scholarly journals Bilge Keel Induced Roll Damping of an FPSO With Sponsons

2016 ◽  
Author(s):  
Babak Ommani ◽  
Nuno Fonseca ◽  
Trygve Kristiansen ◽  
Christopher Hutchison ◽  
Hanne Bakksjø
Keyword(s):  
Free Surface ◽  
Two Dimensions ◽  
The Body ◽  
Proximity Effects ◽  
Roll Damping ◽  
Damping Coefficients ◽  
Free Decay ◽  
Strip Theory ◽  
Decay Tests ◽  
Bilge Keel

The bilge keel induced roll damping of an FPSO with sponsons is investigated numerically and experimentally. The influence of the bilge keel size, on the roll damping is studied. Free decay tests of a three-dimensional ship model, for three different bilge keel sizes are used to determine roll damping coefficients. The dependency of the quadratic roll damping coefficient to the bilge keel height and the vertical location of the rotation center is studied using CFD. A Navier-Stokes solver based on the Finite Volume Method is adopted for solving the laminar flow of incompressible water around a section of the FPSO undergoing forced roll oscillations in two-dimensions. The free-surface condition is linearized by neglecting the nonlinear free-surface terms and the influence of viscous stresses in the free surface zone, while the body-boundary condition is exact. An averaged center of rotation is estimated by comparing the results of the numerical calculations and the free decay tests. The obtained two-dimensional damping coefficients are extrapolated to 3D by use of strip theory argumentations and compared with the experimental results. It is shown that this simplified approach can be used for evaluating the bilge keel induced roll damping with efficiency, considering unconventional ship shapes and free-surface proximity effects.

Effect of Bilge Radius and Bilge Keel Height on Roll Damping Performance of Floating Structures

2021 ◽  
Author(s):  
Chang Seop Kwon ◽  
Joo-Sung Kim ◽  
Hyun Joe Kim
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Kinetic Energy ◽  
Floating Body ◽  
Rectangular Section ◽  
Roll Damping ◽  
Floating Structures ◽  
Free Decay ◽  
Computational Fluid Dynamics Cfd ◽  
Bilge Keel

Abstract A round bilge with a bilge keel structure is a key element which can alleviate roll motions of ships and floating structures by transferring the roll momentum of a floating body into the kinetic energy of water. This study presents a practical guide to properly designing a bilge radius and bilge keel height of a barge-shaped and tanker-shaped FPSOs. A parametric study to figure out the effect of bilge radius and bilge keel height on the roll damping performance is conducted through a series of numerical roll free decay simulations based on Computational Fluid Dynamics (CFD). The bilge radius is normalized by the half breadth of ship, and the bilge keel height is normalized by the maximum bilge keel height which is limited by the molded lines of a side shell and bottom shell. In addition, it is investigated to identify how the roll damping performance of a rectangular section differs from the result of a typical round bilge section with maximum available bilge keel height.

Hydrodynamic Properties of a Suction Can Oscillating Near the Free Surface

2009 ◽  
Author(s):  
C. Plummer ◽  
G. Macfarlane ◽  
Y. Drobyshevski
Keyword(s):  
Free Surface ◽  
Added Mass ◽  
Hydrodynamic Properties ◽  
Noticeable Effect ◽  
Sea Floor ◽  
Notable Effect ◽  
Free Decay ◽  
Decay Tests ◽  
Quadratic Damping ◽  
Motion Amplitude

Offshore operations often require heavy subsea equipment, such as suction piles or cans, to be lowered by a support vessel into the sea. A lifting device must have adequate capacity to withstand the dynamic loads generated by the motions of the vessel and the heave response of the structure. The objective of this study is to determine the added mass and damping of a suction can oscillating in heave near the free surface; knowledge of these hydrodynamic properties is required for the accurate prediction of the dynamic lift forces during the deployment. This project is a logical progression following two similar studies, which investigated these hydrodynamic properties for the suction can in the mid-water position and when approaching the seabed. All three studies involved the conduct of model tests to determine the hydrodynamic properties. Free decay tests were conducted at several heave frequencies, and the added mass, linear and quadratic damping components were determined. In addition, the effect of varying the percentage of open hatch area has been investigated. Test data demonstrates that the heave added mass is strongly dependent on the frequency of motion, and its values are significantly smaller than those measured in the unrestricted flow. From observations, there was no dependency on the motion amplitude, nor did the size of open hatches have notable effect on the added mass. It was observed that when the top plate of the structure was in contact with the free surface a mean “pull down” force appeared. This force is caused by the suction underneath the top plate when the can moves upwards. As opposed to the mid-water position and near the sea floor, the study indicates that the area of open hatches has no noticeable effect on the heave damping when the suction can is oscillating near the free surface.

scholarly journals Bilge-Keel Influence on Free Decay of Roll Motion of a Realistic Hull1

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Yichen Jiang ◽  
Ronald W. Yeung
Keyword(s):  
Free Surface ◽  
Fluid Motion ◽  
Three Dimensional ◽  
Vortex Method ◽  
Roll Motion ◽  
Two Dimensional ◽  
Desktop Computer ◽  
Roll Damping ◽  
Discrete Vortex ◽  
Bilge Keel

The prediction of roll motion of a ship with bilge keels is particularly difficult because of the nonlinear characteristics of the viscous roll damping. Flow separation and vortex shedding caused by bilge keels significantly affect the roll damping and hence the magnitude of the roll response. To predict the ship motion, the Slender-Ship Free-Surface Random-Vortex Method (SSFSRVM) was employed. It is a fast discrete-vortex free-surface viscous-flow solver developed to run on a standard desktop computer. It features a quasi-three-dimensional formulation that allows the decomposition of the three-dimensional ship-hull problem into a series of two-dimensional computational planes, in which the two-dimensional free-surface Navier–Stokes solver Free-Surface Random-Vortex Method (FSRVM) can be applied. In this paper, the effectiveness of SSFSRVM modeling is examined by comparing the time histories of free roll-decay motion resulting from simulations and from experimental measurements. Furthermore, the detailed two-dimensional vorticity distribution near a bilge keel obtained from the numerical model will also be compared with the existing experimental Digital Particle Image Velocimetry (DPIV) images. Next, we will report, based on the time-domain simulation of the coupled hull and fluid motion, how the roll-decay coefficients and the flow field are altered by the span of the bilge keels. Plots of vorticity contour and vorticity isosurface along the three-dimensional hull will be presented to reveal the motion of fluid particles and vortex filaments near the keels.

On the harmonic oscillations of a rigid body on a free surface

1965 ◽  
Vol 21 (3) ◽  
pp. 427-451 ◽  
Author(s):  
W. D. Kim
Keyword(s):  
Integral Equation ◽  
Free Surface ◽  
Rigid Body ◽  
Potential Problem ◽  
Harmonic Oscillation ◽  
Added Mass ◽  
The Body ◽  
Rigorous Solution ◽  
Damping Coefficients

The present paper deals with the practical and rigorous solution of the potential problem associated with the harmonic oscillation of a rigid body on a free surface. The body is assumed to have the form of either an elliptical cylinder or an ellipsoid. The use of Green's function reduces the determination of the potential to the solution of an integral equation. The integral equation is solved numerically and the dependency of the hydrodynamic quantities such as added mass, added moment of inertia and damping coefficients of the rigid body on the frequency of the oscillation is established.

The Influence of Vortex Formation on the Damping of FPSOs With Large Width Bilge Keels

2012 ◽  
Author(s):  
Allan C. de Oliveira ◽  
Antonio Carlos Fernandes ◽  
Hélio Bailly Guimarães
Keyword(s):  
Pressure Distribution ◽  
Vortex Shedding ◽  
Vortex Formation ◽  
Roll Damping ◽  
Flat Bottom ◽  
Long Time ◽  
Large Width ◽  
Decay Tests ◽  
Visualization Techniques ◽  
Bilge Keel

The roll damping of a FPSO assessment is a different subject than the ship case. The fact that the FPSO is not moving changes the flow hydrodynamics in such a way that the well established understanding is no longer applied. This is so at least for certain particularities such as flat bottom, no lift effect due to zero velocity, and so on. Recent researches have proven the strong effect of the vortex shedding on the roll damping of a FPSO mainly when large width bilge keel are present. Although these effects are known by a long time for ships, the increase of the vortex magnitude due the large width bilge keels on a FPSO has let to uncertainties about the behavior of the structures and the situation is challenging. It has been understood that the vortex can modify deeply the pressure distribution along the FPSO hull in such way that the final roll dissipation is higher. Surprisingly, under certain conditions the memory effects are small. The use of visualization techniques allied to the analysis of several decay tests for the same hull can help the understanding of several aspects such as the uncertainty in the measurements and the vortex behavior.

Time-Domain Simulations of Radiation and Diffraction Forces

2010 ◽  
Vol 54 (02) ◽  
pp. 79-94 ◽  
Author(s):  
Xinshu Zhang ◽  
Piotr Bandyk ◽  
Robert F. Beck
Keyword(s):  
Free Surface ◽  
Time Domain ◽  
Three Dimensional ◽  
The Body ◽  
Surface Boundary ◽  
Two Dimensional ◽  
Forward Speed ◽  
Time Step ◽  
Surface Boundary Conditions ◽  
Strip Theory

Large-amplitude, time-domain, wave-body interactions are studied in this paper for problems with forward speed. Both two-dimensional strip theory and three-dimensional computation methods are shown and compared by a number of numerical simulations. In the present approach, an exact body boundary condition and linearized free surface boundary conditions are used. By distributing desingularized sources above the calm water surface and using constant-strength flat panels on the exact body surface, the boundary integral equations are solved numerically at each time step. The strip theory method implements Radial Basis Functions to approximate the longitudinal derivatives of the velocity potential on the body. Once the fluid velocities on the free surface are computed, the free surface elevation and potential are updated by integrating the free surface boundary conditions. After each time step, the body surface and free surface are regrided due to the instantaneous changing wetted body geometry. Extensive results are presented to validate the efficiency of the present methods. These results include the added mass and damping computations for a Wigley III hull and an S-175 hull with forward speed using both two-dimensional and three-dimensional approaches. Exciting forces acting on a Wigley III hull due to regular head seas are obtained and compared using both the fully three-dimensional method and the two-dimensional strip theory. All the computational results are compared with experiments or other numerical solutions.

Experimental Determination of Roll Damping Coefficient for FPSO

2014 ◽  
Author(s):  
Oliver A. Seelis ◽  
Longbin Tao
Keyword(s):  
Experimental Determination ◽  
Cross Section ◽  
Test Data ◽  
Transfer Functions ◽  
Damping Coefficient ◽  
Roll Damping ◽  
Damping Coefficients ◽  
Decay Test ◽  
Bilge Keel

The roll damping coefficient is a crucial parameter for several design and operational aspects of FPSOs. The accurate prediction of the coefficient is not a trivial task and generally performed experimentally. A polynomial linearization of the decay test data has been widely applied in the offshore industry. However, research has indicated that for FPSOs with rectangular cross section and attached bilge keels, this methodology may lead to inaccurate damping coefficients. This paper presents a study on the experimental determination of roll damping coefficients for FPSOs, obtained by free decay tests. For this purpose model tests are executed in the towing tank of the Marine Hydrodynamic Laboratory at Newcastle University. The model is based on the design of a purposely build FPSO, as typically applied in the central North Sea sector. The cross section of the FPSO is boxed shaped with a characteristic knuckle shaped bilge. The tests are conducted using three different bilge keel arrangements. The parametric change in bilge keel size results in the variation of the flow characteristics around the bilge knuckle. The damping coefficients are then established from the decay test data using a polynomial approach, a bi-linear approach and a hyperbolic approach. A comparison between the damping evolutions obtained with the different methodologies is performed for each bilge keel configuration. Further, a numerical model of the FPSO is created using DNVs Sesam software. With the established damping coefficients, damping matrices are manually defined as an input to Sesam and roll transfer functions are numerically established. The computational determined transfer functions are then compared against the RAOs obtained from the model tests in regular waves to determine the most appropriate methodology. The damping coefficient for the bare hull is well established by all three proposed methodologies. However, with the attached bilge keels the bi-linear and the hyperbolic methodologies produce damping coefficients reflecting the experimental results more accurately than the polynomial approach, indicating that the recently developed hyperbolic method is a valid alternative, and in certain cases, is more suitable to determine the roll damping coefficient. The experimental measurements could serve as a benchmark for further research and contribute to the practical application of FPSO roll damping determination.

scholarly journals Estimation of the Motion Performance of a Light Buoy Adopting Ecofriendly and Lightweight Materials in Waves

2020 ◽  
Vol 8 (2) ◽  
pp. 139 ◽  
Author(s):  
Se-Min Jeong ◽  
Bo-Hun Son ◽  
Chang-Yull Lee
Keyword(s):  
Fluid Dynamics ◽  
Wave Motion ◽  
Viscous Damping ◽  
Motion Simulation ◽  
Incoming Wave ◽  
Damping Coefficients ◽  
Free Decay ◽  
Computational Fluid Dynamics Cfd ◽  
Motion Performance ◽  
Decay Tests

In this study, the methods and results of numerical simulations to estimate the motion performance of a newly developed lightweight light buoy in waves and to check the effect of conceptually developed appendages on that performance were introduced. The results from a potential-based motion analysis with viscous damping coefficients obtained from free decay tests using computational fluid dynamics (CFD) and those obtained from wave motion simulation using CFD were compared. From these results, it was confirmed that viscous damping should be considered when the frequency of an incoming wave is close to the natural frequency of the buoy. It was estimated that the pitch and heave motions of the light buoy became smaller when the developed appendages were adopted. Although the quantitative superiority of the appendages was different, the qualitative superiority was similar between both results.

ON THE ROLL DAMPING OF AN FPSO WITH RISER BALCONY AND BILGE KEELS

2021 ◽  
Vol 153 (A2) ◽  
Author(s):  
R van ’t Veer ◽  
F Fathi
Keyword(s):  
Offshore Structures ◽  
Continuous Operation ◽  
Harsh Environments ◽  
Cfd Simulations ◽  
Roll Damping ◽  
Noticeable Effect ◽  
Damping Coefficients ◽  
Fluid Behaviour ◽  
Bilge Keel ◽  
Damping Model

Although the topic of roll damping of vessels at sea is already brought to the attention of naval architects by Froude more than 100 years ago, the physics of it remain intriguing, even today. An accurate prediction of the motions of offshore structures in harsh environments, designed for 25 years continuous operation, is the topic of this paper. Model test experiments for two FPSO’s developed by SBM for Petrobras are discussed. It is shown that the FPSO submerged riser balcony on one side of the vessel contributes to the roll damping through similar physics as the bilge keel does. Flow memory effects are discussed in detail since these are shown to have a noticeable effect on the roll damping coefficients. The paper further employs 3D CFD simulations to enhance the understanding of the fluid behaviour around the FPSO appendages, necessary to construct a rational and accurate roll damping model in the future.

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