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Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2017;139(4):041101-041101-8. 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 computational fluid dynamics (CFD) code REEF3D solves the three-dimensional Navier–Stokes equations on a staggered Cartesian grid. Higher order numerical schemes are used for time and spatial discretization. For the interface capturing, the level set method is selected. 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. A breaking focused wave is simulated and the associated kinematics is investigated. Free surface flow features during the interaction of nonbreaking focused waves with a cylinder and during the breaking process of a focused wave are also investigated along with the numerically captured free surface.

Commentary by Dr. Valentin Fuster

Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2017;139(4):041301-041301-10. 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 center of gravity (KG) is not possible. For vessels with low metacentric height (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.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(4):041302-041302-16. doi:10.1115/1.4036376.

Several procedures have been proposed and developed to overcome the challenge in ultradeepwaters testing. A realistic alternative approach uses a hybrid passive methodology through equivalent truncated mooring systems. Often, the searching for equivalent systems involves using a trial-and-error. As an alternative, researches on the use of optimization techniques to establish truncated mooring system with the required properties have been attempted in recent years. In the literature, it is available only approaches considering nongradient-based algorithms. These algorithms usually involve several parameters which require appropriate tuning to provide good performance. Our approach involves optimization algorithms based on gradient. We use a calibration method to perform a static adjustment of design variables to optimally fit truncated mooring system to full-depth mooring system, which proved efficient. A further feature of this work is related to the study of the influence of design variables on the response, through a methodology based on design of experiments (DOE), avoiding the use of irrelevant variables. It should be emphasized that to the authors' knowledge this DOE methodology presented was not seen in other works in this field. We will show that the methodology proposed in this work makes easy to find an equivalent mooring system on truncated water depth. We will present and discuss two fictitious cases, one case based on the literature and another case based on a real scenario. The results show a good agreement between truncated mooring system and full-depth mooring system for the static adjustment.

Commentary by Dr. Valentin Fuster

Research Papers: Materials Technology

J. Offshore Mech. Arct. Eng. 2017;139(4):041401-041401-10. doi:10.1115/1.4036111.

The aim of this study was to analyze the ultimate strength of stiffened aluminum panels by the nonlinear 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 (HAZ) are investigated. The finite element analysis (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 the ultimate strength of stiffened panels; (2) residual stresses may have positive effect on the ultimate strength; and (3) the new stiffened panels also have good performance on ultimate bearing capacities.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(4):041402-041402-9. 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 molybdenum-disulfide (MoS2)/polytetrafluoroethylene (PTFE)—modified topcoat. The investigations involved the following tests: accelerated corrosion protection/aging tests, coating adhesion tests, scanning electron microscopy (SEM)/energy-dispersive X-ray (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 aging on surface morphology, surface chemistry, and hoarfrost accretion were also investigated.

Commentary by Dr. Valentin Fuster

Research Papers: Polar and Arctic Engineering

J. Offshore Mech. Arct. Eng. 2017;139(4):041501-041501-8. doi:10.1115/1.4035771.

This paper investigates the breaking load of ice sheets up to 6 m 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.

Commentary by Dr. Valentin Fuster

Research Papers: Piper and Riser Technology

J. Offshore Mech. Arct. Eng. 2017;139(4):041701-041701-6. doi:10.1115/1.4036205.

The present paper addresses aspects related to local buckling and instability of tensile armors in flexible pipes. Analytical models for evaluating the tensile armor buckling capacity in both transverse (radial and lateral) directions are presented based on formulating the linearized 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 antibuckling tape are formulated and a combined criterion for radial instability is proposed based on considering radial buckling of the tensile armor, wire yield failure, 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 , Failure , Wire , Stress , Armor
Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(4):041702-041702-7. doi:10.1115/1.4036374.

A sudden reduction of the fluid flow yields a pressure shock, which travels along the pipeline with a high-speed. Due to this transient loading, dynamic hoop stresses are developed that may cause catastrophic damages in pipeline integrity. The vibration of the pipe wall is affected by the flow parameters as well as by the elastic and damping characteristics of the material. Most of the studies on dynamic response of pipelines: (a) neglect the effect of the material damping and (b) are usually limited to harmonic pressure oscillations. The present work is an attempt to fill the above research gap. To achieve this target, an analytic solution of the governing motion equation of pipelines under moving pressure shock is derived. The proposed methodology takes into account both elastic and damping characteristics of the steel. With the aid of Laplace and Fourier integral transforms and generalized function properties, the solution is based on the transformation of the dynamic partial differential equation into an algebraic form. Analytical inversion of the transformed dynamic radial deflection variable is achieved, yielding the final solution. The proposed methodology is implemented in an engineering example; and the results are shown and discussed.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(4):041703-041703-18. doi:10.1115/1.4036370.

A model test of a free-hanging riser under vessel motion and uniform current is performed in the ocean basin at Shanghai Jiao Tong University to address four topics: (1) confirm whether vortex-induced vibration (VIV) can happen due to pure vessel motion; (2) to investigate the equivalent current velocity and Keulegan–Carpenter (KC) number effect on the VIV responses; (3) to obtain the correlations for free-hanging riser VIV under vessel motion with VIV for other compliant risers; and (4) to study the similarities and differences with VIV under uniform current. The top end of the riser is forced to oscillate or move, in order to simulate vessel motion or ocean current effects. Fiber Bragg Grating (FBG) strain sensors are used to measure the riser dynamic responses. Experimental results confirm that the free-hanging riser will experience significant out-of-plane VIV under vessel motion. Meanwhile, vessel motion-induced VIV responses in terms of response amplitude, response frequency, and cross section trajectories under different test cases are further discussed and compared to those under ocean uniform current. Most importantly, the correlation among VIV response frequency, vortex shedding pairs, and maximum KC number KCmax is revealed. The presented work is supposed to provide useful references for gaining a better understanding on VIV of a free-hanging riser and for the development of future prediction models.

Commentary by Dr. Valentin Fuster

Research Papers: CFD and VIV

J. Offshore Mech. Arct. Eng. 2017;139(4):041801-041801-12. 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 Free-Surface Random-Vortex Method (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. Furthermore, the detailed two-dimensional vorticity distribution near a bilge keel obtained from the numerical model will also be compared with the existing experimental Digital Particle Image Velocimetry (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 isosurface along the three-dimensional hull will be presented to reveal the motion of fluid particles and vortex filaments near the keels.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;139(4):041802-041802-9. doi:10.1115/1.4036373.

Production risers as well as drilling risers are often exposed to ocean currents. Vortex-induced vibrations (VIVs) have been observed in the field and can cause fatigue failure and excessive drag on the riser. In order to suppress VIV, fairings are often used. This paper presents qualification tests for two types of fairings: the short-crab claw (SCC) fairings and the AIMS dual flow splitter (ADFS) fairings. The short-crab claw fairing design is a novel design patented by the Norwegian deepwater project (NDP). As will be detailed in this paper, both the SCC and ADFS designs offer very low drag, completely suppress VIV, and are effective even when they are in tandem. A model test campaign was undertaken in the 200-m towing tank facility at the ocean, coastal, and river engineering in St. John's, NF, Canada. A rigid pipe with a diameter of 0.3556 m (14 in) was utilized for the experiments. This corresponds to prototype size for a production riser and a 1:3.8 scaled model for a 1.3716 m (54 in) drilling riser. Given that these tests were conducted at prototype scale, they were used to qualify the fairings for field deployment. Both fairings (SCC and ADFS) were very effective in suppressing VIV and reducing drag. The ADFS fairings are most effective for a span to diameter ratio of 1.75. For all fairing geometries, it was found that a small taper increases the fairing effectiveness considerably.

Commentary by Dr. Valentin Fuster

Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2017;139(4):041901-041901-8. doi:10.1115/1.4035772.

This paper presents numerical studies of the dynamic responses of two jacket-type offshore wind turbines (OWTs) 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., 2012, “Lattice Towers for Bottom-Fixed Offshore Wind Turbines in the Ultimate Limit State: Variation of Some Geo metric Parameters,” ASME J. Offshore Mech. Arct. Eng., 134(2), p. 021202. 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.

Commentary by Dr. Valentin Fuster

Research Papers: Offshore Geotechnics

J. Offshore Mech. Arct. Eng. 2017;139(4):042001-042001-7. 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 freestream 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.

Commentary by Dr. Valentin Fuster

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