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research-article  
Yichen Jiang and Ronald W. Yeung
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4036326
The prediction of roll motion of a ship with bilge keels is particularly difficult because of the nonlinear characteristics of the viscous roll damping. Flow separation and vortex shedding caused by bilge keels significantly affect the roll damping and hence the magnitude of the roll response. To predict the ship motion, the Slender-Ship Free-Surface Random Vortex Method (SSFSRVM) was employed. It is a fast discrete-vortex free-surface viscous- flow solver developed to run on a standard desktop computer. It features a quasi-three dimensional formulation that allows the decomposition of the three-dimensional ship-hull problem into a series of two-dimensional computational planes, in which the two-dimensional free-surface Navier-Stokes solver FSRVM can be applied. In this paper, the effectiveness of SSFSRVM modeling is examined by comparing the time histories of free roll- decay motion resulting from simulations and from experimental measurements. Further, the detailed two-dimensional vorticity distribution near a bilge keel obtained from the numerical model will also be compared with the existing experimental DPIV images. Next, we will report, based on the time-domain simulation of the coupled hull and fluid motion, how the roll decay coefficients and the flow field are altered by the span of the bilge keels. Plots of vorticity contour and vorticity iso-surface along the three-dimensional hull will be presented to reveal the motion of fluid particles and vortex filaments near the keels.
TOPICS: Hull, Keel, Ships, Vorticity, Vortices, Damping, Flow (Dynamics), Fluids, Simulation, Vortex shedding, Particulate matter, Computer simulation, Modeling, Computers, Flow separation
research-article  
Muk Chen Ong, Eirik Trygsland and Dag Myrhaug
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4036208
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 investigate the formation of horseshoe vortex and flow structures around two different bottom-fixed offshore wind turbine foundations in order to provide an assessment of potential scour for engineering design. 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 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 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.
TOPICS: Gravity (Force), Flow (Dynamics), Boundary layers, Wind turbines, Seabed, Vortices, Computational fluid dynamics, Simulation, Slabs, Offshore wind turbines, Shear stress, Engineering design, Eddies (Fluid dynamics), Reynolds number
research-article  
Hans Bihs, Mayilvahanan Alagan Chella, Arun Kamath and Øivind Asgeir Arntsen
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4036206
For the stability of offshore structures, such as offshore wind foundations, extreme wave conditions need to be taken into account. Waves from extreme events are critical from the design perspective. In a numerical wave tank, extreme waves can be modeled using focused waves. Here, linear waves are generated from a wave spectrum. The wave crests of the generated waves coincide at a preselected location and time. Focused wave generation is implemented in the numerical wave tank module of REEF3D, which has been extensively and successfully tested for various wave hydrodynamics and wave-structure interaction problems in particular and for free surface flows in general. The open-source CFD code REEF3D solves the three-dimensional Navier-Stokes equations on a staggered Cartesian grid. Solid boundaries are taken into account with the ghost cell immersed boundary method. Higher order numerical schemes are used for time and spatial discretization. The free surface flow is solved as two-phase fluid system. For the interface capturing, the level set method is selected. The level set function can be discretized with high-order schemes. In order to test the generated waves, the time series of the free surface elevation are compared with experimental benchmark cases. The numerically simulated free surface elevation shows good agreement with experimental data. In further computations, the impact of the focused waves on a vertical circular cylinder is investigated. Free surface flow features during the interaction of focused waves with a cylinder are also investigated along with the numerically captured free surface.
TOPICS: Waves, Cylinders, Flow (Dynamics), Hydrodynamics, Fluids, Offshore structures, Ocean engineering, Navier-Stokes equations, Computational fluid dynamics, Design, Circular cylinders, Computation, Time series, Wind, Stability
research-article  
A.W. Momber, M. Irmer and N. Glück
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4036207
The application of thin-film coatings is a method to protect armatures, accessories and control elements on offshore facilities against corrosion and mechanical damages. The performance of a dual-layer thin-film (30 µm) coating system under simulated Arctic offshore exposure was investigated. The coating system consisted of polyamide-based primer and MoS2/PTFE-modified topcoat. The investigations involved the following tests: accelerated corrosion protection/ageing tests, coating adhesion tests, SEM/EDX inspections, static contact angle measurements, specific surface energy measurements, hoarfrost accretion, and abrasion resistance tests. The test conditions were adapted to Arctic offshore conditions. Effects of accelerated offshore ageing on surface morphology, surface chemistry and hoarfrost accretion were also investigated.
TOPICS: Thin films, Coatings, Ocean engineering, Low temperature, Testing, Arctic region, Corrosion, Surface science, Abrasion, Inspection, Surface energy, Adhesion, Damage, Corrosion protection
research-article  
Svein Savik and Mats J. Thorsen
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4036205
The present paper addresses aspects related to local buckling and instability of tensile armours in flexible pipes. Analytical models for evaluating the tensile armour buckling capacity in both transverse (radial and lateral) directions are presented based on formulating the linearised differential equation describing transverse stability of the thin curved wire assuming no friction. Then analytical models for the ultimate capacity of the outer sheath and anti-buckling tape are formulated and a combined criteria for radial instability is proposed based on considering both the radial buckling capacity of the tensile armour and the ultimate capacity of the outer sheath and tape. Thereafter, a study is performed comparing the proposed models with test data and alternative models available in the literature.
TOPICS: Pipes, Buckling, Armor, Stability, Friction, Wire, Differential equations
research-article  
Chenfeng Li, Zhiyao Zhu, Huilong Ren and Carlos Guedes Soares
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4036111
The aim of this study was to analyze the ultimate strength of stiffened aluminum panels by the non-linear finite element method. A new type of stiffened aluminum alloy panel has been designed which has fixed longitudinal and alternating floating transverse frames. Based on material tensile tests, the material properties of the aluminum alloy were obtained. Then, the simulation method of welding residual stresses and the effect of heat-affected zone are investigated. The FEA software ABAQUS V6.11 is used to estimate the ultimate strength of these stiffened panels under axial compression. The results show that: 1) The mechanical imperfections have significant effect on ultimate strength of stiffened panels; 2) residual stresses may have positive effect on ultimate strength; 3) the new stiffened panels also have good performance on ultimate bearing capacities.
TOPICS: Aluminum, Finite element methods, Tensile strength, Aluminum alloys, Residual stresses, Simulation, Welding, Materials properties, Bearings, Finite element analysis, Compression, Computer software, Heat
research-article  
Ali Fatemi and Shawn Kenny
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4035995
The local buckling response and post-buckling mechanical performance of high strength linepipe subject to combined loading state was evaluated using the finite-element simulator Abaqus/Standard v6.12. The constitutive model parameters were established through laboratory tests and the numerical modeling procedures were verified with large-scale experiments investigating the local buckling response of high strength linepipe. The numerical predictions demonstrated a high level of consistency and correspondence with the measured experimental behaviour with respect to the peak moment, strain capacity, deformation mechanism and local buckling response well into the post-yield range. A parametric study on the local buckling response of high strength plain and girth weld pipelines was conducted. The loading conditions included internal pressure and end rotation. The pipe mechanical response parameters examined included moment-curvature, ovalization, local strain and modal response. The magnitude and distribution of the characteristic geometric imperfections and the end constraint, associated with the boundary conditions and pipe length, had a significant influence on the predicted local buckling response. The importance of material parameters on the local buckling response; including the yield strength (YS), yield strength to tensile strength ratio (Y/T), and anisotropy, was also established through the numerical parameter study. For girth weld linepipe, the study demonstrated the importance of the local high/low misalignment, associated with the circumferential girth weld, on the local buckling response.
TOPICS: Buckling, Pipes, Yield strength, Boundary-value problems, Tensile strength, Pressure, Rotation, Deformation, Computer simulation, Anisotropy, Equipment performance, Constitutive equations, Finite element analysis, Pipelines
research-article  
Jonas W. Ringsberg, Viktor Daun and Fredrik Olsson
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4035996
A method is presented that enables the analysis of weather window assessments for the installation and retrieval phases of a self-elevating unit (SEU). The method takes site-specific parameters, defined as soil type and water depth, into account in addition to vessel-specific and environmental parameters. The inclusion of site-specific parameters is the novel contribution compared to assessment methodologies used today. A simulation model is presented that incorporates a coupled non-linear time-domain analysis of vessel motion and soil-structure interaction. Soil deformation behaviour during impact is described by resistance curves based on a bearing capacity theory. A structural evaluation criterion against which impact forces are compared is used for weather window assessments. The simulation model is applied on a case study utilizing different soil types to study impact forces and the capacity of the structure for withstanding such impacts and eventually performing a weather window assessment. The results show that the jacking operation can be divided into two phases when it comes to loads on the spudcan: a phase dominated by vertical forces followed by a phase dominated by horizontal forces. It is found that including soil deformation behaviour is of paramount importance to the magnitude of the resulting impact forces and that class-recommended practice does indeed produce rather large force estimates. Thus, assessments where site-specific parameters are incorporated could definitely increase the operable weather window for SEUs, and, consequently, increase the economic competitiveness of, for example, the offshore wind industry.
TOPICS: Stress, Jack-up drilling rigs, Soil, Vessels, Deformation, Simulation models, Ocean engineering, Load bearing capacity, Water, Wind, Time-domain analysis
research-article  
Shaoshi Dai, Bassam A. Younis and Hongyang Zhang
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4035957
Predictions are reported of the two-dimensional turbulent flow around a square cylinder with rounded corners at high Reynolds numbers. The effects of rounded corners have proved difficult to predict with conventional turbulence closures and hence the adoption in this study of a two-equation closure that has been specifically adapted to account for the interactions between the organized mean-flow motions due to vortex shedding and the random motions due to turbulence. The computations were performed using OpenFOAM and were validated against data from flows pastcylinders with sharp corners.For the case of rounded corners, only the modified turbulence closure succeeded in capturing the consequences of the delayed flow separation manifested mainly in the reduction of the magnitude of the lift and drag forces relative to the sharp-edged case. These and other results presented here argue in favor of the use of the computationally more efficient unsteady RANS approach to this important class of flows provided that the effects of vortex shedding are properly accounted for in the turbulence closure.
TOPICS: Turbulence, Corners (Structural elements), Cylinders, Flow (Dynamics), Vortex shedding, Flow separation, Reynolds-averaged Navier–Stokes equations, Computation, Drag (Fluid dynamics), Reynolds number
research-article  
Leijian Song, Shixiao Fu, Tie Ren and Ziqi Lu
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4035768
This paper investigates the phase angles of the vibrations and hydrodynamic forces by the model testing of a flexible riser's vortex-induced vibration (VIV) under uniform flow. The VIV displacement of the riser is derived from the measured strains in the cross-flow (CF) and inline (IL) directions. Then, the hydrodynamic forces are obtained by the dynamic equation of an Euler-Bernoulli beam based on the results of VIV displacement. The characteristics of the phase angle of displacement and the hydrodynamic forces are analyzed. The results show that the phase angles of displacement and the hydrodynamic forces are almost identical at different cross-sections of the riser under uniform flow. Moreover, within two adjacent vibration nodes in IL direction, the phase angle almost kept constant, while had a 180 degree change at the two sides of each vibration node. When the reduced velocity varies from 5.25 to 7.5, the phase angles of displacement derived from the flexible riser's VIV are 45 degrees larger than those from the rigid cylinder's self-excited vibration.
TOPICS: Fluid-dynamic forces, Vibration, Vortex-induced vibration, Flexible risers, Displacement, Pipeline risers, Flow (Dynamics), Risers (Casting), Cross section (Physics), Testing, Cross-flow
research-article  
R.M. Chandima Ratnayake and Daniel Dyakov
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4035769
Although novel techniques and high performance material open the way for a generation of high-performance welded joints, the welding operations are inherently quite complex and expensive. This has been the case, especially for super duplex stainless steel (SDSS) welding. If inappropriate parameter combinations are selected, then the welding process degrades the strength and corrosion resistance because of unbalanced ferrite/austenite content in a SDSS welded joint. Therefore, it is vital to determine the optimal combination of parameters such as welding process, rate of shielding gas, heat input, and weld geometry. In addition, the optimal combination of parameter levels plays an important role in maintaining the microstructural and mechanical properties in the weld's metal region. This manuscript illustrates an expert knowledge based methodology for designing the optimal parameter combination, using an engineering robust design approach and related (ERDA) experimentation results. The experiments were performed to investigate the effect of welding factors (i.e. gap geometry, different welding techniques, material transfer and welding processes) on the material properties in the weld and heat affected zone (HAZ). The optimal parameter combination, results of the verification experiment and the metallographic examination results of selected regions of the butt welded joints are presented.
TOPICS: Arc welding, Welded joints, Design, Welding, Geometry, Heat, Metals, Ferrites (Magnetic materials), Corrosion resistance, Materials properties, Mechanical properties, Stainless steel
research-article  
Ivan N. Porciuncula, Claudio A. Rodríguez and Paulo T. T. Esperança
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4035770
Along its lifetime, an offshore unit is subjected to several equipment interventions. These modifications may include large conversions in loco that usually are not adequately documented. Hence, the accurate determination of the platform's KG is not possible. For vessels with low GM, such as semisubmersibles, Classification Societies penalize the platform's KG, inhibiting the installation of new equipment until an accurate measurement of KG is provided, i.e., until an updated inclining test is performed. For an operating semisubmersible, the execution of this type of test is not an alternative because it implies in removing the vessel from its in-service location to sheltered waters. Relatively recently, some methods have been proposed for the estimation of KG for in-service vessels. However, as all of the methods depend on accurate measurements of inclination angles and, eventually, on numerical tools for the simulation of vessel dynamics onboard, they are not straightforward for practical implementation. The objective of the paper is to present a practical methodology for the experimental determination of KG, without the need of accurate measurements of inclinations and/or complex numerical simulations, but based on actual operations that can be performed onboard. Indeed, the proposed methodology relies on the search, identification and execution of a neutral equilibrium condition where, for instance, KG = KM. The method is exemplified using actual data of a typical semisubmersible. The paper also numerically explores and discusses the stability of the platform under various conditions with unstable initial GM, as well as the effect of mooring and risers.
TOPICS: Semi-submersible offshore structures, Center of mass, Vessels, Water, Equilibrium (Physics), Risers (Casting), Ocean engineering, Mooring, Pipeline risers, Dynamics (Mechanics), Stability, Computer simulation, Simulation
research-article  
Fwu Chyi Teo, Leong Hien Poh and Sze Dai Pang
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4035771
This paper investigates the breaking load of ice sheets up to 6m thick, against a sloping structure. The reference model by Croasdale, which the design code is based on, neglects the edge moment arising from the loading eccentricity, as well as a second order bending effect induced by the axial loading in its formulation. In this paper, the model is reformulated to incorporate these effects into the governing equation, as well as to account for the occurrence of local crushing at the point of contact between the ice sheet and sloping structure. For thin ice, predictions from the modified model resemble closely those by Croasdale's model. As the ice thickness increases, however, significant deviations from the reference model can be observed. For thick ice, the terms omitted for brevity in the reference model have a significant influence, without which, the breaking load is under-estimated. It is furthermore demonstrated that against sloping structures, the dominant failure mode is that of flexural, except in very limiting cases where it switches to crushing.
TOPICS: Ice, Failure, Stress, Design, Failure mechanisms, Switches
research-article  
Muk Chen Ong, Erin E. Bachynski and Ole David Økland
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4035772
This paper presents numerical studies of the dynamic responses of two jacket-type offshore wind turbines using both decoupled and coupled models. The investigated structures are the OC4 (Offshore Code Comparison Collaboration Continuation) jacket foundation and a full-lattice support structure presented by Long et al. in 2012. Both structures support the NREL 5-MW wind turbine. Different operational wind and wave loadings at an offshore site with relatively high soil stiffness are investigated. In the decoupled (hydroelastic) model, the thrust and torque from an isolated rotor model were used as wind loads on the decoupled model together with a linear aerodynamic damper. The coupled model is a hydro-servo-aero-elastic representation of the system. The objective of this study is to evaluate the applicability of the computationally efficient linear decoupled model by comparing with the results obtained from the nonlinear coupled model. Good agreement was obtained in the eigen-frequency analysis, decay tests, and wave-only simulations. It was also found that, by applying the thrust force from an isolated rotor model in combination with linear damping, reasonable agreement could be obtained between the decoupled and coupled models in combined wind and wave simulations.
TOPICS: Dynamic response, Offshore wind turbines, Wind, Waves, Ocean engineering, Thrust, Simulation, Engineering simulation, Rotors, Wind turbines, Collaboration, Soil, Stiffness, Stress, Torque, Servomechanisms, Dampers, Damping
research-article  
Ali Nematbakhsh, Zhen Gao and Torgeir Moan
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4035475
A Computational Fluid Dynamics (CFD) based Numerical Wave Tank (NWT) is developed and verified to study wave load effects on fixed and floating offshore structures. The model is based on solving Navier-Stokes equations on a structured grid, level set method for tracking the free surface, and an immersed boundary method for studying wave-structure interaction. This paper deals with establishing and verifying a CFD based NWT. Various concerns that arise during this establishment are discussed, namely effects of wave reflection which might affect the structure response, damping of waves in downstream, and three dimensional effects of the waves. A method is described and verified to predict the time when incoming waves from wave generator are affected by reflecting waves from the structure which can help in better designing the dimensions of NWT. The model is then used to study sway, heave and roll responses of a floating barge which is non-uniform in density and limited in sway by a spring and damper. Also, it is used to study wave loads on a fixed, large diameter, surface piercing cylinder. The numerical results are compared with the experimental and other numerical results, and in general good agreement is observed in all range of studied wave frequencies. It is shown that for the studied cylinder, the Morison equation leads to promising results for wavelength to diameter ratio larger than 2pi (kD < 1), while for shorter wavelengths results in considerable over prediction of wave loads, due to simplification of wave diffraction effects.
TOPICS: Stress, Offshore structures, Waves, Computational fluid dynamics, Wavelength, Dimensions, Reflection, Density, Diffraction, Wave frequency, Navier-Stokes equations, Dampers, Damping, Design, Cylinders, Generators, Springs, Drilling barges, Morison equation, Surface-piercing cylinders, Structural response analysis
research-article  
Jianwei Zhang, Wanqing Wu and Junquan Hu
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4035476
A combination of the moisture content, dynamic energy produced by the waves and the vessel engines along with the weight of the bulk cargo itself may lead the mixture to liquefy. When the liquefaction of the granular bulk cargo occurs, it may behave like a fluid and can cause the vessel to list or even capsize. In this study, based on a CFD solver, a numerical model was developed to simulate the sloshing problem for nickel ore slurries with three different moisture contents. The volume of fluid (VOF) method is adopted to capture the movement of the fluid interface. To validate the present model, the simulation results were compared with experimental data. The numerical results are in good agreement with the experimental results. Finally, the present model was used to investigate the dynamic behavior of nickel ore slurries with different moisture contents combined with non-Newtonian Herschel-Bulkley and Bingham constitutive equations. After taking the grid and time step independence study, the dynamic moment impacted on the cargo hold model boundaries was calculated. The effects of different moisture content, the excitation amplitude, the frequency on the sloshing-induced moment and the free surface deformation were discussed extensively. The results confirm that the proposed model can be used to predict the movement of the nickel ore slurry and analyze its impact moment on the cargo hold model when it takes a roll motion.
TOPICS: Nickel ores, Slurries, Sloshing, Fluids, Vessels, Excitation, Simulation results, Weight (Mass), Liquefaction, Computer simulation, Engines, Surface deformation, Waves, Computational fluid dynamics, Constitutive equations

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