bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimisation of Resistance and Towed Stability of an Offshore Going Barge With CFD
Authors: Toxopeus Serge , Stroo Ko , Muller Bart
Published: November 2013

For the transport of large structures over sea, offshore going barges are one of the preferred means of transportation. The resistance and towed stability of these vessels are of prime importance with respect to the economic feasibility of transport. To ensure acceptable towed stability behaviour, side skegs are generally placed under the aft ship of the barge. However, these skegs give additional resistance and thus undesired fuel consumption.

SMIT and MARIN joined forces with the intention of designing a new ocean going barge with sufficient towed stability and superior resistance characteristics compared to the already low drag Giant 4 barge. The Giant 4, for which favourable tow characteristics were observed during its service, was used as a reference vessel. The first step was to modify the hull form in order to fulfil SMIT’s design requirements. Subsequently, a detailed CFD study into the flow around the barge and the size, shape and positioning of the side skegs was performed leading to a significant resistance reduction. However, this reduction in resistance led to a degradation of the towed stability of the vessel, which was compensated for by adding a centreline skeg. The final design has a 14% percent reduction in resistance compared to the existing barge, with comparable towed stability.

A model testing program was performed in order to validate the CFD computations. The agreement between the CFD and experimental results was found to be very good, demonstrating that CFD tools can be used to optimise the resistance and towed stability of barges.
3D Efficiency Analysis of Cycloidal Wave Energy Converters in Oblique Wave Fields
Authors: Fagley Casey , Siegel Stefan G. , Seidel Jürgen , Schmittner Christian
Published: November 2013

Numerical results from a 3D diffraction model are presented where a Cycloidal Wave Energy Converter (CycWEC) is interacting with an incoming straight crested Airy Wave. The diffraction model was developed in response to experimental observations from 1:10 scale experiments which were conducted in the Texas A&M Offshore Technology Research center wave basin. These experiments were the first investigations involving a CycWEC where three dimensional wave diffraction effects were present due to the fact that the span of the CycWEC was much smaller than the width of the basin. The diffraction model predicted the observed surface wave patterns in the experiment well, and showed that diffraction induced wave focusing increased the recoverable wave power beyond the 2D predictions for small CycWEC spans, while approaching the 2D limit for very large spans. The numerical model was subsequently used to estimate the sensitivity of the CycWEC to misalignment between the incoming waves and the WEC shaft. The loss in efficiency was found to be strongly dependent on the ratio between WEC span and incoming wavelength, where short spans (on the order of one wave length or less) which are realistic for actual ocean deployment showed only minor reductions in efficiency, while very long spans were found to be more sensitive to misalignment.
Effects of a Shielding Skirt for Prevention of Sea Lice on the Flow Past Stocked Salmon Fish Cages
Authors: Frank K. , Gansel L. C. , Lien A. M. , Birkevold J.
Published: November 2013

The effect of a shielding skirt, a tarpaulin mounted from the surface down to 5 meter depth around a net cage, on the flow pattern at a commercially stocked salmon cage was investigated. Dye was used as a tracer for water movement and the dye spreading was monitored using aerial images. Current meters were employed to investigate the flow close to the net inside and outside the cage. Tests were conducted with and without the shielding skirt. The focus was on the effectiveness of the shielding skirt to deflect water around the cage. This study shows that a shielding skirt can reduce horizontal flow components significantly inside a cage, which is related to a reduction of water exchange. The flow towards a cage is divided by a shielding skirt, i.e. some of the water is transported around the cage, while some is passing underneath the shielding skirt. Some water entering the fish cage from underneath the tarpaulin is transported towards the surface inside the cage. The use of a shielding skirt might not prevent interaction of the upper water layers inside and outside of a fish cage completely, but it has the potential to reduce the inflow of surface water into the cage, if deployed properly.
Fatigue of Risers and Pipeline Girth Welds
Authors: Horn Agnes Marie , Wästberg Stig , Cordes Rogers
Published: November 2013

Deep-water tendon and riser systems are often subjected to severe fatigue loading from waves, currents and vessel movements. The girth welds between successive lengths of pipe or at pipe terminations are the locations most vulnerable to fatigue damage and accurate and reliable assessment of the fatigue performance of these welds is of significant importance. These welds are normally designed on the basis of appropriate S-N curves. In addition, it is common practice to perform an Engineering Critical Assessment (ECA) based on fracture mechanics fatigue crack growth principles, principally in order to establish acceptance limits for weld flaws that are consistent with the fracture toughness of the welds and the required fatigue life.

However, for the most important case of fatigue failure from the weld toe, the mode of failure principally covered by the S-N curves, a conventional ECA invariably results in tolerable weld toe flaws that are too small for reliable detection by currently available NDE methods. In fact, this situation is not unreasonable since it is known from careful metallurgical examinations that weld toe fatigue cracks initiate at tiny (<0,5 mm deep) sharp imperfections that are an inevitable consequence of welding, whereas the detection limit for even the most sensitive NDE techniques is around 1–2 mm. Hence this paper discusses different models for including crack initiation and early growth of defects in girths weld, in addition some trends found in full-scale data from an on-going girth weld fatigue JIP is shown.
A Stabilized Edge-Based Finite Element Approach to Wave-Structure Interaction Assessment
Authors: Elias Renato N. , Coutinho Alvaro L. G. A. , Gonçalves Milton A. , Cortês Adriano M. A. , Drummond Alves José L. , Guevara Nestor O. , Silva Carlos E. , Correa Bruno , Rochinha Fernando A. , Guerra Bernadá Gabriel M. , de Carvalho e Silva Daniel F.
Published: November 2013

Complex flows involving waves and free-surfaces occur in several problems in hydrodynamics, such as fuel or water sloshing in tanks, waves breaking in ships, offshore platforms motions, wave action on harbors and coastal areas. The computation of such highly nonlinear flows is challenging since waves and free-surfaces commonly present merging, fragmentation and cusps, leading to the use of interface capturing Arbitrary Lagrangian-Eulerian (ALE) approaches. In such methods the interface between the two fluids is captured by the use of a marking function that is transported in a flow field. In this work we simulate these problems with a 3D incompressible SUPG/PSPG parallel edge-based finite element flow solver associated to the Volume-of-Fluid (VOF) method. The hyperbolic equation for the transport of the marking function is also solved by a fully implicit parallel edge-based SUPG finite element formulation. Global mass conservation is enforced adding or removing mass proportionally to the absolute value of the normal velocity at the interface. All those techniques were successfully implemented in a computational code, which has been suitably used to carry out several studies. The performance and accuracy of the proposed solution method is tested in the simulation waves and in the interaction between waves and a semisubmersible structure. Results count on the establishment of a relaxation zone close to the domain outflow, which partially absorbs incoming waves, avoiding their reflection.
Experimental and Numerical Investigation of Dynamic Positioning in Level Ice
Authors: Metrikin Ivan , Kerkeni Sofien , Jochmann Peter , Løset Sveinung
Published: November 2013

Offshore operations in ice-covered waters are drawing considerable interest from both the public and private sectors. Such operations may involve a requirement for vessels to keep position during various activities, e.g. lifting, installation, crew change, evacuation, and possibly drilling. In deep waters, mooring solutions become uneconomical and therefore dynamic positioning (DP) systems are attractive. However, the ice environment is highly variable with ice features ranging from mild pack ice to pressure ridges and icebergs. This paper focuses on level ice as one of the primary ice types for DP operations in cold waters. The paper presents two major contributions: experimental and numerical. The experimental part is devoted to the description of ice model tests performed at the large ice tank of the Hamburg Ship Model Basin (HSVA) in the summer and autumn of 2012. Experimental design, instrumentation, methods and results are presented and discussed. The numerical part presents a novel model for simulating DP operations in level ice. In the modelling, the vessel and the ice floes are treated as separate independent bodies with 6DOF. The fracture of level ice is calculated on-the-fly based on numerical solution of the ice material failure equations, i.e. the breaking pattern is not pre-calculated. The numerical model is connected to the DP controller and the two systems interchange data dynamically and work in a closed loop. The structures of the models as well as the physical and mathematical assumptions are discussed in the paper. Furthermore, several ice basin experiments are reproduced in the numerical simulation and the results of the physical and numerical tests are compared. To the best of the authors’ knowledge, both accurate ice basin tests of DP in level ice and high fidelity simulations of such experiments are novel and have not been published previously.
An Alternative Methodology to Evaluate the Fatigue Life of Risers Based on Regular Waves
Authors: de Sousa Fernando Jorge Mendes , Rebello Ana Maria Valadão , Sagrilo Luís Volnei Sudati , de Siqueira Marcos Queija
Published: November 2013

In the last years, the increasing knowledge about environmental data has also increased the number of sea states that are used in the fatigue analysis of risers. These analyses demand high computational resources and, consequently, methodologies to reduce this cost become very attractive. One of the most studied alternatives in this area is the utilization of regular waves, represented by block diagrams. Based on distributions such as the one proposed by Longuet-Higgins, irregular waves are decomposed in regular waves, originating deterministic scatter diagrams. The Longuet-Higgins distribution, however, has a tendency to generate an unrealistic number of waves at high periods and, consequently, it is usual do set a maximum cutoff level for the periods. In this work, a different approach to generate the regular waves scatter diagrams is investigated. It is based directly on the generation of time series of sea surface elevations, allowing an easy identification of heights and periods of regular waves. The obtained results indicate the feasibility of the proposed method, based on comparisons with irregular waves results.
High Performance Semisubmersible Design for Dry Tree Applications in Harsh Environment
Authors: Mansour Alaa M. , Kumar Dhiraj
Published: November 2013

The Free Hanging Solid Ballast Semisubmersible (FHSB) Semi is an innovative high performance submersible design that is suitable for the dry tree application because of its extremely low heave motion response. In this paper, the FHSB Semi has been designed to support dry tree application in the Gulf of Mexico (GoM) for similar conditions; payload, environment, water depth and number of risers such as that applied to an existing Spar platform in the GoM. Holstein Spar has been considered for this purpose. Numerical results for the semisubmersible motion response in extreme hurricane and loop current are presented in this paper. The paper also compares the FHSB Semi designs that support certain payload/number of risers in the GoM, in the North West Shelf Australia and in Brazil. Key figures from this comparison are presented in the paper. The paper includes discussions on the proposed project execution plan covering the fabrication, transportation and installation as well as the added risks and risk mitigations associated with the new semisubmersible design and execution.
Installing U-OWC Devices Along Italian Coasts
Authors: Arena Felice , Fiamma Vincenzo , Laface Valentina , Malara Giovanni , Romolo Alessandra , Viviano Antonino , Sannino Gianmaria , Carillo Adriana
Published: November 2013

In the last decades, the research has directed its efforts and resources paper is to investigate towards the possibility to incorporate wave energy converters, into the traditional maritime breakwaters to combine classical use with new opportunities and developments (for example, the Green Ports). Since the nineties, the OWC (Oscillating Water Column) plants were developed at full scale to produce electrical power from ocean waves. For instance, a new plant was built in Mutriku (Spain) recently.

A new kind of OWC caisson, named U-OWC or REWEC3, which has the advantage to obtain an impressive natural resonance without any device for phase control, has been patented by Boccotti [1]. This new U-OWC device gives performances better than those of a conventional OWC either with small wind waves or with high waves [2,3,4]. The properties of the REWEC3 have been verified with two small-scale field experiments carried out in the natural ocean engineering laboratory NOEL of Reggio Calabria off the eastern coast of the Sicily Channel [5–7]. The aim of the present two sites along Italian coasts for possible installations of REWEC3 devices: i) the port of Civitavecchia (Rome, Italy)in the Tyrrhenian sea; ii) the port of Pantelleria, in the Sicily Channel.
Benefit of Measurements and Structural Reliability Analysis for Wellhead Fatigue
Authors: Hørte Torfinn , Russo Massimiliano , Macke Michael , Reinås Lorents
Published: November 2013

Structural Reliability Analysis (SRA) methods have been applied to marine and offshore structures for decades. SRA has proven useful in life extension exercises and inspection planning of existing offshore structures. It is also a useful tool in code development, where the reliability level provided by the code is calculated by SRA and calibrated to a target failure probability.

The current analysis methods for wellhead fatigue are associated with high sensitivity to variations in some input parameters. Some of these input parameters are difficult to assess, and sensitivity screening is often needed and the worst case is then typically used as a basis for the analysis. The degree of conservatism becomes difficult to quantify, and it is therefore equally difficult to find justification to avoid worst case assumptions.

By applying SRA to the problem of wellhead fatigue, the input parameters are accounted for with their associated uncertainty given by probability distributions. In performing SRA all uncertainties are considered simultaneously, and the probability of fatigue failure is estimated and the conservatism is thereby quantified. In addition SRA also provides so-called uncertainty importance factors. These represent a relative quantification of which input parameter uncertainties contribute the most to the overall failure probability, and may serve well as guidance on where possible effort to reduce the uncertainty preferably should be made. For instance, instrumentation may be used to measure the actual structural response and thus eliminate the uncertainty that is associated with response calculations. Clearly measurements obtained from an instrumented system will have its own uncertainty. Other options could be to perform specific fatigue capacity testing or pay increased attention to logging of critical operational parameters such as the cement level in the annulus between the conductor and surface casing.

This article deals with the use of measurements for fatigue life estimation. Continuous measurements of the BOP motion during the drilling operations have been obtained for a subsea well in the North Sea. These measurements are used both in conventional (deterministic) analysis and in SRA (probabilistic analysis) for fatigue in the wellhead system. From the deterministic analysis improved fatigue life results are obtained if the measured response replaces the response obtained by analysis. Furthermore, SRA is used to evaluate the appropriate magnitude of the design fatigue factor when fatigue analysis is based on measured response. It is believed that the benefit from measurements and SRA serve as an improved input to the decision making process in the event of life extension of existing subsea wells.
Numerical Evaluation of PLEM Structure and Foundation Modes of Failure and Failure Consequences
Authors: Abdalla Basel , Mei Haixia
Published: November 2013

For deep water pipeline end manifold (PLEM) subject to complex loading conditions, a combined 3D finite element (FE) model has been developed by the authors to determine the foundation bearing capacity by studying the interaction between individual parts, including the PLEM structure, flowline, mudmat and soil. The advanced numerical techniques have proven that the resulting factor of safety against bearing failure is higher when compared to the classical approach given in API-RP-2A/2GEO. However, the minimum required safety factor of 2.0 is not reached. Therefore, risk assessment using reliability analysis becomes mandatory to assess the probability of failure of the mudmat. As an input to the risk assessment, this paper presents an investigation of the system’s modes of failure and failure consequences on the environment and project cost.

Using the developed 3D FE model, plausible excessive loads beyond design considerations are applied until a failure in soil foundation occurs. Then, loads are increased until the capacity of structural and mechanical components in the system is reached putting the containment of carried hydrocarbons at risk. A direct relationship between the hydrocarbon temperature and the capacity of structural/mechanical components is developed to evaluate the system flexibility and resistance to induced excessive deformations.

The paper demonstrates the advantages of numerical techniques in evaluating the ultimate capacity of a PLEM structure and foundation soil system. The methodology can be used in the design of a new system or to assess the stability of an existing system under future loadings.
On the Design of the In-Plane Bending Fatigue Test
Authors: Agustsson Geir , Bendiksen Erik , Nielsen Christian B.
Published: November 2013

The fatigue design of the steel armour layers is an important factor in flexible riser design. Typically, the riser must be able to endure the loads of a field condition for a period of 20 years. In flexible pipe design, this is analyzed in global and local flexible pipe fatigue modeling, which needs to be supported by proper testing. One of the widespread test methods is the so-called in-plane bending fatigue test, which is one of the important qualification tests for flexible pipes.

API 17B references two purposes for performing in-plane bending fatigue tests. One is to demonstrate that the pipe can endure the loads which it is exposed to during its lifetime (service simulation). The other is to validate the design methodologies (service-life model validation).

The first purpose calls for test conditions being as close to reality as possible. However, testing in conditions close to reality does not necessarily call for field environment conditions being present during the in-plane testing itself. This is because field conditions are covered with a combination of small-, mid- and full-scale tests according to the recommendations given in API 17B.

The second purpose can be, or is even best fulfilled with simplifications compared to field conditions.

This study will go through the options and discuss the pros and cons of simplifications versus a more complicated test. The discussions in the paper are based on the author’s experience and can be supported with examples from actual testing.
System Reliability for Offshore Wind Turbines: Fatigue Failure
Authors: Márquez-Domínguez S. , Sørensen J. D.
Published: November 2013

Deeper waters and harsher environments are the main factors that make the electricity generated by offshore wind turbines (OWTs) expensive due to high costs of the substructure, operation & maintenance and installation. The key goal of development is to decrease the cost of energy (CoE). In consequence, a rational treatment of uncertainties is done in order to assess the reliability of critical details in OWTs. Limit state equations are formulated for fatigue critical details which are not influenced by wake effects generated in offshore wind farms. Furthermore, typical bi-linear S-N curves are considered for reliability verification according to international design standards of OWTs. System effects become important for each substructure with many potential fatigue hot spots. Therefore, in this paper a framework for system effects is presented. This information can be e.g. no detection of cracks in inspections or measurements from condition monitoring systems. Finally, an example is established to illustrate the practical application of this framework for jacket type wind turbine substructure considering system effects.
Non-Destructive Measurement of Residual Stresses by an Ultrasonic Method
Authors: Kleiman Jacob , Kudryavtsev Yuri , Smilenko Volodimir
Published: November 2013

An ultrasonic computerized complex for measurement of residual and applied stresses UltraMARS® was recently introduced. Average through thickness stresses can be measured based on the acoustic-elasticity effect, according to which the velocity of elastic wave propagation in solids is dependent on the mechanical stress. The system was used successfully in numerous applications proving to be a reliable, fast and economical way to evaluate residual and applied stresses in materials and structures. The newly developed complex was used in a number of applications that called for non-destructive evaluation of stresses. Examples of such applications will be discussed in the paper.

The system was further developed to allow for measurement of subsurface and surface stresses in structural materials and made of them structures.
Numerical Simulation of Fatigue Crack Propagation Under Simulated Whipping Loading Arising in Hull Structures
Authors: Matsuda Kazuki , Gotoh Koji
Published: November 2013

Fatigue crack propagation behavior under simulated whipping loadings which contain two different frequency components is highlighted. Numerical simulation of fatigue crack propagation based on an advanced fracture mechanics approach using the RPG (Re-tensile Plastic zone Generating) stress criterion for fatigue crack propagation is improved to enable the extraction of the effective loading history. The critical value of the plastic hysteresis for the stress versus strain relationship occurring in the vicinity of a crack tip is defined as the control parameter for extracting the effective loading history. Comparison of fatigue crack propagation curves obtained from the improved numerical simulations with the measurements which were conducted by Sumi (2010) is performed. These comparisons show the validity of the proposed procedure for extracting the effective loading history from the superposed loading history containing different frequencies. In addition, imperfection of the S-N curves approach, which corresponds to the combination of the linear cumulative damage law and the loading cycle count by the rain flow method, is conducted.
Benchmarking of a Massively Parallel Hybrid CFD Solver for Ocean Applications
Authors: Gerber Andrew G. , Wilcox Kevin W. , Zhang Jian T.
Published: November 2013

This paper presents progress on the development of a CFD program called EXN/Aero specifically designed to exploit performance gains from new hybrid multicore-manycore computer architectures. The hybrid multicore-manycore design is outlined along with performance and validation testing on an underwater vehicle and unsteady vortex shedding applications. It is shown that by revisiting CFD code design with a view to a number of important trends in the high performance computing industry, significant order of magnitude gains in computational power can be achieved.
A Comparison of Different Approximations for Computation of Second Order Roll Motions for a FLNG
Authors: Rezende Flavia C. , de Oliveira Allan C. , Chen Xiao-bo , Menezes Fabio
Published: November 2013

The use of FLNG units for gas exploration and production offshore is a subject in study by some oil companies. More complex and sophisticated than a FPSO production plant, a gas production plant has strict motion criteria in order to have an optimal operational performance. Due to this, designers have been trying hull concepts with small initial stability and higher roll motion periods in order to reduce the unit motions and improve the plant performance. Indeed, the increase of roll natural period dramatically reduces the first order roll motions. However, the unit still responds at its resonance due to second order excitation. These kinds of loads are also more complex and require a great computational power to be evaluated. Due to its complexity, which would involve the solution of a non-homogeneous free surface boundary condition, some approximations are used in order to assess the second order loads and motions. In this paper, the different formulations for the first part of QTF, contributed by first order quantities, are revisited and the differences are highlighted. Furthermore the approximations for the computation of the second part of the QTF, contributed by the second order potential, are benchmarked for the case of a FLNG operating in deep water depth.
Communication in the High North: Supporting Safe Maritime Operations
Authors: Kvamstad Beate
Published: November 2013

The special Arctic conditions with the presence of ice and harsh weather require complex offshore maritime operations compared to more central parts of the world. In order to maintain the safety level of the operations one need to share information amongst numerous actors across large distances. This is especially important in critical situations, when correct decisions need to be made very fast. Having a shared situational awareness becomes crucial. This again requires reliable and robust communication infrastructure such that undisrupted information is received by the end user in time. This represents a major challenge in the Arctic, since communication infrastructure above 75°N is relatively poor.

Ships operating in open seas use maritime communication systems based on geostationary (GEO) satellites which orbit the earth above the equatorial line, such as for example Inmarsat and VSAT. However, they have little or no coverage at all in the Arctic and the low elevation angles makes them vulnerable to external influences. The theoretical coverage limit for GEO systems is 81.3°N, but field tests performed in the Norwegian MarSafe North1 project showed that instability and signal dropouts can be experienced already at 70° N during certain conditions. The only satellite system that provides full coverage in the Arctic area is Iridium, and it offers digital capacity through the Iridium OpenPort services. However, users have reported unstable performance, and the field tests showed that Iridium has unstable digital throughput in Arctic areas. The paper will present results from field tests performed in the MarSafe North project.

The projects MarCom2, MarSafe North and ArctiCOM3 have investigated the state-of-the-art of Arctic communications; they have investigated future needs for satellite communications and proposed potential solutions. This paper will present the main results from these projects, as well as describing the main activities and tentative results achieved in the on-going project MARENOR, which analyses the factors that influence the quality of service of navigation and communication systems in the Arctic.
DRX Wet-Cell and Static Displacement Tests to Understand Salt Deposition and Shale Inhibition Mechanisms
Authors: Khodja Mohamed , Belkadi Mustapha , Kaouane Ahcene , Saber-Khodja Malika , Guellil Khadidja , Khadraoui Sofiane
Published: November 2013

Salts scale is one of the most important problems found in the oil and gas industry. Precipitation of salts can occur near wellbore as well as in the wellbore. The resulting reduces drastically well performance and causes a significant decrease in production rate. In order to remove salt deposits as well as prevent the precipitation of salt, fresh water treatments are performed at regular intervals during production operations. In drilling activity, shale is the most common rock types encountered while drilling wells. The combination of KCl/Polymer in water base mud (WBM) reduced frequency and severity of shale instability problems. In the order, all polymers are not able to resolve problems and inhibition mechanism comprehension still complex. Chemical treatment is an alternative approach to inhibit salts formation and shale instability. Bad inhibitor selections or/and high concentration provide limited performance and will cause damage which mechanism still unclear. In addition, it is extremely difficult to test salts formation and shale inhibition in the laboratory under reel conditions. Research shows that variation on temperature and pressures affect seriously inhibition mechanisms. In this way, our work tends to follow salts crystallization and shale swelling by using two methods: X-ray diffraction wet-cells for the in situ monitoring of fine powder reactions and displacement tests to follow permeability reduction under reservoir conditions. The possible mechanisms of halite and shale inhibition are discussed. Some experiences combining with using fines minerals particles to simulate compacted rock are realized. The following conclusions have been drawn:

- At ambient conditions, the contact between the water mixture and fines reservoir rock shows that crystallization was increased with time and affected by fines particles compaction and surface rugosity,

- Technique of chemical inhibition restrain salt deposit (60% of crystallization reduction) and enhances shale stability,

- A minimum polymer inhibitor concentration effectively was confirmed by X Ray Wet-Cell and Dynamic test under reservoir conditions.
Influence of Striker Shape on the Crack Initiation and Propagation on Laterally Impacted Thin Aluminium Plates
Authors: Liu Bin , Villavicencio Richard , Guedes Soares C.
Published: November 2013

Experimental and numerical results of drop weight impact tests are presented, examining the plastic response and the crack initiation and propagation of small-scale clamped rectangular aluminium plates laterally impacted by different indenter shapes. The experiments are conducted using a fully instrumented impact testing machine. The shape of the deformation of the specimens and the process of initiation and propagation of the material fracture is presented. The obtained force-displacement responses show a good agreement with the simulations performed by the LS-DYNA finite element solver. The strain hardening of the material is defined using experimental data of quasi-static tensile tests and the critical failure strain is evaluated measuring the thickness and the width at fracture of the tensile test pieces. The results show that the absorbed energy to perforate the specimens is highly sensitive to the shape of the striker. Thus, the crack propagation for each striker type is analysed in terms of the force-displacement response. The failure modes are described by the matrix of the infinitesimal strain tensors and the shape of the deformation of the failing elements.
Numerical Study on Vortex Induced Motion of Floating Body by Lattice Boltzmann Method
Authors: Hirabayashi Shinichiro , Suzuki Hideyuki
Published: November 2013

Flow around a solid cylinder is numerically simulated with allowance of the movement of the cylinder by the lattice Boltzmann method. The drag force of the fixed cylinder is in good comparison with the literature showing that the fluid force acting on the cylinder is accurately estimated in the computation. It is confirmed that as the body oscillates, the drag and lift forces increases and decreases, respectively. By changing the mooring tension, the locked-in phenomenon and the nonlinear interaction between the vortex-shedding and the mooring system are found.
Experimental Study on the Interaction Between the Net and the Weight System for a Gravity Type Fish Farm
Authors: Lader Pål , Kristiansen David , Jensen Østen , Fredriksson David W.
Published: November 2013

The escape of salmon from aquaculture farms is considered a major problem for the industry. Structural failure is the dominating cause for escapes. One major issue is the contact, and subsequent abrasion between the sinker tube chain and the net, which has caused several large-scale escapes over the past years. Model tests were performed in a tow tank at the United States Naval Academy to investigate at which combinations of waves and current the net deforms and comes in contact with the sinker tube chain. A model in scale 1:40 was used to represent a fish cage with a circumference of 120 meters and net depth of 40 meters. Building a physical model of such a compliant structure is inherently difficult, so particular attention was given to similarity issues. With the physical modelling approach, two different net design and four different weighting systems were tested. The experiments showed that the sinker tube with the fixed connection to the net performs better than individual weights both for cylindrical and conical nets. For cylindrical nets exposed to current only contact occurred at a velocity of 0.3 m/s when using weights and at a velocity of 0.5 m/s when using either the fixed or sliding sinker tube. The results for conical nets exposed to waves and current were less consistent. Weights (contact at 0.3 m/s) performed better than a sinker tube with sliding connection (contact due to waves only) but not as good as a fixed sinker tube (contact at 0.3 m/s). When exposed to waves and current contact occurred at a higher velocity and over a smaller area when using the conical net compared to the conventional cylindrical. For cylindrical nets contact occurred when exposed to waves alone, whereas for conical nets contact occurred at current velocities of 0 m/s, 0.15 m/s, and 0.3 m/s for the weights, the sliding sinker tube, and the fixed sinker tube respectively.
Roll Decay Model Test of a Post Panamax Container Ship: Experimental and Numerical Analysis
Authors: Piehl Henry , el Moctar Ould
Published: November 2013

The slender shape of a modern ship hull, exhibits a roll motion that is — in contrast to heave and pitch motion — submitted to large amplitudes and weak damping. This effect explains the importance of determining the roll damping of a hull and denotes the difficulty to measure such a small dynamic property.

For the joint research project Best-Roll a series of roll decay tests were conducted with the model of a post panamax container ship. To capture the influence of the draft and the ship velocity, the roll motion was measured for a systematic variation of these parameter. In addition the tests were performed with an initial roll angle of 20 degrees, in order to investigate the nonlinear damping behavior at large roll angles.

The objective of the subsequent study was to test the capability of OpenFOAM to compute the overall roll damping of a ship hull and appendages. For the numerical comparison, the roll decay test was simulated with OpenFOAM, using a transient multiphase solver with a RANS turbulence model, dynamic mesh motion and a rigid body motion model of the ship. The results were compared by calculating the roll damping coefficient for both experiment and numerical simulation by means of time series analysis of the roll angle.
Fracture and Damage During Dynamic Interactions Between Ice and Compliant Structures at Laboratory Scale
Authors: Taylor Rocky , Browne Tom , Jordaan Ian , Gürtner Arne
Published: November 2013

Recent laboratory experiments have been carried out to study the compressive failure of confined polycrystalline ice during indentation by structures of varying compliance at different temperatures. The experiments presented in this paper focus on the indentation of ice using a single indenter mounted on a compliant beam. From these tests it was observed that the stiffness of the structure plays an important role in the development of cyclic loading patterns. Fracture and damage processes in the ice have been linked with the observed loading patterns and failure frequencies as high as 240 Hz have been linked to crushing and extrusion of ice. Temperature and loading rate were observed to significantly influence whether or not cyclic loading occurred and which ice failure mechanisms dominated the interaction process. When cyclic loads were observed, their frequency was found to exhibit a positive relationship with the indentation rate, as well as the structural stiffness. Based on this work it is concluded that both structural feedback and the mechanics of ice failure play an important role in determining the nature of dynamic interactions between ice and compliant structures.
Riser VIV Suppression Device Tests for Application to a Southeast Asia TLP
Authors: Yen Kien Min , Magee Allan , Aramanadka Shankar Bhat , Abdul Malik Adi Maimun , Abdul Ghani Mohamad Pauzi , Ismail Nasrudin Haji , Ahmad Zukni NorBahrain , Teng Yih Jeng
Published: November 2013

The Southeast Asia metocean environment is characterized by moderate wind and waves with relatively strong and persistent currents. The design of a Tension Leg Platform (TLP) for Southeast Asia is strongly affected by the current and resulting drag loads, particularly if many risers are required. To reduce vortex-induced vibrations (VIV) of offshore tubular members i.e. top tension risers (TTRs), vortex suppression devices such as helical strakes and fairings are used.

This paper gives an insight into a set of riser VIV model tests which is carried out to study the effectiveness of an open-back “U”-shaped fairing in suppressing riser VIV. The splitting loads and the drag loads of the fairings are also investigated during these tests. The tests are being done at the Marine Technology Center (MTC), Universiti Teknologi Malaysia (UTM) in support of this application, and also in part to increase local technological capabilities. Two sizes of risers are being tested in three configurations: (a) bare riser without any fairings; (b) riser with weathervaning fairings (normal condition) and (c) riser with fairings fixed at different headings (abnormal condition). The test results show that in the normal condition, the fairing is effective at suppressing VIV.
Mooring Optimization of Offshore Floating Systems Using an Improved Particle Swarm Optimization Method
Authors: da Fonseca Monteiro Bruno , de Lima Junior Mauro Henrique Alves , Albrecht Carl Horst , de Lima Beatriz de Souza Leite Pires , Jacob Breno Pinheiro
Published: November 2013

The offshore oil production industry is currently expanding its activities in even deeper waters, using moored floating platforms submitted to extreme environmental conditions. Therefore, the design of mooring systems to keep the position of such platforms is of vital importance to assure safety and economical feasibility for offshore oil production.

In this context, this work presents optimization procedures to find the minimum offsets for floating platforms under environmental loads, taking radius, azimuth, pre-tension and material of the mooring lines as design variables.

Considering that such optimization procedures require high computational costs, due to the need of nonlinear static and dynamic analyses with Finite Element models for each candidate solution, among the various meta-heuristic algorithms the Particle Swarm Optimization method (PSO) was chosen due to its simplicity, efficiency and parallel capability.

Results of typical systems are presented, which indicate that the method is effective.
Ship-Bank Interaction of a Large Tanker and Related Control Problems
Authors: Mucha Philipp , el Moctar Ould
Published: November 2013

The objective of this work is to establish a synthesis between modern methodology in the field of ship maneuvering and control theory using the example of hydrodynamic ship-bank interactions for a large tanker. Evolving technologies have paved the way for developing increasingly sophisticated modeling techniques to study ship flows. These tecnologies have made it possible to resemble Planar Motion Mechanism (PMM) tests in numerical simulations using Reynolds-averaged Navier-Stokes (RANS) equations. These advances give way for the numerical determination of hydrodynamic derivatives as present in the maneuvering equations. This methodology is adopted in the present investigation to obtain these coefficients for various separation distances to a vertical wall. Likewise, control theory has experienced vital progress enabling engineers to apply elaborate control policies in their systems. Special attention has been payed to the distinct discipline of optimal control theory and the family of Linear Quadratic (LQ) regulators. Among the popular class of conventional Proportional-Integral-Derivative (PID) controllers rather heuristic design procedures are applied; appealing to the practitioners but might not be suitable for special applications. The work presented investigates the suitablity of deriving hydrod-namic properties by means of Virtual Planar Motion Mechanism (VPMM) tests for the KVLCC2 tanker travelling at various distances to a vertical wall of infinite depth. In subsequent maneuvering simulations the performance of the introduced controllers is discussed.
Buckle Interaction in Deep Subsea Pipelines
Authors: Karampour Hassan , Albermani Faris
Published: November 2013

The paper investigates the interaction between propagation buckling and lateral buckling in deep subsea pipelines. Lateral buckling is a possible global buckling mode in long pipelines while the propagation buckling is a local mode that can quickly propagate and damage a long segment of a pipeline in deep water. A numerical study is conducted to simulate buckle interaction in deep subsea pipelines. The interaction produces a significant reduction in the buckle design capacity of the pipeline. This is further exasperated due to the inherent imperfection sensitivity of the problem.
Reliability-Based LRFD Criteria for Unstiffened Panel of Ship Structures
Authors: Bai Yong , Qian Miao-hua
Published: November 2013

It is of significance to do the research of safety and reliability for ship structures, especially for marine structures because of the poor conditions and high risks, future design for ship structures will move toward a more rational and probability-based design. This paper chooses the unstiffened panel of ship structures as the research subject. Based on the MATLAB software, this paper develops the procedures and calculates one limit state equation of the panel, derives partial safety factors (PSF) for the Load and Resistance Factor Design (LRFD) of the panel under different reliability index levels. The PSF may provide a reference for the practical project design.
A Practical O(Δω) Approximation of Low Frequency Wave Loads
Authors: Monroy Charles , de Hauteclocque Guillaume , Chen Xiao-Bo
Published: November 2013

The low-frequency quadratic transfer function (QTF) is defined as the second-order wave loads occurring at the frequency equal to the difference frequency (ω1 − ω2) of two wave frequencies (ω1, ω2) in bi-chromatic waves of unit amplitude. The exact formulation of the QTF which is recalled here is difficult to implement due to numerical convergence problems mainly related to the evaluation of an arduous free surface integral. This is why several approximations have been used for practical engineering studies. They have been the subject of a detailed review in [5]. Following this work, two closely-related formulations are investigated in this paper.

In [2], the classical formulations of QTF are examined by an analysis based on the Taylor development with respect to Δω for Δω ≪ 1 and an expansion of QTF in power of Δω is then obtained. It is shown that the zeroth-order term is a pure real function equal to the drift loads and that the term of order O(Δω) is a pure imaginary function.

The second-order low-frequency wave loading of order O(Δω) contains a free-surface integral representing the second-order corrective forcing on the free surface. Since the integrand is of order O(1/R4) with R as the radial coordinate, the free-surface integral converges rapidly with the radial distance. Unlike what has been assumed in previous studies of particular cases, this free-surface contribution is, in general, not negligible for high Δω compared to other components and the complete QTF.

Depending whether we use the O(Δω) approximation for the whole QTF or only for this free surface integral, it leads to two different approximations. The first one is called original O(Δω) approximation, because it is on this form that the O(Δω) approximation was first described in [2]. If we use the O(Δω) approximation only for the free surface integral, we call this approximation the practical O(Δω) approximation. It is shown in this paper that the original formulation fails to predict the behaviour of the QTF even for small O(Δω). Comparison for the O(Δω) approximation of the free surface integral is performed against the analytical solution and the exact numerical formulation. The results are improved compared to when we neglect this free surface integral for the range of Δω of interest, but still the agreement with the exact solution is not ideal. A path for further improvement is finally proposed.
A Numerical Study on Storm Surge and Sediment Resuspending in Modaomen Estuary During Typhoon Hagupit
Authors: Zhang Zhuo , Song Zhiyao , Lu Fuqiang
Published: November 2013

This study examines the effects of waves and local wind forcing on storm surge by using a coupled numerical model of surge, waves and tide and the effect of waves on sediment resuspending by an imbedded suspended sediment transport model in Modaomen Estuary, China. The hindcast results for Typhoon Hagupit (2008) indicate that although local wind forcing is significant in surge level prediction, only employing local wind forcing without incorporating wave radiation stresses is unable to properly simulate storm surge including water level set-up and the wave-driven alongshore current. This study also finds that the effect of waves on sediment concentration at landfall is significant because the waves play important role in sediment stirring and resuspending during the hurricane. The sudden siltation thickness including wave effect is apparently larger than that without considering waves, especially near the shoal.