130,515 research outputs found
Added Mass and Damping Coefficient Prediction - Results of Different Methods
No full textAdded Mass And Damping Coefficient Prediction - Results Of Different
Methods
Ermina Begovic, University of Naples Federico II, Naples/Italy, [email protected]
Guido Boccadamo, University of Naples Federico II, Naples/Italy, [email protected]
Abstract
The evaluation of ship motions and loads obtained through seakeeping calculations is continuing to
be one of the most important research subject. Numerical procedures used for this purpose are
generally validated by ship motions experiment; evaluation of motion equation coefficients is carried
out experimentally by forced motions and measurement of exciting forces on restrained model in
regular seaway.
Even if the motion prediction by some numerical method is satisfactory, the predicted values of
particular coefficients from the motion equation (added mass and damping coefficients) and of
exciting forces are not always satisfactory. This can affect heavily loads assessment that is
fundamental for structure scantlings. In particular, for the widely studied Wigley based hulls (Journee
(1992)), big discrepancies between numerical and experimental values have been noted.
In this work the review of experimental results for added mass and damping coefficients for heave and
pitch available in literature is given. Fo r two Wigley models (Journee (1992)) and for high speed Blok
and Beukelmann Model 5 (Keuning (1990)) heave and pitch added mass and damping coefficients are
calculated by 2 1⁄2 D high speed theory by Faltinsen and Zhao (1990) with and without cross-flow
corrections as proposed by Authors in previous works. Results are compared with numerical results
of similar works where different 3D time domain calculation methods were used.
Significant differences in some coefficients are found, calling for further investigation on the matter
Surf-Riding Operational Measures for Fast Semidisplacement Naval Hull Form
No full textSurf-riding/broaching failure mode is one of the Second Generation Intact Stability Criteria (SGISC) dealt by IMO. The SGISC are structured with a multi-tiered approach: Level 1, Level 2 and Direct Stability Assessment (DSA). When a ship does not verify one level, the next once must be applied, or the ship design must be modified. If ship changes are not feasible, Operational Measures (OM) can be provided to avoid dangerous situations and reduce the likelihood of stability failures. The OM are divided into Operational Limitations (OL) related to areas or routes and related to maximum significant wave heights and Operational Guidance (OG).
The surf-riding criterion has been applied on the parent hull of the Systematic Series D, a fast semi-displacement naval hull with forms typically vulnerable to surf-riding phenomenon. The 90 m length ship results vulnerable to Level 1 and 2, therefore Operational Measures have been discussed and provided for a hypothetical route in the Mediterranean Sea (Area 26).
Following the OL, in considered Area 26 the ship operations are limited when significant wave heights exceed 3.8 m. The simplified OG define critical ship speeds to be avoided for each considered sea state
Vulnerability assessment of surf-riding-broaching and pure loss of stability for Systematic Series D1 model
No full textThis paper is focused on the first and second level vulnerability assessment of Surf riding/Broaching and Pure Loss of Stability Criteria according to IMO second generation intact stability criteria. The calculations are performed for the semi-displacement twin-screw round-bilge hull form model D1 of the Systematic Series D. This model has hull form and service speed representative of corvettes built in 90-ties. The considered loading condition is obtained from Italian Navy ships statistics. Both criteria are analysed for different operational characteristics to evaluate the speed limits where the ship is not vulnerable. Model D1 is found to be vulnerable to both failure modes at service speed. Performing 2nd level of both criteria 'safe' speed is around 19 knots for surf-riding and around 15, 5 knots for pure loss. The obtained results are commented and compared against similar ships from the relevant state of the art papers
Surf-riding failure mode: from IMO criterion to Direct Assessment procedure and application on Systematic Series D
No full textThe paper follows contemporary development of the second generation IMO intact stability criteria and describes application of vulnerability criteria for surf-riding / broaching to Systematic Series D parent hull. Model D1 is a semi-displacement twin-screw round-bilge hull by Kracht and Jacobsen (1992) representative of several naval ships built during 90ties. The modern hull form and the complete set of resistance and selfpropulsion results available for the Systematic Series D models offer a possible benchmark case to support scientific community for further criteria verification. More in particular, the Direct Assessment of surf-riding failure mode has been addressed by two approaches. The first one is based on the 1 DoF nonlinear differential equation for surge motion solved analytically and the occurrence of homoclinic bifurcation is examined. The second approach is based on a 6DoF ship dynamics simulation taking into account wave, propeller and maneuvering forces and moments. Instantaneous wetted surface is considered for restoring and Froude-Krylov forces while ship resistance, thrust and maneuvering are based on the calm water performances. Calculations are performed for four ship speeds at the wave with /L = 1 for different wave steepness. A condition where the occurrence of the surf-riding by 1DoF has been verified, is further analyzed by 6DoF, exploring the effect of the nonlinearity in the Froude Krylov force. The limit wave steepness is found for each considered ship speed
Second Generation Intact Stability Criteria Fallout on Naval Ships Limiting KG Curves
No full textThe International Maritime Organization (IMO) finalized the Second Generation Intact Stability Criteria (SGISC), in February 2020. They are intended to be included in Part A of the 2008 International Code on Intact Stability in the following years. The SGISC consider five modes of dynamic stability failure in waves: parametric roll, pure loss of stability, surf-riding/broaching to, dead ship condition and excessive acceleration. In this paper, two semi-displacement, round bilge and transom stern hull forms, the parent hull of the Systematic Series D and the ONR Tumblehome, i.e. typical naval hull forms, are examined. Although naval ships are not directly impacted by SGISC, they are sensitive to dynamic stability failure phenomena due to their geometry and range of service speeds. The procedures to assess the ship vulnerability to the dead ship condition and excessive acceleration criteria, referring to the latest drafts of the criteria (SDC 7/5, 2019), were implemented in Matlab (R),. The limiting KG curves associated with this set of criteria were obtained for each vessel. The minimum allowable KG curve associated with the excessive acceleration criterion was compared with the maximum allowable KG curve associated with dead ship condition, to investigate the existence of a safe operational area
Application of surf-riding and broaching criteria for the systematic series D models
No full textThe Second Generation Intact Stability Criteria (SGISC) have been introduced after the occurrence of stabilityrelated
accidents, which clearly demonstrated that “actual” intact stability criteria were not adequate. New
generation criteria are based on the physical and failure mode analysis of parametric roll, pure loss of stability,
surf-riding/broaching, dead ship condition and excessive accelerations.
This paper is focused on the verification of surf-riding/broaching vulnerability criterion of semi-displacement
hull form of the Systematic Series D by Kracht and Jacobsen (1992) representative of several naval ships built
during 90ties. Levels 1 and 2, as amended in IMO relevant documents, have been calculated for all seven D
Systematic Series ships from resistance and propulsion performances measured in calm water. After several and
counteracting changes in the wave celerity definition by IMO, the calculations have been performed for all
models with both linear and non-linear formulation.
The influence of the diffraction component on the wave surging force has been analyzed according to the
works of Feng et al. (2015, 2017). The total surge force, calculated by 3D panel method, has been compared
against actual IMO formulation and calculation of vulnerability has been performed for three hulls.
The limit Froude number for the considered 90m ships is increased by improving the accuracy of 2nd level
calculation, taking into account only nonlinear wave celerity or diffraction component and linear wave celerity
A method for early-stage design current loads determination on drill-ships
No full textThe increasing demand for offshore operations in deep water implies the necessity to predict station-keeping ability of offshore vessels since the early stages of design. To this end, besides developing sufficiently fast and accurate methodologies for the equilibrium resolution of the forces acting on the ship, it is of utmost importance to estimate, in a reliable way, the external forces acting on the vessel. This work focuses on the current loads, aiming at developing a model for fast current load prediction based on high-fidelity Computational Fluid Dynamics (CFD) computations. Selecting the drill-ships as reference vessel-type for the study, starting from the actual fleet operating worldwide, a systematic series of hulls has been generated varying the main hull-form parameters inside the database, according to a Box-Behnken scheme. CFD calculations based on RANS equations have been performed on the whole ship set, for a set of incidence angle varying from 0 to 180 degrees considering the hull symmetric. As numerical analyses are not suitable for fast calculations the results on the systematic series have been used as input for developing a surrogate model based on Multiple Linear Regressions (MLR). The method allows for scaling the results as a function of the Reynolds number, allowing for general and flexible applicability among different vessel dimensions. The results obtained with the developed model are compared with the conventional current loads estimation methods, and the obtained results are compared on the capability plot, highlighting the higher reliability of the proposed model for early-stage predictions
On the Effect of Viscous Forces on the Motion of High Speed Hulls
No full textFor fast slender hullforms, the damping forces due to the generated waves are so small that the damping forces relative to viscosity cannot be neglected in evaluating the vertical motions.
In this paper the numerical and experimental assessment of the vertical motions in head sea for three mono-hulls are presented. The considered hulls are fast slender displacement type with transom stern and L/B ratios 14, 12, 8. The most slender hull is a trimaran main hull, the second one is a catamaran demihull and the last one is the Model 5 (Blok and Beukelman, 1984). The calculations were performed using 2 1⁄2 D high speed theory by Faltinsen and Zhao (1991). To obtain potential flow theory results the viscosity correction from the cross flow was added. The effect of the viscosity from cross flow was considered as reported by Lee (1977), Chan (1992), Centeno (2000). At the Trieste towing tank experimental program was conducted for the catamaran demihull and the trimaran main hull for four different speeds while for the Model 5 experimental data were collected from the literature.
The effect of cross flow coefficients is evaluated and discussed, and empirical coefficients are set. Furthermore, the influence of slenderness ratio and speed is discussed and finally some conclusions are given
Surf-Riding-Broaching and Pure Loss of Stability vulnerability on Systematic Series D1 Model
No full textThe development of the IMO second generation intact stability criteria is based on a multi-level approach. In each level the accuracy of the analysis is increased and if a possible vulnerability is detected the next level is applied. A ship, depending on its characteristics and external conditions, may be considered vulnerable to one or more stability failure modes. For each of the stability failure modes, the study will begin applying the first level of vulnerability, and in case the ship is considered vulnerable, to one or more failure modes, the second level of vulnerability will be applied to specific mode.
This paper is focused on the first and second level vulnerability assessment of the Surf riding/Broaching and Pure Loss of Stability. After testing the procedure on IMO benchmark ships, these two criteria are verified on the semidisplacement twin-screw round-bilge hull forms of the Systematic Series D, by Kracht and Jacobsen (1992), model D1. This model is representative hull form and service speed of Italian Navy ships and the considered loading condition is taken from the naval ship statistics. These two criteria are analysed for different operational characteristics to evaluate the speed limit
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
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