency and direction spreading can be used in design.