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

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GUSTO

(GUSTO

INTERN

CL-....

DOCUMENT DESCRIPTION:

AFSTUDEER VERSLAG - CASE STUDY

IHiC GUSTO ENGINEERING B.V.

P.O. Box 11, 3100 AA Schiedam, Holland _{PART II} 557 's-Gravelandseweg, 3119 XT Schiedam

Telephone : ( +31 10) 246 68 00

Telefax (+31 101 246 69 00 ..::.i..6 1 8 5

, -0:::::::::. 9 5 1 5 3 1 0 Rev. 0

PROJECT

INVESTIGATION INTO TIME RESPONSE UNDER VARYING DYNAMIC POSITIONING LOADS OF THRUSTER / E-MOTOR /;GENERATOR," DIESEL

Rev,

N2

of Prepared_by Checked_by Appr._P.M.

STATUS

/ DATE

Pages _IDC _PFI _C/A _FNL _VFD _APP

0 W. Kuijpers

*Jr

TU Delft

Technische Universiteit Delft

Faculteit der Werktuigbouwkunde en Maritieme Techniek rapport OEMO 96/08

For Information:

I D C Internal Discipline Checking P F I Preliminary for Information

C / A For Comments and/or Approval

FNL Final

V F D Verified

A P P Approved

P M Project Manager

0 Copyright IHC Gusto Engineering B.V. / Technische Universiteit Delft (1996) I

ENGINEERING

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ACKNOWLEDGEMENTS

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

grateful to Ir 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 Jr 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. Acknowledgement is also due to

manufacturers Lips, ABB, Sulzer and Caterpillar for providing model data. W. Kuijpers

November 1996

GUSTO varying dynamic positioning loads of

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

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CONTENTS

Page

1 GENERAL INTRODUCTION 1

2 OVERALL POWER PLANT 3

2.1 GENERAL ARRANGEMENT MV CARDISSA 3

2.1.1 Main particulars of the vessel 3

2.1.2 Dynamic Positioning specifications 3

2.1.3 Power plant 3

2.2 ENERGY STORAGES QUANTIFIED 7

3 MODELLING OF INDIVIDUAL COMPONENTS 8

3.1 ENGINE CONTROL AND MONITORING 11

3.2 THRUSTER 13

3.2.1 General 13

3.2.2 Controllable pitch installation 13

3.2.3 Thruster 13 3.3 ELECTRIC MOTOR 17 3.4 GENERATOR 17 3.4.1 General 19 3.4.2 Generator controller 19 3.4.3 Generator 20 3.5 DIESEL ENGINES 22 3.5.1 General 25 3.5.2 Governor 25 3.5.3 Diesel engine 26 3.5.4 Nomenclature 34

4 PARAMETER VARIATION - SIMULATION RESULTS 40

4.1 GENERAL 40

4.2 CONDITIONS INVESTIGATED 40

4.2.1 Limiting design condition 40

4.2.2 Systematic variation of conditions 41

4.2.3 Simulation runs 42

4.2.4 Power producing plant 42

4.2.5 Positional losses 43

4.2.6 Model limitations 43

4.3 NOMENCLATURE 44

4.4 DESCRIPTION OF RESULTS 45

4.4.1 RUN 1, thrust 100% - time 100% 45

4.4.2 RUN 2, thrust 150% - time 100% 45

4.4.3 RUN 3, thrust 200% - time 100% 46

4.4.4 RUN 4, thrust 150% - time 200% 47

4.4.5 RUN 5, thrust 150% - time 300% 48

4.5 CONCLUSIONS 48

4.6 ABSTRACT FOR FPP-PROPELLERS 49

GUSTO varying dynamic positioning loads of PROJECT 6185

ENGINEERING _thruster

/ E-motor / generator / diesel REVISION 0

TU Delft

GUSTO 6185.9515.310 & OEMO 96/08

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CONTENTS

Page

CONCLUSIONS AND RECOMMENDATIONS 50

5.1 CONCLUSIONS 50 5.2 RECOMMENDATIONS 51 REFERENCES . 52 APPENDICES A - THRUSTER B - ELECTRIC LOAD C - MAIN ENGINE D - DIESEL GENERATOR

E - TURBO COMPRESSOR ANALYSIS F - SIMULATION RESULTS

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GUSTO varying dynamic positioning loads of PROJECT

6185

/ e-motor / generator / diesel REVISION 0

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GUSTO 6185.9515.310 & OEMO 96/08

1 GENERAL INTRODUCTION

Dynamically positioned (DP) vessels keep position by constantly adjusting the

thruster forces. To obtain the fluctuating thruster forces, the thrusters are

controlled by the DP system. As the power absorption of the thrusters

fluctuates (strongly), research on the behaviour of the power generating plant was required. In the study described, the power plant behaviour was studied by means of computer simulations. The power generating plant is modelled to enable simulation of power plant transient behaviour in the time domain. The

simulation models were derived from literature and matched with

manufacturers' data.

In phase 1 and 2 of this research, as described in report [Kuijpers, 1996a], the

overall structure of the power plant model was determined. With the overall

model structure determined, in this report, phase 3, the main components of

the power generating plant (diesel engine, generator, electric motor and

thruster) have been modelled. These component models are linked together in

accordance with the previously determined model structure and have an internal form for which manufacturers' data was available. The power plant

modelled is

the plant

of the shuttle tanker MV Cardissa, which was

investigated as case-study. In phase 4 a simulation environment was created

in Fortran, as specified by Gusto. The Fortran source code is printed in

1Kuijpers, 1996c1. In phase 5, also described in this report, the transient

responses are examined during loading of the vessel at sea, while the ship is kept dynamically in position.

Using the power generating plant model, the research was to include several simulations to represent transient behaviour, parameter variations to examine

sensitivity and conclusions obtained by the simulation results. To determine system dynamics, the power plant is simulated representing a worst-case

scenario (original track, as determined earlier by

Gusto's SIMULA

(Stoep, 1992a1). Furthermore, the power plant is subjected to enlarged signals

(2 and 3x the thruster force required) and accelerated signals (the original track in 50% and 33% of time) the latter to investigate specifically power

plant dynamics.

The report is subdivided in several chapters. Chapter 2 describes

MV "Cardissa" general arrangement and considerations on modelling the overall system. Chapter 3 describes the individual component models, i.e. models for thrusters, electric motor, generator and diesel engine. Chapter 4

describes the actual simulation runs carried out. The report ends with

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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 concluding part of his Master's thesis work.

" a , E. -m.173 a 'al

6185I

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2 OVERALL POWER PLANT

2.1 GENERAL ARRANGEMENT MV CARDISSA [Timmerman, 19921, [Dekkers, 19931

2.1.1 Main particulars of the vessel

Year built

Year converted to shuttle tanker

Deadweight on assigned summer freeboard: Length overall

Length between perpendiculars Beam extreme

Draft on assigned summer freeboard Moulded depth

2.1.2 Dynamic Positioning specifications

Class Notation D.N.V. class

DYNPOSS AU F'

The DP system is designed to keep the bow within a 10,0 m radial offset during loading conditions in the following sea states (The one minute wind gust factor is to be taken as 1.21):

mean hourly wind

(m/s, 10 m above sea level) current (surface m/s)

significant wave height (m)

average wave period (s)

2.1.3 Power plant

Main propulsion (for longitudinal thrust)

Main engine MCR output /rpm

Output /rpm, during Dynamic Positioning rough sea state

calm sea state Main thruster 1983 1992 24,699 170 160 22.72 10.37 13.9 tonnes Sulzer (5RLB66) 8,000kW /140rpm 4,000kW /112rpm 1,000kW /77rpm CPP, Dp =5200mm

ENGINEERING

thruster / e-motor / generator / diesel REVISION 0

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GUSTO 6185.9515.310 & OEMO 96/08

hook up loading 13.0 16.5 0.75 0.8 3.0 5.5 7.0 8.0 I

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Diesel-electric system for side thrust during Dynamic Positioning Power system

Power generating set

Electric output /rpm 2 Tunnel thruster fore

each driven by an asynchronous motor

1 Tunnel thruster aft

driven by an asynchronous motor

660V /60Hz

3 diesel gen. sets

1440kW (each) /1800rpm CPP, Dp =2400mm 1350kW

CPP, Dp = 2100mm

asynchr. 750kW

The maximum electrical load will be supported with any one of the diesel

generators shut down for maintenance and with the running diesel generators

approximately 80% loaded. DP ability will be sufficient with only one tunnel thruster fore running.

Separate diesel-electric system for platform load (before and after conversion)

Power system : 440V /60Hz

Power generating set

during transit : PTO (shaft main engine)

in port : 3 diesel gen. sets

electric output : 1250kW

The platform load is not taken into consideration in this research, as the

system is separated from the power systems that generate thrust during

dynamic positioning.

Detailed information on the main propulsion and the D.P. power system can be found in appendices A, B, C and D, with manufacturers' data. In Figure 1,

the main powerplant components are shown. In Figure 2 a side view of the

vessel is shown.

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varying dynamic positioning loads of GUSTO

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2.2 ENERGY STORAGES QUANTIFIED

In the theory part of this research, as described in [Kuijpers, 1996al, energy

storages in the power plant were considered qualitatively. As shown,

mechanical energy can be stored in a spring and in a (rotating) mass, inertia. Electrical energy can be stored in a capacitor and a coil.

In this paragraph the four energy storages are considered quantitatively for the

MV "Cardissa" powerplant. The differences in the energies stored will be

calculated for two conditions. Though both conditions are chosen rather

arbitrarily, it will be illustrated that the energy storage capacity of the inertia is significantly higher than for any other type of energy storage. The other types will be neglected in the simulation.

For future powerplants to be modelled, the same quantatative consideration

is advised, to check wether for that powerplant neglecting electric energy

storages is valid.

mechanical and electrical energy storages:

In report I Kuijpers, 1996a], the presence of four types of energy storages have been described: two mechanic and two electric energy reservoirs. Now, those energy reservoirs will be related quantitatively, in order to estimate the need

of modelling. The first three equations underneath relate the mechanic

properties, equations four to six, their electric equivalents.

Consider the energy stored in an arbitrary period of time, t1..t2, for both the

mechanic and electric properties of a system: mechanic properties

torque - rotation angle (stiffness, spring):

M = k.4 -

kfo.dt

energy stored: t2 t2 2

Taw dt

= f M.1 dMdt 1 2k(M2 -M12)

k dt

4ti

torque - rotation speed (absorption, damper): del)

M

dt

absorption (no storage)

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GUSTO 6185.9515.310 & OEMO 96/08

= =

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torque - changing rotation speed (inertia, mass):

d24) do)

M

- J.

dt2 dt

electric properties (assume powerfactor = 1)

voltage - electric charge (capacitance, capacitor):

= =

f idt

1 C t energy stored: r2 r2 = f4 U- I dt

f

U. C dt = -1c(u22-u12) dt 2

voltage - current (absorption, resistance):

U

- R

dq

=R/

dt

absorption (no storage)

voltage 'changing' current (inductance, coil):

d2q = dt2 dt energy stored: t2 t2 E =

f

U. 1dt

=dl

1dt - L(I22 112 dt ) 2 tr ti ti ti

ENGINEERING

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energy stored:

r2 t2

E = f M.o.) dt IJ. d(1)

-0J(ca2-6),)

dt = ii 2 2 2 dt = U

E

=

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Energy storages MV "Cardissa" quantified

Consider now part of the power plant of the Cardissa: 1 bow thruster

1 electric motor (bow)

1 generator

1 diesel engine (1800 rpm)

Assume the thrust required is zero (0%) at the beginning of the period

considered (t1) and full (100%) at the end (t2). The time between t1 and t2

is short, causing a frequency drop of 3%. (The classification societies demand

a maximum frequency drop of 5..10%).

So, for the components installed (see also appendix D 'Diesel generator'), the current and rotation speed change approximately as shown underneath:

1(t1)

= 250A

1(t2) = 1575 A (nominal current)

= 1800 rpm /60 2r7 = 190 rad/s (nominal speed) = 185 rad/s (3% frequency drop)

The period considered is a period large enough to adjust the propeller pitch from 0% to 100%, but too small for the governor and diesel engine to react properly, about 5 seconds. The pitch has changed, causing the generator

current and rotation speed to change.

By making 3 comparisons underneath, it is illustrated that the energy stored

in the inductance is small in comparison with the energy stored in the inertia.

The energy storage ability of a spring and a capacitor is even less.

stiffness versus inertia

Simulating transient behaviour of engine rooms, the stiffness of steel shafts etcetera is generally considered infinite. This assumption is also made without further consideration now.

capacitance versus inductance

The powerfactor was assumed to be 1. (see above), in general the

capacitance is about the same or smaller than the inductance in the electric system aboard (C < = L). The voltage may change a little, but will remain within tight limits of its nominal value. So, U(t2) is = U(t1).

Consequently, the energy stored in the capacitor is less and may even be

negligible compared with the energy stored in the coil.

In case of variable frequency as in case of frequency converters, the influence of the capacitance may need further consideration.

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inertia versus inductance

The change of kinetic energy depends on the change of rotation speed

and the inertia of the system. The inertia

is about 150 kgm2 (See

appendix D 'Diesel generator'.) The applicability of the inductance

calculated needs preferably further consideration.

E = 1/2 150 (1902 - 1852) = 140-103 J

The energy stored due to inductance depends on the change of current

and the inductance of the system. The inductance is about 0.0031 H (This value is higher than realistic, see appendix D 'Diesel generator'.) Calculating the change of energy stored in a coil:

E = 1/2 0.0031 Fl 575' - 2502) = 3.8.103 J

Conclusion: the energy storage of the electric system is roughly 2.5% of the inertia energy storage. The inductance is only roughly determined, but as

illustrated,

its influence is small in the simulation considered.

It

will be

neglected in the simulations.

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3 MODELLING OF INDIVIDUAL COMPONENTS ENGINE CONTROL AND MONITORING

general

Practice has shown that transient behaviour of a CPP thruster

- engine

combination can considerably be improved by the application of a pitch limiter. In case the pitch is adjusted fast, the diesel engine may be overloaded due to lack of scavenge air pressure. Smoke emission during transient loading can be reduced considerably by the combined application of a scavenging air pressure

fuel limiter and a pitch limiter. Both limiters are required to obtain the

maximum benefit. Two systems are considered, of which the second one is

preferred [Sulzer ZA40S]:

System A. Fuel limiter in governor:

The charge air fuel limiter in the speed governor limits the governor output as a function of charge air pressure. The governor also sends a signal (open/close contact) to the pitch control system which regulates the pitch according to the

contact position. During transient loading of the engine the pitch

will momentarily be stopped and reduced until the required charge air pressure is obtained.

System B. Fuel limiter fully integrated in pitch control system.

In this configuration, which is preferable, the charge air pressure and fuelrack position are continuously and directly monitored by the pitch control system.

Pitch control and fuel limitation are then directly controlled as a function of

charge air pressure by this system. (For redundancy, the charge air pressure

fuel limiter in the speed governor should not be omitted.)

Simulation model

The pitch control will be limited according to system A. In the simulations it

is shown that this system is, as said, not preferable and further investigation

on simulating system B is advisable for correct representing optimal system

dynamics.

If the charge air is insufficient for complete combustion of the fuel, the

governor sends a binary signal (open/close contact) to the pitch control

system. During an open contact, the pitch will momentarily be stopped and

reduced until the charge air pressure required is obtained.

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From simulations, expectations and operational experience on the Cardissa, it

was found that DP capability of the diesel-electric plant is not limiting its

operations. Transient behaviour of the main engine tends to be more

restricting on DP ability than the diesel-electric

plant. Simulating DP operations, the power management system of the diesel-electric plant is simplified to a charge air restriction. In case of an overload signal from the

diesel engine, the pitch is reduced.

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3.2 THRUSTER

3.2.1 General

The Cardissa is equipped with controllable pitch propellers solely. See § 2.1

'General Arrangement Cardissa'. Propellers are LIPS C-type. CPP rotate at (almost) constant speed and control the thrust by regulating the pitch.

3.2.2 Controllable pitch installation

A functional scheme of the control system is shown in Figure 3 (in Dutch).

The pitch control system 'olieinvoereenheid in asleiding' is modelled in

[Klein Woud, 1993a1. The pitch angle is linearized as the maximum pitch is no

more than 20 degrees, giving the block diagram as shown in Figure 4.

Absolute pitch setting and pitch change speed is limited, in accordance with

manufacturers data.

3.2.3 Thruster

The thrust and torque are determined primarily by the pitch angle. Afterwards,

both are compensated for the rotation speed and dynamic influences by energy storages. The thrusters are assumed to remain constantly and fully submerged. The significant loss of thruster thrust that may be expected due

to air suction is

not taken into consideration. See [Stoep, 1992b] for

calculations on the Cardissa.

Pitch dependency. Lips provided data for thrust, power and inertia versus propeller pitch for nominal speed. The data is approximated by polinoms. Speed dependency. A basic dependency on rotation speed is modelled

for thrust and power.Regarding thrust coefficients for four quadrant

diagrams I Kuiper, 19911:

= Cy Y2p(Va2 + (0.7u nD)2-1/4u-D2) - thrust;

= Co- 1/2p(Va2 + (0.7u nD)2-1/1rt.D3) - torque;

Assuming V, small (Dynamic Positioning mode) and CT and Co about

constant, both thrust and torque are proportional to rotation speed

square (n2). Power is proportional to n3.

GUSTO varying dynamic positioning loads of PROJECT

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Energy storage. Due to water dynamics, the thrust will not immediately

be the quasi-static thrust if the pitch angle is adjusted [Made, --J. See Figure 5. Though "the curves represent results of model tests and are typical for the tunnel configuration and change rate of this particular

arrangement only" (3000 hp.), a dynamic influence on thrust reversal is demonstrated. This influence is simulated by using a first order system,

with a time delay dependent on pitch reversal time. The relation pitch

reversal time, thrust reversal time is shown in Figure 6.

Accelerating torque calculation is explained in [Kuijpers, 1996a1. (The inertia includes the inertia of the water.)

Simulation parameters and manufacturer's data used to extract these

parameters is inserted in appendix A 'Thruster'.

;7 olieleidingen door holle asleiding restr:ctiekle (40 bar) /ischroefnaaf SChroefspoedterugmeldstang

door holle asleiding

olleinvoereenheld in asleldin9 spoed aanwi)zing volgscnuif V A 't4-;derla servospoed aanvipzing xervocylinder1 retour tank electrische bedlenlng

u OWN

Fig. 3 _{Controllable pitch scheme}

olletanA rpcmtluchtinq hogere plaatsing dan schroefnaaf evt. lekkage duplex zuigfilters ntlast-lep (140 bar) duplex persfilters koeler X hydrauli Pen (schroef'ype)

/e-motor / generator / diesel REVISION

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ervo-pitch control system

control logic

thruster curves Tpit

pitch pitch control + thruster curves Ppjtch pitch thruster torque

0

ni

0

co co CTI 0 C.31 pitch

thruster

thruster curves Tpitch._ (CPP) ' Tstotic energy storage ' WaCerAlow thrust obtained cn cy) oo is) cri

< cl) 2 cr. co cs3 Ct. (1) "C 0 I 3 a'

3 5

0

0 o

Tplh p it I speed \2 1st order system tominol speeil 4. rotation speed accelerating torque (prediction) co "0 DO "V _{> M XI} C)

<

0

Fig. 4 Thruster model mc-)3 L

5 z

thrust required Ts PMS security logic

0

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TURNING TIME RPM

4.1

TURNING TIME TORQUE Cs

c)

TIME DELAY

TURNING TIME THRUST

TIME lcm= 5sec

Record of torque and thrust reversal owing to rpm reversal

Fig. 5 _{Record of torque and thrust reversal}

16-12 sec L L reversal TIME lcm 5sec I 2 6 8 10 12 1'4

Influence of pitch reversal time and reversal time of propeller revolutions on thrust reversal time. T/ = Propeller pitch reversal time or reversal time propeller revolutions. T2 = Reversal time thrust. = Controllable-pitch propeller. 0 = Fixed pitch propeller with rpm regulation

Fig. 6 _{Influence of pitch reversal time or propeller revolutions} on thrust reversal time

TURNING TIME PITCH

TURNING TIME TORQUE

TIME DELAY

TURNING TIME THRUST

Record of torque and thrust reversal owing to pitch

GUSTO varying dynamic positioning loads of 6185

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3.3 ELECTRIC MOTOR

The asynchronous electric motor produces torque based on the difference

between rotation speed of the rotor and the rotating magnetic field due to the

alternating current in the stator. The model used in shown in Figure 7.

The model is

based on the equations for an asynchronous machine

[Hamels, 1992] and further expanded with manufacturer's data.

Kloss formula, relating torque and slip approximately: 2 Mk Me S k us Is Sf si, skis : torque : maximum torque slip

: slip at maximum torque

3p Us2 (1-a)

47t fs2 Lb

C us2 , C = constant

ff

: stator voltage

stator frequency = synchronous frequency

Modelling the electric energy storage was shown to be small in § 2.2 'Energy

storages quantified' and is not modelled. Modelling the rotor speed and

synchronous frequency has been described by lKuijpers, 1996a].

Simulation parameters and manufacturer's data used to extract these

parameters is inserted in appendix B 'Electric load'.

Mk

Investigation into time response under

varying dynamic positioning loads of PROJECT 6185

a

GUSTO

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+

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Me Mk

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thruster torque

asynchronous electric motor

mechanical energy Pm - Mw

energy storage -iner

tia-Mocc

ca

Fig. 7

Electric motor model

energy transformation and dissipation Pm

-.Pe slip ')1 Mkip P1/3 electrical energy _Pe -sqr 1(3) U I.pf Inorre f ) sqrt(3).0 PeCi.

frequency voltage current power factor

0

rn

0

co

0

CD -63

"

accelerating torque prediction error rotation speed "0 XI 1:1 (prediction)

>

M

<

Pei) PC/ 3/7 I/ 1Sdyn SC/7 Sstot energy storage -coil. - ./ Mkip,nom Unom 0-Pin.no Cr C.31

0

20 0 01

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3.4 GENERATOR

3.4.1 General

A generator transforms the mechanical energy generated in the diesels into

electrical energy. The generator controller is designed to control the generator voltage.

The generator is assumed to be in quasi-static electric equilibrium constantly.

As mentioned by [Kuijpers, 1996a1, the load fluctuations have periods of

about 20 seconds, enabling a quasi-static approach. The model is only valid under these restrictions.

At first the generator controller will be explained. Secondly, the generator

itself.

3.4.2 Generator controller

General

The generator controller regulates the field excitation. By regulating the field

excitation, the current is controlled.

A generator feeding an infinitely strong electrical circuit (i.e. constant voltage and frequency, independent of the current delivered), will deliver current to the

circuit depending on the diesel engine torque. The generator controller will

regulate the field excitation in order to have the electrical torque equal to the shaft torque (and constant speed).

In local electrical circuits, as aboard ships, the electrical circuit is not strong. The voltage varies depending on the current generated. See also paragraph

'Mechanical and Electrical Equilibria' in [Kuijpers, 1996a1. By controlling the

field excitation, the busbar voltage is regulated. Two control strategies are applied: constant voltage or a constant voltage/frequency ratio. Constant

voltage is most common these days.

/ e-motor / generator / diesel REVISION

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simulation model

Describing the generator model in

§ 3.4.3, it will be shown that the field

excitation is not the control input used, but a simplification: 'control signal', which is proportional to the generator current.

Due to this simplification, the generator controller will be modelled as a

straightforward Fl-controller, with required voltage being either a constant voltage or a constant voltage/frequency ratio:

Urequired = U nominal

or _Urequired _{= U nominal.} frequency / frequencyneminal

The controller is not limited for overload. Active power is limited by the diesel engine controller (governor).

3.4.3 Generator

General

The upper graph of Figure 8.

shows the conceptual functioning of a

(cylindrical-rotor machine) generator. As shown, both the current delivered and

the electrical torque depend on the phase angle '6' between the (rotating) magnetic field of the stator and the field of the rotor. lie, though both the stator rotation speed and the rotor rotation speed are equal (synchronous

machine), the angle '6' (being the integrated difference between the rotor and stator speed) varies depending on the load applied. The maximum torque will

be developed if 5=900. At higher torques, the generator fails to function. At

negative values of 6, the diesel engine is a load instead of an engine

[Hoeijmakers, 19951.

The graph shows the conceptual functioning of a cylindrical-rotor generator. More in general (cylindrical-rotor and salient pole meachines), the generator torque developed can be written as [Hamels, 19911:

X, -X

U_S -sin 6 + `7 Us2 sin26

Me(8) 3- 2 Xd-Xq

cos

As shown in [Hamels, 1991], for the cylindrical-rotor machine of Figure 8, the second term can be neglected, resulting in the equation used in the figure.

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-The general equation can also be written 11-lamels, 19911 as:

3. Us.1k.sino

CU2sin2ö

s

With and Cr:

lk : shortcircuit current

Cr : machine constant, which

machines, but not in general.

With typical generator efficiencies of 96%, lk is about the total current(I),

while the current needed for field excitation can be accounted for by

efficiency(n). So, cos cp of a cilinderrotor-type generator can be derived as follows:

P,(ö) =-- Me(S)-Ws = 3- Us- lk- sine mechanical power

Po(6) =

3- U f con

= n.- Pm(S) electrical power

cosq) = A/3- (n- Us/U-

yo.

sind

= -V3- r7- sins powerf actor

The generator electrical power and mechanical power are thus related

physically a.o. by phase angle S. So, under static conditions cos cb is directly related with. sind.

simulation model

Obtaining a model for the generator, the first subject considered will be the interaction generator - generator controller, in order to generate sufficient

current to keep the busbar voltage within required limits. Then, having

established voltage(U) and current(I) for the generator, the generator model

will be completed by equalizing electric and mechanic power.

1.) Establish voltage U and current I.

In order to keep the voltage within tight boundaries, the current is varied

by the generator controller. By adjusting the field excitation LP, the

current is controlled. As shown in the upper diagram of Figure 8, the

current depends further on: voltage U, angle cp, angle 6, stator speed w

and inductivity L which is a machine constant.

can be neglected for cylindrical rotor

PAGE 21

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Firstly assume superposition for current I under small variations of

voltage U. Then the current is the sum of the current obtained by

controlling the field excitation and a voltage dependent current:

Current obtained by controlling the field excitation. Assume that the generator controller is capable of fulfilling its task properly, i.e. the

controller regulates field excitation tlis, to control current I

to a

required value, taking into account variations of U,40, d and w. Then

the controller establishes a field excitation which controls the

current to be exactly the required current, established by the

generator controller. Under this assumption, modelling the generator

-

generator controller combination, the field excitation may be

skipped and may be replaced by some 'control signal'. 'Control signal' is proportional to the current:

I contiol control signal- Inomma,

Voltage dependent current. See also paragraph 'Mechanical and Electrical Equilibrium' in lKuijpers, 1996aI for the dependency of

voltage and current. The necessity of

simulating a voltage

dependant current is understood by realising that electric equilibrium will also be accomplished if the generator is badly controlled (e.g.

constant field excitation).

Figure 8, 'generator concept' shows an approximate relationship:

U + sing). Xs- I = Ep- cosd

differentiating this equation, shows an approximate relationship:

Al

AU AU

-

cos24)-X,

The model used is further simplified. Xs is taken constant (constant

busbar frequency),

see appendix D

'Diesel generator'. The

powerfactor is taken 0.8 (cos),.

AU _U-U,equireth with Urequired as defined while describing the generator controller before.

PAGE 22

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2.) Equalise the mechanic and electric power.

In [Kuijpers, 1996a] it is determined that the rotor and the stator of

synchronous machines are simulated to rotate at equal speed. So,

simulating as described in the theory, one cannot obtain 6 by integrating rotation speed differences. Therefore, one cannot calculate cos 0 from its physical relation with sin& Knowing current(I), voltage(U), torque(M) and

speed(w), the powerfactor(coscb) can also be calculated directly. (The

relationship between cos 0 and sine. was illustrated in the introduction of the paragraph.)

In general, for 3-phase AC electric-mechanic energy conversion, the

relations underneath are valid:

= V3.

I apparent power

Pe

= V3.

coscb electric power

Pm

= M w

mechanic power

17 = Pe/Pm efficiency

So, cos q) can be calculated without sina:

cos)

= r

Pm/S powerfactor

In the lower graph of Figure 8, the equations above are implemented.

Obtaining the accelerating torque, simulating the energy storage capacity of

the rotor inertia, was explained by [Kuijpers, 1996a1. Obtaining the energy stored in a coil was explained in paragraph 'Electrical Energy Storage' of

I Kuijpers, 1996al.

Simulation parameters and manufacturer's data used to extract these

parameters is inserted in appendix D 'Diesel generator'.

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TU Delft

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energy storage

-coil

-current field excitation (controller) voltage power factor frequency _voltage _required _{y oil age} _{power f actor}

generator concept

(i.e.

cilinderf °tor -type, otherwise o 'sin 28

should be included furthermore)

electrical energy Pe -sqr 3).U.I pf EU cos 0 electrical energy Pe -sqrtl3).U.1 pf Xs Ep j.w.'isf

generator model

Icontrol I .4) + 'Al Xs.I Fig. 8 Generator model sc t(3)u.i Ep energy transform Pe S energy storoge -coil -Mkip Ss tot Sdyn mechanical energy Pm -mkip 77 P1 energy transformation Pe -77 f(Sdyn, Pm) Pm ' Sdyn Mstatic

energy storage ?inertia=

Neff

mechanical energy Pm

-m.

Pm

Ms to tic

energy storage -iner tia- MacC

generator = diesel engine torque

torque roto ion accelerating torque speed (prediction) Melt

diesel engine torque

°rotor

Ostotor

''rotor

rotation speed rotor

'stator

current control signal

co

0

co cri 23

0

co co

CD

00 r

(29)

3.5 DIESEL ENGINES

3.5.1 General

The diesel engines modelled aboard the Cardissa, are 3 high speed Caterpillars

(1489 kW at 1800 rpm. each) and a low speed diesel engine running at

112 rpm in rough weather DP mode (77 rpm in good weather).

3.5.2 Governor

general

A governor is an automation device designed to control the speed of the diesel-engine. The governor's function is to regulate the flow of fuel into the

diesel-engine to control its speed.

The basic structure of most governors is quite similar. The main functional

components are the control unit, the

actuator and fuelrack

limiters

[VanDeMark, 19841, [Blanke, 1990], [Sulzer ZA40S]:

The control unit. The required rotation speed is compared with the actual rotation speed (speed sensor). The difference between both should be eliminated by regulating the engine output power, i.e. by regulating the

flow of fuel into the engine.

The actuator is a device which exerts the force required on the fuel rods which cause them to move to the position determined by the control unit, the fuelrack position.

Limiters. To protect the engine from overload situations, the fuelrack position is limited as a function of:

scavenging air pressure (charge air pressure limiter)

engine speed (torque limiter, electronic speed control only) - maximum offset (torque limiter)

A load is either driven by one single engine (stand alone), or by several

engines (load sharing). Aboard the Cardissa, the main engine drives the main thruster (stand alone). The combined load of the side thrusters is driven by the

3 generator sets (load sharing). To enable load sharing of several diesel

engines one may either use governors with speed droop (about 4%) or

isochronous governors (electronic) in combination with centralized load sharing

control. In case of 4% speed droop, the engine speed at 0% load, is 4%

higher than it is at 100% load. With isochronous governors, the engine speed is independent of engine load.

ENGINEERING _thruster _/_e-motor _/ _generator

/ diesel REVISION 0

PAGE 25

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-See for controlling and monitoring the entire system (fuelrack position, limiters

and load) also § 3.1 'Engine Control and Monitoring'.

A mechanical-hydraulic governor with speed droop and extra stabilisation is

shown in Figure .9. The (rotating) mass-spring combination, determines the

difference between required and actual shaft speed. The speed droop is due

to the upper beam. The extra stabilisation is obtained by the needle valve

construction, a construction also used for isochronous, stand alone governors [Klein Woud, 19881.

simulation model

The governor shown in Figure 9, is modelled conceptually in the upper block

diagram of Figure 10. The mass-spring combination,

is linearized. This combination is the control unit mentioned before. The position of the main piston, the fuelrack position, is the limited integrator block. The integrator is

limited according to scavenging air pressure and engine speed.

The model used for simulation is the lower block diagram. It is identical to the

upper diagram with the flow through the needle valve linearized. Neglecting the limiters, the transfer function of the governor model is:

cl

FR 1 +

ci

(c2+ c3

cac4

1 +

with c1 # 0

FR : fuelrack position (z in Figure 10)

E : rotation speed error

The transfer function is identical to the transfer function in [Kyrtatos, 199-], with the transfer function of the flyball system linearized.

ENGINEERING

41,e thruster / e-motor / generator / diesel

REVISION 0

PAGE 26

TU Delft

GUSTO 6185.9515.310 & OEMO 96/08

cl .s + c4

s2 cl-s(c2+c3) +

c4(s+1)

s s

(31)

nickgeldhrter Kolben

nickfiihrender Kolben

Rig. 9

_{Governor with permanent speed &mop and}

_extra stabilisation

ENGINEERING _thruster _/

e-motor / generator / diesel REVISION, 0

dirt

I PAGE

27 TU Delft

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1

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required shaft speed wreq required shaft spee wreq

governor concept

moss- spring combination (linearized)

wreq shaft speed

governor model

shaft %peed 'Governor model limiter C4 imited nteigrato rncx 1 min limiter

pressure flIelrock position fz)

limited inte rotor

I uelrock position 12) co co' (A)

0

co a) co cn

0

r4 .7 al C P.+ 5 nadelventc 777 flow 1 Fig: / k-m.(ureq).(11/12)

(33)

3.5.3 Diesel engine General

The effective power depends on the combustion power developed and the efficiency of the engine. The combustion power depends on the amount of

fuel burnt per cycle. Dynamics of the diesel engine are merely determined by two dynamics, i.e. the fuel injected which depends on governor dynamics as described above and the amount of air (oxygen) available, as will be described underneath. A dieselengine overall scheme is shown in Figure 11.

The engine should not be over-fuelled. To fully burn 1 kg. of fuel, 14.5 kg air is needed [Klein VVoud, 19881. As combustion time is short, only part of the air injected is consumed, giving need to a surplus of air. At an air-fuel ratio of

20:1, no excessive smoke was expected for a particular low speed engine. Exceeding a 25:1 ratio, more than sufficient air is available [Kyrtatos, 199-].

To increase the amount of air in the combustion space, the intake air is

compressed. Generally, the compression power is obtained from a turbine in

the exhaust gas flow: a turbo charger. The turbo charger is dimensioned to

obtain sufficient air in normal operation. The intake air is first compressed and cooled afterwards to further increase air density. The operating field of turbo

charged diesels is

accordingly limited to those regions with sufficient

compressed air. Figure 72 shows the operating field of the Wartsila VASA 46,

which is typical for turbo charged diesels.

This operating field is valid under steady operation. Under rapidly changing power demands, the turbo compressor inertia may cause a shortage of air

available, as compressor speed and so compressed air pressure increase is delayed.

Except for combustion, the (same) compressed air is used for combustion

space refreshment (scavenging). Especially two-stroke engines (Cardissa main

engine) need sufficient scavenging air to drive the exhaust gases out of the

combustion space. In the lower power region, the Cardissa main engine has an auxiliary blower installed (beneath 25% power).

PAGE 29

(34)

Fig. 11 Overall scheme of diesel engine

Operating field for CP-propeller, rated speed

500 RPM, (4V93L0519, fig_2-2) minim

(kW)

6

2

CLUTCH SLIP TIME 3-5 sac

Remark! Restrictions for low load operation to be observed.

Fig. 12 Operating field for diesel engine

/ e-motor / / diesel REVISION 0

.4<e generator _PAGE ₃₀

TU Delft

GUSTO 6185.9515.310 & OEMO 96/08

JUG 100 &CHAN CAL FUEL STOP 300

I il

lt

r

41

RANGE FOR TEMPORARY OPERA T ION ONLY

A!, ali, 44% .114110/11111111110* 700 ,00 RANGE CONT FOR 1 WOOSOPERAT 11

Nal

L MD 00 ION .,.4*/ MO RIP1141. '00

olio

OD 0

IN Ell

irn

... 150 200 ?SO 350 400 450 500 SPEED (rpa) RAN Cylinders

Exhaust gas receiver

Cooler 'Compressor Air inlet Exhaust Turbine I

(35)

Simulation Model

The aim of the diesel engine model is to represent the torque developed by the

diesel engine. A variety of models was found in literature, briefly outlined

underneath:

A model presented by [Klein Woud, 1993a]. It is easy to handle, but has the disadvantage that though only few data are needed, these data are

not known if the engine behaviour is not known. (How much torque is immediately available ?) Especially for the Cardissa main engine, the

predictive behaviour would be poor as the engine runs at 112 rpm, while

it was designed initially for 140rpm MCR (135rpm ECR1). This is why

diesel engine matching with comparable engines would have low

predictive value as the engines to match with, run basically at design

speed/load.

Models that make use of maps with operating fields for both the effective torque and air pressure. These maps are not available.

In recent years, articles have been published with diesel engine models

based on modelling each diesel engine component, giving good

resemblance with test bed measurements. [Woodward 19841, [Klein

Woud 1993b], 1Kyrtatos 199-], ILarmi 19931, [Lan 1996]. These models

need however quite extensive data, data which is not available in this

research (a.o. operating maps for the turbo compressor). Furthermore,

even if available, matching the engine would take days [Klein Woud

1993131. The degree of accuracy is

(far) beyond the scope of this

research.

ENGINEERING

f)..' e _PAGE ₃₁

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Unfortunately, none of the models above is considered appropriate due to lack

of data. In the simulation model used, the calculation of the air pressure is

restricted to a global relationship between pressure, airflow and power in the exhaust gases. The nomenclature used is given in § 3.5.4. The block diagram in Figure 13 shows the calculation order of the dieselengine model used. The

numbers in the block diagram reference to the equation numbers, the

timeconstants T-rc, TE) reference to first order delays.

Intake manifold pressure:

Intake air pressure after turbo compressor: By evaluating steady state

pressure, airflow and power, an approximate relationship can be noticed

for the two diesel engines considered and a third engine tested for

[Lan, 1996]. See Figure 14. Values used are inserted in appendix E

'Turbo compressor analysis' The main engine is Sulzer under propeller load (P = Ne3), the generators are caterpillars (Ne constant). The pressure

is the pressure immediately after the compressor, which is the intake manifold pressure plus the pressure drop over the air cooler. Turbo

compressor data is insertes in appendix E 'Turbo compressor analysis'.

Pc- Po

equation 1.

Pressure drop over air cooler: The pressure drop over the air cooler is assumed 15% of the pressure at nominal load. (A rough calculation based on [Coulson, --1 gives 0.5 bar for Sulzer.) The pressure drop is [Kyrtatos, 199-]:

APcool

equation 2.

Intake manifold pressure:

Pp Pc P cool

equation 3.

The pressure is simulated with a first order delay for turbo compressor delay. An auxiliary blower is not modelled. See [Woodward, 19841 for

models.

2.5

Pexhaust

=C2 TC tn. 2 Pc

varying dynamic positioning loads of

ftJGUSTO

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-Exhaust manifold pressure:

From the two equations underneath, equation 4 is extracted which is used to in the model to calculate exhaust manifold pressure:

turbo charger compressor torque [Lan, 1996]: Cp To I(Pc)k-I

21T cNTc _Po

turbo charger turbine torque [Lan, 1996]:

mEC Ten

pe [1 (t-'0 ko 1ke

Nrc _PE

Exhaust manifold pressure:

ke-1 k-1 = 1 -

C m T

r 0

ke P -1] p o PE PO 11TC C pe thE TE equation 4.

To represent the influence on dynamic behaviour of the exhaust

manifold, the inputs pl and TE are determined as in steady-state. !or is then obtained after a first order delay for exhaust manifold influence.

Air flow in a two stroke engine:

Flow of

air

through a two-stroke engine can

be approximated

[Woodward, 1984] by:

(C313 4- C4) PI

PE equation 5.

The cylinder displacement air flow is the air volume trapped in the

cylinders during a combustion cycle. For a two stroke model this is

[Ne = rps, p1 = N/m^2]: Z NE pt

R7,

V trap equation 6. Mc M

/ generator / diesel REVISION o

4.f PAGE 33

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The oxygen available for combustion is in general less then the cylinder

displacement air flow (oxygen), as part of the exhaust gases stays

behind. The ratio of fresh air is approximated. The scavenging efficiency

is shown in Figure 15 [Kuijper, 1954] for two stroke engines. (In the

figure, As is the ratio scavenge air per cycle / cylinder volume.)

The air flow through a four stroke engine is the sum of a scavenging air

flow as expressed above, and a cylinder displacement air flow:

rn pi pi z Vc NE pi

PE

IT;

equation 7. Engine power:

The power developed by the engine is proportional with the fuel burnt.

The fuel burnt is the fuel injected as long as sufficient oxygen is available

for combustion. Insufficient air reduces the power developed and is modelled using a cylinder efficiency. Calculation of power is based on

mean effective pressures:

1MEP

= Cs.

FR- !icy

equation 8.

The calculation of the engine frictional losses is an empirical correlation by I Kyrtatos, 199-L

FMEP = CeNE + 0.047 IMEP + 0.453

equation 9.

BMEP = IMEP - FMEP

equation 10.

The brake power [Nle = rps, BMEP = N/mA2]:

BMEP VG.NEZ

PE

equation 17.

The exhaust gas heat flow is the heat induced minus brake power and

losses in cooling water, lubrication oil and radiation. At rated power are

the losses approximately 15%. The exhaust gas heat flow is used to

calculate the intake manifold pressure and is calculated roughly:

'exhaust

fuel- It

P,

equation 12.

8

varying dynamic positioning loads of GUSTO

/ e-motor / generator / diesel REVISION,

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= +

2RT"

(39)

Engine speed:

The calculation of the accelerating torque was explained in I Kuijpers,

1996a].

Modelling the two-stroke model was found more difficult than modelling the

four stroke model due to the large influence of exhaust manifold pressure on

the air flow through the cylinders. Especially the exhaust manifold pressure

was difficult matching, as no measurements were available, while its influence on mass air flow can not be neglected. Exhaust manifold pressure for the main engine has poorly been modelled.

Simulation parameters and manufacturer's data used to extract these

parameters is inserted in appendix C 'Main engine' and appendix D 'Diesel

generator'.

ENGINEERING _thruster _/ _e-motor _/ _generator REVISION 0

any,- PAGE 35

(40)

3.5.4 Nomenclature

BMEP : brake mean effective pressure [bar]

: constant pressure specific heat intake IJ/kgKI

Cp : constant pressure specific heat exhaust [J/kgK]

FMEP : frictional mean effective pressure [bar]

fuel : fuel flow [g/s]

: fuel caloric value [kJ/kg]

IMEP : induced mean effective pressure [bar]

Jdm : diesel engine inertia [kgm'2]

: ratio of specific heats

ke : ratio of specific heats

mass flow rate intake air _[kg/s]

m'E : mass flow rate exhaust gas [kg/s]

: torque [Nm]

NE : engine speed [rpm]

NTC : turbocharger speed [rpm]

Po : ambient pressure [K]

Pc : air pressure after compressor [bar]

PE : pressure at turbine nozzle [bar]

: intake manifold pressure [bar]

: effective power [W]

Pexh : power exhaust gases (heat flow) 1W1

: air constant [J/kgK]

To : ambient temperature [KM

Tc : temperature after compressor [K]

TE : temperature at turbine nozzle [K]

: intake manifold temperature [K]

: cylinder displacement volume

Vtrap : trapped air volume [m's3]

: (volume at closing the exhaust gate) : number of cylinders

6 : coefficient for 2 stroke engine. li

: coefficient for 4 stroke engine 2

: pressure drop over air cooler [bar]

qc : compressor efficiency

cylinder efficiency

fIr : turbine efficiency

TTC : time constant turbo compressor

rE : time constant exhaust manifold

PAGE 36

TU Delft

GUSTO 6185.9515.310 & OEMO 96/08

H

k

m'

T1

(41)

ef,

(42)

90 7 6 5 4 3 2 .1avimsoollmg 9_mkeltrsp9glin 80

aw,

PAGE 38

TU Delft

GUSTO 6185.9515.310 & OEMO 96/08

I ( I . , _,.-- -e ...' --.4-," ./ .war Doling

rff

Fr Q2 0,4 06 0,8 1,0 12 1,4 16 18 an Ai

poele f feet btf diverse spoelsystemen.

Fig. 15 Scavenging efficiency (2-stroke)

(43)

7 100% CO 80% o_ a) 100% Ct _60% 120% 60% 40% 20% 120% 40% 20% 0%0% 120% 0%0% 60% 40% 20% SULZER 5RLB66

propeller law + 85% load at 100% rpm

40% BO% 120%

Pexh ^ 2.5/air flow

CATERPILLAR 3516 generator load

40% 80% 120%

Pexh 2 5/air flow

MATSUI MU323DGSC [Lan, 19961 unknown speed/load relation

0%0% 40% 80% 120%

Pexh " 2 5/air flow

Fig. 14 Turbo compressor analysis

PAGE 39

TU Delft

GUSTO 6185.9515.310 & OEMO 96/08

a)

100%

(44)

4 PARAMETER VARIATION - 'SIMULATION RESULTS

4.11 GENERAL

In this chapter, only the simulation runs on the total' power generating plant are discussed. The simulations testing individual components of the power

generating plant are inserted in the respective appendices (Appendix AS - 16, C16 - 19 and D19). The simulations represent the vessel during DP operation.

The bow thrust, the stern thrust and the main thrust are simulated

representing a twenty minutes period. The thrust required in the first run is the

thrust required under the most onerous design condition. For this condition, the power generating plant appears to be well able to develop the thrust

required.

As the research is aimed investigating system dynamics of the generating

plant, thrust required is multiplied by 1.5 and 2, examining system sensitivity

on the magnitude of thrust required. System dynamics is calculated andmain

characteristics are graphically shown in appendix Fl The influence on

positional losses of differences between thrust required and thrust obtained

is also 'represented in the graphs.

To study specifically the deviations encountered due to system dynamics, the

reaction time influencing parameters are multiplied by 1.5 and 2.. The track studied is the original track multiplied by 1.5..

4.2

CONDITIONS INVESTIGATED

4.21

Limiting design condition

In order to simulate a sensible number of conditions, the power plant model is investigated under limiting design conditions and under systematically more severe conditions. Only more severe conditions have been investigated, as the

MV Cardissa power plant as modelled is well able producing the thrust

required. By systematically varying the conditions, the number of variables and the number of simulation runs is reduced, while main system capabilities are.

represented.

Limiting "design conditions are obtained from the conversion specificationi. The limiting weather conditions in combination with maximum positional

losses determine the thrusts required. The thrusts required are obtained

from [ Stoep, 1992a1t. The forces on the vessel were then believed to represent the forces due to the most onerous combination of vessel loading and environmental condition. The vessel is fully loaded and subjected to the "loading" limiting environment (see § 2.1.2,, § 2.1.2

'dynamic positioning specifications),.

ENGINEERING _thruster_/ _e-motor _/ _generator _/ _diesel REVISION(

0

PAGE 40

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2. The power plant is designed to produce the thrusts as required with a

number of components shut down. As mentioned in § 2.1.3, the

maximum electrical load is to be supported with one of the diesel

generators shut down for maintenance and with the running

diesel

generators approximately 80% loaded. Furthermore, DP ability should be

sufficient with only one tunnel thruster fore running.

4.2.2 Systematic variation of conditions

Starting from the design condition, several (more severe) conditions have been examined. At first, the absolute magnitude of the thrusts required is increased

by multiplying the thrust required under design conditions by 1.5 and 2. Besides the magnitude of the maximum thrust, also the slope - change of

thrust per time equivalent - is increased. The ability of the power generating

plant to generate the thrust required is both statically and dynamically

determined. (With other words, is the plant capable of producing the thrust

required in the end and is the plant reaction time short enough to produce the thrust at the moment needed.)

To examine exclusively the dynamic capabilities of the power generating plant, a second variation has been examined. By compressing the thrust required signal in time, the magnitude remains unchanged, while the slope is increased

change of thrust per time equivalent -. This variation may have any of the

following three practical causes mentioned underneath:

The modelled system represents a power plant, with the same nominal output as MV Cardissa power plant output, but the system reaction time is delayed. This is, the power plant time constants are enlarged, rotating

components inertia is increased and the maximum allowed changing speed is reduced (e.g. pitch changing speed). The original 20 minutes track remains a 20 minutes track.

The modelled system represents a power plant subjected to environmental forces with a higher frequency (a.o. frequency of the

drifting forces). The original 20 minutes track represents a shorter period

(200% time 10 minutes, 300% time 6 minutes 40 seconds).

The modelled system represents a power plant, with a tighter DP control system. This is, smaller positional losses are allowed, causing MV Cardissa to keep the bow within a smaller radial offset (most likely

in combination with an increased magnitude). The original 20 minutes

track represents a shorter period.

PROJECT 6185

PAGE 41

(46)

The variation of parameters is thus basically limited to dynamic parameters.

I.e. as the aim of this research is to examine the transient behaviour of a

power plant under DP loads. Furthermore, the models used are matched with manufacturers' data, and consequently the models should be re-matched in

case of a different nominal output.

4.2.3 Simulation runs

As mentioned in

the previous paragraphs, the power plant model

is

investigated under limiting design conditions and under systematically more

severe conditions.

In the simulations, either the magnitude or the time

compression of the thrust required is varied. Five simulation runs have been

carried out, with variations in time compression and magnitude increase as

illustrated in the table underneath:

Simulation tracks are shown in appendix F 'Simulation results'.

4.2.4 Power producing plant

As mentioned before, the power plant is designed to produce the thrusts as required with a number of components shut down. The maximum electrical

load is to be supported with one of the diesel generators shut down for

maintenance and with the running diesel generators approximately 80% loaded. Furthermore, DP ability should be sufficient with only one tunnel thruster fore running.

The conditions simulated are calculated for the MV Cardissa power plant with

one of the three diesel generator sets shut down and also one of the two tunnel thrusters fore shut down (frame 80 shut down).

The components in service are consequently

- stern thruster with electric motor

bow thruster frame 86 with electric motor

2 diesel generator sets (governors with speeddroop)

main thruster and main diesel engine (governor without permanent

speeddroop)

/ generator diesel REVISION

PAGE 42

TU Delft

GUSTO 6185.9515.310 & OEMO 96/08

RUN 1 2 3 4 5 time simulated (seconds) 1200 1200 1200 magnitude thrust required (percentage) 100 150 200 150 150

time compression ratio (percentage) 100 100 100 200 300 - --, '

(47)

The individual components have been modelled as described in chapter 3.

Extra losses due to friction in bearings etcetera have been neglected.

4.2.5 Positional losses

The DP control system regulating the positional losses,

is taken from

[Stoep, 1993a] as shown in Fig. 15 of [Kuijpers, 1996a1. Under the limiting design conditions mentioned in 4.2.1, the thrust obtained resembles closely

the thrust required. See Run 1. Therefor, the thrusts obtained are not fed back

as forces on the vessel (dashed line in Fig. 15, [Kuijpers,1996a]).

The differences between the thrust required and obtained, cause the vessel to lose position. This loss of position is an extra loss, as the main loss of position is regulated by the DP control system. As the thrust obtained is not fed back as force on the vessel, the DP control system has no knowledge of the extra positional loss and consequently the extra positional losses are not regulated.

The extra positional losses are expressed as losses in the surge, sway and yaw direction and have not been transformed in a radial offset at the bow. The vessel inertia is in accordance with [Stoep, 1992a]. That is, with the

added mass of the water included (approximately):

surge direction: 30,000 ton

sway direction: 60,000 ton

yaw (around midship): 80,000,000 ton.m

4.2.6 Model limitations

The simulation results represent the dynamic behaviour of power generation.

Correct functioning of the simulated plant, does not automatically express however all harming circumstances, by which parts of the power plant may

be operated in unsound conditions. E.g. the main engine is not modelled for predicting vibrations or excessive wear and tear due to low load operation (low temperatures).

PROJECT 6185

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4.3 NOMENCLATURE

The nomenclature which is used in the time trace plots in Appendix F is as

follows:

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TU Delft

GUSTO 6185.9515.310 & OEMO 96/08

Name Dimension Description

Tstern req Tstern obt

-150.. +150 kN -150.. +150 kN

force required stern thruster

force obtained stern thruster

pitch stern -22.. +22 ° pitch stern thruster

power stern

0..800 kW power stern thruster

Tbow86 req -400.. +400 kN force required bowthruster frame 86

Tbow86 obt -400.. +400 kN force obtained bowthruster frame 86

pitch bow86 -20.. +20 ° pitch thruster frame 86

power bow86

0..1600 kW power thruster frame 86

frequency 56..64 Hz busbar frequency

voltage 650..670 V busbar voltage

current 0..2500 A (total) current on busbar

powerfactor 0..1 powerfactor

P electric

0..2880 kW electric power on busbar

Tmain req 0..1000 kN force required main thruster

Tmain obt 0..1000 kN force obtained main thruster

pitch main 0..18 ° pitch main thruster

power main

0..4200 kW power main thruster

rotat speed 104..120 rpm rotation speed main thruster

extra surge rn uncontrolled positional loss - surge

extra sway m uncontrolled positional loss - sway

extra yaw degrees (°) uncontrolled positional loss - yaw

I