bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Modelling of Breaking Wave Interaction With a Group of Four Vertical Slender Cylinders in Square Arrangements
Authors: Alagan Chella Mayilvahanan , Bihs Hans , Kamath Arun , Myrhaug Dag , Arnsten Øivind Asgeir
Published: October 2016

The main purpose of the study is to investigate the breaking wave interaction with a group of four circular cylinders. The physical process of wave breaking involves many parameters and an accurate numerical modelling of breaking waves and the interaction with a structure remain a challenge. In the present study, the open-source (Computational Fluid Dynamics) CFD model REEF3D is used to simulate the breaking wave interaction with the multiple cylinders. The numerical model is based on the incompressible Reynolds Averaged Navier-Stokes (RANS) equations, the level set method for the free surface and the k–ω model for turbulence. The model uses a 5th-order conservative finite difference WENO scheme for the convective discretization and a 3rd-order Runge-Kutta scheme for time discretization.

The numerical model is validated with experimental data of large-scale experiments for the free surface elevation and the breaking wave force on a single cylinder. A good agreement is seen between the numerical results and experimental data. Two different configurations with four cylinders are examined: in-line square configuration and diamond square configuration. The breaking wave forces on each cylinder in the group are computed for the two cases and the results are compared with the breaking wave force on a single isolated cylinder. Further, the study investigates the water surface elevations and the free surface flow features around the cylinders. In general, the cylinders in both configurations experience the maximum forces lower than the maximum force on a single cylinder. The results of the present study show that the interference effects from the neighbouring cylinders in a group strongly influence the kinematics around and the breaking wave forces on them.
On the Energy Absorption of Combined Foam-Honeycomb Layered Structures
Authors: Karagiozova Dora , Alves Marcilio
Published: October 2016

Analytical and numerical analyses are carried in order to reveal the importance of the topology of the cellular materials for their dynamic compaction. The aim is to distinguish between the deformation mechanisms and energy absorption of materials, which exhibit structural softening, such as out-of-plane loaded honeycomb, and structural hardening (foam). It is shown that the dynamic compaction of honeycombs does not obey the law of shock wave propagation and a new phenomenological model of the velocity attenuation in out-of plane loaded honeycomb is proposed. Comparisons with some currently available theoretical models of the dynamic compaction of cellular materials are discussed when paying attention to the effect of the material homogenization of the honeycomb on their response to impact loading.

A numerical analysis of a bi-layer cellular structure comprising layers with dissimilar constitutive properties is carried out to reveal the possibility for the peak load reduction in cellular structures when subjected to impact loading. In the reported examples, a foam material (Alporas with density of 245 kg/m3) and hexagonal honeycomb made of aluminium alloy AA5056 and having densities of 60.46 kg/m3 and 96 kg/m3 are used.
The Velocity Field Underneath Linear and Nonlinear Breaking Rogue Waves
Authors: Alberello Alberto , Chabchoub Amin , Babanin Alexander V. , Monty Jason P. , Elsnab John , Lee Jung H. , Bitner-Gregersen Elzbieta M. , Toffoli Alessandro
Published: October 2016

During the past decades, a large number of waves of extreme height and abnormal shape, also known as freak or rogue waves, have been recorded in the ocean. Velocities and related forces can be enormous and jeopardise the safety of marine structures. Here, we present an experimental study devoted to investigate the velocity field underneath a breaking rogue wave. The latter is replicated in the laboratory by means of dispersive focussing methods such as the New Wave Theory and nonlinear focussing techniques based on the Nonlinear Schrödinger equation. While the former is basically a liner method, the nonlinear focussing fully accounts for the dynamical evolution of the wave field. Experiments were carried out in the Extreme Air-Sea Interaction flume of the University of Melbourne using a Particle Image Velocimetry (PIV) system to measure the velocity field below the water surface. Measurements show that the mechanism of generation affects the shape of the breaking waves as well as the kinematic field and associated hydrodynamic forces. Particularly, the New Wave Theory leads to higher velocities and a more energetic breaker than the nonlinear focussing.
Current Drag From Tow and Wind Tunnel Tests of a FLNG Hull
Authors: Huang Zhenjia (Jerry) , Kim Jang , Jang Hyunchul , Slocum Scott T.
Published: October 2016

In this paper, the current drag of a barge-shaped floating liquefied natural gas (FLNG) vessel was studied. Three model tests were performed — a wind tunnel model test, a submerged double-body tow test and a surface tow test. Computational fluid dynamics (CFD) simulations were carried out to gain further insights into the test results. During testing, the tow speed was kept low to avoid surface waves. When the current heading was around the beam current direction, the transverse drag coefficient measured from the wind tunnel test was significantly lower than those of the submerged tow and surface tow tests. The submerged tow and the surface tow provided similar drag coefficients. Results presented in this paper indicated that the difference between the wind tunnel test and the tow tests was caused by the wind tunnel boundary layer effect on the incoming wind profile and formation of a recirculation zone on the upstream side of the model, with a possible additional contribution from the wind tunnel floor constraint on the flow in the wake. Such effects are not accounted for with the simple corrections based on flow velocity reduction in the wind tunnel boundary layer. When conducting future wind tunnel model tests for barge-shaped FLNG hulls, one should consider the potential under-measurement of the transverse drag. In this paper, details of the FLNG model, test setup, test quality assurance (QA), measurement and CFD simulation results are presented, as well as discussions and recommendations for model testing.
Numerical Simulation of Icebreaking Process and Parameters Sensitivity Analysis
Authors: Qiao Yue , Han Duanfeng , Teng Mange , Wang Guoliang , Liu Feng
Published: October 2016

The interdependence exists between ship response and ice load during the ship-ice interaction process. According to the ice properties and the velocity between a ship and ice can make influences on ice load, taking a Russian icebreaker as model, a series of numerical simulations were performed to research the ice load variation tendency rules in 3 axial directions and the total force direction. On the basis of regarding ice elements as elastic materials, the theory of a semi-infinite elastic plate resting on an elastic foundation was used to simulate ice sheet.

By means of finite element model, stress and strain contours of typical moment and ice load history curve in four directions were obtained. Furthermore, the effect of important parameters such as elasticity modulus of ice, ice thickness and ship speed on the impact force were also analyzed.

The results indicate that ice sheet will crack in the form of a circle centered on the collision point which accord with actual circumstance, and there are some certain variation relationships between each calculation parameters and ice load values. At the same time, the study can also provide technological support for ice load research.
A New Method to Investigate the Failure Envelopes of Offshore Foundations
Authors: Tian Yinghui , Zheng Tianyuan , Zhou Tao , Cassidy Mark J.
Published: October 2016

This paper presents an alternative numerical method in addition to the traditional ‘probe test’ to investigate the combined loading failure envelopes of foundations in soil. In the ‘probe test’ method, the foundation is displaced with a specified displacement path and eventually the soil resistance force reaches a stabilised point sitting on the failure envelope in the load space. While the displacement paths are arbitrarily or empirically set, the positions of the stabilised loads on the failure envelope can not be predetermined or planned. This paper’s new method, however, can specify the load paths, which directly shoot onto the failure envelope. This allows the investigation of the failure envelope can be better achieved with planned load paths. In addition, this new method is advantageous in checking the plastic flow conditions (i.e. normality of the failure envelope) as the load path directions are predetermined.
Experimental Study of Flow Field Characteristics in Tidal Stream Turbine Arrays
Authors: Nuernberg Martin , Tao Longbin
Published: October 2016

Tidal currents at many locations around the world have great potential to be used as a large scale renewable energy resource in the future. For large tidal turbine arrays to be commercially viable, the interactions of large devices operating in a confined operating environment need to be understood to optimise the layout of arrays to maximise electricity generation.

This study presents results from a comprehensive experimental investigation of the flow field characteristics within tidal turbine arrays across a number of array layout configurations and current velocities. Up to four small scale turbines were placed in a circulating water channel to investigate the effects of changing array configuration and wake interaction on the flow velocity and turbulence characteristics in small array layouts. Detailed account of the resulting flow field characteristics has been taken by particle image velocimetry measurements at a number of locations within the wake of the array thus providing a large set of instantaneous flow recordings for further analysis of flow features and wake characteristics.

Results are shown for experimental studies of single, three and four turbine arrays and some preliminary comparison between experimental measurements and numerical results are made.
Application Study of Cartesian Grid in Numerical Prediction of a Marine Propeller
Authors: Gu Lang , Wang Chao , Hu Jian
Published: October 2016

Cartesian grid was used in open water performance prediction, cavitation performance prediction and flow field characteristics of a propeller to research the applicability of the Cartesian grid in the numerical simulations of marine propellers. The comparisons of calculated results with the previous research and experimental results verify the accuracy of calculations with the grid on the prediction of thrust and torque coefficient and the simulation of cavitation distribution, wake velocity distribution and the vortex structure trajectory. Meanwhile the propulsive performances of Cartesian grid are better than other types of grid with the similar number of nodes. And the turning point of crash performance under cavitation condition and the phenomenon of vortex merging with neighboring vortex structure are excellent agreement with experiments and references.
Large Containerships’ Fatigue Analysis due to Springing and Whipping
Authors: Ren Huilong , Zhang Kaihong , Li Hui , Wang Di
Published: October 2016

As the sea transport demand increases constantly, marine corporations around the world are pursuing solutions with large scale and low cost, which makes ultra large containerships’ construction consequentially. Ultra large containerships are more flexible relatively, and the 2-node natural frequency can easily fall into the encountered spectrum frequency range of normal sea state. Meanwhile, as the speed of containerships is high and its large bow flare, when sailing with high speed, the bow structures may suffer severe slamming forces which can increase the design wave loads’ level and the fatigue damage. The importance of hydroelastic analysis of large and flexible containerships of today has been pointed out for structure design. Rules of Many Classification Society have made changes on design wave loads’ value and fatigue influence factor modification. The paper firstly introduced 3-D linear hydroelasticity theory to calculate the Response Amplitude Operator (RAO) in frequency domain, and then described 3-D nonlinear hydroelasticity theory to obtain the nonlinear wave loads time history in irregular waves in time domain, considering large amplitude motion and slamming force due to severe relative motion between ship hull and wave. Based on the theories, computer programs are made to conduct the calculations under specified load case, and some calculation and statistical results are compared with experimental results to verify the accuracy and stability of the programs secondly. The paper focused on the influence of springing and whipping on fatigue damages of 8500TEU and 10000TEU containerships in different loading cases, using spectrum analysis method and time domain statistical analysis method. The spectrum analysis method can calculate fatigue damage due to low-frequency wave loads and high-frequency springing separately, while the time domain statistical analysis can calculate fatigue damage due to the high-frequency damping whipping additionally, based on 3-D time domain nonlinear hydroelasticity wave loads’ time series simulation in irregular waves and rain flow counting method. Finally, discussions on influence factor of springing and whipping with different loading cases are made. Based on these two containerships in example, the fatigue damage due to whipping can be the same as the fatigue damage due to springing and even sometimes can be larger than the springing damage. According to the wave loads influence factor, the fatigue assessment of different position on midship section is done on the basis of nominal stress. Besides, some suggestions on calculating load case selection are made to minimize the quantity of work in frequency and time domain. Thus the tools for fatigue influence factor modification are provided to meet the demand of IACS’ UR[1].
Wind Turbine Controller to Mitigate Structural Loads on a Floating Wind Turbine Platform
Authors: Fleming Paul A. , Peiffer Antoine , Schlipf David
Published: October 2016

This paper summarizes the control design work that was performed to optimize the controller of a wind turbine on the WindFloat structure. The WindFloat is a semi-submersible floating platform designed to be a support structure for a multi-megawatt power-generating wind turbine. A controller developed for a bottom-fixed wind turbine configuration was modified for use when the turbine is mounted on the WindFloat platform. This results in an efficient platform heel resonance mitigation scheme. In addition several control modules, designed with a coupled linear model, were added to the fixed-bottom baseline controller. The approach was tested in a fully coupled nonlinear aero-hydroelastic simulation tool in which wind and wave disturbances were modeled. This testing yielded significant improvements in platform global performance and tower-base-bending loading.
Measurement of Lined Pipe Liner Imperfections and the Effect on Wrinkling and Collapse Under Bending
Authors: Harrison Benjamin , Yuan Lin , Kyriakides Stelios
Published: October 2016

Carbon steel pipe is often lined with a thin layer of non-corrosive material to protect it against corrosion from sour hydrocarbons. The product is commonly assembled by mechanical expansion of a liner shell bringing it into contact with the inner surface of a seamless steel pipe. During installation and operation lined pipelines can experience bending or compression deformations large enough to cause the liner to buckle and collapse inside an intact outer pipe. It has been demonstrated that such buckling instabilities are very sensitive to small initial geometric imperfections in the liner [1–3]. Liner imperfections in 8- and 12-inch lined pipes have been measured using custom scanning devices and have been characterized by trigonometric Fourier series. Imperfection amplitudes large enough to significantly influence the collapse of the liner have been detected. The main source of the imperfections is the internal surface relief in the seamless outer pipe left behind by piercing, rolling and external finishing of the pipe. Dominant circumferential and axial imperfection waves are used in finite element models to demonstrate the detrimental effect of such imperfections on liner collapse under bending.
Structural Reliability Analysis Using Quadratic Polynomial Response Surface and Finite Element in MATLAB
Authors: Wahab M. M. A. , Kurian V. J. , Liew M. S. , Nizamani Z. , Kim D. K.
Published: October 2016

Many aging jacket platforms are being pushed for continued use beyond their design life due to advancement in oil extracting technology and economic reasons. Thus reassessment to determine the platform safety is vital. But no exact guide on methods to assess the safety of the aging platforms is available. Hence, development of reliability analysis methodologies is an active research area. Meanwhile, reassessment deals with numerous uncertainties especially in the load and resistance variables of a jacket platform. Response Surface methodology, a limit state approximation technique was deployed by many in many engineering fields to apprehend inherent uncertainty. Hence in this work, a reliability analysis methodology that combines simple response surface and finite element approach in MATLAB is adopted. The approach assumes a physical transfer function utilizing explicit multivariate expressions and random variables. Also, this avoids large number of finite element simulation required for any probabilistic analysis. The methodology developed allows for reliability analysis to be performed based on easy to program procedures. It also addresses the platform and environmental specific uncertainty variables to distinguish the distinctive characteristics of the cases. Upon execution of multiple finite element simulations, response of components and systems are formulated using quadratic polynomial response surface expressions, which eventually are utilized in the limit state functions. Later using numerical techniques in a separate computational routine the reliability indices are estimated. Utilizing the developed method, component and system level reliability indices are obtained. The developed methodology also has been verified with method currently available in the practice as well as with typical simulation method i.e. Monte Carlo Simulation.
Alternative Stress Models With Focus on Full FE Model for Flexible Risers
Authors: Wang Lidong , Sævik Svein , Ye Naiquan , Yue Qianjin , Yang Zhixun , Chen Jinlong
Published: October 2016

Flexible risers are exposed to complicated loading conditions including internal/external pressure, wave and current loads, as well as floater movements. The combination of these three loads may lead to instability of the tensile armor wire in both longitudinal and transverse directions, which may result in different stress state based on different numerical models in BFLEX software. Simplified models have been implemented to calculate the stress in tensile armor wires with moderate accuracy, with only considering the longitudinal slip of wires.

In order to investigate how the transverse slip will influence the stress state, a full finite element (FullFE) modeling option has been implemented recently to capture the interaction between different layers more precisely thus giving more accurate stress prediction. Different stress components will be compared with already existing simplified approaches. In addition, geodesic and loxodromic slip assumptions will be applied to study the slip behavior of the tensile armor layers.
Nonlinear Dynamics in Immersed Tunnel Installation Under Regular Waves
Authors: Hu Zhihuan , Li Xin , Lu Haining , Li Jun , Wu Shaobo , Zhang Fan
Published: October 2016

Immersed tunnel technique has been developed and successfully applied to the installation of immersed tunnel. During the installation, the tunnel is connected to the dual pontoon floating structures (DPFS) by cables and lowered slowly to the tunnel trench. Such installations generally involve multi-body dynamics with wave-induced motions and nonlinear constraints. This paper establishes a time domain model for the immersed tunnel installation based on a linear potential flow approach. The mooring lines are simplified as linear undamped springs with constant stiffness. More attention has been paid to simulate the marine cables, which are modelled by the lumped mass method as well as by bilinear undamped spring. The effects of cable model on the wave-induced dynamics of the given system are discussed. In the numerical simulation, the slack phenomena and snap load of cables are observed and reveal the rich and complex dynamical behaviors of the immersed tunnel installation.
Effect of Relative Density on the Dynamic Impact Behaviors of Closed-Cell Foam
Authors: Wang Shilong , Ding Yuanyuan , Wang Changfeng , Zheng Zhijun , Yu Jilin
Published: October 2016

Dynamic behaviors of closed-cell foam are investigated with cell-based finite element models based on the 3D Voronoi technique. The typical deformation feature of cellular structures under high-velocity impact is layer by layer collapse, like a shock wave propagating in the specimen. The one-dimensional velocity distribution of the structure is calculated to characterize the propagation of shock front and thus the shock wave speed is determined quantitatively. It is found that the shock wave speed has intense dependence on the impact velocity for a specific relative density. The difference between the shock wave speed and the impact velocity is asymptotic to a constant as the impact velocity increases. This constant can be therefore regarded as a dynamic material parameter. The influence of relative density on this dynamic material parameter is investigated. The results show that the shock wave speed at a specific impact velocity increases with the increase of the relative density of cellular structure in a certain extent. An expression of the shock wave speed with respect to the impact velocity and relative density is obtained. The dynamic strain hardening parameter is lower than that in the quasi-static one, which indicates different mechanisms of the deformation under high-velocity and quasi-static loadings.
Reducing Operational Risks by On-Board Phase Resolved Prediction of Wave Induced Ship Motions
Authors: Naaijen P. , Roozen D. K. , Huijsmans R. H. M.
Published: October 2016

For numerous offshore operations, wave induced vessel motions form a limitation for operability: Installation of wind turbines, removal and placements of top sites on/from jackets, landing of helicopters etc. can only be done safely in relatively benign wave conditions. In many cases the actually critical phase takes no more than some tens of seconds. An on-board prediction of vessel motions would enable crew to anticipate on these near future vessel motions and avoid dangerous situations resulting from large ship motions. This paper presents results from a field campaign in which non-coherent raw X-band navigation radar data was used as input for a procedure that inverts the radar data into a phase resolved estimation of the wave elevation. In combination with a wave propagation and vessel response model, this procedure can compute a prediction of phase resolved vessel motions, some tens of seconds up to minutes into the future, depending on radar range and sea state. We compare predictions obtained this way with actual measurements of a well intervention vessel that were obtained during a sea trial performed at the North Sea. It was concluded that the method results in very accurate predictions: correlations between 0.8–0.9 were obtained for predicted ship motions of/around the COG and the vertical motions of the helicopter deck.
Vibration Behavior of Pipelines Conveying Gas-Liquid Two-Phase Flow Supported on the Seabed
Authors: An Chen , Duan Menglan , Su Jian
Published: October 2016

Pipe-soil interaction has been intensively investigated both theoretically and experimentally due to its importance in the design of seabed pipelines. In recently years, the effects of single phase, steady internal flow on the dynamic response of seabed pipelines have received increasing attention. However, the effects of transient two-phase flow on the vibration behavior of seabed pipelines have been seldom studied. In this work, a physical model for analyzing the dynamic behavior of a seabed pipeline conveying gas-liquid two-phase flow is proposed. Slug flow regime is considered as it causes most violent vibrations. An analytical model is adopted for the prediction of important flow characteristics of the gas-liquid slug flow. The dynamic behavior of pipelines is analytically and numerically investigated by using the generalized integral transform technique (GITT), by which the transverse vibration equation is transformed into a coupled system of second order differential equations in the temporal variable. Good convergence behavior of the proposed eigenfunction expansions is demonstrated for calculating the transverse displacement at various points of pipelines conveying gas-liquid slug flow. Parametric studies are performed to analyze the effect of the internal two-phase flow on the dynamic behavior of pipelines.
Prediction of the Damage Extents of Ship’s Double Hull Side Structures Subjected to Lateral Collisions
Authors: Cho Sang-Rai , Kim Kyeong-Ryun , Song Seung-Uck , Park Sang-Hyun , Lee Joo Sung , Lee Jin Tae
Published: October 2016

Lateral collision tests were conducted on two steel double-hull side structure models, models DH-3 and DH-4. The models were approximately one-seventh scale of a VLCC (Very Large Crude Carrier), and the small longitudinal stiffeners of a VLCC were smeared into the shell plates. The collision tests were performed using a car crash testing facility. The striker had a knife-edge type header and its mass was 1,350 kg. The collision velocities were 7.224 m/s and 10.0 m/s for models DH-3 and DH-4, respectively. The collision test results are briefly reported in this paper.

Using a commercial FEA package, numerical collision analyses were then conducted for the two new models together with another two models of one-tenth scale, which were reported elsewhere. In the numerical analyses the strain and strain-rate hardenings were considered. The convergence test was also performed to obtain an optimal mesh size. The numerical predicted damage extents were compared with those of the experiments. The prediction errors were in the range of −5.7% to +6.8%.

An analytical approach was also attempted to predict the damage extents of double-hull structures subjected to lateral collisions. Compared with the experimental and numerical predictions the analytical predictions provide acceptable accuracies for the initial design stage. For a more rational ship structural design against collision, future studies are proposed to improve the prediction accuracy and reliability.
Experimental and Numerical Study on Flow Past Four Rectangular Columns in Diamond Configuration
Authors: Liang Yibo , Tao Longbin , Xiao Longfei , Liu Mingyue
Published: October 2016

Vortex-Induced-Motions (VIM) is a cyclic rigid body motion that is induced by vortex shedding of a large sized floating structure (Spar, Semi-Submersible and Tension-Leg Platform) in deep water. Since the potential impact of VIM on fatigue life of mooring and riser systems can be critical, the study of the VIM phenomenon has drawn considerable attention among offshore engineering community. Despite the effort, there is still lack of understanding the complex fluid-structure interaction phenomenon. To date, it is very much relying on the simplified empirical approach in practical design.

Most of the Semi-Submersibles consist of four rounded-cornered rectangular columns. To investigate the flow interaction between each columns can reveal some physics behind the VIM. In this study, flow past four stationary rounded-corner rectangular columns are tested in the circulating channel and numerically simulated by the Star-CCM+ CFD package. Forces on the columns and the flow characteristics behind the structures are present in this study. A 2-D particle imaging velocimetry (PIV) technic has been adopted in present study to obtain the flow characteristics. The aim of this study is to investigate the interactions between four square columns.
Fatigue Life Analysis Method of Upper-Hinge Joints of FPSO SYMS Based on Real-Time Proto-Type Monitoring Technique
Authors: Wu Wenhua , Lv Baicheng , Li Wenyuan , Wang Yanlin , Yue Qianjin , Zhang Yantao
Published: October 2016

At present, the complexities of distributive characteristics in temporal and spatial domain of ocean environmental loading contribute to the difficulties of the fatigue life estimations of marine structures.

In shallow water, soft yoke mooring system (SYMS) is considered to be the best mooring system, and has been widely used in oil development in the Bohai Bay and the Gulf of Mexico. Soft yoke mooring system establishes the mooring functions via the multi-dynamic mechanism of thirteen hinge joints. The accuracy of fatigue life of the hinge joints is important to ensure the safety of mooring system. The damage failure of hinge joints would cause great financial loss.

In 2012, Dalian University of Technology set up a full coupled proto-type monitoring system which consisted of the four sub-monitoring systems, that is, ocean environmental parameters sub-system including wind, current and wave factor, motions and attitudes of the FPSO including six degree freedom of vessel motions, motions and the mooring force monitoring system of the mooring leg. The massive monitoring information is obtained by the integrated software with continuous. The present paper proposes a real-time fatigue life prediction method of upper hinge joint of SYMS based on the prototype monitoring technique.

The friction parameter of hinge joints contact surface is increased in long-term service and reduced by adding lubricant. In the SYMS design phase, there is no effective analysis of the repeated friction parameter changes. The variations of friction coefficients caused by long-term cycle stress and maintenance are considered in the fatigue calculation. The stress distribution of hinge joints under design parameter is carried out by using ABAQUS. Through calculation and comparison, the equivalent stress and fatigue damage variable of KPA (Key Process Area, large deformation units and easy wear area) units in the condition of the friction coefficient is 0.15 (design parameter) and 0.95. We found that the friction coefficient change due to long-term service will speed up the fatigue failure of the hinge joints.

The relationship between friction coefficients and KPA regional stress of mooring legs swinging angle are established through the finite element simulation. Through prototype monitoring software analysis the marine environment loading, structural response and KPA regional stress information, the abrasion of the hinge node and fatigue damage variable Dθμ can be real-time predicted.

The present fatigue life analysis method based on monitoring technique exhibits good advantages and research value for the fatigue life estimation of offshore structure subject to wave induced motions.
On the Relative Importance of Loads Acting on a Floating Vertical-Axis Wind Turbine System When Evaluating the Global System Response
Authors: Collu Maurizio , Borg Michael , Manuel Lance
Published: October 2016

Interest in offshore floating wind turbines has been growing over the last decade. While a number of studies have been conducted to model the dynamics of offshore floating HAWT systems (e.g. OC3-Phase IV, OC4-Phase II), relatively few studies have been conducted on floating VAWT systems, despite their potential advantages. Due to the substantial differences between HAWT and VAWT systems, analysis procedures employed for a floating HAWT analyses cannot be extended to use for floating VAWT systems. Here, the main aim is to provide a systematic analysis and comparison of the forces acting on a reference offshore floating VAWT, considering a turbulent wind field and stochastically generated waves, to assess the more critical loads and distinguish them from those with negligible effect, when estimating the global system response. The floating VAWT system considered is comprised of a 5MW rotor supported by the OC4-Phase II semi-submersible. Using the coupled model of dynamics for VAWT “FloVAWT”, the global response of the system is estimated for a set of load cases, allowing the assessment of the contributions of individual force components. In particular, the simulations allow us to assess the impact of the VAWT aerodynamic forces, the platform hydrodynamic forces, and the mooring forces. The results help evaluate the relative importance of hydrodynamic with respect to aerodynamic forces, depending on the loading condition. A deeper insight into the aerodynamic forces is provided, which shows the impact of a) the roll/pitch inclination and b) the roll/pitch motion velocities on the rotor aerodynamic response and, eventually, on the global response of the platform.
Numerical Study of Spudcan Penetration in Clay-Sand-Clay Soil
Authors: Yin Qilin , Dong Sheng , Zhai Jinjin
Published: October 2016

Aiming at simulating the preloading process of jack-up rig at drilling locations with typical stratum combination near the Yellow River estuary, FE models of interaction between spudcan and sub-soil are set up based on a large deformation finite element method known as Remeshing and Interpolation Technique with Small Strain (RITSS). The ABAQUS finite element package is used to calculate small strain solutions following each step. The seabed can be simplified as a three-layer deposits with soft clay layer, sand layer and soft clay layer from surface to bottom according to geological data. The backfill phenomenon during preloading is described. The results show that the surface soft clay falls onto the upper surface of spudcan very soon after the preloading process begins, afterwards the backfill tendency occurs in the middle sand layer. SNAME recommends two sets of formulas for calculating penetration resistance respectively for backfill penetration and penetration with no backfill. In this study the model with no backfill is realized by condition control in program codes and the penetration resistance is compared with the results of practical backfill model. The variance proves the shortcomings of the method given by SNAME.
Real-Time Hybrid Model Tests of a Braceless Semi-Submersible Wind Turbine: Part III — Calibration of a Numerical Model
Authors: Berthelsen Petter Andreas , Bachynski Erin E. , Karimirad Madjid , Thys Maxime
Published: October 2016

In this paper, a numerical model of a braceless semi-submersible floating wind turbine (FWT) is calibrated against model test data. Experimental data from a 1:30 scaled model tested at MARINTEK’s Ocean Basin in 2015 using real-time hybrid model testing (ReaTHM) is used for the calibration of the time-domain simulation model. In these tests, aerodynamic loads were simulated in real-time and applied to the physical model. The simulation model is based on the as-built model at full scale. The hull and turbine are considered as rigid, while bar elements are used to model the mooring system in a coupled finite element approach. Frequency-dependent added mass, radiation damping, and excitation forces/moments are evaluated using a panel method based on potential theory. Distributed viscous forces on the hull and mooring lines are added to the numerical model applying Morison’s equation. The viscous drag coefficients in Morison’s equation are calibrated against selected test data, including decay tests in calm water and test with irregular waves. Simulations show that the drag coefficients change when waves are present. Aerodynamic loads are included as time varying loads applied directly at the hub based on the actual physical loads from the experiment. This way, uncertainties related to the aerodynamic loads in the calibrations are removed. The calibrated numerical model shows good agreement with experimental data.
Numerical Study of Seabed Boundary Layer Flow Around Monopile and Gravity-Based Wind Turbine Foundations
Authors: Ong Muk Chen , Trygsland Eirik , Myrhaug Dag
Published: October 2016

Computational fluid dynamics (CFD) has been used to study the seabed boundary layer flow around monopile and gravity-based offshore wind turbine foundations. The gravity-based foundation has a hexagonal bottom slab (bottom part). The objective of the present study is to study the flow structures around the bottom-fixed offshore wind turbine foundations in order to provide essential hydrodynamic coefficients for engineering design and an assessment of potential scour erosion. Three-dimensional CFD simulations have been performed using Spalart-Allmaras Delayed Detached Eddy Simulation (SADDES) at a Reynolds number 4×106 based on the free stream velocity and the diameter of the monopile foundation, D. A seabed boundary layer flow with a boundary layer thickness D is assumed for all the simulations. Vortical structures, time-averaged results of velocity distributions and bed shear stresses are computed. The numerical results are discussed by studying the difference in flows around the monopile and the gravity-based foundations. A distinct horseshoe vortex is found in front (upstream side) of the monopile foundation. Two small horseshoe vortices are found in front of the hexagonal gravity-based foundation, i.e. one is on the top of the bottom slab and one is near the seabed in front of the bottom slab. The horseshoe vortex size for the hexagonal gravity-based foundation (computed as the distance from the separation point to the foundation surface along the centerline on the seabed), is found to be smaller than that for the monopile foundation. The effects of different foundation geometries on destroying the formation of horseshoe vortices (which is the main cause of scour problems) are discussed.
Hardware-in-the-Loop Simulation for a Heave Compensator of an Offshore Support Vessel
Authors: Zhao Luman , Roh Myung-Il , Ham Seung-Ho
Published: October 2016

Tune and verification of control system algorithms for offshore installation operations involving complex and advanced machinery and difficult due to its safety factor. It may be also very costly or even impossible to establish certain test conditions in the physical process environment of the control system. To solve this problem, the Hardware-In-the-Loop-Simulation (HILS) can be regarded as an effective method for testing the control system prior to its final development. The sophisticated HILS is composed of a control system and a HIL simulator which is a simulation model of the offshore plant developed by software. In this study, we focus on the application of HILS for a heave compensator which is used to keep the position or the lowing speed of a lifting object. This study contains three components. Firstly, a physics-based analysis component is used to develop a simulation model of an offshore plant, that is, a HIL simulator. Secondly, the programmable logic controller (PLC) component, that is a control system, is used to regulate the offshore plant model, including a proportional-integral-derivative (PID) feedback controller which aims to control the position or lowering speed of the lifting object. Thirdly, an interface component is developed to communicate the data between the HIL simulator and the control system in real-time. To evaluate the applicability of HILS for a heave compensator, it was applied to an example of an offshore support vessel (OSV) crane. In order to verify the control system for the crane operation in case of heave stabilization of the lifting object, two simulation processes had been established with both a software PLC (software-in-the-loop) and a hardware PLC (hardware-in-the-loop). HILS makes it possible to test the heave compensator without building costly prototypes and without endangering natural environment.
Correlation Between Stress Concentration Degree and Residual Magnetic Field
Authors: Bao Sheng , Mustapha Ashri , Lou Huangjie
Published: October 2016

In this research, the correlation between the stress concentration degree and the residual magnetic field (RMF) of ferromagnetic steels is investigated. Tensile tests were carried out to measure the RMF signals on the surface of steel specimens with different stress concentration degrees. The effectivity of the metal magnetic memory (MMM) technique in defective specimens was investigated in comparison with a non-defective specimen. Analysis of the qualitative relationship between the stress concentration degree and the RMF components was subsequently performed on two types of defective specimens. The qualitative correlation between the planar distributions of the RMF components and the stress concentration degree was further studied. It was validated that both the RMF signal and its planar distribution are effective in characterizing the stress concentration degree. The results obtained in this research will be a complement to the MMM technique.
Motion Analysis of TAD and TLP Platforms Operating Side by Side
Authors: Hannan Mohammed Abdul , Wei Bai , Magee Allan Ross , Choudhary Ankit , Jain Amit , Adaikalaraj Peter Francis Bernad , Hussain Anis
Published: October 2016

A Tender Assisted Drilling (TAD) is typically a support vessel that serves support of a drilling rig. The TAD acts as a platform for supplies and is stationed alongside the drilling rig from which the rig will work off to reduce the loads on the actual rig. While operating, the TAD needs to be structurally coupled with the floating production platform. The coupling makes sure that the Tender does not drift away from the platform and provides stiffness to the relative motions between the TAD and the production platform. Current practice is to couple the TAD and the platform by using nylon hawser ropes. The hawsers provide adequate elasticity to accommodate the low frequency motions and do not allow the TAD to drift away from the production platform. In this study the individual free floating TAD and Tension Leg Platform (TLP) system is model first using the commercial software Hydrostar. After that the technique of developing equivalent stiffness matrix to represent the TLP tendons are applied in order to simulate the influence of the tendons on TLP motions. The response of these individual models are then calculated numerically and validated against existing data. After validation is performed, coupled interactions of this two body is simulated while both the bodies are freely floating side by side. Several possible side by side orientations of the TAD and TLP are tested and the results are compared for this first phase of the study. Later, the freely floating bodies are connected to each other by means of springs. The connection between the two floating bodies is modeled using equivalent stiffness matrix for connections.

The results obtained from this first phase frequency domain analysis will help to clearly understand the coupled behavior of such system under various orientations. In later phases, this model will be further developed in order to perform the time domain analysis and several innovative types of connections other than simple hawsers will be tested in order to couple this TAD and TLP system.
Water Deluge System Design for Fire Fighting
Authors: Lee Jin , Kim Sang Hwan , Seo Jung Kwan , Paik Jeom Kee
Published: October 2016

The ships and offshore structures are exposed to inherently the risk of fire and explosion. These fire and explosion, accident caused by grave consequences not only to the ships and offshore platforms on the sea but the environment all mankind. The aim of this paper is to focus on an optimization of water deluge and mist spray system locations subjected to jet on the ships and offshore platforms. A trustworthy set of fire scenarios is identified and classified using probabilistic sampling methods calling for Latin Hyper Sampling. These events of fire are numerically calculated for selected scenarios by the computational Fluid Dynamic (CFD) code using a KFX. The Water Deluge Location Index (WLI) is then calculated by using the frequency and consequence of fire scenarios. And then, WLI are utilized to prioritize the optimal locations of water deluge and mist spray systems. The recommended methodology believes that can increase to certainties in the design procedure of unreliability and can regard the cost-effectiveness of safety design.
Numerical Analysis for Damage Characteristics Caused by Ice Collision on Side Structure
Authors: Cao Bo , Bae Dong-Myung , Sohn Jung-Min , Prabowo Aditya Rio , Chen Tuo Han , Li Huaiji
Published: October 2016

Ships operating in ice region have to meet more rigorous environment and more accidents occur such as hull-ice collision accidents or low temperature damage. The problem of hull-ice collision is a serious one about those. Compared with the other parts such as stern and stem, the side structure of a ship may be regarded as the weakest part in the collision accidents, due to its relative less safe distance. However, side structure has the biggest share among the whole structure, which means that it has the higher ratio to be collided with ice. It is worthy much to avoids the serious damages and improve the survivability of ship in the collision accident. This research simulated the collision between side structure and ice. It had analyzed the characteristics of damaged structure based on the energy method when the collision happened. And then it also discussed the importance of material properties especially for ice properties and compared the difference in the end. This paper can be taken as a period summary and were benefit to the research in next step. It was trying to find a way that can simulate the collision fast and accurately as well.
Combined Experimental and Numerical Studies of Ocean Basin Inlet Design
Authors: Magee Allan R. , Dao My Ha , Zheng Yingying , Jaiman Rajeev Kumar , Lim Kian Yew
Published: October 2016

Current generation systems consisting of pumps or impellers are conventionally used in deepwater model test facilities for producing realistic full-depth steady current flows. A typical current inlet design involves the diversion of current into a mixing chamber which is then channeled into the basin through an array of flow-conditioning filters (screens) installed along the width of the basin. The screens are used to reduce turbulence and create an overburden pressure within the mixing chamber to enhance inflow uniformity. However, one undesirable consequence of this setup is that it increases energy loss to the system which leads to a higher operating cost.

In order to address this problem, the present work is aimed at establishing an effective and energy-efficient inlet design and screen configuration to improve spatial uniformity of inflow with minimal energy loss to the system. A wind tunnel study is carried out to determine an optimal tapered culvert angle and honeycomb screen configuration to achieve the aforementioned objectives. Uniformly-distributed wind flow of up to 20 m/s is channeled into a Perspex chamber and allowed to escape sideways through plastic honeycomb screens. Pressure drops are measured with an array of transducers at locations upstream and downstream from the honeycomb screen, while streamwise velocity distribution across the width of the chamber is captured with a pitot-static tube mounted on a traversing mechanism. Three cases of flow resistance are investigated, i.e. no screen, thin honeycomb screen (thickness = 12mm), and thick honeycomb screen (thickness = 50mm). For each case, the culvert angle is varied between 7°, 13° and 21°. A CFD model is developed with OpenFoam and compared with the present experimental data. The validated model is expected to be used to investigate more complex inlet configurations and assess the performance of realistic ocean basin geometry.