SIMULATION OF FLOW AROUND FLOATING STRUCTURES: SHIPS AND PLATFORMS

Transcrição

1 2013 CAE NAVAL & OFFSHORE Windsor Guanabara, Rio de Janeiro/RJ Brasil 13 de Junho de 2013 SIMULATION OF FLOW AROUND FLOATING STRUCTURES: SHIPS AND PLATFORMS Alexandre T. P. Alho Laboratório de Sistemas de Propulsão DENO/POLI, UFRJ

2 INTRODUCTION Preliminary Considerations Growing demand for high efficiency systems Demand for accurate predictions in less time and at low costs. Accurate CFD models: designers can rely on as an effective design tool. CFD model must be developed based on a good compromise between the quality of the numerical result and the computational effort. Performance prediction of ships and offshore platforms Experimental methods are well-established, but are usually expensive and time-consuming. Optimization process is virtually impossible based on experimental methods: very high costs.

3 INTRODUCTION Examples of CFD Projects CFD Predictions of the Hull Resistance and the Wave System of a Catamaran. Investigate the performance of passive damping foils on heave response of a catamaran. Develop a CFD model to study the effectiveness of passive damping devices on heave motions of mono-column platforms. Methodology The flow around vessel/platform hulls was simulated by means of commercial CFD code (ANSYS CFX). Results are validated against experimental data (if available).

4 CFD PROJECTS Resistance & Wave Cut Motivation Growing demand for high speed multihull vessels. Catamaran/SWATH concept has been received special attention good performance in terms of speed and transversal stability. Objective Validate a CFD model in terms of its performance on estimating hull resistance and calculating the wave cuts generated by the hull. Main Particulars Length (BP): Beam (each hull): 27.6 m 2.97 m Draft (design load): 1.5 m Block coefficient: 0.653

5 IF CFD PROJECTS Resistance & Wave Cut Main Particulars Length (BP): Beam (each hull): 27.6 m 2.97 m Draft (design load): 1.5 m Block coefficient: ,55 IF. Sep 22 Demihull separation 2.75 m (22), 5.25 m (42) and 7.75 m (62): 0,45 0,35 0,25 0,15 IF. Sep.42 IF. Sep B. 0,05 Significant interference effects -0,05-0,15 0,1 0,2 0,3 0,4 0,5 0,6 Fn

6 Resistance (gf) CFD PROJECTS Resistance & Wave Cut Hull Resistance In most cases, numerical errors are lower than 5.0% (max. 7.2%) Exp. CFD Hump & hollow behavior well described Unable to resolve wave-breaking. 0 0,25 0,3 0,35 0,4 0,45 Fn

7 Wave Elevation Wave Elevation Wave Elevation CFD PROJECTS Resistance & Wave Cut Free surface elevations 0,03 0,02 FN = Exp. CFD 0,01 0-0,01-0,02 0,03 0,02 FN = Exp. CFD 0,01-0, ,5 0 0,5 1 1, ,5 3 3,5 x-position -0,01-0,02-0,03 0,04 0,03 0,02 FN = Exp. CFD 0,01-0, ,5 0 0,5 1 1, ,5 3 3,5 x-position -0,01 Good correlation upstream and along the hull. -0,02-0,03-0, ,5 0 0,5 1 1,5 2 2,5 3 3,5 x-position

8 CFD PROJECTS Heave Response Objective Investigate the performance of passive damping foils on heave response of a catamaran viscous damping coefficient. Main Particulars Length (BP): 27.6 m Beam (each hull): 2.97 m Draft (design load): 1.5 m Block coefficient: Passive damping foil.

9 CFD PROJECTS Heave Response Heave Response Without Damping Foil With Damping Foil

10 CFD PROJECTS Heave Response Heave Response Without Damping Foil With Damping Foil

11 CFD PROJECTS Heave Response Objective Develop a CFD model to study the effectiveness of passive damping devices on heave motions of mono-column platforms. Vertical Circular Cylinder External dia.: 110 m Moonpool dia.: 50 m Central Moonpool Devised to improve response in waves. External skirt: damping device

12 CFD PROJECTS Heave Response Free Decay Simulation: Original Skirt

13 Vertical displacement [Norm.] CFD PROJECTS Heave Response Validation: Original Skirt Decay period: good correlation! Numerical (CFD) Experimental Over-estimated amplitude: numerical simulation did not include the damping effect of mooring lines, risers, etc. Time [s]

14 CFD PROJECTS Heave Response Free Decay Simulation: Alternative Skirt Geometry Alternative B

15 CFD PROJECTS Seft-propulsion Test Objective Develop a CFD model dedicated to estimate the propulsion factors and to simulate the self-propulsion test of a hull. Focus Design applications. Main Particulars: Length (Loa): Length (Lpp): Breath (B): Design draught (T): Service Speed (V S ): 73.4 m 70.6 m 14.8 m 2.6 m 9.5 knt

16 CFD PROJECTS Seft-propulsion Test Hull Performance Test speed (V S ): 9.5 knt Total resistance (R T ): 50.6 kn Wake coefficient (w): 0.153

17 CFD PROJECTS Seft-propulsion Test Test Results Propeller revolutions (N): Propeller thrust (T req ): 433 rpm 65.3 kn N = 420 rpm

18 CFD PROJECTS Seft-propulsion Test Results Evaluation Comparison against statistical estimation. Wake fraction, thrust deduction fraction and relative-rotative efficiency predictions based on Holtrop & Mennen (1984). Statistical Numerical Dif. Propeller Revolutions % rpm Propeller Thrust % kn Wake Fraction % --- Thrust Deduction Fraction % --- Relative-rotative Efficiency % ---

19 FINAL REMARKS The overall performance achieved suggests that the CFD numerical models were able to resolve the physics of the flow around vessel/platform hulls. The comparison against experimental results showed that the numerical models were able to provide reasonable performance predictions, suggesting that designers can rely on CFD models as an effective design tool.