Raphael I. Tsukada

Model Test of a Steel Catenary Riser in a Towing Tank

The objective of the present work is the study of the dynamic behavior of steel catenary risers (SCRs), focusing on the contribution of vortex-induced vibration (VIV), through model test in a towing tank. Nowadays, a great deal of effort is being spent in order to better understand VIV’s contribution in the dynamics of riser structures through experiments, analytical analysis and numerical predictions. In the present work, the design of a SCR model test, along with its setup in a towing tank, will be described in detail and discussions of main results from the experiments will be presented. The experiment has been conducted under several simulated environmental condition combinations, varying the towing speed, riser top forced oscillation amplitudes, waves amplitudes and periods. Very promising results have been observed from the experiment. Riser oscillations due to high harmonics of vortex shedding were observed. Analysis of the experimental results, coupled with the support of numerical tools, showed the influence of the phenomena of traveling waves in the cross-flow response as is reported from the literature.Copyright

Numerical Simulation of a Submersible Buoy Using a Wake Oscillator Model Calibrated for VIM

New discoveries of petroleum reservoirs in ultra deep-water depths, like Pre-salt fields in Santos Basin, are demanding new riser systems concepts. In this scenario, the Free-Standing Hybrid Riser (FSHR) system is a viable choice. A submersible buoy connected by rigid and flexible risers constitutes this riser system. The sea current can cause the Vortex-Induced Motion (VIM) of the buoy, which can increase significantly the riser fatigue damage. Although the VIM phenomenon is similar to Vortex-Induced Vibration (VIV), it generally occurs in rigid bodies with low aspect ratio, where end effects causes tridimensional flow behavior. Therefore, the vortex wake characteristics and the hydrodynamics coefficients found for VIV is no longer valid for VIM. In this context, wake oscillator models used for VIV prediction in actual form is not adequate for the VIM prediction of the buoys. In this paper, a VIV wake oscillator model is calibrated for VIM, through hydrodynamic coefficients found in the technical literature. In order to verify accuracy, the VIM calibrated wake oscillator model is used to reproduce some FSHR reduced model tests. The results of amplitude and frequency of oscillation against the reduced velocity obtained from the numerical simulation are compared with the experimental results. The numerical results presented the same trend with some differences in amplitude. The amplitude deviation could be related to the hydrodynamics coefficients used in the calibration of the wake oscillator model.© 2015 ASME

A Numerical Simulation Procedure for Vortex-Induced Vibration of Pipelines With Free Span

Long subsea pipelines are generally used to connect an offshore petroleum production facility and a petroleum terminal at the coast, to export produced oil or gas. In the pipeline way though the sea bottom, free pipeline spans happen due to the uneven and irregularities of ocean ground. In this portion of the pipeline length, the sea current can cause forces in the pipe, as the drag and vortex-induced vibration forces, respectively. These forces can have great influence in the structural stress and fatigue damage. Therefore, they must be carefully analyzed and considered in the pipeline design. The present work aims to introduce a numerical simulation procedure based on a semi-empirical VIV model to predict dynamic response of a pipeline with free span. Computations were carried out in time domain using finite element method. Beam elements considering large displacements and rotation were used to represent the pipe behavior. The VIV forces are calculated based on hydrodynamic coefficients, like added mass, lift and drag coefficients. In order to verify the accuracy of the VIV estimation procedure, comparisons with experimental results are presented.Copyright

Pipeline VIV: Analytical Solution, Experiments and Parameter Identification

The study presents a closed-form solution for the vibration of a simply-supported beam due to vortex shedding, assuming linear elasticity and considering fluid damping. The in-line and cross-flow fluid forces are coupled to the beam equation as harmonic nonhomogeneous terms. Experimental results of 2 DOF VIV of a flexible small scale pipe in a uniform stream are presented for perpendicular an oblique (at 60 degrees of the translation direction) pipe. The range of relative velocity is from 1 to 10. The performance of two fluid damping models (Venugopal, 1996; Blevins – modified, 1990) is evaluated by comparing their predictions to the measurements of the in-line and cross-flow oscillations. Finally, ranges for in-line and cross-flow force coefficients are proposed and compared to the literature.Copyright

Numerical Simulations of Ocean Drilling System Behavior With a Surface or a Subsea BOP in Waves and Current

Subsea equipment such as the drilling riser and the subsea Blow-Out Preventer (BOP) are mandatory in traditional systems used in deep sea drilling for ocean floor research and petroleum wellbore construction. The drilling riser is the vertical steel pipe that transfers and guides the drill column and attached drilling bit into a wellbore at the sea bottom. The BOP is used to protect the wellbore against uncontrolled well pressures during the offshore drilling operation. Presently, there is a high level of drilling activity worldwide and in particular in deeper and ultra-deeper waters. This shift in depth necessitates not only faster drilling systems but drilling rigs upgraded with a capacity to drill in the deep water. In this scenario, two general drilling systems are today considered as alternatives: the traditional system with the subsea BOP and the alternate system with the surface BOP. In the present paper, the two systems are initially described in detail, and a numerical simulation in time domain to estimate the system behavior is presented. Simulations of a floating drilling rig coupled with the subsea and surface BOP in waves and current are carried out for a comparison between the two methods. Results are shown for riser and BOP displacements. Critical riser issues for the systems are discussed, comparing results from both drilling system calculations. Conclusions are addressed showing advantages and disadvantages of each drilling system, and indicating how to correct the problems detected on each system.© 2008 ASME

Isothermal and Non-Isothermal Transient Gas Flow in Submarine Pipeline

The discoveries of the Pre-salt oilfields have driven the development of new technologies to enable the production of the deepwater reservoirs. In this scenario, subsea pipelines play an important role. Analysis of the steady and transient flow inside the pipes should be addressed in the design, considering the variation of the fluid properties. In this context, a pipe flow simulator project has been developed to attend gas flow analysis for petroleum industry. In this project, the fluid compressibility factor (Z-factor) and the viscosity are considered function of the pressure, temperature and gas composition. The non-isothermal transient gas flow were calculated using the Method of Characteristics (MOC). The results shown the difference of the isothermal and non-isothermal steady state and transient flow.Copyright

Parametric Study of the VIM Response of Submersible Buoy of the FSHR System

Nowadays, hybrid risers systems have been continuously adopted for deep and ultra-deep water petroleum fields. This riser system consists of a submersible buoy connected to flexible and rigid risers. Commonly, the buoy is installed 200 meters depth from the free surface to avoid the wave forces. Therefore, the main forces acting in this part of the system is due to the current flow. In this case, the VIM force is one of the forces that induce oscillations in the buoy, which can cause significantly fatigue damage to the riser system. In this scenario, understand the VIM response of the buoys is important for the system design. In this work, the semi-empirical approach presented by Tsukada et al. (2014), Numerical Simulation of VIM Response of a Submersible Buoy Using a Semi-Empirical Approach, OMAE2014–24187, is applied to perform a parametric study varying the riser length and the properties and dimensions of the buoy of a FSHR system. The main objective of this study is to show the trends of the response due to the variation of this parameters, which assists in understanding the real riser system response presented later in this paper. The real riser system properties and dimensions are obtained from the technical literature and used in the simulations. The results of amplitude and frequency of oscillation of the buoy are compatible with the results of FSHR model test experiments.Copyright

Numerical Simulation of VIM Response of a Submersible Buoy Using a Semi-Empirical Approach

Nowadays, discoveries of petroleum reservoirs are located in ultra deep-water depths. In this scenario, risers systems generally demand submersible buoys to support the riser, in order to reduce weight in floating platform and riser tensioners. Usually, these buoys are installed below 200 meters depth to avoid the wave forces. However, in this condition the sea current cause the Vortex-Induced Motion (VIM) of the buoys, which can increase significantly the riser fatigue damage. Although the VIM phenomenon is similar to Vortex-Induced Vibration (VIV), it generally occurs in rigid bodies with low aspect ratio, where end effects causes tridimensional flow behavior. Therefore, the vortex wake characteristics and the hydrodynamics coefficients found for VIV is no longer valid for VIM. It makes complex the prediction of VIM in buoys. In this paper, a semi-empirical model using hydrodynamic coefficients found for low aspect ratio cylinders are presented. In order to verify accuracy of numerical simulations, results are compared with experimental data presented in the literature and a good agreement is found.Copyright

Evaluation of Numerical Methods Applied in the Analysis of the Transient Flow of Pipeline Systems

Brazilian subsea exploration is increasing specially after the post salt petroleum field discovery. Several challenges have been imposed for the production of those fields. In this scenery, the transport of oil and gas from the production field to the continent is a problem, especially when the fields are located at a great distance from the coast. A possible solution could be the use of subsea pipeline systems, for the transportation of the fluids produced from the petroleum wells. For the pipeline system design it is highly recommended the evaluation of the transient flow, considering the water hammer phenomenon. The definition for this phenomenon is given by the pressure variation due to operation singularities in the pipe system. The disruption in the flow originated by the operation of valves or failure of a pump can be listed as some of the main causes of the water hammer. The basic equations to model the water hammer in fluid mechanics comes from two partial differential equations, the equation of continuity and momentum. The solution of those equations can be obtained by different numerical methods. In this context, this work seeks to contrast results obtained by finite difference method (FDM), the method of characteristics (MOC) and finite elements method (FEM) solutions for the water hammer problem. Those numerical methods were implemented and used to solve a simple system, which are composed of an infinite reservoir, a pipeline and a valve. In this case the valve is closed, originating the water hammer phenomenon. Although it can be considered a simple problem, it allows the evaluation of those numerical methods. Performance, convergence and accuracy were evaluated in order to support the choice of the best numerical method for the development of a numerical simulator used in complex and greater pipeline system design.Copyright