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Accepted Manuscripts

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research-article  
Yu Ping Li, Jiang tao Yi and Fook Hou Lee
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039372
Up to now, the post-consolidation bearing capacity enhancement of jack-up spudcan foundation has been explored using centrifuge model tests and numerical analyses, which however ignored the realistic jack-up lattice leg. This paper investigates both typical lattice leg and sleeve effects on the post-consolidation spudcan bearing capacity using centrifuge model tests, by replicating the entire process of spudcan in normally consolidated clay: "penetration-unloading-consolidation-re-penetration". The experimental results show that the lattice leg and sleeve affect the spudcan bearing capacity in two sides compared with spudcan without leg. Firstly, it increases the transient bearing capacity during initial spudcan penetration; Secondly, less post-consolidation bearing capacity improvement is yielded by the presence of the leg. The former effect is of importance on the prediction of jack-up leg penetration, and the latter effect would suggest a lower risk of spudcan punch-through for realistic offshore jack-up rig during preloading and operation period.
TOPICS: Load bearing capacity, Jack-up drilling rigs, Risk, Numerical analysis, Transients (Dynamics), Ocean engineering
research-article  
Hui Li, Muk Chen Ong, Bernt J. Leira and Dag Myrhaug
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039297
This paper presents an engineering approach to study the effects of soil profile variation and scour on structural response of an offshore monopile wind turbine. A wind-wave model for finite water depth is proposed to obtain the corresponding sea-state based on the incident wind. Different wind, wave and current loads on the wind turbine for the operational conditions are considered. The interaction between the foundation and the soil is simulated by nonlinear springs, for which stiffness properties are obtained from the axial load transfer curve, the tip load-displacement curve and the lateral load-deflection curve. Four types of soil conditions are considered, i.e., 100% sand layer, 50% sand layer (top) and 50% clay layer (bottom), 50% clay layer (top) and 50% sand layer (bottom), as well as 100% clay layer. For a given current speed, the variations of the structural response of the wind turbine due to the effects of different wind-wave load combinations, soil conditions and scour have been investigated. Different wind-wave load combinations directly affect the mean internal bending moment and mean displacement vertically along the support structure. Different soil conditions change the eigen-frequency of the structure significantly. The top layer of the soil appears to have a strong influence on the mean internal bending moment and the mean shear force distribution along the foundation. Moreover, the effect of scour alters the eigen-frequency of the structure significantly. The maximum mean bending moment and displacement increase for the cases with scour hole as compared to the cases with scour protection.
TOPICS: Ocean engineering, Soil, Wind turbines, Stress, Wind waves, Sands, Displacement, Seas, Shear (Mechanics), Deflection, Springs, Stiffness, Water, Wind
research-article  
Felipe Lopes de Souza, Eduardo Aoun Tannuri, Pedro Cardozo de Mello, Guilherme R. Franzini, Jordi Mas-Soler and Alexandre N. Simos
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039263
The measurement of the directional wave spectrum in oceans has been done by different approaches, mainly wave-buoys, satellite imagery and radar technologies; these methods, however, present some inherent drawbacks, e.g., difficult maintenance, low-resolution around areas of interest and high-cost. In order to overcome those problems, recent works proposed a motion-based estimation procedure using the vessel as a wave sensor; nevertheless, this strategy suffers from low estimation capabilities of the spectral energy coming from periods lower than the cut-off period of the systems, which are important for the drift effect predictions. This work studies the usage of wave-probes installed on the hull of a moored vessel to enhance the estimation capabilities of the motion-based strategy, using a high-order estimation method based on Bayesian statistics. Firstly, the measurements from the wave-probes are incorporated to the dynamic system of the vessel as new degrees-of-freedom, thus the Bayesian method can be expanded without additional reasoning. Secondly, the proposal is validated by experiments conducted in a wave-basin with a scale model, concluding that the approach is able to improve not only the estimation of spectra with low peak period, but also the estimation in the entire range of expected spectra. Lastly, some drawbacks are discussed, as the effect of the non-linear roll motion, which must be taken in account when calculating the wave-probe response; and the poor mean-direction estimation capability in some particular wave directions and low peak periods.
TOPICS: Waves, Probes, Vessels, Spectra (Spectroscopy), Sensors, Maintenance, Radar, Resolution (Optics), Degrees of freedom, Dynamic systems, Mooring, Oceans, Hull, Buoys, Satellites, Statistics as topic
research-article  
Ying Zhao, Xiaohan Jia, Yian Zhang and Xueyuan Peng
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039262
This paper presents the substructure-based dynamic analysis of an offshore platform with compressor packages. Three typical substructure methods, the Guyan condensation method, the fixed-interface component mode synthesis (CMS) method and the free-interface CMS method, were compared to identify the appropriate substructure method for this application. A mode truncation criterion was proposed to ensure the accuracy of the recommended substructure method. The results indicated that the free-interface CMS method could generate almost the same results as the fully coupled method, and save more than 50% in calculation time and more than 60% in storage space. When the same amount of time was used, the free-interface CMS method obtained more accurate results than the fixed-interface CMS method and Guyan condensation method; thus, the use of this method for evaluating the dynamics of an offshore platform with compressor packages was recommended. The cut-off frequency of the substructure was suggested to be 1.25 times the highest frequency of interest when conducting a dynamic analysis of an offshore platform with compressor packages using the free-interface CMS method. In addition, the offshore platform is a flexible structure with low and dense mechanical natural frequencies (MNFs), with approximately 4500 orders vibration modes in the frequency range of 0-40 Hz, and the displacement response at the area around the compressor package exceeded the allowable value under the excitation of the compressor package.
TOPICS: Compressors, Dynamic analysis, Offshore platforms, Condensation, Dynamics (Mechanics), Excitation, Vibration, Displacement, Flexible structures, Storage
research-article  
Monica Campos Silva, Marcelo de Araujo Vitola, Luis Eca, Paulo de Tarso Themistocles Esperança and Sergio H. Sphaier
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039260
In recent decades, the use of computational fluid dynamics (CFD) in many areas of engineering as a research and development tool has seen remarkable growth. Recently, an increasing concern with the assessment of the quality of CFD results has been observed. Wave modeling is an important task in many ocean engineering applications. Although numerical modeling studies of waves can be found in the literature for many applications, it is hard to find studies that present the numerical uncertainties of the results. In this study, the numerical uncertainties in mean wave parameters simulated using a viscous model were estimated using the procedure established by [1], which is based on grid/time refinement studies and power series expansions. StarCCM+ software was used to simulate wave propagation. The computational domain was discretized using a trimmer mesh. The results obtained for a regular wave with a wave steepness (H/L) equal to 0.025 are presented. The numerical uncertainties in mean wave height and mean wave period were estimated along the computational domain. The results indicate that the convergence properties of the mean wave parameters with the grid refinement depended on both position in the domain and the selected wave parameter.
TOPICS: Waves, Modeling, Uncertainty analysis, Computational fluid dynamics, Uncertainty, Computer software, Ocean engineering, Wave propagation, Computer simulation, Industrial research
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  
Hui Li, Di Wang, Chengming Zhou, Kaihong Zhang and Huilong Ren
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039160
In hydroelastic model tests, segmented ship models are usually used to make sure that the model scale and full size ship satisfy the similarity law of structural natural frequency and distribution of ship bending stiffness. However, springing barely occurs in those tests because the natural frequency of segmented ship models are too high for the regular waves required to be generated in a tank. In order to investigate the springing effect, three sets of backbone of variable cross section are adopted in the test. One set of the backbones is satisfied with the similarity law of natural frequency, and two extra sets of low stiffness backbone are used so that the springing effect can appear and be measured. Experimental results show that the springing occurs when the wave encounter frequency coincides with the first elastic natural frequency of the ship, or with half or one-third of it. A good agreement has also been obtained between the experimental and the numerical results by a 3D hydroelastic method. Based on these results, the contribution of the springing responses to the fatigue damage of the ship is estimated and analyzed.
TOPICS: Waves, Discrete wavelet transforms, Fatigue damage, Ships, Stiffness
research-article  
Alireza Ebrahimi, Shawn Kenny and Amgad Hussein
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039132
omposite flexible pipe is used in the offshore oil and gas industry for the transport of hydrocarbons, jumpers connecting subsea infrastructure, and risers with surface platforms and facilities. Although the material fabrication costs are high, there are technical advantages with respect to installation and performance envelope (e.g. fatigue). Flexible pipe have a complex, composite section with each layer addressing a specific function (e.g. pressure containment, axial load). Continuum finite element modelling procedures are developed to examine the mechanical response of an unbonded flexible pipe subject to axisymmetric loading conditions. A parameter study examined the effects of: (1) pure torsion, (2) interlayer friction factor, (3) axial tension, and (4) external and internal pressure on the pipe mechanical response. The results demonstrated a coupled global-local mechanism with a bifurcation path for positive angles of twist relative to the tensile armour wire pitch angle. These results indicated idealized analytical and structural-based numerical models may be incomplete or may provide an accurate prediction of the pipe mechanical response. The importance of using an implicit solver to predict the bifurcation response, and simulate contact mechanics between layers was highlighted.
TOPICS: Wire, Finite element analysis, Pipes, Armor, Bifurcation, Ocean engineering, Pressure, Fatigue, Friction, Composite materials, Computer simulation, Manufacturing, Stress, Torsion, Risers (Casting), Contact mechanics, Modeling, Petroleum industry, Pipeline risers, Tension, Containment
research-article  
Lin Li, Zhiyu Jiang, Andreas Vangdal H⊘iland and Muk Chen Ong
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4039131
The aquaculture industry is aiming to move fish farms from nearshore areas to open seas because of many attractive advantages in the open water. However, one major challenge is to design the structure to withstand the environmental loads due to wind, waves and current. The purpose of this paper is to study a vessel-shaped fish farm concept for open sea applications. The structure includes a vessel-shaped hull, a mooring system and fish cages. The shape of the hull minimizes the wave loads coming from the bow, and the single-point mooring system is connected to the turret at the vessel bow. Such a system allows the whole fish farm to rotate freely about the turret, reduces the environmental loads on the structure and increases the spread area of fish wastes. A basic geometry of the vessel hull was considered and the hydrodynamic properties were obtained from the frequency domain analysis. A mooring system with six mooring lines was designed to avoid possible interactions with the fish cages. Time domain simulations were performed by coupling the hull with the mooring system. A simplified rigid model of the fish cages was considered. The global responses of the system and the mooring line loads were compared under various wave and current conditions. The effects due to misalignment of wave and current directions on the responses were discussed. Finally, the responses using flexible and rigid net models were compared under steady current conditions.
TOPICS: Ocean engineering, Numerical analysis, Vessels, Mooring, Stress, Hull, Waves, Seas, Simulation, Water, Wind waves, Design, Engineering simulation, Shapes, Frequency-domain analysis, Geometry
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
research-article  
Chunning Ji, Wanhai Xu, Hai Sun, Rui Wang, Chunhui Ma and Michael M. Bernitsas
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4038936
Flow-induced vibrations of two elastically mounted circular cylinders in staggered arrangement were experimentally investigated. The Reynolds number range for all experiments (2.5x104
TOPICS: Flow (Dynamics), Flow-induced vibrations, Cylinders, Vibration, Springs, Stiffness, Water, Circular cylinders, Displacement, Fluids, Turbulence, Reynolds number, Shear (Mechanics), Fluid-dynamic forces, Dampers, Damping

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