CFD ANALYSIS FOR OFFSHORE SYSTEMS: ? multidisciplinary simulation tools for several engineering ...

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4th Latin American CFD Workshop Applied to the Oil and Gas Industry July 12-13, 2010 - Rio de Janeiro - Brazil Organization: Support: CFD ANALYSIS FOR OFFSHORE SYSTEMS: VALIDATION AND APPLICATIONS Daniel F. C. e Silva*, Paulo R. Pagot PETROBRAS Av. Horcio Macedo 950, Prdio 32 Cidade Universitria Ilha do Fundo Rio de Janeiro RJ CEP: 21941-915 *e-mail: danielfc@petrobras.com.br e-mail: pagot@petrobras.com.br ABSTRACT The Scientific Methods group in Petrobras Research Center works on development and application of multidisciplinary simulation tools for several engineering problems related mainly to offshore systems. Recently, there were many different cases where Computational Fluid Dynamics was successfully employed. This work presents a collection of cases where CFD simulations were validated against experimental data and directly used to guide solutions for practical problems. The case#01 consists on calculating the current loads on a FPSO and investigating the influence of appendices such as bilge keels and rudders on the near flow field. In a very similar way, case#02 extends this procedure to wind loads determination. Case#03 presents the calculation of hydrodynamic forces on a torpedo anchor during its installation. The simulation results coupled with a simplified dynamic model allows evaluating the directional stability to different torpedo models. A whole FPSO topside geometry is modeled on case#04, which investigates the flow pattern near the FPSO Helideck. The simulation velocity and turbulence profiles were compared to wind tunnel measurements. These cases, which will be briefly presented, show how CFD tools can be suitable for many practical problems solutions. All simulations here presented were performed using ANSYS CFX. Case #01 Determination of current loads on offshore units In order to predict the dynamic behavior of floating units as well as calculate the mooring lines loads, it is necessary to determine the loads promoted by the sea water flow around the unit hull. Since a low Froude number regime can be considered, only the vessel underwater portion need to be modeled. All current generated forces can be evaluated using a steady-state simulation for each heading angle[1]. Several headings were automatically evaluated using ModeFrontier. Figure 1 presents the transversal force coefficients, compared to towing tank experiments[2]. This validation case encourages the use of this procedure for other vessels. Figure 1 Current transversal force coefficients compared to IPT (Instituto de Pesquisas tecnolgicas) towing tank experiments. For the 90 and 270 headings, the streamlines in a transversal plane are presented. -1-0.8-0.6-0.4-0.200.20.40.60.810 50 100 150 200 250 300 350H ead ingCyyIPT Experiments C FX Simulation4th Latin American CFD Workshop Applied to the Oil and Gas Industry July 12-13, 2010 - Rio de Janeiro - Brazil Organization: Support: Case #02 Determination of wind loads on offshore units The wind loads also need to be evaluated for offshore units for the same reasons described on the previous case. Therefore, a very similar procedure was developed to calculate the forces and moments induced by the airflow around offshore units[1]. Using a domain starting from the mean surface level (equivalent to the wind tunnel floor) and considering only the platform geometry above water, again a steady state simulation was conducted for each individual heading angle condition. Figure 2 shows a comparison between the simulation and the experiments from wind tunnel measurements[3]. Case #03 Torpedo anchor directional stability during the installation process The geometry of offshore anchors is usually determined from a geotechnical perspective. This case presents the proposal of simple modifications on a torpedo anchor to improve its hydrodynamic performance concerning its directional stability. Additionally, the study of a real case torpedo deformation is presented. Although the directional stability investigation requires a dynamic analysis, the CFD simulations were performed on a steady state regime to show for which inclinations the anchor tends to recover the vertical position. The trajectory was investigated through a simplified dynamic model and compared with field measurements[4, 5]. On the Figure 3, pressure contours and flow streamlines on the region near the torpedo deformed fin can be seen. Case #04 Investigation of the airflow around an offshore helideck The offshore helideck wind flow is usually subject to the platform interference. The helideck airspace velocity and turbulence fields are important issues to promote safe helicopter take-off and landing operations. This case brings some CFD results of a helideck wind flow 3D-field defined by the local conditions and constrained by the FPSO structure. A discussion about the chosen CFD boundary conditions is also presented. These CFD results are compared with the wind tunnel model-scale velocity and turbulence measurements[6, 7]. The atmospheric boundary layer wind tunnel measurements were performed with use of two different techniques: Particle Image Velocimetry (PIV) and Constant Temperature Anemometry (CTA). The British standard CAP437[8] is examined and suggestions are made herein. Figure 4 shows a contour of vertical velocity near the Helideck airspace. -1.5-1-0.500.511.50 60 120 180 240 300 360Heading [deg]CoefficientsCMz ExperimentsCMz SimulationCx ExperimentsCx SimulationCy ExperimentsCy SimulationFigure 2 Wind force and moment coefficients for several headings compared to experiments. Figure 3 Pressure contours and streamlines for a torpedo anchor with damaged fins. Figure 4 Contours of vertical velocity in a plane over the helideck airspace. 4th Latin American CFD Workshop Applied to the Oil and Gas Industry July 12-13, 2010 - Rio de Janeiro - Brazil Organization: Support: References [1] SILVA, D. F. C., Validao do Clculo de Esforos de Correnteza e Vento em Embarcaes via CFD, Comunicao Tcnica PETROBRAS, CT MC001/2010. [2] NISHIMOTO, K., Ensaios de Validao e Obteno de Dados para Anlise de Sistema de Ancoragem do P-50 Utilizando DYNASIM2001, Relatrio no 2, DENO/USP, 2001. [3] CLAUSEN, J. W., Wind Tunnel Tests with PIPA II FPSO, DMI-Force 107-32335 Final Report, 2008. [4] SILVA, D. F. C., Anlise Hidrodinmica do Lanamento de Estacas Torpedo, Relatrio Tcnico PETROBRAS, RT MC001/2010. [5] SILVA, D. F. C., CFD Hydrodynamic Analysis of a Torpedo Anchor Directional Stability, OMAE2010-20687, ASME 29th International Conference on Offshore Mechanics and Arctic Engineering, 2010. [6] SILVA, D. F. C.; PAGOT, P. R., Validao de Ferramenta de CFD para Simulao Numrica do Escoamento de ar sobre Helipontos Offshore, Relatrio Tcnico PETROBRAS, RT MC078/2009. [7] SILVA, D. F. C.; PAGOT, P. R.; NADER, G.; JABARDO, P. J. S., CFD Simulation and Wind Tunnel Investigation of a FPSO Offshore Helideck Turbulent Flow, OMAE2010-20686, ASME 29th International Conference on Offshore Mechanics and Arctic Engineering, 2010. [8] CIVIL AVIATION AUTHORITY, Offshore Helidecks Landing Areas Guidance on Standards, CAP437, sixth edition, December 2008.

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