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Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2019;142(1):011601-011601-15. doi:10.1115/1.4043805.

In the present study, the wave interaction with the very large floating structures (VLFSs) is analyzed considering the small amplitude wave theory. The VLFS is modeled as a 2D floating elastic plate with infinite width based on Timoshenko–Mindlin plate theory. The eigenfunction expansion method along with mode-coupling relation is used to analyze the hydroelastic behavior of VLFSs in finite water depth. The contour plots for the plate covered dispersion relation are presented to illustrate the complexity in the roots of the dispersion relation. The wave scattering behavior in the form of reflection and transmission coefficients are studied in detail. The hydroelastic performance of the elastic plate interacting with the ocean wave is analyzed for deflection, strain, bending moment, and shear force along the elastic plate. Further, the study is extended for shallow water approximation, and the results are compared for both Timoshenko–Mindlin plate theory and Kirchhoff’s plate theory. The significance and importance of rotary inertia and shear deformation in analyzing the hydroelastic characteristics of VLFSs are presented. The study will be helpful for scientists and engineers in the design and analysis of the VLFSs.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;142(1):011602-011602-12. doi:10.1115/1.4044074.

A rigid jumper is an important part of a subsea production system, and it may experience significant vortex-induced vibrations (VIVs) if subjected to current. It has normally a non-straight geometry shape in three-dimensional space. Consequently, the response of a rigid jumper under VIVs is much more complicated compared with straight pipeline structures. Currently, there are very limited studies and design guidelines including methods on how to assess the fatigue damage of rigid jumpers under VIVs. The methodology used for straight pipelines is often applied by ignoring the non-straight geometry characteristics and the multiaxial stress states. However, both experimental and numerical results show that the torsional stress does exist besides the flexural stress for rigid jumpers under VIVs. The objective of this study is to do a fatigue assessment practice based on the state-of-the-art calculation methods to a rigid jumper on model scale. The VIV response is obtained from experimental tests and numerical calculations by either force or response model methods. The influence of torsional stress on fatigue assessment is studied. Two approaches have been investigated. In the first approach, the flexural and torsional stresses are evaluated separately. The second approach uses the first principal stress to calculate the fatigue damage; thus, the flexural and torsional stresses are evaluated together. It appears that the use of the first principal stress gives higher fatigue damage if the torsional stress contribution is significant. Furthermore, the principal stress method is also less time-consuming in processing the results.

Commentary by Dr. Valentin Fuster

Research Papers: CFD and VIV

J. Offshore Mech. Arct. Eng. 2019;142(1):011801-011801-11. doi:10.1115/1.4043943.

In this study, the ducted propeller has been numerically investigated under oblique flow, which is crucial and challenging for the design and safe operation of the thruster driven vessel and dynamic positioning (DP) system. A Reynolds-averaged Navier–Stokes (RANS) model has been first evaluated in the quasi-steady investigation on a single ducted propeller operating in open water condition, and then a hybrid RANS/LES model is adapted for the transient sliding mesh computations. A representative test geometry considered here is a marine model thruster, which is discretized with structured hexahedral cells, and the gap between the blade tip and nozzle is carefully meshed to capture the flow dynamics. The computational results are assessed by a systematic grid convergence study and compared with the available experimental data. As a part of the novel contribution, multiple incidence angles from 15 deg to 60 deg have been analyzed with different advance coefficients. The main emphasis has been placed on the hydrodynamic loads that act on the propeller blades and nozzle as well as their variation with different configurations. The results reveal that while the nozzle absorbs much effort from the oblique flow, the imbalance between blades at different positions is still noticeable. Such unbalance flow dynamics on the blades, and the nozzle has a direct implication on the variation of thrust and torque of a marine thruster.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2019;142(1):011802-011802-11. doi:10.1115/1.4044073.

The duck's webbed feet are observed by using electron microscopy, and observations indicate that the edges of the webbed feet are the shape of protuberances. Therefore, the rudder with leading-edge protuberances is numerically studied in the present investigation. The rudder has a sinusoidal leading-edge profile along the spanwise direction. The hydrodynamic performance of rudder is analyzed under the influence of leading-edge protuberances. The present investigations are carried out at Re = 3.2 × 105 and 8 × 105. In the case of Re = 3.2 × 105, the curves of lift coefficient illustrate that the protuberant leading-edge scarcely affects the lift coefficient of bionic rudder. However, the drag coefficient of the bionic rudder is markedly lower than that of the unmodified rudder. Therefore, the lift-to-drag ratio of the bionic rudder is obviously higher than the unmodified rudder. In another case of Re = 8 × 105, the advantageous behavior of the bionic rudder with leading-edge protuberances is mainly performed in the post-stall regime. The flow mechanism of the significantly increased efficiency by the protuberant leading-edge is explored. It is obvious that the pairs of counter-rotating vortices are presented over the suction surface of bionic rudder, and therefore, the flow is more likely to adhere to the suction surface of bionic rudder.

Commentary by Dr. Valentin Fuster

Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2019;142(1):011901-011901-9. doi:10.1115/1.4044075.

The dynamic response of a tension leg platform (TLP) floating offshore wind turbine (FOWT) was analyzed with considering the aero-hydro characteristic of the whole floating wind turbine system including the wind turbine, TLP platform, and tethers. The “aero-hydro” coupled dynamic analysis was conducted in ansys-aqwa with a dynamic link library (DLL) calculating the aerodynamics loading at every steptime based on the blade element momentum theory. Results from the coupled dynamic analysis of TLP FOWT under the condition of turbulent wind and regular wave show that the wind loads influence mainly the low-frequency response of the TLP FOWT. The wind loads have a large impact on the offsets of the TLP away from the initial position while the wave loads influence mainly the fluctuation amplitude of the TLP FOWT. The average TLP pitch response under the wind load is significantly larger due to the large wind-induced heeling moment on the wind turbine. In addition, the tension of tethers at the upwind end is greater than that at the downwind end. The wind loads could reduce effectively the average tension of the tethers, and the tension of tethers is significantly affected by the pitch motion. Results from the coupled dynamic analysis of TLP FOWT under the condition of turbulent wind and irregular wave show that the surge and pitch of TLP result in an obvious increase of thrust of the turbine and the amplitude of torque fluctuation, more attention should be paid to the pitch and surge motion of TLP FOWT.

Commentary by Dr. Valentin Fuster

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