Free-running Model Tests Research Articles

Experimental study on turning characteristics of KVLCC2 tanker in long-crested irregular waves

In this paper, turning characteristics of KVLCC2 tanker in long-crested irregular waves are investigated by 1/100 scaled free-running model tests. Three kinds of model propulsion points are determined in calm water tests, which correspond to the full-scale calm water speeds of 4.0, 7.0, and 15.5 knots. Turning circle tests are performed in long-crested irregular head waves of sea state 5 and 6, with variations of the model propulsion points. In order to obtain the statistical data on the manoeuvring behaviours of the model in irregular waves, irregular waves per each sea state are generated with five seeds, in other words, five kinds of superposition of regular waves. Initial and steady turning characteristics of the model ship are analyzed by using the indices proposed in the previous studies. The deviations of initial turn trajectories depending on sea state are relatively small, compared to the deviations of steady turn trajectories. Drifting distances of steady turns increase with increasing sea states, and they are remarkably increased with decreasing ship advance speeds. With the lowest model propulsion point corresponding to full-scale 4 knots, the model ship cannot turn in most runs under sea state 6 waves. Next, with the model propulsion point corresponding to full-scale speed of 7 knots, turning circle tests are performed in regular waves in order to compare the trajectory drifts in regular waves with those in irregular waves. Five kinds of regular waves are generated following the representative parameters of sea state 6 irregular waves such as significant, mean heights, modal, mean, and energy periods. Turning trajectories in regular waves are most similar to those in irregular waves in case that the regular waves have the mean height and the energy period of irregular waves. Finally, course-keeping and +35° initial turning tests with lowest propulsion point are carried out in sea state 6 long-crested irregular waves, in order to investigate low speed course-keeping and turning abilities depending on the wave incident angles. Wave incident angles are varied from 30° to 330° at intervals of 30°. In port beam and port stern quartering waves, the steering moments by rudder cannot overcome the wave drifting forces and moments, so the model ship has problems on course-keeping and turning manoeuvres.

ESTIMATION OF HYDRODYNAMIC COEFFICIENTS OF A NONLINEAR MANOEUVRING MATHEMATICAL MODEL WITH FREE-RUNNING SHIP MODEL TESTS

Free-running model tests have been carried out based on a scaled chemical tanker ship model, having a guidance, control and navigation system developed and implemented in LabVIEW. In order to make the modelling more flexible and physically more realistic, a modified version of Abkowitz model was introduced. During the identification process, the model’s structure is fixed and its parameters have been obtained using system identification. A global optimization algorithm has been used to search the optimum values and minimize the loss functions. In order to reduce the effect of noise in the variables, different loss functions considering the empirical errors and generalization performance have been defined and implemented in the system identification program. The hydrodynamic coefficients have been identified based on the manoeuvring test data of free-running ship model. Validations of the system identification algorithm were also carried out and the comparisons with experiments demonstrated the effectiveness of the proposed system identification method.

6DOF manoeuvring model of KCS with full roll coupling

In the present article a novel mathematical model formulation is introduced to account for the effect of heeling of the ship while manoeuvring in calm water, harbour conditions. The mathematical model is based on a comprehensive captive manoeuvring test program, carried out with the well-known benchmark hull KCS, where the roll motion was forced with a newly installed harmonic roll actuator.The new mathematical model was implemented in the ship manoeuvring simulator and compared against free running model tests. The simulator model, after slight tuning, seems capable of capturing the manoeuvring motion of the ship in 6 DOF, however, additional research is planned with respect to the effect of the transverse stability and the draft of the ship.

Comparative study on analysis methods for added resistance of four ships in head and oblique waves

This paper presents a comparative study of wave-induced motion responses and the added resistance of a ship. Four representative types of ships are adopted as test models: LNG carrier, tanker, containership, and bulk carrier. Two experimental techniques—captive and free-running model tests—are conducted under regular head and oblique wave conditions to create benchmark data. Several numerical computation methods (asymptotic formula, 2D strip theory, 3D panel method, and CFD) are applied to perform the seakeeping analysis. The comparison results indicate that the accuracy and reliability of each analysis technique are validated, and its characteristics and limitations are investigated with respect to the physical aspects of the added resistance caused by a wave. The analysis results are compared based on how steady flow-induced coupling effects are considered. Further, the sensitivities of seakeeping quantities with respect to wave steepness were examined based on the results of linear and fully nonlinear computations. The overall tendency of the added resistance in accordance with the incident direction of a wave is discussed.

Numerical simulation of 6-degrees-of-freedom motions for a manoeuvring ship in regular waves

In this study, the authors propose a numerical simulation method to estimate 6-degrees-of-freedom (DOF) motions for a manoeuvring ship in regular waves. The proposed method is classified as a unified method; that is, it directly solves a family of equations of motion for the resultant 6-DOF ship motions, without separating the motions according to frequency. The authors’ mathematical model is based on an earlier model proposed by Hamamoto and Kim (1993); however, the authors primarily improve the external wave forces and the inertial and damping components of the hydrodynamic forces induced by ship motion form those of their model, such that the authors’ model can be applied to general manoeuvres (e.g., turning) and short waves. Using the proposed method, the authors conducted numerical simulations for tanker- and container-ship models under course-keeping and turning manoeuvres in regular waves. To validate the proposed method, the authors performed a free-running model test and compared the estimations with the test results. The comparisons clarify that the proposed method can approximately estimate the manoeuvre characteristics (e.g., mean ship speed, drift angle, and check helm) under course-keeping manoeuvres and turning trajectories, as well as the 6-DOF ship motions induced by waves, although room for improvement remains. The proposed method is applicable to both blunt and slender ships.

THE VERTICAL FORCE REQUIRED FROM THE AFT CONTROL SURFACES TO ENABLE A SUBMARINE TO MAINTAIN A LEVEL TURN

When a submarine turns in the horizontal plane it experiences a complex flow around the hull, casing and sail. This results in out-of-plane loads in heave and pitch. To design the Aft Control Surfaces (ACS) such that adequate force can be applied to counteract these out-of-plane loads, it is necessary to be able to accurately predict the magnitude of these forces during the turn. In this paper, predictions of the BB2 generic submarine with X-plane ACSs turning in the horizontal plane using a Coefficient Based Model (CBM) were compared with those from a fully coupled Computational Fluid Dynamics (CFD) model, and results from free running model tests. The CFD predicted the measured ACS angles well, whereas the CBM predicted that much larger angles were required. This is possibly due to the CBM not accurately predicting the out-of-plane loads on the turning submarine. Further work is required to investigate this issue.

Experimental investigation of single blade loads by captive model tests in pure oblique flow. Part II: Propeller in-plane loads and preliminary comparison of single blade loads during transient phases

Off–design and realistic operative conditions of the propulsion system have been progressively considered a frontier topic for the development of novel and successful design procedures. The availability of high accurate data on propeller performance, both experimental or numerical, is pivotal for the development of reliable tools and the enhancement of traditional ones. In this context, this paper continues the investigation described in (Ortolani et al., 2020), that was dedicated to analyze the single blade loads obtained by oblique towing tests and assess their consistency with those measured during the steady turning phase, reproduced by free running model tests. In this paper, the same analysis is broadened to two aspects. At first, the in–plane forces and moments that are obtained by a conversion from the rotating frame of the measure to the fixed frame are discussed. These loads need quantification, because they are the primary cause of damages of the shafting structure, vibratory loads and also contribute to dynamic response of the ship. Then, the study is switched to the comparison of single blade loads during transient motions of the turning maneuvers, at weak and tight rudder angles, performed at the same reference speed as the captive model test. The analysis lays the basis for the enhancement of ship system (comprehensive) mathematical models, already used in ship design, to real–time analysis of vibratory loads and emitted noise.

Numerical Estimation of Hull Hydrodynamic Derivatives in Ship Maneuvering Prediction

Abstract Prediction of the maneuvering characteristics of a ship at the design stage can be done by means of model tests, computational simulations or a combination of both. The model tests can be realized as a direct simulation of the standard maneuvers with the free running model, which gives the most accurate results but is also the least affordable, as it requires a very large tank or natural lake, as well as the complex equipment of the model. Alternatively, a captive model test can be used to identify the hydrodynamic characteristics of the hull, which can be used to simulate the standard maneuvers with the use of dedicated software. Two types of captive model tests are distinguished: circular motion tests (CMT) and planar motion mechanism tests (PMM). The paper presents an attempt to develop a computational method for ship maneuverability prediction in which the hydrodynamic characteristics of the hull are identified by means of computational fluid dynamics (CFD). The CFD analyses presented here directly simulate the circular motion test. The resulting hull characteristics are verified against the available literature data, and the results of the simulations are verified against the results of free running model tests. Reasonable agreement shows the large potential of the proposed method.

Estimation of wave-induced steady force using system identification, model tests, and numerical approach

This paper addresses the method for estimating maneuvering mathematical models in waves, and uses captive model tests to verify it. To obtain the wave force and moment, two kinds of captive model tests are performed. One is straight towing tests with different towing directions to consider various incident wave angles, while the other is circular motion tests at different regular wave frequency. These captive model tests provide added resistance and wave-induced steady force considering both different wave direction and ship speed, and different wave drift angles. The wave drift forces and moment obtained are validated by performing the maneuvering simulation in waves, and comparison with the results of free running model tests. This paper introduces the application of the Estimation-Before-Modeling (EBM) approach, one of the System Identification methods, to estimate mathematical models for ship maneuverability targeting KRISO Very Large Crude oil Carrier (KVLCC2). The most important force component for the prediction of the maneuverability of a ship is the accuracy of the wave-induced steady force acting on the ship. Here, the EBM approach is divided into two steps: First, the motion variables and forces are estimated by Extended Kalman Filter and Modified Bryson-Frazier smoother. Second, the wave-induced steady forces are determined by removing the hydrodynamic force acting on the ship during motion. Also, during the second step of the EBM approach, the database consisting of wave length and incident wave angles is constructed. All estimated variables are based on data obtained by free running model tests in regular waves. Finally, the estimated wave-induced steady force is interpolated, added to the hydrodynamic force component of the 3-DoF equations of motion, and the turning simulation of the ship in waves is then performed. Comparison of the results with the results of free running model tests confirms the validity of the established model.

Effects of water depth and speed on ship motion control from medium deep to very shallow water

To extend the knowledge on ship manoeuvring and predict ship's behaviour accurately from medium deep to very shallow water, this paper comprehensively investigates water depth and speed effects on ship's manoeuvrability, steering model and motion control through experimental studies. Free running model tests at four water depths and six speeds were conducted using a scaled ship model in the towing tank at Flanders Hydraulics Research (FHR). The influence of under keel clearance and speed on the acceleration tests and zigzag manoeuvres were firstly analysed. Secondly, the shallow water effect on ship steering model was discussed via theoretical analysis and experimental validation. Then, the investigation of the effects of speed and under keel clearance on the ship motion controller's parameters and performance was executed, and a novel adaptive controller was proposed. To improve controller's performance in shallow water, a scheme was proposed to optimize its parameters. The results indicate that both speed and water depth have considerable influence on ship's manoeuvrability and controllability. Especially, water depth restrictions change the hydrodynamic forces, reduce the propulsion efficiency and manoeuvreability, the ship becomes more difficult to manoeuvre and control in shallow water. These impacts on ship manoeuvring, modelling and motion control cannot be neglected.

A 2D+t approach for the transverse viscous loads in a modular maneuvering model

The importance of the transverse viscous loads, in a modular maneuvering model, is investigated. A method to estimate steady sectional drag coefficients is first presented. A 2D+t approach, which accounts for forward-speed effects, is also presented. The time-varying drag coefficients, in the 2D+t method, are estimated with three methods: two simplified methods using results from the literature directly, and one more sophisticated method which uses time-derivatives, from time-dependent drag coefficients for hull forms in the literature, and integrates the drag coefficients along the hull. Turning circles with 25∘ and 35∘ rudder angle are simulated in calm water and regular waves for a range of wavelengths between λ∕Lpp=0.281 and 1.120, with wave steepness H∕λ=1∕40, and initial head sea. The Duisburg Test Case (DTC) is used as a test ship. The numerical simulations are compared with free-running model tests. Overall, the 2D+t method, with integrated drag coefficients, shows a better match with the experiments compared to the cross-flow approach. However, both methods capture the main trends considering tactical diameter and advance for the tested wave conditions. Furthermore, using scaled time-varying drag coefficients for a circular cylinder can be a good starting point in a 2D+t approach.

EXPERIMENTAL AND NUMERICAL INVESTIGATION ON MANEUVERING PERFORMANCE OF SMALL WATERPLANE AREA TWIN HULL

Free running model tests and a system-based method are employed to evaluate maneuvering performance for a Small Waterplane Area Twin Hull (SWATH) ship in this paper. A 3 degrees of freedom Maneuvering Modeling Group (MMG) model is implemented to numerically simulate the maneuvering motions in calm water. Virtual captive model tests are performed by using a Reynolds-averaged Navier-Stokes (RANS) method to acquire hydrodynamic derivatives, after a convergence study to check the numerical accuracy. The turning and zigzag maneuvers are simulated by solving the maneuvering motion model and the predicted results agree well with the experimental data. Moreover, free running model tests are carried out for three lateral separations and the influence of the lateral separations on maneuvering performance is investigated. The research results of this paper will be helpful for the maneuvering prediction of the small waterplane area twin hull ship.

Validation of 6-DOF motion simulations for ship turning in regular waves

In this study, a six degrees of freedom (6-DOF) motion simulation method of a ship steering in regular waves is validated. The proposed simulation model is based on the two-time scale concept where the 6-DOF motions are expressed as the sum of the low-frequency maneuvering motions and high-frequency wave-induced motions. Turning simulations of a KCS container ship model with a rudder angle of $$\pm 35^\circ$$ in calm water and regular waves are performed and the obtained results are compared with the results of a free-running model test. The model tests were conducted using a ship model of length 3.057 m in a square tank at the National Research Institute of Fisheries Engineering, Japan. The wave conditions were as follows: the wave height was 3.6 m at full-scale, ratio of wavelength to ship length was 1.0, and the ship approached in the head wave direction before it was steered. The present method can simulate both the turning motion and wave-induced motions in regular waves with practical accuracy.

Investigating the Performance of a Ship by Matching the Stern Hull Form to Propeller and Engine Power

Designing the form of the ship stern hull could have some impacts on the efficiency of ship propeller and the requirement of the ship speed. Therefore, stern hull form of a ship matched to its propeller and engine power is important consideration in preliminary ship design stage. The main objective of this study is to investigate ship performance by matching the stern hull shape to the propeller diameter and engine power toward high speed. This study was conducted by free running model test and Maxsurf Resistance application. The stern forms were employed U-shape and V-shape. In addition, the fixed pitch propeller (FPP) with three blades was used and the diameter is varied into three sizes 0.032 m, 0.040 m, and 0.048 m. The results show the increase of propeller diameter increases model’s speed for both U-shape and V-shape stern and the effect of the propeller diameter on the speed could be described by using the equations of second-order polynomial. The optimum propeller diameter could be determined taking into account stern hull form, stern shape, tip clearance, and proper speed where then propeller diameter related to draft is given by 0.79T with tip clearance 10%Dp for both U-shape and V-shape. The ship resistances of U-shape stern at Fr 0.221 and V-shape at 0.208 are obtained approximately 89.797 KN and 77.10 KN respectively. Furthermore, the powers of ship for both U-shape and V-shape at those Fr are obtained 904,374 KW and 726,807 KW respectively. Finally, the best stern hull form matched to propeller diameter and engine power is selected and given by U-shape stern.

Effect of waterjet intake plane shape on course-keeping stability of a planing boat

Abstract The course-keeping stability of a high speed planing boat should be considered at the design stage for its safe operations. The shape of waterjet intake plane is one of important design parameters of a waterjet propelled planing boat. That has significant influences on the stern flow patterns and pressure distributions. In this study, the effects of the waterjet intake shapes of planing boats on the course-keeping stabilities are investigated. Two kinds of designed planing boats have the same dimensions, but there are differences in waterjet intake plane shapes. Captive and free-running model tests, Computational Fluid Dynamics (CFD) analyses are carried out in order to estimate their hydrodynamic performances including course-keeping stabilities. The results show that the flat and wide waterjet intake plane of the initially designed boat makes the course-keeping stability worse. The waterjet intake shape is re-designed to improve the course-keeping stability. The improved performances are confirmed by free-running model tests and full-scale trials.

Estimation of the manoeuvrability of the KVLCC2 in calm water using free running simulation based on CFD

There are three different well-known methods for predicting the manoeuvrability of ships: (1) free running model test, (2) direct manoeuvring simulation using CFD and (3) system-based manoeuvring simulation. In this paper, the manoeuvrability of the KVLCC2 was estimated using CFD with rigid body motion and body force propeller method. The free running manoeuvre at the different time steps were also simulated. The yaw checking ability and the turning ability of KVLCC2 were predicted using CFD and could have been confirmed that the IMO criteria was satisfied. When the results were compared with the model test and system-based method, the free running simulation showed better agreement to that of the model test. It could also be confirmed that the results vary depending on the time step. Overall, the CFD results using the body force propeller method estimated most accurately the test results.

P.845The effect of antipsychotic therapy on the social functioning of patients with paranoid schizophrenia

A ship encounters wave loads in actual seas, it is necessary to estimate the ship's manoeuvring performance in waves for its safe operations. In this study, turning characteristics of a KCS model ship in regular waves were investigated by free-running model tests. 35° port and starboard turning circle tests in regular waves were carried out in KRISO Ocean Engineering Basin with variations of regular wave heights, lengths, directions and model approach speeds. Propeller revolution rate was mostly fixed at the constant value corresponding to the full-scale speed of 16 knots in calm water. Ship's turning behaviours in waves can be divided into initial and steady turn phases. Firstly, during initial turns, the ship's safety which is related to avoiding a fixed front obstacle is focused on. ‘DMINw’ is defined as the minimum distance between the fixed front obstacle and the ship initial trajectory. And safety index ‘STw’ is proposed, which is formulated by using the ratio between DMINw in calm water and in waves. The ship becomes safer in head waves than calm water due to small advances. In starboard beam waves, when the ship turns to starboard, that becomes dangerous due to small transfers. In port beam waves, the ship which turns to starboard is mostly safe. But, in short port beam waves, yaw angle is slowly increased and the advance is relatively large. It is hypothesized that the wave drift yaw moments prevent the ship from turning to starboard. Secondly, drifting distances and angles in steady turns are quantified. The drifting distances increase with decreasing wave lengths, when the wave lengths are varied from 0.5 to 1.0 ship length. The drifting distances are approximately proportional to the wave steepness. Finally, approximate formula of STw is proposed. That is formulated as the function of ship advance and transfer in calm water and waves only. DMINw can be simply obtained if the initial trajectory is approximated as the ellipsoidal quadrant. Based on several parametric simulations, it was confirmed that STw is proportional to the cube of the transfer in waves and is inversely proportional to the advance in waves. STw is formulated so that it can be applied to arbitrary ships.

Initial and steady turning characteristics of KCS in regular waves

A ship encounters wave loads in actual seas, it is necessary to estimate the ship's manoeuvring performance in waves for its safe operations. In this study, turning characteristics of a KCS model ship in regular waves were investigated by free-running model tests. 35° port and starboard turning circle tests in regular waves were carried out in KRISO Ocean Engineering Basin with variations of regular wave heights, lengths, directions and model approach speeds. Propeller revolution rate was mostly fixed at the constant value corresponding to the full-scale speed of 16 knots in calm water. Ship's turning behaviours in waves can be divided into initial and steady turn phases. Firstly, during initial turns, the ship's safety which is related to avoiding a fixed front obstacle is focused on. ‘DMINw’ is defined as the minimum distance between the fixed front obstacle and the ship initial trajectory. And safety index ‘STw’ is proposed, which is formulated by using the ratio between DMINw in calm water and in waves. The ship becomes safer in head waves than calm water due to small advances. In starboard beam waves, when the ship turns to starboard, that becomes dangerous due to small transfers. In port beam waves, the ship which turns to starboard is mostly safe. But, in short port beam waves, yaw angle is slowly increased and the advance is relatively large. It is hypothesized that the wave drift yaw moments prevent the ship from turning to starboard. Secondly, drifting distances and angles in steady turns are quantified. The drifting distances increase with decreasing wave lengths, when the wave lengths are varied from 0.5 to 1.0 ship length. The drifting distances are approximately proportional to the wave steepness. Finally, approximate formula of STw is proposed. That is formulated as the function of ship advance and transfer in calm water and waves only. DMINw can be simply obtained if the initial trajectory is approximated as the ellipsoidal quadrant. Based on several parametric simulations, it was confirmed that STw is proportional to the cube of the transfer in waves and is inversely proportional to the advance in waves. STw is formulated so that it can be applied to arbitrary ships.

Experimental investigation of practical autopilots for maritime autonomous surface ships in shallow water

To evaluate the performance of autopilots in real scenarios for maritime autonomous surface ships (MASS), this article investigates four autopilots via the experimental study instead of the theoretical analysis and discusses their application capabilities. Free running model tests with four autopilot algorithms were performed using a scale model of a Liquefied Natural Gas (LNG) carrier in the towing tank at Flanders Hydraulics Research (FHR), and they were tested at different forward speeds and Under Keel Clearances (UKCs) focusing on the autopilot performance in shallow water. Furthermore, an innovative method combining the model evaluation mechanism and the grey relational decision-making theory was proposed to comprehensively evaluate the performance of autopilots. Finally, we discussed the application potential of the autopilots for controlling a surface ship in shallow water and presented shallow water effects on autopilots’ performance. The four autopilots presented in this research could successfully follow the desired trajectories with satisfactory control effects in shallow water, but their performance varied in different scenarios. Our results indicate the tested controllers can be applied in the towing tank, but for optimum selection of autopilot algorithms, we should take into consideration multiple factors including controller parameters, trajectory, and test condition, etc.

선박 조종성능 평가를 위한 옥외 자유항주모형시험의 불확실성 해석

An outdoor free-running model test system was designed for assessing ship maneuverability with test uncertainty. The test model was a surface combatant of tumblehome hull geometry. The straight forward tests were conducted first to obtain the relationship between the propeller revolution rate and advance speed. During the outdoor tests, the propeller revolution rate to achieve a certain Froude number condition was higher than that in the indoor free-running model tests. Turning circle and zigzag tests for evaluating ship maneuverability criteria were carried out at the propeller revolution rate determined by the straight forward test results. The random and systematic standard uncertainties of maneuvering criteria were obtained by repeated tests and comparison with the indoor free-running model test results, respectively. The test uncertainty was largely dominated by the systematic standard uncertainty, while the random standard uncertainty was small with good repeatability.