Investigation into Time Response under varying dynamic postioning loads of thruster/e-motor/generator/diesel

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GUSTO

ENGINEERING DOCUMENT DESCRIPTION;

AFSTUDEER. VERSLAG - 'THEORY I,H C 'GUSTO 'ENGINEERING .B.V.

P.O. Box 11, 3100 AA Schiedam', Holland _{PART I} 557 's-Gravelandseweg, 3119 XT Schiedam: Telephone : (+31 101 24668 Or/ Telefax :: (,+3110) 246 69 00

6185

::-

9515

I

300

Rev. '0 . '''''', PROJECT:

INVESTIGATION INTO TIME RESPONSE UNDER

VARYING DYNAMIC POSITIONING LOADS OF

THRUSTER ill &MOTOR f GENERATOR I DIESEL

I

W of Prepared Checked' A ppr.

STATUS DATE

I Rev., by by P.M. Pages

libc PFI C/A IFNI_ VFD APP

0 W. Kuijpers

,u

I

4iff

TUf Delft

Technische Universiteit Delft

Faculteit der Werktuigbouwkunde en' Maritreme Techniek rapport OEMO 95/20

For Information:

D C Internal Discipline Checking

P IF II Preliminary for Information

C / A, For 'Comments and/or Approval

F N L Final

V F D Verified A P 'P Approved

P M Project Manager

CLIENT

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r

ACKNOWLEDGEMENTS

The author wishes to thank Prof. lr J. Klein Woud for his 'guidance. He is

grateful to

It J.D. Wilgenhof for his time and effort spent in enlarging

clearness and completeness of the study and 'especially the report. Special

thanks are due to Dr Ir C. van de Stoep and Ir S.A.W. Janse for their

assistance and guidance during the course of the research. He also wishes to

thank the section OEMO at Delft University and IHC Gusto Engineering in providing the opportunity to perform this research work.

W. Kuijpers

November 1996 investigation into time response under

,

GUSTO varying dynamic positioning loads of PROJECT 6185

ENGINEERING _thruster

/ E-motor/ generator / diesel REVISION

0

GUSTO 6185.9515.300 & OEMO 95/20

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CONTENTS

Page

1 GENERAL INTRODUCTION 1

2 INTRODUCTION ON DYNAMIC POSITIONING 3

2.1 GENERAL 3

2.2 HISTORIC OVERVIEW 4

2.3 FORCES AND MOTIONS 6

2.4 DP SYSTEM CONFIGURATION 10

2.4.1 General 10

2.4.2 Position reference system 10

2.4.3 Control system 14

2.5 DP MACHINERY PLANT CONFIGURATION 16

2.5.1 General 16

2.5.2 Thrusters 17

2.5.3 Diesel-electric plant 19

2.6 SUMMARY 32

3 OVERALL SIMULATION STRUCTURE ₃₃

3.1 GENERAL 33

3.2 COMMUNICATION WITH SIMULA 35

3.2.1 About SIMULA ₃₅

3.2.2 Relationship of SIMULA with current research ₃₅

3.2.3 Data transfer ₃₅

3.3 COMPONENT CAUSALITY ₃₈

3.4 MECHANICAL SIMULATION STRUCTURE 39

3.4.1 General ₃₉

3.4.2 Physics ₄₀

3.4.3 Classical Method ₄₃

3.4.4 Method Proposed ₄₅

3.4.5 Conclusions 48

3.5 ELECTRICAL SIMULATION STRUCTURE ₄₉

3.5.1 General ₄₉

3.5.2 Busbar model ₅₃

3.5.3 Component in- and outputs 53

3.6 MECHANICAL AND ELECTRICAL EQUILIBRIA SUMMARIZED ₅₅

3.7 MODEL LIMITATIONS ₅₇

3.8 SUMMARY ₅₈

4 CONCLUSIONS AND RECOMMENDATIONS ₆₀

4.1 CONCLUSIONS ₆₀

4.2 RECOMMENDATIONS FOR FURTHER RESEARCH ₆₀

,

investigation into time response under

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GUSTO 6185.9515.300 & OEMO 95/20

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REFERENCES all APPENDICES:

A - THESIS ASSIGNMENT

B - DESCRIPTION OF THE SIMULATION PACKAGE '"SIMULAh

C - COMPONENT IN- AND OUTPUTS

ID - DP RAPID PROTOTYPE

IE MECHANICAL MODEL, example: mass - spring

IF - 'MECHANICAL MODEL, example: clutch

- ELECTRICAL MODEL, energy storage in conk

CONTENTS

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investigation into time response under varying dynamic positioning loads of

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1 GENERAL INTRODUCTION

Vessels that are capable of keeping position exclusively by constantly

adjusting the thruster forces, are called dynamically positioned (DP) vessels.

The station keeping ability of the vessel may limit the operational capability for which the vessel is intended, if this station keeping ability is insufficient.

As the shipowner prescribes the operational capability,

characteristics

indicating the station keeping ability of the vessel are required in DP vessel design. The station keeping ability will be influenced in the first place by the

hull of the vessel and the thruster configuration. Secondly, the transient

behaviour of the machinery plant driving the thrusters, may have a non

negligible influence.

The aim of the current research is to indicate the influence of the machinery plant transient behaviour on the station keeping ability of the vessel. In case the influence should turn out not to be negligible, the research should provide

a simulation package, indicating

the difference between required and

obtainable thruster forces for the vessel in design.

The project is subdivided in three phases. The first two are presented in this

report. The third phase will be looked at inlet continuation of the project.

Phase 1

The field studied is roughly stipulated. That is,

what kind of

machinery plant should be able to represent and which loading profile is most relevant for the subject investigated. In chapter 2

this phase is described.

Phase 2 Described in chapter 3, an overall structure is determined which

enables the components of a DP machinery plant to be linked in a sequence that can be calculated. The overall structure determines

the in- and outputs for the models to be used to represent the

components in a DP machinery plant. By linking models in the order determined, any DP machinery plant can be simulated. The specific

internal structure of the models is not described in this report. This report is concluded by a summary of the results accomplished and by

recommendations for investigation.

Phase 3 Starting from this phase, the research is focussed on a case-study. In phase 3, the specific internal structure of the component models required are determined. These models are linked by their respective

in- and outputs as determined in the overall structure established (phase 2). The case-study is modelling the power plant of shuttle

tanker MV Cardissa, designed at IHC Gusto.

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Phase 4

To enable simulation, a simulation environment needed to be

programmed in Fortran. In the simulation environment programmed,

the models determined in phase 3 are implemented. Fortran

simulation environment and model code

is printed in report [Kuijpers, 1996c].

Phase 5

The dynamic behaviour of the MV Cardissa power plant

is

determined by simulating power plant behaviour under varying

environmental loading conditions. The results of the simulation runs

are shown and conclusions are drawn for specifically the MV

Cardissa power plant, together with more general conclusions.

Phase 3 and 5 are described in report [Kuijpers, 19961o].

The study project is carried out for IHC Gusto Engineering, a Dutch offshore

engineering company. The project is performed in cooperation with Delft

University of Technology, department of Naval Architecture and Marine

Engineering, The Netherlands. This report has been written by the author as

the first part of his Master's thesis work.

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2 INTRODUCTION ON DYNAMIC POSITIONING

2.1 GENERAL

In order to obtain a model capable of representing transient behaviour of a DP

machinery plant, at first the principle of DP is studied, the environmental

forces, the control system and the machinery plant components. The research described in this chapter is to the extend of getting sufficient idea on the area studied to enable a general model to be developed.

To place the principle of dynamic positioning in its context, the chapter is started by a historic overview (§ 2.2). To get a notion on the environment in

which the machinery plant should cooperate, the behaviour of the forces and

motions is studied (§ 2.3) and the system allocating the required thrust that is to be obtained by the machinery plant is considered (§ 2.4). Knowing the

environment of the machinery plant, the individual components of the

machinery plant are described in § 2.5.

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2.2 HISTORIC OVERVIEW

The concept of dynamic positioning was already mentioned in Jules Verne's "Vile a helice" in 1867. Nevertheless, launching the first DP ship lasted until 1961 [Fay, 1990]. In 1968, considerable scientific research was started with regard to dynamic positioning. Much scientific interest in DP can be noticed between 1975 and 1985, regarding the amount of articles being published. Basic-scientific interest has largely faded at time of writing, leaving some

interest in a feed forward control system for wave excitations by Pinkster a.o.

[Gu, 1992].

The last decade has witnessed a steady increase in demand for vessels that are equipped with DP systems. Up to recently, they were mostly employed for

the positioning of drill vessels in deep waters where conventional mooring

systems were considered to be inadequate or cumbersome. At present the use of such systems has increased to shuttle tankers, FPSO-s, pipelaying vessels,

diving support vessels, maintenance and survey vessels, etc. Including use

aboard cruise vessels - much like Jules Verne's Lille a hello°. See Figure 1. investigation into time response under

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Since, in order to preserve the swinging ofthe

island,it wasnecessary to keep it at a sufficient distance

from land, it was not "moored" in the strict sense of the

term. In other words, anchors were not used, as this

would have been impossible at depths of one hundred meters or more. Thus, by means of the machines, _which

maneuver_{ahead and astern throughout its stay, it is kept}

in place, as immobile as the eight main islands of the Hawaiian Archipelago.

Fig. 1 Illustration and text of the novel by Jules Verne, "Vile

/ E-motor / generator/ diesel REVISION

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2.3 FORCES AND MOTIONS

A brief summary will be given below, further information and references to mathematical models are given in [Fay, 19901 and [Morgan, 1978].

The dynamic positioning system controls the thruster forces to counteract the

disturbing (environmental) forces, in order to maintain a reference point

aboard, in the vicinity of a desired position or track (pipelaying) in the

horizontal plane. The disturbing forces causing horizontal ship motions are the

environmental forces due to wind, current and waves and the ship induced

forces. These induced forces are the forces on the ship structure like a

mooring system or a pipe being laid astern of the vessel. The motions in the

horizontal plane are the longitudinal and transverse motions surge and sway,

together with a change of heading, yaw.

Of the environmental forces, the wave forces tend to be highest (although exceptions on this statement occur frequent, depending on operation and

environment). Wave forces may be subdivided into two classes

[Pinkster, 1971]. The first of these being the first order forces proportional to

the wave height. These forces are of importance in determining the motions

of the vessel such as the heaving, pitching and rolling. These forces are

important in respect to the station keeping ability, however, not in the

dynamic positioning system. This is as it is practically unable to counteract these forces, since the frequency of the oscillating motion of the vessel due

to these forces would require forces, which are far in excess of the attainable,

while the mean force is zero.

The second class of wave forces are the slowly varying drifting forces (second order forces). These forces are proportional to the square of the wave height.

Drifting forces are due to the reflexion, by the vessel, of incoming waves. In

regular waves of constant height, the wave force manifests itself as a

constant but small force in the direction of propagation of the waves. In

irregular waves in which the wave height is constantly varying, the drifting force also varies. The wave force may then be assumed to be the sum of a constant part and a slowly oscillating part. The slowly oscillating part is due to the occurrence of wave groups.

The thruster forces have to counteract these environmental forces. As the

thruster force may be assumed to be the sum of a constant and an oscillating part, the DP power requirements vary accordingly. A typical power variation,

absorbed by the motors driving the thrusters(CPP), is recorded aboard the

dynamically positioned offshore production facility "Seillean" [Williams, 1993]. See Figures 2 and 3.

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Fig. 2 Typical record of parameters at limit of MV "Seillean"

GUSTO varying dynamic positioning loads of

PROJECT 6185

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GUSTO 6185.9515.300 & OEMO 95/20

t

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Fig. 3 _{Typical record of parameters at limit of MV} _{"Seillean" (enlarged)}

varying dynamic positioning loads of PROJECT 6185

/ E-motor / generator / diesel REVISION

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POIVER 1 11143 1 0 It -14 " 1 i

-rT

30 SEC 11.4 GUSTO

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A long term evaluation using statistical data (supplied by Det Norske Veritas),

shows that in typical DP operation, during at least 60% of the time, the

required thrust is less than 15% of the installed thrust [Lindman, 1989].

Furthermore, it shows the average thrust force over one year being only 17%.

See Figure 4.

I Time as percent of one year

T Thrust

Estimated thrust requirements for DP, statistical data investigation into time response under

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2.4 DP SYSTEM CONFIGURATION

2.4.1 General

As mentioned previously the DP system controls the three degrees of

manoeuvrability of the vessel, i.e. surge, sway and yaw. See Figure 5. To

control the vessel, the DP system consists of three elementsfPinkster, 1971]

and [Fay, 1990]:

a system to measure the position of the vessel;

a system to determine the force to be applied in order to rectify errors in

position (control system);

a system of thrusters, driven by a power plant to develop the force

required.

The first two systems mentioned are beyond the actual scope of this research.

These systems will be described only briefly underneath. They are the time

varying input to the current research. A model to simulate these control

systems has already been developed at IHC Gusto. The structure of that model is briefly described in § 3.2 'Communication with SIMULA' as far as it

directly influences the current research model, and more extensively in

appendix B 'Description of the simulation package "SIMULA".

The power generating system, that actually develops the thrust to remain in,

or regain a lost position are described in the next paragraph 'DP machinery

plant configuration'. It is this thrust generating system that is going to be

modelled in this research.

2.4.2 Position reference system

A short description of commonly used systems to measure the position is

given underneath. See Figure 6. A more extensive overview can be found in

[Lough, 1985] and ]Fay, 1990]. As this research does not involve the DP

system itself (position measuring, thruster allocation), a superficial summary will do.

Inclinometers

The taut wire inclinometer consists of a cable under tension and a sinker

weight. By measuring the angle between the cable and the vertical, the

position of the ship relative to the sinker weight can be determined. For

greater depths more sophisticated inclinometers have been developed.

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Surge

HYDRO ACOUS77C SYSTEMS

Or o or::=.7,7.=e....=!=tOr NO or:.,:110===r-11110

1411.,===7.1041 414.

Yaw

Fig. 5 Modes of motion

Fig. 6 Position reference systems

OP

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Sway

TAUT WIRE SYSTEMS

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IN

rrar,,:111,-investigation into time response under

varying dynamic positioning loads of PROJECT 6185 ENGINEERING _thruster

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SATELLITEsysTsms1

'SU/Vila/ROWSYSTEMS

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Hydroacoustic Position Reference System (HPR)

The system consists of acoustic beacons and receivers, some on the sea-bed

and others attached to the hull. From the time lag between transmitting and

receiving the signals, the position of the vessel can be determined. The

Systems used are the long -, the short - and the ultra-short baseline system. Radio Position Reference System

The most common form of radio position reference system fitted on DP ships

is commonly referred to as the "Artemis", which is a registered trademark. Artemis uses an automatic tracking microwave link between a fixed station (usually a platform) placed at any convenient point above sea level and a mobile station mounted on board the vessel and provides measurements of

range and bearing of the vessel from the fixed station. Signal interruption may be encountered due to passing ships, heavy rains or snow showers, or dead

zones. The positions of these dead zones are very sensitive to the exact height of the antennas above the water surface.

Wave

forces

ICommandedposition /heading

Reference

positron heading

Position/heading control algorithm

Tunnel thruster

Taut Tunnel thrusters

Thrusters wire

'CT Beacon Sea bed

Fig. 7 Dynamic positioning - basic concept

Wnd compensation Transponder Thruster allocation logic DP control system Wind force Surface current force

ENGINEERING

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platform)'

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Satellite system

A Doppler Log and the more recently developed

Global Positioning

System(GPS) are available. The recently developed Differential GPS (DGPS) is already widespread. DGPS corrects the GPS position with the corresponding

fault encountered from a nearby fixed station.

A comparison of position measuring accuracy is given in the following table for the systems mentioned:

Besides position determination, a position reference system makes use of: gyrocompasses for heading determination;

vertical reference units for roll and pitch angles;

wind sensors (anemometers) for wind speed and direction.

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Position Reference System Normal Operating Range Typical Accuracy Taut Wire Artemis H.P.R. Doppler Log GPS DGPS up to 300 m water depth 50 m to 10 kms

up to 1000 m water

depth

within the range of a geostationary orbit almost worldwide

where GPS is

operational with a fixed position nearby

0,5% water depth 1,5 m, 2' arc 1,0% water depth 90 m 40 - 50 m 2 - 10 m (depends on distance with fixed

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2.4

Control system

The position of the vessel', as supplied by the, previous discussed systems of

measurement, serves as input to the control system. This is, the wind force

is 'usually compensated for in a feed forward loop. The force for compensating

the wave force (drifting force) and current force, are generally calculated' historically using RID control (feedback) and more recently using Kalman filters. To decrease power requirements of the thrusters, the high frequent 11-IF) variances are filtered out. The result is a low frequent (LA' thruster

demand (with characteristic periods longer than 20 seconds), resembling the sum of the current force and the wave, wind drifting forces. As mentioned in § 2.2 'Historic Overview', research is also undertaken to compensate for the wave drifting force in at feed forward loop..

A simplified graphical representation of a DP control system. is, shown in Figure 7. Depending on the redundancy demanded for the vessel operating

mode, the basic concept will] be more or less expanded..

To give an idea on the accuracy required' the paragraph will be concluded by tabulating typical accuracy requirements [Lough, 1985]. The typical accuracy requirements are tabulated together with the worst environmental conditions in which such accuracy would 'be expected to be maintained:

Significant wave height in meters. It should be noted that for these wave

heights operations may have ceased since the danger level] to personnel may have been reached, but the vessel will still operate in a ]DP mode so

that operations can resume quickly when the weather conditions

improve.

;investigation into time response under

GUSTO varying 'dynamic positioning loads of

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'operating mode environmental] conditions

wind waves current.

(knots) (m) (knots), accuracy. I '

, -

i diving support i ' 30 4.5 _1: 1

±3 m, heading ±2°

1 1 I drilling ' 25 3.9

3

±7 rti or 3% water

depth whichever

iS-'greater

fire fighting!

40+

2.5 1

±15 m

platform support I 20 ,2.0i 1!

heading ± 1°,

excursion 1.5 im.

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To finalize, a more specific accuracy requirement: the DP system of shuttle tanker "Cardissa" (designed by IHC Gusto) was specified to be capable of

adequate operational station keeping in the following sea states

[Stoep, 19921:

Notes:

The one minute wind gust factor is to be taken as 1.21

Adequate operational station keeping was defined during design as

keeping the bow within a 10,0 m radial offset during loading conditions investigation into time response under

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hook up loading

mean hourly wind (m/s, la mt above sea level) 13.0 16.5

current (surface m/s) 0.75 0.8

significant wave height On; 3.0 5.5

average wave period (s) 7.0 8.0

I

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2.5 DP MACHINERY PLANT CONFIGURATION' General

A DP machinery plant will be looked upon as being the total installation that

enables developing thrust as determined by the DP control system and

furthermore provides the auxiliary power demanded for platform load. Finally, it includes a monitoring system that should prevent a system black-out. What

the platform load is actually composed of, remains beyond consideration. It

is only the total platform load that should be developed by the machinery plant configuration. The platform load is defined as being the hotel load and the load needed on the work floor.

Having defined the function of the machinery plant configuration, the main

elements entailing the machinery plant can be stipulated.

These are:

thrusters, producing the thrust required;

control units to adjust the pitch and/or azimuth of the thrusters,

according to the thrust demanded;

electric motors, producing the torque for the thrusters (diesel-electric

only);

frequency converters, to adjust the electric motor speed in accordance

with the thrust required and rectifiers to convert alternating current to

direct current (both, diesel-electric only);

busbar, distributing the electrical power among various electrical loads (including the platform load, diesel-electric only);

generators, converting the mechanical power into

electrical power

(diesel- electric only);

combustion engines (diesel, gas) to generate the power demand; power management system, monitoring the load in order to (a.o.) prevent a system black-out.

Having stipulated the main elements, the functioning of each of these

elements is explained in the total conjunction of a DP machinery plant. A

detailed description of its basic functioning is beyond the scope of this

paragraph, and does not need consideration until the element is

being

modeled.

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2.5.2 Thrusters

The amount of force for the propeller type thrusters, may be adjusted by two different strategies. Firstly, the Controllable Pitch Propeller (CPP) has a more or less constant rotation speed and the thrust is adjusted by variances in pitch of the propeller(CPP). Secondly, the Fixed Pitch Propeller (FPP). This thruster type has fixed blades and the rotation speed is controlled, adjusting the thrust

obtained to the thrust required.

Hydrodynamically, the FPP is beneficial. Especially in DP operations where low thrust requirements are frequent, energy savings are considerable. That is, at

zero required thrust, the FPP has zero speed and accordingly no energy

consumption. A CPP on the contrary, keeps its nominal speed while the pitch

is reduced to zero. The rotating thruster causes friction by the water,

absorbing energy.

At partial loads, the lower energy consumption of a FPP is especially of

interest in DP operations. The system will often be at partial load, as

mentioned in § 2.3 'Forces and Motions'. Plotting the occurrence of the thrust required in a year against the propeller curves for CPP and FPP, the difference

in the energy required for both thruster types can be shown over a one year

period, showing the FPP supremacy (hydrodynamically). See Figure 8.

A wide choice of specific thruster designs is available, though the propeller type thrusters will almost always be used [Made, and [Fay, 1990]. The

thruster designs available are: Non-steerable thrusters

thruster with fixed or controllable pitch (FPP/CPP), with or without

nozzle,

tunnel mounted thrusters (FPP/CFP);

jet thrusters housed in the hull (rarely used for DP), with the water flow direction controlled either to port or to starboard.

Steerable thrusters

azimuth thrusters (standard steerable thrusters - FPP/CPP);

cycloidal or Voight-Schneider thrusters; jet thrusters (steerable).

Some of these designs may be available in retractable and/or containerized

configurations.

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Generally, thrusters are mechanically driven (one buys a thruster module with

a shaft to be connected to a motor). Some are electrically driven, e.g. the

Azipod system by ABB and again others are hydraulically driven. Use of

hydraulically driven thrusters for DP has not been found and are believed to be more appropriate for smaller powers. Hydraulic power conversion will

remain disregarded.

Pitch and azimuth control units

In order to adjust the amount of thrust to the required thrust, one should

adjust the propeller pitch (CPP only). The same accounts for the direction of

the thrust in case of a steerable thruster. Generally, the pitch and azimuth are

hydraulically adjusted. The control unit will take an amount of time to adjust the pitch/azimuth to the setting required. Common values for adjusting the

pitch from zero to full pitch are between 10 and 20 seconds. The control unit may be a simple bang-bang controller or a more refined one.

DAYS ENERGY CURVES 240 200 160 120 1,0 POWER 0,75 0,5 0,25 120 0,5 160 200 240 DAYS

-4-0,25 0,5 FPP 0,75 1,0

THRUST REQUIREMENT PROFILE

Fig. 8 Difference in energy consumption, FPP versus CPR investigation into time response under

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2.5.3 Diesel-electric plant

For a DP machinery plant, one often opts for a diesel-electric installation. Having a diesel-electric plant, one has far more flexibility in distributing the available power over the power consumers (e.g. thrusters) than one has for

a conventional (purely mechanical) installation. [Galan Garcia, 1995],

[Hackman, 19941

Having a conventional installation one should install in the vicinity of each

consumer a combustion engine, with sufficient power to deliver the maximum

design load. Having a number of thrusters installed at several positions

aboard, one should install accordingly a large number of relatively small

combustion engines throughout the ship, sufficient to feed any individual

maximum load. As long as one wants a thruster operational, one should have the connected combustion engine running. As mentioned in § 2.3 'Forces and

Motions', the maximum design load for the thruster will however rarely be

needed, causing each of these engines to run mostly at low outputs, choking

the engines. To get redundancy, one is almost bound to install a second

engine just next to the first one. To enable repair with the truster running,_one

is bound to install the second engine.

Having a diesel -electric plant one is free to transport and distribute energy throughout the vessel, enabling a centralized power plant. The centralized

power plant will be formed by a limited number of diesel generators, powerful

enough to generate the total design power (which is less than the _{sum of each}

individual design power). To adjust the energy generated to the

energy

required, one can easily switch a diesel-generator sets on and off. Having this

freedom, system redundancy is easy obtainable. Figure 9 shows a typical

diesel-electric power plant for DP.

With the main considerations mentioned why to install a diesel-electric plant,

the individual component description will continue.

key _PAGE ₁₉

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X X

X Circuit breaker Tx 6 Diesel alternators each rated at 4 MVA Main HT 6.6 kV switchboard 2.24 MW Main propeller (Port) Port forward } Clutch

0

Fire pump Port aft Fig. 9

Typical electrical distribution

arrangements Thrusters 1.5 MW CtZ> Starboard Starboard aft forward

Main 415 V switchboard Main propeller

(Starboard)

0 Motor 6.6 kV

Tx Transformer 6.6 kV/415 V 1-00 r-+ Qo

0

rn

0

co NJ

0

Tz. m

< 0

m (7) wi

00 2I

) 03

0 0 cri

Port sets Starboard sets 1.5 MW 2.24 Tx

(25)

Electric drives

Basically three types of drives exist, each with their advantages and

disadvantages. DC machine

Due to the independency of frequency (as it is for AC drives), the drives can be speed controlled solely by controlling the voltage over the drive. To do so,

electrical converters (AC/DC, DC/DC), may be quite simple (see paragraph 'electrical converters' below). This is why until recently, drives for variable

speed over a large range, were of this type. However, due to the principle of

a DC machine, an electrical power equivalent to the full mechanical load is

transferred by means of brushes. These brushes are mechanical contacts, used to carry the power from the static to the rotating part of the engine. These brushes lower efficiency and require maintenance. Furthermore, DC

engines above 1 MW are significantly larger than their AC equivalences. Due

to these drawbacks, the use of DC drives decreases with the developmentof

frequency converters.

Asynchronous machine (AC)

The beauty of this machine

is

the 'Keep

It Stupidly Simple (KISS)'

construction. The 3-phase AC develops in the static part a rotating magnetic field, causing a squirrel cage inside the static part to rotate, driving the load.

The squirrel cage rotates at a speed that is almost the frequency of the

electrical circuit. The torque depends on the difference in these rotating

speeds (slip). Though the speed is not exactly fixed with the feeding

frequency, the speed varying range is limited, as in nominal operation the slip

should be kept within a small range. The ratio between the synchronous rotating speed and the feeding frequency depends on the number of

pares. Two-speed motors can be built by combining different numbers of pole-pares within one motor.

The advantage over both other electric drives is the lack of brushes, causing

at minimum of maintenance. The drawback of this machine is that the

asynchronous machine needs an electrical feed to generate voltage. (I.e. at

mechanical driven squirrel cage may rotate without magnetic resistance.) This implies that this machine can not be used as a stand-alone generator and for use as a variable speed driving machine, the frequency converters need to be

more sophisticated than those for a synchronous machine. [Flame's, 1992]

(names found are asynchronous -, squirrel cage - and induction machine) Synchronous machine (AC)

The rotation speed of the synchronous machine is fixed with the frequency of the feeding electricity. The machine does have brushes as in the DC motor,

investigation into time response under -:...,,, ,GUSTO varying dynamic positioning loads of

PROJECT 6185

PAGE 21

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but only for the field excitation (magnetic field), so they can be kept small and

will not need as much maintenance as in DC machines. Compared to the

asynchronous machine, this one is applicable as a generator without needing external electricity. Furthermore, frequency converters of a simpler type can

be used. It will rotate at an exact, fixed ratio of the feeding frequency,

depending on the number of pole-pares, just as the synchronous speed of an asynchronous motor.

Busbar

The busbar is the set of conductors connecting the generators with the

electrical load. By opening or closing circuit breakers, one may connect or

disconnect certain elements from the busbar.

The busbar is almost always of the three phase, alternating current (A.C.), voltage source type. Each phase is shifted in time over a third period, the

voltage and the frequency of the busbar are controlled to remain within tight

limits (about 5.. ± 10%, demanded by society rules, preventing

malfunctioning of electrical devices). Generally the nominal value is a 50 or 60 Hz frequency. The voltage has many different nominal rates (also dependent

on the frequency), of which 380, 440, 660, 720, 3300 and 6600 Volt are

some of the most commonly applied.

Generally, one can say that the load on the electrical components (motors,

generators, busbar, etc.) is proportional to the current through the

components. A higher current, causes more energy to be dissipated in the

elements by the internal resistance. The energy dissipated should be removed, as an energy build-up, gives rise of the temperature and finally will destroy the component by melt. The amount of (electrical) insulation needed depends on

the voltage over the components, avoiding short circuits.

The powerfactor (cos) of the busbar,

is the phase shift between the

instantaneous voltage and instantaneous current in the circuit. Its value is

between one(1), current is in-phase with voltage, and zero(0), instantaneous

current is zero when the instantaneous voltage has its maximum. The

instantaneous power is the product of instantaneous current and

instantaneous voltage. The effective power, being the power that

is

transformable in mechanical power is the mean instantaneous power. As a result, it can be understood that the effective power is proportional to both

the phase shift between the instantaneous current and voltage and to

furthermore to the product of the effective current and voltage. In case of three-phase alternating current, the effective power is:

Pelectric 1/3' U" I" COS0 (3-phase AC)

Concluding, having a effective voltage chosen for the circuit, one can obtain the same mechanical power by conversion under complete different electrical

ENGINEERING

thruster / E-motor / generator / diesel REVISION 0

*el _PAGE 22

TU Delft

GUSTO 6185.9515.300 & OEMO 95/20

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loads. That is, the electrical load of a component (proportional to the current

is proportionally reversed to the value of cos.

To obtain high effective power, with a minimum of electrical load, one should

keep the powerfactor in the vicinity of one(1).

To end the busbar characteristics, it should be remarked that generally the ideal wave pattern for the instantaneous alternating current is a sinusoid. A

transformed pattern will be formed under the influence of disturbances.

Sensitive electronic devices will fail if the disturbances are high. Furthermore, the instantaneous maximum voltage will increase, increasing the insulation to

prevent short circuits.

Typical electrical drive configurations

Having mentioned the available thrusters, the busbar characteristics and the

electric motor types, one may now understand the three main choices in

electrical drive configurations. These are:

a constant speed asynchronous motor drive Cl or 2- speed) which requires a CPP;

a variable speed DC motor drive which works with a FPP;

a variable speed frequency converter and AC motor drive with a FPP. Before continuing, the advantages and disadvantages will be mentioned. option (1)

advantages: the system is controlled by a well known pitch controller,

the electric motor is maintenance friendly

disadvantages: high power consumption due to constant speed

option (2)

advantages: the system is controlled by an easy to control DC motor,

low power consumption due to variable speed

disadvantages: the DC motor is large and takes a lot of maintenance

option (3)

advantages: the electric motors are maintenance friendly, low power

consumption due to variable speed

disadvantages: the system is controlled by relatively unknown

(distrusted) and expensive frequency converters investigation into time response under

PAGE 23

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Electrical converters

Using an AC busbar, for both the options 2 and 3 of the before mentioned

typical electrical drive configurations, an electrical converter should be applied to transform the energy available at the busbar into a form appropriate for the

electric motor.

[Godfroid, 1986],

[Graham, 1988], [Hofer, 19861,

[Maas, 1987], [Rautelin, 19901 Rectifiers

Using a DC-motor, opting for the second option, one should rectify the

alternating current of the busbar before connecting it to the motor. By using

a set of diodes or thyristors, the alternating current on the busbar side of the rectifier can be transformed into direct current on the output (DC-motor) side. Using thyristors, one can control the (DC) voltage, enabling control of speed of a DC-motor.

In general, a mayor advantages of using diodes instead of thyristors are the

better characteristics on the busbar. That is, the powerfactor (cos) will be

almost one(1) for the whole power region, while the harmful disturbances on

the busbar remain lower. If the input port is controlled by thyristors (or

transistors), the powerfactor will be poor especially at low power consumption

and disturbances will get introduced into the busbar. As explained at the

description of the thrusters, the power consumption at DP mode is often low. Having a constant (i.e. uncontrolled) DC-voltage, one may use choppers to get a controlled DC-voltage.

Frequency converters

Choosing the third option, one needs to control the frequency. As it was explained, alternating current electric motors have an almost fixed relation

between the busbar frequency and the rotation speed. As the frequency of the busbar is constant (ideally), a frequency converter should intersect the electric motor and the busbar, varying the frequency of the electric motor. One should vary the rotation speed (and so the feeding frequency) in case one wants to

apply fixed pitch thrusters to be used in DP mode. Making an appropriate

choice between converter and electric motor to be used, is beyond the scope of this research. Still, the major considerations are mentioned:

A basic distinction should be made between frequency converters with and

those without a direct current circuit intermediate. Of the first type the

cycloconverter is the one used. By controlling a high number of thyristors, the output frequency is controlled. The output frequency can only be significantly lower than the input frequency, consequently one needs low speed motors, which are expensive and large. So, the cycloconverter is especially of use in

the high power region (above ±8 MW). In lower power regions, this converter investigation into time response under

ENGINEERING

ak, thruster / E-motor / generator / diesel

REVISION 0

PAGE 24

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type itself is relatively expensive due to the large number of diodes/thyristors

applied and because of

the

costs of the

electric motor needed. A

cycloconverter is

generally used with a synchronous motor, though an

asynchronous one could be applied as well. A typical current / voltage record

of a cycloconverter is shown in Figure 10.

The type with a direct current circuit intermediate, has on the busbar port a

rectifier as was mentioned above. On the electric motor port, an inverter

transforms direct current into alternating current with variable frequency.

Either the current in the intermediate circuit is (almost) constant (by means of a strong coil), or the voltage (by means of a strong condenser). Furthermore, the energy flow in the intermediate circuit may be controlled by the input port

(rectifier) or by the output port (inverter). In case the energy flow in the

intermediate circuit is controlled by the output port (inverter), the input port can be a diode-type rectifier, with the advantages mentioned.

Of the configurations with a circuit intermediate, the ones most commonly

found are:

current source (synchro-, or load commutated frequency converter); voltage source;

pulse width modulated (PWM). See Figure 11.

A drawback of the synchro-converter is the motor type: a synchronous one

should be connected, as the asynchronous one will not function.

(As

mentioned when describing the different motor types: the asynchronous

machine is under certain conditions unable to generate its own field

excitation). Furthermore, the conjugate converter-synchronous motor should

accurately be configured for satisfying service.

Evaluating the figure, it should be noted that the torque developed by the engine is dependant on the current through the engine. On a 'macro'-scale, the PWM current resembles closely the sinusoid, the current source type is

alright,

while the voltage source type is

performing bad, by possibly

introducing (unwanted) vibrations. Furthermore, the busbar port is controlled

on the busbar side of the converter, with the disadvantages mentioned.

PWM makes use of just recently developed transistors, causing the converter

not to be fully trusted by shipowners. Its advantages though are high:

any motor can be connected to the converter, increasing redundancy by

flexibility;

the powerfactor is close to one(1) over the full output range;

the torque developed is smooth.

PAGE 25

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elliCIONOWAIX

010101414101471K

Fig. 10 Cycloconverter principle

Current source

Voltage source

Pulse width modulated

Voltage

CUrTtnl.

Voltage Current

Fig. 11 Frequency converters principles (with circuit intermediate)

'

-GUSTO

ENGINEERING _thruster _/ _E-motor _/ _generator _{/ diesel} REVISION o

arr.- PAGE 26

TU Delft

GUSTO 6185.9515.300 & OEMO 95/20

Voltage Current

(31)

Generators

The generators will transform the mechanical power from the combustion

engines into electrical power. Usually a synchronous electric drive will be used

(see before). The generator is an inverted synchronous motor: transforming mechanical power into electrical power, instead of transforming electrical power into mechanical power.

As a generator is a synchronous machine, the busbar frequency is proportional

to the rotation speed of its driving shaft. The busbar voltage should be

controlled in order to keep the voltage close to its design value. A controller measures the voltage and adjusts the excitation field accordingly.

A low powerfactor will

fully load the generator electrically, while the

converted mechanical power may be extremely low, giving need to oversized

generators in comparison with the effective power. A common value for

cos cp = 0,8.

One may use

mechanically unconnected generators to

compensate the load, increasing coscp. Combustion engines

As mentioned describing the generators, the shaft driving the generator is proportional to the busbar frequency in case of a diesel-electric plant. By controlling the output of the motor (diesel, gas), one should keep the busbar frequency within close limits. Having a mechanically coupled thruster, one should control the speed according to the appropriate thruster strategy. In order to enable load sharing, the installed controllers (governors) should be either electronically linked or allow sufficient speed-droop (unknown if the solution is sufficient for diesel-electric plants).

investigation into time response under ,

,

ENGINEERING _thruster _/ _E-motor _/ _generator _{/ diesel} REVISION 0 ...-,

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GUSTO 6185.9515.300 & OEMO 95/20

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Power management system (PMS)

A power management system (PMS) should prevent overloading the engines,

ultimately causing a system black out. By tripping off non essential users or

reducing load, the voltage and frequency of the electric circuit should remain

within sound boundaries.

In Figure 12 [ABB], the control and monitoring system is shown in relation

with a complete diesel-electric plant. The power management system monitors

the engines conditions and communicates with the thruster assignment. In

Figure 13 this flow of information is shown schematically. Information

between the bridge console and the power management system is either an

alarm situation or a human feed-forward: prepare for a significant load

increase by starting a diesel engine.

As mentioned, PMS main task is to control the attached power units in order

to prevent a total system black-out. PMS should see to it that the electrical

voltage and frequency remain within limits required. PMS will do this by

tripping of non-essential consumers if necessary.

PMS may be included in a more comprehensive electronical control system,

with functions as [van Rietschoten en Houwens, DIGICOM 2000]: automatic synchronisation;

constant frequency regulation;

active symmetrical and a-symmetrical load sharing; reactive load sharing;

engine fault detection;

load depending start/stop;

selection of stand by generators;

etcetera. See also [Buchanan, 1993].

Figures 72 and 14 show typical overall views of the main components, linked together, for comprehensive DP machinery plants.

ENGINEERING _thruster _/ _E-motor

/ generator/ diesel REVISION

PAGE 28

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POWER MANAGEMENT ENGINE MONITORING CON4Ot E 2 x 5730 WA 2550 kW 2550 kW BALLAST BILGE CONTROL MAN-MACHINE COMMUNICATION -Fteati)011, WING-' A TM. rIPEOT ONSOLE 1 1-1 MACHINERY 'UT TION SYSTEM

ADP BACK-UP COMPUTER

, THRUSTER 1 -ASSIGNMENT 11 BACK-UP

POWER _MANAGE- MENT

co

0

cn co (.31 co. tto

0

C/1

0

TJ '13

>

M C)

<

0

M Efj RETRACTABLE AZIMUTH MAIN TUNNEL AZIMUTH AZIMUTH GENERATING DC)

z

THRUSTER PROPULSION THRUSTER THRUSTER THRUSTER SETS 2550 kW 2 x 3000 kW 1000 kW 3000 kW 3000 kW 2 x 5730 WA TUNNEL TUNNEL THRUSTER THRUSTER GENERATING SETS Fig. 12

Typical control and monitoring system

NJ

OD

(..0 0 CTI

0 0

(34)

1

7 engineroom

control

engineroom

ADP computer bridge console auto pilot

thrustei ossignnlent

required forces in surge,

sway and yaw directions

thruster allocation thrusts required security checks

.10

speed! control

Si ends

speed control system

pitch control

signals

pitch control system

Fig. 13

Typical control and monitoring system, schematically

power availability and requirements

T

power

management

system

block-out prevention engine monitoring power availability and requirements

CD Z

00 Af/7_

(20

DP system control

z

C 0

(SIMUL A ) man

/

machine communication

CD '1;1

0

rn

0

(35)

ER 4 AZIMUTH THRUSTER CONTROL

ER-3 AND ER-4 _{*POWER DISTRIBUTION} _{"ALARM MONITORING} FLUID MANAGEMENT _{AUXILLARy CONTROL} FIRE HI/AC

'UTILITY

ER-3 AZIMUTH THRUSTER CONTROL FIBRE OPTIC DATA HIGHWAY

SOLITAIRE PRINCIPAL INSTALLATION DRAWING

ER-3 AVM OPERATOR STATION

HYDRO ACOUSTIC POSITION REFERENCE SYSTEM HARBOUR CONTROL ROOM AVM OPERATOR STATION

ER-2 AZIMUTH THRUSTER CONTROLS

Fig. 14

Vessel integrated automation system layout MV "Solitaire"

OP BRIDGE ADP ATC AVM REF SYST.

FRI AND ER-2 POWER MANAGEMENT ALARM AND MONITORING FLUID MANAGEMENT AUXILIARY CONTROL FIRE WAG 'UTILITY

ER-1 AZIMUTH THRUSTER CONTROLS

BRIDGE *AND .POYSTICK AND DISPLAY NAVIGATION BRIDGE ATC AVM ADP BRIDGE WP4G JOYSTICK AND DISPLAY ER AIM OPERATOR STATION

(f)

0

cr) co cri CA.)

0 0

90

0

rn

0

co NJ

0

TAUT WIRES

I

(36)

2.6 SUMMARY

To obtain the force necessary to keep a DP vessel in position, a power plant

consisting of thrusters and a power generating plant will be installed. In this chapter, the power plant itself and the surrounding of the power plant have

been described.

The power generating plant of a DP vessel will generally be a diesel-electric

plant, consisting of diesel engines or gas turbines, generators and electric

motors with or without electrical converters. The power generating plant

drives the thrusters installed. Purely mechanical plants are applied too,

consisting of an engine, gearbox, thruster combination.

DP load variances on the power plant are controlled by a DP control system,

allocating the thruster force in order to keep the vessel in position. , _{investigation into time response under}

kg: _PAGE ₃₂

(37)

3 OVERALL SIMULATION STRUCTURE:

GENERAL

In the research phase described in this chapter, the overall system structure

used will be explained. Special attention has been paid to use a modular structure, enabling any DP plant configuration to be modelled. Furthermore, the number of non-causal calculation constructions is minimized, increasing

simulation run stability and at the same time decreasing calculation time. Developping the overall structure, two statements have been borne in mind, restricting applicability, see § 3.7 'Model limitations' (both statements copied from [Smith, 19841):

A further restriction on the models used is created by the availability of

data. The models used must be in a form for which manufacturers' data are readily available, or are easily derived, if the simulation program is to

be of general application. This restriction must be borne in mind in the selection of the particular mathematical formulation.

In the formulation of the digital simulation model, the availability and

accuracy of the specified data are crucial factors in determining the detail worth including in the model. It is expedient to make compromises in the

modelling detail, to the extent where an improved representation is

commensurate with the additional computational effort expended and

where improvement in the simulated responses is evident.

The components accounted for are those described in § 2.5 'DP Machinery Plant Configuration'. The components may be linked in any order physically possible. The structure worked out is capable to order these components

causally, except for the interconnecting busbar(s) that is(are) calculated using

a root-finding algorithm.

For each component the in- and outputs are

established, while the actual simulation models will be looked upon for now

as being black-boxes with internally characteristics depending on manufacture. These specific characteristics are of later concern.

ENGINEERING _thruster _/ _E-motor _/ _generator

/ diesel REVISION 0

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To evolve the overall structure to be used, § 3.2 "'Communication with Sithult describes the environment in which the machinery plant simulation should fit.

hi 5 3.3 'Component causality' physics of component groups are described,

highlighting why and which components are basically causal and why others are physically (rather) non-causal. § 3.4 'Mechanical simulation structure' and'

§ 3.5 'Electrical simulation structure' unfold the system structure for those

component groups that are physically 'non-causal, as these should get

structured somehow to enable calculations. 5 3.6 'Mechanical and electrical' equilibria summarized' compares the model approaches of mechanics and electrics. § 3.7 deals with the restrictions on the simulation applicability. As

the structure is specifically designed to represent the effects of load changes due to DP control, abrupt power 'changes may not be represented accurately.

GUSTO

varying dynamic positioning loads of PROJECT 6185 ENGINEERING'

thruster / E-motor / generator / diesel REVISION o

.ice _PAGE ₃₄

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-3.2

COMMUNICATION WITH SIMULA

3.2.1 About SIMULA

SIMULA is a ready-available program developed at IHC Gusto, able to simulate (a.o.) the dynamic positioning capability (behaviour) of the vessel in the time

domain. The motions of a vessel, that are caused by environmental forces

acting on the vessel are simulated. The vessel is kept in place by controlling

the thruster force available, in a way to counteract these forces.

By making use of spectra, a realistic random environment is being produced

for a typical of one hour. By calculating the responding forces/motions (HF

and LF) the behaviour of the vessel is simulated. The required thruster force per unit is determined, given a certain set-point (position and heading, which are obtained by an optimization routine). Ideally, the thruster force obtained

would be identical to the force required.

See for a more detailed description of SIMULA, appendix B 'Description of the Simulation Package SIMULA'.

3.2.2 Relationship of SIMULA with current research

It was recognised by IHC Gusto, that the available thruster force may be

different from the thruster force required. Firstly, the machinery power

installed may be insufficient (statically limited) and secondly because the

transient behaviour of the machinery plant may have a non negligible influence on the station keeping ability of the vessel, caused by some kind of time lag (dynamically limited, it will take a CPP for example, about 10 seconds to have

the propeller blades adjusted from zero to full pitch, and so increase thrust from zero to full).

The aim of the current research is to simulate the transient behaviour of the machinery plant, in order to notify the influence of engine dynamics on the

station keeping ability of the vessel. In case the influence should turn out not

to be negligible, the research should provide a tool, i.e. a user adjustable simulation program, for determining the specific influence for vessels to be

designed at IHC Gusto in the future. As SIMULA simulates in the time domain,

the transient behaviour of the machinery plant will also be simulated in the

time domain.

3.2.3 Data transfer

One needs to bear in mind that periods shorter than about 20 seconds are highly frequent with the forces/motions ( § 2.3 'Forces and Motions'). An

accurate calculation of the high frequent motions is of minor importance in the data transfer between SIMULA and the machinery plant model: it is important for the error in set-point, but has no influence on the required thruster forces.

PAGE 35

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At a simulation time step of approximately one second, the LF forces/motions

will have sufficient simulation steps to acquire accurate results. Typically, SIMULA is producing, as mentioned above, records of a one hour period.

Simulating at one second time steps, the calculation runs slightly faster than real-time.

The data transfer is thus limited accordingly to time steps of about one

second, if the simulation of the machinery plant is to be used practically

during the design of a DP vessel at all.

Data transfer between SIMULA and the machinery plant simulation program,

may be limited to the required and obtained thruster forces. Considerable

preciser results may be expected however, when the angles and motions of

the vessel are taken into account too. This is valid for both the amount and direction of the force developed by the thrusters.

Figure 15 shows the relation of the simulation models used: SIMULA and the machinery plant model. SIMULA determines the thrusts required as determined by the thruster assignment. The machinery plant returns thrusts obtained.

k

GUSTO

PAGE 36

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environmental and DP system model SIMULA , constant timestep (r.J 1 sec.) wavet

'fl

_flCurrent wind vessel

wind forces LF + HF motions

24

I

setpoint

25_

Fig. 1.6

Data transfer between

the poworplant model and Sirribla

C'

0 - - orn ni

0

CD

r

In

00

thrusts required

machinery plant Model, variable timeStep

thruster assignment

forces required (LF) Fx,Fy,M

I

--a

--0

(I) --1

0

co Cn cri cri

0

sza

0

CD

PO (1) 2 m

<

C_Dy Q. rn "5 3

g

2.

000 -I.0

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-4. 2), thruster allocation 28 DP system controller 27 L

thrust 1, thrust 2, thrusts obtained thrust 1,

I.

thrust 2

ti

(42)

3.3 COMPONENT CAUSALITY

Causality in general [Bosch, 1994]. Mathematical relations describe the mutual

influence of several variables.

For instance, Ohm's law defines the

dependency between the current i and voltage u for resistance R by 'u/i = Re,

as Newton does for the dependency between force, mass and acceleration IF m. a'.

The relation 'u = R. P suggests that a current i will introduce a voltage u.

Still, the same relation is used to state that a voltage u will determine a

current i through the resistor by 'i u/R1

From a mathematical point of view, both equations are the same. They differ in the way cause and effort or input and output are assigned. Assigning cause and effect is called introducing causality into a model. Causality is introduced artificially in order to allow calculations. Causality is not physically determined

("reality doesn't bother about causality").

Using simulation techniques, one should establish somehow the mutual

interaction between physical variables, in order to represent the

ACTION-=-- REACTION relationship.

Sometimes, however, the return action is negligible, as it is for controllers in

general. A controller will steer an actuator, e.g. a valve. For this controller, the

electronic design will be such that the return action of the valve on the

controller electronics can be neglected [Hamels, 1992].

In this chapter, special attention has been paid to establishing the connection

between different components by their

respective physical variables.

Especially those physical variables that have a mutual interaction have been looked upon: P_{mechanic =} M al and D_{electric= UI.}

GUSTO

ftinvestigation

into time response under

..

.Pc f

PAGE 38

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=

(43)

3.4 MECHANICAL SIMULATION STRUCTURE

3.4.1 General

In this paragraph the simulation structure that will be used to simulate the mechanical power transmission (_.Prnech = cum) is outlined. The method proposed is a slight variation on the simulation method generally used. The

simulation

structure applied, has certain

benefits due to an (almost)

unequivocal torque transmission from one component to the other, both for synchronous as asynchronous links. This,

in contrast with the method

generally used.

The specific relevance of the method proposed is due to the presence of both

synchronous and asynchronous electric motors, both connected to the

busbar(s). Having both systems (synchronous, asynchronous) unequivocally

structured, offers two advantages:

It offers high flexibility in representing several diesel-electric

configurations by enabling a modular structure.

Passing on (approximately) physical correct torques, enables one to

calculate the electrical

power using

effective values (U,I,coscb,

frequency). Having the effective values, the electrical components can

be modelled with rather accurate models.

To explain the method proposed, two 'thruster-shaft-diesel engine' combinations are described physically, one with a rigid shaft (synchronous), the other with a flexible shaft (asynchronous). Both the 'Classical Method' and

the 'Method Proposed' will be compared to the physical behaviour of the 'thruster-rigid shaft-diesel'-combination. It will be shown that the 'Method

Proposed' resembles closer the physical behaviour of the synchronous system,

than the 'Classical Method'.

In Appendix D, 'Mechanical Simulation Structure - Example', an elaborated case-study of al 'thruster-clutch-diesel engine' combination is inserted. The

clutch can be either a synchronous or an asynchronous link between the diesel

and the thruster, depending on the loads applied. The clutch simulation

illustrates the

almost unequivocal structure

of the synchronous and

asynchronous structures for the method proposed. investigation into time response under

ENGINEERING _thruster _/ _E-motor_/ _generator _/ _diesel REVISION 0

Ar'e PAGE 39

TU Delft

GUSTO 6185.9515.300 & OEMO 95/20