Simulations on Hydrodynamic Coefficients of Stationary Cactus-Shaped Cylinders at a Low Reynolds Number

Flexible cylinders, such as marine risers, often experience sustained vortex-induced vibration (VIV). Both helical strakes and fairings are demonstrated to be effective in suppressing VIV, while, helical strakes result in large drag, which increases the rotational angle and bending moment at the riser hang-off location and, fairings are cumbersome in term of storage, installation and maintenance. This study was inspired by the giant Saguaro Cacti which grow in desert region. Saguaro Cacti have shallow root system, but can grow up to fifty feet in height and can withstand very high wind velocities. In this study, numerical simulations of flow past a stationary cactus-shaped cylinder are performed in two-dimensional field at a low Reynolds number of 200. The hydrodynamic coefficients and the vortex-shedding patterns of a cactus-shaped cylinder are compared with those of a circular cylinder. In addition, a set of two cactus-shaped cylinders of tandem arrangement are also studied to investigate the effects of wake. Results showed that a cactus-shaped cylinder can reduce the drag, lift, and Strouhal number, which suggests its potential as an alternative technology to suppress VIV of a riser.

3-D Numerical Simulations of Subsea Jumper Transporting Intermittent Slug Flows

Subsea jumper is the steel pipe structure to connect wellhead and subsea facilities such as manifolds or processing units in order to transport the produced multiphase flows. Generally, the jumper consists of a goalpost with two loop structures and a straight pipe between them, carrying the multiphase oil and gas from the producing well. Due to the jumper pipe characteristic geometry and multi-fluid properties, slug flows may take place, creating problematic fluctuating forces causing the jumper oscillations. Severe dynamic fluctuations cause the risk of pipe deformations and resonances resulting from the hydrodynamic momentum/pressure forces which can lead to unstable operating pressure and decreased production rate. Despite the necessity to design subsea jumper with precise prediction on the process condition and the awareness of slug flow risks, it is challenging to experimentally evaluate, identify and improve the modified design in terms of the facility scale, time and cost efficiency. With increasing high computational performance, numerical analysis provides an alternative approach to simulate multiphase flow-induced force effects on the jumper. The present paper discusses the modelling of 3-D flow simulations in a subsea jumper for understanding the development process of internal slug flows causing hydrodynamic forces acting on the pipe walls and bends. Based on the fluctuating pressure calculated by the fluid solver, dynamic responses of the jumper pipe are assessed by a one-way interaction approach to evaluate deformation and stress. A potential resonance is discussed with the jumper modal analysis. Results from the structural response analyses show dominant multi-modal frequencies due to intermittent slug flow frequencies. Numerical results and observed behaviors may be useful for a comparison with other simulation and experiment.

Experimental and Numerical Study of Vortex Induced Vibrations on a Spool Model

The problem of Vortex-Induced Vibrations (VIV) on spool and jumper geometries is known to present several drawbacks when approached with conventional engineering tools used in the study of VIV on risers. Current recommended practices can lead to over-conservatism that the industry needs to quantify and minimize within notably cost reduction objectives. Within this purpose, the paper will present a brief critical review of the Industry standards and more particularly focus on both experimental and Computational Fluid Dynamic (CFD) approaches. Both qualitative and quantitative comparisons between basin tests and CFD results for a 2D ‘M-shape’ spool model will be detailed. The results presented here are part of a larger experimental and numerical campaign which considered a number of current velocities, heading and geometry configurations. The vibratory response of the model will be investigated for one of the current velocities and compared with the results obtained through recommended practices (e.g. Shear7 and DNV guidelines). The strategy used by the software K-FSI to solve the fluid-structure interaction (FSI) problem is a partitioned coupling solver between fluid solver (FINE™/Marine) and structural solvers (ARA). FINE™/Marine solves the Reynolds-Averaged Navier-Stokes Equations in a conservative way via the finite volume method and can work on structured or unstructured meshes with arbitrary polyhedrons, while ARA is a nonlinear finite element solver with a large displacement formulation. The experiments were conducted in the BGO FIRST facility located in La Seyne sur Mer, France. Particular attention was paid towards the model design, fabrication, instrumentation and characterization, to ensure an excellent agreement between the structural numerical model and the actual physical model. This included the use of a material with low structural damping, the performance of stiffness and decay tests in air and in still water, plus the rationalization of the instrumentation to be able to capture the response with the minimum flow perturbation or interaction due to instrumentation.

Large Eddy Simulation of Cross Flow in Pipe Junction

Mixing in pipe junctions can play an important role in exciting force and distribution of flow in pipe network. This paper investigated the cross pipe junction and proposed an improved plan, Y-shaped pipe junction. The numerical study of a three-dimensional pipe junction was performed for calculation and improved understanding of flow feature in pipe. The filtered Navier–Stokes equations were used to perform the large-eddy simulation of the unsteady incompressible flow in pipe. From the analysis of these results, it clearly appears that the vortex strength and velocity non-uniformity of centerline, can be reduced by Y-shaped junction. The Y-shaped junction not only has better flow characteristic, but also reduces head loss and exciting force. The results of the three-dimensional improvement analysis of junction can be used in the design of pipe network for industry.

Numerical Investigation of Local Scour Around Submerged Pipeline in Shoaling Conditions

Water waves play an important role in local scour around subsea pipelines laid on the sandy seabed, especially in shallow water regions. In this paper, a two-dimensional numerical model is employed to predict local scour around submarine pipelines under water waves in shoaling condition. The motion of water under waves is simulated by solving the Reynolds Averaged Navier-Stokes (RANS) equations. The evolution of the seabed surface near the pipeline is predicted by solving the conservation of the sediment mass, which transport in the water in the forms of bed load and suspended load. The main aim of this study is to investigate the effect of the seabed slope on the scour profiles and scour depth. To achieve this aim, numerical simulations of scour around a pipeline on a flat seabed and on a slope seabed with a slope angle of 15° are conducted for various wave conditions.

Generation of Regular and Irregular Waves in Navier-Stokes CFD Solvers by Matching With the Nonlinear Potential Wave Solution at the Boundaries

The capability of wave generation and absorption in a viscous flow solver becomes important for achieving realistic simulations in naval and offshore fields. This study presents an efficient generation of nonlinear wave fields in the viscous flow solver by using a nonlinear potential solver called higher-order spectral method (HOS). The advantages of using a fully nonlinear potential solver for the generation of irregular waves are discussed. In particular, it is shown that the proposed method allows the CFD simulation to start at the time and over the space of interest, retrieved from the potential flow solution. The viscous flow solver is based on the open source library OpenFOAM. The potential solvers used to generate waves are the open source solvers HOS-Ocean and HOS-NWT (Numerical Wave Tank). Several simulation parameters in the CFD solver are investigated in the present study. A HOS wrapper program is newly developed to regenerate wave fields in the viscous flow solver. The wrapper program is validated with OpenFOAM for 2D and 3D regular and irregular waves using relaxation zones. Finally, the extreme waves corresponding to the 1000 year return period condition in the Gulf of Mexico are simulated with the viscous flow solver and the wave elevation is compared with the experiments.

Numerical Simulation of the Resistance and Self-Propulsion Model Tests

The paper proposes a series of numerical investigations performed to test and demonstrate the capabilities of a RANS solver in the area of complex ship flow simulations. Focus is on a complete numerical model for hull, propeller and rudder that can account for the mutual interaction between these components. The paper presents the results of a complex investigation of the flow computations around the hull model of the 3600 TEU MOERI containership (KCS hereafter). The resistance for the hull equipped with rudder, the POW computations as well as the self-propulsion simulation are presented. Comparisons with the experimental data provided at the Tokyo 2015 Workshop on CFD in Ship Hydrodynamics are given to validate the numerical approach in terms of the total and wave resistance coefficients, sinkage and trim, thrust and torque coefficients, propeller efficiency and local flow features. Verification and validation based on the grid convergence tests are performed for each computational case. Discussions on the efficiency of the turbulence models used in the computations as well as on the main flow features are provided aimed at clarifying the complex structure of the flow around the stern.

Full-Scale Reynolds Number VIV Testing of Tri-Helically Grooved Drill Riser Buoyancy Module

A newly developed Tri-Helically Grooved drilling riser buoyancy module design was tested in the towing tank of SINTEF Ocean in June 2017. This new design aims to reduce riser drag loading and suppress vortex-induced vibrations (VIV). Objectives of the test program were two-fold: to assess the hydrodynamic performance of the design allowing for validation of previous computational fluid dynamics (CFD) studies through empirical measurements, and, to develop a hydrodynamic force coefficient database to be used in numerical simulations to evaluate drilling riser deformation due to drag loading and fatigue lives when subjected to VIV. This paper provides the parameters of the testing program and a discussion of the results from the various testing configurations assessed. Tests were performed using large scale, rigid cylinder test models at Reynolds numbers in the super-critical flow regime, defined as starting at a Reynolds number of Re = 3.5 × 105 – 5.0 × 105 (depending on various literatures) and continuing until Re = 3 × 106. Towing tests, with fixed and freely oscillating test models, were completed with both a bare test cylinder and a test cylinder with the Tri-Helical Groove design. Additional forced motion tests were performed on the helically grooved model to calculate lift and added mass coefficients at various amplitudes and frequencies of oscillation for the generation of a hydrodynamic force coefficient database for VIV prediction software. Significant differences were observed in the hydrodynamic performance of the bare and helically grooved test models considering both in-line (IL) drag and cross-flow (CF) cylinder excitation and oscillation amplitude. For the helically grooved model, measured static drag shows a strong independence from Reynolds number and elimination of the drag crisis region with an average drag coefficient of 0.63. Effective elimination of VIV and subsequent drag amplification was observed at relatively higher reduced velocities, where the bare test model shows a significant dynamic response. A small level of expected response for the helically grooved model was seen across the lower range of reduced velocities. However, disruption of vortex correlation still occurs in this range and non-sinusoidal and highly amplitude-modulated responses were observed.

Fluid Structure Interaction Simulations Involving Free Surface

The aim of this paper is to evaluate how much affects the presence of gravity and free-surface to a flexible structure in a classical fluid structure interaction (FSI) problem typically found in off-shore problems and other oceanic applications. The base problem selected is the Turek benchmark case where a deformable plate is attached to the wake of a circular cylinder. To focus on the differences of considering free surface, a simple geometry has been selected and two different situations have been studied: the first one is the classical Turek benchmark, the second is a similar geometry but adding gravity and free surface. The free surface problem was studied placing the structure at different depths and monitoring the deformation and forces on the structure.

FSI Analysis the Dynamic Performance of Composite Propeller

The marine propeller is regarded as critical component with regard to the performance of the ships and torpedoes. Traditionally marine propellers are made of manganese-nickel-aluminum-bronze (MAB) or nickel-aluminum-bronze (NAB) for superior corrosion resistance, high-yield strength, reliability, and affordability. Since the composite materials can offer the potential benefits of reduced corrosion and cavitation damage, improved fatigue performance, lower noise, improved material damping properties, and reduced lifetime maintenance cost, Many researches on the application of the composite materials for marine propeller had been conducted. In this work, the INSEAN 1619 large screw 7 bladed propeller is analyzed, to explore the hydrodynamic and structural performance of composite materials effect on propeller’s performances, The commercial software ANSYS Workbench was used in this research. The coupled FSI method was used to analysis the dynamic performance of INSEAN 1619 large screw 7 bladed propeller made of different materials. The simulation results show that the effect of fluid–structure interaction in the analysis of flexible composite propellers should be considered.