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research-article  
Shangmao Ai, Liping Sun, Longbin Tao and Solomon Yim
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040049
Dynamic position (DP) control and the pipeline dynamics are two main parts of the deepwater S-lay simulation model. In this study, a fully coupled analysis tool for deepwater S-lay deployment by dynamically positioned vessels is developed to integrate the major aspects related to the numerical simulation including coupled pipeline motion and roller contact forces. The roller-pipe interaction is incorporated in the S-lay pipeline model using a contact search method based on a lumped-mass (LM) formulation in global coordinates. Proportional-integration-differentiation (PID) controller and Kalman filter are applied in the vessel motion equation to calculate the thrust allocation of the DP system in time domain. Numerical simulation results showed that the dynamic effect have the significant contribution to the tension, but little influence on the maximum pipe stress and strain. The dynamic response of the coupled S-lay and DP pipeline deployment system increases the demand on the tensioner load carrying capability as well as the maximum DP thruster power.
TOPICS: Simulation, Dynamic positioning systems, Modeling, Pipelines, Pipes, Rollers, Vessels, Stress, Computer simulation, Thrust, Dynamics (Mechanics), Control equipment, Equations of motion, Simulation models, Tension, Dynamic response, Kalman filters
research-article  
Jassiel Vladimir Hernández-Fontes, Marcelo de Araujo Vitola, Monica Campos Silva, Paulo de Tarso Themistocles Esperança and Sergio H. Sphaier
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040050
Green water occurs when an incoming wave exceeds the freeboard and propagates onto the deck of naval/offshore structures, such as FPSOs and platforms. This water can affect the integrity of facilities and equipment that are installed on the deck, compromise the safety of the crew and affect the dynamic stability of the structure. Traditionally, wave trains have been used to study the green water problem, which is a good approach to analyzing consecutive green water events. However, to carry out systematic studies that allow local details to be identified for different types of green water, an alternative method is to study isolated events generated by a single incoming wave. The purpose of this paper is to experimentally investigate the generation of different types of isolated green water events using the wet dam-break approach as an alternative to generating the incoming wave. Tests were carried out in a rectangular tank with a fixed internal structure. Different freeboard conditions were tested for two aspect ratios of the wet dam-break (h0/h1=0.40 and 0.6). Conventional wave probes were used to measure the water levels in the tank, and a high-speed camera was set to capture details of the generated green water events. The results demonstrated the ability of this approach to represent different types of green water, similar to those obtained with unbroken regular waves in barge-shaped fixed structures, including dam-break, plunging-dam-break and hammer-fist.
TOPICS: Dams, Water, Waves, Wave packets, Dynamic stability, Probes, Safety, Hammers, Offshore structures
research-article  
Yan Li, Liqin Liu, Qiang Zhu, Ying Guo, Zhiqiang Hu and Yougang Tang
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4040048
The nonlinear coupling effect between degree-of-freedom (DOFs) and the influence of vortex induced loads on the motion of SPAR-type FOWT are studied based on an aero-hydro-vortex-mooring coupled model. Both first- and second-order wave loads are calculated based on the three-dimensional (3D) potential theory. The aerodynamic loads on the rotor are acquired with the blade element momentum theory. The vortex induced loads are simulated with computational fluid dynamics (CFD) approach. The mooring forces are solved by the catenary theory and the nonlinear stiffness provided by the SPAR buoy are also considered. The coupled model is set up and a numerical code is developed for calculating the dynamic response of a Hywind SPAR-type floating offshore wind turbine (FOWT) under the combined sea states of wind, wave and current. It shows that the amplitudes of sway and roll are dominated by lift loads induced by vortex shedding, and the oscillations in roll reach the same level of pitch in some scenarios. The mean value of surge is changed under the drag loads, but the mean position in pitch, as well as the oscillations in surge and pitch, is little affected by the current. Due to the coupling effects, the heave motion is also influenced by vortex-induced forces. When vortex-shedding frequency is close to the natural frequency in roll, the motions are increased. Due to nonlinear stiffness, super-harmonic response occurs in heave, which may lead to internal resonance.
TOPICS: Stress, Vortices, Mooring, Wind turbines, Vortex shedding, Stiffness, Surges, Computational fluid dynamics, Oscillations, Resonance, Momentum, Drag (Fluid dynamics), Waves, Potential theory (Physics), Degrees of freedom, Rotors, Blades, Dynamic response, Offshore wind turbines, Seas, Wind waves, Buoys
research-article  
Vladimir Shigunov
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039955
The paper describes a numerical approach to predict required added power for propulsion in waves. Such predictions are important to address fuel consumption in seaway and define suitable operating point and sea margin, as well as for routing optimization and hull performance monitoring. Added resistance and in general drift forces and moments due to waves are key input parameters for added power requirements. The 3d Rankine source-patch method was used to compute them. The method solves the problem in the frequency domain, linearizing wave-induced motions around the fully nonlinear steady flow. The added power software combines added resistance and drift forces and moments in irregular waves with wind forces and moments, calm-water manoeuvring forces and moments, rudder and propeller forces and propulsion and engine model and provides associated resistance and power as well as changes in ship propulsion in waves. The approach is demonstrated for a container ship to compare predictions with full-scale data.
TOPICS: Flow (Dynamics), Containers, Engines, Propulsion, Waves, Optimization, Computer software, Propellers, Ships, Water, Wind, Seas, Hull, Fuel consumption
research-article  
Guangyao Wang, Ye Tian and Spyros A. Kinnas
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039948
This work focuses on the study of flow around a rigid cylinder with both Particle Image Velocimetry (PIV) experiment and Computational Fluid Dynamics (CFD) simulation. PIV measurements of the flow field at the downstream of the cylinder are first presented. The boundary conditions for CFD simulations are measured in the PIV experiment. Then the PIV flow is compared with both RANS (2D) and LES (3D) simulations performed with ANSYS Fluent. The velocity vector fields and time histories of velocity are analyzed. In addition, the time-averaged velocity profiles and Reynolds stresses are analyzed. It is found that, in general, LES (3D) gives a better prediction of flow characteristics than RANS (2D).
TOPICS: Flow (Dynamics), Simulation, Computational fluid dynamics, Cylinders, Reynolds-averaged Navier–Stokes equations, Boundary-value problems, Stress, Particulate matter
research-article  
Rodrigo Provasi, Fernando Geremias Toni and Clovis de Arruda Martins
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039923
Flexible pipes are structures composed by many layers that vary in composition and shapes. The structural behavior of each layer is defined by the role it must play. The construction of flexible pipes is such that the layers are unbounded, with relative movement between them. Even though this characteristic is what enables its high bending compliant behavior, if the displacements involved are small, a bonded analysis is interesting to grasp the general characteristics of the problem. The bonded hypothesis means that there is no movement relative between layers, which is fine for a small displacement analysis. It also creates a lower bound for the movement, since when considering increasingly friction coefficient values, it tends to the bonded situation. The main advantage of such hypothesis is that the system become linear, leading to fast solving problems (when compared to full frictional analysis) and giving insights to the pipe behavior. The authors have previously developed a finite element based one called macroelements. This model enables a fast-solving problem with less memory consumption when compared to multipurpose software. The reason behind it is the inclusion of physical characteristics of the problem, enabling the reduction in both number of elements and memory used and, since there is less elements and degrees of freedom, faster solved problems. In this article, the advantages of such model are shown by using examples that are representative of a simplified, although realistic, flexible pipe. Comparisons between the macroelement model and commercial software are done to show its capabilities.
TOPICS: Pipes, Computer software, Displacement, Shapes, Friction, Construction, Degrees of freedom, Finite element analysis
research-article  
Chang-Wook Park, Jeonghwa Seo and Shin Hyung Rhee
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039866
A series of model tests of a caisson in wet towing were conducted in a towing tank to assess the stability and effective power requirement in calm water and head sea conditions. The scale ratio of the model was 1/30, and the model-length-based Froude number in the tests ranged from 0.061 to 0.122, which is equivalent to 2 and 4 knots in the full scale, respectively. During the towing of the model, tension on the towline and six-degrees-of-freedom motion of the model were measured. Under the calm water condition, the effects of towing speed, draft, and initial trim variation on the towing stability and effective power were investigated. Initial trim improved stability and reduced required towing power. In head seas, effective power and towing stability were changed with the wave length. It increased as the wave length became longer, but the added resistance in long waves also stabilized the model with reduced yaw motion.
TOPICS: Stability, Caissons, Waves, Water, Seas, Model basin, Yaw, Tension
research-article  
Rodrigo Provasi, Fernando Geremias Toni and Clovis de Arruda Martins
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039795
The layers of unbounded flexible pipes have relative movement, enhancing its capabilities to handle curvatures and moment loads. In a simplified approach those pipes can be described using bonded elements but to really capture this behavior, a frictional contact is utterly needed. In general, dealing with contact problems in computational mechanics is complicated, since it involves the constant evaluation of its status and can lead to convergence problems or simulation failure, due to intrinsically problematic and inefficient contact models or due to contact models that are insufficient to capture the desired details. The macroelement formulation, which was created to deal with flexible pipes in a simplified way, needed a frictional contact element to enhance the quality of results and closeness to real behavior. The major drawback for developing such element is the different nature of the nodal displacements descriptions. The first approach possible is the simplest contact model: it involves only the nodes in each contacting elements. The gap information and distances are evaluated using exclusively the nodal information. This kind of model provides good results with minimum computational effort, especially when considering small displacements. This paper proposes such element: a node-to-node contact formulation for macroelements. It considers that the nodal displacements of both nodes are in cylindrical coordinates, with one of them using Fourier series to describe the displacements. To show model effectiveness, a case study with a cylinder using Fourier series and multiple helical elements connected with the contact element is done and show great results.
TOPICS: Modeling, Pipes, Fourier series, Cylinders, Failure, Simulation, Stress, Computational mechanics, Contact mechanics
research-article  
Jianping Zhang, Zhen Gong, Liang Guo and Helen Wu
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039717
For large-scale offshore wind turbine rotating blades (NREL 5MW), the theoretical model of fluid-structure interaction (FSI) vibration was established, and the blade load and the basic equations for modal analysis were given. Based on ANSYS Workbench platform, the blade modal characteristics at different rotating speeds were analyzed, and further research on dynamic stability was carried out. The results indicate that the FSI and the blade rotation have a great influence on modal frequencies, which increase with the rotating speed of the blade under FSI. When the frequency of the periodic wind speed is close to the first order natural frequency of the blade, the maximum flapping displacement and the maximum Mises stress both increase with time, and the vibration divergence appears. At the safe distance of 4.50m, the critical value of the blade maximum Mises stress basically presents a linear upward trend with the increase of the elasticity modulus, which can provide technical references for optimization design and safe operation of wind turbine blades.
TOPICS: Dynamic stability, Wind turbines, Rotating blades, Blades, Fluid structure interaction, Stress, Vibration, Modal analysis, Design, Optimization, Rotation, Wind velocity, Displacement, Elastic moduli, Offshore wind turbines
research-article  
Finn-Idar G Giske, Bernt J. Leira and Ole Øiseth
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039718
In this paper the first order reliability method (FORM) found in connection with structural reliability analysis is first used in an inverse manner to efficiently obtain an approximate solution of the full long-term extreme response of marine structures. A new method is then proposed where the second order reliability method (SORM) is used to improve the accuracy of the approximation, resulting in an inverse SORM (ISORM) approach. This method is compared with exact results obtained using full numerical integration. The new method is seen to achieve significantly improved accuracy, yet keep the number of required short-term response analyses within acceptable levels.
TOPICS: Marine structures, Reliability, Event history analysis, Approximation
research-article  
Mona Golbabaei-Asl, Alex Povitsky and Lev Ring
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039565
The study presents a one-dimensional numerical model of wave propagation as well as transmission/reflection phenomena in Newtonian and non-Newtonian drilling mud flow associated with oil/gas drilling activities. Propagation of wave formed due to back pressure changes by means of a choke is investigated. In general, the reflection and transmission of pressure waves at intersection of conduits with different cross sections or in case of partial blockage typical of drilling practices is multi- dimensional and caused by non-uniform boundary conditions over the cross section. The one-dimensional approach is investigated to approximate the multi-dimensional reflection and transmission of pressure pulses at areal discontinuity in conduit. The approach is facilitated by introduction of a local force exerted by solid wall on the fluid at the intersection of the conduits into conservative form of conservation of momentum equation. In addition, non-conservative formulation of momentum equation was explored. To solve the differential equations, MacCormack numerical scheme with second-order accuracy is applied to the non-linear Euler and 1-D viscous conservation equations. A grid refinement study is performed. It is shown that non-conservative form of the conservation laws results in more accurate prediction of transmission and reflection in case of areal discontinuity. The results of the numerical modeling are presented in terms of pressure wave propagation and attenuation upon reflection and transmission at consequent interfaces.
TOPICS: Wave propagation, Fluids, Drilling, Modeling, Reflection, Pressure, Momentum, Computer simulation, Waves, Cross section (Physics), Differential equations, Flow (Dynamics), Boundary-value problems
research-article  
Zhang Jian, Oleg Gaidai and Junliang Gao
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039564
This paper presents a generic Monte Carlo based approach for bivariate extreme response prediction for fixed offshore structures, particularly Jacket type. The bivariate analysis of extremes is often poorly understood and generally not adequately considered in most practical measurements/situations, that is why it is an important to utilize the recently developed bivariate ACER (average conditional exceedance rate) method. According to the current literature study, there is not yet a direct application of the bivariate ACER method to coupled offshore jacket stresses. This study aims at being first to apply bivariate ACER method to jacket critical stresses, aiming at contributing to safety and reliability studies for a wide class of fixed offshore structures. An operating Jacket located in the Bohai bay was taken as an example to demonstrate the proposed methodology. Satellite measured global wave statistics was used to obtain realistic wave scatter diagram in the Jacket location area. Second order wave load effects were taken into account, while simulating Jacket structural response. An accurate finite element ANSYS model was used to model Jacket response dynamics, subject to non-linear hydrodynamic wave and sea current loads. Offshore structure design values are often based on univariate statistical analysis, while actually multivariate statistics is more appropriate for modeling the whole structure. This paper studies extreme stresses, that are simultaneously measured/simulated at two different Jacket locations. Due to less than full correlation between stresses in different critical Jacket locations, application of the multivariate (or at least bivariate) extreme value theory is of practical engineering interest.
TOPICS: Stress, Offshore platform jackets, Statistics as topic, Waves, Offshore structures, Dynamics (Mechanics), Safety, Reliability, Electromagnetic scattering, Finite element analysis, Modeling, Statistical analysis, Seas, Satellites, Offshore structural design
research-article  
Leilei Dong, Qi Zhang, Y. Huang, G. Liu and Zhiyuan Li
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039524
This paper deals with the effect of termination restraint due to end fitting on the stress evaluation of tensile armors in unbonded flexible pipes under axial tension. The problem is characterized by one single armoring tendon helically wound on a cylindrical supporting surface subjected to traction. The deviation from the initial helical angle is taken to describe the armor wire path as the pipe is stretched. The integral of this angle change gives the lateral displacement of the wire, which is determined by minimizing the energy functional that consists of the strain energy due to axial strain, local bending and torsion, and the energy dissipated by friction, leading to a variational problem with a variable endpoint. The governing differential equation of the wire lateral displacement, together with the supplementary condition, is derived using the variational method and solved analytically. The developed model is verified with a finite element simulation. Comparisons between the model predictions and the finite element results in terms of the change in helical angle and transverse bending stress show good correlations. The verified model is then applied to study the effects of imposed tension and friction coefficient on the maximum bending stress. The results show that the response to tension is linear and friction could significantly increase the stress at the end fitting compared with the frictionless case.
TOPICS: Stress, Pipes, Fittings, Tension, Friction, Wire, Displacement, Bending (Stress), Finite element analysis, Traction, Variational techniques, Tendons, Armor, Differential equations, Simulation, Torsion
research-article  
James Doherty and Barry Lehane
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039523
Pile foundation design is conventionally conducted using a process of trial and error, where dimensions of a pile are estimated and the performance is computed. The performance is compared with design criteria and dimensions varied to achieve a safe and economical design. In this paper, this time consuming and labour intensive process is replaced with an automated approach using the example case of an offshore monopile supporting a wind turbine. The optimum length and diameter of the monopile is determined with the aim of minimising the pile weight while satisfying both serviceability and ultimate limit states. The approach handles general soil and loading conditions and includes an ability to incorporate cyclic loading.
TOPICS: Design, Offshore wind turbines, Dimensions, Pilings (Building), Maintainability, Ocean engineering, Soil, Wind turbines, Errors, Weight (Mass)
research-article  
Chee K. Wong and Thomas Brown
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039261
Offshore structures constructed in waters where ice cover is prevalent for several months a year are subjected to ice loading. Some of these structures are conical or sloped-faced in shape, where flexural failure becomes the dominant mode of failure for the ice sheet. The flexural failure mode reduces the magnitude of ice-structure interaction loads in comparison to other modes of failure. Various researchers have devised flexural failure models for ice-conical structure interactions. Each model shares the same principle of the ice sheet being modelled as a beam on an elastic foundation, but each model has different limitations in precisely simulating the interaction. Some models do not incorporate the ice rubble pile, while other models make over-simplified assumptions for three-dimensional behaviour. The proposed three-dimensional model aims to reduce some of these limitations by modelling the geometry of the ice rubble pile around the conical pier using the results of small-scale tests, modelling the loads exerted by the ice rubble pile on the conical structure and ice sheet with a rigorous method of slices, adding driving forces in keeping the rubble pile intact and in upward motion during the interaction, accounting for eccentric offsetting moments at the ice-structure contacts, and modelling the flexural behaviour of the ice sheet subject to ice rubble loads using finite element method. The proposed model is used to analyze the interaction events recorded at the conical piers of the Confederation Bridge over a period of 11 years.
TOPICS: Bridges (Structures), Piers (Structural), Ice, Ice rubble, Three-dimensional models, Failure, Modeling, Stress, Offshore structures, Finite element methods, Ice-structure interaction, Failure mechanisms, Accounting, Flexural behavior, Geometry, Shapes, Water
research-article  
Ravi Challa and Solomon Yim
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4038939
This study provides an evaluation of predictive capabilities and computational efficiency of a finite-element (FE) method and a smoothed particle hydrodynamics (SPH) technique in modeling a fully coupled fluid-flexible structure interaction (FSI) problem under a unified solution methodology and computational platform. The two numerical methods are validated with experimental data of an elastic gate subjected to a rapidly varying flow. An arbitrary Lagrangian-Eulerian (ALE) formulation is employed in the FE model for efficient large-deformation interface tracking. While the rapidly varying fluid flow is modeled using both the ALE based FE and the SPH formulation, the deformation of an elastic gate (flexible structure) is modeled using a standard Lagrangian FE method in both FSI models. In both numerical solutions, the fluid flow is governed by the Navier-Stokes equation and the structure is modeled as elastic. Numerical simulation results show that the ALE-FE/FE continuum approach not only captures the dynamic behavior properly, but also predicts the water-free surface profiles and the elastic gate deformations accurately. On the other hand, the coupled purely Lagrangian approach of the SPH/FE under an identical computational platform is found to be less accurate and efficient in predicting the dynamics of the elastic gate motion and the water free-surface profiles.
TOPICS: Hydrodynamics, Particulate matter, Finite element analysis, Modeling, Fluid structure interaction, Gates (Closures), Deformation, Fluid dynamics, Water, Dynamics (Mechanics), Numerical analysis, Finite element model, Flexible structures, Flow (Dynamics), Fluids, Navier-Stokes equations, Computer simulation
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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