bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Anisotropic Shape on Ship Wakes in Presence of Shear Current of Uniform Vorticity
Authors: Li Yan , Ellingsen Simen Å.
Published: October 2016

We analyze the interactions between a subsurface shear current of uniform vorticity and a moving surface disturbance of anisotropic shape which generates surface gravity waves. The problem extends previous analysis of ship waves in the presence of a shear current varying linearly with depth, now also accounting for the three dimensional shape of real ships, in order to study the interplay of aspect ratio and the shear current. Based on general solutions derived previously, we apply an elliptical Gaussian pressure disturbance at the surface moving at constant velocity as a model for a real “ship”. Wave contributions in the far field and expressions for the Mach angle (of maximum wave amplitude) based on asymptotic expressions for high Froude numbers, are derived thereafter. Through numerical calculations we present wave patterns, as well as Kelvin and Mach angles, at moderate Froude numbers under different shear strenghts and aspect ratios. Results show that the aspect ratio has negligible effect on the value of the critical shear vorticity and Kelvin angle, whereas a subtle interplay of aspect ratio and shear strenght is found to affect the Mach angle at moderate Froude numbers.
An Appreciation of Professor Norman Jones’ Contributions to Impact Engineering
Authors: Li Q. M.
Published: October 2016

This article summarises Professor Norman Jones’ academic career and his scholarly contributions to impact engineering. In the past 50 years, Professor Jones has performed profound research on a wide range of impact engineering problems, supervised postgraduate students, researchers and academic visitors from all over the world, initiated international research networks and conferences, and has played important roles in consulting government bodies and in generally serving the academic community. Due to his research excellence and achievements, Professor Jones has received numerous prestigious awards and titles including Fellowship of the Royal Academy of Engineering and Foreign Fellowship of the Indian National Academy of Engineering.
A Review of the Metal Magnetic Memory Technique
Authors: Bao Sheng , Fu Meili , Hu Shengnan , Gu Yibin , Lou Huangjie
Published: October 2016

Metal magnetic memory (MMM) is a newly developed non-destructive testing (NDT) technique. It has potentials to detect early failure, such as stress concentration, micro-crack and fatigue damage of ferromagnetic components. This paper outlines the recent developments of the mechanism studies on the MMM technique. Several advances in experimental investigations on the MMM technique are also summarized, e.g. the factors which can influence the detection signals, the criteria for judging the damage state and the magnetic parameters which can be used to assess the testing results. Finally, some future development trends about this technique are suggested.
Study on Fluid Flow and Heat Transfer Characteristics of Molten Pool Considering the Effects of Turbulence
Authors: Liang Rong , Chen Zhen , Luo Yu
Published: October 2016

Recently, the welding of dissimilar material using high energy beam has gained greater popularity. During the process of welding, the differences in physical properties of the materials and high concentration of energy have a great influence on the related fluid flow and heat transfer characteristics in the molten pool. For the welding of a similar material, the fluid flow in the molten pool is often assumed to be laminar in nature. However, in the dissimilar material welding, the fluid flow in the molten pool is more intensive and complicated. It is necessary to take into account the effects of turbulence in the numerical model. In this paper, the simulation of laser welding of low carbon steel and stainless steel couple is carried out. A three-dimensional, transient numerical model is developed using computational fluid dynamics (CFD) method. The characteristics of heat, mass and momentum transports in the molten metal pool are investigated using both laminar and turbulent flow models under identical welding conditions of laser power and moving speed. To improve calculation accuracy, the turbulence effects are taken into account by employing a suitably modified k–ε model. Melting and solidification were simulated not by tracking the solid-liquid interface but using the Enthalpy-Porosity model to save calculation time. Results show that the diffusive transport is enhanced in the turbulent model. This is reflected in the reduction in the maximum values of temperature and velocity magnitude in the turbulent model in comparison to those in the laminar model. The influence of turbulence on the species transport in the molten pool is significant. The phase distribution in the turbulent simulation is found to be more uniform than that obtained in the laminar simulation. Good agreements between the experimental observations and simulation results are obtained by the proposed method. This study has laid a solid foundation for the analysis of welded joint by coupled thermo-hydro-mechanical method.
Applying CFD for In-Line Structure Hydrodynamics in Pipeline Installation Analysis
Authors: Hoekstra Carel , Smienk Henk , van Drunen Joris , Pistidda Alessio
Published: October 2016

Over the last decade Heerema Marine Contractors (HMC) has successfully performed multiple installation campaigns of large sized in-line structures (ILS) with Deep Water Construction Vessels (DCV) Aegir and Balder. Nowadays steady increase in size and weight of ILS have made these special operations even more complex. Presence of large sized ILS and accompanying buoyancy modules in the catenary have proven to play a dominant role in pipeline integrity. Originally hydrodynamic force formulations in finite element analysis are solely designated for the pipeline itself. These computations comprehend the application of the Morison equation using constant hydrodynamic coefficients of basic shapes in steady flow.

Therefore hydrodynamic forces acting on the ILS, characterized by irregular relative motions of a complex shaped and perforated structure, are highly simplified while playing a dominant role in the analyses. Validity of applying the standard Morison equation is debatable, since large ILS cannot be assumed slender. Nonetheless Morison type formulations can provide reasonable results depending on the accuracy of the hydrodynamic coefficients. Deriving these coefficients for complex shaped structures using industry standards is a highly interpretive process involving an accumulation of assumptions. This approach yields varying coefficients, which are applied conservatively in installation analyses, resulting in an unnecessary reduction of DCV offshore workability.

To improve workability of these complex installations, HMC has implemented an ILS specific hydrodynamic profile from Computational Fluid Dynamics (CFD) analysis into the installation analyses. This is effectuated by the development of an enhanced methodology with a dedicated hydrodynamic formulation for large perforated ILS. Dependencies on Keulegan-Carpenter (KC) number and local angle of attack are addressed in this formulation to respectively cover the inertia dominated oscillating motions and complex geometric composition. The applied hydrodynamic formulation is based on work of Molin et al. which showed a good agreement to the CFD analysis performed for this study. Development and application of this methodology is initiated as a first assessment towards more accurate ILS installation analyses.

Analysis of a study case shows reductions up to 50% of maximum bending strain in a specific regular wave analysis. From the work presented it is concluded that the industry practice vastly overestimates hydrodynamic forcing on large sized ILS. Complementary research is needed on the topics of oscillations for low (<1.0) KC number, effects of relative fluid velocity and finally the implementation of irregular waves.
The Detailed Safety Lifecycle for Offshore Safety System Design
Authors: Park Jin Hyung
Published: October 2016

Safety Lifecycle is described in IEC61508 and IEC61511, but more detailed engineering procedure than IEC61508 and IEC61511 is required for real design and manufacture of safety system for offshore project. For the turret national research project, more detailed design procedure was applied for the design of turret. First of all, the nodes are defined based on EUC (Equipment Under Control) units and the hazards are analysed by 3 methodologies — Hazard Checklist, HAZOP and FMEA during hazard analysis phase. Then the risk are analysed quantitatively by LOPA methodology. To produce the safety requirement specification for function, the cause & effect, the logic diagram and IO list will be produced. The isolation countermeasures in case of explosion and fire and the SIFs (Safety Instrumented Function) in LOPA report and the process shutdown interlocks will be included in the cause & effect and the logic diagram will be developed based on this cause & effect. The safety requirement specification for safety will be developed by exSILentia from exida. Based on the safety requirement specification, the Functional Design Specification, the Safety Validation Plan including the test specification and test plan, the system configuration will be developed during the Basic Design phase and then IO list & terminal allocation, application software implementation, panel drawing will be developed during the detailed design.
The Influence of External Boundary Conditions on Ice Loads in Ice-Structure Interactions
Authors: Sopper Regina , Daley Claude , Colbourne Bruce , Bruneau Stephen
Published: October 2016

Design ice loads are generally derived from field measurements or laboratory experiments. The latter commonly neglect the circumstance that most ice-structure interactions occur underwater, despite the fact that studies report higher ice loads if water is present. Other than a few studies on ice extrusion processes, most investigations on ice loads also do not specifically consider the presence of snow or granular ice at the ice-structure interface. To elucidate the influence of water, snow and crushed ice, as external boundary conditions, on ice load magnitude, 71 small-scale laboratory tests were carried out. Testing involved a hydraulic material testing system (MTS machine) located in a cold room at −7°C. Ice specimens were conical shaped with 25 cm in diameter and with 20° and 30° cone angles. Those were impacted with a flat indentation plate at 1 mm/s, 10mm/s and 100 mm/s indentation rates. Time-penetration and time-force histories from the MTS machine, as well as qualitative contact area and local pressure measurements from tactile pressure sensors were collected. Tests were also recorded with a high speed camera and monitored with still photos. The effect of submergence was most evident at high indentation rate, yielding high ice loads. Snow and granular ice caused comparably high ice loads at the high indentation rate. Moreover, the snow and granular ice conditions also significantly increased loads at the low indentation rate. In all cases, higher ice loads were associated with increased effective contact areas.
Assessment of Helicopter Emergency Response Capacity in the Barents Sea
Authors: Jakobsen Marion , Milaković Aleksandar-Saša , Ehlers Sören
Published: October 2016

As the global demand for energy is increasing, oil and gas exploration is moving further north to more remote areas. Offshore activity in these areas is challenging. Arctic-specific environmental conditions, long distances from onshore facilities and general lack of infrastructure are some of the challenges faced. Therefore, new and more robust solutions — both on technological and operational side — are required before commencing operations safely in these areas.

In this paper, a helicopter emergency response capacity — with respect to prevailing wind conditions — for operations in the Barents Sea is studied and a method for mapping the rescue capacity in the given area is presented. The goal is to develop a method capable of assessing the probability of a successful rescue at different locations within given time requirements and under prevailing wind conditions. This is accomplished using a simulation model capable to determine how the wind speed and direction affect the search and rescue helicopter operations in the Barents Sea.

The simulation model uses historical wind data along a potential route as input for evaluating the flying time to different locations in an area under the given wind conditions. In addition to the wind conditions, the variation in recovery time, and mobilisation time is implemented into the simulation model. By running the simulation model multiple times, probability distributions of the number of personnel which can be recovered within the given time requirements are established. This information is then used to plot isocurves of equal rescue probability on top of a map of the Barents Sea.

Based on the results, it is concluded that wind conditions have significant effect on rescue capacity of a helicopter, and thus thorough weather observations should be made before establishing a search and rescue system for a given area.
Fatigue Considerations for Subsea Well Systems
Authors: Baker David A. , Walker Karen M.
Published: October 2016

Fatigue assessments are a central component of subsea well system analysis and are essential in understanding overall well system performance. In recent years there has been an increased focus on understanding fatigue in well systems. This focus has taken on many forms — a broad industry JIP, technical studies by individual operators, development of a variety of technologies for subsea instrumentation and monitoring and case studies implementing this technology. An overarching objective of all these efforts is to clarify some of the complexity that is inherent to well system fatigue.

As with any analysis, well system fatigue analysis benefits from appropriately defined inputs. These inputs affect both sides of the analysis equation — loading considerations and resistance considerations. A majority of the efforts to date have examined loading considerations. Inputs such as metocean criteria and soil properties can have a dramatic impact on the analysis results. Accurate characterization of these inputs is desired, though often difficult to achieve. As a result, large safety factors are often employed to provide a margin against this uncertainty.

Often overlooked, however, is the resistance half of the assessment, particularly the accurate characterization of fatigue hotspots such as welds and connectors. Conservative analysis of hotspots is frequently employed as these components are not amenable to inspection, however the conservative assumptions can result in non-realistic assessment of fatigue performance.

This paper presents some additional considerations for minimizing the uncertainty in well system fatigue analysis, through thorough materials characterization and design analysis, focusing on connector hotspots. The information presented is representative of actual considerations utilized for ExxonMobil projects. Fatigue performance is improved through finite element characterization of the connector design and “hand picking” component match-ups to minimize the resulting stress amplification. Attention to the total life fatigue properties, including experimental assessment of the material used, is needed to provide an accurate representation of fatigue performance. Finally, in the absence of this information, mitigation of fatigue risks can be obtained by displacing fatigue hotspots.
Reliable Reservoir Characterization and History Matching Using a Pattern Recognition Based Distance
Authors: Lee Jiyoon , Choe Jonggeun
Published: October 2016

A distance is defined as a measure of dissimilarity between two reservoir models. There have been many distances proposed for fast modeling. However, some distances cause distortion or loss in original permeability distribution of models. To avoid such problems, this study proposes a pattern recognition based distance.

The distance is defined by the difference of correlation coefficients between ensemble models. From multi-dimensional scaling, initial 400 ensembles are presented on 2D plane using the distance. Then 10 groups are made by K-medoids clustering. After comparing oil production from each centroid and that of the reference field, 100 models are selected around the best centroid.

We validate the clustering by comparing the uncertainty range of 100, 50, and 20 ensemble members sampled from the initial 400 models in box plots and cumulative distribution functions. For a history matching and reservoir characterization, ensemble smoother is applied to the 100 models selected. The proposed method takes only 25% time for simulation showing reliable results compared with the initial 400 models.
Global Performance and Mooring Analysis Using Long Term Analysis Methodology
Authors: Xu Mubing , Sablok Anil , Dalane Oddgeir
Published: October 2016

The long term analysis is performed to predict the hull global motion and mooring strength of a Spar platform and the results from long term analysis are compared with the predictions from the short term analysis. The long term motions are also used to investigate the long term riser strength response in Ref. [5]. The results includes the lateral offset, heel angle, heave motion and the mooring line tension at the fairleads. In the short term analysis, the environment events with various return periods and various realizations are considered for each environment load. The Gumbel fitting is used to predict the extreme response. In the long term analysis methodology, the prediction of the global performance and the mooring tension are based on 56-year hindcast wind and wave data with 3-hour intervals. Weibull fitting is used to predict the extreme response for various return periods. The comparison between the long term and the short term predictions indicate that the short term predictions are generally conservative compared to the more accurate, but computationally expensive long term analysis method. The long term methodology is not widely adopted currently due to the computation inefficiency. However, it is expected that this long term methodology could provide a better option in the future with the consideration of its accuracy and the application of high speed computer.
Numerical Study on the Maneuvering of a Ship in Waves Based on Unified State Space Model
Authors: Thayale Veedu Rameesha , Krishnankutty Parameswaran
Published: October 2016

Ship maneuvering performance is usually predicted in calm water conditions, which provide valuable information about ship’s turning ability and its directional stability in the early design stages. Investigation of maneuvering simulation in waves is more realistic since the ship usually sails through waves. So it is important to study the effect of waves on the turning ability of a ship. This paper presents the maneuvering simulation for a container ship in presence of regular waves based on unified state space model for ship maneuvering. Standard maneuvers like turning circle and zigzag maneuver are simulated for the head sea condition and the same are compared with calm water maneuvers. The present study shows that wave significantly affects the maneuvering characteristics of the ship and hence cannot be neglected.
Statistical Method to Requalify Steel Grades During Conversion of Tankers to FPSO
Authors: Cambos Philippe , Parmentier Guy
Published: October 2016

During ship life, operating conditions may change, tanker may be converted into FPSO, and flag requirements may be modified. Generally these modifications have few impacts on existing structures; flag requirements only rarely are to be applied retroactively. Nevertheless in some cases modifications of operating condition may induce considerable consequences, making in the worst cases impossible any reengineering. For example converting a common tanker, built with plain steel of grade A into an Offshore Floating Unit able operating in cold region, may require a grade change corresponding to a grade B. It is obviously meaningless to replace all material just because material certificates.

Steels used by shipyards have to fulfill Classification society’s requirements dealing with mechanical strength; generally shipbuilding corresponds to a small part of steelmaker’s production. For this reason steelmakers are reluctant to produce steels with mechanical properties corresponding exactly to the minima required. They generally deliver steels already in stock, with higher mechanical characteristics than required. In this case it can be taken advantage of this common practice. In order to demonstrate that the material fulfill the requirements of grade B it has been decided to adopt a statistic approach. At this stage there are two main issues, the first one is that it is needed to provide evidences that the actual material Charpy V characteristics fulfill the requirements of grade B; the second one is to provide these evidences with a minimum testing. To assess this assumption a random check has been carried out. Different probabilistic model have been tested in order to check common approaches and probabilistic model based on physical considerations. In the paper the main assumptions for estimating the minimum Charpy value main assumption in the probabilistic models are recalled, the behavior of empirical sample is examined, the parameters of probability laws fitting the empirical distribution and definitely as accuracy of probability law parameters determination is not perfect with a finite number of specimens the uncertainty in the determination of parameters is taken into account with confidence limits.

According to the selected probabilistic model the minimum value corresponds to an acceptable probability of failure, taking into account the target confidence level, or is independent of any acceptable probability of failure and is defined with the same confidence level. At the end it is concluded that a random check with a data treatment assuming a random distribution of Charpy V test results distributed according to a Weibull probability law of the minimum allows providing evidences that with a sufficient confidence level the steel used for the considered structure fulfill the requirements of the new operating conditions.
An Investigation of Passive and Semi-Active Tuned Mass Dampers for a Tension Leg Platform Floating Offshore Wind Turbine in ULS Conditions
Authors: Park Semyung , Lackner Matthew A. , Cross-Whiter John , Tsouroukdissian A. Rodriguez , La Cava William
Published: October 2016

Floating offshore wind turbines are able to access deeper waters with stronger winds, but also have more complicated dynamic behavior than fixed-bottom offshore turbines, potentially resulting in larger loads. Structural control using tuned mass dampers (TMD) is a promising method for mitigating these loads. Previous research on structural control in wind turbines has typically considered passive devices and operational conditions. In this study, the effects of a passive tuned mass damper and a semi-active tuned mass damper, located at the tower top, are analyzed and simulated for the GE Haliade 150–6MW wind turbine located on the Glosten Pelastar tension-leg platform (TLP). The system is simulated using FASTv8, the wind turbine aero-elastic wind turbine simulator developed by NREL, which includes a TMD module capable of modeling passive and semi-active devices. A pendulum-type TMD developed by ESM GmbH, which can oscillate in the fore-aft and side-side directions, is modelled with non-linear position constraints. Semi-active control is defined using an “on-off” TMD damping based on a “ground-hook” control law. Ultimate limit state (ULS) conditions with a parked rotor are simulated, for two different water depths. The results are analyzed in terms of the load reductions at the tower base, nacelle acceleration reduction, and tendon tensions for the various configurations. The impact of TMD stroke limitations and the sensitivity of the results to water depth are investigated. The results will show that structural control can reduce ULS loads in deep water configurations, but are less effective in shallow water. The dynamics of the system that cause this result will be elucidated. The results will also demonstrate that semi-active control can be an effective strategy to further reduce loads and reduce the TMD stroke.
Study on the Hydrodynamic Characteristic of a Spar Type Floating Foundation Which Used to Support a Vertical Axis Wind Turbine
Authors: Liu Liqin , Zhang Xiaorui , Guo Ying , Qiu Yu
Published: October 2016

The vertical axis wind turbine has more advantages in the development of the large-scale offshore wind power, on account of that its drive system is placed on the bottom of the turbine and has little influence on the tower. This paper presents a preliminary design of a Spar foundation, which can support a Darrieus vertical axis wind turbine with a generated power of 5 MW. The influences of structure’s parameters of the floating foundation, such as the radius of flotation tank, the radius and height of upper ballast tank, the height of bottom ballast tank and other factors on the motions of the floating wind turbine system are analyzed. The initial structure parameters of the floating foundation are determined, and the hydrodynamic parameters are calculated as well. It is found that the radius of floatation tank and bottom ballast tank have more impact on the Response Amplitude Operators (RAOs) and natural period of heave of the floating foundation. The interval of center of gravity and center of buoyancy and the height of upper ballast tank influence obviously on the RAOs and natural period of pitch of the floating foundation. Heave and pitch motions of the floating foundation designed here are relatively small. The natural periods of heave and pitch of the floating foundation are away from the main wave energy periods.
Structural Integrity Assessment of Free-Fall Lifeboats by Combining Fast Monte-Carlo Simulations With CFD by Means of Proxy Load Variables
Authors: Fouques Sébastien , Hermundstad Ole Andreas
Published: October 2016

The paper is concerned with the launch of free-fall lifeboats (FFL). It proposes a method that complies with the DNV-OS-E406 standard in order to select characteristic launches from Monte Carlo simulations for further structural load assessment with CFD and FEM. Proxy variables derived from kinematic parameters and aiming at predicting pressure load indicators are computed with the VARUNA launch simulator developed by MARINTEK. The statistical distributions of the proxy variables obtained from the Monte Carlo simulations are used to identify critical scenarios, and characteristic launches can then be selected from a chosen probability level. The feasibility of the proposed method is documented in the paper for several types of pressure loads. Existing model test data from various FFL-launch campaigns in calm water and in waves are used to compute the proxy variables as it would be done in the VARUNA simulator. Scatter diagrams showing the correlation with actual measured pressure load indicators are then established to assess the quality of the chosen proxy variables.
Evaluation of Wind Loads on FPSO Topsides Using a Numerical Wind Tunnel
Authors: Barcarolo Daniel , Andrillon Yann , Jacquin Erwan , Ledoux Alain
Published: October 2016

The accurate evaluation of wind loads applied on floating offshore structures is extremely important as they are in specific conditions one of the dimensioning criteria for the mooring design. Nowadays these loads are mainly assessed through wind tunnel tests performed at model scale. Estimating realistic wind loads however, remains a big challenge. The complexity and associated simplification level of FPSO topside structures, the scale effects and the establishment of the atmospheric boundary layer imply that many simplifications are to be made. Typically, the FPSO topside is greatly simplified and equivalent blocs of wired frame are used. Today with the evolution of CFD software, and the increase of the meshing capacity, new scopes open to CFD. Aerodynamic simulations on complex FPSO structures are therefore now possible, but need specific developments and validations that are presented in this paper.

The main objective of the work presented is to investigate the ability of CFD to evaluate wind loads on complex FPSOs topsides and to provide information on the impact of model simplifications made in wind tunnels. In a first stage, the numerical model was intensively validated by comparing its results to a wind tunnel test case. The numerical model was developed in order to ensure the quality of the results and enable a relevant comparison that was obtained with grids density up to 30 million cells. For this purpose, the geometric model used corresponds to the one used in wind tunnel. The same Atmospheric Boundary Layer was simulated and a thorough effort was performed to ensure the mesh convergence. In a second stage, more physical aspects of the wind tunnel methodology were investigated. Typically the accuracy of the blockage effect correction was evaluated by performing computations with and without blockage, and results were compared with classical corrections applied in wind tunnel. The impacts of the Atmospheric Boundary Layer on wind loads have also been investigated. Finally, the wind load contribution of each component of the FPSO was evaluated.
Examination of Effect of Lateral Loads on the Hull Girder Ultimate Strength of Large Container Ships
Authors: Matsumoto Toshiyuki , Shigemi Toshiyuki , Kidogawa Mitsuhiko , Ishibashi Kinya , Sugimoto Kei
Published: October 2016

It is known that the hull girder ultimate strength with consideration of lateral loads such as bottom sea pressures and/or cargo loads generally decreases than that without consideration of the lateral loads (i.e. the effect of lateral loads).

In this study a series of elasto-plastic analyses of three cargo holds models, which can reproduce the collapse behavior of the hold structures subjected to both vertical bending moment and lateral loads such as bottom sea pressures, container cargo loads etc., were carried out on a number of container ships with various sizes, and the hull girder ultimate strength obtained through the analyses were comparatively examined focusing on the effect of the lateral loads.

As results of the examination, it has been concluded that local strength of the double bottom structure against the lateral loads is closely related to the hull girder ultimate strength in the case of container ships, the effect of the lateral loads on the hull girder ultimate strength varies among container ships due to the difference of construction of the double bottom structure and it is important to assess the hull girder ultimate strength explicitly taking into consideration the effect of the lateral loads for large container ships such as Post-Panamax sizes.
Examining Field Measurements of Deep-Water Crest Statistics
Authors: Makri Isavella M. , Rose Stephen M. , Christou Marios , Gibson Richard , Feld Graham
Published: October 2016

This paper concerns the field measurements of wave crest elevations and wind speed obtained from fixed platforms in the North Sea during the period of 2011 to 2015. An improved quality control process is proposed by the authors aiming to maximise the amount of waves that are not discarded, whilst maintaining a reliable database. Applying this revised methodology, the total number of waves in the quality-controlled dataset is approximately 184 million. In total, the measurements correspond to 32 years of continuous data and contain approximately 0.0054% of rogue waves, according to the definition of Haver (2000) [1]. As the sea-state steepness increases, it is observed that the measured crest elevations are poorly modelled by the Forristall (2000) [2] distribution; similar behaviour is observed for sea-states with high wind speeds. The model by Tayfun and Fedele (2007) [3] is shown to be a better alternative in these cases, demonstrating that non-linear amplification beyond second-order cannot be neglected.
Scaling Effect on Saturated Impulse for Square Plates Under Rectangular Pulse Loading
Authors: Zhu Ling , He Xu , Yu T. X. , Chen F. L. , Li Yinggang
Published: October 2016

In this paper, the saturated impulse phenomenon is investigated for elastic-plastic square plates under rectangular pulse loading. Dynamic elastic-plastic response (e.g. the deflection-time curve) of square plates is simulated by employing the finite element (FE) code ABAQUS. Numerical results show that two saturated impulses exist for an elastic-plastic plate: one is for maximum deflection and the other is for permanent deflection, and are compared with that obtained from rigid-plastic analysis. The saturated impulse for permanent deflection in the high load range lie between the upper and lower bounds of the saturated impulse for maximum deflection obtained from rigid-plastic analysis. Furthermore, scaling effect on saturated impulse is explored. When the length-thickness ratio is constant, a linear relationship exists between the non-dimensional saturated impulse for permanent deflection and the non-dimensional pressure amplitude and it is independent of the absolute size of square plates. Finally, the formulae of the permanent deflection and that of the saturated duration for loading pulse of square plates are proposed to facilitate the anti-blast design for plate structures.
Experimental Evaluation of Pure In-Line Vortex Induced Vibration (VIVx) of Low Mass-Damping Ratio Cylinder
Authors: Mobasher Amini Mohammad , Fernandes Antonio Carlos
Published: October 2016

Numerous experimental and numerical studies have been carried out to better understand and to improve prediction of cylinder VIV (vortex Induced Vibration) phenomenon. The behavior of cylinder due to in-line vibration (VIVx) has been neglected in the earlier studies because of its lower amplitude in comparison with cross flow vibration (VIVy). However, some researchers have studied VIVx in 2DOF along with VIVy. Recent investigations show that response amplitude of structure caused by VIVx is large enough to bring it to consideration. This study focuses on understanding the origin and prediction of VIVx amplitude exclusively in 1DOF and subcritical flow regime. The experiments were performed in current channel on bare circular cylinder with low mass-damping ratio in Reynolds number range Re = 10000 ∼ 45000.
Reliability Modelling of Subsea Cluster Manifolds Based on the Fault Tree Analysis
Authors: Wang Yingying , Li Fangqiu , Duan Menglan , Liu Houfa , Gu Jiandong
Published: October 2016

The cluster manifold becomes an essential part in a subsea production system and it has been widely used in the development of ultra-deepwater oil and gas fields. One hand, it can gather the production fluid from subsea wells. On the other hand, it can distribute water, gas and chemical agents from the floating production system to each subsea well. Hence, the failure of subsea cluster manifolds may not only lead to the stagnation of production wells and economic losses but also cause environmental pollution and human health in severe cases. Therefore, the reliability of subsea cluster manifolds is quite of importance and it should be studied for the safety service of the subsea production system. Based on the fault tree analysis (FTA), this paper will discuss the failure cause of six well slots subsea cluster manifolds in LiWan 3-1 gas field in China South Sea. Considering the pipeline structure, control system and flow assurance, the fault trees of subsea cluster manifolds are built. Meanwhile, the importance degrees of each elementary event are ranked orderly and the minimum cut sets are obtained through analyzing the structure important of the FTA. The failure major reasons are obtained and the preventive measures are proposed, which could have some guiding significance for the operations of subsea cluster manifolds system in China South Sea.
Benchmark Study of Numerical Approaches for Wave-Current Interaction Problem of Offshore Floaters
Authors: Pan Zhiyuan , Vada Torgeir , Finne Styrk , Nestegård Arne , Hoff Jan Roger , Hermundstad Elin Marita , Stansberg Carl Trygve
Published: October 2016

The influence from a current on the relative motions and wave drift forces for moored floater can be quite significant. In this paper, a benchmark study is carried out for three programs, MULDIF, WADAM and WASIM, with the focus on their capability on handling the wave-current interaction problem. A semi-submersible model Troll B and a tanker model KVLCC2 are used for this study. The motions, free surface elevation at specified off-body points and mean drift forces are calculated by the programs in different current or forward speed conditions. Analysis results are compared and discussed, with the aim to evaluate the numerical performance of each programs and their validity range in terms of current speed.
Validation of Hydro-Structure Analysis Using Partial Structural Model
Authors: Jagite George , Xu Xiang-Dong , Chen Xiao-Bo , Malenica Sime
Published: October 2016

Nowadays direct Finite Element Method (FEM) calculation using partial or full length model is necessary for checking the structural integrity of ship and offshore structures under given environmental conditions. The main advantage of using hydro-structure analysis on partial model is to obtain better accuracy than usual computation based on rule loads and also a consistent decrease of the time necessary to build a complete ship model. The comparison of different three cargo hold models with the complete ship model and the improvement of our partial FEM models are the main objectives of the work. Unlike the classical partial FEM models approach, our hydro-structure analysis is based on creating an equivalent full FEM model from the partial model. The equivalent full FEM model is built by adding to the partial model two concentrated masses in the center of gravity of missing aft and fore parts. The mass and inertia properties of the equivalent full FEM model are the same as full ship FEM model. By using an equivalent full FEM model the problem of balancing the partial model transforms into the same problem for the corresponding full model. Instead of using the traditional method for interpolating the pressure from hydrodynamic mesh to structural mesh, the pressure components are recalculated over structural mesh. The inertial loads are then determined by motion equations integrating all pressure loads. In this way, the structural model is fully balanced. The balancing of the 3D FEM structural models represents one important issue to avoid unphysical structural response induced by an unbalanced structural model. This paper is focused on the validation of hydro-structure analysis methodology by comparing the results on a FSO unit using an equivalent full FEM model and a complete ship model.
On the Performance of a Dual-Cylinder Wave-Energy Converter: Single Versus Two Degrees of Freedom
Authors: Wang Lu , Son Daewoong , Yeung Ronald W.
Published: October 2016

A recently developed dual (coaxial-)cylinder wave-energy converter (WEC) consists of inner and outer-cylinders, with the outer one sliding over the inner one. An effective design was to tension-tether the inner cylinder (Son and Yeung, OMAE2014-#24582) while the outer cylinder acts as a floater heaving in response to incident waves. Even though the idea was a success, there was significant scientific curiosity in our early stage of the design in the following context: if both cylinders were allowed to heave simultaneously and independently, what would be the implications on the energy-extraction performance and power-take-off constraints? In this paper, we report the detailed analysis conducted at the time of the design. To begin with, the hydrodynamic coefficients, namely, the added mass, radiation-damping, and wave-exciting force for the individually moving cylinders were solved using the method of matched eigenfunction expansions (Chau and Yeung, OMAE2012-#83987). We expanded that capability to allow coupling or interference hydro-dynamic coefficients to be computed in the current work. This coupling is shown to lead to two degrees of freedom of motion, one for each cylinder, with excitation forces on each based on reciprocity (Haskind’s) relations. The resulting relative heave motion between the cylinders is used to drive the permanent magnet linear generator (PMLG) to capture electrical energy. The performance of the WEC, in terms of capture width, is calculated for both regular-wave and irregular-wave conditions and is compared with that for the one degree-of-freedom system, fixed inner cylinder and heaving outer cylinder. The change in WEC performance in response to changing generator damping was found to be very different for the two cases. This behavior leads to very different optimal generator damping values in regular and irregular waves. The advantages and shortcomings of the two systems are compared and explained.
Numerical and Experimental Study of a Multi-Use Platform
Authors: Armesto José A. , Sarmiento Javier , Ayllón Víctor , Iturrioz Arantza , Jurado Alfonso , Guanche Raúl , Losada Iñigo J.
Published: October 2016

A complete hydrodynamic study of a Multi Use Platform designed to harvest energy from waves and wind is presented. The steps given in the study, and presented in this paper are: (i) the design of the platform, (ii) the laboratory tests performed at the Cantabria Coastal and Ocean Basin, (iii) the development of a new in-house time domain numerical model that integrates all components and (iv) the first validation cases of the numerical model.

The development of time domain numerical model that coupled the behaviour of the platform and the OWCs based in Cummins equation is presented. The model includes nonlinear friction forces and a quasi-static approach to model the mooring system. The obtained system is extended to include the variation of the pressure inside each chamber. The effect of the wind turbine in the movements and power production of the platform is studied using also a quasi-static approach.
Real-Time Hybrid Model Testing of a Braceless Semi-Submersible Wind Turbine: Part II — Experimental Results
Authors: Bachynski Erin E. , Thys Maxime , Sauder Thomas , Chabaud Valentin , Sæther Lars Ove
Published: October 2016

Real-Time Hybrid Model (ReaTHM) tests of a braceless semi-submersible wind turbine were carried out at MARINTEK’s Ocean Basin in 2015. The tests sought to evaluate the performance of the floating wind turbine (FWT) structure in environmental conditions representative of the Northern North Sea. In order to do so, the tests employed a new hybrid testing method, wherein simulated aerodynamic loads were applied to the physical structure in the laboratory. The test method was found to work well, and is documented in [1].

The present work describes some of the experimental results. The test results showed a high level of repeatability, and permitted accurate investigation of the coupled responses of a FWT, including unique conditions such as blade pitch faults. For example, the influence of the wind turbine controller can be seen in decay tests in pitch and surge. In regular waves, aerodynamic loads due to constant wind had little influence on the structure motions (except for the mean offsets). Tests in irregular waves with and without turbulent wind are compared directly, and the influence of the wave-frequency motions on the aerodynamic damping of wind-induced low-frequency motions can be observed.
On the Streamwise Oscillations of Freely Vibrating Cylinder Near a Stationary Plane Wall in Steady Flow
Authors: Li Zhong , Yao Weigang , Jaiman Rajeev K. , Khoo Boo Cheong
Published: October 2016

A partitioned iterative scheme based on Petrov-Galerkin formulation [1] has been employed for simulating flow past a freely vibrating circular cylinder placed in proximity to a stationary plane wall in both two-dimension (2D) and three-dimension (3D). In the first part of this work, effects of wall proximity on the vortex-induced vibration (VIV) of an elastically mounted circular cylinder with two degree-of-freedom (2-DoF) are systematically studied in 2D by investigating the hydrodynamic forces acting on the cylinder, the vibration amplitudes, the phase differences between the forces and displacements, the response frequencies as well as the vortex shedding dynamics. For that purpose, a careful comparison has been established for the isolated and near-wall cylinders, in which the gap ratio, e/D (where e denotes the gap between the cylinder and the wall and D denotes the diameter of the cylinder), is set to be 0.9, at Re = 200. Our 2D simulations have revealed that larger streamwise vibration amplitude and smaller streamwise vibration frequency can be observed in VIV of the near-wall cylinder compared to its isolated counterpart. We then focus on the explanation of the enhanced streamwise vibration amplitude when the cylinder is placed in the vicinity of the plane wall. It is found that the wall proximity largely amplifies the streamwise vibration amplitude due to net energy transfer from the fluid to the cylinder in the pre-lock-in region as well as the initial branch of the lock-in region, while reduces the streamwise vibration frequency to the level of the transverse vibration frequency. In the second part, the main focus of this article, following Tham et al. (2015) [2] where 2D results were systematically reported, we perform 3D simulations of VIV of a circular cylinder for both isolated and near-wall cases (e/D = 0.9) at Re = 1000 to compare the hydrodynamic forces and vibration characteristics in 3D with the results corresponding to the 2D study. We show that wall proximity effects on VIV are also pronounced in 3D with the following observations: (1) the wall proximity increases the mean lift to a lesser extent compared to 2D, while also enhances the mean drag unlike in 2D; (2) the wall proximity enhances the streamwise oscillation as well owing to a combined effect of increased drag force together with energy transfer from fluid to structure as in 2D; (3) in terms of the flow field, the wall proximity increases the wavelength of streamwise vorticity blob; and (4) similarly with the mechanism of vortex suppression in 2D, wall boundary layer vorticity strongly strengthens the negative vorticity shed from upper surface of cylinder, stretching and suppressing the positive vorticity shed from the bottom surface of cylinder.
Design and Structural Strength Analysis of the Gate-Type Tidal Current Energy Converter
Authors: Ma Yong , Li Bingqiang , Xu Yan , Dong Yue
Published: October 2016

Ocean tidal current energy is a kind of clear renewable energy with its great utilization and potential. In this paper, it is a research on the new gate-type tidal current energy converter. According to the sea condition around ZhouShan Island, general design has been carried out. Based on FEM, the wave load and the evaluation of structure strength have been worked out. And at the same time, the characteristic of the load in different work conditions and the response of structure has been analyzed. As the result, the gate-type tidal current energy converter can satisfy the work requirement of 300kW water turbine, and the structure itself also satisfies the rules of China Classification Society. The design and results in this paper can provide the reference for the development and application of a gate-type tidal current energy converter.