ASME Conference Presenter Attendance Policy and Archival Proceedings

2016;():V002T00A001. doi:10.1115/OMAE2016-NS2.

This online compilation of papers from the ASME 2016 35th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2016) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

CFD and VIV: Advanced Computation

2016;():V002T08A001. doi:10.1115/OMAE2016-54005.

Offshore and Naval engineering have relied on physical models, i.e. experimental fluid dynamics (EFD), for several decades. Although the role of experiments in engineering is still unquestionable, some of the limitations of physical models, as for example domain size (blockage and scale effects), can be addressed using mathematical models, i.e. computational fluid dynamics (CFD). However, to gain confidence in the use of CFD it is fundamental to determine the modelling accuracy, i.e. to determine the difference between the “physical reality” and the selected mathematical model. The quantification of the modelling error is the goal of Validation. It must be emphasized that Validation applies to the mathematical model (and not the code) and is performed for selected flow quantities (the so-called quantities of interest).

Ideally, Validation would be performed comparing an exact measurement of the “physical reality” with the exact solution of the selected mathematical model. However, exact measurements do not exist and mathematical models for turbulent flows do not have analytical solutions. Therefore, procedures must be developed to take into account experimental and numerical uncertainties. Furthermore, the exact values of the flow parameters as for example Reynolds number, fluid viscosity or inlet turbulence quantities are often unknown, which leads to the so-called parameter uncertainty that also has to be dealt within the assessment of the modelling error.

The main goal of this paper is to demonstrate that the very popular designation of “code X is validated” is meaningless without saying what is the mathematical model embedded in the code, what are the quantities of interest for the specific application and what is the Validation uncertainty imposed by the experimental, numerical and parameter uncertainties. Furthermore, we also illustrate that Validation is not a pass or fail exercise. A modelling error of 10% may be acceptable for a given application, whereas 1% may not be enough for a different one.

To this end, we present the application of the ASME V&V 20 Validation procedure for local set points and the metric for multiple set points to several practical test cases: prediction of transition from laminar to turbulent regime for the flow over a flat plate; flow around a circular cylinder; flow around the KVLCC2 tanker and current loads in shallow water for a LNG carrier. In most of these exercises, parameter uncertainty is assumed to be zero, which is an assumption often required for the so-called practical calculations due to the computational effort required to address it. Nonetheless, as an illustration of its application, the flow over the flat plate includes parameter uncertainty for the specification of the inlet turbulence quantities.

Commentary by Dr. Valentin Fuster
2016;():V002T08A002. doi:10.1115/OMAE2016-54008.

Hybrid RANS-LES methods are gaining popularity for the simulation of the complex bluff body flows at high Reynolds numbers due to their reduced computational cost and good accuracy. A number of such methods have been proposed in the literature. Each of these methods have enjoyed varying degree of success for different applications. One of the most important parameter which determines the switching between near-wall RANS region and off-body LES region is the length scale parameter. This parameter can be grid based or physics based and numerous choices exist for defining this parameter. This study proposes to investigate the effect of this parameter on the size of the RANS and LES regions and also on the solution accuracy. Four test problems are chosen covering attached, mildly separated and massively separated flow regimes. Results will help us to identify length scale definitions to be used for different flow scenarios.

Commentary by Dr. Valentin Fuster
2016;():V002T08A003. doi:10.1115/OMAE2016-54074.

Computational fluid dynamics (CFD) plays an important role in predicting the fluid characteristics throughout the engineering practice. With the developing of computers and CFD software, it has become a powerful tool in the hydrodynamics area. In this paper, the hydrodynamic characteristics of dredging dustpan internal flow field on the coastal engineering are further studied using CFD as a modeling and calculation tool. It is implemented in such way that the ICEM CFD software has been firstly employed to establish the full dredging dustpan model and gridding, the numerical simulation of internal flow field is then accurately performed by the FLUENT code under the conditions of different slurry concentration and particle diameter working on the dredging dustpan. Based on the calculation results, the effect of different slurry concentration and particle diameter on the dredging and transporting efficiency of dustpan is presented and discussed. It is shown that when the slurry concentration is low, particle diameter plays a leading role, and suction efficiency is proportional to particle diameter. However, the slurry concentration plays an important role and is inversely proportional to the suction efficiency as it is in a high level. It has been demonstrated that the present study is efficient and accurate for the numerical simulation of the dredging dustpan internal flow field.

Commentary by Dr. Valentin Fuster
2016;():V002T08A004. doi:10.1115/OMAE2016-54075.

The main focus of this paper is on the uncertainty analysis methodology and procedure in CFD recommended by 22nd ITTC and the benchmark database for the verification and validation of the results of dredging dustpan’s inlet and outlet cross-section velocity ratio coefficient viur. Compared with the previous uncertainty analysis of CFD focused on the fluid grid-convergence in the steady flow, which is less to consider other factors that may affect the accuracy of the results of numerical simulation, this study compensates for this deficiency and implements the grid-convergence and time-step-size-convergence studies respectively by using three types of grids and time step sizes with refinement ratio under the condition of unsteady flow. Through confirming the validity of CFD uncertainty analysis, the agreement between the numerical simulation correction values from the grid-convergence and time-step-size-convergence and the benchmark test data is found to be quite satisfactory. The results obtained in this study have shown that it is indispensable to carry out the time-step-size-convergence studies for CFD uncertainty analysis during the unsteady flow calculation because the numerical simulation errors respectively caused by the grid and time-step-size in the convergence study have the same order of magnitude. In further the present study of simultaneously conducting both grid-convergence and time-step-size-convergence is demonstrated efficient and effective in the CFD uncertainty analysis.

Commentary by Dr. Valentin Fuster
2016;():V002T08A005. doi:10.1115/OMAE2016-54259.

A CFD study to understand the hydrodynamics and fluid flow around a chordwise flexible hydrofoil with combined sway and yaw motion which imitates the caudal fin flapping in thunniforms, is presented. The dependency of motion parameters of the flexible flapping hydrofoil to its propulsive performance is studied by carrying out the analyses over a Strouhal number range of 0.1 to 0.4 in steps of 0.025 at three maximum angle of attacks viz. 10°,15°,20°. Qualitative observations of the wake field and trailing jet is presented using velocity magnitude contours and vorticity contours. The analyses carried out at 40,000 Reynolds number and sway amplitude of 0.75 chordlength, revealed that the average thrust coefficient increases with increase in Strouhal number and maximum angle of attacks. The highest efficiency is achieved when the maximum angle of attack is 15° and Strouhal number is 0.225.

Commentary by Dr. Valentin Fuster
2016;():V002T08A006. doi:10.1115/OMAE2016-54346.

The accurate evaluation of wind loads applied on floating offshore structures is extremely important as they are in specific conditions one of the dimensioning criteria for the mooring design. Nowadays these loads are mainly assessed through wind tunnel tests performed at model scale. Estimating realistic wind loads however, remains a big challenge. The complexity and associated simplification level of FPSO topside structures, the scale effects and the establishment of the atmospheric boundary layer imply that many simplifications are to be made. Typically, the FPSO topside is greatly simplified and equivalent blocs of wired frame are used. Today with the evolution of CFD software, and the increase of the meshing capacity, new scopes open to CFD. Aerodynamic simulations on complex FPSO structures are therefore now possible, but need specific developments and validations that are presented in this paper.

The main objective of the work presented is to investigate the ability of CFD to evaluate wind loads on complex FPSOs topsides and to provide information on the impact of model simplifications made in wind tunnels. In a first stage, the numerical model was intensively validated by comparing its results to a wind tunnel test case. The numerical model was developed in order to ensure the quality of the results and enable a relevant comparison that was obtained with grids density up to 30 million cells. For this purpose, the geometric model used corresponds to the one used in wind tunnel. The same Atmospheric Boundary Layer was simulated and a thorough effort was performed to ensure the mesh convergence. In a second stage, more physical aspects of the wind tunnel methodology were investigated. Typically the accuracy of the blockage effect correction was evaluated by performing computations with and without blockage, and results were compared with classical corrections applied in wind tunnel. The impacts of the Atmospheric Boundary Layer on wind loads have also been investigated. Finally, the wind load contribution of each component of the FPSO was evaluated.

Topics: Stress , FPSO , Wind , Wind tunnels
Commentary by Dr. Valentin Fuster
2016;():V002T08A007. doi:10.1115/OMAE2016-54414.

The present work investigates the transitional flow around a smooth circular cylinder at Reynolds number Re = 140,000. The flow is resolved using the viscous-flow solver ReFRESCO, and distinct mathematical models are applied to assess their ability to handle transitional flows. The selected mathematical models are the Reynolds-Averaged Navier-Stokes equations (RANS), Scale-Adaptive Simulation (SAS), Delayed Detached-Eddy Simulation (DDES), eXtra Large-Eddy Simulation (XLES) and Partially-Averaged Navier-Stokes (PANS) equations. The RANS equations are supplemented with the k–ω Shear-Stress Transport (SST) with and without the Local Correlation Transition Model (LCTM). The numerical simulations are carried out using structured grids ranging from 9.32 × 104 to 2.24 × 107 cells, and a dimensionless time-step of 1.50 × 10−3. As expected, the outcome demonstrates that transition from laminar to turbulent regime is incorrectly predicted by the k–ω SST model. Transition occurs upstream of the flow separation, which is typical of the supercritical regime and so the flow physics is incorrectly modelled. Naturally, all Scale-Resolving Simulation (SRS) models that rely on RANS to solve the boundary-layer, called hybrid models, will exhibit a similar trend. On the other hand, mathematical models capable to resolve part of the turbulence field in the boundary layer (PANS) lead to a better agreement with the experimental data. Furthermore, the k–ω SST LCTM is also able to improve the modelling accuracy when compared to the k–ω SST. Therefore, it might be a valuable engineering tool if its computational demands are considered (in the RANS context). Therefore, the results confirm that the choice of the most appropriate mathematical model for the simulation of turbulent flows is not straightforward and it may depend on the details of the flow physics.

Commentary by Dr. Valentin Fuster
2016;():V002T08A008. doi:10.1115/OMAE2016-54442.

A partitioned iterative scheme based on Petrov-Galerkin formulation [1] has been employed for simulating flow past a freely vibrating circular cylinder placed in proximity to a stationary plane wall in both two-dimension (2D) and three-dimension (3D). In the first part of this work, effects of wall proximity on the vortex-induced vibration (VIV) of an elastically mounted circular cylinder with two degree-of-freedom (2-DoF) are systematically studied in 2D by investigating the hydrodynamic forces acting on the cylinder, the vibration amplitudes, the phase differences between the forces and displacements, the response frequencies as well as the vortex shedding dynamics. For that purpose, a careful comparison has been established for the isolated and near-wall cylinders, in which the gap ratio, e/D (where e denotes the gap between the cylinder and the wall and D denotes the diameter of the cylinder), is set to be 0.9, at Re = 200. Our 2D simulations have revealed that larger streamwise vibration amplitude and smaller streamwise vibration frequency can be observed in VIV of the near-wall cylinder compared to its isolated counterpart. We then focus on the explanation of the enhanced streamwise vibration amplitude when the cylinder is placed in the vicinity of the plane wall. It is found that the wall proximity largely amplifies the streamwise vibration amplitude due to net energy transfer from the fluid to the cylinder in the pre-lock-in region as well as the initial branch of the lock-in region, while reduces the streamwise vibration frequency to the level of the transverse vibration frequency. In the second part, the main focus of this article, following Tham et al. (2015) [2] where 2D results were systematically reported, we perform 3D simulations of VIV of a circular cylinder for both isolated and near-wall cases (e/D = 0.9) at Re = 1000 to compare the hydrodynamic forces and vibration characteristics in 3D with the results corresponding to the 2D study. We show that wall proximity effects on VIV are also pronounced in 3D with the following observations: (1) the wall proximity increases the mean lift to a lesser extent compared to 2D, while also enhances the mean drag unlike in 2D; (2) the wall proximity enhances the streamwise oscillation as well owing to a combined effect of increased drag force together with energy transfer from fluid to structure as in 2D; (3) in terms of the flow field, the wall proximity increases the wavelength of streamwise vorticity blob; and (4) similarly with the mechanism of vortex suppression in 2D, wall boundary layer vorticity strongly strengthens the negative vorticity shed from upper surface of cylinder, stretching and suppressing the positive vorticity shed from the bottom surface of cylinder.

Commentary by Dr. Valentin Fuster
2016;():V002T08A009. doi:10.1115/OMAE2016-54512.

The flow around a two-dimensional smooth square column with rounded corners at subcritical Reynolds number is simulated using the finite-volume open-source software OpenFOAM. The effects of turbulence are accounted for by using a two-equation turbulence closure specifically adapted to account for the interactions between periodic vortex shedding and random turbulence. The computational approach is first validated against data for the flow past a sharp square column. The purpose of this paper is to assess the model’s applicability to the more challenging case where the column corners are rounded. Of special interest is the prediction of the pressure distribution and the unsteady force coefficients induced by periodic vortex shedding from this geometry. The computed results show that the effects of rounded corners are to progressively reduce the drag and lift coefficients, while increasing the base pressure levels relative to the case of sharp corners.

Commentary by Dr. Valentin Fuster
2016;():V002T08A010. doi:10.1115/OMAE2016-54520.

This paper presents currently available methods for assessment of flow-induced vibration (FIV). Approaches considered include the application of the Energy Institute’s guidance [1], as well as advanced simulation; where flow simulation using computational fluid dynamics (CFD) and structural response using finite element (FE) analysis are coupled. Using a test case where initial screening by the guidelines results in a recommendation for further analysis, an approach using both CFD and vibro-acoustic analysis is described. This process enables the practitioner to quantify the different vibration mechanisms present, as well as to understand the risk of fatigue failure and what corrective action, if any, is required.

Commentary by Dr. Valentin Fuster
2016;():V002T08A011. doi:10.1115/OMAE2016-54643.

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 study the flow structures around the bottom-fixed offshore wind turbine foundations in order to provide essential hydrodynamic coefficients for engineering design and an assessment of potential scour erosion. Three-dimensional CFD simulations have been performed using Spalart-Allmaras Delayed Detached Eddy Simulation (SADDES) at a Reynolds number 4×106 based on the free stream 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 (upstream side) 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 (computed as the distance from the separation point to the foundation surface along the centerline on the seabed), 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
2016;():V002T08A012. doi:10.1115/OMAE2016-54695.

When a riser array system is subjected to a uniform flow, an unstable flow-induced vibration commonly occurs among cylinders, generally called fluid-elastic instability. It can cause long-term or short-term damage to the riser array system. A numerical investigation has been performed in the present study. Generally, flow-induced vibrations include vortex-induced vibration (VIV), wake-induced vibration (WIV), jet switching, turbulent buffeting and fluid-elastic instability. The dynamic interactions among the fluid-induced vibrations, wake interference and proximity interference pose difficulties in the design and operation of the riser array system. The dynamics of a riser array system is very different from that of basic canonical configurations such as side-by-side, tandem and staggered arrangements. In a riser array system, the interferences come from all possible nearby constituent risers. There is a synchronization phenomenon among the cylinders, which may lead to detrimental collisions and short-term failures. It is known that the vortex-induced vibration (VIV) of an isolated circular cylinder is self-limiting. An extensive vibration occurs in the lock-in region within which the frequency of the vortex shedding matches the structural frequency of the immersed structure. In a riser array system, there is a point at which the vibration of cylinder suddenly increases. The vibration of the constituent risers increases without bound with the increment of the free-stream velocity. This free-stream velocity is defined as the critical velocity. The interference not only comes from the inline and cross-flow directions, but also the wake interference from the diagonal upstream risers. In a riser array system, each riser vibrates independently. However, there is symmetry of frequency spectrum observed about the inline direction along the middle row of the risers.

In this study, the dynamic response of the different risers in the array system is investigated with the help of the amplitude response results from the canonical arrangements (side-by-side and tandem) and wake flow structures. The long top-tensioned riser system can be idealized by two-dimensional elastically mounted cylinders to solve the complex fluid-structure interaction problem. The dynamic response of a typical riser array system has been analyzed at low and high Reynolds number. It is encouraging to see that the results reported in the present investigation can provide useful insight and suggestions in the design and optimization of riser systems to avoid collisions and various long-or short-term failures.

Commentary by Dr. Valentin Fuster
2016;():V002T08A013. doi:10.1115/OMAE2016-54736.

A numerical investigation of vortex-induced vibration (VIV) of a pair of identical circular cylinders placed side by side in an uniform flow has been performed. One of the cylinder is elastically mounted and only vibrates in the transverse direction, while its counterpart remains stationary. When two cylinders are placed sufficiently close to each other, a flip-flopping phenomenon can be an additional time-dependent disturbance in the range of 0.2 ≲ g* ≲ 1.2. This phenomenon was well-reported by the experimental work of Bearman and Wadcock [1] in a side-by-side circular cylinder arrangement, in which the gap flow biased toward one of the cylinders and switched the sides intermittently. Albeit one of the two cylinders is free to vibrate, this flip-flopping during VIV dynamics can still be observed. In the side-by-side arrangement, the lock-in region shrinks due to the presence of its stationary counterpart and occurs prematurely compared to that of an isolated counterpart. Similar to the tandem cylinder arrangement, in the post lock-in region, the vibration amplitude is amplified compared to the isolated counterpart. For the vibrating cylinder in the side-by-side arrangement, the biased gap flow shows a quasi-stable flow regime within the lock-in region, instead of a bi-stable regime which is reported in the stationary side-by-side arrangement. When these factors take place simultaneously, the dynamics of freely vibrating cylinder becomes complex and such a side-by-side canonical arrangement is common in offshore engineering applications, for example a floating platform operating in the side of FPSO, arrays of riser and pipelines, ships travelling in rows within close proximity and many other side-by-side operations. The chaotic fluctuation and large vibration may occur when two bluff bodies are placed closely. It often causes inevitable damages and potential risks to the offshore structures and may leads to a collision or long-term fatigue failure associated with flow-induced vibrations.

Commentary by Dr. Valentin Fuster
2016;():V002T08A014. doi:10.1115/OMAE2016-54853.

It is well known that fairing devices are better alternatives than helical strakes due to their low-drag performance while suppressing vortex-induced vibration (VIV). Our objective is to present a systematic numerical study to understand the hydrodynamic performance and physical mechanism of fairing configurations and then propose a new device for suppressing VIV and reducing drag force. In this work, we simplify our investigation by allowing the cylinder-fairing system to oscillate in cross-flow direction without rotation. Firstly, we present a set of simulations of vortex-induced vibration for Short Crab Claw (SCC) fairings [1] with different nondimensional length (Lf/D), where Lf is the length of fairing and D denotes the diameter of cylinder. To establish the relation between the length of fairing and the performance with respect to VIV suppression and drag reduction, we consider the length ratio Lf/D = 1.25, 1.50, 2.00. The underlying VIV suppression mechanism is investigated with the aid of force and amplitude variations, wake flow structures and frequency ratios. Our results show that the SCC fairing with longer length performs better by suppressing the amplitude up to 84% and reduces the drag coefficient by 40%. This finding implies that by offsetting the vortices shed away from the main cylinder, it lowers the influence of vortex interactions, which leads to the suppression of VIV and net reduction in the drag force generation. Based on this mechanism, we propose a new design of fairing, namely the “Hinged C-shaped”, which consists of a thin splitter plate (connected at the base of main cylinder) bifurcating into a C-shaped geometry after a certain distance. Through our numerical study on its hydrodynamic performance, it is shown to be efficient with respect to VIV suppression and drag reduction. To understand the VIV suppression physics, the numerical study is conducted in two-dimension for the cylinder-fairing mounted elastically with mass ratio m* = 2.6 and the damping ξ = 0.001 at low Reynolds number. We further demonstrate the performance of the new fairing device in three-dimension at sub-critical Reynolds number.

Commentary by Dr. Valentin Fuster
2016;():V002T08A015. doi:10.1115/OMAE2016-55054.

Unsteady flow around an oscillating plate cascade has been computationally studied, aimed at examining the predictive ability of a non-linear frequency solution method for hydro-elasticity analysis compared with a standard analytical solution. The comparison of computational and analytical solutions for flow around an oscillating plate configuration demonstrates the capabilities of the frequency domain method compared with the analytical solution in capturing the unsteady flow. It also shows the great advantage of significant CPU time saving by the frequency methods over the nonlinear time method. This approach is based on casting the unsteady flow equations into a set of steady-like equations at a series of phases of a period of unsteadiness. So, One of the advantages of this method compared with other conventional time-linearized frequency domain methods is that any steady flow solution method can be easily used in a straightforward simple method for modelling unsteady perturbations.

Commentary by Dr. Valentin Fuster
2016;():V002T08A016. doi:10.1115/OMAE2016-55114.

The study of flow around circular cylinders is one of the challenges in marine hydrodynamics. Many offshore structures are of cylindrical shape such as spar hull, risers and pipelines. Furthermore, the flow around cylinder open the path for studying more complex shape structures like ellipses, flat plat and other complex geometries. The aim of the study is to understand the flow field around these structures and calculation of forces acting on them. In current study, flow over a circular cylinder is investigated numerically using the turbulent model Large Eddy simulation (LES) and SIMPLE pressure-velocity coupling. In LES option, Smagorinksy-lilly subgrid scale model with dynamic stress is used. Mesh is generated using ICEM CFD tool and analysis is performed using ANSYS fluent. Velocity profiles are investigated in the cylinder wake up to ten diameters and compared with the experimental results. Hydrodynamic values and pressure distribution on the cylinder wall are also analyzed.

The numerical results extracted from the simulation agreed well with the experimental results available in the literature. Furthermore, the spanwise mesh resolution study shows that flow around cylinder at Reynold number Re = 3900 is highly 3dimensional study and 2D study or coarse mesh in spanwise direction will deteriorate the results.

Commentary by Dr. Valentin Fuster

CFD and VIV: Free Surface Flows

2016;():V002T08A017. doi:10.1115/OMAE2016-54423.

In the present study, the irregular wave forces on a fully submerged circular cylinder are investigated using the open-source computational fluid dynamics (CFD) model REEF3D. A complete three dimensional representation of the ocean waves requires the consideration of the sea surface as an irregular wave train with the random characteristics. The numerical model uses the incompressible Reynolds-averaged Navier-Stokes (RANS) equations together with the continuity equation to solve the fluid flow problem. Turbulence modeling is carried out using the two equation k-ω model. Spatial discretization is done using an uniform Cartesian grid. The level set method is used for computing the free surface. For time discretization, third-order total variation diminishing (TVD) Runge Kutta scheme is used. Ghost cell boundary method is used for implementing the complex geometries in the numerical model. MPI is used for the exchange of the value of a ghost cell. Relaxation method is used for the wave generation. The numerical model is validated for the irregular waves for a wave tank without any structure. Further, the numerical model is validated by comparing the numerical results with the experimental data for a fully submerged circular cylinder under regular waves and irregular waves. The numerical results are in a good agreement with the experimental data for the regular and irregular wave forces. The JONSWAP spectrum is used for the wave generation. The free surface features and kinematics around the cylinder is also presented and discussed.

Commentary by Dr. Valentin Fuster
2016;():V002T08A018. doi:10.1115/OMAE2016-54451.

The main purpose of the study is to investigate the breaking wave interaction with a group of four circular cylinders. The physical process of wave breaking involves many parameters and an accurate numerical modelling of breaking waves and the interaction with a structure remain a challenge. In the present study, the open-source (Computational Fluid Dynamics) CFD model REEF3D is used to simulate the breaking wave interaction with the multiple cylinders. The numerical model is based on the incompressible Reynolds Averaged Navier-Stokes (RANS) equations, the level set method for the free surface and the k–ω model for turbulence. The model uses a 5th-order conservative finite difference WENO scheme for the convective discretization and a 3rd-order Runge-Kutta scheme for time discretization.

The numerical model is validated with experimental data of large-scale experiments for the free surface elevation and the breaking wave force on a single cylinder. A good agreement is seen between the numerical results and experimental data. Two different configurations with four cylinders are examined: in-line square configuration and diamond square configuration. The breaking wave forces on each cylinder in the group are computed for the two cases and the results are compared with the breaking wave force on a single isolated cylinder. Further, the study investigates the water surface elevations and the free surface flow features around the cylinders. In general, the cylinders in both configurations experience the maximum forces lower than the maximum force on a single cylinder. The results of the present study show that the interference effects from the neighbouring cylinders in a group strongly influence the kinematics around and the breaking wave forces on them.

Topics: Waves , Modeling , Cylinders
Commentary by Dr. Valentin Fuster
2016;():V002T08A019. doi:10.1115/OMAE2016-54533.

In this paper, numerical simulations of non-linear sloshing in rectangular tanks are presented. Model implementations in the open source software REEF3D are tested and results compared with experimental data. Three different conditions are compared with experiments in 2D. First, the free surface time-evolution is compared for both linear and non-linear sloshing. In the last case, video images from the SPHERIC project are compared with simulations images of the free surface. A condition with lateral wave impacts in sloshing, with a frequency closer to the natural frequency of the first mode, can be found in this case. The non-linear sloshing, case 2, is also simulated in 3D. The numerical model is solving the RANS equations with the k-ω turbulence model. The level set method is used to capture the interface. Higher order discretization schemes are implemented to handle time-evolution and convective fluxes. A ghost cell method is used to account for solid boundaries and multiple grids for parallel computations. It is found that the limiting factor for the eddy-viscosity has significant influence in case 2 and 3. As the sloshing becomes more violent, the increased strain at the gas-liquid interface overproduces turbulence energy with unrealistically high damping of the motion. 3D simulations are only performed in case 2, which shows slightly better comparison than with 2D. Due to non-linearities and small damping, the time to reach steady-state may take several cycles, but no information is given in the paper [1]. The last case shows promising results for the global motion. As expected, the break up of the liquid surface makes it difficult to resolve each phase. But overall, the numerical model predicts the sloshing motion reasonably well.

Commentary by Dr. Valentin Fuster
2016;():V002T08A020. doi:10.1115/OMAE2016-54675.

Many offshore constructions and operations involve water impact problems such as water slamming onto a structure or free fall of objects with subsequent water entry and emergence. Wave slamming on semi-submersibles, vertical members of jacket structures, crane operation of a diving bell and dropping of free fall lifeboats are some notable examples. The slamming and water entry problems lead to large instantaneous impact pressures on the structure, accompanied with complex free surface deformations. These need to be studied in detail in order to obtain a better understanding of the fluid physics involved and develop safe and economical design. In the special case of free-fall lifeboats, model testing can be expensive and time consuming. Here, numerical modelling can make useful contributions to the design process. The slamming of a free falling body into water involves several complex hydrodynamic features after its free-fall such as water entry, submergence into water and resurfacing. The water entry and submergence lead to formation of water jets and air cavities in the water resulting in large impact forces on the object. In order to evaluate the forces and hydrodynamics involved, the numerical model should be able to account for the complex free surface features, the instantaneous pressure changes around the lifeboat and accurately evaluate the loads on the lifeboat. As a step towards simulating free-fall lifeboats, water entry of a free-falling wedge into water is studied in this paper using a CFD model. The vertical velocity of the wedge during the process of free fall and water impact are calculated for different cases and the free surface deformations are captured in detail. Numerical results are compared with experimental data and a good agreement is seen. The open-source CFD model REEF3D is used in this study. The model solves the Reynolds-Averaged Navier-Stokes equations to evaluate the fluid flow. The convective terms are discretized using a 5th-order conservative finite difference WENO scheme. Time discretization is carried out using a 3rd-order Runge-Kutta scheme. Pressure discretization is carried out using Chorins projection method. The Poisson pressure equation is solved using a pre-conditioned BiCGStab algorithm. A sharp representation of the free surface is obtained using the level set method. The falling wedge is represented using the level set paradigm as well, avoiding the need for re-meshing during the simulation. Turbulence modeling is carried out using the k-ω model. Computational performance of the numerical model is improved by parallel processing using the MPI library.

Commentary by Dr. Valentin Fuster
2016;():V002T08A021. doi:10.1115/OMAE2016-54780.

A time-domain seakeeping numerical model based on a computational fluid dynamics (CFD) software FINE/Marine has been developed for nonlinear steady and unsteady viscous flows. Simulation of multi-phase flows around a Wigley hull with forward speed is performed by solving the Reynolds-average Navier-Stokes (RANS) and continuity equations with k-ω (SST-Menter) turbulence model. The water free surface is captured by Blend Reconstruction Interface Capturing Scheme (BRICS). Both steady and unsteady problems including wave-making, radiation and diffraction problems are simulated. Ship waves generated by the Wigley model advancing at a constant forward speed in calm water or incident waves are computed. The numerical results including the wave-making resistance and wave patterns for steady problem, hydrodynamic coefficients and forces for unsteady problems are illustrated and compared with experimental measurements in good agreement. It is confirmed that the present numerical model has the capability of evaluating the seakeeping performance of ships.

Commentary by Dr. Valentin Fuster
2016;():V002T08A022. doi:10.1115/OMAE2016-54808.

Numerical simulation of free surface water waves using various wave models is studied. The performance of the wave models is compared in three test cases in 2D, all of which include a focusing event. Then the attention is focused on coupling one of these wave models to a URANS/VOF code, ReFRESCO. Here the strong suits of both solvers are utilized to simulate nonlinear water waves in an accurate and efficient manner. Even though only simulating nonlinear waves is discussed in this paper, the main purpose of the coupling approach is to simulate hydrodynamic wave loading on structures and wave-structure interaction. Computational cost, convergence behavior and effects of grid refinement are some of the topics that are also discussed throughout the paper.

Commentary by Dr. Valentin Fuster
2016;():V002T08A023. doi:10.1115/OMAE2016-54827.

A numerical wave tank that specialized to Navier-Stokes equation was established by a CIP (Constrained Interpolation Profile) method in present study. The numerical model is used to simulate the strongly nonlinear interaction between the solitary wave and a horizontal two-dimensional plate. Most of all, hydrodynamic forces acting on the plate due to solitary wave are investigated in both submerged cases and elevated cases. Sufficient numerical simulations with parameters varied, including different water depths, submergence depths, elevations above the still water level and wave amplitudes are carried out. The time series for hydrodynamic forces and the extremum of horizontal and vertical force are presented in this paper for some significant conclusions. The numerical results are compared with experimental data is in good agreement.

Commentary by Dr. Valentin Fuster
2016;():V002T08A024. doi:10.1115/OMAE2016-54869.

Recently, a method for numerical reproduction of measured irregular wave events has been developed. The measured motion of the wave maker flaps defines the wave kinematics at the boundary of the numerical simulation in order to generate the waves as described in (Pakozdi, Kendon, & Stansberg, 2011) and (Ostman, Pakozdi, Stansberg, Fagertun, & Vestbostad, 2015).

When such data are not available, the control signal of the wave maker can, instead, be generated from a given free surface elevation following the same procedure as in model tests. Following this procedure automatically gives the possibility to subsequently reproduce the numerical wave experimentally using the obtained control signal.

The latter procedure is applied to a model test case with extreme irregular wave events and resulting nonlinear global wave loads on a vertical cylinder (Stansberg, 1997), with the focus on higher-order ringing excitation. The purpose of the investigation is two-fold: 1) to validate the wave reconstruction procedure, and 2) to validate the resulting CFD ringing loads with the given waves. The results show generally good agreement both in the waves and in the loads; discrepancies found in the loads are considered to be mainly originating from corresponding uncertainties in the wave reconstruction. Wave breaking may be one source of uncertainty.

Commentary by Dr. Valentin Fuster
2016;():V002T08A025. doi:10.1115/OMAE2016-54874.

Flow past a circular cylinder close to a free surface at low Reynolds number is investigated numerically in this paper extending the work done in previous 2014 and 2015 OMAE papers [1, 2]. In the former, the dependence of the flow with the submergence was discussed and in the latter the flow at high Froude numbers was investigated. It was found that shedding is blocked as the cylinder approaches de free surface with an increasing value of the net lift coefficient and a reduction of the lift alternate oscillations due to such shedding. This variation on the lift pattern suggests that if a VIVACE device [3] is set under these flow characteristics, its performance will significantly differ from that in deep water conditions. These devices take advantage of flow induced motions in bluff bodies (such as the cylinder) to generate electric power. Singh & Mittal [4] coupled VIV system is studied in present paper in the presence of a free surface, selecting a configuration with large response in unbounded flow and adding a free-surface to such case. Results are presented and discussed.

Commentary by Dr. Valentin Fuster
2016;():V002T08A026. doi:10.1115/OMAE2016-54876.

Breaking wave-induced loads on offshore structures can be extremely severe. The air entrainment mechanism during the breaking process plays a not well-known role in the exerted forces. This paper present a CFD solver, developed in the Open-FOAM environment, capable of simulating the wave breaking-induced air entrainment. Firstly the model was validated against a bubble column flow. Then it was employed to compute the inline force exerted by a spilling breaking wave on a vertical cylinder in a 3D domain at a laboratory scale. Results showed that the entrained bubbles affected the magnitude of the force partially. Further analyses on the interaction of the bubble plume with the flow around the cylinder are needed.

Commentary by Dr. Valentin Fuster
2016;():V002T08A027. doi:10.1115/OMAE2016-54917.

For the stability of offshore structures, such as offshore wind foundations, extreme wave conditions need to be taken into account. Waves from extreme events can become critical from design perspective. In a numerical wave tank, extreme waves can be generated through focussed waves. Here, linear waves are generated from a wave spectrum. The wave crests of the generated waves coincide at a pre-selected location and time. 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 shows good agreement with the measurements from experiments. In further computations, the wave impact of the focussed waves on a vertical circular cylinder is investigated. The focussed 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 CFD code REEF3D solves the three-dimensional Navier-Stokes equations on a staggered Cartesian grid. Solid boundaries are taken into account with the ghost cell immersed boundary method. For the discretization of the convection terms of the momentum equations, the conservative finite difference version of the fifth-order WENO (weighted essentially non-oscillatory) scheme is used. For temporal treatment, the third-order TVD (total variation diminishing) Runge-Kutta scheme is employed. For the pressure, the projection method is used. The free surface flow is solved as two-phase fluid system. For the interface capturing, the level set method is selected. The level set function can be discretized with high-order differencing schemes. This makes it the appropriate solution for wave propagation problems based on Navier-Stokes solvers, which requires high-order numerical methods to avoid artificial wave damping. The numerical model is fully parallelized based on the domain decomposition, using MPI (message passing interface) for internode communication.

Topics: Waves
Commentary by Dr. Valentin Fuster
2016;():V002T08A028. doi:10.1115/OMAE2016-55065.

Waves generated by passing ships have potential adverse impacts on the environment (beach erosion, ecological disturbance, structures damage) and other waterway users (navigations, moored ships) in the coastal and sheltered areas. But issues related to waves of ships were addressed rarely in China until now. Accurate prediction of wash waves is the first step to control the washes from passing ships and it’s significant to reduce the effects of washes. A coupled method is used in this paper to simulate the washes and its effects caused by the passing ship. A potential flow theory method is adopted as the stationary wave generation model; a non-hydrostatic wave flow model is used as the wave propagation model; a time domain method is chosen as the model for simulating the forces and moments of mooring ship. The waves calculated by a potential flow theory method in the near field are used as the input for the non-hydrostatic wave-flow model to obtain the far field waves. A time-domain representation of the wave-cut at the location of the passing vessel is transformed to the frequency-domain and is used as the input for the diffraction computations. Parts of the calculated results are validated experimentally, satisfactory agreement is demonstrated.

Topics: Ships , Water
Commentary by Dr. Valentin Fuster

CFD and VIV: Risers and Pipelines

2016;():V002T08A029. doi:10.1115/OMAE2016-54085.

This paper describes an open source numerical investigation into slugging flow in a typical two-dimensional pipeline-riser for the first time. CFD tools Gmsh, OpenFOAM and ParaView are employed for mesh generation, numerical simulation and post process respectively. Original OpenFOAM solver ‘twoPhaseEulerFoam’ is used to simulate the gas-liquid flow in the system consisting of inclined pipeline and vertical riser. By comparing the numerical results of slugging phenomena and pressure fluctuation periods to previous experimental observations, it can be confirmed that it is possible to carry out such simulations in a complete open source way. Based on case studies, pressure fluctuation features in a typical single slugging cycle is also discussed in details. Furthermore, temperature variation of the internal flow due to air compressibility is found to have similar fluctuation period as that of pressure. In the end, the impacts of fluid properties on system pressure variations are discussed too. To future numerical investigations of subsea pipeline-riser induced slugging, present work is a basis for further open source solvers development.

Commentary by Dr. Valentin Fuster
2016;():V002T08A030. doi:10.1115/OMAE2016-54273.

Over the last decade Heerema Marine Contractors (HMC) has successfully performed multiple installation campaigns of large sized in-line structures (ILS) with Deep Water Construction Vessels (DCV) Aegir and Balder. Nowadays steady increase in size and weight of ILS have made these special operations even more complex. Presence of large sized ILS and accompanying buoyancy modules in the catenary have proven to play a dominant role in pipeline integrity. Originally hydrodynamic force formulations in finite element analysis are solely designated for the pipeline itself. These computations comprehend the application of the Morison equation using constant hydrodynamic coefficients of basic shapes in steady flow.

Therefore hydrodynamic forces acting on the ILS, characterized by irregular relative motions of a complex shaped and perforated structure, are highly simplified while playing a dominant role in the analyses. Validity of applying the standard Morison equation is debatable, since large ILS cannot be assumed slender. Nonetheless Morison type formulations can provide reasonable results depending on the accuracy of the hydrodynamic coefficients. Deriving these coefficients for complex shaped structures using industry standards is a highly interpretive process involving an accumulation of assumptions. This approach yields varying coefficients, which are applied conservatively in installation analyses, resulting in an unnecessary reduction of DCV offshore workability.

To improve workability of these complex installations, HMC has implemented an ILS specific hydrodynamic profile from Computational Fluid Dynamics (CFD) analysis into the installation analyses. This is effectuated by the development of an enhanced methodology with a dedicated hydrodynamic formulation for large perforated ILS. Dependencies on Keulegan-Carpenter (KC) number and local angle of attack are addressed in this formulation to respectively cover the inertia dominated oscillating motions and complex geometric composition. The applied hydrodynamic formulation is based on work of Molin et al. which showed a good agreement to the CFD analysis performed for this study. Development and application of this methodology is initiated as a first assessment towards more accurate ILS installation analyses.

Analysis of a study case shows reductions up to 50% of maximum bending strain in a specific regular wave analysis. From the work presented it is concluded that the industry practice vastly overestimates hydrodynamic forcing on large sized ILS. Complementary research is needed on the topics of oscillations for low (<1.0) KC number, effects of relative fluid velocity and finally the implementation of irregular waves.

Commentary by Dr. Valentin Fuster
2016;():V002T08A031. doi:10.1115/OMAE2016-54321.

The Steel Lazy Wave Riser (SLWR) configuration is considered as one of the favorable solutions for deepwater applications. This is due to the SLWR’s capability to effectively absorb the dynamic motions from the vessel and the relatively lower cost that it offers. However, the application of SLWRs has its own challenges when it comes to fatigue due to Vortex-Induced Vibrations (VIV). The buoyancy section and the touchdown area may be critical to VIV due to curvature and current exposure onto the area. Furthermore, there is only limited previous research on VIV responses of a SLWR configuration.

This paper presents a parametric study of VIV responses of various SLWR systems. The study location is the deeper area of the Norwegian Continental Shelf. The main parametric variations are water depth, the length of the buoyancy section, the dimension of the buoyancy modules and the hydrocarbon content. These variations result in different lazy wave configurations, which give different trends of VIV responses. The analysis works are performed using the computer programs Riflex and VIVANA.

The observation results show that, for the type of current used in this study, riser configurations at the same water depth tend to have the same level of fatigue life. In addition, risers in shallower water have a lower fatigue life compared to risers in deeper water due to the current profiles used in this study and the higher system stiffness of the risers at shallower water.

The results also show that the buoyant section has only little to modest influence on the VIV-fatigue life. The influence will only become apparent when the buoyancy section creates a sufficiently high arch shape in the lazy wave configuration.

The riser’s selfweight mainly affects the riser’s fatigue life at the upper catenary part of the riser. Although the upper catenary part of the riser generally has sufficient VIV-fatigue lifetime, a riser with lighter content has a lower fatigue life at this part compared to a riser with heavier content.

Commentary by Dr. Valentin Fuster
2016;():V002T08A032. doi:10.1115/OMAE2016-54323.

Marine drilling riser is subject to complicated environmental loads which include top motions due to Mobile Offshore Drilling Unit (MODU), wave loads and current loads. Cyclic dynamic loads will cause severe fatigue accumulation along the drilling riser system, especially at the subsea well head (WH).

Statoil and BP have carried out a comprehensive model test program on drilling riser in MARINTEK’s Towing Tank in February 2015. The objective is to validate and verify software predictions of drilling riser behaviour under various environmental conditions by use of model test data.

Six drilling riser configurations were tested, including different components such as Upper Flex Joint (UFJ), tensioner, marine riser, Lower Marine Riser Package (LMRP), Blow-Out Preventer (BOP), Lower Flex Joint (LFJ), buoyancy elements and seabed boundary model.

The drilling riser models were tested in different load conditions:

1. Forced top motion tests

2. Regular wave test

3. Combined regular wave and towing test

4. Irregular wave test

5. Combined irregular wave and towing test

6. Towing test (VIV)

Measurements were made of micro bending strains and accelerations along the riser in both In-Line (IL) and Cross-Flow (CF) directions. Video recordings were made both above and under water.

In this paper, the test set-up and test program are presented. Comparisons of results between model test and RIFLEX simulation are presented on selected cases. Preliminary results show that the drilling riser model tests are able to capture the typical dynamic responses observed from field measurement, and the comparison between model test and RIFLEX simulation is promising.

Commentary by Dr. Valentin Fuster
2016;():V002T08A033. doi:10.1115/OMAE2016-54338.

This paper presents a numerical study of flow around an elastically mounted circular cylinder in close proximity to a plane boundary vibrating in the transverse and inline directions in an oscillatory flow. The Reynolds-Averaged Navier-Stokes (RANS) equations and the SST k-ω turbulent equations are solved using the Arbitrary Langrangian-Eulerian (ALE) scheme and Petrov-Galerkin Finite Element Method for simulating the flow. The equation of motion is solved using the fourth-order Runge-Kutta method to find the displacements of the cylinder in the transverse and inline directions. The numerical model is validated against the previous results of vortex-induced vibration of an isolated circular cylinder in both cross-flow and inline directions. The flow model is further extended to study the vortex-induced vibration of a cylinder near a plane boundary with a very small gap ratio (e/D) of 0.01, with D and e being the diameter and the gap between the cylinder and the plane boundary, respectively. Simulations are carried out for two Keulegan-Carpenter (KC) numbers of 5 and 10 and a wide range of reduced velocities. It is observed that both the KC number and the reduced velocity affect the vibration of the cylinder significantly.

Commentary by Dr. Valentin Fuster
2016;():V002T08A034. doi:10.1115/OMAE2016-54374.

Numerous experimental and numerical studies have been carried out to better understand and to improve prediction of cylinder VIV (vortex Induced Vibration) phenomenon. The behavior of cylinder due to in-line vibration (VIVx) has been neglected in the earlier studies because of its lower amplitude in comparison with cross flow vibration (VIVy). However, some researchers have studied VIVx in 2DOF along with VIVy. Recent investigations show that response amplitude of structure caused by VIVx is large enough to bring it to consideration. This study focuses on understanding the origin and prediction of VIVx amplitude exclusively in 1DOF and subcritical flow regime. The experiments were performed in current channel on bare circular cylinder with low mass-damping ratio in Reynolds number range Re = 10000 ∼ 45000.

Commentary by Dr. Valentin Fuster
2016;():V002T08A035. doi:10.1115/OMAE2016-54502.

Steel lazy-wave riser (SLWR) are attractive deepwater applications for offshore oil and gas industry. When subjected to current, both the buoyancy elements and the riser may experience Vortex Induced Vibrations (VIV). Such vibrations are the result of the periodic hydrodynamic forces that are induced by the interaction of slender bodies and external fluid flow. If the vibration period is close to the natural period of the system, it can lead to fast accumulation of fatigue damage to the risers and amplified drag loads. There is a competition between the vortex induced forces acting on the buoyancy element and the riser segment due to its different diameters. The interaction of the vortex shedding from the riser and the buoyancy element depends on many parameters, such as the arrangement of the buoyancy element, aspect ratio of the buoyancy element, etc.

Shell Oil Company conducted VIV model tests with a straight flexible cylinder and staggered buoyancy elements corresponding to a buoyant section of a SLWR in MARINTEK in 2011. Five different buoyancy element configurations were tested. The test data has been extensively studied (Rao, et al 2015 and Jhingran, et al 2012). The interaction of the buoyancy elements and bare riser and its influence on the riser response (frequency, displacement and fatigue damage) have been investigated.

Semi-empirical VIV prediction software, such as VIVANA [4], SHEAR7 [13] and VIVA [11] are most commonly used by the offshore industry in the riser systems design against VIV loads. However, these software are not purposely designed to account for the interaction of the bare riser section and the buoyancy elements. It is of great interest to evaluate the prediction accuracy. The purpose of this study is to benchmark the VIV prediction of riser with buoyancy elements using VIVANA. The prediction is compared with Shell model test results with focus on CF responses. Uncertainty and improvement of the prediction are also discussed.

Commentary by Dr. Valentin Fuster
2016;():V002T08A036. doi:10.1115/OMAE2016-54503.

The Norwegian Deepwater Program (NDP) has over several years working on conceptual development and design of new steel riser solutions for deep water and harsh environment. For the steel lazy wave riser (SLWR) design, the buoyancy section is a critical segment. When subjected to current, both the buoyancy elements and the riser may experience vortex induced vibrations (VIV), which can lead to fast accumulation of fatigue damage and amplified drag loads. As part of developing robust SLWR, it is crucial to understand the current induced loads and responses on various staggered buoyancy configurations.

In order to investigate the interaction of bare pipe section and buoyancy elements and its effect on VIV, hydrodynamic model tests were carried out in MARINTEK’s ocean basin in 2014. The test pipe is 38 m in length and 3 cm in diameter. The length of each buoyancy element 0.15 m and its diameter is 0.15 m. The ratio between buoyancy element diameter and riser diameter is 5. VIV response may depend on the spacing ratio of the length of the buoyancy segments and the gaps between two adjacent buoyancy elements, aspect ratio of the buoyancy element, and coverage percentage of buoyancy elements. Hence these parameters were varied and 6 different staggered buoyancy configurations were tested in uniform flows. The use of strakes to suppress VIV was also evaluated. The primary objective is to determine a range for these parameters that leads to the smallest VIV curvature (and hence an optimal riser fatigue design).

The shedding frequency of the bare riser section is significantly higher than the buoyancy element due to its smaller diameter. Therefore, the bare riser section will excite much higher modes. As observed from the present tests, the fatigue damage is dominated by the bare riser component for all of the test configurations. The CF displacement of cases with the highest buoyancy coverage (50%) is often dominated by the vortex shedding of the buoyancy element. The use of stakes can effectively suppress the vortex shedding and leads to lowest fatigue damage in the test. In addition, the non-dimensional frequency of the buoyancy element is low (about 0.087–0.122) due to its small aspect ratios (1/1 and 2/1) in present tests. The vortex shedding of the buoyancy element seems to be weakened when the spacing ratio is larger than 1/1, which is different than earlier tests using buoyancy element with larger aspect ratio (≥5/1). The impact on the SLWR design is evaluated based on both present and other relevant model tests results.

Commentary by Dr. Valentin Fuster
2016;():V002T08A037. doi:10.1115/OMAE2016-54511.

In this paper a new wake oscillator model with nonlinear coupling term is proposed to model the vortex-induced vibration of an elastically supported rigid cylinder constrained to vibrate in the cross-flow direction. The superiority of this new model lies in its ability to satisfy at the same time both free and forced vibration experiments. The new wake oscillator model is based on an existing van der Pol wake oscillator model and nonlinear coupling terms are added to improve its performance in the modelling of forced vibration. The tuning of this new model to the forced vibration shows good agreement with experiments in terms of the added damping but failed to capture the negative added mass at high reduced velocities. To eliminate this discrepancy the model is further enhanced by relaxing the assumption of constant potential added mass. Using the parameters obtained from the forced vibration experiments, the free vibration simulation is conducted and results are compared with the experiments. Comparison indicates good agreement between simulation and experiments, and the main features of VIV are captured.

Commentary by Dr. Valentin Fuster
2016;():V002T08A038. doi:10.1115/OMAE2016-54617.

A model test of a free-hanging riser under vessel motion was performed in the ocean basin at Shanghai Jiao Tong University to confirm whether vortex-induced vibration (VIV) can happen due to pure vessel motion, to investigate the equivalent current velocity and Keulegan–Carpenter (KC) number effect on the VIV responses and to obtain the correlations for free-hanging riser VIV under vessel motion with VIV for other compliant risers. Top end of the riser was forced to oscillate at given vessel motion trajectories. Fiber Brag Grating (FBG) strain sensors were used to measure the riser dynamic responses. Experimental results confirmed that the free-hanging riser would experience significant out-of-plane VIV. Meanwhile, VIV responses in terms of response amplitude, response frequency and cross-section trajectories under different test cases were further compared and discussed. Most importantly, the correlation among VIV response frequency, vortex shedding pairs and maximum KC number KCmax was revealed. The presented work is supposed to provide useful references for gaining a better understanding on VIV induced by vessel motion, and for the development of future prediction models.

Commentary by Dr. Valentin Fuster
2016;():V002T08A039. doi:10.1115/OMAE2016-54700.

The design of top-tensioned risers has to face a number of challenges in deepwater. One of the challenges comes from the heave motion of the platform that leads to the fluctuation of the axial tension in the risers, which is called parametric excitation (PE). Vortex-induced vibration (VIV), induced by sea current, is also a notorious challenge, especially for high aspect ratio risers. Usually a marine riser is subjected to parametric excitation and vortex-shedding, simultaneously. However, so far only a limited number of VIV studies has considered axial parametric excitation. Thus, the objective of this paper is to analyze the influence of parametric excitation on the riser VIV. To simulate the fluid-structure interaction, a marine riser is modeled as a Bernoulli-Euler beam with periodical time-varying tension in the axial, while the lifting force generated by vortex-shedding is represented by a distributed wake oscillator model that is coupled with the vibration acceleration of the riser structure. The VIV model is represented by a set of coupled partial differential equations. Such coupling model is solved with a standard centered finite difference method so as to formulate a reduced-order system which is then numerically integrated in the time domain. The dynamic behavior of a TTR system is investigated with several combinations of amplitudes and frequencies in the heave motion. Interesting results are achieved as they are much different from the ones undergoing VIV only. It is revealed that the periodically varying top tension accounts for the artificial transition of vibration modes. In cases where PE frequency is distant away from the vortex shedding frequency, the amplitude of VIV response under dynamic axial tension is smaller than that under static tension, while in the special cases where PE frequency is close to the vortex shedding frequency, the response of VIV is significantly amplified. The sum and difference frequencies of the parametric and vortex-shedding excitation are noticeable in the power spectrum density of riser’s lateral displacement. Under the combined excitations, when the heave motion is mild, vortex shedding dominates riser’s response. However, in a severe sea state with remarkable heave motion amplitudes, the parametric excitation contributes more to the TTR dynamic response, because in this scenario the parametric instability disrupts the generation of vortex-shedding at the spanwise locations of riser.

Commentary by Dr. Valentin Fuster
2016;():V002T08A040. doi:10.1115/OMAE2016-54701.

In this paper, we focus on vortex-induced vibration (VIV) of a free-hanging riser attached to a vessel under irregular wave conditions. The global in-plane responses of the hanging riser are firstly studied numerically in order to generate the equivalent current profile under vessel motion, and a simplified irregular vessel motion-induced VIV prediction methodology is then proposed based on the understanding from previous experimental observations and literature review. Further comparison on irregular vessel motion-induced VIV and ocean current-induced VIV at the same operation site with the same return period is performed to emphasize the importance of vessel motion-induced VIV. Numerical results highlight that vessel motion-induced VIV can cause similar stresses, fatigue damage and drag amplification similar to the steady ocean current cases, especially to the operation site like Norwegian Sea where strong wave field exists with mild current condition. It should be mentioned that although the simplified methodology proposed in this paper requires further experimental validation, it is believed that the presented numerical pre-study would help the industry and the researchers to have initial understanding on the possible occurrence of vessel motion-induced VIV. We further show the similarities and differences of vessel motion-induced VIV with respect to the ocean current-induced VIV and its implications on riser design and operation.

Commentary by Dr. Valentin Fuster
2016;():V002T08A041. doi:10.1115/OMAE2016-54750.

A group of circular cylinders exists in many engineering practices, such as offshore drilling riser system. Due to the interference between the riser main tube and auxiliary lines, the hydrodynamic forces acting on the riser system is much different from those on a single circular cylinder. It is very rare in the publication and still not certain in the determination of the forces in the drilling riser design of the industry. Particularly, it is unclear of the hydrodynamic forces when the Reynolds number is very high which is quite common in the real ocean fields. In this paper, the stationary riser system consisting of a group of six circular cylinders with unequal diameters is considered. The hydrodynamic forces acting on the main cylinder in the Reynolds number ranging from 105 to 2×106 are numerically calculated by solving the Reynolds averaged Navier-Stokes (RANS) equations. The Spalart-Allmaras RANS model is employed to account for the turbulence effect. It is found that drag coefficients are close to 1 when the incoming flow is symmetrical with respect to the configuration of the cylinders and are dramatically reduced when the incoming flow is asymmetrical. No “drag crisis”, which is a well-known phenomenon in a single cylinder case, is found in this particular range of Reynolds numbers. A detailed analysis, including the flow field and pressure distribution around the main tube, is also presented in the present work. The numerical result of the hydrodynamic forces on the main line is very helpful for the engineers to determine the drag coefficients in the practice of drilling riser system design, under the guidance of API-RP-16Q.

Commentary by Dr. Valentin Fuster
2016;():V002T08A042. doi:10.1115/OMAE2016-54816.

Ocean currents may cause vortex induced vibrations (VIV) of deep-water umbilicals. The VIV response may give significant contributions to the total fatigue damage. Good estimations of the VIV response and damage are therefore important for the design of deep-water umbilicals. As VIV response is very sensitive to the structural damping, good response and fatigue estimates will be dependent on good estimates of the damping and that they are included in the VIV response analysis in a consistent way.

A complex cross section such as an umbilical or a flexible riser will have two sources of structural damping; damping due to the strain variation in the individual materials that make up the cross sections, and damping due to the different layers slipping against one another. The first may be denoted material damping and is present at all response levels, and will be particularly important at low response levels. The second may be denoted slip damping and will contribute when the curvature exceeds the initial slip curvature.

Ideally, accurate data for both the material and the slip damping are available. Unfortunately, this is not always the case and the damping parameters must then be estimated. The material damping may be estimated from the material properties of the various layers in the cross section, taking operating conditions such as temperature into account. The slip damping may be estimated from detailed cross-sectional analyses.

As the slip damping is dependent on the curvature, iterations are needed to ensure that the applied damping and the calculated response are consistent with each other. A procedure to include these iterations within a VIV response calculation is proposed.

A case study is presented demonstrating the use of the proposed procedure for a deep-water umbilical in a lazy wave configuration. For the case studied, the maximum curvatures caused by VIV are significantly reduced.

Commentary by Dr. Valentin Fuster
2016;():V002T08A043. doi:10.1115/OMAE2016-54891.

Vortex induced vibrations (VIV) and slug flow are two important aspects for marine risers conveying a multiphase flow, and should be carefully examined due to the influence on the fatigue life of the structure. This article examines a truncated riser exposed to VIV with an internal two-phase slug flow. The main focus of the article was to examine the effect of internal slug flow on the VIV of a riser. The VIV were simulated in time domain with a linear structural model with constant pretension. Approximately 150 vortex shedding periods were simulated after the response reached steady state. An internal two-fluid flow was introduced, with constant internal velocity, pressure and uniform slug lengths. From the numerical study it was apparent that the slug velocity and slug length had an influence on the response pattern, amplitude and frequency. An analytical model that predicts additional response frequencies due to slug flow was also compared to the numerical studies. The analytical study produced similar additional response frequencies as the numerical study.

The slug length and internal velocity can influence the response of the riser, and should be considered for marine risers conveying multiphase flow.

Commentary by Dr. Valentin Fuster
2016;():V002T08A044. doi:10.1115/OMAE2016-54932.

Blow-out preventers (BOPs) can be subjected to vortex induced vibration and also oscillating wave-induced loads from attached risers. Typically, the response of the BOP is estimated using heuristic codes and assumed hydrodynamic coefficients (added mass and drag) of the BOP and attached structures. In current practice, complex structures such as a BOP are tested in a tow basin using physical models to find these coefficients. An alternative, which is explored here, is to use computational fluid dynamics (CFD). Two BOP designs have been tested in 1:12 scale. We use CFD to simulate these experiments and to calculate forces on the models the resulting force histories. Added mass and drag coefficients are calculated from the CFD results at various Keulegan-Carpenter numbers and the results are compared with data derived from tow tank experiments. It is shown that the CFD simulations can provide an alternative to tow tank experiments over a wide range of experimental parameters. CFD simulations are also used to predict the hydrodynamic coefficients of a full scale structure and the results compared with the model scale results.

Commentary by Dr. Valentin Fuster
2016;():V002T08A045. doi:10.1115/OMAE2016-54945.

The steel lazy wave riser is an emerging solution for deepwater applications in harsh conditions. The addition of buoyancy to provide the unique “lazy wave” shape reduces the dynamic stresses at the touchdown zone due to vessel motions and waves and results in improved performance. However, as the buoyant region cannot be easily fitted with VIV suppression, VIV becomes a critical aspect of the design. The present study progresses the modeling effort presented in [2] to model and understand the global response of a deepwater lazy wave riser using computational fluid dynamics (CFD). An industry first CFD simulation of a steel lazy wave riser under in-plane currents is presented and validated against experiments with two different configurations. Results show good agreement between CFD and experiments and provide an initial understanding of the riser response under in-plane currents. The CFD method developed has been validated and will be an important tool for the design of lazy wave risers.

Commentary by Dr. Valentin Fuster
2016;():V002T08A046. doi:10.1115/OMAE2016-54990.

As meta-stable motions transverse to fluid flow in slender bluff-bodied structures, Vortex-Induced Vibrations (VIV) are mostly determined by three-dimensional (3D) geometric and relativistic changes that evolve in the structure. Simplistic models of the structure ignore these key physical principles.

In a 2014 OMAE paper, we introduced the key physical concepts for simulating VIV in a horizontal-oriented slender structure (pipeline). In a 2015 OMAE paper, we re-oriented the same structure vertically to simulate VIV in a vertical riser.

In this paper, one or more of the following variations in the vertically-oriented riser will be made, in order to judge the physical effect each variation has on the character and distribution of VIV along the riser:

• Cyclically move the upper end of the vertical riser

• Change into an S-shaped riser by adding weight/buoyancy

• Disconnect the lower end of the S-shaped riser

The simulations help show and reinforce the following mechanical concepts of VIV

• How gravity and fluid drag evolves a 3D shape in the riser

• How this shape creates specific structural flexibilities

• How these flexibilities set the stage for specific VIV

• How tension and end conditions are vital to VIV behavior

Commentary by Dr. Valentin Fuster
2016;():V002T08A047. doi:10.1115/OMAE2016-55000.

Vortex induced vibrations (VIV) are generally less critical in wave dominated flow conditions than in pure current flows. A steady state response needs time to build up and continuous variation in flow velocity generally reduces the vibration amplitudes. For low Keulegan-Carpenter (KC) flows in-line VIV is generally removed entirely, replaced by forced response at the wave frequency, while cross-flow vibration amplitudes are reduced. For large KC numbers, the wave induced flow behaves similarly to a current and the oscillatory nature of the flow no longer influences the VIV response as much.

Empirical models to predict the influence of waves in VIV design of offshore cylindrical structures are formulated in offshore design codes. For most flow regimes the models are sound and supported by a solid base of empirical test data. There are, however, exceptions — particularly for the low KC number regime, defined here as LKCR. For such flows the oscillating pressure differentials which cause vortex induced vibrations are no longer governed by traditional dimensionless parameters such as the reduced velocity, but instead the oscillating lift is governed by the ratio between the eigen-frequency of the structure and the wave frequency. Particularly, the frequency of the lift force is twice the wave frequency in regular waves. In irregular wave conditions there are necessarily also spectral peaks at both the actual wave frequency and at three times the wave frequency, but the governing spectral density is concentrated at two times the wave frequency.

The present study introduces a novel response model to conservatively assess cross-flow VIV in LKCR.

Commentary by Dr. Valentin Fuster

CFD and VIV: Ship and Floating Systems

2016;():V002T08A048. doi:10.1115/OMAE2016-54119.

Drillships have specific features which make ship resistance and propulsion characteristics difficult to predict with simple empirical methods. In this paper, RANSE CFD was used to optimize the hull and appendages of a drillship for speed or fuel consumption. An innovative thruster arrangement was initially designed for dynamic positioning and its performance in transit was verified with the use of CFD. Furthermore a special hydrodynamically shaped moonpool has been developed to eliminate moonpool sloshing during transit, further reducing resistance and increasing safety on board.

CFD was also used to determine the propulsion efficiencies of the vessel. An innovative way to use an actuator disc approach was used to calculate effective wake factors.

The various CFD calculations and the resulting design modifications are verified and confirmed with calm water resistance and self-propulsion tests.

The bow shape modifications reduce the bare hull resistance by 12% and the headbox modifications reduce the added headbox resistance by 26% of the bare hull resistance. The Callirrhoe moonpool design reduces the added moonpool resistance by 37% compared to a conventional moonpool while at the same time eliminates sloshing in transit. The propulsion efficiency is above 60% when the aft thrusters are used, which is deemed high for such a vessel.

Commentary by Dr. Valentin Fuster
2016;():V002T08A049. doi:10.1115/OMAE2016-54295.

The paper presents computations on predicting the hydrodynamics of a generic floating liquefied natural gas (FLNG) hull form in regular head sea waves using unsteady Reynolds-Averaged Navier-Stokes (URANS) solver StarCCM+. Initially, model scale simulations were conducted at model test basin water depth (d = 0.8m), with detailed verification and validation study performed to estimate numerical uncertainties. The simulation results were compared with potential flow solutions and validated against experimental studies. Using the verified numerical setup, ship hydrodynamics including wave induced loads, moments as well as ship motion responses in deep water waves (d = 8.0m) have been studied. The computed time history results were decomposed by Fourier series to obtain force/moment and motion transfer functions on the frequency domain. From the obtained results, the presented URANS approach demonstrates slightly better accuracy compared with potential flow (PF) solutions. It is also found that water depth has great influences on the computed wave force and ship motion transfer functions for certain range of wave frequencies.

Commentary by Dr. Valentin Fuster
2016;():V002T08A050. doi:10.1115/OMAE2016-54342.

The prediction of nonlinear roll motion of ships depends highly on the accurate estimation of roll damping. The nonlinear nature of roll damping arises from the viscous flow and the associated phenomenon of flow separation around the ship hull. Roll damping changes noticeably with a slight change in the ship hull geometry and appendages. The estimation methods employed in industry are highly empirical in nature. These empirical methods were derived from combinations of model tests conducted in wave flumes and basins, and some selected formulae used in fluid dynamics. These methods have limitations and the roll damping prediction show large variation with change in the ship parameters. The advances made in Computational Fluid Dynamics (CFD) in recent times, and validation of the CFD results using experimental studies, can help in predicting roll motion and damping more accurately. The present work uses CFD as a tool to estimate roll damping of a ship mid-section with bilge keel with validation using published experimental results.

Commentary by Dr. Valentin Fuster
2016;():V002T08A051. doi:10.1115/OMAE2016-54344.

When two vessels are positioned close to each other in a current, significant shielding or interaction effects can be observed. In this paper the current loads are determined for a LNG carrier alone, a Shuttle tanker alone and both vessels in side-by-side configuration. The current loads are determined by means of tow tests in a water basin at scale 1:60 and by CFD calculations at model-scale and full-scale Reynolds number.

The objective of the measurements was to obtain reference data including shielding effects. CFD calculations at model-scale Reynolds number are carried out and compared with the model test results to determine the capability of CFD to predict the side-by-side current load coefficients. Furthermore, CFD calculations at full-scale Reynolds number are performed to determine the scale effects on current loads.

We estimate that the experimental uncertainty ranges between 3% and 5% for the force coefficients CY and CMZ and between 3% and 10% for CX. Based on a grid sensitivity study the numerical sensitivity is estimated to be below 5%. Considering the uncertainties mentioned above, we assume that a good agreement between experiments and CFD calculations is obtained when the difference is within 10%.

The best agreement between the model test results and the CFD results for model-scale Reynolds number is obtained for the CY coefficient with differences around 5%. For the CX coefficient the difference can be larger as this coefficient is mainly dominated by the friction component. In the model tests this force is small and therefore difficult to measure. In the CFD calculations the turbulence model used may not be suitable to capture transition from laminar to turbulent flow. A good agreement (around 5% difference) is obtained for the moment coefficient for headings without shielding effects. With shielding effects larger differences can be obtained as for these headings a slight deviation in the wake behind the upstream vessel may result in a large difference for the moment coefficient.

Comparing the CFD results at full-scale Reynolds number with the CFD results at model-scale Reynolds number significant differences are found for friction dominated forces. For the CX coefficient a reduction up to 50% can be observed at full-scale Reynolds number. The differences for pressure dominated forces are smaller. For the CY coefficient 5–10% lower values are obtained at full-scale Reynolds number. The moment coefficient CMZ is also dominated by the pressure force, but up to 30% lower values are found at full-scale Reynolds number. The shielding effects appear to be slightly smaller at full-scale Reynolds number as the wake from the upstream vessel is slightly smaller in size resulting in larger forces on the downstream vessel.

Commentary by Dr. Valentin Fuster
2016;():V002T08A052. doi:10.1115/OMAE2016-54404.

Linear damping models have been used in the past for solving floating body dynamics, especially for simple geometries such as spar. However, due to the addition of heave damping elements to spar such as heave plate, complex flow around these elements may change the relationship between damping and velocity of the body to nonlinear. The damping plays a major role in accurate determination of motion response of spars, especially the heave. Free decay tests have been carried out for spar with and without heave plate in calm water condition. The Computational Fluid Dynamics (CFD) simulation of heave decay is carried out using ANSYS FLUENT and validated by free decay test results using scale models. Mesh convergence study has been conducted to determine the optimum mesh size. The heave motion obtained from CFD are used to derive the damping terms by matching the heave motion obtained using equation of motion by changing the damping term with linear, quadratic and the combination of linear and quadratic. The heave motion obtained from linear damping model matches well with that obtained from measured motion and CFD simulation for spar without heave plate. However, the linear / quadratic damping models alone are not suitable for spar with heave plate. Hence a combination of linear and quadratic damping model is proposed for spar with heave plate. The heave motion computed using a combination of linear and quadratic damping model matches well with that obtained from experimental studies and CFD simulations thus indicating the complexity of flow around heave plate in comparison to the spar alone. Further, the vortices around the spar models obtained from CFD simulations are also presented and discussed with regard to the higher order damping.

Commentary by Dr. Valentin Fuster
2016;():V002T08A053. doi:10.1115/OMAE2016-54429.

This paper presents numerical computation of added mass and damping coefficients of a slender horizontal cylinder in the free surface zone, which typically serves as a fish cage floater. A fully viscous two phase flow solver in OpenFOAM was employed in the numerical computation. The purpose was to validate the capability of this solver and dynamic mesh functionality. A two dimensional numerical wave tank was set up, and two wave relaxation zones were used to reduce the size of the computational domain. Harmonic forced oscillations of the cylinder were performed at different frequencies and amplitudes. The mesh at free surface zone was refined based on the radiated wave heights at different oscillation frequencies in order to properly resolve the radiated waves. The result shows that in most frequency ranges, the numerical computation agreed well with the experimental data and analytical solution. However at low frequency range for added mass coefficient in heave motion, deviations were observed, and it was due to the effect of finite water depth. In addition for sway motion at high frequency range, the damping coefficient was underestimated comparing with analytical solution. This was believed to be as a result of high steepness of the radiated waves.

Commentary by Dr. Valentin Fuster
2016;():V002T08A054. doi:10.1115/OMAE2016-54465.

A deepwater Spar Drilling Production Storage Offloading (SDPSO) floating system that consists of a classic Spar hull with dry–tree production, oil storage and offloading has been proposed for offshore oil exploitation. One of the key features of the SDPSO is the oil storage system, which includes the mid–section of the classic spar hull for oil storage providing a large storage capacity of more than 500,000 barrels of oil. In the working condition, oil storage tank is fully filled with oil and seawater, the storage and offloading of oil are achieved by seawater displacement and oil–water separation. However, a seawater displacement oil storage system does raise the problems of stability of the oil–water interface, possible sludge contamination of the water and corrosion of the internal surface of the storage tank during oil filling and offloading operations. It is clear that sludge deposit and corrosion effect are closely related to the temperature distribution of crude oil and seawater inside the tank. Therefore, it is necessary to investigate hot–oil/cold–water heat transfer in the SDPSO oil storage tank during both operations and storm conditions. For this purpose, CFD modeling and numerical studies were performed for a simplified oil storage system of the SDPSO platform in an effort to gain better understanding of the heat transfer problem.

Commentary by Dr. Valentin Fuster
2016;():V002T08A055. doi:10.1115/OMAE2016-54470.

Drillship is a marine vessel designed for drilling purposes of oil and gas wells. This kind of vessel has what is called a moonpool that is an opening on the base of the hull used for drilling operation. Nowadays, in search of better process efficiency, some of these drillships are being constructed with dual-derrick and are in need of a larger moonpool, which results in some effects on the floating structure, such as resistance increase. The main objective of this paper is to investigate the influence of shape and size of the moonpool on the resistance of a drillship dual derrick, while in transit. It presents the application of a commercial CFD (Computational Fluid Dynamics) software as a numerical approach to calculate the flow around a drillship without neglecting free surface effects. Throughout this work, the made assumptions, applied boundary conditions and appropriate mesh density studies are thoroughly discussed. Verification assessment is part of the work. In addition to the numerical study, some experimental tests were done at LOC/UFRJ (Laboratório de Ondas e Correntes – Laboratory of Waves and Currents) to validate the numerical approach. The comparison between simulation results and experiments allows the analysis of the present CFD models benefits and limitations, providing guidelines for similar future studies. The overall match between laboratory and virtual tests results supports the expansion of this procedure to other vessels and offshore floating units. The results of this work clarify the motion inside the moonpool and its effects. Furthermore, it gives the results of several different moonpool profiles that were optimized for this specific hull.

Commentary by Dr. Valentin Fuster
2016;():V002T08A056. doi:10.1115/OMAE2016-54522.

Cartesian grid was used in open water performance prediction, cavitation performance prediction and flow field characteristics of a propeller to research the applicability of the Cartesian grid in the numerical simulations of marine propellers. The comparisons of calculated results with the previous research and experimental results verify the accuracy of calculations with the grid on the prediction of thrust and torque coefficient and the simulation of cavitation distribution, wake velocity distribution and the vortex structure trajectory. Meanwhile the propulsive performances of Cartesian grid are better than other types of grid with the similar number of nodes. And the turning point of crash performance under cavitation condition and the phenomenon of vortex merging with neighboring vortex structure are excellent agreement with experiments and references.

Commentary by Dr. Valentin Fuster
2016;():V002T08A057. doi:10.1115/OMAE2016-54528.

As bank effect has a remarkable influence on the maneuverability of a ship proceeding close to a vertical bank, the assessment of ship maneuvering stability is of great importance. The hydrodynamic derivatives of a ship can reflect the change of the ship’s maneuverability and they are determined with the method of planar motion mechanism (PMM) tests. This paper presents a numerical way to simulate the PMM captive model tests for the ship KVLCC2. A general purpose viscous flow solver was adopted to solve unsteady Reynolds averaged Navier Stokes (RANS) equations in conjunction with a RNG k-ε turbulence model. A hybrid dynamic mesh technique is developed to update the mesh volume around the ship hull when the ship is undertaking pure yaw motions and it turns out efficient and effective to solve the limitation of small ship-bank distance to the mesh configuration and remeshing.. The numerical simulations and the accuracy of the numerical method was validated in comparison with the results of PMM tests in a circulating water channel. Then a series of distances between ship and bank together with different water depths were set for simulating the PMM tests of the KVLCC2 model in proximity to a vertical bank. The first order hydrodynamic derivatives of the ship were analyzed from the time history of lateral force and yaw moment according to the multiple-run simulating procedure. The values of derivatives in different lateral proximities to the bank and variant water depths were compared and it showed some favorable trends for predicting the ship’s maneuverability in the restricted waterways. For example, the influence of velocity derivatives on lateral force reduces while that of velocity derivatives on yaw moment strengthens and this is partly due to the suction force and bow-out moment caused by bank wall effect. The straight line stability and directional stability in terms of the calculated hydrodynamic derivatives were also discussed based on the MMG model for ship maneuvering. Results indicate that the ship is inherently unstable without control and the enhancement of bank effect makes the condition even worse. Moreover, a stable or unstable zone of PD controller parameters focusing on the directional stability was illustrated and setting the values of controller parameters in the range of “Control with high sensitivity” is recommended for cases of the ship navigating in very close proximity to a bank.

Topics: Stability , Ships
Commentary by Dr. Valentin Fuster
2016;():V002T08A058. doi:10.1115/OMAE2016-54530.

Recently, a growing interest has been seen in vortex induced motions of offshore units. These induced motions are significantly relevant for the design of mooring systems and risers of offshore platforms.

This work analyzes the vortex induced motions (VIM) of a Tension Leg Platform (TLP) when submitted to currents. The model tests were carried out in the fKN@LOC/COPPE-UFRJ laboratory facilities. Furthermore, a novel arrangement to represent the TLP tendons in shallow current channels is presented and discussed. This experimental set-up is innovative since, to avoid the set down, a tower has been implemented. TLP vertical motions are restrained and necessary stiffness for horizontal modes is provided by springs.

The ultra-reduced model with four square columns and four pontoons in closed configuration was built using a scale of 1:200. The range of current velocities was from 0.33 to 2.59 m/s. and five angles of attack were considered: 0, 11.25, 22.50, 33.75, 45 degrees. The reduced velocity reached a very high value of 32 for 45 degrees of heading.

Results have shown that the induced motions are dependent on the angle of attack and the current speed. For zero degrees of heading a typical bell curve of VIM was observed. On the other hand, for 45 degrees, motions increase steadily with current speed.

Commentary by Dr. Valentin Fuster
2016;():V002T08A059. doi:10.1115/OMAE2016-54746.

Floating offshore structures, such as production semi-submersibles and spars, can exhibit significant in-line and transverse oscillatory motions under current conditions. When caused by vortex shedding from the floater, such motions are generally called Vortex-Induced Motions (VIM). For semi-submersibles these motions could have a strong impact on the fatigue life of mooring and riser systems. Some field development studies indicate that the VIM induced fatigue damage for larger diameter Steel Catenary Risers (SCRs) can have a magnitude equal to or larger than the wave-induced fatigue damage.

The VIM phenomenon for multi-column floaters is characterized by complex interactions between the flow and the motions of the floater. Presently, model tests are the preferred method to predict the VIM response of a multi-column floater. However, several studies indicate that the observed VIM response in the field is less than what is observed in model test campaigns: typical model test results are very conservative. Using such test results in the development of mooring and riser design can easily result in very conservative designs which can have a significant impact on mooring and riser cost, or even affect SCR selection and/or feasibility.

The primary objective of the VIM JIP was to increase the physical insight into the VIM phenomenon. This knowledge is then used to address possible areas that could explain the differences between the results from model tests and field observations. To address these objectives, the JIP focused on model testing and CFD studies. A key segment of the JIP was the use of identical semi-submersible hull geometries for the numerical and experimental studies thereby facilitating the interpretation of the various response comparisons.

The JIP identified that a CFD model, at model-scale Reynolds number, can reasonably well reproduce the VIM response observed in model tests. However, to have confidence in the CFD results extensive numerical verification studies have to be carried out. The effect of external damping was investigated in model tests and in CFD calculations. Both the numerical and experimental results show that external damping significantly reduces the VIM response. Comparisons between CFD results at model- and full-scale Reynolds number indicate that Froude scaling is applicable, with minor scale effects identified on the amplitudes of the VIM motions. Changing the mass ratio of the floater has a small influence on the VIM response. Experimentally it was found that VIM response under inline or transverse waves is slightly smaller than without the presence of waves and is wave heading and wave height dependent. The presence of waves does not explain the observed differences between model test results and field observations. The effect of unsteady current on the VIM response is minimal.

Based on the results from the JIP it is concluded that increased external damping reduces the VIM response. The questions that remain are if the increased external damping is actually present in full-scale conditions and if the mooring and riser systems provide the required damping to reduce the VIM amplitudes.

Commentary by Dr. Valentin Fuster
2016;():V002T08A060. doi:10.1115/OMAE2016-54784.

The commercial CFD code ANSYS Fluent is used for the three-dimensional estimation of wave loads and the dynamic response of a floating single point wave energy converter of the SINN Power wave power plant due to non-breaking and unidirectional waves in coastal waters. The VoF method is used to model the free surface and wave theories to set up the boundary conditions at the inlet for regular waves. The wave induced vertical motions of the floating module are computed by a sixDoF solver. Preliminary 2D and 3D studies to set up boundary conditions, mesh densities and solver settings were performed. The numerical results were compared to analytical solutions in form of water surface elevations and wave kinematics which showed good agreement.

The paper presents the dynamic response of the floating module for different load cases in terms of non-breaking waves. The resulting horizontal and vertical forces at the floating module will be presented and explained by the flow dynamics. Time and space depending velocities and pressure distributions including details on vortex separation will be given, which reveal valuable insights on the contribution of inertia and drag forces leading to the dynamic structural response of the floating devices.

Commentary by Dr. Valentin Fuster
2016;():V002T08A061. doi:10.1115/OMAE2016-54813.

Experiments regarding flow-induced vibration on floating rounded squared section cylinders with low aspect ratio were carried out in an ocean basin equipped with a rotating-arm apparatus. Floating squared section cylinders with rounded edges and aspect ratios of L/D = 2.0 were elastically supported by a set of linear springs in order to provide low structural damping to the system. Two different incidence angles were tested, namely 0 and 45 degrees. The Reynolds numbers covered the range from 2,000 to 30,000. The aim was to understand the flow-induced vibrations around single columns, gathering information for further understanding the causes for the Vortex-Induced Motions in semi-submersible and TLP platforms. Experiments on circular and squared sections cylinders (without rounded edges) were also carried out to compare the results with the rounded square section cylinders (with rounded edges). The amplitude results for in-line, transverse and yaw amplitude for 0-degree models showed to be higher for squared section cylinders compared to those for the rounded square section cylinders. No significant difference between the 45-degree models was observed. The results of ratio between frequency of motion in the transverse direction and natural frequency in still water confirmed the vortex-induced vibration behavior for the squared and rounded square section cylinders for 45-degree incidence; and also the galloping characteristics for 0-degree incidence cases. The rounded effect on the square section cylinders showed to be important only for reduced velocity larger than 8, which is probably related to the position of the separation point that changes around the rounded edge, behavior that did not occurr for the squared edge that fixed the separation point for any reduced velocity.

Commentary by Dr. Valentin Fuster
2016;():V002T08A062. doi:10.1115/OMAE2016-54847.

Assuring a ship’s maneuverability under diverse conditions is a fundamental requirement for safe and economic ship operations. Considering the introduction of the Energy Efficiency Design Index (EEDI) for new ships and the related decreasing installed power on ships, the necessity arose to more accurately predict the maneuverability of ships in severe seas, strong winds, and confined waters. To address these issues, extensive experimental and numerical investigations were performed within the European funded Project SHOPERA. Here, second order forces and moments for a containership and a tanker were measured in model tests and computed by solving the Reynolds-Averaged Navier-Stokes (RANS) equations. Generally, these measured and computed second order loads (drift forces and yaw moments, added resistance) compared favorably. Furthermore, the effects of waves on zig-zag and turning circle maneuvers were investigated.

Topics: Waves , Ships
Commentary by Dr. Valentin Fuster
2016;():V002T08A063. doi:10.1115/OMAE2016-54926.

The impact of plastic pollution on marine ecosystems and global economy has been drawing public concern since the end of the 20th century. To mitigate this issue, The Ocean Cleanup (TOC) Foundation is developing technologies to extract, prevent, and intercept plastic debris from coastal and oceanic environments. The core technology being optimized is the use of floating booms placed perpendicular to the main ocean plastic flow so it can concentrate plastic debris to a point where it can be extracted, shipped and processed in a cost-effective manner.

In order to optimize the system’s field efficiency (i.e. mass of ocean plastic captured per length of floating boom), a multi-scale approach has been elaborated, where temporal and spatial scales span over several orders of magnitude. Here we introduce this general multi-scale method alongside its assumptions and multi-scale models. We then describe two application examples, the first corresponding to our coastal pilot in the Japanese island of Tsushima and the second related to our main cleanup target area: the so-called Great Pacific Garbage Patch, situated between Hawaii and the US west coast.

Commentary by Dr. Valentin Fuster
2016;():V002T08A064. doi:10.1115/OMAE2016-54987.

The vortex-induced motions (VIM) of offshore platforms stand as an intriguing and challenging engineering problem, drawing attention from industry, universities and research institutes. Field observations, model tests and calculations have extensively showed that the complex fluid-structure interaction can result in appreciable motions and increased fatigue of mooring and risers. It is thus a very relevant issue from the engineering standpoint. A large volume of experimental research has been carried out, mainly to verify designs and characterize the occurrence of VIM. Conversely, the numerical investigations applying CFD tools have shown to be a more flexible approach enabling better understanding of the physics at play due to the possibility of investigating the effects of different parameters upon the vortex induced motions of floating platforms. Moreover, the CFD calculations enable investigation of the full-scale behavior of the platforms under VIM, a very controversial issue presently. Bearing upon these issues, the VIM Joint Industry Project aims at increasing physical insight of this phenomenon by means of investigating the influence of geometric design variations, flow conditions and scale effects with the objective of improving practical knowledge that can be applied in the design stage of floating platforms. In this paper, we present some of the CFD studies, results and observations carried out within the JIP, regarding the VIM of a semi-submersible with circular columns in 0 and 45 degrees and over a wide range of reduced velocities. It is confirmed that the 0 degree incidence results in larger motions than the 45 degrees-incidence case, in contrast to the VIM behavior of a semi-submersible with square columns. The tests campaign carried out at the University of São Paulo for the same platform agree with these results. Within the lock-in range, the frequency synchronization of the lift forces on columns and pontoons cause large net transverse forces. Appreciable sway motions thus result. For larger reduced velocities, synchronization of the flow around the columns cease, but the forces on the pontoons then largely contribute to the total force. In this high-reduced velocity range, the phasing between total force and motion is such that energy transfer from the fluid to the body occurs, causing the amplification of the motions.

Commentary by Dr. Valentin Fuster
2016;():V002T08A065. doi:10.1115/OMAE2016-55038.

The flow field characteristics of 2-D flap style rudders with and without gap are analyzed through 4 models. To explore the influence of different filling styles, one flap rudder with gap and three flap rudders without gap are simulated from 0 to 30 degrees angle of attack with k-omega SST turbulence model.

Validation is done by comparing the results with EFD data from reference and the mesh independence verification is also made. Then lift and drag coefficients are compared among four models. Pressure, velocity distributions are given to explain the difference on hydrodynamic characteristics among them. Unsteady computation method is used to investigate the fluctuation characteristics of drag coefficients at large angle of attack. Stream lines are shown to better understand the vortex system on the suction surface.

Commentary by Dr. Valentin Fuster
2016;():V002T08A066. doi:10.1115/OMAE2016-55090.

Assessment of wave-structure interaction in terms of combined hydrodynamic stability and structural survivability is paramount in extreme wave conditions. Components of CFD methodologies needed for accurately capturing the detailed motion of a floating wind turbine (FWT) in survival sea-state is the focus of the study. Physical wave tank tests of a Tension Leg Platform (TLP) concept with four moorings are applied as a first validation, due to its simplicity from a CFD point of view. Two different codes have been objects of study, namely the open source code OpenFOAM® with a flexible mesh approach and the commercial CFD code StarCCM+ with the overset mesh method. The influence of the surface capturing algorithm (VOF method) and the two-way coupling of the six degrees-of-freedom body motion solver and the hydrodynamic solver have been identified as the crucial components in CFD simulation of the FWT. A major advantage of StarCCM+ was that it does not suffer from the same sensitivity as OpenFOAM to the fact that motion of the floating body is strongly coupled to the solution of the hydrodynamics (a stiff FSI problem) which led to instability of the numerical solution. The results obtained with StarCCM+ are comparable with the measured motion of and tension forces on the TLP in both in regular waves and irregular waves.

Commentary by Dr. Valentin Fuster

CFD and VIV: VIV Physics and Suppression

2016;():V002T08A067. doi:10.1115/OMAE2016-54062.

The flow-induced vibration fatigue of an array of tubes is a complex problem of practical significance in the offshore oil and gas industry. Simple analytical tools for analyzing isolated tubes lack the capability of directly addressing the array problem, so they require some sort of calibration if they are to be used for this application. Computational fluid dynamics (CFD) and coupled computational fluid-structure interaction programs can also be utilized to address the problem in more detail, but at a significant cost in computing time. In either case, understanding of the phenomena is limited, and relatively little relevant data are available to verify the accuracy of these programs for this application. This paper documents a physical model test performed at the University of California-Berkeley Richmond Field Station Tow Basin with the following objectives: to improve confidence in the understanding of the dynamic performance and fatigue demand on both bare and straked tubes in an arrayed configuration; to estimate the influence of an external super-structure (e.g., the truss section of a floating truss spar platform) on the vibrations of the tubes in the array; and, to generate data for verification or calibration of state-of-the-art or emerging analysis tools. The findings provide new, useful information on both the fatigue of tubes in complex configurations and the effectiveness of suppression devices in these scenarios for fatigue mitigation.

Commentary by Dr. Valentin Fuster
2016;():V002T08A068. doi:10.1115/OMAE2016-54190.

Bluff body structures exposed to ocean current can undergo vortex-induced motion (VIM) for certain geometric and physical conditions. Recently, the study of VIM has been gaining attention for many engineering applications, in particular offshore structures such as buoys, FPSOs, semi-submersibles, Spars and TLPs. The present work is a part of a systematic effort to investigate the VIM response of multi-columns floating platform. In real sea condition, floating platforms are in high Reynolds numbers region and flow patterns around structures are turbulent in nature. For the purpose of investigating and simulating accurately the nonlinear dynamic processes of vortex shedding, transport and wake interactions with the bluff body, the fundamental study of VIM around a square column at moderate Reynolds numbers (1500 ≤ Re ≤ 14000) is firstly investigated. In the present work, the transient flow pattern around a free vibrating square cylinder at moderate Reynolds numbers is numerically investigated by an open source CFD toolbox, OpenFOAM. Good consistency and agreement are found between the present numerical findings and that of experiments. The cylinder, with a blockage area of 4.2%, is mounted on an elastic support for free vibration in the transverse direction. Hybrid RANS-LES turbulence models are considered here for accurate prediction of massively separated turbulent wake flow while maintaining the reasonable computational cost. Three hybrid turbulence models, the DDES (Delayed Detached Eddy Simulation, the k-ω SST-DES (Detached Eddy Simulation), and the k–ω SST-SAS (Scale Adaptive Simulation), are studied and their results are compared with the reported experimental measurements. It is shown that the result of simulation with the k–ω SST-SAS model is closer to the reported literature than the other two and therefore, subsequently adopted for all the simulations of our study in this paper. The scaling effect of cylinder length in the spanwise direction is also studied with the objective to reduce the computational cost. From the comparison with the recent experimental measurements, the discrepancy between the present simulations of reducing cylinder length and the experiment increases only when Re ≥ 4000. This might stem from the increase in wavelength of some vortex shedding modes and turbulence intensity variation in the spanwise direction near the cylinder as Re ≥ 4000. The detailed flow patterns, 3D vortex structures (based on Q-criterion) and vortex-shedding modes are presented in this work as well.

Commentary by Dr. Valentin Fuster
2016;():V002T08A069. doi:10.1115/OMAE2016-54517.

This paper presents a set of numerical simulations of flow-induced vibrations (FIV) and coupled wake flow behind two identical square columns in a side-by-side configuration. To observe the four regimes as a function of different gap ratios, the computational results of the configuration at low Reynolds number Display FormulaRe=ρfUDμf in stationary condition are firstly compared with existing experimental data of moderate Reynolds number. We next investigate the configuration of elastically mounted square columns, which are free to oscillate in both streamwise and transverse directions. The simulations are performed by the Petrov-Galerkin finite-element method and Arbitrary Lagrangian-Eulerian technique to account for the fluid mesh motion. The four regimes of stationary side-by-side configuration follow the same trend of the experimental data conducted at moderate Reynolds number, while the ranges of each regime differ due to the turbulent wake properties. For the freely vibrating condition, all the simulations are computed at low Reynolds number (Re = 200), mass ratio equal to Display Formula10m*=Mmf and reduced velocity in the range of Ur ∈ [1,50] where Display FormulaUr=UfND and in free-damping condition Display Formulaζ=C2KM=0. The four regimes in vibrating condition are investigated as a function of gap ratios g* = g/D, which is the ratio of spacing between the inner column surfaces to the diameter of the column. The effects of reduced velocity on the force variations, the vibration amplitudes and the vorticity contours are analyzed systematically to understand the underlying FIV physics of side-by-side columns in the four regimes. Finally, we present a FIV study of the full semi-submersible model at moderate Reynolds number Re = 20,000 which can be considered as the application of side-by-side configuration.

Commentary by Dr. Valentin Fuster
2016;():V002T08A070. doi:10.1115/OMAE2016-54564.

Wake induced vibration is a distinctive phenomena of fluid-elastic instability arising from interactions of a body in the wakes of another bluff body and characteristically different from the well-understood vortex induced vibrations. This work presents a fluid-structure interactions numerical model as an alternative tool for investigation of wake induced vibrations. In an attempt to better understand mechanisms of wake induced motions, a simplified model of two cylinders in tandem arrangement with different diameters under cross flow was considered in this work. Cross flow velocity conditions vary from moderate to high Reynolds number (Re = 2 × 103–5 × 104) in the same range as many experiment reported recently in literature. A hybrid detached eddy simulation approach is used for turbulence modelling at those high Reynolds number conditions in order to resolve complex near body flow features as well as in the wake regions. The proposed model is first validated through extensive benchmarking with experimental studies for responses of tandem cylinders at the same flow conditions as in physical experiment. With good agreement to experimental data, the model was extended for simulations of cylinders of different diameters in tandem arrangement. For different diameters between upstream and downstream cylinders, the fundamental frequencies of shedded vortices from the cylinders are essentially different. It is observed from the present study that responses of the downstream cylinder are characterized not only the geometrical parameters such as distances and diameter differences between the cylinders but also the Reynolds number. As contrast to many experimental studies, at constant Reynolds number, downstream cylinders are found to have multiple lock-in regions in a wide range of reduced velocities. This distinctive behaviour of the cylinders at constant Reynolds numbers and diameter ratios suggests strong evidence of complicated mechanism of wake-induced vibrations phenomena. Further analysis of results from high fidelity numerical simulations were carried out for detailed investigations of force amplitudes and frequencies. The current analysis revealed multiple frequency content of the force; thus explaining high response amplitudes of the downstream cylinder at high reduced velocity.

Commentary by Dr. Valentin Fuster
2016;():V002T08A071. doi:10.1115/OMAE2016-54592.

The use of the Lattice Boltzmann Method (LBM) for fluid-flow simulation has been the subject of several recent studies where it was reported that the method offers many advantages such as high accuracy coupled with computational efficiency. In this paper, we report on the use of this method, in conjunction with Large-Eddy Simulations, to study an interesting phenomenon related to the suppression of vortex shedding from circular cylinders. Specifically, it has been observed in experiments that vortex shedding from a cylinder can be drastically reduced by the injection of a fluid jet into the approach flow. We first present results for a cylinder without jet injection in order to quantify the suitability of the LBM for use in such flows. Thereafter, we present results for the case with jet injection where we consider the case where Re = 55,440. Preliminary results conclusively demonstrate that the presence of jet injection does indeed lead to substantial reduction in the magnitude of lift forces on the cylinder. This strongly argues in favor of further research to understand the dynamics of this phenomenon, and the range of parameters needed to maximize its beneficial effects.

Commentary by Dr. Valentin Fuster
2016;():V002T08A072. doi:10.1115/OMAE2016-54623.

Vortex-induced vibration (VIV) can lead to significant fatigue damage accumulation in deepwater marine risers. In order to assess the effects of VIV and to ensure riser integrity, field monitoring campaigns are often conducted wherein riser response is recorded by a few data sensors distributed along the length of the riser. It is possible to empirically estimate the fatigue damage at “key” critical fatigue-sensitive locations, where sensors may not be available as part of the spatially distributed discrete measurements. In this study, two empirical techniques — Proper Orthogonal Decomposition (POD) and Weighted Waveform Analysis (WWA) — are sequentially applied to the data; together, they offer a novel empirical procedure for fatigue damage estimation in an instrumented riser. The procedures are briefly described as follows: first, POD is used to extract the most energetic spatial modes of the riser response from the measurements. Often, only a few dominant POD modes preserve most of the riser motion kinetic energy; other modes are less important. Identified POD mode shapes are discrete as they are defined only at the available sensor locations. Accordingly, a second step in the proposed procedure uses WWA to express each dominant POD mode as a series of riser natural modes that are continuous spatial functions defined over the entire riser length. Based on the above empirically identified modal information, the riser response over the entire length is reconstructed using backward procedures — i.e., compose identified natural modes into the POD modes and, then, assemble all these dominant POD modal response components into the derived riser response. The POD procedure empirically extracts the energetic dynamic response characteristics without any assumptions and effectively cleans the data of noisy or less important features, which makes it possible for WWA to identify dominant riser natural modes — all this is possible using the limited number of available measurements from sensor locations. Application of the entire procedure is demonstrated using experimental data from the Norwegian Deepwater Programme (NDP) model riser.

Commentary by Dr. Valentin Fuster
2016;():V002T08A073. doi:10.1115/OMAE2016-54632.

Experiments regarding free-end effects on vortex-induced vibration (VIV) of floating circular cylinders with low aspect ratio were carried out in a towing tank. Four cylinders with low aspect of ratio, L/D = 2 (Length / Diameter) were tested with different free end corner shape types, namely by the relation between chamfer rounding radius (r) divided by the radius of cylinder (R) (r/R = 0.0, 0.25, 0.5 and 1.0). For the initial case, r/R = 0.0 represents flat tip and r/R = 1.0 the hemispherical tip. The aims were to understand the effect of different free-end types on VIV behavior of cylinders. The floating circular cylinders, i.e. unit mass ratio m* = 1(structural mass/displaced fluid mass) were elastically supported by a set of linear springs to provide low structural damping on the system and allow six degrees of freedom. The range of Reynolds number covered 3,000 ≤ Re ≤ 20,000. To conclude, cylinder with r/R = 0.25, shows lower amplitudes in transverse direction. The same occurs for the cylinder r/R = 0, but for amplitudes of vibration in in-line direction. Behaviors of the vibration frequencies in in-line and transverse direction don’t have significantly differences. Regarding force coefficient, flat tip cylinder (r/R = 0) presents higher values compared to the others however, for the lift coefficient, results converge in similar values for the same velocities that were observed higher transverse amplitudes. The visualization experiments show an expressive reduction of the recirculation bubble for r/R = 0.5 model compared with the flat tip, can therefore justify the lower values for this model obtained in draft amplitudes and drag coefficient compared with the flat tip model.

Commentary by Dr. Valentin Fuster
2016;():V002T08A074. doi:10.1115/OMAE2016-54662.

In this work, we investigate the combined translation and rotational flow-induced vibration (FIV) of elastically mounted square cylinder in a free-stream at zero incidence angle. We employ a partitioned iterative scheme to solve coupled fluid-rigid body interaction using unstructured grid. The fluid-solid coupled solver and the mesh is verified by investigating pure translational motion cases at zero incidence against published data for a laminar flow past a square cylinder. Further analysis revealed that the increase of mass ratio shifts the lock-in to higher reduced velocity region. The influence of of the torsional motion parameters is analyzed for a pure rotational case. The combined 3-DOF motion is next considered while keeping the above two analyses as reference. It was evident that, even small yaw vibrations adds circulation to the flow and thus increases the vortex intensity. This phenomenon is identified to be responsible for the differences of motion parameters between the isolated DOF cases and combined 3DOF cases. Finally, for the completeness of the study, the influence of 3D effects is estimated for the same geometry and also a high Re case is presented.

Commentary by Dr. Valentin Fuster
2016;():V002T08A075. doi:10.1115/OMAE2016-54689.

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)1. 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.

Commentary by Dr. Valentin Fuster
2016;():V002T08A076. doi:10.1115/OMAE2016-54810.

Vortex Induced Motions (VIM) of semi-submersibles occur when the vortex shedding frequency is close to the natural frequency of the semi-submersible in the direction transverse to the current. Recent studies suggest that the magnitude of VIM predicted during model tests is higher than that observed in the field. Among others, the damping effect provided by the risers and mooring lines is regarded as one of the reasons for this difference. In this paper the setup and results are presented for model tests to investigate the influence of damping on VIM.

For these model tests an active damping system was developed, which introduces an actively controlled external force mimicking a damping force. This applied damping force is based on the floater sway motion and sway velocity. With this system the introduced damping level can easily be controlled and verified without changing the stiffness of the system. In this paper the advantages and disadvantages of this active damping system are presented.

The VIM tests were conducted for two semi-submersibles: a paired-column semi with eight columns and a four column semi. Reduced velocities ranged from Ur=3 to Ur=10 and different levels of additional linear damping were applied to the floaters in the direction transverse to the current direction. Damping was found to reduce the VIM motions significantly: reductions of more than 60% were observed in the nominal A/D response for 25% equivalent linear damping. This indicates that damping has a significant effect on the global VIM response and thus should be considered in the design phase of the risers and mooring lines of the semi-submersibles.

To improve the understanding of the driving mechanism of VIM and also to provide validation data for CFD analyses, forces were measured on each column of the four column semi. Column force measurements indicate that for the four column semi for 45 degrees heading, i.e. the heading with largest VIM responses, the upstream, the portside and the starboard side columns are exciting the VIM motions. For 22.5 degrees, the downstream, the portside and the starboard side columns excite the VIM motions. For all tested headings the pontoon always damps the VIM response.

Commentary by Dr. Valentin Fuster
2016;():V002T08A077. doi:10.1115/OMAE2016-54815.

Flow-induced motions (FIM) of an elastically mounted circular cylinder with roughness strips at different angles of attack are investigated by 2-D URANS simulations based on the open source CFD tool OpenFOAM. The cylinder is constrained to one degree of freedom motion in the transverse direction. Two selectively distributed roughness strips on cylinder surface are used to enhance the FIM of the cylinder. Simulations are conducted at Re = 60,000 and Re = 100,000 with the angle of attack, αattack, varies from 0° to 90°. Amplitude response, frequency response, and near-wake structures of the cylinder are discussed in numerical results. The simulation results indicate that both suppression and enhancement of FIM are observed for the cylinder as the angle of attack rises from 0° to 90°. For αattack≥ 55°, the near-wake region becomes quite slender and vortex shedding is suppressed. The FIM of the cylinder is greatly inhibited when αattack ≥55°. In addition, a complex vortex reattachment observed in the near-wake region intensifies the fluid-structure interactions and causes obviously higher amplitude.

Commentary by Dr. Valentin Fuster
2016;():V002T08A078. doi:10.1115/OMAE2016-54989.

The cylinder flow is a canonical problem for Computational Fluid Dynamics (CFD), as it can display several of the most relevant issues for a wide class of flows, such as boundary layer separation, vortex shedding, flow instabilities, laminar-turbulent transition and others. Several applications also display these features justifying the amount of energy invested in studying this problem in a wide range of Reynolds numbers. The Unsteady Reynolds Averaged Navier Stokes (URANS) equations combined with simplifying assumptions for turbulence have been shown inappropriate for the captive cylinder flow in an important range of Reynolds numbers. For that reason, recent improvements in turbulence modeling has been one of the most important lines of research within that issue, aiming at better prediction of flow and loads, mainly targeting the three-dimensional effects and laminar-turbulent transition, which are so important for blunt bodies. In contrast, a much smaller amount of work is observed concerning the investigation of turbulent effects when the cylinder moves with driven or free motions. Evidently, larger understanding of the contribution of turbulence in those situations can lead to more precise mathematical and numerical modeling of the flow around a moving cylinder. In this paper, we present CFD calculations in a range of moderate Reynolds numbers with different turbulence models and considering a cylinder in captive condition, in driven and in free motions. The results corroborate an intuitive notion that the inertial effects indeed play very important role in determining loads and motions. The flow also seems to adapt to the motions in such a way that vortices are more correlated and less influenced by turbulence effects. Due to good comparison of the numerical and experimental results for the moving-cylinder cases, it is observed that the choice of turbulence model for driven and free motions calculations is markedly less decisive than for the captive cylinder case.

Commentary by Dr. Valentin Fuster

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