Bilge Keels Research Articles

A Numerical Study of the Roll Damping for Double-Symmetric Bodies

Roll damping is essential for describing properly the motions of a ship, particularly when operating in rough sea conditions, being determinant for parametric or synchronous roll phenomena. Roll damping is a complex process of energy transfer from the hull to the water, which affects the amplitude of motion. Roll damping is dominated by viscous effects as well as by the interaction of the ship with the free surface. Numerical simulations of the free roll decay are carried out in this paper for a double-symmetric floating hull with one or two bilge keels based on an unsteady viscous flow solver. The numerical solutions reported in here are computed with the ISIS-CFD viscous flow solver, part of the Numeca FineTM/Marine suite. Turbulent flow is simulated by solving the unsteady equations of flow. Closure to the turbulence is achieved through the Shear Stress Transport (SST hereafter) based Detached Eddy Simulation (DES), which provides the accuracy of LES for highly separated flow regions and a computational efficiency of RANS in the near-wall region. Free roll decay is studied for various initial roll angles at a given advancing speed of the main hull. The influences of the bilge keel surface and initial roll angle on the roll damping coefficient are discussed.

Most often instantaneous rotation center (MOIRC) for roll damping assessment in the free decay test of a FPSO

The previous study of Fernandes et al. [1] showed that rather than a fixed rotation center, the phenomenology can be better represented by the concept of a MOIRC. This study contributes to understanding Instantaneous Rotation Centers (IRC) and Most Often Instantaneous Rotation Centers (MOIRC) behavior during the free roll decay tests of a FPSO by following the time series of the IRC. It is important from a practical standpoint to understand that IRC behavior leads to distinct damping values. The work shows that the time series trace of the vertical position of the IRC resembles a tangent-form for lower damping. But when the damping increases, the time series trace changes to double parabolic type (one parabola with concavity downwards and another with concavity upwards). Consequently, the MOIRC changes its positions. The effect of the MOIRC on roll damping as a consequence of vortex generation on bilge keels has for half cycles. It explains why the damping is different from the clockwise to the counterclockwise oscillation, finally a point of finding out that considering the IRC is important for roll damping assessment. The linear and nonlinear analyses, also presented, have qualitatively confirmed the observations from the experimental analysis.

Experimental Study on Estimation of Roll Damping for Various Midship Sections

The magnitude of the roll motion of a floating structure depends on the roll damping acting on the body. In other words, the roll damping of a floating structure must be accurately obtained in order to precisely evaluate the roll motion. Various methods are used to evaluate the roll damping of a floating structure, such as the linear potential theory, computational fluid dynamics (CFD), and model tests. However, it is difficult to evaluate the roll motion of a floating structure with appendages such as a bilge keel and riser slot due to the limitation of ignoring the viscous effects in the linear potential theory. Among these methods, a model test based on a free decay test and harmonic excited roll motion (HERM) is known to be the most reliable method to estimate the roll damping of the floating structures. In this study, model tests using free decay and HERM techniques were performed in the Ocean Engineering Basin (OEB) of KRISO with various types of midship sections. The roll damping results were estimated based on post-processing methods using both techniques, and the roll damping results were compared.

A sliding grid based method for the roll decay simulation

The paper describes a novel technique meant to support the numerical prediction of the roll decay for a ship hull equipped with bilge keels in an attempt of establishing a methodology for accurate prediction of the damping forces which occur during the oscillation. The numerical simulation is achieved by making use of the ISIS-CFD solver, part of the FineTM/Marine package available in the NUMECA suite. The solver is based on finite volume method to build the spatial discretization for the governing equation to solve the incompressible unsteady RANSE in a global approach. Closure to the turbulence is achieved by making use of the detached eddy simulation model which has been found as being enough accurate. All the computations are performed in respect to an earth-fixed Cartesian reference frame with only one degree of freedom i.e. roll motion. The validation of the computed solution is done through the comparisons with the experimental data.

Prediction of roll hydrodynamic characteristics for a ship hull section with a high-order finite volume solver

This work predicts hydrodynamic characteristics for a rolling ship hull section. Flows about the mid-section of a KVLCC2 under harmonic excited roll motions are simulated with solutions of Navier-Stokes (NS) equations filtered by a large eddy simulation (LES) with a finite volume (FV) solver. This solver applies a fourth-order compact scheme to discrete spatial terms, a fourth-order Runge-Kutta scheme to advance equations, a momentum interpolation method (MIM) to avoid pressure and velocity decoupling, a volume of fluid (VOF) method to capture free surface, and a ghost cell immersed boundary method (GCIBM) to simulate rolling ship. Firstly, verification is implemented to confirm the convergency. Validation is followed by comparison with experimental data. Then, systematic assessments are made on the effects of roll amplitude, frequency and bilge keels by analyzing the tendency of roll moment, damping and added mass moment of inertia as well as flow structures. Wave radiation and viscous effects are elucidated through flow structures. As a result, viscous effect is dominant. Roll amplitude, frequency and bilge keels influence roll hydrodynamic characteristics substantially. Besides, causes of vortex generation, shedding and movement, variation of dynamic pressure field and mechanism of bilge keel to mitigate roll motion are disclosed.

Numerical prediction of roll damping of ships with and without bilge keel

Estimation of roll damping of ships from free roll decay experiments or using appropriate semi-empirical formulae is a common practice. The total roll damping of a ship or a barge can be strongly nonlinear consisting of linear and nonlinear components; especially when bilge keel is fitted to the hull as a damping device and also when angles of roll are large. CFD simulations of nonlinear roll motion with experimental validation is gaining acceptance in the recent past.The present work reports three dimensional simulations of roll decay of a ship model, both with and without bilge keel, with experimental validation. A good comparisons of roll damping, natural period and added mass with experiments are obtained. The use of large eddy simulation on this class of problems is explored and commented upon. Ikeda's semi-empirical method over-predicts roll damping at larger roll angles. The pressure aft of bilge keel is found to be deviating whereas the pressure in front of bilge keel showed a close resemblance with semi-empirical calculations. Effect of bilge keel thickness, degrees of freedom and scaling on roll motion and damping is found to be insignificant for a barge-like ship model chosen in this study.

Effects of the asymmetric riser and bilge keel arrangements on FPSO green water assessment

This article aims to report important findings on how the asymmetric riser and bilge keel arrangements affect the motion response and green water assessment by using a real FPSO conversion project. Recently, the authors have proposed a practical approach for short-term and long-term green water prediction. In this paper, the method has been further extended to include the effect of truncated bilge keel by using Morrison elements. Numerical studies are conducted focusing on the effect induced by asymmetric riser arrangement and truncated bilge keels. Comparisons of short-term and long-term results between different models indicate that the FPSO’s motion is significantly affected by asymmetrically arranged appendages and attachments in a complicated way. The relative wave elevation is also affected by appendages and attachments, but not the same trend as the motion response. The effect of the asymmetric arrangement of risers and bilge keel on long-term relative wave elevation response has been captured by both traditional contour line approach and response-based analysis, but some discrepancy identified between the results from the two methods indicates the limitation of the traditional contour line approach.

Theoretical investigation on the seakeeping performance of a supply vessel with X-bow

The X-bow concept was developed by Ulstein company, in order to increase the ship hydrodynamic performance. Low resistance, reduced fuel consumption, smoother motion and low acceleration levels are some of the hydrodynamics improvements. A theoretical analysis on the seakeeping performance of a supply vessel with X-bow form was performed by using Aveva Marine Engineering Software and it is presented in this paper. The ship motions and accelerations on irregular waves were calculated and the influence of the roll gyradius and bilge keel was analysed. The conclusion reveals the important influence of the mentioned elements on the ship motions and accelerations.

Resistance Performance Map through Studies on the Dimension Parameters of the Bilge Keel attached on the Fishing Vessel

Resistance Performance Map through Studies on the Dimension Parameters of the Bilge Keel attached on the Fishing Vessel

Ship Manoeuvring Prediction Based on Numerical Towing Tank Technique

A practical procedure is proposed in this paper to predict ship manoeuvrability. A three degrees of freedom MMG (Japanese Manoeuvring Mathematical Modelling Group)-type model is established to simulate rudder manoeuver. Propeller thrust and rudder loads are calculated by empirical formulas, whereas the hull forces as well as moment are determined with hydrodynamic derivatives which are derived from CFD (Computational Fluid Dynamics) computations. An own developed RANS (Reynolds-Averaged Naiver-Stokes) solver on the base of OpenFOAM is applied to simulate a range of PMM (Planar Motion Mechanism) tests and Fourier analyses of the computed results are carried out to obtain the required derivatives. In order to demonstrate the effectivity of the whole procedure and the RANS computations, the US (United States) combatant DTMB 5415 is taken as a sample for an application. Forced motions of surge, sway, yaw and yaw with drift for the bare hull with bilge keels are simulated. Thereafter, simulations of standard rudder manoeuvers, i.e. turning and zigzag, are performed by applying the computed derivatives. The results are compared with available measured data. It has been shown that the present procedure together with the RANS method can be used to evaluate the manoeuvrability of a ship since general good agreements between the simulated results and measured data are achieved.

Bilge keel design for the traditional fishing boats of Indonesia's East Java

Seakeeping, especially for the roll motions, is of critical importance to the safe operation of fishing boats in Indonesia. In this study, a traditional East Java Fishing Boat (EJFB) has been analysed in terms of its seakeeping performance. Furthermore, a bilge keel was designed to reduce the roll motions of the EJFB using multiple stages approach. After installing the designed bilge keels, it was shown that up to 11.78% and 4.87% reduction in the roll response of irregular seaways and the total resistance under the design speed, respectively. It was concluded that the roll-stabilized-EJFB will enhance the well-being of the fisherman and contribute to the boats' safe operation, especially in extreme weather conditions. Moreover, the total resistance reduction of the EJFB due to the installation of the designed bilge keels also resulted in increased operational efficiency and reduced fuel costs and fuel emissions for local stakeholders.

Towing-tank experiment and analysis of nonlinear roll damping for a drillship with different appendages

Offshore platforms with ship-like hull structures have relatively low roll damping moment. In this paper, variations of roll reduction appendages based on increasing damping strategy are proposed. The appendages are an ordinary bilge keel, a newly adopted variation involving extended platforms with ship-like hull structures, such as drillships or reversed flaps, and a stinger type. To validate the roll damping performance of newly designed appendages, a series of free-decay data for the roll motion in calm water and in regular waves are obtained from towing tank tests. The free-decay data for the roll motion is analyzed using a linear-plus-quadratic damping method as well as an alternative method based on the Hilbert transform in order to overcome the possible problem of nonlinearity for non-conventional roll stabilizers. The Hilbert transform technique has the advantage over polynomial damping models for not assuming the form of the roll characteristics. A comparison of the analysis methods for the different appendages is carried out for linear, nonlinear, and equivalent linear damping methods. The results are crosschecked in regular beam waves for different amplitudes near their resonant frequencies. From the experimental analysis, the linear and nonlinear roll characteristics for various appendages are discussed.

Using Length of Bilge Keel to Length of Waterline Ratio to Reduce Ship Rolling Motion

Using Length of Bilge Keel to Length of Waterline Ratio to Reduce Ship Rolling Motion

Using Length of Bilge Keel to Length of Waterline Ratio to Reduce Ship Rolling Motion

Fishing vessels must have good stability and manoeuvrability. Hull with round bottom shape has a relatively poor rolling duration compared to other forms. Rolling duration reduction will improve the quality of stability of the ship which can be obtained with bilge keel installation. The objectives of this research are 1) to compare each parameter value on model ship by using bilge keel and 2) to determine the minimum ratio of bilge keel's length toward waterline length on the model ship which still has the ability to reduce rolling motion on the ship. The method used was giving treatment to model ship and observing its rolling motion with different lengths of bilge keel. Based on the result of the research, it can be concluded that bilge keel installation with some length ratio toward length of waterline has different results significantly, and bilge keel with length ratio 0.2 still have effective capability in reducing the rolling motion of the model ship.

A Body-Nonlinear Green’s Function Method with Viscous Dissipation Effects for Large-Amplitude Roll of Floating Bodies

A novel time-domain body-nonlinear Green’s function method is developed for evaluating large-amplitude roll damping of two-dimensional floating bodies with consideration of viscous dissipation effects. In the method, the instantaneous wetted surface of floating bodies is accurately considered, and the viscous dissipation effects are taken into account based on the “fairly perfect fluid” model. As compared to the method based on the existing inviscid body-nonlinear Green’s function, the newly proposed method can give a more accurate damping coefficient of floating bodies rolling on the free surface with large amplitudes according to the numerical tests and comparison with experimental data for a few cases related to ship hull sections with bilge keels.

Design of Fishing Boat for Pelabuhanratu Fishermen as One of Effort to Increase Production of Capture Fisheries

Design of fishing boat for Pelabuhanratu fisherman as one of effort to increase production of capture fisheries. The fishing boat should be proper for the characteristic of its service area, as; capacity of fishing boat up to 60 GT, the fishing boat has minimum 6 fish holds and location of fish hold in the middle body, the fishing boat hull has the bilge keel plate, and the material of hull fishing boat to be made of wooden, steel, aluminium, or fiberglass. Main dimesion of fishing boat is Length Over All = 25.436 m, Breadth = 4.55 m, Draft = 1.6 m, Speed = 12.5 knots. The research had been known every thing that will be supporting the production of capture fisheries like ; amount of fish production = 25.030 ton per day, the fishing port capacity approximately 268.957GT per day, the area of fishing port < 30 hectares, the zone of fish processing industry had not completed, therefore all data research result less than standard of Oceanic Fising Port. So Pelabuhanratu National Fishing Port can not be changed to Oceanic Fishing Port.

Numerical simulations of free roll decay of DTMB 5415

Computational fluid dynamics (CFD) that implement finite volume method are offering accurate and precise solutions for ship roll problem. In this study, the free roll decay of the benchmark DTMB 5415 hull with bilge keels in calm water was numerically solved by applying this type of code. Contributions of viscosity, wave, eddy and forward ship speed were separately investigated for ship roll damping and these were integral part of experimental setups. Numerical results were first validated with experiments. Additionally, mathematical derivations of ship roll response were used where experiments were impractical or inapplicable. Related literature still lacks a validation procedure. Therefore, a detailed analysis was made for the numerical estimation of roll damping. The weaknesses of CFD and the mathematical model were identified and discussed by comparing the obtained results. The results indicated that although the numerical simulations successfully captured the decay coefficients, these simulations lacked accuracy in calculating the natural roll frequency.

Analysis on Rolling Damping of a Conventional Boat fitted with T-shaped Bilge Keels

This work presents a numerical study on the effect of T-shaped bilge keels on the roll damping of a conventional boat. A scaled boat model with the same dimensions as that of Irkal et al. [2] was fitted with two T-shaped bilge keels at the edges of the model. Computational Fluid Dynamics method was employed to simulate the roll decay motion of the boat. The motion of the boat is captured using a 6DOF model and the Overset grid approach. Comparison was performed on the damping characteristics of the conventional I-shaped and the T-shaped bilge keels. In addition, the impact of the aspect ratio of the keel bilges on the roll damping of the boat was evaluated. It was found that the bilge keel aspect ratio influences the damping coefficient non-linearly. Sufficiently large aspect ratio, i.e. an aspect ratio greater than 2, is necessary in order to obtain an effective damping on the peak angle.

안티롤링 진자를 장치한 선박의 점성감쇠계 해석에 대한 연구

The rolling motion of a floating body makes crews and passengers exhausted and/or applies forces to the structure to cause damage; it might even upset the body. Therefore, almost all ships are equipped with bilge keels for anti-rolling; in special cases, an anti-rolling tank(ART) or fin stabilizer or gyroscope could be installed. But an ART requires a large capacity to install it, and a fin stabilizer and gyroscope need great costs to install and also many expenses to operate. The authors suggest the use of an anti-rolling pendulum(ARP) ,and they showed that the ARP is effective to reduce rolling by experiments and via a Runge-Kutta analysis. This paper introduces the linearized 2 degrees of freedom with a viscous damping system for a ship equipped with ARP; it also shows the validation of the linearized analysis for the ship’s roll motion. The paper proposes an optimum ARP on the basis of the justified model. The case of the 7.7kg model with ship 20g ARP of a mass ratio of 0.26%, is the most effective for reducing roll motion. The paper shows the ARPs with various mass ratios are effective for reducing the roll motion of a ship by free decaying roll experiments.

Numerical study of forced roll oscillation of FPSO with bilge keel

This work shows a study about the bilge keel and its influence, width and form, on the roll damping of FPSO sections. A computational code was developed to simulate forced roll oscillation of a middle section of FPSO with different bilge keels types. The computational code calculates the solution of the Navier-Stokes equations in 2D, using the finite volume method and the upwind TVD scheme of Roe-Sweby. The effect of wave radiation is neglected. This work shows how the generated vortices influence the pressure distribution around the middle section of FPSO. The numerical results are compared with experimental results to validate the computational code. New bilge keel configurations were studied to improve the roll damping of FPSO.