Accepted Manuscripts

A. Spicer Bak and Michael E. McCormick
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037951
Results of experimental and computational fluid dynamics (CFD) studies conducted to compare the flow-energy-attenuating performances of two different buoy configurations are presented. A finned-body was experimentally studied in two orientations - one with a splitter and rotated 22.5° (with no splitter). The finned-buoy is designed for both wave and current attenuation; however, only the current application is discussed here. Scaled models were subjected to wind tunnel testing and CFD analyses. For this study, the steady-state drag coefficient (CD) is considered to be the performance measure. The CFD model is used to match the physical testing by utilizing the k-? turbulence model. Reynolds numbers (based on the tip-to-tip fin diameter) approaching the drag crisis are used to evaluate the bodies of interest, both of which have an aspect ratio (draft-to-diameter) of 1.85. The finned-bodies do encounter a drag crisis (as commonly seen with a cylinder), since the fins cause the buoys to act as a bluff body. The flow structures around the bodies are examined and compared to those predicted by established theories. For the finned-body, the 22.5° yaw orientation is found to have a consistently higher drag than the splitter orientation. The drag enhancement is explained by two phenomena. The first is a low-pressure area located in pockets adjacent to the upstream fins. The second is the absence of the drag-crisis, due to fixed separation points at the fin tips for all Reynolds numbers.
TOPICS: Steady state, Buoys, Drag (Fluid dynamics), Computational fluid dynamics, Testing, Fins, Reynolds number, Flow (Dynamics), Separation (Technology), Turbulence, Waves, Pressure, Cylinders, Wind tunnels, Yaw
Arturo Ortega, Ausberto Rivera and Carl M. Larsen
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037842
Flexible risers provide optimum solutions for deep water offshore fields. Reliable dynamic analysis of this kind of slender structure is crucial to ensure safety against long time fatigue failure. Beyond the effects from wave loads, the influence from transient internal slug flow on the slender structure dynamics should also be taken into account. In this study two coupled in-house codes were used in order to identify and quantify the effects of an internal slug flow and wave loads on the flexible riser dynamics. One code carries out a global dynamic analysis of the slender structure displacements using a finite element formulation. The other program simulates the behaviour of the internal slug flow using a finite volume method. The slug flow is influenced by the dynamic shape of the riser, while the time varying forces from internal slug flow plus external waves will influence the shape. Hence, a fully coupled analysis is needed in order to solve the coupled problem. By means of the distributed simulation these two programs run synchronously and exchange information during the time integration process. A test case using hydrodynamic forces according to the linear Airy wave theory, coupled with an internal unstable slug flow was analysed and the results shown amplification of the dynamic response due to the interaction between the two load types, effects on the effective tension caused by the internal two-phase flow, and influence on the internal slug flow caused by the wave induced response.
TOPICS: Waves, Slug flows, Flexible risers, Stress, Shapes, Dynamics (Mechanics), Dynamic analysis, Finite element analysis, Two-phase flow, Dynamic response, Finite volume methods, Pipeline risers, Transients (Dynamics), Risers (Casting), Ocean engineering, Fluid-dynamic forces, Safety, Simulation, Linear wave theory, Fatigue failure, Tension, Water
Yanbing Zhao and Haixiao Liu
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037843
With the application of innovative anchor concepts and advanced technologies in deepwater moorings, anchor behaviors in the seabed are becoming more complicated, such as 360-degree rotation of the anchor arm, gravity installation of anchors with high soil strain rate, and keying and diving (or penetration) of anchors. With moving of the AHV or platform, anchor line produces a space movement, and forms a reverse catenary shape and even a three-dimensional profile in the soil. Numerical analysis on the behaviors of anchor lines and deepwater anchors requires techniques that can deal with large strains and deformations of the soil, track changes in soil strength due to soil deformation, strain rate and strain softening effects, appropriately describe anchor-soil friction, and construct structures with connector elements to conform to their characteristics. Being an effective tool of large deformation finite element analysis, the coupled Eulerian-Lagrangian (CEL) method is advantageous in handling geotechnical problems with large deformations. This paper gives an overview of several key techniques in the CEL analysis of comprehensive behaviors of deepwater anchors. Numerical probe tests and comparative studies are also presented to examine the robustness and accuracy of the proposed techniques. The aim of this paper is to provide an effective numerical framework to analyze the comprehensive behaviors of anchor lines and deepwater anchors.
TOPICS: Deformation, Finite element analysis, Soil, Seabed, Numerical analysis, Mooring, Probes, Robustness, Shapes, Friction, Rotation, Gravity (Force)
Technical Brief  
Ki-Su Kim, Myung-Il Roh, Sung-Min Lee, Han-Sung Kim and Hyunsik Ahn
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037828
With the recent international economic downturn, most EPC (Engineering, Procurement, and Construction) contractors are incurring deficits in their FPSO (Floating, Production, Storage, and Offloading unit) projects. Numerous reasons underpin these situations. One of the most important reasons is the cost-estimation failure. The cost estimation is the key contractual point and mainly depends on a weight estimation of the FPSO topsides. Because the topsides contain a lot of equipment and complex structures, it is very difficult to make an estimation at the contractual stage. To overcome this problem, many methods have been proposed to estimate the weight of offshore topsides; however, most of the methods involve the top-down approach, making it difficult to obtain a sufficiently accurate prediction for field-work usage in terms of the weight estimation. Therefore, a WBS (Work Breakdown Structure) for the performance of the weight-estimation process is proposed in this study. Using the WBS of the FPSO topsides, the corresponding presentation of the weight-estimation process makes the process usable in the field work regarding the WBS-item estimations. Accordingly, estimates of the detailed units (disciplines, modules, and areas) inside the topside that were previously not possible were performed. In addition, a prototype program was developed using the proposed method, and the applicability of the proposed method was evaluated through the application of three projects.
TOPICS: Weight (Mass), FPSO, Construction, Engineering prototypes, Ocean engineering, Underpinning, Engineering disciplines, Failure
Jithin Jose, Olga Podrazka, Ove T. Gudmestad and Witold Cieslikiewicz
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037829
Wave breaking is one of the major concerns for offshore structures installed in shallow waters. Impulsive breaking wave forces sometimes govern the design of such structures, particularly in areas with a sloping sea bottom. Most of the existing offshore wind turbines were installed in shallow water regions. Among fixed-type support structures for offshore wind turbines, jacket structures have become popular in recent times as the water depth for fixed offshore wind structures increases. However, there are many uncertainties in estimating breaking wave forces on a jacket structure, as only a limited number of past studies have estimated these forces. Present study is based on the WaveSlam experiment carried out in 2013, in which a jacket structure of 1:8 scale was tested for several breaking wave conditions. The total and local wave slamming forces are obtained from the experimental measured forces, using two different filtering methods. The total wave slamming forces are filtered from the measured forces using the Empirical Mode Decomposition method, and local slamming forces are obtained by the Frequency Response Function method. From these results, the peak slamming forces and slamming coefficients on the jacket members are estimated. The breaking wave forces are found to be dependent on various breaking wave parameters such as breaking wave height, wave period, wave front asymmetry and wave breaking positions. These wave parameters are estimated from the wave gauge measurements taken during the experiment. The dependency of the wave slamming forces on these estimated wave parameters is also investigated.
TOPICS: Waves, Water, Wave forces, Offshore wind turbines, Uncertainty, Wind, Seas, Filtration, Gages, Offshore structures, Ocean engineering, Design, Frequency response
Raul Urbina, James M. Newton, Matthew P. Cameron, Richard W. Kimball, Andrew J. Goupee and Krish Thiagarajan
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037826
Environmental conditions created by winds blowing oblique to the direction of the waves are necessary to conduct some survivability tests of offshore wind turbines. However, some facilities lack the capability to generate quality waves at a wide range of angles. Thus, having a wind generation system that can be rotated makes generating winds that blow oblique to the waves possible during survivability tests. Rotating the wind generation system may disrupt the flow generated by the fans because of the effect of adjacent walls. Closed or semi-closed wind tunnels may eliminate the issue of wall effects, but these types of wind tunnels could be difficult to position within a wave basin. In this work, a prototype wind generation system that can be adapted for offshore wind turbine testing is investigated. The wind generation system presented in this work has a return that minimizes the effect that the walls could potentially have on the fans. This study characterizes the configuration of a wind generation system using measurements of the velocity field detailing mean velocities, flow directionality and turbulence intensities. Measurements were taken downstream to evaluate the expected area of turbine operation and the shear zone. The dataset has aided in the identification of conditions that could potentially prevent the production of the desired flows. Therefore, this work provides a useful dataset that could be used in the design of wind generation systems and in the evaluation of the benefits of recirculating wind generation systems for offshore wind turbine research.
TOPICS: Offshore wind turbines, Wind, Waves, Flow (Dynamics), Fans, Wind tunnels, Testing, Turbines, Turbulence, Shear (Mechanics), Engineering prototypes, Design
Kanhua Su, Jianming Yang and Stephen D. Butt
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037830
Deepwater environments raise higher demand on the bearing capacity of the drilling conductor below the mud line. To deal with this problem, a conductor bearing capacity enhancement device was designed. The setting capability of this device and its axial bearing capacity were analyzed with soil mechanics and pile theory. Furthermore, a calculation model is developed, and a numerical method is used to solve for the lateral capacity, coupled with drilling platform, riser, conductor, seabed soft soil and the device. The result shows that proper setting depth of the device is primarily determined by the property of the soil, rather than the conductor jetting operation. Additionally, appropriate determination of the diameter and wall thickness of this device can greatly improve the lateral bearing capacity of the conductor.
TOPICS: Underwater drilling, Load bearing capacity, Soil, Drilling, Wall thickness, Seabed, Numerical analysis, Pipeline risers, Soil mechanics, Risers (Casting)
Qiuhao Hu and Ye Li
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037696
This paper presents our recent numerical simulations of a high solidity Wells turbine under both steady and unsteady conditions by solving Reynolds-averaged Navier-Stokes (RANS) equations. For steady conditions, the equations are solved in a reference frame with the same angular velocity of the turbine. Good agreement between numerical simulation result and experimental data has been obtained in operational region and incipient stall conditions. The exact value of stall point has been accurately predicted. Through analyzing the detailed fluid fields, we find that the stall occurs near the tip of blade while the boundary layer keeps attached near the hub, due to the effect of radial flow. For unsteady conditions, two types of control methods are studied, constant angular velocity or constant damping moment. For the constant angular velocity, the behaviors of the turbine under both high and low sea wave frequency are calculated to compare with those obtained by quasi-steady method. The hysteresis characteristic can be observed and deeply affects the behaviors of the Wells turbine with high wave frequency. For the constant damping moment, the turbine angular velocity is time-dependent. Under sinusoidal flow, the incident flow velocity in the operational region can be improved to avoid the stall.
TOPICS: Wells, Engineering simulation, Turbines, Simulation, Reynolds-averaged Navier–Stokes equations, Unsteady flow, Damping, Computer simulation, Flow (Dynamics), Wave frequency, Fluids, Blades, Ocean waves, Radial flow, Boundary layers
Luca Bonfiglio and Stefano Brizzolara
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4037487
Near field flow characteristics around catamarans close to resonant conditions involve violent viscous flow such as energetic vortex shedding and steep wave making. This paper presents a systematic and comprehensive numerical investigation of these phenomena at various oscillating frequencies and separation distances of twin sections. The numerical model is based on the solution of Navier-Stokes equations assuming laminar-flow conditions with a volume of fluid approach which has proven to be particularly effective in predicting strongly non-linear radiated waves which directly affect the magnitude of the hydrodynamic forces around resonant frequencies. Considered non-linear effects include wave breaking, vortex shedding and wave-body wave-wave interactions. The method is first validated using available experiments on twin circular sections: the agreement in a very wide frequency range is improved over traditional linear potential flow based solutions. Particular attention is given to the prediction of added mass and damping coefficients at resonant conditions where linear potential flow methods fail, if empirical viscous corrections are not included. The results of the systematic investigation show for the first time how the so-called piston-mode motion characteristics are nonlinearly dependent on the gap width and motion amplitude. At low oscillation amplitudes flow velocity reduces and so does the energy lost for viscous effects. On the other hand for higher oscillation amplitude the internal free surface breaks dissipating energy hence reducing the piston mode amplitude. These effects can not be numerically demonstrated without a computational technique able to capture free surface non linearity and viscous effects.
TOPICS: Resonance, Flow (Dynamics), Numerical analysis, Pistons, Waves, Vortex shedding, Oscillations, Viscous flow, Fluid-dynamic forces, Navier-Stokes equations, Damping, Separation (Technology), Fluids, Computer simulation, Laminar flow, Oscillating frequencies

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