Subharmonic oscillations in nonlinear rolling

(.kewi Engug. Vol. II. No. 6. pp. 663.-669. 194. UU29()1/4 S3.(1() - ((0 Printed in Great Britain. © 194 Pergamon Press Ltd.

SUBHARMONIC OSCILLATIONS IN NONLINEAR ROLLING

ArroN10

CARDO,* _{ALBERTO FRANCESCUTTOt and RAD.OSLAV NABERGOJ}

lstitut() di Architettura Navale. Via A. Valerio 10. 34129 Trieste. 1stituto di Fisica. Via A. Valcrio 2.34129 Trieste and 1stituto di Fisica. Viale Ungheria 43. 33100 Udine. Italy

AbstractEn this paper the rolling motion of a ship is examined with particular regard to the possibility of obtaining oscillations 'hich are suhharmonic to the ecitation frequency. Three different mechanisms are found to he responsible for this phenomenon. the importance of which

has already been recognized in the context of ship stability. The first is related to a stroni

symmetric or non smmetric nonlinLantv in the righting arm The second is 1inLLd to the

harmonic composition of sea waves and the third to the well known parametric excitation caused h' coupling between Thfferent ship motions in a following sea.

The onset.of suhharmonics is related to a threshold value [or the excitation stronilv depenUin on damping. The more appropriate analytical methods for i theoretical study of each mechanism are suggested.

[.

INTRODUCTION

A MORE or less marked rolling oscillation is always present in the motion of a ship. This

aspect is clear in everyday experience concerning the behaviour of different types of

'essels and ocean vehicles.

The situation appears quite complicated when quantitative evaluations on rolling are

sought. In fact, the enormous progress obtained through theoretical investigations and

experimental trials in seakeeping during the past thirty years essentially concerns the ship

motion in moderate seas. In these conditions the linear approach is reasonably valid and

the superposition of the effects can be adopted. Thus, there are no particular problems in

Luantitativeiy handling different wave spectra or any confused sea (St Denis and Pierson.

1953).

The knowledge of ship dynamics in strong or extreme sea conditions is on the other

iand very limited notwithstanding the continuous efforts made by research workers in

:Fi,is field. In particular. the correct evaluation of the rolling period and its dependence on

the oscillation amplitude still remains a partially unsolved prOblem. The main problem of

Dractical interest concerns the prediction of the maximum roll amplitudes in different sea

bonditions. Its importance is clear as soon as one considers the need of having a

reliminarv knowledge of rolling extrema before every rational tentative formulation of

:t) criteria. It is useless to mention the paramount importance of handling the problem

f maximum rolling amplitudes which is related not only to the well-being of the

passengers and crew members hut also to the resistence of the structures and machinery..

he operative efficiency of warships and so on.

Very often. the study of the behaviour of the ship in the seaway can be simplified if

rolling is considered as an independent oscillation. However, it still remains complicated

LIue to the nonlinearities present in every mathematical model proposed for its realistic

.1escription(Cardo et al.. 1981: 1982; 1983a).

As

a result, other resonance conditions are

prediLtahle apart trom synLhronlsm In particular tor a ship in regular he im seas these

bre verified when the period T of the excitation draws sufficientk' close to a multiple or

(o3

Deift University of TechnclOY

Ship HydromeclenICS Laboratory

Library

Mekelweg 2- 2628 CD Deift

The Netherlands

and Rrosi...v

sub-multiple integer of the ship's natural period T. Two distinct phenomena can so he

observed, and these are the ultraharmonic resonances for T

,iT and the suhharmonic

resonances for T - To/n. where n is an integer.

Moreover the nonlinearit of the righting arm gives

to the characteristic shift of

the reonance frequency which becomes.a function of the oscillation amplitude. Thus.

the ship presents large frequency windows of particular sensitivity in the sea spectrum.

There are situations in which she ma\' present two stable steady-state oscillations for the

same excitation intensit\ (Cardo er a!

1983h)

These have difterent amplitudes

depending on initial conditions or on possible parameter changes in the system.

Let us now turn Our attention towards subharmonic oscillations. These, together with

synchronism constitute the most peculiar means of capsizing It is not sufficient to have

a

correct frequency tuning to ensure the existence of suhharmonic oscillations, the

excitation ifltensitv must exceed a given threshold so as to bring about their setting in.

These kind of oscillations are produced through a coupJin between the free and forced

oscillatory modes of the ship by means of the nonlinearit

in the righting moment

Generally, a more thorough investigation of the ship roll motion shows that suhharmonic

oscillations ma be found in several cases.

This paper is devoted to a qualitative investigation of the physical mechanisms which

bring about the latter situations. To this effect, the behaviour of a ship in a series of

typical conditions of particular significance will he considered.

2.

EQUATION OF MOTION

The independent, rolling can he described by the following, differential equation

I th .* D(d). d) + Mr (d). r) = E(t).

(I)

where th is the rolling angle with respect to the calm sea surface. / is the mass moment of

inertia including the added mass. D(th..d) is the dissipative term, Mr(th. r) is the righting

arm. and E(t) is the heeling moment due to external forces.

As known. Equation (1) can be written in adimensional terj:ns h using suitable angle

and time scales for the problem and i.e. b,, and T,, respectively. With the suhstitutionsx =

th/d,, and T = nT,,. one obtains

I + dft.

) ± mr(X. T) = e(T).

(2)

Here the meaning of the various terms is obviously unchanged. Later on. the rightin,g

arm will be considered as a function of the angle only. except for Section 5 where a

coupling between different oscillations is introduced through a dependence on tune.

Moreover, it is usual to assume a power series expansion in x and

for the dissipative

term. A realistic damping model taking both the speed and the angle nonlinearities into

account, has been 'recently considered in a series of papers (Cardo etal.. 1982: Ikeda.

1983).

3.

SUBHARMONICS DUE TO NONLINEARITY OF THE RIGHTING MOMENT

Let us consider the rolling of a ship in a regular beam sea. In this case, for the

excitation pne has

Subharmonic oscillations in nonlinear rolling

665

e (T) = e

cos(WT±'y).

(3)

Foremost, a ship in an upright equilibrium position will be examined. The righting

moment will be expressed by a power series expansion including only odd terms in

the

angle. In particular, a cubic polynomial may be used for a sufficiently realistic analysis.

So, the equation of motion becomes

I + d(x) + ox + ax = e

cos (orr+y),

(4)

where w is the angular

frequency" of the small free oscillations of the ship

Now, let us examine the case of a ship in a heeled equilibrium position This situation

can be described by different theoretical models due to the particular nature of the

heeling and i e to initial instability, to an internal or an external cause In the case of

initial instability, it is commonly assumed that the righting moment is no longer an odd

function of the heeling angle. Limiting the approximation to a cubic polynomial, one can

write

I ± d(x,±) + ux + a,x2 +

= e.cos(wi+y)..

(5)

If, on the other hand, the ship is heeled by an intetnal cause (cargo shifting. asymmetric

damage, etc) or by an external cause (wind towing etc ) one still takes Equation (4)

into consideration to represent the roll motion. In this case, however, the excitation

includes a further term which may be considered as constant. Thus, the equation of

motion is

x + d(x,) + wx +

= e() + e1cos(w'r+y);

(6)

It can be proved that Equation

(5)

and Equation (6) are equivalent, in as much as the

first is reduced to the second by a change of variable and vice versa (Hayashi. 1964).

Therefore, in the following discussion, only Equation

(5) will

be considered.

It has been shown (Cardo et aL, 1981) that in a first-order analysis the model described

by Equation (4) presents a subbarmonic resonance when the ratio of the excitation to the

synchronism frequency is close to 3, and i.e. o/w0.= 3. This is one of the so-called

predominant resonances and is known as subharmonic 1/3. On the other hand. Equation

(5)

possesses apart from the subharmonic 1/3 also a subharmonic resonance when the

ratio of the excitation to the synchronism frequency is close to 2

1 e W/W()

2 This is

the subharmonic 1/2. Generally speaking, Equations (4) and (5) possess an infinite

number of subharmonics, the. importance of which progressively decreases.

It must be noted that this kind of subharmonic oscillations, is brought about by the

coupling of the natural oscillation mode with the forced one, and therefore are direct1y

supplied by the nonlinearity in the righting moment of the ship.Consequently. the choice

of a linear mathematical model or of an equivalent linearisation procedure givs way to

the impossibility of investigating this kind of phenomena. Futthetmore, it must be

observed that the appearance of a subharmonic component in the ship's response is

linked to a threshold level for the excitation intensity. This is due to. the fact that in the

subharmoñic frequency region. the synchronous component of the oscillation is generally

stable. The threshold depends both on damping and on the. righting moment nonlinearity

(Carclo et al., 1984). Wright and Marshfield (1980). have proved the existence of a

subharmonic resonance in the above conditions by means of model experimental trials

- The theoretical investigation of the subharmonic. resonances in Equations (4) and (5)

can be carried out in two different ways and i.e. by:

- stability analysis of the resonant oscillation in synchronism;

-explicitresearch of resonant solutions through perturbative techniques;

The first possibility is referred to the classical theory developed by Floquet (Stoker.

1950) concerning the analysis of the conditions necessary for the development of strong

disturbances. As a result a Mathieu or a Hill equation is obtained, giving the stability of

the resonant oscillation with respect to ultraharmonics of order 17=2. 3. etc. In particular.

as the righting moment in Equation (5) is not an odd function of the heeling angle. the

analysis also includes the semi-integer ultraharmonics The first of these is the subharmonic

1/2. i.e. a resonance with half the driving frequency. This result is theoretically correct.

although obtained in the particular case of a stability analysis which does not foresee

a

priori' the existence of subharmonic components. Such an analysis predicts neither the

existence of subharrnonic 1/3 in Equation (4). nor the existence of subbarmonic 1/3 in

Equation (5).

The second method is undoubtedly superior due to its constructive features. It also

supplies the equations giving the response of the system in the subharmonic frequenày

regions (Francescutto and Nabergoj. 1983) and therefore is now generally preferred in

theoretical investigations.

4.

SUBHARMONICS DUE TO HARMONIC WAVE COMPOSITION

As known (Kinsman, 1965), finite amplitude waves may be expressed by means of

trigonometric polynomials of different order of approximation. The main sinusoidal

component has amplitude a and wave length K, whereas the profile modulation is given

by successive components the wave length of which is an integer submultiple of the first

component. An approximation often adopted for a progressive wave is the following

(r,

T)

_a

/ a

₃

_,

_/

= ---i--- cos (krwr) +

cos2(krwr) -

ir

cos3(krurr),

(7)

where (r, r) is the waver amplitude with respect to the calm sea surface, k=2Tr/K is the

wave number and r is the position vector. The wave expressed by Equation (7) is the one

known as Stokes' wave, including the first three terms of wave lengths K, K/2 and K/3

respectively.

.- .

Let us consider a ship under the influence of a transverse trochoidal wave of frequency

o.

The components of the profile appear as higher harmonics of the natural

frequency of the ship and therefore, they can produce subharmonic oscillations in the

rolling. Thus, when the ship is in synchronism with the first wave component, she will he

in a subharmonic resonance with respect to the second or to the third component.

according to her initial stability conditions. The intensity of the higher wave components

is highly reduced with tespect to the main component. But, the higher components act on

a ship which is already subject to large amplitude oscillations.

At this stage, it is impossible to ascertain whether this subharmonic phenomenon can

be relevant or not. Anyhow, its entity is not foreseable in linear terms. i.e..

with the

application of the superposition principle. This is due to the fact that the total response of

the ship cannot consist in a simple summing of different oscillation modes, one of

which

-is by all means resonant (Vedeler, 1925). A more correct investigation of the problem

may .be carried out with the application

of the recently developed perturbation

techniques with multi-frequency excitation (.Nayfeh and Mook, 1979), as the latter

correctly introduce a kind of nonlinear superposition to the effects.

Due to the particular type of excitation concerned, the subharmonic resonance

brought about by the wave composition is similar to that described previously. Here

again a threshold level for the onset is found.

5.

SUBHARMONICS DUE TO COUPL NO IN THE SHIP MOTIONS

A subharmonic component in the ship oscillation can be present due to the coupling

between different motions. In particular, pitching and/or heaving with rolling may give

way to the above phenomenon in the case of following seas. The latter can produce large

rolling amplitudes rendering the safe running of the ship particularly dangerous. This is

the first example of subharmonic oscillations discovered and deeply studied (Grim.

1952).

The mechanism of this subharmonic phenomenon is found in the existence of

parametric resonance conditions, i.e. in the presence of an efficient way of transferring

energy from pitching and/or heaving to rolling. As known, a following wave produces a

periodic change in the righting moment if the-encounter frequency is suitably tuned.

When the excitation intensity exceeds a threshold value, we observe a rolling oscillation

whose predominant frequency component is subharmonic 1/2 with respect to the

excitation. The threshold level i.s increased by the damping.

This phenomenon is already predictable in a linear analysis of the ship's motion. at

least in qualitative terms. It may be efficiently investigated by means of the classical

theory of Floquet (Stoker, 1950). Thus, a system of two coupled nonlinear equations is

found. one of which can be reduced to the well-known equation of Mathieu or Hill. A

major quantitative correspondence is obtained by considenng the nonlinearity effects

Consequently. a nonlinear modelling produces a nonlinear Mathieu or Hill equation with

a forcing term, due to the wave excitation. The analysis of an equation like this can be

suitably carried out with different perturbation techniques (Nayfeh et al.. 1973).

6.

DISCUSSION AND CONCLUSIONS

We have herewith examined different physical mechanisms which may give rise to

subharmonic oscillations in the rolling of a ship. For each subharmonic considered, we

have then pointed out the most suitable methodology fOr the analytical solution of the

particular problem concerned. In Fig. 1, the wave period necessary for subharmonic

excitation, relative to both conditions of initial stability is shown

It is difficult at this stage of knowledge, to quantify once and for all the entity of these

effects. Certainly, not all of the phenomena considered constitute a true mechanism for

Fia. 1. Schematic representation of tuning factor values for the subharmonic rolling due to different physical

mechanisms. Both the cases of upright and heeled initial equilibrium position for the ship are considered.

capsizing, i.e. a risk avoiding the limitation of the ship motion within the stability

domain. As a consequence, an extensive study must be tackled quantitatively:

- to acquire a deeper understanding of the oscillatory motions of the ship in relation to

manoeuvrability, seakeeping, etc.;

- for a better analysis of the structural excitations which the ship is subject to:

- to develop adequate experiments for the research of more realistic models which

correctly take into account all the physical effects, and in particular inertia with added

mass, damping, etc

Lastly, new investigations must be made for the understanding of the transient state of

motion with particular regard to the effects produced by impulsive heeling excitations

We feel that a suitable combination of different excitations gives way to situations which

are still to a certain extent unknown and which may therefore result to be extremely

dangerous for the safety of the ship.

AcknovIedgemeusThjs work

was supported by the National Research Council under contribution C.T.

82.01399.07.

REFERENCES

CARDO. A.. FRANCESCUTTO.

A and

NABERGOJ.R. 1981. Ultraharmonics and suhharmonics in the rolling motion

of a ship: steadv-stte solution. hit. Shipbldg Prog. 28. 234.

CARDO

A..

FRANCESCUrO.A. and NABERGOJ. R. 1982. On damping models in free and forced rollin motion. Ocean Engng 9. 171.

CARDO A FRANCESCUTrO

A and

NABERGOJ

R 1983a On thc maximum amplitudes

rn nonhinLar rolling

Proc. 2nd 1,ii. co,,f.: on Stability of Ships wid Ocean Vehicles, Tokyo. The Society of Naval Architects of Japan.

V RIGHTING ARM NONLINEARITY o WAVE COMPOSITION o PARAMETRIC EXCITATION

vO

0

V3 1/2

iT/TO

UPRIGHT SHIP

0

V

V

0

1/2

1T/T0

HEELED SHIP

Subharmonic oscillatiOns in nonlinear rolling 669

CARDO, FRANCESCUTrO, A. and NABERGOJ, R. 1983b. Rolling response in synchronism: transient and

steady-state behaviour. Proc. 12th Scientific and Methodological Seminar on Ship Hydrodynamics. Varna

(Edited, by Bogdanov, P. A.).

CARDO, A., FRANCESCUTrO, A. and NABERGOJ, R. 1984.. The excitatiOn threshold and the onset of subharmonic

oscillations in nonlinear rolling, submitted for publication.

FRANCESCUTrO A and NABERGOJ, R 1983 Steady-state oscillations of gas bubbles in liquids explicit formulas

for frequency response curves J acoust Soc Am 73 457

GRIr, 0. 1952. Rollschwingungen, Stabilitãt und Sicherheit 'mi Seegang. Schiffstechnik 1, 21. HAYASHI C 1964 Nonlinear Osczllatwns in Physical Systems McGraw-Hill New York

ITCEDA. Y. 1983. On the form of nonlinear roll damping of ships. ISM-Bericht 83/15, Technische Universität Berlin, Berlin.

KINSMAN. B. 1965. Wind Waves. Prentice-Hall, New York.

NAYFEH, A.H. and Mooic, D.T. 1979. Nonlinear Oscillations. Wiley-lnterscience. New York.

NAYFEH, A.H., MooK, D.T. and MARSHALL, L.R. 1973.. Nonlinear coupling of pitch and roll modes in ship motions. J. Hydronautics 7, 145.

ST DENTS, M. and PIERSoN, Wi. 1953. On the mOtiOns Of ships in confused seas. Trans. Am. Soc. mech. Engrs

61.280.

STOKER. J .J. 1950. Nonlinear Vibrations in Mechanical and Electrical Systems. lnterscience. New YOrk. VEDELER, G. 1925. 'Notes on the rolling of ships. Trans. lnstn nay. Archit. 47. 166.

WRIGHT, J.H.G. and MARSHFIELD, W.B. 1980. Ship roll response and capsize behaviour in beam seas. Trans. R. lnstn nay. Archit; 122, 129.

DEPARTMENT OF TRANSPORTATION

UNITED STATES COAST GUARD

MAILING ADDRESS:, - --U.5.COASTGUARD

--WASHINGTQ QC ,2p5)3

PHONE: _(ZU2) 4Zb-2197

1-7 SEP 1980

Subj: Report to the President on Large Tank Vessel Maneuvering and Stopping Abilities

Gentlemen:

Enclosed is a copy of Coast Guard Report CG-M-4-79, "Report to the President on an Evaluation of Devices and Techniques to Improve Maneuvering and Stopping Abilities of Large Tank Vessels." Not only does the study report on the maneuvering ability of tank vessels, but it also proposes that performance requirements for stopping and maneuvering abilities be established

This report is the final in a series of' five that were initiated by the

President in his 17 March 1977 message to Congress on the reduction of marine oil pollution The report presents the basic definitions of' maneuvering and stopping abilities and an assessment of the maneuvering and stopping abilities of existing tank vessels as a function of deadweight, compared to those of dry cargo vessels Using the methods for evaluating maneuvering abilities that are outlined, va"ious devices that have been proposed were examined The initial evaluation was based on the deg"ee of improvement in the maneuvering ability, the cost and the effect on the design of the vessel for each device. A

mathematical (simulat2on) model was enipioyed to further evaluate the most promising devices A description of each operational technique has been

included. The evaluation of the operational techniques was performed primarily by reporting on various full scale tests The findings Indicate that good maneuvering characteristics can be achieved for tank vessels, but without some guidance Or requirement, this is not generally considered in the design cycle for new ship construction.

4ar

ti

£?L

ames C. Card

Commander, U.S.Coast Guard

- Chief, Ship Design Branch

Merchant Marine Technical Division By direction of the Commandant

S

SPEtD

LIM IT

'.55

it's a law we

Increased Astern Power - Increased astern power calls for increasing the output of the astern turbine in a steam propulsion plant. The designer can add more stages to the astern section or increase the steam flow through the astern nozzles. The primary effect on maneuverability is increased astern horsepower. This device is studied further, including an analysis of operating experience. Compared to steam powered ships, diesel vessels have increased astern power. The degree of astern power is limited to 80 percent of the ahead power because of the considerably lower propeller efficiency in reverse.

Steerable Kort Nozzle. - A steerable Kort nozzle is shown installed on a 25,000 DWT Great Lakes bulk carrier, Figure 20. The large ring serves as a device to direct the flow from the propeller. This device has a limited maximum rudder angle. Most installations of a steerable Kort nozzle have

increased the propulsive :efficiency, reducing the Operating costs somewhat. As can be seen from the indices In Table 9 this device is in development and could eventually be applied to large tank vessels. This device is studied further.

Steerable Propeller This device is similar to an outboard motor that can be rotated through 360 degrees. The propeller unIt Is rotated to direct the thrust. Figure 21 shows one instaIled'on a harbor tugboat. Both this and the Voith-SchneIder (vertical axis) units can only be used on tugs, ferries and small coastal vessels because of power limitations. No further study is conducted.

Voith-Schnieder (vertical axis) Propulsion - The vertical axis propulsor shown in Figure 22 and the steerable propeller can providethrust in any direction and are used without a rudder. The Voith-Schneider has been

successfully installed on tugs, workboats, and the Staten Island Ferries under construction. This device is not examIned further for large tankers because of power limitations.

Tunnel Thruster - The tunnel thruster has been installed on the largest tankers for use in maneuvering around docks, buoys, and to offshore mooring systems. They are ducts that have an impeller in the middle, and they are installed transversely in the ends of the vessel to provide the greatest amount of turning force. Thrusters are not effective above forward speeds of 2 to knots. They are most effective at zero speed, which is when the rudder is completely ineffective. This device is studied in more detail.

Stern Anchor .- The stern anchors that are currently being installed on tankers and bulk carriers are designed to prevent a vessel from swinging within a channel or confined area. Employing a stern anchor as a device to reduce stopping distance requires extensive redesign of the anchor handling equipment. This may be practical for small tankers. It is not considered further for application to large tankers because suitable machinery is beyond the present technology. Since it may be a viable device for small tankers, it is examined in the simulation study.

Drag Augmentation Devices - Although these devices are not considered further, Figure 23 showing three drag augmenting devices Is of interest. The

-54

Figure 21 -SteerabIe PrôpeIIeron the tugA.I Allah.

55

(b) POSITION OF PARACHUTES ON SIDE OF TANKER

(c) POSITION OF biJCtSIN BOW OF TANKE

Figure 23 Schematic Views of the Three Braking Devices

splayed rudder device is somewhat similar to braking flaps. It requires a control system modification to a twin rudder vessel to simultaneously provide full right rudder on the starboard rudder and full left on the port rudder These devices provide only slight, improvements in stopping ability and are not

considered further.

-In this discussion and Tables 8 through 10 most of the devices were found to be impractical for large tankers The following devices are studied further using a simulation model:

Active Rudder

Twin Screw and Twin Rudder

* _{Increased Astern Power}

Steerable Kort Nozzle ¶ Tunnel Thruster

Stern Anchor (for Smaller tankers)

-The concepts of increased rudder area and increased rudder angle are important but require extensive model tests. Of the two, increased rudder area does not appear particularly promising, but a detailed study of rudder and hull interaction needs to be cOnducted to answer the increased area question.

DETAILS OF THE VESSEL AND DEVIES APPLIED TO T} EXPANDED MARITIME ADMINISTRATION FULL FORM HULL CONFIGURATION

The initial study using mathematical simulations of maneuvering devices was performed by Hydronautics, Inc , for the Maritime Administration This

study began in 1978 with emphasis on expanding the hydrodynamic coefficients for the low L/B (length divided by beam) hull forms to include twin screw and twin rudder configurations These full forms were developed and tested in shallow water since full form hulls are much wider and shallower than

conventional tanker hull forms The purpose was to seek an efficient hull form ror the'shallow'harborsañd waterways of the United States.

The study obtained hydrodynamic coefficients for a Maritime Ainistration Standard series full form bulk carrier modified with a twin screw, twin rudder configuration With this information the survey of maneuvering devices could include a twin screw configuration The selection of devices for this study was based primarily on the potential effect on an 811,00bDWT tanker and are

listed below: '

'Ti.zln Propellers and 'Rudders

Increased Astern Power

' Maneuvering P'opulsion Devlèes including: Tunnel Thruster

Active Rudder

High Lift Rudders including: Flapped Ruddè'

Rotating Cylinder Rudder

Thrust Vectoring Devices including: Steering Kort Nozzle

Kitchen Rudder

Although the details of this study are in a later section, a summary of the Hydronautles findins is appropriate. These results pertain to vessels of the 84,O00 DWT size:

generally the baseline vessel (8',0OO DWT without devices) was able to maneuver nearly as well as with devices

twin screw,, twin rudder configuration provides only marginal improvement in the ability to maneuver.

the stern anchor shows considerable promise to improve stopping abIlity.

the best performer. was the steerable Kort nozzle.

some of the highly mechanical devices showed significant improvements inmaneuverability.

The Coast Guard added to the Mar1time Administration effort on the 8,000 DWT tanker by sponsoring mathematical simulation studies of t0,000 DWT and 280,000 DWT tankers In this study the same full hull form was used To obtain maneuvering data for a larger and a smaller vessel the geometric

characteristics of the 81,000 DWT vessel were scaled appropriately This scaling procedure is commonly used in model testing The characteristics o

the three tanker's are shOwn in Table 11.

This hull form is considered' reasonable fot' the study. of a large tanker since its proportions are suitable for a design intended to carry maximum deadweight in restricted water depths This size was selected since it represents the size of new tankers which may be constructed for service to U. S. ports which, ih general, have relatively shallow water.

Recent articles in the maritime press have reported that many companies have ordered 80,000 DWT vessels to satisfy pollution requirements of the 1978

Port and Tanker Safety Act. Some companies have sold large 'vessels and purchased 80,000 DWT vessels In their place. H. P. Drewry, a compiler of

Table 11

Principal Characteristics of Three Tank Vessels Used for Maneuvering Devices Simulation Study

(Based on MarAd Standard Series)

kO,000 DWT

8k,ôoo

DWT

280,000

DWT Length Between Perpendiculars

_160.9

_206.1

_307.9

lnm(ft)

(527.9)

(675.9)

(1009.9)

Beam, m(ft)

32.2

kl .2

_61.6

(105.6)

(135.2)

(201.9)

Draft, m (ft) 10.7

13.7

20.5

59

Block Coefficient

0.85

0.85

_0.85

L/B

5.0

5.0

5.0

B/T

_3.0

_3.0

_3.0

(35.2)

(k5.1)

(67.3)

Displacement, Long tons

k7,600.

100,000.

_333,300.

Rudder Area, Movable

57.k

_{9k .2}

_210,2

in m (ft)

(617.9)

(1013.)

(2261.)

Rudder Area

the horizon:

"The recent boom in orders for tankers of' about 80,000 DWT has provided much welcome relief to a few shipyards and the market in general. It appears that both owners and

charterers alike are looking on this size of tanker as the optimum size for profitable tanker operations in the

1980's... These new generation tankers are specially designed for shallow draught operations in restricted areas, such as the U. S. Gulf and U. S. East Coast."

The p0,000 DWT tanker is representative of a size common to U. S. coastwise trade; it carries out numerous delivery chores and amounts to 24 percent of' the U. S. flag tankship fleet. The 280,000 DWT tankship on the other hand, represents one of the largest size vessels that trades in the U S Eleven vessels in the 20.0,000 DWT and above range have been constructed at U. S. shipyards, with the largest (390,000 DWT) delivered in 1979. There are presently few harbors or ports that can accept a vessel of this size, even if it has a shallow draft configuration.

The selection of concepts for simulation is based on the evaluation in Tables 8 through 10. The concepts selected are based on potential application to a large (280,000 DWT) tanker, and include concepts simulated for the 81,000 DWT tanker in the initial Maritime Administration study. Certain maneuvers do not provide any information on some devices .as the following matrix shows:

* Applicable to smaller tankships only.

The simulations are for an approach speed of 8 knots for all maneuvers, except the accelerating which starts at slow speed. All maneuvers were in shallow water with a water depth to tanker draft ratio (PIT.) of' 1.2.

Examination of devices in shallow water conditions provides conservative answers compared to the deep water conditions of' Section IV. It also

represents the most common environment fOr ship maneuvering: confined arid congested coastal waters and harbors.

Details relating to hydrodynamic performance and mathematical, modeling of the concepts may be found in the Hydronautics report. In selecting size or

60 Concept. Turniñg Circle Accelerg Turn Crash Stopping 20-20 Stop Maneuver Z-Maneuver

Basic Ship Yes Yes Yes Yes Yes

Twin Screw/Rudder Yes Yes . Yes Yes Yes

Increased Astern - - Yes Yes

Stern Anchor (') - - Yes Yes

Tunnel Thruster Yes Yes - - Yes

Active Rudder Yes Yes - Yes

number of units for the various maneuvering devices, the following items were emphasized: realistic, dimensions, locations, and capabililties for eaOh device, and design and arrangement in accordance with manufacturer's specifications

The costs were obtained from the Maritime Administration, and are reflective of the costs that would be charged by U. S. shipyards for

incorporating the device into new construction. They are current Oosts based on completion of the vessel before the end of 1981. The costs are for

different tankers than those used in the simulation analysis, but they are representative and are referred to as follows:

MarAd Tanker Deadweight Initial Designation Long tons Costs

Twin Rudders/Twin Propellers - The concept was investigated in great detail. Planar motion mechanism tests were carried out to obtain hydrodynamic coefficients for the mathematical simulation model.

A short feasibility study was carried out to define the twin screw

configuration. The study concluded that twin screw machinery could fit within a conventional single screw hull, and that propellers turning outboard over the top and a centerline skeg with open shaft and strut arrangement should be used Propeller diameter was based on expected propulsion coefficients. The overall maneuverability performance of' twin screws and rudders is slightly better than

the conventional sIngle rudder and propeller arrangement. This concept suffers from higher Initial and operating costs. The twin screw configuration requires 25 per cent more power for a nominal 16 knot speed, the horsepower requirements

and costs for new construction are: Horsepower

Single Twin Cost

Optimizing the hull configuration may provide better powering relative to the single screw baseline, but further research and development is required.

Increased Astern Horsepower - Increased astern power reduces stopping time and distance. In typical steathturbine plants, the' astern turbine is capable Of generating about 140 percent of ahead power.

There are basically two methods which can be used to increase astern power. In the first, the turbine efficiency is improved by providing more stages or higher speeds. Besides an Increase in turbine size and costs, there

61 140,000 DWT 11 ,050 114,1130 $3,510,000 814,000 DWT 18,1140 23,500 $1; ,700 ,000 280,000 DWT 140,000 51 ,600 $6 , 1400, 000 T6 37,000 $I9,100,00O T8 91,800 $69,000,000 TlO 265,000 $136,900,000

gases In the astern stages cause losses which detract from the ahead

efficiency. _{In the second method, increased astern power can be obtained by} simply increasing steam flow without changing either the number of' stages.or

the blade speed. Studies indicate that the astern steam flow could be

Increased to 150 percent of the rated full power ahead throttle _{flow without} affecting the size of the turbine. _{Furthermore, various combinations of' the..'}

two methods are possible The second method was used in the simulations t

is estimated that the capital cost of' new construction and retrofit wOuld be.:

One method for evaluating maneuvering devices is to check the operations of one class of' vessels with the device in comparison to those without the device. _{To perform such experiments, conditions must be closely controlled..} Unfortunately, no such experiments have been performed in this field, However, in a sense there is one device which is Installed on a significant number of tank vessels which are motor propelled - that of increased astern horsepower compared to steam ships. Because of' their design, large motor propelled

tankers have approxImately 80 percent of' ahead power available astern oompared to 14Opercent for steam tankers1 In comparing the operating experience of steam versus motor propelled tankers, accident rates were compared. _{Table 12} shows accident rate broken down by vessel size. The same information is shown graphically in Figure 214. This shows that motor propelled tankships and steam propelled tankships have approximately the same accident rate when considered over the whole deadweight range: .0148 steam tankers as compared to .0142 for motor tankers. _{Table 12 indicates that increased astern horsepower may reduce} CRG accidents for tankers under 150,000 DWT, but there are so many other

factors Involved in tanker accidents that such a conclusion cannot be drawn with certainty. The number of accidents for motor propelled tankers over

200,000 DWT Is so small that no statistical inference can be made about the effect of increased astern horsepower on large tankers.

Stern Anchor - Since most CRG casualties occur in restricted and shallow water, the use of an anchor system as a drag where favorable conditions exist has some potential for reducing stopping times and distances for smaller

tankers, but only as a last resort. Results are similar for bow and stern anchors. A stern anchor allows more direôtional control during maneuvers. For the baseline ship, classification society rules require the following anchors

for the 140,000 DWT and 814,000 DWT vessels.

Weight of Anchor Length of Chain in Long Tons in m (ft)

110,000 DWT 5 290 (950)

814,000 DWT 9 335 (1100)

62

Tanker New Retrofit

T6 $1410,000 $5,850,000

T8 $'410,000 _$8,190,000

Table 12 - Accident Rates for Steam Propelled and Motor Propelled Vessels for the Years 1969 through 1977

Size Range 100,000 149,999 150,000-199,999 200,000-249,999 250,000 Up Total & Tanker Accidents 37 20 92 38 187 Steam Propelled Operating Years 590 425 1742 1175 3932 Accident Rate .063 .047 .053 .O32 .048 Tanker Accidents 60 13 3 1 77 Motor Propelled Operating Years 1350. 281 104 8,9 1824 Accident Rate .044 .096 .029 .011 .042

C,) .06 >. _C

WI-C,)

I-.W

20.

Wo

9:

C.) w

o

.( a

I-.02 COMBINED STEA M PROPELLED MOTOR PROPELLED 100 000 150000 200 000 250 000 149999 199;999 249 999 AND UP

Figure 24 - Accident Rate of Steam and Motor Powered Tank Vessels Larger

Than 100,000 DWT for the Years 1969 through 1977

Since the anchor would be used. as an emergency stopping device, the anchOr windlass and handling gear must be designed for rapid operation and high loads Deck machinery that meets such requirements is not within the practical limits of size, complexity, and operation for marine use. In addition, tunnels, cable crossings, pipelines, and other submerged objects may be damaged, or they may damage the anchor system if' it is deployed around them.

Tunnel Thruster - Tunnel thrusters are useful at low ship speeds when the effectiveness of the conventional rudder is reduced. For the purpose of' this study each of the ships was equipped with standard size bow thrusters. They are designed to provide a turning rate of 8 to 9 degrees per minute at low

speeds. The number and size of the unIts fOr each vessel is as follows: No of HP per

Thrusters Thruster

Costs for 1981 delivery dates for new and retro-fitting of thrusters for representative Maritime Administration vessels aPe:

Tanker New Retro-fit

T6 $350,000 $1170,000

T8 $650,000 $89o,000

Tl0

$1,760,000

$2,3110,000

Active Rudder - The active rudder shown4n Figure 17 consists of a submerged electric motor contained in a streamlined casing, set within the normal rudder The active rudder's small propeller is usually encased in a Kort nozzle duct. The unit improves maneuverIng performance at low and zero

speeds, both ahead and astern The tandem arrangement of an active rudder unit directly behind the propeller increases efficiency compared to the propeller alone. The horsepower for the active rudder units are:

Horsepower for Active Rudder

140,000 DWT 400

814,000 DWT 600

280,000 DWT 900

Controllable Pitch Propellers - Most diesel and gas turbine powered vessels are equipped with controllable pitch propellers because their' engines have limited RPM ranges or cannot be run in reverse. The propeller blades can be adjusted to reverse thrust while the engines and propeller continue to rotate in the same direction. This device Is similar in effect to increased

65

140,000 DWT 1 2000

811,000 DWT 1 3000

to the propeller, the efficiency of the blades is lower in the astern mode, so reverse thrust ià approximately 80 percent of forward thrust. The accident analysis for diesel and steam tankers applies equally well to tankers having contràl][able pitch propellers. This device can be used in large tankers and other high power applications. The initial and maintenance costs for the propeller and controls are high. Typical initial costs are:

Tanker Cost

T6 $700,000

T8 $1,170,000

T10 $2,3'IO,OOO

Steerable Kort Nozzle - This device provides higher efficiency of the hull and propeller than the conventional arrangement and gives significantly better maneuvering qualities. The steerable., Kort nozzle used for the 84,000 DWT tanker simulation was scaled for the 110,000 DWT and 280,000 DWT ships. This device requiries a complicated design procedure, larger steering gear, and higher initial cost.. For large tankers, constructIon and operation of this nozzle are serious problerns, and it has not been applied to large tankers. RESULTS OF MANEUVERING DEVICES FOR THE MARITIME ADMINISTARTION STANDARD SERIES

The maneuvering device study for a large tanker (280,000 DWT) is In response to the Presidentlal.Initiative. It is apparent that few of these devices can make significant improvements to large tankers. However, the devices could improve the maneuverability of small vessels. Since the overall objective' of' this effort is to reduce oil outflow from CRG accidents, smaller tankers were also examined to provide an appreciation of device effectiveness for a range of tankers Standard maneuvers are explained and the measure of controllability that it portrays is presented.

Turning Circles - These maneuvers are simulated for an approach speed of' 8 knots in shallow water. Each of the Figures 25, 26, 27 shows the basic

trajectory of the vessel, along with the values non-dimensionalized by dividing by ship length. The trajectory of the baseline vessel and the vessel with various maneuvering devices in a hardover turn are superimposed. I can be seen from the three figures that the most effective device is the Kort nozzle. Due to the speed of advance a bow thruster has little effect, while the twin screw and rudders results in some increased turning ability as seen by

decreases in advance and radius of' turn.

Figure 28 provides the relationship of the maximum advance, defined in Figure 14 as a function of tanker deadweight and illustrates the reduction in

TWIN SCREW, 2 RUDDERS

SINGLE SCREW, NORMAL RUDDER

(BASELINE SHIP)

400

200

u-I

Figure 25- Comparison of Paths for 40,000 DWT Ship

Turning in Shallow Water

SINGLE SCREW, NORMAL RUDDER

WITH ACTIVE RUDDER

(SAME WITH BOW THRUSTERS )

SINGLE SCREW, STEERING KORT NOZZLE

t

800

600

400

-200

\

1'

200k

N

400

600

SINGLE SCREW,NORMAL RUDDER (BASELINE SHIP )

SINGLE SCREW, NORMAL RUDDER PLUS BOW THRUSTER

(ABOUT SAME AS A RUDDER PLUS ACTIVE RUDDER)

TWIN SCREW, 2 RUDDERS

Figure 26 - Comparison of Paths for 84,000 DWT Ship Turning in Shallow Water

SINGLE SCREW,

STEERABLE KORT NOZZLE

1000

800

-600

z.

400-200

-400.'

/

/

/

/

-I

SINGLE, STEERABLE KORT NOZZLE

Figure 27 - Comparison of Paths for 280,000 DWT Ship Turning in Shallow Water

SINGLE,NORMAL RUDDER (BASIC SHIP )

SINGLE, NORMAL RUDDER

WITH BOW THRUSTERS

(SAME AS NORMAL RUDDER

AND ACTIVE RUDDER)

1WIN, 2 RUDDERS

/

/

I I I I

Vf

I

j

200

'400

600

800

1000 1200 1400 _{1600 SIDE REACH IN METERS}

/

750 500 250 LE BASED ON MARAD SIMULATION FULL FORMSERIES OF 0 50 100 150 200 250 300

DEADWEIGHT (IN THOUSAND ) TONS

Figure 28 - Maximum Advance from Turning Maneuver from

advance that can be achieved with steerable Kort nozzles. These results are tabulated below:

Reduction in Length Reduction!

Advance (in) (in) Length

Accelerating Turn - These results are presented in Figures 29 through 31 for the three vessel sizes and combined in Figure 32 as a function of

deadweight. There is no clearly effective device for reducing the advance, considering the magnitude of the reductions compared to vessel length and beam Although these figures illustrate execution of the maneuver at zero speed the results are similar if executed at slow (maneuvering) speeds. The comparitive results of Figure 32 show that the reduction in advance with the bow thruster is an improvement of only about one-third of a ship length over the entire deadweight range:

Reduction in Length Reduction!

Advance (in) (in) Length

Crash Stop - The results of the crash stop, (Figures

33

through

36)

i11utrate that twin screws and rudders are mOre effective than increased astern horsepower as a stopping device. The table below stnmarizes the significant, results of the figures:

Length * * * Reduction in Advance Cm) * * *

(in) Increased by Twin by

Astern HP Length Screw Length

71 '10,000 DWT 160 230 1.11. 250 1.6 814,000 DWT 206 260 1.3 300 1.5 280,000 DWT 308 300 1.0 4O0 1.3 '0,00O DWT 55 160 0.3' 81,000 DWT 60 206 0.29 280,000 DWT 90 308 0.29 i0,000 DWT 75 160 0.'46 8z4,000 DWT 110 206

0.53

280,000 DWT

130

308

0.12

-I

SINGLE SCREW,

STEERABLE KORT NOZZLE

4

-300-

-i,"

--

I

-100

100

_:200

300

-200

200

-500

Figuie 29- Comparison of Paths of 40,000 DWT Ship in an Accelerating Turn

in ShallowWater

:

(BASELINE SHIP )

SINGLE SCREW,NORMAL RUDDER

PLUS BOW THRUSTER

TWIN SCREW, 2 RUDDERS

SINGLE SCREW, NORMAL RUDDER

PLUS ACTIVE RUDDER:

I

-400

/

/

200

-200

SINGLE SHIP, NORMAL RUDDER

( BASELINE SHIP )

SINGLE, STEERABLE KORT NOZZLE

SINGLE SHIP, NORMAL. RUDDER

AND BOW THRUSTER

boo:

/1

N

-600

- -

I

g

600

SIDE REACH IN METERS

SINGLE SHIP,

NORMAL RUDDER

AND ACTIVE RUDDER

Figure 30- Comparisons of Paths for 84,000 DWT Ship in

-400

200--20Q.-,

-400-.

600...IIb

-800-'

-1000

200 400

600

SINGLE, NORMAL RUDDER

(BASELINE SH:I'P )

SINGLE, NORMAL RUDDER

PLUS BOW THRUSTERS

SINGLE, NORMAL RUDDER

PLUS ACTIVE RUDDER

..800

Figure 31 - Comparison of Paths for.280,000 DWT Ship in an Accelerating Turn in Shallow Water

SIDE REACH IN METERS

TWIN SCREW, TWO RUDDERS

250 200 150 100 50 BASELINE SHIP H

WITH BOW THRUSTER

0 50 100 150 200 250 300

DEADWEIGHT (IN THOUSAND) TONS

Figure 32 - Maximum' Advance from Accelerating Turn for

1600

1400

1200

1000

TWIN SCREW, 2 RUDDERS,

I

1.1 MIN, 20 SEC

H H

z

800

_{SINGLE,NORMALRUDDER}

'''ASTERNANCHOR,

if

Tst

=5 MIN, 22 SEC

600

400

200

I S S

SINGLE, NORMAL RUDDER

( BASELINE SHIP )

I

=14MIN;2OSEC

mm

SINGLE, INCREASED ASTERN POWER,

1

=11 MIN,55 SEC

st

SINGLE, STEERABLE KORT NOZZLE,

111MIN,35SEC

st

200

400

SIDEREACHIN METERS

Figure 33- Comparison of Paths for 40,000 DWT Ship during

a Crash Stop in Shallow Water

TWIN SCREW, 2 RUDDERS,

1000

I

= 12 MINI 5 SEC

St LU

I-LI.I 800

SINGLE SCREW, NORMAL RUDDER

PLUS STERN ANCHOR,

ii

I

st

=5MIN,45SEC

600

LU

I

400

200:

SINGLE SCREW, NORMAL RUDDER

.( BASELINE SHIP )

=15MIN, 1OSEC

st

SINGLE SCREW, NORMAL RUDDER,

INCREASED ASTERN POWER,

I

=12MIN, 3.5SEC

St

SINGLE SCREW, STEERABLE KORT NOZZLE,

1200

I

=12MIN,15SEC

st

200

400 SIDE REACH IN METERS

Figure 34- CompariSon of Paths for 84,000 DWT Ship during

a Crash Stop :in Shallow Water

77 1600

SINGLE SCREW1 CONVENTIONAL

RUDDER (BASELINE SHIP ),

Tst =

16 MINI 30 SEC

SINGLE SCREW WITH CONVENTIONAL RUDDER

AND INCREASED ASTERN POWER,

I

=13MIN,355EC

st

INGLL SCR[V. STEERABLE KCIT r.0 'Zl.E

I

13 MIN, 1') 'EC

st

TWIN SCREW AND RUDDE:RS,

T

= 1:; MIN, S SEC

200 400 S i.E REAC H IN METERS

Figure 35 - Comparisor P LhS to 280,000 DV

Shii' t'

iii

a Crah Sto

i

hallovi W.rter

2.0 1.5 1.0 0.5 STERN ANCHOR BASELINE SHIP TWIN SCREW 0 50 100 150 200 250 300

DEADWEIGHT (IN THOUSAND ) TONS

Figure 36- Maximum Advance from Crash Astern from

Speed of 8 Knots ifl Shallow Water

OPERATIONAL TECHNIQUES AND METHODS

GENERAL DISCUSSION

Operational techniques arid methods are generally independent of vessel equipment, design, or construction. They can be applied to any vessel with varying derees .of.success. Some of the methods, such as transiting a channel at reduced 'speed increase Operating costs. Some of the methods require

additional training of the officers, and this is an area in which a real time simulator can be used effectively.

Table 13 lists maneuvering techniques, along with a subjective evaluation 'of utility. The measure of how well a technique wörk is the. same as for the

devices. The second index is based on the difficulty of the maneuver rather than the size or cost. The third index in the table pertains to whether or not the technique has been used on large tankers.

-Slower Approach Speed - The common sense approach to reducing stopping time and dtstance is to be going slow when a situation arises that demands that the vessel be stopped. When reducing the speed of the vessel, however, a

period of reduced controllability occurs due to the loss of flow over the

rudder. Figure 37 displays the effect of vessel size On stopping distance. It shows that the distance required to stop a 200,000 DWT vessel from 6 knots is 1220 in (14000 ft.) and from 15.5 knots it is 14250 m (111,000 feet). Reducing approach speed by 61-percent reduces stopping distance by 71.percent.

This has been recognized.. Figure 38 has been extracted' from a. training publication for ship officers.. The lower curve shows the stopping distance as a function of approach speed for an 18,000 DWT:tanker, and the upper curve is for a 210,000 DWT vessel. Representative values from this figure are:

Stopping Distance, meters

II knots 8 knots

18,000 DWT, 325 ' 7140

210,000 DWT 550 1670

This shows that a .50 per cent reduction in pproach speed reduces the stopping distance by 67 per cent for the larger tanker arid 56 per cent for the smaller tanker. Two important conclusions regarding the relationship of vessel size to stopping distance are seen from these figures:

Slower approach speeds yield greater reduction in stopping distance for larger vessels.

Slower approach speed reduces stopping distances for all size

vessels. .

large Table 13

Performance Indices for Operational Techniques and Methods to Improve Maneuverability and Controllability

81

ability cuity tankers

Slower Approach Speed A A A

Hard-over Rudder A

B.

A

Propeller Kick B

Rudder Cycling C C B

Traditional Tug B B

Tug Used as Rudder or Brake A B Alongside Tug A Improvement in Index Maneuverability Difficulty to Perform Technique Applied on Large Tankers

A Significant Low Yes, Operational

B Moderate Moderate Yes, Experimental

0

0

100,000

200,000

300,000

TANKER DEADWEIGHT, L. TONS

HYDRONAUTICS INC. SIMULATIONS

IN SHALLOW WATER

Figure 37- General Effect of Ship Size on Stopping Head Reach

/

o £550 TANKERS

INDUSTRY TANKERS AU. SHIPS STOPPING FROM SERVICE SPEEDS WHICH

AVERAGE IS KNOTS ItI.5)

0 IDEMITSU MARU COMPUTED-APPROACH--1/ NORWAY SPEED 16 KNOTS

/

_os ASSUMINGSTRAIGHT FMALAYSIA PATH 0 BERNICIA

I-

S SFEEO

-

ABOUT 15.3 KNOTS /OLIBYA P'PIAMPSHIRE

CuBA 9PANAMA °NISO

0 Q /

OBARCEIONA BALTIMORE ARUBA

/

ATHENIC

I

/

ORIENTAL PIONEER MONTEVIDEO 0 SUEZ

- --

----'COMPUTEO

APPROACH SPEED 6 KNOTS

82

8 KNOTS

20'

LU LU

U-o

15

z

I,,

0

=

I-I

U

LU LU

=

0

z

0

6 5 C,) uJ

I-

uJ

0

-I

z

w

(33

z

I-

0

Z2

0

I-

C,) 1 6

a

10 12 14 16

APPR0ACH SPEED IN KNOTS

Figure 38

This is illustrated by the slope of the two curves in Figure

38.

curve for the 210,000 DWT vessel is much steeper than that for the

18,000

DWT tanker. The two curves

(8

knots and 6 knots) at the lower portion of Figure 37 are nearly flat, as compared to the steeper dashed curve representing the

computed stopping distance for an approach speed of 16 knots.

There are some problems with slower approach speeds. Wind and current conditions require vessels to increase speed to maintain desired track. Steam turbine power plants can operate at any speed, but diesel engines cannot

operate below about 70 percent of rated RPM. The net effect is that at slow to moderate speeds, a direct drive single screw motor propelled vessel must

continually stop and start to maintain a low speed. The normal practice is to proceed at higher speed where continous Operation can be maintained and to use braking tugs in tight situations.

Hard-Over Rudder - This technique utilizes the large drift angle of a tanker to reduce the forward speed by executing a maximum rudder angle turn. It reduces the advance s:ignificantly but requires two ship lengths of sea room to steer the vessel, off Its original track. Both results are potential

collision avoidance procedures. This is not discussed further but is

illustrated in Figure

39.

A five fold reduction in head reach or advance can be achieved by execution of a hard-over turn.

Propeller-Kick - This technique is especially effective for large tank vessels because. It does not increase vessel speed. The technique Is used at dead slow or slow speeds to move the vessel quickly. It is executed by first applying hard rudder, then ordering about half speed revolutions. The

increased flow over the rudder generates an extremely large force which results in the rapid turning of the vessel. This technique is considered further.

Rudder Cyclln& - This technique, when first proposed appeared very

effective, but subsequent trials and studies have failed to substantiate this. It is Intended to reduce the stopping distance and time from full speed. There are various types of cycling but the most common requires complicated throttle and helm commands and close attention to the vessel's course, heading, and turning rate. For a 191,000 DWT tank ship approaching at 16 knots the rudder cycling maneuver takes about 10 minutes. Recent trials on the ESSO OSAKA have found the technique provides very little Improvement in stopping time or

distance. No further consideration of this technique is given.

Tugs - Recently there has been interest in expanding the traditional use

of' tugs. The proposal hasbeen to see how tugs could assist the

propulsion/rudder system of large tankers in normal and emergency maneuvering situations at moderate speeds. The Rudder Tug, Braking Tug, and Alongside Tug are arrangements for tug assistance. The use of tugs is summarized below.

Traditional Tug - This technique Is commonly used in ship berthing at vessel speeds below two or three knots. It does not require special thrusting or powering arrangements for the tug. No further discussions of this will be provided.

4000 3000 2000

I-uJ w 1000

z

0

I->

Lu

0

-J 1000

I-E 0 2000 3000

- 4.2

KNOTS

ENGINE ORDERED "CRASH ASTERN' (STOPPING)

/16KN0Ts

--VESSEL TRIAL DRAFT DISPL., L. TONS V. APPROACH RPM ASTERN RPM TIME-LAG TIME TO STOP

HEAD REACH, FEET

LATERAL.DEVIATION,FEET - 6.1 KNOTS 4.3 KNOTS. 4.8 KNOTS CALCULATED 191 MDWT 60-5W' 221,400 16.0 KNOTS :55 92 SECONDS 20.0 MINUTES 60'-5W' 221,400 16DKNOTS .54 44 SECONDS. 17.6 MINUTES 60-5W' 221,400 16.5 KNOTS 56 102 SECONDS 18.0 MINUTES

55'3Y"

201,000 16.6 KNOTS 55 65 SECONDS 21.3.MINUTES 191 MDWT CLASS (CALCULATED) 191 M DWT ESSO NORWAY 191 M DWT ESSO BERNICIA 191 MDWT ESSO MALAYSIA

/

I I - I . I I I

I.

I I I

I

3000 4000 5000 6000 .7000 8000 9000 10,000 11,000 12.000 13000 14,000 15000

HEADREACH,:FEET..-Figure 39 - Comparison of Calculated "Crash Astern" Maneuvers with Full Scale TrialResults

(ESSO 191,000 DWT Tanker, Load Condition)

13,550 14,400 15,050 16.600

1,650 1,650 .2,900 1,750

ESSO MALAYSIA ESSO NORWAY ESSO BERNICIA

4000 I I

attached Should an emergency arise, the tugs would be near the vessel to lend assistance.

Assistance - This function Involves tugs attached to or in contact with a vessel for the purpose. of braking, stopping, or keeping a vessel within a desired swept path. Tug assistance can be accomplis.hed through one or more of the foilowingthree arrangements:

Alongside Tu - The alongside arrangement Is an effective way to provide immediate auxiliary power and maneuverability to a ship transiting a congested or restricted waterway. This arrangement can augment the vessel's steering and braking for vessel speeds below three knots, and it can augment braking up to speeds of 6 to 8 knots. Tests have shown that in winds up to O knots and at speeds up to about 8 knots, control of the ship may be maintained in some cases by tugs, even if the ship's rudder is stuck in a hard-over position. At least one tug must be provided on each side of the tanker, and more tugs may be required depending on individual conditions. This arrangement is also an effective way to reduce stopping distance.

Braking Tug - This arrangement is generally considered to be with the use of a tension line (hawser) attached to the stern of the ship, with the other end through a bridle or special winching arrangement on the tug, which faces in the opposite direction. Ahead thrust by the tug applies a retarding foroe on the tanker. With additional equipment on the tug it may provide turning assistance by pulling at an angle relative to the ship. Additional studies have been conducted in Japan, and the practice is common In some ports.

Rudder Tu - This technique incorporates various concepts to form a large rudder and stern attachment for turning thrust. With the tug secured to the aftermost part of the tanker, application of thrust and rudder by the tug imparts a turning moment to the tanker. Special winches on the tug permit expanded flexibility in control by the tug. Rudder tugs have been tested and are in common use In the Panama Canal. The technique was used in San Francisco harbor a number of years ago. This concept was also tested in two Coast Guard sponsored programs to assess ship maneuverability after equipment failure. This is discussed in more detail later in this report. It has been proposed that a tug equipped with flanking rudders could alsoprovide controlled braking.

EVALUATION OF TUG ARRANGEMENTS

The studies undertaken by the Coast Guard, relate to the Coast Guard's responsibility to investigate the maneuvering capabilities of' tank vessels within enclosed and confined waterways such as Puget Sound, Port Valdez, or the Chesapeake Bay, and to determné the need for regulations governing their

passage. These regulations might involve:

* _{Recommended sale maximum or minimum ship speeds under specified}

environmental conditions (wind, current)..

* _{Critical harbor areas where spec-ial precautions might be} required (tug escort) or constraints based on environmental conditions Or ship displacement and tonnage.

* _{The need for tugs.}

Procedures to be adopted in case of equipment failure.

There are two major arrangements of the tugs/vessel that are discussed. In each case the tug arrangement schemes were evaluated, although by different means, within the framework presented in Section III Since the two test and evaluation programs that are reported here address one or more of the tug arrangement schemes, it is much clearer to discuss each of the arrangements

within the study. .

AN INVESTIGATION INTO SAFETY OF PASSAGE OF LARQE. TANKERS IN THE PUGET SOUND AREA

The study investigated safety of passage under maximum credible adverse environmental conditions (140 knot winds, up to 6 krot currents) as follows

* _{Track keeping runs in critical portions of four passages in the}

Puget Sound area,. wi.thout the assistance of tugs. This provided the baseline vessel performance for subsequent comparison with tug arrangement schemes.

* _{Runs with engine and rudder failures with no tugs, and with two}

or four tugs assisting the vessel by providing astern thrust parallel to the ship's centerline. This examined Alongside Tug arrangements.

These techniques were evaluated using a mathematical simulation model which incorporated the human factor. The simulations were conducted at the

CAORF facility of the Maritime Administration located at Kings Point, New York. These runs were performed on a computer with maneuvers dictated .by a programmed autopilot These runs were followed by a manned simulation of several of the runs to examine the results of the computer simulation Five different tankers ranging from 140,000 DWT to 1400,000 DWT were used. The conclusions of the. initial assessment of unassisted track keeping apply to the most severe tidal current conditions .and a wind of 140 knots:

* _{Vessel size is not a primary variable affecting track keeping}

capability: the 80,000 DWT and 1400,000 DWT vessels held track about equally well. The ratio of rudder area to immersed profile area of the hull appeared to be an. important physoal factor.

*

Very high crab angles (vessel not aimed in the directiónof

vessel speeds. Although the autopilot could cope with these conditions, this situation may be considered unacceptable by a human pilot. However, it Is expected that a human pilot would periodically Increase engine RPM without significantly

increasing ship speed to achieve better control and to avoid large crab angles.

The conclusions for the unassisted vessel which experienced engine failure but retained rudder control (with current and 110 knot winds):

When. engine failure occurred at l knots, and sometimes at 6 knots, the wind consistently overpowered the rudder and could turn the vessel in a direction opposite to that desired.

Following currents created the greatest difficulty for vessels. The current carried the vessel along While it was attempting, often unsuccessfully, to turn. Changes In course were

impractical; :very.large advances occurèd, and speed over the ground remained too high to attempt anchoring.

' _{With a head-on current, the vessels also could not follow the}

desired course. By turning into the current these vessels were generally able to reduce their speed over the ground to speeds at which anchoring might be feasible. Varying the delay time. before heading into the current demonstrated that increased delay in the time at which the vessel turned up into the current resulted In greater transfer (side reach) and also reduced the amount of time available for anchoring. The larger the vessel, the longer the time delay it could tolerate before a turn into the current became of little or no advantage.

The inability of all the vessels to consistently establish speeds over the ground at which anchoring may be attempted, and the difficulty of' maintaining control In a turn, suggest that tug support Is needed to guarantee safety in the event of' engine failure.

The simulation runs with Braking Tugs assistance upon engine and rudder failure concluded that:

The use of tugboats to retard the forward motion of the vessel results In an appreciable reduction in the distance traversed and the transfer In particular.

High magnitudes of transfer occur at ship speeds through the water of' 8 knots or more. Tugboat utilization strategies other than pure retardation, which was the only strategy simulated,

are required if lower transfers are to be achieved at these speeds. The impact of the use of' modern tugs, such as tractor

tugs which can exert apprediable lateral forces at high speeds may be advantageous.

*

At speeds less than 8 knots, reasonable magnitudes of transfer can be achieved with retarding tugs. However, these lower

speeds may conflict with the requirements for Satisfactory track keeping when extremes of current and wind exist. Vessels should be equipped with instrumentation to determine the Speed through the water

EXPLORATORY TANKER/TUG MANEUVERING TESTS

In July

1978, a

series of exploratory tests were conducted in the waters of Port Valdez to explore the effectiveness of a tug in controlling the movement of a loaded tanker subject to the simultaneous lossóf propulsive

power and steering. The techniques that are to be evaluated here are the tug performing as a rudder and the tug augmenting the vessel's braking effort, and they were evaluated in full scale trials The two vessels participating were a

120,000 DWT tanker and a

5,750

horsepower tug. Nine tests were run to evaluate the use of the-tug. There were three major objectives of these full scale

tests:

* _{To ascertain the ability of the tug, pushing and also acting -as}

a partial rudder at the stern, to counter the turning moment of the tanker when it suffered a simultaneous loss of propulsive power and of steering with the rudder in a hard-over position.

* _{To ascertain the ability of the tug, pushing and also acting as}

a partial rudder at the stern, to turn the tanker in a tight turn when it suffered a simultanious loss of propulsive power

- and of steering with the rudder in a hard-over position.

* _{To ascertain the abilIty of the tug (initially travelling}

-unattached as escort) to counter the tanker's turn by pushing in - the forward half length after simultaneous loss of propulsive

power and steering with the rudder in a hard-over position.

For the runs where the tug was secured, the tug was snugged up until its ow fenders were in contact with the tanker's hull. Figure 110 illustrates the results of this arrangement.:

* _{The tug when secured to the stern of the tanker- at its}

-centerline, upon simulation of failure of tanker propulsion and rudder at hard left, was able to limit the transfer to port from the following initial speeds:

Speed Transfer, m (ft)

11

55

(180)

6 11110 (111110)

8" DACRON LINE

Figure 40 - Diagram of Tanker and Tug Connection Arrangements

for Port Valdez Full Scale Evaluation

90

1ANKER

STERN

AFT CENTERLINE CHOCK