An effective method for treating the stability of marine vehicles

Paper 22:

AN EFFECTIVE METHOD FOR TREATING THE STABILITY OF MARINE VEHICLES

Q

This paper puts forward a rational strategy for dealing with the com-plex problem of marine vehicle stability for which a comprehensive solution is difficult, or even impossible,although practical situations call for the full use of the solutions which are available. One application of this strategy is demonstrated, in relation to ships, by the use of practical ship stability criteria into which dynamic effects have been incorporated. The key conclu-sion is that the use of this approach would enhance the existing treatment of vessel stability and thus improve overall marine safety.

1 INTRODUCTION

In today's world the success of any marine vehicle, whether ship, semi-submersible or other form of floating structure, is measured by its ability to do a given task in the most effective way. We can therefore expect to find a radical difference between the principal characteristics of a ship designed to carry bulk cargo from one part of the world to another and those of a floating structure developed to drill for oil at a given location in a hostile offshore environment. Practical experience, however, soon reveals that the ideal selection of parameters for any particular marine vehicle can never be

achieved. A ship, for example, designed with a very light hull mass for speed purposes, is unlikely to be the cheapest to construct. And when a high standard of "safety" is to be one of the principal features of the marine vehicle then the problem of assigning the correct weighting to each of the key parameters becomes extremely

difficult.

We all acknowledge the paramount importance of safety in the operation of marine vehicles

but

are nothing like as unanimous about the means to be used to achieve an acceptable standard. The problem of stability provides an excellent illustration of this dilemma although we may agree that a vessel with good stability is one for which there is tittle danger of capsize in a

Dein University of Technology

Ship itydramechanics Laboratory

Library

2 -.2528 CD Delft

7lands

;.;. - Fax: 31 15 781838 C KUO, D VASSALOS and J G ALEXANDER

9

University of Strathclyde, Glasgow, Scotland

22/2.

realistic environment even when damaged. The stability of marine vehicles, however, is a very complex subject which involves not only the vessel's

ability to remain upright in hazardous seaways but also how it meets its opera-tional requirements without imposing undesirable constraints on other design parameters, see ref. 1. In view of this, there must be a rational procedure available for meeting the design and regulatory requirements.

What possibilities are open to us in this situation? In order to answer this question we shall have to examine the principal approaches that may be adopted to deal with it. The first of these is to design vessels that

are inherently very stable, by increasing, for example, the:beam/length ratio. This is an acceptable approach in certain cases, but results in vessels that

are very uncompetitive in terms of mobility.

The second approach is to accept the statistical facts that it' is

impossible to design a competitive vessel which is completely safe and that a certain '(we hope very small) percentage of the total fleet will inevitably be lost: over a period of time. In the light of this we could admit a degree of capsize risk in the vessel's design. This is regarded as an unsatisfactory solution because "risk" is hard to define, and the approach is a somewhat

negative one..

The third possibility is simply to ensure that a particular vessel meets the existing stability regulations. This is the practical and realistic approach but it Is still far from satisfactory. It

is

a recognised fact, that some ships have capsized while fully meeting the rules of the International Maritime Organisation (IMO) while others are operating "safely" although not fulfilling these requirements, see ref. 2. Furthermore, it takes time to implement new regulations and they usually lag behind practice.

Finally we can concentrate as much research effort as possible on fin-ding the ideal combination of parameters for the highest standard of vessel-stability. This is certainly the long-term answer, but because of the the complex nature of the subject early results cannot be expected. Further, even when theoretical solutions have been achieved it will still take a long time to develop and make available a truly practical procedure.

Safety, however, is of paramount importance and it is essential to develop interim procedures that will enhance the existing treatment especially in such key areas as stability and vessel station keeping.

The strategy proposed in this paper stems from earlier _{ship stability} studies, ref. 3, and is based on the concept of relating the evaluation of a ship's stability to the quantity and quality ot _{the information available on} the vessel and the areas of operation in which it is _{operating. The} back-ground to this strategy is outlined and our application of the procedure is demonstrated. _{The usefulness of such an approach is emphasised and areas of} future development are indicated.

2 OUR STRATEGY

Our ability to obtain satisfactory solutions to a technological problem depends principally on two factors: _{the state of our knowledge at}

a given point in time; _{and the quality, as well as the quantity,}

of the information available. When this basic consideration is _{not recognised or well} under-stood a considerable amount of unnecessary effort can be expended on unpro-ductive approaches.

Some of the advanced theoretical treatments _{of ship stability indicate} that recent research in this _{area provides an outstanding example of this.} References 4 and 5 _{show that a great deal of attention has been devoted to} developing the "state-of-the-art" without any regard to whether the design and operational data are available or can be obtained with a sufficient degree of accuracy. _{In fact there is a continuous evolution operating in both of these} areas since fresh methods of solving a problem lead to further advances and new approaches to data-gathering have a similar result.

Since we do need to have the "best" design _{tool at all times, it is} essential to have a clearly-defined _{strategy for dealing with technological} problems at the design stage. _{Such a strategy must be directed}

at the simultaneous achievement of the following three basic objectives:

To make the most effective use of available knowledge

To ensure that any decision on the use of a particular method,treatment or aspect of knowledge, has taken into _{consideration the quantity and} quality of the information available.

To be able to offer a range of solutions for various stages of _{a design} or evaluation procedure.

22/4

Clearly it is essential to develop a practical and yet logical proce-dure if we are to achieve these basic aims. We shall now outline such a procedure for dealing with the stability of marine vehicles.

3 AN EFFECTIVE PROCEDURE

The proposed procedure is illustrated in fig 1 and essentially the

choice of method for evaluating the stability of a ship or ocean vehicle at any given stage is based on the concept of "levels of stability", which is governed by the amount of data available at the time. Indeed, it is. accepted within the procedure that complete data on the vessel cannot be available, for example, in the early stages of design, or before the operational loading conditions have been identified.

The starting point is Stability-Evaluation-Level A which is based. on a minimum quantity of data. As soon as additional information is provided we can go on to Stability-Evaluation-Level B, and the process can continue to the Nth level, depending on the degree of sophistication required and advances in knowledge, as well as

on

the quality of the information provided.

Several key features of the procedure deserve further examination.

First, the actual "levels" of the procedure are determined by the user, and it is not necessary or desirable to have universally accepted standards for each level. The decisive factor is always the quantity and quality of the information required to implement a given level of stability.

A second point is that although a particular level. of stability may have been selected as the starting or reference point it is Always advisable to apply the criteria of the level one or two below this as a cross-check. Ideally we would expect the higher levels to provide a more realistic and reliable assessment of the vessel's stability, as they take into account some of the more important stability factors. However, lower levels can reveal features which may not be apparent in the higher ones, and this procedure

readily allow the user to make use of any of the lower levels at any time..

Thirdly, we should not expect all the levels to give identical results. It is possible in practice, under certain circumstances, for one level to con-tradict another. With only the principal dimensions of a design, for in-stance, it is possible to give certain general guidance on how to ensure its

stability. As information becomes available on the _{hape characteristics or} the planned operational conditions it may well be seen that different _measures will be required for this purpose.

4 PRACTICAL APPLICATION

With the proposed procedure we can readily utilise a series of stabi-lity criteria which will provide useful guidance in the assessment of the sta-bility of a vessel. Table 1 _{illustrates how the available information can be} related to the various stability criteria suggested, ranging from the ele-mentary to the most advanced level. Level A, for example, offers very approximate guidance to _{a vessel's stability characteristics by using an}

estimated value of the metacentric height and selecting a reasonable value for the freeboard in relation to total depth. _{As additional information becomes} available more accurate guidance can be provided and the _{degree of} sophis-tication possible and the amount of dynamic information that can be included are obvious from a consideration of the proposals at Level D. _{At this stage} use can be made of the information available for all the previous _{levels as} well. _{Information regarding the likely operating conditions will}

include the expected height and direction of regular waves and the wave encounter frequencies.

The procedure is based on using the time-varying roll _{restoring level GZ(4),t)} as the "characteristic property" for assessing the stability of the vessel _in the operating environment and applying tests to identify _{predispositicns of} the vessel to unacceptably large motions. _{The steps involved in the procedure} are shown in fig. 2 and may be explained as follows:

Step 1_- fig. 2a:

A representative regular wave is allowed to propagate along the selec-ted heading and is "frozen" at prescribed times _{ti (i = 0,N). Under free trim} conditions GZ values are calculated for _{a number of heeling angles covering} the complete range of interest. _{One typical GZ curve is illustrated for time}

PrincipaI dimeisions, -:operating draughts -' data as similar VesSela 4. _AVATLABLE, . INFORMATICN

Hull

lines Approximate data 41telY . con-ditions

xt4

onlyind' loading_and gusting Data on the .:Operating.seaWaya N -Full characteristics of vessel an&operi-tinganvironMent 22/6 TABLE-BASTC.,1 PARAMETERS EVALUATED Metadentric Height (GM),_ Freeboard; . Righting arm curvei.7tinimum GM static levers Estimate . haturdiperiod Rightineand ,heeling-moment curves Righting, damping and excitation., -vessel_ -roiling in regular wax* Righting; :damPing7' and excitation. moments of

aAreS-sol folling-in,.

randbm seaways . . Motion response of the Vedi011n seaways -Positive -- SUGGESTED 'STABILITY CRITERIA.

20% of

ttital

depth Meating_Statical stability require-ments Check

for'

readnances ; . .

Area ratio ejedecle

4 t'ertaiti

'value

(weather Work/energy balance . , - - --critatia (parametric reso-nance), Balancing of erWrgy

09a,r. a time span

(probabilistic

esti-Mates)

"Stability 'criteria

based

on vessel

Step 2 - fig. 2b:

By systematically evaluating a number of GZ curves at different intervals and applying surface fitting techniques the function GZ(.,t) _{can be} represented as a surface - see fig. 2b for the following-sea example.

Step

3 -

fig. 2c:

In order to incorporate the effects of rolling motion into stability assessment, trial functions with pre-specified initial conditions are used to represent a typical roll cycle, although only half this cycle is considered in the assessment. The important parameters defining the roll cycle are a fea-sible windward angle .1, a dangerous leeward angle

2

(the downblooding angle, .f, being taken as the _{extreme leeward angle), the period of encounter, T}

E' and the time t at which the roll cycle begins.

Step 4 - fig. 2d:

The critical roll cycle is the one during which the ship has the minimum amount of roll restoring energy and this is found by shifting the half roll cycle along the axis under the GZ surface.

Step 5 - fig. 2e:

The critical roll cycle is then projected on to the GZ(.,t) surface, see fig. 2e.

Step 6 - fig. 2f:

The roll cycle on the GZ surface is further projected _{on to the GZ-.} plane to give a curve which indicates the restoring characteristics _{of the} vessel in waves during the critical roll cycle. _{This curve is known as a}

"butterfly diagram", see fig. 2f.

All other moments or levers can be included by _{expressing them as} functions of via the trial function. Figure 3 is a typical result of the procedure outlined above, with the wind heeling lever _{and the roll damping} lever introduced. _{The work done by the restoring moment during the half roll} is then compared with the work done by the excitation effects and conclusions are drawn as to the stability or otherwise of the situation.

On the basis of this procedure several stability _{criteria can be} pro-posed and one will be used here to demonstrate the potential of the method . To this end .1 is evaluated by the best available _{means, 2 is computed so} that an area balance is achieved and compared to .f. _{The proposed stability} criterion is that _{2 < f ($f being assumed to be 50°).}

. .

, u8E,9g STABILITY:-CH±TERIA

o . Considers:further the proposed stability' criterion We

_look

_{again at}

Level D discnieedin...;the foregoing

section,

_{we now .:demonstrate their}

. _

practical use in following waves on two

trawlers.

_{of :Which particulars are}

given in Table 2. and whose 'approximate lines plans are given _{in fig. 4.}

Length (M) Breadth (m) , .

epth(m)

iáãe

kó -(m),

q.LCG(mY'

.(m) (8):

Damping.Lever '(m) (matirpnii)

Wind ,iteeling Lever

aye Height (m) WaVe Length (m) TABLE ..21.1--7ES T SHTP. 'PARTICULAR'S:- . Shin"

50.85

. . . . 1500:.00 0.280 maiimum)

0:129

4 .57,

''56

160.00 2.58: -09.1 . -5 '.0.070

Figures:, 5, and 6 proyide the basic information _on 'their 'Stability characte-ristics .- --From these figures we note Several: ..points of interest. t

Firstly, ;--we",can'obtain some indication of the stability' of i vessel by qualitative assessment of the GZ(4),t) surface, ,4êe tigd: ::5-a and 6a. For

Ship A,:

the

surface ,eibits-3-considerable irregularity

_whereas'

for ship B it

is sthooth., With no sharp .changes of curvature. _{This atiggests that Ship A is}

Secondly, from the general appearance of the diagrams we see that the excitation moment area of Ship A, fig. 5b, exceeds its restoring moment _area, but the reverse is the case for Ship B, fig. 6b. _{It should be noted that Ship} A is much larger than Ship B, yet it is Ship A which is the casualty, both in mathematical terms and in reality.

If we now apply the proposed criterion we note that _{no balance of} energy is possible for Ship A up to the downflooding angle,

Mf,with

MI =

-16.4. However, for Ship B, with _{(pi = -32.0, this balance is achieved at a}

leeward roll angle of 43a.

If the same argument is followed for the _{case of quartering waves, the} stability assessment is less straightforward and depends to _{a great extent on} the phase relationship between the roll motion and the waves. Figure 7 illus-trates four relative positions for the rolling ship and the wave and we note that in one case (fig. 7d) the stability criterion outlined in the foregoing is not met. _{However, from practical experience of Ship B we know that it is a} stable vessel and behaves well in the seaways. _{This means that a modification} of the stability criterion is needed. _{A possible solution is to accept what} is demonstrated numerically here that every ship will fail in some positions, and aim to keep the percentage of failure during _{the encounter period at an} acceptable level, i.e., to use a form of the _{risk analysis approach.} Alter-natively we can overcome the phasing problem by using random waves. Further discussion of this treatment will appear in _{our future publications.}

6 DISCUSSION

Having outlined an effective strategy for treating the complex subject of marine vehicle stability and demonstrated its practical application it will be useful for us to discuss briefly the following points of interest:

a) Basic Objectives

The main difficulty associted with _{a rational treatment of marine} vehicle stability is that too many parameters _{are closely linked together.} There is no doubt about the need to advance in knowledge of the subject but it is important that long-term research objectives should be planned to evolve in such a way as to maximise the interim benefits at the earliest possible stage. This is why we must identify our objectives clearly at the beginning and de-rive logical approaches to achieving our goals. The strategy we have outlined

is:directedat fthe,.achievement of just each goals and also overcomes the

prob-lem enCountered in. prectide'-Where one ..woUld., like an ailrembracing solution that ia_itonceaccurate; reliable, and simple to

apply!

-Vole

Oftducation

,

designing Safety systems which eliminate the need for human intervention but

, .

-until such systems are available education has -a '.,key role to playin this

--.Possible ways

of achieving greater understanding of stability through

. edUCation7Can.,ringe from practical training

on

marine vehicle behaviour

in

the

Seaways with the aid'of'Computer software,

_{_}

practical.,C6urSes:On Stability,

_

and on the application of microcomputers and advanced instrumentation on board vessels..

22/10

--Potential for our Strategy-:

:The strategideScribed-in Section 3

is

directed .,at the evaluation of marine:, vehicle stability and its practical application,but, the 'sae approach

,

is equally aPplicablefOr..dealing with anrcomplei,subjedt on which only a

_{_}

.

-

, _ .

limited amount

of

knowledge and information currently:. exist., Typical areas where suCh-A treatment would be helpful

include

dynamic positioning of .vessels near :flied structures .e0 as to avoid collision, gt.11e operation of underwater vehicles, the lifting of heavy cargoes offshore,

fatigusand

fracture Of_ off-shore structures.

J..

-. A s with safety incgeneral, marine vehiCls-stability-iS7WmUch affected

_by-human,action and:respOnses that,7.even with the

Most Up..,to-Aate.

aids, it is not 4misibls.to cover ail .eventualities completely.: :A.

case c0-16 argued

for

d) The Need for Expertise

Safety influences many aspects of marine vehicle performance and it is

e

recognised practice in Many areas to ensure that only minimum regulatory

,requirements are Jmet. In the case of stability, however,-.-it is important to have a number of realistic assessments. developed.

through

cOoperation. between

_

experts on the subject,- ' designers and practitioners, because the regulations themselves cover only minimum requirements but because they have

_

also proved themselves inadequate in practice Mareover;the consequences of a poor-assessment- could be' catastrophic, esPecially in

7 FUTURE DEVELOPMENTS

The philosophy and the results outlined in this paper provide a logical way of dealing with the complex subject of ship and ocean vehicle stability. However, we are still in the lower levels of stability evaluation because _it is essential to incorporate _{as much dynamic information as practicable}

into the assessment. _{Our future research efforts, will}

therefore be principally directed towards the following aspects:

a). _{A Practical Tool for Stability Assessment}

Our research effort will be devoted to _{developing practical stability} criteria for ships, semisubmersibles and other _{ocean vehicles. We are} planning to use data on marine vehicles known to have good stability characteristics and data on those that lack them, and to do assessments of the vessels at various _{stability levels. The developed criteria} will then be formulated _{so that their practical application is made}

as

simple as possible.

A Realistic Representation of Seaways

So far we have incorporated dynamic _{information into the procedure by} calculating the restoring moment which acts on the vessel by virtue of its instantaneous position and _{attitude in incident regular}

waves of arbitrary length, height and direction. The main directions of

interest here are following and _{quartering seas.}

We are now in the process of enhancing the regular wave approach _{by introducing random} waves based on the use of the wave spectrum. _{It is our aim to adapt}

this concept to dealing with _{stability at higher levels.}

Improved Procedures for Stability Assessment

A number of possible _{procedures for assessing stability have}

been produced. _{So far they have not fulfilled their initial promise either} because of the inapplicability of the basic theory involved, _{or because} of the restrictive nature of the _{final form of the criteria but}

once a higher quality and greater quantity of information is available it will be possible to develop _{some of our theoretically-based}

methods into effective stability assessment procedures that take full account _{of the} dynamic aspects, see for example _{refs 6 and 7.}

22/12

Practical Application

It is essential to Apply selected levels of stability to as many ships as possible if we are to achieve practical benefit from this _research. This will enable _{us to assess in a broad context what role the} pro-cedure can play In design and regulatory _{assessment. As practical} experience is gained we hope to be able to improve the overall _safety

of ships- and ()dean Vehicles.

8 CONCLUSIONS

The following conclusions can be drawn on the .basis of this brief cOnsi-' deration of our research studies into marine vehicle _stability.t.

As 'with other engineering problems it. Is very important to relate the reliability, of the _{stability guidance expected to the quantity and} quality of the information available as well as to the current state of knowledge.

A logical procedure has been outlined which makes _{use of the concept of} "levels 'of stability" and will readily _{incorporate both theoretical} advances

and

better data AS these are achieved..

The examples given in this paper indicate that it _{is now possible to} have practical ship stability criteria which incorporate dynamic effects and offer potential. to differentiate between Ships with good and poor stability characteristics.

TO improve the capsize-safety of marine vehicles it would: be logical to

make extensive, practical application of the proposed procedure and evaluate the feedback.

ACKNOWLEDGEMENMTS-Part -of this work was carried out under contract to'the United Kingdom Department .ofTrade, whose support is gratefully acknowledged. We also wish to thank Miss C Hutcheon for her help with the preparation of this paper.

REFERENCES

Kuo, C, Welaya, Y _{"A Review of Intact Ship Stability Research and} Criteria", _{Ocean Engineering, Pergamon Press, Vol 8 No 1, Jan. 1981.} Bird, H, Odabasi, Y "State of Art: Past, Present and Future", First International Conference on the Stability of Ships and Ocean Vehicles, University of Strathclyde, Glasgow, March 1975.

"Ship Stability Research Workshops No 9" _{September 1979, Ross Priory,} University of Strathclyde.

Proceedings of the Second International Conference _{on the Stability of} Ships and Ocean Vehicles, Society of Naval Architects _{of Japan, Tokyo,} October 1982.

Kuo, C (editor), Proceedings of the First International Confeence on the Strability of Ships and Ocean Vehicles", _{University of Strathclyde,} Glasgow, March, 1975.

Martin, J, Kuo, C, Welaya, Y "Ship Stability Criteria Based on Time Varying Roll Restoring Moments", Second International _{Conference on the} Stability of Ships and Ocean Vehicles, Tokyo, October 1982.

Welaya, Y "Application of Time Dependent Restoring for Stability Assessment of Ships and Ocean Vehicles", PhD Thesis, University of Strathclyde, 1980.

STABILITY

EVALUATION.

'LEVEL N

YES

22/14

e. PROJECTION OF THE CRITICAL ROLL CYCLE ON THE GZ SURFACE

FIG.2

_{BASIC STEPS IN DETERMINING TIME VARYING RESTORING MOMENT}

f. _{PROJECTION OF THE SHIP RESTORING} CHARACTERISTIC ON THE GZ-0 PLANE

C.

TYPICAL ROLL CYCLE ON 0 - t PLANE

0.2 -11011111111/.11

Trr

1&,

II juIIIIIIII

11:-1 I -14444 I 1 1 0 2 I I ----1,--, 4-, -1 --1-1,7-t-- -77-4-,---

---i--30 -25 -20 -45;1:1110:

illi,

10 15 20 25 :30 . 35 .. 40 y,,-,451 Ba 11.-1.111111 RESTORING 1.10 1,1.1 I

-WIND HEELING

II

1'1I;" -0.; IT *--- WINDHEELING-DAMPING 1:0'1 . .: i .11!11 .. , 1 ic- uf I' . 1

r

70.e

F I G .

TYP ICAL R:EPRESENTATII0N OF EXCITATION AND RESTOR LNG MOMENTS

FIG1L

_{LINES SKETCHES OF TRAWLERS}

_

SHIP A

22/18 : r ' .7.'', 7+1

.1 t

1 I ii;rr-.... .. s1 _{,:i '1.11 I CI} ,11.111 ir,e'l

in

1 I II

r

.4 11 .4 I g I I I I. I I I g - _{-;1,,.." ...:: P} - III14.4 I_41.011:11 . . 30 20 ' TE.0 , 0 441114? I :le) 1, ', lo -10 20 as - so'..:. 36 40 415 ,- 50 ". ' .

'-'

.

;it's,

_{-...- .-- RESTORING}

FIG. 5

STABILITY CHARACTERISTICS OF SHI:0=-.

,WIND HEELING

WINOHEEINGOANPING':

b.

LEVER (m)

"r"PT _Is,r-tar_{goo .14,} .r-rrT 11111

141;

11,1/11-1 hit ; rep. ,1-pp Fp( 5 i0' 15 20 25 30 35 .40 45 30 10 !!! t :1111"1: :: ! ,, 111 el _{ilft:Ittliti:11111;;11} ,

'It

, Ii.! IIi _RESTORING WINO HEELING

MINI:NEEL INGOAMPI NEI

43

LEVER(0)-. 0.52'-LEVER:(m). Fr"17;77,771111:'"-riiiit'M .44,1-44" 111111 11 1.1111 ,111 .1:1N: 10.. 15 20 23 50 35- .40 45 _50 RESTORING .WINO- HEELING WINDHEELING-Dii4PIN9 22/20 ..TirTrr7Trrrm.rr " Iii71/7311"'7"1 ;I30 rfct s ' I ' ... ..-. 5 10, /5 20 25 55 55 40 45 50 REMRING . --- ' WIND HEELING ----' WINOMEELING-DAN id 29 30 95 40 45 50

-

RESTORTNEI WIND HEELING WINCIHEELING-04