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Accepted Manuscripts

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research-article  
Erlend Liavaag Grotle, Hans Bihs, Vilmar Æs⊘y and Eilif Pedersen
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040560
In this paper, numerical simulations of non-linear sloshing in rectangular tanks are presented. Model implementations in the open source software REEF3D are tested and results compared with experimental data with three different conditions. The interface location is compared for both linear and non-linear sloshing. The non-linear sloshing is simulated in both 2D and 3D. Video images from the SPHERIC project are compared with simulations for the interface. A condition with lateral wave impacts in sloshing, with a frequency close to the natural frequency of the first mode, can be found in this case. The numerical model is solving the RANS equations with the k??w turbulence model. The level set method is used to capture the interface. Higher order discretization schemes are implemented to handle time-evolution and convective fluxes. A ghost cell method is used to account for solid boundaries and parallel computations. It is found that the limiting factor for the eddy-viscosity has significant influence in the non-linear sloshing cases. As the sloshing becomes more violent, the increased strain at the gas-liquid interface overproduces turbulence energy with unrealistically high damping of the motion. 3D simulations show slightly better comparison than 2D. Due to non-linearities and small damping, the time to reach steady-state may take several cycles. The last case shows promising results for the global motion. As expected, the break up of the liquid surface makes it difficult to resolve each phase. But overall, the numerical model predicts the sloshing motion reasonably well.
TOPICS: Simulation, Computational fluid dynamics, Engineering simulation, Sloshing, Computer simulation, Turbulence, Damping, Waves, Sferics, Eddies (Fluid dynamics), Viscosity, Flux (Metallurgy), Steady state, Computation, Computer software, Cycles, Reynolds-averaged Navier–Stokes equations
research-article  
Heidi Moe F⊘re, Stine Wiborg Dahle and Rune Harald Gaarder
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040562
This paper presents a study of traditional netting materials subjected to disinfecting chemicals during fish farming and treatment of net cages. A series of tests were performed in order to study the effect of various concentrations of disinfecting chemicals on the tensile strength of Raschel knitted Nylon netting materials. Simulated spill of diluted hydrogen peroxide to the jump fence during de-lousing did not affect the strength of the applied new and used knotless nylon netting samples. Hydrogen peroxide reacted with biofouling forming gas bubbles, but this did not result in reduced netting strength. The performed tests did not indicate any effect on netting strength from a simulated single, traditional bath disinfection as performed at service stations applying the disinfectant Aqua Des containing peracetic acid. However, increasing the Aqua Des concentration from 1 to 10 % resulted in a strength reduction of 3-6 %. Simulated spill of concentrated Aqua Des on the jump fence of a net with copper coating residuals resulted in a severe reduction in strength of 45 %. This strength loss was probably a consequence of chemical reaction between copper and Aqua Des, and uncoated netting did not experience any loss in strength subjected to the same chemical exposure. These findings from application of Aqua Des should also apply to other peracetic acid disinfection chemicals with trade names as for example Perfectoxid and Addi Aqua.
TOPICS: Nylon fabrics, Tensile strength, Hydrogen, Copper, Coating processes, Coatings, Bubbles, Biofouling, Chemical reactions
research-article  
Zhengshun Cheng, Zhen Gao and Torgeir Moan
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040561
Designing reliable and cost-effective floating bridges for wide and deep fjords is very challenging. The floating bridge is subjected to various environmental loads, such as wind, wave, and current loads. All these loads and associated load effects should be properly evaluated for ultimate limit state design check. In this study, the wind-, wave-, and current-induced load effects are comprehensively investigated for an end-anchored curved floating bridge, which was an early concept for crossing the Bj⊘rnafjorden. The considered floating bridge is about 4600 m long and consists of a cable-stayed high bridge part and a pontoon-supported low bridge part. It also has a number of eigen-modes, which might be excited by the environmental loads. Modeling of wind loads on the bridge girder is firstly studied, indicating that apart from aerodynamic drag force, aerodynamic lift and moment on the bridge girder should also be considered due to their significant contribution to axial force. Turbulent wind spectrum and spatial coherence play an important role and should also be properly determined. The sway motion, axial force and strong axis bending moment of the bridge girder are mainly induced by wind loads, while the heave motion, weak axis bending moment and torsional moment are mainly induced by wave loads. Turbulent wind can cause significant larger low-frequency eigen-mode resonant responses than the second-order difference frequency wave loads. Current loads mainly contribute damping and reduce the variations of sway motion, axial force and strong axis bending moment.
TOPICS: Computer simulation, Stress, Bridges (Structures), Dynamic analysis, Wind waves, Wind, Girders, Waves, Turbulence, Design, Resonance, Modeling, Damping, Drag (Fluid dynamics), Lift (Fluid dynamics), Cables
research-article  
Mengmeng Zhang, Shixiao Fu, Leijian Song, Jie Wu, Halvor Lie and Hanwen Hu
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040509
Flexible pipe with staggered buoyancy elements such as lazy wave riser, drilling riser etc., has been widely used in ocean engineering. Under the influence of sea current, both of the buoyancy elements and the riser may experience vortex induced vibrations (VIV). However, when VIV occurs, hydrodynamic characteristics of the buoyancy elements and its influence on hydrodynamic force of the bare pipe still need investigation. The purpose of this paper is to reveal the hydrodynamic characteristics of flexible pipe with staggered buoyancy elements undergoing VIV. The cross flow hydrodynamic coefficients of the flexible pipe with 25%, 50% and 100% coverage of staggered buoyancy are obtained from model tests, using hydrodynamic forces and coefficients identification method. Then, the characteristics of added mass coefficients and excitation coefficients in CF direction are analyzed. The results show that the added-mass coefficients of bare pipe are relatively larger than those of buoyancy module, while the total mass per unit length (sum of structural mass and added mass) is consistent along the pipe. Similarly, the range of excitation coefficient on the buoyancy elements is smaller than that on the bare pipe, and their ratio is equal to the reciprocal of diameter ratio 2.5.
TOPICS: Buoyancy, Hydrodynamics, Pipes, Vortex-induced vibration, Pipeline risers, Fluid-dynamic forces, Excitation, Seas, Cross-flow, Waves, Ocean engineering
research-article  
Athul Sasikumar, Arun Kamath, Onno Musch, Hans Bihs and Øivind Asgeir Arntsen
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040508
Harbours are important infrastructures for an offshore production chain. These harbours are protected from the actions of sea by breakwaters to ensure safe loading, unloading of vessels and also to protect the infrastructure. In current literature, research regarding the design of these structures is majorly based on physical model tests. In this study a new tool, a three-dimensional numerical model is introduced. The open-source CFD model REEF3D is used to study the design of berm breakwaters. The model uses the Volume averaged Reynolds Averaged Navier-Stokes (VRANS) equations to solve the porous flows. At first the VRANS approach in REEF3D is validated for flow through porous media. A dam break case is simulated and comparisons are made for the free surface both inside and outside the porous medium. The numerical model REEF3D is applied to show how to extend the database obtained with purely numerical results, simulating different structural alternatives for the berm in a berm breakwater. Different simulations are conducted with varying berm geometry. The influence of the berm geometry on the pore pressure and velocities are studied. The resulting optimal berm geometry is compared to the geometry according to empirical formulations.
TOPICS: Breakwaters, Modeling, Optimization, Geometry, Design, Flow (Dynamics), Porous materials, Computer simulation, Simulation, Dams, Ocean engineering, Chain, Computational fluid dynamics, Engineering simulation, Databases, Vessels, Seas, Pressure
research-article  
Xingchen Tang, Daming Li, Xiao Wang and Yanqing Li
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040507
This paper derived the continuity and momentum equations of solid-liquid two-phase flows using infinitesimal body analysis and obtained well-posed equations of two-dimensional (2D) flow-sediment movement. Based on the theory of solid-liquid two-phase flow, the momentum equations of the bedload sediment were deduced and a closed form of the 2D total sediment model equations was established. Then, the exchange mechanism of suspended sediment and bedload sediment and their computational method were elaborated on in great detail. Combined with the basic theory of one-dimensional (1D) flow-sediment movement, a flow-sediment numerical model of one and two dimensions was established for the region of Yongding New Estuary. A series of model verifications were carried out, which showed that the model can be adopted to simulate the flow-sediment movement in this region. This model was then applied for the environmental assessment of Taida Sea Reclamation Project. The conclusions indicate that the backwater effects of the proposed construction scheme would be fairly small and that the deposition amount in the river would be greatly reduced owing to the source of alongshore sediment transport being blocked by the project. This study provides a scientific model and method for the feasibility study and environmental assessments of construction projects.
TOPICS: Flow (Dynamics), Computer simulation, Dimensions, Sediments, Construction, Two-phase flow, Momentum, Seas, Rivers, Computational methods
research-article  
Hossein Gholami, Behrouz Asgarian and Saeed Asil Gharebaghi
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040505
Corrosion is identified as one of the most important deterioration factors for structural integrity of offshore platforms. For reliability analysis of these platforms, a probabilistic model for prediction of long-term corrosion loss as a function of time is essential. The purpose of this study is to propose a novel model for steel corrosion of jacket platforms in the Persian Gulf region. Field measurements for members in seawater are collected and statistically analyzed to identify the probability function for corrosion loss at different times. A new model with time-dependent parameters is suggested, based on the statistical analysis results. Application of above model in the reliability analysis of jacket platforms is investigated by introducing a new reliability analysis framework. This framework is a general solution for probabilistic analysis of jacket platforms with several stochastic variables, which can be used for the platforms with different configuration and loads. In this framework, direct analysis is performed in each stage of FORM instead of using the response surface method, which is a common approach to obtaining the required response. This framework is applied to three jackets and the annual probability of failure over the platforms service life is computed. Comparison of results revealed that among the years beyond the platform design life, the amount of annual P_f is increased in parabolic function. In addition, studying the results is illustrated that in the case of ignoring the corrosion loss as a stochastic variable, P_f is estimated 7% lower than values obtained in actual condition.
TOPICS: Event history analysis, Corrosion, Probability, Response surface methodology, Seawater, Statistical analysis, Offshore platforms, Steel corrosion, Service life (Equipment), Stress, Design, Failure
research-article  
Severin Sadjina, Lars Tandle Kyllingstad, Martin Rindar⊘y, Stian Skjong, Vilmar Æs⊘y and Eilif Pedersen
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040473
Here, we present the concept of an open virtual prototyping framework for maritime systems and operations that enables its users to develop re-usable component or subsystem models, and combine them in full-system simulations for prototyping, verification, training, and performance studies. This framework consists of a set of guidelines for model coupling, high-level and low-level coupling interfaces to guarantee interoperability, a full-system simulation software, and example models and demonstrators. We discuss the requirements for such a framework, address the challenges and the possibilities in fulfilling them, and aim to give a list of best practices for modular and efficient virtual prototyping and full-system simulation. The context of our work is within maritime systems and operations, but the issues and solutions we present here are general enough to be of interest to a much broader audience, both industrial and scientific.
TOPICS: Simulation, Computer software
research-article  
Erin E. Bachynski, Csaba Pakozdi, Anders Östman and Carl Trygve Stansberg
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040442
Recently, a method for numerical reproduction of measured irregular wave events has been developed. The measured motion of the wave maker flaps defines the wave kinematics at the boundary of the numerical simulation in order to generate the waves. When such data are not available, the control signal of the wave maker can, instead, be generated from a given free surface elevation following the same procedure as in model tests. This procedure is applied to a model test case with extreme irregular wave events and resulting nonlinear global wave loads on a vertical cylinder, focusing on higher-order ringing excitation. The purpose of the investigation is two-fold: 1) to validate the wave reconstruction procedure, and 2) to validate the resulting CFD ringing loads with the given waves. In order to better understand the frequency content in the CFD-generated loads, wavelet analysis as well as the response of a single degree-of-freedom (SDOF) oscillator are examined and compared with the corresponding results for the 3rd order wave forcing based on the MacCamy-Fuchs (MF) and Faltinsen, Newman, Vinje (FNV) formulations. The results show generally good agreement between CFD and experiment both in the waves and in the loads; discrepancies found in the loads mainly originate from corresponding uncertainties in the wave reconstruction. Wave breaking may be one source of uncertainty. The MF+FNV formulation showed reasonable prediction of the maximum responses of an SDOF oscillator, but could not capture the loads well at all of the important frequencies.
TOPICS: Stress, Waves, Computational fluid dynamics, Uncertainty, Excitation, Cylinders, Signals, Wavelets, Degrees of freedom, Kinematics, Computer simulation
research-article  
Mete Mutlu, Yingjie Tang, Matthew A. Franchek, Rob Turlak and Jose Gutierrez
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040391
Presented is the performance analysis of annular blowout preventer reciprocating elastomer hydraulic seals operating in subsea environments. The method is based on a systems level model that combines the effects of friction, material mechanical properties of the seal, installation compression, subsea hydrostatic pressure, and control system dynamics into one model. The model is calibrated using data from tests conducted on the surface and then validated on subsea operational data. Through model simulations, it will be shown that insufficient installation squeeze of the seal in combination with low elasticity seal material result in cases where the seal does not leak at the surface but show substantial internal leakage in subsea conditions. Leakage is also observed under dynamic operation when the walls of the seal groove do not energize the seal. The proposed model based analysis method in conjunction with surface level testing offer a new paradigm in evaluating reciprocating seal subsea performance a priori of subsea operation thereby avoiding costly downtimes and subsea failures.
TOPICS: Dynamics (Mechanics), Elasticity, Control systems, Elastomers, Friction materials, Simulation, Sealants, Hydrostatic pressure, Ocean engineering, Mechanical properties, Engineering simulation, Testing, Compression, Failure, Leakage
research-article  
Lianhai Zhang, Wei Ma and Chengsong Yang
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040375
Freeze-thaw action changes soil microstructure and thus has a great influence on physical and mechanical properties of soils, which is closely correlated to pore water pressure (PWP). Herein, the PWPs of sandy soil and silty clay were measured in laboratory during freeze-thaw cycles (FTC). Experimental results showed that PWP was influenced by temperature, freeze-thaw history (i.e. number of freeze-thaw cycle), soil type and others. The PWP experienced a periodical change as temperatures periodically changes during the FTC testing, the PWP decreased during freezing and increased during thawing. Soil type has a slight influence on the variation of PWP, both in character and extent. A theoretical analysis of PWP in frozen soil was given to explain the PWP changes. In addition, the PWP depression during freezing was a major driving force for water migration. The PWP variations are highly relevant to the changes in soil microstructure such as soil particle (grain size composition and mineral composition), pore structure and particle arrangement, which will be the focus of further research.
TOPICS: Soil, Particulate matter, Temperature, Freezing, Cycles, Grain size, Water pressure, Thawing, Mechanical properties, Testing, Theoretical analysis, Water, Minerals
research-article  
Ilias Zilakos and Michael Toulios
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040303
Inflatable devices that provide reserve buoyancy to damaged ships, preventing capsizing and/or sinking, along with lifting wreckages from the seabed, were studied within the framework of the European funded project "SuSy" (Surfacing System for Ship Recovery). Part of the work involved material evaluation and testing as well as simulations of the structural response. This paper first describes an orthotropic hyperelastic constitutive model for a candidate material also used in the fabrication of prototype inflatable devices. A strain energy density function is proposed that is further used to derive the stress and elasticity tensors required for the numerical implementation of the model in the user-defined subroutine (UMAT) of ABAQUS/Standard. The second part of the paper presents the FE simulation of the latter stages of inflation of two salvage devices inside an actual double bottom structure. The numerical results are in good agreement with tests conducted in dry land and under water, with the structure raised following the inflation of the devices. The evolving stress state in both the devices and the double bottom structure under increased contact interaction leads to useful conclusions for future use in the development of this salvage system.
TOPICS: Simulation, Modeling, Ships, Salvage (Wastes), Stress, Engineering prototypes, Tensors, Constitutive equations, Density, Buoyancy, Elasticity, Manufacturing, Water, Seabed, Testing
research-article  
Jiafeng Xu, Zhengru Ren, Yue Li, Roger Skjetne and Karl Halse
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040235
Ship roll motion is critical for offshore operations due to its lack of damping mechanism. This paper demonstrates a dynamic simulation scheme of an active roll reduction system using free-flooding tanks controlled by vacuum pumps. A tank is installed on each side of a catamaran. Both the tank hatches are opened to the sea and the air chambers of both tanks are connected by an air duct. Vacuum pumps and air valve stabilized the wave-induced roll motion by controlling the water levels in the tanks through a feedback controller. The catamaran is a dynamic model with single degree of freedom in roll, and its hydrodynamic behavior is calculated using potential theory by SHIPX. The air chambers are modelled as isothermal processes of ideal gas. The behavior of the liquid flow in a tank is simulated by incompressible Reynolds-averaged Navier-Stokes solver with the volume of fluid model, then summarized as a response function for the vessel model. A simplified control plant model for the vacuum pumps is proposed where higher order behaviors are neglected and the external wave-induced load is unknown. A parameter-dependent observer and a backstepping controller are adopted to estimate the external load and reduce the roll motion. The system stability is proved by Lyapunov's direct method. The performance of the entire system is evaluated in terms of roll reduction capability and power cost. The system is more suitable for roll reduction in low-speed or resting conditions.
TOPICS: Simulation, Floods, Vacuum pumps, Stress, Waves, Control equipment, Potential theory (Physics), Ocean engineering, Thermodynamic processes, Degrees of freedom, Damping, Valves, Ducts, Feedback, Vessels, Water, Seas, Dynamic models, Ships, Stability, Flow (Dynamics), Fluids
research-article  
Kan Ye and Jinchen Ji
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040240
Different from the fixed-based wind turbines, the floating type wind turbines are regarded as under a free-free end operating condition. The tower structure of a floating offshore wind turbine is an integrated part connecting the nacelle and support platform. An analytic solution is presented in this technical brief for the free-vibration of the tower structure of a spar-type offshore wind turbine. The tower structure is modelled as a free-free beam based on Euler-Bernoulli beam-column theory. The platform and the nacelle are considered as two large mass components connected by torsion springs at two tower ends with different stiffness. The effects of system parameters on the natural frequencies are investigated under a range of variables, including the tower structure parameters, platform and nacelle parameters, and the connection types. Non-linear relationships between those variables and the natural frequency of the tower structure are numerically found and some design issues are discussed for the spar-type floating wind turbines.
TOPICS: Offshore wind turbines, Spar platforms, Wing spars, Wind turbines, Floating wind turbines, Torsion, Design, Free vibrations, Springs, Stiffness
research-article  
Hongchao Wang, Scott Draper, Wenhua Zhao, Hugh Wolgamot and Liang Cheng
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040242
This paper expounds the process of successfully establishing a Computational Fluid Dynamics (CFD) model to accurately reproduce experimental results of three-dimensional (3D) gap resonance between two fixed ship-shaped boxes. The ship-shaped boxes with round bilges were arranged in a side-by-side configuration to represent an FLNG offloading scenario and were subjected to NewWave-type transient wave groups. We employ the open-source CFD package OpenFOAM to develop the numerical model. 3D gap resonance differs from its 2D counterpart in allowing spatial structure along the gap and hence multiple modes can easily be excited in the gap by waves of moderate spectral bandwidth. In terms of numerical setup and computational cost, a 3D simulation is much more challenging than a 2D simulation and requires careful selection of relevant parameters. In this respect, the mesh topology and size, domain size and boundary conditions are systematically optimized. It is shown that to accurately reproduce the experimental results in this case the cell size must be adequate to resolve both the undisturbed incident waves and near-wall boundary layer. By using a linear iterative method, the NewWave-type transient wave group used in the experiment is accurately recreated in the numerical wave tank (NWT). Numerical results including time series of gap responses, resonant amplitudes and frequencies, and mode shapes show excellent agreement with experimental data.
TOPICS: Resonance, Computational fluid dynamics, Surface waves (Fluid), Waves, Transients (Dynamics), Simulation, Ships, Boundary-value problems, Iterative methods, Boundary layers, Computer simulation, Time series, Topology, Mode shapes
research-article  
Hayden Marcollo, Andrew E. Potts, Daniel Johnstone, Peter Pezet and Phillip Kurts
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4038933
Drilling risers are regularly deployed in deep water (over 1500 m) with large sections covered in buoyancy modules. The smooth cylindrical shape of these modules can result in significant vortex-induced vibration (VIV) response, causing an overall amplification of drag experienced by the riser. Operations can be suspended due to the total drag adversely affecting top and bottom angles. Although suppression technologies exist to reduce VIV (such as helical strakes or fairings), and therefore reduce VIV-induced amplification of drag, only fairings are able to be installed onto buoyancy modules for practical reasons, and fairings themselves have significant penalties related to installation, removal, and reliability. An innovative solution has been developed to address this gap; LGS (Longitudinally Grooved Suppression). Two model testing campaigns were undertaken; small scale (sub-critical Reynolds Number flow), and large scale (post-critical Reynolds Number flow) to test and confirm the performance benefits of LGS. The testing campaigns found substantial benefits measured in hydrodynamic performance that will be realized when LGS modules are deployed by operators for deepwater drilling operations.
TOPICS: Buoyancy, Drag reduction, Pipeline risers, Vortex-induced vibration, Drag (Fluid dynamics), Reynolds number, Flow (Dynamics), Testing, Shapes, Reliability, Underwater drilling, Water

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