Ship Squat Research Articles

Case Study of Ship Squat in Sulina Channel Using NTPRO 5000 Navigational Simulator

paper presents a case study of ships passing through shallow and narrow waters in Sulina Channel. The method used in this research was simulation using NTPRO 5000 navigational simulator and it was studied the variation of ship squat, draft and under keel clearance for several military ships similar to Romanian Navy vessels, sailing with different speeds. The purpose of this research was to find out which ship types could navigate through Sulina Channel and what is the maximum speed they could pass without grounding due to ship squat. The integration of the trials results showed what the safe speed for each vessel was and it was concluded that the maximum speeds were 8, 12, and 16 knots. Also, a difference between desired and real speed was observed, caused by the restrictive navigation condition. Numerous researches are being done in ship squat domain and Sulina Channel is one of the canals where this phenomenon is studied, but squat simulation on military ships has never been done before in this canal.

A numerical investigation of the squat and resistance of ships advancing through a canal using CFD

As a ship approaches shallow water, a number of changes arise owing to the hydrodynamic interaction between the bottom of the ship’s hull and the seafloor. The flow velocity between the bottom of the hull and the seafloor increases, which leads to an increase in sinkage, trim and resistance. As the ship travels forward, squat of the ship may occur, stemming from this increase in sinkage and trim. Knowledge of a ship’s squat is necessary when navigating vessels through shallow water regions, such as rivers, channels and harbours. Accurate prediction of a ship’s squat is therefore essential, to minimize the risk of grounding for ships. Similarly, predicting a ship’s resistance in shallow water is equally important, to be able to calculate its power requirements. The key objective of this study was to perform fully nonlinear unsteady RANS simulations to predict the squat and resistance of a model-scale Duisburg Test Case container ship advancing in a canal. The analyses were carried out in different ship drafts at various speeds, utilizing a commercial CFD software package. The squat results obtained by CFD were then compared with available experimental data.

Technical Note: PreSquat – Workshop on Numerical Prediction of Ship Squat in Restricted Waters

In September 2013, the PreSquat workshop on numerical prediction of dynamic squat of ships in shallow and restricted waters was held by the University of Duisburg-Essen, the Federal Waterways Engineering and Research Institute (BAW) and Germanischer Lloyd (GL) in Mülheim, Germany. The purpose of the workshop was to benchmark the capabilities of available numerical methods for squat prediction through comparison with experimental data for a large container carrier of Post-Panamax type. With the exception of one test case the comparison with model test results was blind, i.e. the test results were not provided prior to the workshop. This technical note describes the model tests and the numerical results submitted to the workshop.

KCS선형의 천수영역에서의 자세 변화에 대한 실험적 연구

When a ship sails in shallow water, it is well known that an additional sinkage and trim of the ship(squat) is caused by change of hydrodynamic force between the seabed and the bottom of a ship. In this paper, to examine this phenomenon by model tests, the squat of KCS model ship at a low speed is measured by the vision based ship motion measurement system during HPMM tests. Various combinations of a ship speed, a rudder angle and a drift angle were tested at three depth conditions(H/T = 1.2, 1.5 & 2.0). As a result, increase of the ship's speed and ship's drift angle caused an increase in ship squat, but the ship's rudder angle did not. The rate of increase in ship squat was the most at H/T = 1.2 condition. Lastly these experimental results are compared to the results by three empirical formulas and two CFD methods. The tendency of ship squat measured by experiment is similar to those of empirical formulas.

Experimental Study of a Tanker Ship Squat in Shallow Water

One of the phenomena restricting the tanker navigation in shallow waters is reduction of under keel clearance in the terms of sinkage and dynamic trim that is called squatting. According to the complexity of flow around ship hull, one of the best methods to predict the ship squat is experimental approach based on model tests in the towing tank. In this study model tests for tanker ship model had been held in the towing tank and squat of the model are measured and analyzed. Based on experimental results suitable formulae for prediction of these types of ship squat in fairways are obtained.

Experimental evaluation of ship squat in shallow waters

Enlargement of ship size in recent decades and no change in the harbors and approach channels have resulted in global attention toward navigation in shallow and confined waters. A phenomenon which restricts ship navigation in shallow waters is reduction of under-keel clearance in terms of sinkage and dynamic trim, which is called squatting. Due to the complexity of flow around the ship hull, one of the best methods for predicting the ship squat is the experimental approach based on systematic model tests in the towing tank. In this study, model tests for tanker ship model and traditional Persian Gulf and Oman Sea vessel called dhow had been performed in the towing tank and the squat of the models were measured and analyzed. Based on the experimental results, suitable formulae for the prediction of these types of ship squat in fairways are obtained.

Hydrodynamic interaction among hull, rudder and bank for a ship sailing along a bank in restricted waters

By applying a CFD tool to solve the RANS equations, the viscous flow around a model of hull-rudder system towed along a bank in shallow water is numerically simulated. Hydrodynamic forces and moments acting on the ship are calculated for different ship-bank distances and rudder angles. A container ship, KCS, is taken as an example for the numerical study. Under the assumption of low ship speed, the influences of free surface elevation and ship squat are assumed to be negligible. Based on the calculation results, the hydrodynamic interaction among the hull, rudder and bank is analyzed.

Comparison of Measured Ship Squat with Numerical and Empirical Methods

The Beck, Newman and Tuck (BNT) numerical predictions are used in the Coastal and Hydraulics Laboratory (CHL) Channel Analysis and Design Evaluation Tool (CADET) model for predicting underkeel clearance (UKC) resulting from ship motions and squat. The Ankudinov empirical squat prediction formula has been used in the CHL ship simulator and was recently updated. The World Association for Waterborne Transport Infrastructure (formerly The Permanent International Association of Navigation Congresses, PIANC) has recommended several empirical and physics-based formulas for the prediction of ship squat. Some of the most widely used formulas include those of Barrass, Eryuzlu, Huuska, International Commission for the Reception of Large Ships (ICORELS), Romisch, Tuck, and Yoshimura. The purpose of this article is to compare BNT, Ankudinov, and PIANC predictions with measured differential global positioning system (DGPS) squat data from the Panama Canal for four ships. These comparisons demonstrate that the BNT, Ankudinov, and PIANC predictions fall within the range of squat measurements and can be used with confidence in deep draft channel design.

Comparison of Measured Ship Squat with Numerical and Empirical Methods

The Beck, Newman and Tuck (BNT) numerical predictions are used in the Coastal and Hydraulics Laboratory (CHL) Channel Analysis and Design Evaluation Tool (CADET) model for predicting underkeel clearance (UKC) resulting from ship motions and squat. The Ankudinov empirical squat prediction formula has been used in the CHL ship simulator and was recently updated. The World Association for Waterborne Transport Infrastructure (formerly The Permanent International Association of Navigation Congresses, PIANC) has recommended several empirical and physics-based formulas for the prediction of ship squat. Some of the most widely used formulas include those of Barrass, Eryuzlu, Huuska, ICORELS, Romisch, Tuck, and Yoshimura. The purpose of this article is to compare BNT, Ankudinov, and PIANC predictions with measured DGPS squat data from the Panama Canal for four ships. These comparisons demonstrate that the BNT, Ankudinov, and PIANC predictions fall within the range of squat measurements and can be used with confidence in deep draft channel design.

전산유체역학을 이용한 제한수로에서의 선박 침하 해석

When a ship proceeds in confined water, like canal, the water ahead of ship is pushed by hull. This pushed water returns to the side and under the hull, and this returned water will make fluid velocity higher at the side and under the hull, compared to the case in the infinite water depth. Due to the higher velocity, the pressure under the hull will decrease, resulting in the ship drop. This phenomenon is called "ship squat" and ship squat will result in various marine accidents. In this paper, for predicting ship squat, numerical calculation was carried out using commercial CFD code, FLUENT. To confirm wave pattern profile around the ship, VOF(Volume of Fluid) method was applied. The calculated results were compared with other paper's results and empirical methods.

Improved Model of Deep-Draft Ship Squat in Shallow Waterways Using Stepwise Regression Trees

To maintain an optimum balance between security and efficiency of maritime transport in shallow waterways with a lot of deep-draft ship traffic such as in the St. Lawrence Waterway, it is particularly important to accurately estimate the ship squat, which is the reduction of the underkeel clearance between a vessel at rest and in motion. Recently, a squat model based on a regression tree was developed. The skill of this model to predict squat in the St. Lawrence Waterway exceeded the performance of 10 empirical models commonly used by the operational and regularity agencies. Although this approach is promising, two main problems were noticed: (1) the predictions obtained by the regression tree are not smooth and (2) the squat predicted with this model is not always monotonically increasing with ship speed (Froude number). In this paper, a stepwise regression tree algorithm is used to model squat. This approach has the same advantages as the regression tree (allowing the representation of complex and nonlinear relationships) and solves both of the aforementioned problems. Furthermore, the squat predictions of the new stepwise regression model outperform the predictions of the regression tree model and the Eryuzlu model, which is currently used by the Canadian Coast Guard. This new model could provide a handy tool for mariners to get real-time squat predictions in the St. Lawrence River. We also provide an algorithm that can be used to fit a squat model for any other economically important shallow waterway.

Calculation of ship squat in restricted waterways by using a 3D panel method

A panel method is applied to calculate the sinkage and trim of a ship sailing in a shallow channel. The hull surface, free surface and channel wall surfaces are discretized into panels on which Rankine sources of constant strengths are distributed. An iterative scheme is used to deal with the nonlinear boundary conditions on free surface. The boundary condition on water bottom is satisfied by the method of images, whereas the radiation condition is satisfied by raised panels above free surface. The vertical force and pitching moment are obtained by integrating the hydrodynamic pressure over the hull surface, and the sinkage and trim are calculated according to dynamic equilibrium. Numerical calculations are performed for Series 60 (CB=0.6) ship. The numerical results are in good agreement with experimental data.

Numerical Investigation to Predict Ship Squat

As empirical formulas do not succeed in predicting the ship squat with sufficient reliability, a numerical modeling system was developed. It is based on a three-model coupling to reproduce the physics of the phenomenon. The numerical modeling system was also used to set up a new formula. Finally, a comparison between measurements, empirical formulas, and the numerical modeling is made for three examples.

Hydrodynamic effects on fast monohulls or catamarans travelling through the critical speed in shallow water

The speed of linear waves in shallow water approaches a constant value as the wavelength becomes large relative to the water depth. For ships travelling in shallow water, this speed is known as the `critical speed' and acts as a barrier, similar to the speed of sound for aeroplanes. The possibility of travelling at transcritical and supercritical speeds is discussed for existing monohull and catamaran ships. We explore the predicted linearised flow around a ship as it approaches the critical speed, and the singularities that result. Experimental results show the actual flow patterns that occur at ship speeds close to the critical speed, for monohulls or catamarans, in open water or confined channels. References Ang, W. T. 1993 Nonlinear sinkage and trim for a slender ship in shallow water of finite width . Internal report, University of Adelaide. Chen, X. N. 1999 Hydrodynamics of wave-making in shallow water . Ph.D. thesis, University of Stuttgart. Chen, X. N., Sharma, S. D. and Stuntz, N. 2003 Wave reduction by S-Catamaran at supercritical speeds. Journal of Ship Research 47 , No. 1, pp. 1--10. Constantine, T. 1960 On the movement of ships in restricted waterways. Journal of Fluid Mechanics 9 , pp. 247--256. doi:10.1017/S0022112060001080 Dand, I. W., Dinham-Peren, T. A. and King, L. 1999 Hydrodynamic aspects of a fast catamaran operating in shallow water. Proceedings, Hydrodynamics of High Speed Craft , London, November 1999. Gourlay, T. P. 2000 Mathematical and Computational Techniques for Predicting the Squat of Ships . Ph.D. thesis, University of Adelaide. Gourlay, T. P. 2001 The supercritical bore produced by a high-speed ship in a channel. Journal of Fluid Mechanics 434 , pp. 399--409. doi:10.1017/S002211200100372X Gourlay, T. P. and Tuck, E. O. 2001 The maximum sinkage of a ship. Journal of Ship Research 45 , No. 1, pp. 50--58. Gourlay, T. P., Duffy, J. T. and Forbes, A. 2005 The bore produced between the hulls of a high-speed catamaran in shallow water. International Journal of Maritime Engineering 147 , Part A3, pp. 1--8. Gourlay, T. P. 2006 A simple method for predicting the maximum squat of a high-speed displacement ship. Marine Technology 43 , No. 3, pp. 146--151. Gourlay, T. P. 2008 Sinkage and trim of a fast displacement catamaran in shallow water. Journal of Ship Research 52 , No. 3, pp. 175--183. Graff, W., Kracht, A. and Weinblum, G. 1964 Some extensions of D. W. Taylor's standard series. Trans. SNAME 72 , 374--401. Huang, D. B., Sibul, O. J., Webster, W. C., Wehausen, J. V., Wu, D. M. and Wu, T. Y. 1982 Ships moving in the transcritical speed range. Proceedings, Conference on Behaviour of Ships in Restricted Waters, Varna , Vol. II, pp. 1--10. Insel, M. 1990 An investigation into the resistance components of high speed catamarans. Ph.D. thesis, Department of Ship Science, University of Southampton. Lea, G. K. and Feldman, J. P. 1972 Transcritical flow past slender ships. Proceedings, 9th Symposium on Naval Hydrodynamics , ONR, Washington D.C., p1527. Mei, C. C. 1976 Flow around a thin body moving in shallow water. Journal of Fluid Mechanics 77 , pp. 737--751. doi:10.1017/S0022112076002863 Michell, J. H. 1898 The wave resistance of a ship. Philosophical Magazine 45 , pp. 106--123. Mueller-Graf, B. 1995 General resistance aspects of advanced fast marine vehicles. Proceedings, Design of Advanced Fast Marine Vehicles , September, Glasgow. Newman, J. N. 1977 Marine Hydrodynamics , MIT Press. Stoker, J. J. 1957 Water Waves . Interscience. Tuck, E. O. Shallow water flows past slender bodies. Journal of Fluid Mechanics 26 , pp. 81--95. doi:10.1017/S0022112066001101 Tuck, E. O. 1974 One-dimensional flows as slender-body problems, with applications to ships moving in channels. Proceedings, Workshop on Slender Body Theory , Ann Arbor. University of Michigan Report NAME 164, pp. 27--35.

Transition effects on ship sinkage in highly restricted waterways

In this paper, a mathematical model for ship squat in highly restricted waterways is proposed. This model, which is based on the extension of Schijf's (1949) theory to dynamic effects, indicates stable and unstable squat positions for a ship. Furthermore, the transition effects are integrated into this model to simulate the behaviour of a vessel during and after an acceleration phase. This model shows that, although the vessel speed is lower than the Schijf limiting speed, grounding due to the oscillations generated at the end of the acceleration phase may occur when the unstable equilibrium position is reached. The results given by the mathematical model are validated by a numerical model based on finite element analysis.

Statistical Approach to Model the Deep Draft Ships’ Squat in the St. Lawrence Waterway

In shallow waterways such as the St. Lawrence River, an accurate prediction of the squat is important to ensure a balance between the security and the efficiency of traffic. The Canadian Coast Guard is now studying the squat phenomenon and considering to reassess the actual underkeel clearance standards of the St. Lawrence Waterway. Hence, a field campaign was conducted with 12 deep draft ship sailings, during which the maximal squat was measured with on-the-fly global positioning system. All the variables that may influence the squat (speed, draught, water level, etc.) were also measured. Twenty of the empirical models that are used in practice to predict the squat were tested and the Canadian Coast Guard recommended to either optimize these models or develop new models. Therefore, statistical approaches to model the squat of deep draft ships that navigate on the St. Lawrence Waterway are proposed in this paper. The Eryuzlu model, which is presently used by the Canadian Coast Guard, was optimized by modeling its errors with a stepwise regression. New models were also developed with the regression tree technique. The performance of the statistical models was better than 10 empirical models that are considered the most suitable to predict the maximal squat in the St. Lawrence Waterway. The models built by regression tree gave the best predictions.

Considerations on the squat of unevenly trimmed ships

In existing publications about empirical studies of ship squat trim is only dealt with implicitly by focussing the concern on the maximum squat. This restriction is only sufficient if a non-zero static trim is always increased dynamically, which, however, turns out to be wrong. Instead of maximum squat, we suggest a more suitable definition of the quantity relevant for under-keel clearance. In an example, it is shown that a non-zero static trim can actually be reduced dynamically, indicating a possible optimisation of trim. For a correct estimation of the squat effect on under-keel clearance, trim must be treated as an explicit parameter. Our arguments are based on full-scale experiments as well as results from numerical simulations.

Assessment of ship squat in shallow water using CFD

In the recent years much research effort in ship hydromechanics is devoted to the practical navigation problems in getting larger ships safely into existing harbours or to the appropriate design of expansion of harbours for safe accommodation of larger ships. Both these problems are directly related to the safety of navigation and therefore they receive high attention from ship designers, harbour designers, ship operators and maritime administration. The starting point of any navigational or design analysis lies in the accurate determination of the hydrodynamic forces generated on the ship hull moving in confined waters. The analysis of such ship motion should include the effects of shallow water, horizontal restrictions, asymmetric channels, muddy bottoms, ship squat, ship to ship interactions etc. It is natural to use advanced Computational Fluid Dynamics methods for this purpose. The paper includes a wide introduction into the problems of modelling of the restricted water effects on ship motion using CFD. This presentation is illustrated with the examples of calculations performed using the commercial system Fluent.

Slender-body methods for predicting ship squat

A review is made of linear slender-body methods for predicting the squat of a ship in shallow open water, dredged channels or canals. The results are summarized into a general formula based on Fourier transforms, and the method is extended to cater to stepped canals. An approximate solution for canals of arbitrary cross-section is proposed.

A Simple Method for Predicting the Maximum Squat of a High-Speed Displacement Ship

A simple formula is developed for predicting the maximum squat of a displacement ship as it passes through the transcritical speed range. This is given in terms of a maximum sinkage coefficient, which is almost constant across a wide range of hull forms. Satisfactory agreement is shown with model test results, and it is shown that large stern sinkages in the order of 3 to 6 meters are predicted for frigate and destroyer type hulls in shallow calm water.