Simulation – An essential tool in the design of motion control systems

Simulation – An Essential Tool in the Design of Motion

Control Systems

CR Swanton 1 AJ Haywood 2 BH Schaub 3

Simulation is regarhd as an essential tool in the design process within Maritime @namics, Inc. (MDjl With over twenty years of experience in motion control of high speed vessels MDI has refined its design tools in its @orts to produce more efective motion control systems. An example of this rejhement process is the development of a suite of simulation programs that have provided excellent service to the engineering design teams.

l%e main uses of simulation at MDI are: prediction of motions of the vessel without risk control; prediction of motions of the vessel with active ri& control using dt~erent control surjhces; comparison of dl~erent control strategies; prediction of Motion Sickness Incidence (MS~ levels; comparisons of dt~erent vessels on the same route; feasibility studies for new designs of vessels and control concepts.

INTRODUCTION

The production and development of high-speed vessels has been increasing at a dramatic rate in the past few decades. For many of these vessels, such as high-speed fmies and patrol craft, superior seakeeping performance is critical to their operation.

The need for a ride control system (RCS) in the fist f~ industry is clear: if passengers become ill during a transit, they may not choose to ride that vessel again, instead they may seek alternatives such as air or road travel. A properly configured RCS can greatly reduce the motions of a vessel and therefore improve passenger comfort and safdy and route financial viability.

Well-designed ride control systems can allow a patrol craft to continue high-speed operations in large seas. This is essential for pursuit and search and rescue operations.

The design of large high-speed vessels poses many new design challenges. Due to the limited experience with these vessels, scale model testing and prototype development can be costly and time consuming. Modern computer simulations can accurately predict the motions of these vessels due to irregular wave forces, saving money and time.

Simulations can be conducted at an early stage in the design process, indicating problems at a point where they can be properly addressed. Diffkrent solutions can be suggested and simulated to find the best alternative for superior seakeeping performance.

MODELING OF DYNAMIC SYSTEMS

Defining a set of equations of motion for a vessel in a seaway is a complex task where there is no general consensus on the correct form. In many cases the problem is divided into seakeeping and maneuvering. This division is not appropriate for a complete RCS system incorporating roll, pitch, heave and yaw control. The MDI approach is to combine the linearized maneuvering equations with their velocity dependent coefficients together with strip theory to define added mass and damping coefficients. A typical set of maneuvering equations is shown in Lewis et al (1989).

This combination method has proved successful over many years based on good correlation with tank tests and full-scale trials. Figure 1 illustrates the comparison between the simulation program and tank tests for a high-speed mormhull.

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Figure 1: Comparison of Tank Tests and Simulation for a Large Monohull in a Bow Quartering Sea

The ad&ion of control surfaws introduces non-linearities into the equations. Limits are imposed by the control deviws, either mechanical or hydrodynamic or both. For fins, large angles are possible at low speed but as the speed increases these angles must be reduced to prevent cavitation. T-foils also require reduced maximum angles with increased speed.

The simulation programs must take into account these limits. For time domain soluti~ this can readily be achieved whereas m the fiequemy domarn an iterative solution is required. This is achieved by using a describing fiction technique.

GENERAL SIMULATION PROCESS

The initial step in most simulation studies is to analyze the seakeeping pdormance of a bare hull vessel i.e. the hull without active control surfims. Depending on a vessel’s application, its size and uncontrolled motions there are several options available to minimize critical motions.

As an example consider a high-speed vessel with a roll problem. The first option is to add a pair of roll fins to the simulation. Fms are generally of low aspect ratio (less than 1.0), although higher aspect ratio retractable fins are available. Figure 2 shows a typical fin installation. In this particular application,

the fins not only control roll, but due to their afl location, are also an integral part of the autopilot for yaw damping and heading control.

If additional roll stabilization is requir~ active trim tabs can be added. The advantage of trim tabs is their ability to control vertical plane motions such as pitch and heave in addition to roll. One of the key outcomes of a simulation study is to determine the minimum control system that can meet the client’s objectives. For example, can trim tabs generate sufficient roll control to obviate the need for the fins, or vice versa? It is dMicult for fins to generate enough force to greatly reduce vertical motions, if this is important then trim tabs may be the solution. In addition, thwe is no resistance penalty with trim tabs, which can actdy increase speed by attaining optimum trim under various load condhions. Cusanelli & Karafiath (1997) present results born long term research on fixed wedges and flaps on US Naval vessels where significant improvements in performance were obtained. Figure 2 also shows a typical trim tab application on a large monohull.

Figure 2:Fin and Trim Tab Arrangement on Large Mcmohtil Fast Ferry

Another example is where a * owner wishes to extend the operational envelope of a high-speed monohull. In this case trim tabs alone would be insutlicient and additional vertical motion reduction is achieved using a T-foil. These are becoming commonplace on high-speed ferries where vertical motion reduction is essential to passenger comfort. T-foils are located forward along the centerline of the hull. Due to their position, T-foils mounted on monohulls cannot be used for active roll reduction.

There are three main types of T-foil: the bolt-on or fixed foil with trailing edge flap; a pivoting single piece foil; and pivoting foil with trailing edge flap. The complexity of the T-foil and associated structure follows the same order but their effectiveness also increases. Figure 3 shows an application of a typical pivoting T-foil with trailing edge flap.

Figure 3:T-foil Arrangement on Large Monohull Fast Ferry

PROGRAM SUITE

The Maritime Dynamics simulation suite comprises several program% including those for seakeeping studies of monohulls, catamarans and surface effect ships (SES) and another fm general maneuvering.

The Catamaran Seakeeping Simulation Program was developed in support of the US Navy air supported catamarans (SES). The program is applicable to any h@-speed catamaran hull form with or without air support. The monohull program was developed for analysis of transom stern vessels. *keeping Programs

The seakeeping programs have many common attributes that are detailed in this section. Both programs are linearized five-degree of tkedom representations with the lateral and longitudinal vertical planes de-coupled Surge is neglected but heave, pitch, rol~ yaw and sway are modeled Hull added mass and damping coefficients are derived from strip theory (potential flow) analysis of the hull lines. Control devices can include but are not Iimited to; transom mounted trim tabs, fins or T-foils at any station, active waterjet thrust vectoring and vent and lift flow control devices fix air supported vessels.

To properly simulate a specific vessel, input data must include hull lines, mass Pmpertim descriptions of the various propulsiq Iifl (if any) and control devices, and details of desired output and simulation parameters. Depending on the design stage of the vessel, some vessel characteristics may still be unknovwL such as the roll radius of gyation and metacentric height and in such - estimates will be made.

In other cases data maybe available from model testa or sea bids data and it is them incaporated into

the model. One example where this is especially beneficial for roll motion since the wave making damping predicted for the potential flow around most hull forms is only a small tlaction of the total roll damping that is experienced in reality (Lloyd 1998). Skin fiction, eddy making and appendage forces make large contributions to the total roll damping. On the other han~ vertical motion damping is dominated by wave making.

If a route analysis is desirecl the input may also include course information and historic data of sea spectra in the proposed operating area. Sea conditions can be specified by custom spectra taken tlom wave rider buoy data or by Pierson-Moskowitz, JONSWAP or ITTC spectra characterized by a specilied significant wave height and in the case of JONSWAP or ITTC spectr% the period of maximum energy in the spectrum. A range of headings must be simulated to filly characterize the vessel’s seakeeping performance. The seas can be long crested (coming from a single direction) or the more typical short crested or “mixed sea” case. Long crested waves, which occur when the predominant wind direction is stable across the fill fi%ch as in offshore winds in coastal region% produce the most conservative basis for predkting seakeeping.

Output data can include RMS values of all of the motion variables including lateral and vertical accelerations. Slam or broaching statistics for a given reference point including the expected duration and probability of water entry within a given velocity range are also available. Eigenvectors of the system matrices can be obtained as well as response amplitude operators, power spectra and transfm flmctions of the state variables. Power spectra of vertical and lateral accelerations and one-third octave spectra of vertical acceleration at any specified point are available.

Maneuvering Simulation

A time-domain maneuvering program has been developed to act as a test bed fw autopilot controllers and as a means of checking the effect of ride control devices on the maneuvering parameters of a ship design. This program is a four degree of freedom non-linear solution of the coupled surge, sway, yaw and roll equations. Other non-linear et%cts such as cross flow drag are My accounted in the model. Full speed range models of the wattm jets or propellers are rncluded and these are essential for determining the vessel response to standard maneuvers such as turning circles and zig-zags.

Typical simukition studies will compare the maneuvering parametas of a vessel, obtained either from fill-scale measurements or tank tests, with the simulations. A certain amount of tuning of the

coefficients is usually required to obtain good comparisons of the tactical diameter, advance, overshoot CtC.. Having obtained a good model the simulations are repeated with the addition of the ride control system. -, _-. –

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Figure 4: Simulation Validation of Fast Cargo Ship at Design Speed in IITC Spectrum H,n = 7.5m T,= 7s

Extensions of the simulation to include operations in waves are used to test the operation of autopilots. Directional stability in large following seas is a common problem of high-speed vessels and such simulations as discussed here can be used to test a range of solutions including fixed skegs, movable skegs, fidl autopilots etc..

RIDE CONTROL SIMULATIONS Fast Cargo ship

Seakeeping simulations have been performed on a large, fast cargo vessel with and without ride control. This vessel was well along in the design spira~ so most of the necessary parameters were known eliminating the need for extensive estimations. This aided the accuracy of the predictions, as bare hull simulations matched self propelled tank test results as shown in Figure 4. Here

a comparison is made between the simulation and tank test results in heave, pitch, and roll.

The next stage of this study was to estimate the necessary size of effecters to control the ship’s motion. The wave slope capacity, which is the ability of the effecter at its maximum angle of deflection to heel the vessel, is one measure used to estimate the effectiveness of various ride control devices. It is defined as the ratio of the generated moment of the effecter to the vessel’s static righting moment. It should be noted that this essentially static technique should be regarded as a first approximation. The motions of a vessel are a dynamic problem and as such require a dynamic analysis to obtain a true measure of the merits of the control system. In this study, the size of the vessel precluded the control of pitch, so roll reduction was determined to be ton priority.

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Figure 5: Wave Slope Capacity of Ride Control Configurations for Fast Cargo Ship

In this application, the trim tab area was 50?? larger than that of the fins. The size of these effecters was maximized based the structural arrangement. Figure 5 shows the wave slope capacity comparison where it is seen that at the lower speeds, the fins would be more effective in roll reduction. However, at the higher speeds, such as the design speed, the trim tabs are clearly more effective.

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Figure 6: Predieted RMS Roll Motion of Fast Cargo Ship with Diffixwnt Ride Control Configurations at Design Speed

To test this premise dynamically, a simulation was Perf-ed and the result was confirmed. Figure 6 shows that the trim tabs out performed the fins by a large margin at the design spee4 reducing the predicted RMS roll motion by 50°A compared to 3(Y?4 for the tins.

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Figure 7: Correlation of 50m Minesweeper; RMS Roll Motion With and Whhout Anti-Roll T&k

Minesweeper Anti-RoU Tank

Apart from dynamically controlled ride control systems, which require high speeds to be effective, systems for slow to zero speed operation are a requirement for many types of vessels. While such vessels are out of place in a & ship forum, there is a growing requirement for good perfbnnance over a wide speed range requiring vessels to be fitted with more than one control system, as will be demonstrated in the next section. Therefme the authors think it appropriate to include a short section on anti-roll tanks.

A 50m minesweeper was recently tested in a model basin and simulated using the monohull seakeeping program. In most anti-roll tank applications, vessel speed is slow, such as a minesweeper. Figure 7 compares the predicted RMS roll motion tiom the simulation and that measured during the model tests. As can be seen, the results are quite similar. The percentage of roll reduction in this case was almost identical between the model tests and the simulation. This trend is shown again in Figure 8, which illustrates the beam sea, zero speed roll RAO fbr this vessel. In addition, the response plot indicates that the simulation matched the natural roll period measured during the tank tests.

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Figure 8: Correlation of 50m Minesweeper; Sea Roll RAO

Pilot vessel

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Some vessels spend much of their time at slow speed but require higher speed operation for short periods. Pilot vessels must wait on stand-by for harbor traffic requiring their services. The ever-increasing speed of some merchant ships and a reluctance to slow down fw pilot transf= means that pilot vessels must be capable of maintaining the speed of the merchant ship while pilot transfix operati~s are undertaken. Such a vessel would require both low and transfer speed motion controls.

The proposed solution is a passive anti-roll tank for roll stabilization atslow speed and a dynamically controlled b Stablltilon system for transfm speed.

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Figure 9: Comparison of Two Pilot Vessels; With and Without Anti-Roll Tanks at Station Keeping Speed

This proposed vessel was to replace an existing pilot boat enjoyed by the pilots for many years. One of their requirements was to investigate and compare the ride quality of their existing boat with the proposed boat. The existing vessel did have an anti-roll tanlL but no motion damping system for pilot transfix operations. ,-. ,, ●✎ ‘. ,’ ‘.●_. ...-,’ -. . . ..-,“ .- . . . . I I —.———— 5 10 15 20 EncommterPeriod(WC)

Figure 10: Comparison of Two Pilot Vessels. Effectiveness of Fins at Pilot Transfer Speed

Figure 9 shows the simulated beam sea roll response of each vessel at slow speed both with and without a roll tank. The natural roll period shift is attributed to diffimmt metacentric heights and roll radii of gyration. The tank was sized and arranged to achieve similar roll reduction realized on the existing vessel.

Pilot transfw operations cm the new vessel will be safer wilh the fin roll control system. Figure 10 shows that tie new vessel’s beam sea roll response at transfer speed is fhr less than the existing vessel.

Catamaran Ferry Comparison

The old adage of bigger is better doesn’t necessarily apply to the seakeeping periiormance of a fast ferry. It is true that a larger vessel will generally have better seakeeping characteristics, but if the larger vessel is not conducive to a proper ride control system, the smaller vessel fitted with a ride control system may out perform the larger vesseel.

This can be seen by comparing an existing 30m fhst ferry catamaran with a proposed 40m catamaran destined to operate on the same route as the smaller vessel. A usefbl indicator of the severity and frequency of vessel accelerations is motion sickness incidence (MSI). MSI is an empirkml value of the percentage of passengers becoming ill over a defined exposure period. By converting the vertical accelerations into one-third octave bands and comparing them to the criteria proposed by O’ Hanlon and McCauley (1974), motion sickness incidence can be calculated. Certain wave periods, headings and vessel speeds will result in motion occurring at frequencies that passengers fmd uncomfortable, n~ely between 0.1 to 0.63 Hz. 0.01 ~ \ —+-- +--— 0.1 EmmrherFrequency(Hz) 1

Figure 11: Motion Sickness Incidence (MSI) Curves; 10% Sickness for Different Exposure Periods

Figure 11 shows the motion sickness incidence curves for 10?!0 passenger illness for different

exposure periods. The RMS acceleration tolerance decreases with increasing exposure time. Each curve has a minimum at 0.16 Hz.

Figure 12 shows the MSI comparison in the uncontrolled condition. This plot of motion sickness incidence as a fimction of headiig is particularly usefid when coupled with a route analysis that

predicts the probability of emmuntering a specific sea condition on a particular headiig. Oflen several course options are available which may be favored for specific occurrences of a given sea &ndition and direction. When operators are knowledgeable about

the options they have, they are able to set a course which avoids the highest incidence of motion sickness. Passenger comfort and thus repeat business can be maximized. For these vessels, seas forward of the beam should be avoided in this sea state.

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Figure 12: Catamaran Comparison; MSI at Cruise Spee@ without Ride Control

Figure 12 shows a comparison of MSI of the two vessels without a ride control system. The vessel MSI are fairly comparable with the smaller vessel experiencing more severe MSI in bow quartering and head seas as would be expected.

However, a restrictive transom arrangement on the 40m vessel meant that the largest trim tabs that could be fitted were smaller than those on the 30m vessel. Since these vessels travel at about the same speed, these small trim tabs could not generate the forces necessmy to significantly reduce the motions of the vessel. Figure 13 shows that the 30m cat with trim tabs had better seakeeping performance than the larger vessel with its small trim tabs.

As a result, the 40m catamaran required additional ride control devices to at least meet the ride quality standard established by the smaller vessel. T-foils were fitted forward on each hull of the 40m vessel. Figure 13 shows that the addition of the T-foils greatly reduced the moticms of the larger vessel.

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Figore 13: Catamaran Comparison. MSI at Cruise Speed with Ride Control System

MonohuU Motor Yacht

The luxury motor yacht market is another arena fm ride control systems. Although roll stabilizers have been fitted to yachts for many years, till ride control systems are a relatively new commodity.

In a recent ride control study on a 50m motor yacht, several ride control options were investigated. The owner was interested in roll stabilization at low speed as well as roll and pitch control at various e.

For low speed roll reduction, an anti-roll tank was considered. However, due to the large metacentric height (GMT), and associated low natural roll Perid the anti-roll would have been required to be located on an upper declq reducing the dynamic stability below the accepted criteria. Generally, the weight of the fluid in an anti-roll tank should be approximately I-2°A of the cratl’s displacement with successfid applications reachrng 5V0 of the total displacement (Sellars and Martin 1992). However, this vmsel would have required a tank over 10?? of the total displacement to achieve appreciable roll reduction, clearly too large to be practical.

Motion reduction at speed was accomplished with fins and trim tabs. Figure 14 shows the motion reducticm that ride control configurations attained.

Figure 15 is a vertical acceleration comparison

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Figure 14 50m Motor yacht. Effect of Ride Control system

High Speed Monolmll Ferry

Most correlation studies between simulation and model tests are cxmducted in the bare hull condition. It is seldom that a vessel fitted with a dynamic ride control system can be physically tested in a controlled environment. However, recent model tests have been conducted with a dynamic ride control system installed on a scale model of a high-speed monohull ferry.

Simulations were conducted prior to the tests in order to establish the arrangement of the system aud the size of the individual devices.

The RCS consisted of trim tabs, a T-foil, and two pairs of fins. One pair of tins predominantly controlling roll and the other pair were primarily used for yaw control.

Model tests were conducted in head seas and bow quartering seas, where the simulations predicted the largest amount of passenger motion sickness. The model was flee to heave, pitch and roll. The three-dimensional pitchholl gimbal was pivoted at the center of gravity. Accelerometers were placed at the fixward perpendicular, center of gravity and the afi perpendicular. Special atkmtion was paid to the model’s ballast arrangement to ensure the proper pitch and roll natural periods were achieved.

between the model tests and the simulations. Two wave headiis and accelerometer locations are shown. The agreement is very good.

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The Eff&t of Rkle Control on Maneuvering

The seakeeping of an existing naval vessel had two characteristics firstly, its roll peribrmance was Poor, in particular, it experienced large roll motions in light to moderate seas and it heeled outboard in turnw ana secondly it was highly maneuverable. A simulation study was conducted to assess the effects of adding a ride control system on these two characteristics. Would the improvements in roll due to addhion of fins and or trim tabs adversely affect the maneuverability?

Within this paper there have been several case studies demonstrating the use of simulation in assessment of roll and therefwe only the maneuvering is considered in this section. The first step in the process was to build a model of the vessel and compare the results of the simulation with some fill-scale data of a zig-zag maneuver. The results of this are shown in Figure 16 where it can be seen that excellent correlation was achieved.

Once the ride control system had been selected following a seakeeping simulation study, the maneuvering simulation was used in a series of maneuvers. In a circle test, it was fbund that the tactical diameter increased by 7°/0, which was considered acceptable. The steady state roll in the turn was reduced by over 750A.

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Figure 16: Comparison of Full Scale Trials and Simulation of a Zig-zag Maneuver of a Corvette

FUTURE DEVELOPMENTS OF SIMULATION Simulation is likely to play an increasingly important role in the development of high-speed vessels. The builders and operators are pushing the boundaries to higher speed and greater displacement. Innovation generally is synonymous with risk and simulation is one method of reducing the risk by testing the concept at an early stage. The simulation models will continue to improve and the ever increasing computer power will enable more sophisticated models to be generated that can be executed within reasonable time.

Another major advance will be in visualization. The advent of high performance graphical tools will enable realistic animations of vessel motion. Such visualization is an aid to design engineers and to the titure operators of a vessel.

CONCLUSION

This paper has outlined the application of seakeeping simulations in the design of ride control systems. ‘he accuracy of the programs has been demonstrated with comparisons to tank test results and fill-wale trials. The range of simulation tools required in order to model the diverse vessels and control options has been highlighted.

Optimintion of a ride control system ean be accomplished in a short period of time by simulating diffkrent &viees and sizes of effecters. It has also

been shown that simulations can highlight potential

problem areas and belie generali~ti~ns ‘such as “bigger is better”.

This paper has demonstrated the power of simulation tools and in the authors’ opinion shown that it is indeed an essential tool for development of control systems for high-speed vessels.

REFERENCES 1. 2. 3. 4. 5.

Cusanelli, D.S. and Karafiath, G. Integrated Wedge-Flap for Enhanced Powwing Peflormance. Fast 97, Sydney.

Lewis, et. al. Principals of Naval Architecture Volume III Motions in Waves and Controllability. SNAME, 1989.

Lloyd, A.RJ.M. SeakeePinK Ship Behaviour in ROUt#IWeather. LIoy~ 1998.

O’Hanlon, J.F. and McCauley, M.E. Motion Sickness Incidence as a Function of Frequency and Acceleration of Vertical Sinusoidld Motion. Aerospace MM]cine, April 1974.

Sellars, F.H and Martin, J.P. Selection and Evaluation of Ship Roll Stabilization Systems. Marine Technology, April 1992.