Jan 10, 1985 - Centre (SMTC), where the development of manoeuvring standards has been ...... If the navigators' requirements coincide with naval architects'.
O 1990 The Royal Institution of Naval Architects.
A NEW APPROACH TO DEVELOPING SHIP MANOEUVRING STANDARDS by D.Vassalos*, BSc., Ph.D, (Member)and K. Spyrou*, Dipl.Ing. Rezd in London at a meeting of the Royal Institution of N m l Architects on April 24, 1990, Pmfessor K.J. R m o n , M.Sc.,
R.C.N.C.,F.Eng (Vice-Prdent) in
the Chair.
Ship manoeuvrability has been at the forefront of research interest for many years but reliable quantified interpretations of ship manoeuvring capabilities are far from established. Addressing the problem, this paper presents the findings of recent research at Strathclyde University's Marine Technology Centre (SMTC), where the development of manoeuvring standards has been tackled by adopting a dynamic systems approach. By means of this approach distinct manoeuvring characteristics have been identified that can be used as measures of manoeuvrability. These are combined in a hierarchical procedure whereby it is possible to quantify manoeuvrability and hence propose objective manoeuvring standards. The credibility of the overall approach is demonstrated by considering its practical application. SUMMARY:
One of the most important revelations of the first international conference on ship manoeuvrability, which took place in May 1987, was the paradox that in manoeuvrability we are "trying to solve a problem we cannot define" ( 1 ) . Nonetheless, it was generally agreed that unti1,that problem is studied and a valid and satisfactory solution is found it would be counter-productive to enforce rating standards for ship manoeuvrability. Notwithstanding this, ship manoeuvring has recelved attention in many parts of the world over many years, a fact manifested by the extensive published literature on the subject. It is only recently, however, that the necessity for developing reliable manoeuvring standards has been recognised as a top priority, following a number of accidents directly attributable to poor manoeuvrability.
A number of reasons have been suggested for the absence of quantified interpretations of manoeuvring capabilities. These include the inadequacy, until rec~ntly,of the mathematical models and the lack of economic incentives for owners to improve the manoeuvrability of their ships (2). It appears, however, that the main obstacles to progress in this direction are gaps in understanding, even at the conceptual level, coupled with reliance on empiricallyderived conventional procedures. The efforts of Lloyd's Register ( L R ) towards effective assessment of ship manoeuvrability should be noted (3), and their endeavours to establish standards of manoeuvrability are to be commended. It was through discussions with LR that research was recently initiated at SMTC with the aim of improving the existing situation by addressing the problem from fundamental principles. The approach adopted is based on the idea that an effective procedure for developing standards would need to provide an integration of the various manoeuvring requirements which are intuitively perceived as distinct. For example, a vessel intended to maintain her course successfully
* Marine Technology Centre, University of Strathclyde
should also be able to respond quickly to rudder commands and turn within the minimum of space. The main emphasis of the adopted approach is on inherent manoeuvring characteristics deriving solely from the hull-propeller-rudder combination. An essential feature of this approach is a non-linear mathematical model which is assumed to give a reliable description of the behaviour of a manoeuvring vessel. The growing confidence in predictions 'based on computer simulations, coupled with advances in other fields of non-linear dynamics, offers the possibility of gaining much insight into ship manoeuvrability. For example, behaviour such as the hysteresis and jumps observed during direct spiral manoeuvring tests of directionally unstable vessels can readily be given meaningful interpretations in the wider context of a non-linear dynamical system. Based on research carried out at SMTC since April 1988, the main objective o f the present paper is to describe .and explain in detail the steps of the approach adopted, primarily in order to develop objective manoeuvring standards. The major outcome of the work so far has been the identification of manoeuvring criteria, measures, and associated procedures assimilated into a hierarchical structure whereby by integrating a number of different aspects of manoeuvring for specific vessel types a global picture of manoeuvring behaviour can be obtained. Simplified practical applications have demonstrated the credibility of the adopted approach and its potential for providing a means of objective evaluation of the manoeuvring -qualities of different vessel types. The wider application of the new approach, however. still awaits further research. 2 CRITICAL REVIEW 2.1 Activities of Established Authorities
a)
E: I M O 1 s interest in ship manoeuvring assessment dates back to 1968. However, it is only recently that the initiative was taken of recommending a number of performance-related characteristics as representative of manoeuvrability,
to be proved, however, is the objectivity of the suggested values in reflecting adequate manoeuvrability.
see Table 1, ( 4 ) While this was a considerable improvement on the existing situation, the crucial step of assigning specific numerical values to the criteria, and thereby quantifying manoeuvrability, was not attempted. There is, therefore, still no measure against which to compare the manoeuvring performance of ships, whether in operation or at the design stage. The approach of IMO to the subject merits some additional comment. Although the criteria suggested constitute a gathering of everything related to manoeuvring, they are of an empirical nature, based rather on common trials-testing practices than on the fundamental qualities of a vessel's behaviour. The outcome of their application can be misleading in some ways. For example, "initial turning ability" and "turning circle" appear in two different criteria, while a on the other hand, "slow steaming ability" characteristic mainly related to the ship's engines - is classified along with primarily hydrodynamic criteria.
-
b) US Coast Guard: The US Coast Guard has been actively involved with the problem of assessing manoeuvring capability. Following its "Report to the President" (5). a large-scale study was undertaken, aimed at producing manoeuvring standards (61, involving the collection and analysis of data from 603 ship trials. Three manoeuvring characteristics were examined: The stops, turns and zig-zag performance. approach selected comprised a three-level process whereby available data were used to establish "Criteria", "Measures" and "Levels" of performance. In practical terms, the work involved the statistical compilation of a large amount of data, the identification of the "average" performance for each manoeuvring characteristic and - on the basis of distance from that average - the designation of the manoeuvring capabilities of the vessel as "Superior", "Above Average", "Average", "Below Average", or "Marginal". Although praiseworthy, this system did not succeed in establishing itself as a widelyaccepted standard. The following are possible reasons for this:
-
-
d) Others: For the sake of completeness, reference must be made to the pioneering, naval ships based, Handling Quality Criteria from DTMB, developed as early as 1960 ( 8 ) ; also to the very recent standards proposed by the Chinese Society of Naval Architecture and Marine Engineering, concerning tankers, passenger and cargo ships of less than 100,000t dwt (9). e) Current Practice: Existing approaches for developing manoeuvring standards appear to be based on information from trials. Table 3 gives the manoeuvring trials recommended by various organisation (10).
2.2 Linear Theory An alternative approach to measuring manoeuvrability would be the theoretical one based on linear or non-linear models of manoeuvring motion. In the case of linear theory, Ref. (11) represents a thorough investigation on three manoeuvring qualities, including the specification of numerical values for judging manoeauvrability. In particular:
on
directional
stability
c) Federal Republic of Germany: Another attempt to standardise manoeuvrability that is worth mentioning is the Federal Republic of Germany's contribution to IMO, submitted in 1982 (7). In this case, numerical values were determined, on the basis of which "satisfactory" or "inadequate" performance could be judged. Table 2 summarises the recommendations of this paper. What remains
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Directional stability as judged on the basis of the sign of the stability index which is attributed to Abkowitz but originated from the work of Davidson-Schiff(l2) or, even earlier, of Contensou (13) .
This work, although comprehensive, suffers from the limited scope of any linear-theory-based treatment of the basically non-linear problem of manoeuvring. The following points should also be noted:
was
A study based on the evaluation of manoeuvring characteristics which, in most cases, are not influenced directly by the designer would not necessarily reveal objectively average performances. It could, therefore, hardly be associated with manoeuvrability levels.
Turning characteristics measured by Norrbin's index p ,for which it was defined that it should be greater than 0.3 for any ship (earlier tentatively suggested by Norrbin) with the exception of tankers where it could be reduced to 0.2 (as Norrbin-Nomoto's work showed).
- "Phase margin", accounting for the amount of manual control needed, should be greater than -30° in the case of a directionally unstable vessel.
Since the system is based exclusively on ship-trials' material the conventional methods of measuring performance are used and a number of important features are inevitably neglected. No standard recommended.
-
L
-
There is no reference to important qualities such as stopping.
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The p-turning index is adopted for all vessels although its meaning and applicability for directionally-unstable vessels is questionable.
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The closed loop of ship control system/operator is modelled and a standard is provided there. This, however, could be considered at a higher level following the standardisation of inherent ship manoeuvring capabilities.
-3 Non-Linear Theory
Non-linear manoeuvring theory should be the most appropriate one on which to base a rational assessment of manoeuvring characteristics. So far, however, it has not been associated with any standard development procedure. This can be attributed to two factors: - The complex nature of the ship manoeuvring problem - which would call for a much more rigorous approach if non-linearities were to be considered.
-
The fact that until recently the non-linear manoeuvring mathematical models were of a purely I1regression" type. Although a particular mathematical model could be very successful for the specific ship examined it would not allow generalisation to other similar ships. Furthermore,.the individual characteristics of hull, propeller and rudder could not easily be taken into consideration by this means.
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The advent of reliable non-linear modular manoeuvring models, combined with the existence of high-speed computers, offered new possibilities for understanding, predicting and measuring manoeuvrability objectively.
Investigate possible alternatives for the global assessment of steady and transient behaviour of the manoeuvring ship, as a non-linear dynamical system. Link the practical needs with the theoretical treatment in order to produce quantitative information relating ship parameters and essential manoeuvring characteristics.
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Select a non-linear mathematical model capable of predicting manoeuvring motion as close to reality as existing knowledge allows, and capable of discrete treatment of the hull, propeller and rudder, and their interactions. Subsequently, develop the computational tools which will allow a speedy derivation of the quantities required in the standards' development process.
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Set up a procedure which can effectively combine conflicting requirements, and is able to produce optimum solutions and to reveal overall manoeuvring performance tendencies.
A major difficulty in developing standards, however. lies in the need to define "how much" manoeuvrability is sufficient for a particular vessel type. A sensible approach would be to guarantee safety, satisfy economic requirements and link these to the manoeuvring characteristics and eventually to ship parameters. However, approaches of this type, although very promising, do not appear to have produced information in a form which would stimulate the setting up of a procedure for developing standards, (14). This may be caused by the subjectivity of the safety requirements accruing from the interests of the parties involved. A new approach is introduced in this paper, a key advantage of which is that, while information of the kind discussed above can be effectively used, it is not actually necessary. Attention is directed mainly towards studying motion patterns and identifying ranges of ship parameters which would ensure good manoeuvring characteristics. The furthering of knowledge on these points could make the setting of reliable standards possible as described in the following sections.
3
AWPTING A NEW APPROACH
The approach adopted at Strathclyde University's Marine Technology Centre (SMTC) is based on the realisation that to devise an objective standards' development procedure, investigations have to be pursued in two different directions. The first need is to concentrate on understanding manoeuvring behaviour before attempting to answer the question, "What constitutes good manoeuvring capability?". The main tasks in this area are to identify which performancerelated qualities are capable of adequately defining manoeuvrability (manoeuvring criteria) and to develop reliable measures of performance, i.e., quantities and related procedures which can effectively assess manoeuvrability in terms of the above criteria and which can provide quantitative information regarding any particular vessel. Having achieved the above, the second need is to find efficient techniques for combining individual characteristics in order to provide an overall manoeuvrability index for any ship, which will distinguish it from others of similar operational requirements.
various operating conditions, and the sense in which they are affected by the mission of a particular vessel type.
The following sections describe in detail the four phases of work outlined above.
4 CLASSIFICATION OF VESSEL TYPES
To define manoeuvring requirements in a form which allows for systematic research is by no means an easy task. As a first step the following generalised concept of ship manoeuvrability was adopted, according to which the manoeuvring assessment must be based upon turning performance, directional stability and stopping ability. The ideal vessel should therefore be one that turns and stops predictably, safely and quickly within the minimum of space whilst maintaining her course with minimum use of the steering mechanism. The above qualities must be observed for the widest possible operability range of the particular vessel type. While it is understood that performance characteristics depend on aspects such as the closed loop man-machine steering control function, bridge visibility, reliability of bridge equipment, steering gear and main engine characteristics, it is believed that at this stage standardisation based on the inherently-related features of hull, propeller and rudder should be the first priority. Various vessel types have been studied and, taking into consideration the above points, the essential manoeuvring characteristics for each type were identified. From this consideration, and also on the basis of the numbers available from each type and the significance of the implications from possible involvement in an accident, existing ships were classified into five manoeuvring groups:
I
Based on these considerations the following approach was adopted:
- Critically examine the progress in the scientific and legislative fields of ship manoeuvring assessment and identify the perceived concept of a "manoeuvrable" vessel; acquire understanding of the requirements for this, how they relate to
Ships with full hull forms, e.g., tankers and bulk carriers I1 Container vessels 111 Ferries and Ro-Ro's IV General cargo ships V Others, e.g., tugs, fishing and offshore supply vessels.
5
NON-LINEAR TREATMENT OF MANOEUVRING BEHAVIOUR
The s h i p manoeuvring system dynamics c a n be r e p r e s e n t e d i n t h e s t a t e s p a c e by a s e t o f f i r s t - o r d e r o r d i n a r y d i f f e r e n t i a l equations
where X , a
X,A
x€XCRk
represent the s t a t e and c o n t r o l variable vectors respectively t h e s t a t e and c o n t r o l v a r l a b l e s p a c e s o f dimens i o n k , l r e s p e c t i v e l y , and t , t i m e . model
adopted
To o b t a i n t h e s t e a d y - s t a t e s o l u t i o n m a t h e m a t i c a l l y , system ( 3 ) is u s e d , w i t h t h e l e f t - h a n d s i d e s e t t o zero: i.e..
is d i s -
I n calm w a t e r c o n d i t i o n s manoeuvring motion can b e d e s c r i b e d by t h e f o l l o w i n g two s e t s o f v a r i a b l e s : a ) State Variables - s u r g e and sway v e l o c i t i e s , ~ v - yaw and r o l l a n g u l a r v e l o c i t i e s , T , p - heeling angle - t h e c o o r d i n a t e s zo, Yo and t h e h e a d i n g a n g l e $0 of t h e v e s s e l ' s o r i g i n , according t o a n e a r t h f i x e d system.
,
b ) Control Variables - The r u d d e r ( s ) a n-e l e . 6 - the propeller(s) r a t e of rotation , n i and p i t c h , (when c o n t r o l l a b l e ) , Pi A d d i t i o n a l c o n t r o l s can be i n t r o d u c e d by c o n s i d e r i n g p o s s i b l e use o f t h r u s t e r s o r by m o d e l l i n g any t u g l i n e s , w h i l e t r i m and d r a u g h t can a l s o be taken a s controls. 2
( i ) S t e a d y t u r n i n g performance ( i i ) Directional s t a b i l i t y . The r e v e r s e s p i r a l c u r v e can be c o n s i d e r e d as r e p r e s e n t a t i v e f o r b o t h q u a l i t i e s and t h e r e f o r e knowledge o f its p r o p e r t i e s i s fundamental f o r any a s s e s s m e n t procedure.
, ff€AGR1
The m a t h e m a t i c a l manoeuvring c u s s e d i n Appendix I .
f o l l o w e d by some form o f long-term r e c u r r e n t behaviour. I n a s s e s s i n g s t i l l - w a t e r m a n o e u v r a b i l i t y , two a s p e c t s r e l a t e d t o s t e a d y notion can be i d e n t i f i e d :
-
I t i s e v i d e n t t h a t t h e manoeuvring s h i p can be d e s c r i b e d by e i g h t s t a t e v a r i a b l e s and up t o t e n o r eleven controls. I n most c a s e s , however, two c o n t r o l s a r e enough, t h e r u d d e r d e f l e c t i o n a n g l e 6 and t h e p r o p e l l e r r a t e n Alternatively the p r o p e l l e r r a t e can be added t o t h e s t a t e v a r i a b l e s w h i l e t a k i n g a s c o n t r o l t h e f u e l consumption r a t e q o .
.
If t h e r e is no e x p l i c i t t i m e dependence ( u s u a l l y a p p e a r i n g i n waves o r i n a t i m e - v a r y i n g w i n d / c u r r e n t e n v i r o n m e n t ) t h e system ( 1 ) c a n have a n autonomous form :
I f , f u r t h e r , t h e v e s s e l ' s position according t o an earth-fixed system i s n o t i m p o r t a n t , manoeuvring motion c a n be d e s c r i b e d by t h e e q u a t i o n
where t h e s t a t e v a r i a b l e s v e c t o r h now i n c l u d e s o n l y v e l o c i t i e s ( l i n e a r o r a n g u l a r ) and t h e h e e l i n g a n g l e . The m a t h e m a t i c a l model a d o p t e d can be b r o u g h t i n t o t h e form o f e q u a t i o n ( 2 ) o r ( 3 ) , depending on t h e s p e c i f i c c a s e examined ( s e e Appendix I ) . The a s s e s s ment o f s h i p manoeuvring performance can be c a r r i e d behaviour and out in terms of steady-state t r a n s i e n t - s t a t e b e h a v i o u r , a s d e s c r i b e d below.
S i n c e ( 4 ) r e p r e s e n t s a system o f n o n - l i n e a r equat i o n s , t h e s o l u t i o n need n o t be unique. I t is t h e r e f o r e p o s s i b l e t h a t , f o r some v a l u e s o f t h e c o n t r o l v a r i a b l e s , more t h a n one s t e a d y - s t a t e s o l u t i o n c o u l d exist. I n s u c h a c a s e , t h e motion p a t t e r n t o be f o l l o w e d w i l l depend on t h e i n i t i a l c o n d i t i o n s o f t h e system and t h e s t a b i l i t y p r o p e r t i e s o f t h e s o l u t i o n , s e e F i g . 1. ("continuation") is presentl y employed which can e f f i c i e n t l y t r a c e t h e dependence o f s t e a d y - s t a t e s o l u t i o n s on any p a r a m e t e r and s o q u i c k l y produce r e v e r s e s p i r a l c u r v e s . F u r t h e r m o r e , s t a b i l i t y a n a l y s i s i s performed on each point-solution, t o i d e n t l f y t h e s t e a d y - s t a t e s which a r e s t a b l e ( a t t r a c t o r s ) and t h o s e which a r e n o t (repellors). A mathematical technique
The above c o n s i d e r a t i o n s e s s e n t i a l l y b e l o n g t o t h e f i e l d o f b i f u r c a t i o n a n a l y s i s , c o n c e p t s o f which s u c h a s t h e s t r u c t u r a l s t a b i l i t y o f a s y s t e m - can c o n s i d e r a b l y improve o u r u n d e r s t a n d i n g o f manoeuvring b e h a v i o u r , (15).
5.2 Transient-state Behaviour T r a n s i e n t motion can be t h e r e s u l t of a c t i o n t a k e n on o n e , two o r more c o n t r o l s . I n the- f i r s t c a s e , t h e r u d d e r o r p r o p e l l e r s e t t i n g i s changed under s t e a d y o r unsteady i n i t i a l c o n d i t i o n s . I n t h e second, a p r o p e l l e r command is e x e c u t e d t h a t c a u s e s t h e s h i p t o a c c e l e r a t e o r d e c e l e r a t e , and a t t h e same moment t h e Table r u d d e r is d e f l e c t e d from i t s c u r r e n t p o s i t i o n . 4 presents the usual manoeuvring procedures a s s o c i a t e d with t h e above c a s e s . An investigation of steady-state and transient b e h a v i o u r s can p r o v i d e many m e a n i n g f u l manoeuvring c r i t e r i a r e l a t e d t o each s p e c i f i c s i t u a t i o n . A s e l e c t i o n o f c r i t e r i a , made on t h e b a s i s o f minimum d e v i a t i o n from e x i s t i n g p r a c t i c e s , can b e summarised a s follows:
-
Directional s t a b i l i t y
- Steady t u r n i n g c a p a b i l i t y
-
I n i t i a l t u r n i n g and yaw-checking A c c e l e r a t i n g and c o a s t i n g t u r n i n g Crash-astern s t o p p i n g c a p a b i l i t y .
The n e x t g o a l is t o i d e n t i f y e f f i c i e n t ways o f measuring these.
5.1 E y - s t a t e Behaviour 6
With t y p i c a l d i s s i p a t i v e dynamical s y s t e m s s u c h a s a that after a manoeuvring s h i p , i t i s well-known change i n t h e v a l u e o f some o f t h e i r control v a r i a b l e s t h e y e x h i b i t an i n i t i a l t r a n s i e n t p h a s e ,
IDEIYTIFICATION OF MEASURE OF PERFORMANCE
6.1 Control Settings S i n c e t h e c o n t r o l s v e c t o r i s f r e e t o vary i n s i d e i t s
s p a c e , a n i n f i n i t e number o f o p e r a t i n g c o n d i t i o n s is made p o s s i b l e . I t would, however, be s u g g e s t e d t h a t t h e s y s t e m ' s performance can, i n g e n e r a l , be s a t i s f a c t o r i 1 . y o b s e r v e d t h r o u g h a l i m i t e d number o f s e t t i n g s , which - b e c a u s e o f t h e i r dominance o v e r o t h e r s , t h e i r p o s i t i o n i n t h e c o n t r o l s p a c e , and t h e c h a r a c t e r o f t h e s y s t e m - c a n d e f i n e and d i s t i n g u i s h p e r f o r m a n c e i n a g l o b a l manner. Such s e t t i n g s inc l u d e p r o p e l l e r r a t e s and r u d d e r a n g l e s . The a c t u a l v a l u e s s d l e c t e d f o r them a r e p r e s e n t e d i n T a b l e 5. A problem c o u l d a r i s e i f t h e 5 O r u d d e r a n g l e was i n s i d e t h e i n s t a b i l i t y l o o p , b u t i t was p r e f e r r e d t o r e t a i n a s p e c i f i c v a l u e , a s it is more c o n v e n i e n t f o r comparisons.
l
6.2 Steady-state Performance Measures
Steady-state performance i s "recorded" i n t h e r e v e r s e s p i r a l curves corresponding t o t h e four p r o p e l l e r r a t e s p r e v i o u s l y d e f i n e d , and is d i s c u s s e d n e x t . a)
]
h)
a t e d with t h e q u a n t i t i e s mentioned i n F i g . a l s o (15)).
3 (see
b ) C r a s h - a s t e r n S t o p p i n g : The u s u a l m e a s u r e s o f headr e a c h and l a t e r a l d e v i a t i o n a r e a d o p t e d , a s s u m i n g t h a t t h e t i m e t o s t o p i s a c c o u n t e d f o r by t h e distance travelled. The manoeuvre i s p e r f o r m e d from i n i t i a l s e r v i c e s p e e d . C ) A c c e l e r a t i n g and C o a s t i n g T u r n i n g : For t h e accel e r a t i n g t u r n c a s e a l o c a l maximum o f t h e t u r n i n g r a t e c o u l d e x i s t ( s e e F i g . 4 ) . The p o i n t o f maximum c u r v a t u r e o f t h e ( r , t ) c u r v e , is t a k e n a s characteristic. For t h e c o a s t i n g t u r n c a s e , t h e peak o f t h e r a t e o f t u r n and t h e t o t a l c h a n g e o f h e a d i n g a n g l e d u r i n g t h e manoeuvre ( s e e F i g . 5 ) a r e two p o s s i b l e measures.
Directional Stability: I f directional instabilit y i s o b s e r v e d , i t c a n b e measured by t h e s l o p e of t h e curve a t zero, C, t h e h y s t e r e s i s loopw i d t h , l w , and h e i g h t , l h , and a l s o from o t h e r q u a n t i t i e s , such a s surfaces A o r B ( s e e Fig 2 ) . Of t h e s e m e a s u r e s , loop-width i s t h e most import a n t b e c a u s e it d e f i n e s t h e r a n g e o f r u d d e r a n g l e s w i t h i n which a " p e c u l i a r " r e s p o n s e c a n be expected. I n t h e p a s t t h e loop-height received c o n s i d e r a b l e a t t e n t i o n - p e r h a p s b e c a u s e i t was e a s i l y i d e n t i f i a b l e b o t h c o m p u t a t i o n a l l y and d u r i n g t r i a l s b u t i n f a c t it d o e s n o t p r o v i d e A large lh i s , for much u s e f u l i n f o r m a t i o n . e x a m p l e , i n e v i t a b l e f o r a v e s s e l which i s d i r e c t i o n a l l y unstable b u t turns well.
A basic difficulty i n attempting t o standardise manoeuvring p e r f o r m a n c e a r i s e s from t h e f a c t t h a t a l t h o u g h t u r n i n g , d i r e c t i o n a l s t a b i l i t y and s t o p p i n g a r e usually treated individually, the f i n a l vessel d e s i g n n e e d s t o r e p r e s e n t a compromise o f t h e s e distinct qualities. T h i s means t h a t no m e a n i n g f u l a c c e p t a b i l i t y l e v e l s can be derived f o r t h e d i f f e r e n t manoeuvring q u a l i t i e s w i t h o u t c o n s i d e r a t i o n o f t h e i r inter-rel.ationships a s well. I t is necessary t o e n s u r e t h a t t h e numerical v a l u e s proposed i n t h e s t a n d a r d s a r e c o m p a t i b l e w i t h e a c h o t h e r and p r a c t i c a l l y achievable f o r t h e s p e c i f i c s h i p type.
Steady Turning: To measure s t e a d y t u r n i n g capability, t h e r e q u i r e m e n t s f o r minimurn t i m e and s p a c e must be t a k e n i n t o c o n s i d e r a t i o n . In t h i s r e s p e c t a number o f q u a n t i t i e s h a v e been i d e n t i f i e d a s a p p r o p r i a t e and t h e i r r e l e v a n c e h a s been i n v e s t i g a t e d . The p a i r o f d i m e n s i o n a l and non-dimensional r a t e s o f t u r n were found t o be b o t h s i m p l e and s a t i s f a c t o r y . The l a t t e r was non-dimensionalised with r e s p e c t t o instantaneous speed.
The b a s i c i d e a b e h i n d t h e p r e s e n t a p p r o a c h is t h a t e v e r y s h i p i n e a c h g r o u p c a n be a s s i g n e d a s p e c i f i c v a l u e o f a n o b j e c t i v e f u n c t i o n which r e f l e c t s a n d combines t u r n i n g , d i r e c t i o n a l s t a b i l i t y ar?d s t o p p i n g q u a l i t i e s , having a s variables a well-selected s e t o f "important" s h i p parameters. The s i n g l e v a l u e o f t h i s o b j e c t i v e f u n c t i o n would t h e n d e f i n e t h e l e v e l o f t h e v e s s e l ' s o v e r a l l manoeuvring c a p a b i l i t i e s , and a s o l i d b a s i s f o r comparisons and q u a n t i t a t i v e r a n k i n g would b e e s t a b l i s h e d .
6.3 Transient-state Performance Measures
T r a n s i e n t - s t a t e manoeuvring i s i n v e s t i g a t e d t h r o u g h a s y s t e m a t i c s t u d y o f t i m e - h i s t o r i e s and p h a s e - p l a n e diagrams of t h e motion. a ) I n i t i a l T u r n i n g and Yaw-Checking: To combine i n i t i a l t u r n i n g w i t h yaw-checking c h a r a c t e r i s t i c s t h e f o l l o w i n g manoeuvre was f o u n d t o p r o v i d e t h e necessary information: "From s t e a d y i n i t i a l f o r w a r d m o t i o n , s e t 6 = hi a t z e r o t i m e , where 6; i s one o f t h e p r e d e f i n e d r u d d e r a n g l e s . Once t h e maximum v a l u e o f r a t e o f T , , , ~ ~s ,e t 6 = - 6 ; t u r n is reached, and w a i t u n t i l t h e r a t e o f (see Fig. 3 ) .
turn f a l l s
t o zero:'
Even when t h e problem is r e d u c e d t o a s p e c i f i c and r e l a t i v e l y s i m p l e manoeuvre, t h e v a r i e t y o f a l t e r n a t i v e m e a s u r e s is r a t h e r l a r g e . To r a t i o n a l i s e t h e s e , t h e s i g n i f i c a n c e o f each one is assesed t h r o u g h c o m p a r i s o n s and by e x a m i n i n g t h e r e l e v a n c e o f t h e i r meaning t o t h e r e q u i r e m e n t s o f d i f f e r e n t vessel-types.
A t t h e f i r s t s t a g e t h e c r i t e r i o n h a s been a s s o c i -
7 . A PROCEDURE LEADING TO OBJECTIVE MANOEUVRING STANDARDS
To a c h i e v e s u c h a g o a l a s t r u c t u r e d f o r m u l a t i o n was s e l e c t e d i n which t h e c r i t e r i a an'd m e a s u r e s u s e d a r e h i e r a r c h i c a l l y combined f o r v a r i o u s o p e r a t i n g conditions of the vessel. The p a r t i c u l a r manoeuvring r e q u i r e m e n t s o f t h e g r o u p t o which t h e v e s s e l b e l o n g s a r e used a s an i n p u t t o d e f i n e p r i o r i t i e s i n t h e structure. Appendix I1 d e s c r i b e s , by means o f a n e x a m p l e , how s u c h a n a s s e s s m e n t p r o c e d u r e c a n b e developed. To p u t t h e s e i d e a s i n t o p r a c t i c e , a c o l l e c t i o n of r e p r e s e n t a t i v e h u l l forms h a s f i r s t t o be i d e n t i f i e d f o r each group s p e c i f i e d . Standard- s e r i e s h u l l s , a p p r o p r i a t e f o r t h e g r o u p i n q u e s t i o n would b e a c o n v e n i e n t c h o i c e . The r e c e n t p u b l i c a t i o n o f M a r a d ' s S e r i e s , f o r example, p r o v i d e s a good b a s i s f o r f u l l h u l l f o r m s (Group I ) , (16). The n e x t s t e p i s t o i d e n t i f y s h i p p a r a m e t e r s which p l a y a n i m p o r t a n t r o l e i n manoeuvring. These c a n b e g i v e n i n two g r o u p s : a ) Main d i m e n s i o n s , L , B , T, C
b
and t r i m t .
h ) Rudder a r e a , a s p e c t r a t i o and p o s i t i o n , s t e r n prof i l e a r e a , LCQ, p r o p e l l e r t y p e and c h a r a c t e r i s t i c s , e n g i n e power ( p a r t i c u l a r l y i m p o r t a n t f o r s t o p p i n g ) , and o t h e r parameters. A l l t h e parameters
i n ( a ) a r e i m p o r t a n t f o r manoeu-
v r i n g a l t h o u g h t h e i r range o f v a r i a t i o n a s an o p t i o n On t h e o t h e r hand, from during design is limited. group ( b ) , o n l y t h e r u d d e r - r e l a t e d p a r a m e t e r s and the stern profile were selected as the most important.
I t is now p o s s i b l e t o f o r m u l a t e a p r o c e d u r e developing objective standards, a s follows:
for
- F o r each vessel-group, o f C b , L/B, B/T these variables.
s e t t h e range of variation and s e l e c t a s t e p - i n c r e m e n t f o r
-
A s s o c i a t e t h e p r i n c i p a l dimensions with a s t a n d a r d d e a d w e i g h t and t h e r e b y d e f i n e L , B , T , C f o r e a c h b case.
-
S e l e c t a s t a n d a r d r u d d e r and s t e r n a r e a f o r e a c h A l o c a l v a r i a t i o n is t h e n c a r r i e d o u t t o case. study t h e i r e f f e c t .
-
F o r e a c h s p e c i f i c s e t o f t h e v a r i a b l e s manoeuvring p e r f o r m a n c e is t h e n a s s e s s e d t h r o u g h t h e s e l e c t e d c r i t e r i a and m e a s u r e s .
-
To v a l i d a t e t h e p r e d i c t i o n s , t h e o b s e r v e d p e r f o r mance i s compared, whenever p o s s i b l e , w i t h o t h e r p e r f o r m a n c e r e c o r d s drawn from s h i p t r i a l s , t e s t s o r simulations.
Two c a s e s were examined. In the f i r s t a l l the w e i g h t i n g f a c t o r s were f i x e d a t 0.333. I n t h e second 0 . 5 was g i v e n f o r t u r n i n g , 0 . 2 f o r d i r e c t i o n a l s t a b i l i t y and 0 . 3 f o r s t o p p i n g . The r e s u l t s a r e p r e s e n t e d i n a 3-D form i n F i g s . 6 and 7 . The h o r i z o n t a l a x e s g i v e t h e v a r i a t i o n o f L and B , w h i l e t h e v e r t i c a l g i v e t h e v a l u e s of t h e o v e r a l l o b j e c t i v e function. F i g . 8 g i v e s f u r t h e r t h e equal-weights s u r f a c e f o r t h e c a s e o f 0 . 8 d e g r e e s trimmed c o n d i t i o n by t h e bow. 8 . 2 Non-linear Theory I n t h e s e c o n d example, t h e n o n - l i n e a r m a t h e m a t i c a l Two c r i t e r i a a r e c o n s i d e r e d : initial model is u s e d . turning/yaw-checking and d i r e c t i o n a l s t a b i l i t y , w h i l e t h e measures s e l e c t e d a r e p r e s e n t e d i n T a b l e 7 . The s h i p s examined a r e 11 t a n k e r s b a s e d on M a r a d ' s Series. T h e i r main d i m e n s i o n s and r u d d e r a r e a s a r e The r u d d e r a r e a f o r e a c h c a s e is g i v e n i n T a b l e 8. c a l c u l a t e d on t h e b a s i s o f t h e DnV f o r m u l a :
Its s p a n and and c h o r d a r e t h e n c a l c u l a t e d t o f i t t h e s t e r n configuration, allowing f o r the necessary margins w i t h t h e h u l l and t h e p r o p e l l e r . Table 9 p r e s e n t s t h e v a l u e s r e c o r d e d f o r e a c h measure a f t e r t h e s e have been n o r m a l i s e d .
- Once p e r f o r m a n c e s f o r a l l p o s s i b l e c o m b i n a t i o n s o f t h e v a r i a b l e s a r e r e c o r d e d , i d e n t i f y t h e maximum and minimum v a l u e f o r e a c h s p e c i f i e d measure and normalise the values.
The s t r u c t u r e d f o r m u l a t i o n o f t h e a s s e s s m e n t proced u r e is g i v e n i n F i g . 9 , where t h e v a l u e s used f o r t h e weighting c o e f f i c i e n t s a l s o appear.
-
D e r i v e w e i g h t i n g c o e f f i c i e n t s on t h e b a s i s o f p r e f e r e n c e s f o r s p e c i f i c manoeuvring c h a r a c t e r i s t i c s , s u c h p r e f e r e n c e s b e i n g t h e outcome o f independ e n t l y c o n d u c t e d s t u d i e s ( s e e Appendix II!.
-
T a b l e 1 0 p r e s e n t s f i r s t t h e n u m e r i c a l v a l u e s of t h e i n i t i a l turning/yaw-checking and d i r e c t i o n a l s t a b i l i t y f u n c t i o n s , f o l l o w e d by t h e v a l u e s o f t h e o v e r a l l manoeuvring f u n c t i o n , f o r a l l e l e v e n s h i p s examined (See a l s o Fig. 1 0 ) .
Combine h i e r a r c h i c a l l y t h e v a r i o u s a s p e c t s o f manoeuvring performance p a r t i c u l a r t o t h e v e s s e l group i n q u e s t i o n , i n o r d e r t o o b t a i n an o v e r a l l performance index. The v a l u e o f t h i s i n d e x i s t h e n used f o r r a t i n g t h e v e s s e l ' s manoeuvrability ( s e e Appendix 1 1 ) .
On t h e b a s i s o f t h i s l a s t r e s u l t an a c c e p t a b i l i t y norm c a n now be e s t a b l i s h e d . For example, a r e q u i r e ment f o r F(M) > 0 . 5 would r e n d e r s h i p s S 6 , S7, S10 and S11 u n a c c e p t a b l e . Examination o f t h e s h i p s ' d i m e n s i o n s would s u g g e s t t h a t t h e outcome is i n f a v o u r o f more s l e n d e r f o r m s .
8. PRACTICAL APPLICATIONS I n t h i s s e c t i o n two s i m p l i f i e d c a s e s a r e examined t o d e m o n s t r a t e t h e c a p a b i l i t y of t h e p r o c e d u r e f o r p r o d u c i n g meaningful r e s u l t s . 8 . 1 Linear Theory I n t h e f i r s t c a s e , t h r e e manoeuvring q u a l i t i e s - t u r n i n g , d i r e c t i o n a l s t a b i l i t y and s t o p p i n g - a r e c o n s i d e r e d , a s measured r e s p e c t i v e l y by N o r r b i n ' s t u r n i n g index p , t h e s l o p e a t z e r o o f t h e r e v e r s e s p i r a l c u r v e and t h e e m p i r i c a l f o r m u l a :
The a p p l i c a b i l i t y o f t h e p-index t o d i r e c t i o n a l l y u n s t a b l e v e s s e l s i s q u e s t i o n a b l e , b u t it is a d o p t e d f o r i l l u s t r a t i o n purposes. The main p a r t i c u l a r s of a l l t h e v e s s e l s examined i s g i v e n i n T a b l e 6 . The p - i n d e x was found t o v a r y between 0.234 and 0.336 w h i l e t h e a n g l e a t z e r o o f t h e r e v e r s e s p i r a l t o 107.4O (even k e e l c u r v e v a r i e d from 85.34O A s t h e c o e f f i c i e n t s were t a k e n from condition). ( 1 1 ) . however, t h e v a l u e s would be r a t h e r c o n s e r vative (see (17)).
9. CONCLUDING REMARKS The p r e s e n t p a p e r was p r e p a r e d w i t h t h e i n t e n t i o n o f i n t r o d u c i n g an a p p r o a c h f o r d e v e l o p i n g manoeuvring s t a n d a r d s and d e m o n s t r a t i n g i t s c r e d i b i l i t y . The problem was t a c k l e d by c o n c e n t r a t i n g f i r s t on g a i n i n g a q u a l i t a t i v e u n d e r s t a n d i n g o f manoeuvring motion under v a r i o u s c i r c u m s t a n c e s , and t h e r e b y i d e n t i f y i n g a number o f r e l i a b l e m a n o e u v r a b i l i t y c r i t e r i a and measures. I n p a r a l l e l , a h i e r a r c h i c a l p r o c e d u r e was a d o p t e d which can e f f e c t i v e l y combine c o n f l i c t i n g c h a r a c t e r i s t i c s and a l l o w f o r a n o v e r a l l manoeuvrab i l i t y assessment. While i t is u n d e r s t o o d t h a t t h e s e t t i n g o f s t a n d a r d s w i l l r e q u i r e much f u r t h e r r e s e a r c h , t h e a d v a n t a g e s s h o u l d be n o t e d o f f o l l o w i n g a p l a n of a c t i o n whereby topics requiring further investigation are readily identified. The p a p e r s e r v e s t o d e m o n s t r a t e t h a t t h e development o f o b j e c t i v e manoeuvring s t a n d a r d s i s p o s s i b l e . It must be s t r o n g l y emphasised, however, t h a t f o r w i d e l y a c c e p t e d s t a n d a r d s t o be a c h i e v e d s o o n , t h e f u l l e s t p o s s i b l e agreement on common p r o c e d u r e s w i l l f i r s t have t o be o b t a i n e d from t h e b o d i e s c o n c e r n e d .
ACKNOWLEDGEMENTS
Full-form Ship Models.
The support of Lloyd's Register of Shipping in providing SMTC with access to their manoeuvring simulator and the financial support of the University of Strathclyde are gratefully acknowledge. Miss Christine Hutcheon's gallant effort in the preparation of the paper is much appreciated.
17
Procs. Int. Conf. on Ship Manoeuvrability, Prediction and Achievement. RINA, London 1987. "A Procedure for the Prediction of Ship Manoeuvring Response for Initial Design", Proc. Int. Conf. on Computer Applications in the Automation of Shipyard Operation and Ship Design (ICCAS 85). Pub. North Holland, 1985.
19
INOUE, S , HIRANO, M and KIJIMA, K: "Hydrodynamic Derivatives on Ship Manoeuvring". International Shipbuilding Progress. Vol. 3 8, No 321, 1981.
20
FEDYAYEVSKIY, K K and SOBOLEV, G V: "Control and Stability in Ship Design." Translation of the U S Department of Commerce, 1964.
21
TATANO, H , KASHIWADANI, T and TAKEHARA, M: "On the Effects of Stern-profile on the Course Stability of full-bodied Ships". Journal of the Kansai Society of Naval Architects, No. 162, 1976.
2 MIKELIS, N:
3 MIKELIS, N: "The Classification of Ship Manoeuvrability", Proc. Int. Conf. on Ship Manoeuvrability, Prediction and Achievement. RINA, London, 1987. 4
22 VAN LAMMEREN, W P A et al:
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"The Wageningen BSNAME Transactions, 1969.
23
HIRANO, M and MORIYA, S: "A Practical Prediction Method of Ship Motion and its Applcation". Int. Conf. on Ship Manoeuvrability, Prediction and Achievement, RINA, 1987.
Report to the President on an Evaluation of Devices and Techniques to Improve Maneuvering and Stopping Abilities of Large Tank Vessels. US Coast Guard, Report No. CG-M-4-79. 1979.
24
MATSUMOTO, N and SUEMITSU , K: "Interference Effects Between the Hull, Propeller and Rudder of a Hydrodynamic Mathematical Model in Manoeuvring Motion". Naval. Architecture and Ocean Engineering, Vol 22, 1984.
BARR, R et a: "Technical. Basis for Maneuvering Performance Standards", Submitted by the US to IMO, 1982.
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IMO: Interim Guidelines for Estimating Manoeuvring Performance in Ship design, MSC/Circ., 389. 10th January 1985.
5 CARD, J C et al:
6
BARR, R A: "An Increased Role of Controllability in Ship Design". Marine Technology, Vol 24, No 4 , 1987.
18 McCREIGHT: "Ship Manoeuvring in Waves". 16th Symposium on Naval Hydrodynamics, Berkeley, 1986.
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BORCHERDING, K and WINTERFELDT, D: "The Effect of Varying Value Trees on Multi-attribute Evaluations". Advances in Decision Research, North Holland, 1988.
8
GERTLER, M and GOVER, S C: "Handling Quality Criteria for Surface Ships", First Symposium on Ship Maneuverability, DTMB, 1960.
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SAATY, T: "The Analytic Hierarchy Process". McGraw-Hill, 1980.
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LLOYD'S REGISTER OF SHIPPING: Ship Manoeuvring Requirements (Personal Correspondence).
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KUBICEK, M and MAREK, M: "Computational Methods in Bifurcation Theory and Dissipative Structures". Springer-Verlag, 1983.
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10 LANDSBURG, A C et al: "Design and Verification Trans. for Adequate Ship Maneuverabili ty" . SNAME, Vol. 91, 1983. 11
CLARKE, D, GEDLING, P , HINE, G: "The Application of Manoeuvring Criteria in Hull Design Using Linear Theory". Trans. RINA, Vol. 125, 1983.
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"Mecanique du Navire en Route et en Giration". ATMA, 1938. 14 HAMPEL, B: "Minimum Manoeuvring Range as a Criterion to Evaluate the Risk of Collision between Ships". Maritime Simulation Proceedings of the First Intercontinental Symposium, Springer-Verlag, 1983. 15
16
SPYROU, K and VASSALOS, D: "Recent Advances in the Development of Ship Manoeuvring Standards". MARSIM & ICSM 90, Tokyo. ROSEMAN, D (ed):
"The Marad Systematic Series of
APPENDIX I
:
MATHEMATICAL MODEL
Two cartesian coordinate systems are used, the one earth-fixed and the other moving with the ship and positioned on the free surface at the middle of the ship. Assuming that the ship can be treated as a rigid body and that moments of inertia around the vertical and transverse ship's axes are approximately equal, the application of Newton's second law provides the following equations of motion , in standard nomenclature , (18):
Sway
m(ir
+ r u - pw + z c ( q r - p ) + x ~ ( q +p +)l=
APPENDIX I1 : DEMONSTRATION OF THE STRUCTURED FORMULATION
(Al)
Roll
I,p - mzG(i
+ TU - pw) - m z ~ z ~ ( p+q i.)=
The techqique of combining individual manoeuvring characteristics can be demonstrated by using a simplified example. It is assumed that turning can be adequately assessed on the basis of the initial turning/yaw-checking manoeuvre, measured by four quantities: maximum rate , of turn, rmar , time needed to reach it, ,,,,t sway speed at the moment whenr,,,occurs ,,,,v and the time needed to check yaw t, - trm., These parameters are recorded during an initial turning/ 3). yaw-checking manoeuvre (See Fig.
.
The right-hand-side terms of (All represent "external" forces of a quasi-steady nature. Memory effects in the fluid due to the presence of the free surface or vortex shedding are therefore neglected. As usual, subscripts H, P, R, 0 , indicate hull hydrodynamic reaction (accounting for ideal fluid, hull lifting, resistance and cross-flow drag), propeller, rudder and "other" forces (current, wave, wind, thruster etc), respectively. In calm water conditions, the heave and pitch equations are usually neglected and the heave and pitch velocities and accelerations in the remaining equations are set to zero. The surge, sway, yaw and roll equations are then explicitly solved for the accelerations, transforming them into:
It is further assumed that ( n i , 6 , ) ere the important control vector settings for the specific ship type examined, ( j = l , k 1 , ( j = l , l ) with k and 1 the maximum number of alternatives for the propeller and the rudder, respectively. A function can now be built combining the maximum rates of turn for the prescribed control settings:
where a:ij are weighting coefficients reflecting the importance attributed to the specific case of the assessment. Similar functions can be built for the other turning measures, e.g.:
The turning performance as a combination of the four measures can be obtained as: F(T) = (b i=1,4 with being weighting coefficients again, with similar meaning.
C jT) .
by)
The system (A2) together with the equation (3 = p , is used for the study of steady-state behaviour, with all dotted terms being set to zero. If the intention is to study transient-state, the kinematic relations connecting velocities with displacements must be introduced and combined with (Al) to yield a system of eight differential equations, with unknowns ~,~,~,P,'P,$'o,~o,Yo The control variables are then given suitable values and the system is integrated numerically by using an appropriate algorithm for STIFF systems of ordinary differential equations. For the evaluation of hydrodynamic coefficients, references (11). (16) and (19) are used. Resistance is calculated on the basis of the Holtrop and Mennen method. If Model tests are available, the expression is preferred. Res(u) = aou alulu( azu3
+
+
Trim effects on the linear derivatives are available from two sources, (19) and (20). Also, information regarding the effect of stern profile modifications can be derived from (20) and (21). The propeller and rudder force and moment expressions adopted are based on (22) and (23). Finally, the hull-rudder interaction force was taken into consideration in the way suggested in (24).
Following the same pattern, directional stability and F(DS) , stopping functions can be built, F(S) , respectively, and combining them with F(T) one level up, yields:
The specific value of the function provides an overall quantified assessment of the characteristics of interest and therefore the values in the vicinity of its maximum are most desirable. Fig. 1.1 summarizes the simplified procedure. The weighting coefficients must be linked to the mission of the vessel-type and the conditions under which she is expected to operate. To obtain the preferences for specific characteristics and then transform them into weighting coefficients, a number of techniques are available, (251, (261, (27). For an application, see (151, where the weighting factors were derived by adopting a pairwise comparisons technique whilst testing the final result for sensitivity. It is envisaged that for wider application data generation should be based on expert judgement using for example a properly constructed questionnaire.
(
l
Criterion
Associated manoeuvres
(
Eseentlal information
..
-
Yow checking
~
\
~~
- Ernfoatcha$,.y"" Initiol turning tlme to mnrond -..- a--u---d---
10-/ 10' and 20-/ 20.1 zlg-rags
- per Chonge of mhip'r hwdinq un8t rudder angle - Oiatonee tmvsllad --W
I ~ Iturning ~ ~ I
-
Couns keeplng
1
Turns with 10'. 20. rudder angles
~~~~~~
-
' - Cmeh astern
Stopplng
k t V h W l 8 IOOD
- Tmck
rwch
TABLE 1 Summary of IMD's Guidelines
Steady turning
Initial
turning The ship length p a w d through for a &an of 10'heodCg should be. leas than
The turning The flrsl m wenhoot angle d%~mihus durln 10710' f o m L/R and !0'/20"dpmhould not be zags mhould lee8 than : be lsss than :
d-10'
.
Course
Yaw checking
d-20-
d-10'
d-20'
d-10'
Cargo vusels
1.75
1.25
0.20
0.30
10'
Bulk carrlera
1.75
1.25
0.25
0.55
8'
Conuinm
1.75
1.25
0.20
0.30
8'
sp.Clal
1.75
1.25
0.28
0.58
10'
keeping
The time to check yaw, meaaumd In ahlplengths paased through durina lO;/t0'bnd 20/2OS dg-rags, 8hould g the vessel be less than : *ble no adlon to be d-20~ d=10" d-20" taken 15' 1.3 - R n o t the 1.3 amount of manual atwrlng 1.5 1.5 12' for coune12' 1.5 1.5 -wrrectingto be Investigated 15' 1.8 1.8
TABLE 2 Recommendations of Federal Republic of Germany
ORGANIZATIONS
IMO
LLDWS ( LMA 1
Cmsh-stop (AV) a t full speed Crash-stops ~t medium speed Stopping trlol d low 8peed Sbpping fmm low spaad with pmpulslon etopped C.oading stop teat Crash-stop (M) Stopping by use of rudder Turnlng toat Turning te& Turnlng test Turnlng tsst Turnlng teat
at full speed d medium speed at slow speed
with pmpulslon stopped from zem speed
-
I
Pull out Weave manoeuvre Zig-zag Man overboatd (Willlamson turn)
I
StatJstlcal method Change o f heading Later01 thruater : Turning tsst Zlg-rag tssf ahead Zig-zag ttst astern Course kesplng test. a t e r n
--
-
TABLE 3 Recommended Manoeuvres
X
BMT
SNAME
DnV
10th
14th
~TTC
~TTC
Number of controls
Involving one control variable
Initial condltlons
Steady
*M taken
!%%on
1
Unsteady
Propeller
/1
Rudder
I ;orlatlon I
1
Steody
1
Unsteady
Propeller
deflection h:$
Or
PROPELLER RATES
Involving two control variables
variation
'Tfllcal
1
servfcd
.
(course correcting)
,
'Manosoulng' Acelerating
'Moderote'
estobllshed
or chamcteristlu. -Tum[n9 commenta circle
= Cb
V-,
a common
X
v-
Lorge' 35 (turnlng)
(around 5 KN)
TABLE 4
20'
. .
1 ports apemtlons l -
l
. .
(IMO'a auggeatlon
Characterlatlc astern'
Transient Manoeuvring Cases
(at max contlnuoua a a t m propeller rate)
TABLE 5
Criterion Measures
0.85
BLOCK COEFFIClENT
Control settings
1
I
Definition of Control Settings
Initial turning
.
n = n-
.
TABLE 7
TABLE 6
Length
tm)
TABLE 8
Breadth
(m1
Draught (m)
Block Coefficient
Rudder Area(rn21.
Main Dimensions of the Ships Examined
TABLE 9
The Normalised Values for the Ships Examined
Initial
Directianal stability
tr,
Iw
corresponding t o
16.0 k n forward speed
20'
. 35'
n = ,n-
Criteria and Measures in the Second Application
Dimensions of the Vessels Examined Ships
Ships
yaw checking
,tr,lts-
rrnextvr,
d = 5'
-
Turning
-
Yau
Checking
I
~irectionai Stability 1
;hips
Turning function FP)
Dir. Stability function ~ ( 0 s )
Overall Manoeuver function F(M)
S1
0.0000
1.0000
0.6000
S2
0.2961
0.9100
0.6644
S3
0.2388
0.6985
0.5146
54
0.9953
0.9070
0.9423
S5
0.7212
0.7607
0.7449
S6
0.3311
0.5378
0.4551
S7
0.2209
0.2751
0.2534
S8
1.0000
0.6920
0.8152
S9
0.7053
0.4767
0.5681
S10
0.3316
0.2017
0.2536
S11
0.4674
0.0000
0.1870
TABLE 10 Values of the Manoeuvring Functions
l
TInE
SEC
l
)
Fig. 1 Convergence from Different Initial Conditions with 4 O Rudder
Fig. 2 Definition of Measures for Directional Stability
S
H
I P A SHIP B
0.m 0
230
500
750
l000 TInE
Fig. 3 Initial Turning/Yaw Checking Manoeuvre
Fig. 4
l500
1750
l
TInE
Fig. 5
Fig. 6
(
SEC
T r i m 0.8O
I
HEAOINB ANGLE
by the Bow
Fig. 7
Different Weights
)
,
, ,
I ki%%q INW TURNING
- YAW CUECKINC
0.3, rmm
Vrmox
t rmor
tm
- trmx
Iwp
- width
Fig. 10(a)
O v e r a l l Manouevring P e r f o r m a n c e
Fig. 9 The S t r u c t u r e o f t h e S i m p l i f i e d Assessment
Fig. 10(b)
T u r n i n g Performance
Fig. 10(c)
D i r e c t i o n a l S t a b i l i t y Performance
Fig. 1-1 The Assessment S t r u c t u r e o f t h e Simplified Procedure
DISCUSSION Mr N.E. Mikelis, M.Sc., Ph.D. (Member): I feel that the authors have made a worthwhile contribution to the field of ship manoeuvrability with the development of a rationally based manoeuvring standard. Turning, course keeping and stopping abilities are all interrelated and, for this reason, a single expression for the required standard (depending on ship type and mission) should be preferable to the three separate standards currently being developed at IMO. I would wish to ask the authors to apply their procedure in order to verify the classification society rule which they have used in their paper for the selection of rudder area:
As a useful application of their rational procedurethe authors could now check, and possibly improve, this empirical rule for the selection of rudder area.
The Chairman, Professor K.J. Rawson: It strikes me that there is going to be a fairly large data base needed because the people who specify what they want in terms of manoeuvrability, normally say: 'Oh, rather like the last ship', or 'Like Joe's Ship'. If you are going to make this objective, a designer has to survey a large number of parameters of other ships and guess what sort of selection is going to be sensible. It is really not going to be a very precise rationale without comparisons of other ships, accepting of course that the system is more logical. But these large data bases are not readily available except perhaps to the classification societies.
Mr A.L. Dorey, O.B.E., BSc. (Fellow): It is very interesting to note the search for one numbbr which can represent ship manoeuvrability, since it has always seemed to me that aspects of manoeuvrability of ships can and should be divided up. Would the authors comment on why they think this to be necessary, because I would have thought it preferable that one did divide it up. A standard of turning circle should not be too difficult to devise, and a standard for stopping likewise for those who know what certain types of ships are expected to do. In the warship field we do have some criteria for what is acceptable and what is not acceptable and I would have thought that was true in the merchant world too. Directional stability is more difficult. If I produced a directionally unstable warship I would be very ashamed, and I think the owners would not like it at all. In fact, directional instability in warships would be regarded as unacceptable. Do the authors believe that directional stability is predictable for merchant ships? Is it not so that in many cases stability is a function of trim and is thus in principle, adjustable? Would it not therefore be possible to legislate that merchant ships, which after all are very large and potentially very
dangerous, must be directionally stable and that this could be achieved, in practice, by trimming them. Thus, if a naval architect produced a ship which was directionally unstable, it would have to bear the penalty of carrying the trim.
Mr U.K. Gerry, D.Ae. (Member): I should like to raise a point I have mentioned at meetings here before, and that is, why do we insist on manoeuvring ships by rudders? There are alternative means and possibly better means. The method that I have experimented with is using the vast area of the sides of the hull and inducing very small pressure differences on large areas instead of using high forces on a very small rudder. The advantage of this approach is that you can use it when the propeller is reversed for going astern and in cases of power failure.
I have mentioned this to various people but there has never been particular interest in taking it further, largely, I believe, because manoeuvring standards have not been considered to be definable and there has been no basis for assessing improvements. Thus money spent in finding a better way of manoeuvring ships would probably be wasted because there is no motive to apply it. One of the outcomes of this paper is a movement in the direction of defining manoeuvrability, and therefore the possibility of an interest in improving it. Thus something along the lines that I have experimented on might prove in time to be of value to ship manoeuvrability. I wonder if the authors could comment on that. Mr M.N. Parker, B.Eng. (Fellow): The paper is a welcome contribution to the identification of manoeuvring criteria. However, I find a little difficulty in understanding how the proposed overall performance index would be applied in practice. Isee that, with appropriate weightings for ship type and service, the indices for the various manoeuvring characteristics may be combined to provide a factor of merit, but 1 feel that turning ability, directional stability and stopping characteristics, although physically interrelated, generally need to be assessed separately. If we take a vessel for which a specified turning ability is required, then, however you arrange the weighting factors, no excess of directional stability or stopping ability in that vessel is going to compensate for inadequate turning qualities. Professor R.O. Goss, M.A., Ph.D. (Companion): I am going to ask the simplest of questions: how many sailors have the authors asked about this research? The Chairman then proposed a vote of thanks to the authors which was carried with acclamation.
WRllTEN DISCUSSION Professor M. Fujino: I would like to congratulate the authors warmly for their stimulating approach to developing a quantitative measure for ship manoeuvrability standards. In particular, I am interested in the introduction of an objective function for assessing the general character of an individual ship. I would welcome the authors' opinions regarding the following questions: 1.
I conceive that if the manoeuvring standards are developed to enhance the safety of navigation, the values of the weighting factors included in the objective function, which are introduced by the authors, should be associated closely with consideration of avoiding ship casualties. For instance, these values should be connected deeply with a relationship between casualty statistics and major ship manoeuvrability defects causing such casualties. At the present, the International Maritime Organisation is eager to establish manoeuvring standards in order to exclude ships with extremely poor manoeuvrability. Iwould like to know how the authors draw a marginal line by which ship manoeuvrability should be distinguished; in other words, how the authors establish manoeuvring standards in terms of the objective function.
2.
Moreover, when a marginal line of manoeuvring performance is drawn, due consideration should be paid to the confidence of the numerical values of the objective function allocated to each individual ship; in other words, the confidence of the assessed manoeuvrability of each individual ship. As meaningful manoeuvring criteria, the authors propose five kinds of steady-state and transient performance measures in order to identify the manoeuvring performance of each individual ship. However, I conceive that the confidence of each performance measure assessed at the design stage is not the same because of differences in prediction accuracy of each performance measure. For instance, the prediction accuracy of stopping capability is not identical to that of steady turning capability. Therefore, at the design stage, the objective function consisting of five different measures with different confidences should be considered to include uncertainty. How would the authors treat such an uncertainty when developing the manoeuvring standards?
Professor J.W. Doerffer: Participating for many years in the work of the International Maritime Organisation (IMO), recently for 11 years in the capacity of the chairman of the Subcommittee on Ship Design and Equipment. which is responsible for drafting the international manoeuvring standards, I became very safety minded. Any research that is undertaken in this field should help in establishing these
standards keeping in mind that the safety of navigation will depend to a very large extent on the manoeuvring characteristics of the ship. When the skipper discovers his navigational error, he should be able to stop or turn the ship in order to avoid a collision or grounding. The interest of IMO is manoeuvring characteristics of ships dates back to 1968, when the introduction of large ships, especially tankers and bulk carriers, and the considerable increase of service speed caused the increase in the number of ship casualties. The stopping distance exceeded 10 ship lengths with a side transfer of 3 ship lengths and sometimes even more. The turning circle diameter exceeded 5 ship lengths. General opinion put the blame on the so-called 'human factor', but the naval architects knew that there existed great possibilities for improving the inborn manoeuvring characteristics of individual ships. The need for this became more evident with increasing density of traffic in some sea areas and with routeing the sea lanes. But no deeper thought was ever given to standardisation of manoeuvring characteristics and to improving the steering arrangements, the balanced rudder being the generally adopted type. The only additional device implemented on a limited number of ships was the lateral thruster, which at speeds above 4 knots becomes quite ineffective. There are many special types of rudders which improve greatly the manoeuvrability of ships, some even enabling all the manoeuvres (turning at manoeuvring speeds with zero turning circle diameter, stopping and backing with full course control) but none of these steering devices has been implemented on a large scale. The following manoeuvres have direct impact on the safety of navigation and thus IMO is mainly interested in standardisation of these characteristics: crash stop at service and manoeuvring speeds; turning at service and manoeuvring speeds; pull-out. Other manoeuvres are of lesser importance for the safety of navigation, although they are very important to the efficiency of navigation. There are two ways of setting the standards: the navigators should express their view, what standards they would require to considerably improve safety at sea; the naval architects should work out the proposed standards, based on theoretical considerations and practical results of sea trials, collected from a great number of new ships. If the navigators' requirements coincide with naval architects' propositions, then agreement could be easily reached. But if there were a substantial difference, the navigators asking for more stringent standards, then the present requirements for steering arrangements should be reconsidered and new types of rudders should be introduced. In drafting the standards, future technological developments should be considered. The present day average is not sufficiently satisfactorily. There could be individual standards adopted
for different sizes and types of ships, but there must be a limiting extreme value assessed in a similar manner to that for road vehicles and aeroplanes. The final form of the standards and the indices should be clearly readable and easily understood by all those who will have to use them.
Mr G.H. Fuller, R.C.N.C. (Fellow): 'The lack of numerical standards and the use of empirical, often subjective, criteria is all too frequent in many areas of ship design, test and evaluation. Manoeuvring is certainly one of the more complex areas, albeit for surface ships in the air-water interface so basically a two-dimensional problem. Similar problems exist in stability, strength and even resistance and propulsion where huge effort is spent in model to ship correlation but very much less on research into fouling, roughness and even the effect of windage and the real seaway. Nevertheless, the authors are to be congratulated for attacking this difficult area.
available for publication. Reply to the Chairman, Professor K.J. Rawson Professor Rawson emphasised the need for more, widely acceptable and systematically classified data, a point with which the authors fully concur. R is, however, worth mentioning that a great deal of data already exist from which useful information could be extracted, see for example (6), (31)Professor Rawson goes on to suggest that if an assessment procedure is to be objective, extensive comparisons must be carried out with existing ships. This, of course has been the trend so far. Although data are still needed to validate the proposed methodology, the emphasis in our approach has been on the reduction of the necessity for empirical and subjective interventions by developing the means for an objective assessment of ship manoeuvring capability.
Two questions arise: Reply to Mr A.L Dorey Five ship types are proposed; although these form common operational groups on a cargolperhaps route criterion, there can be wide variations within each group for operational reasons as there are between the loaded and ballast conditions. Could the authors elaborate on their methods of arriving at these groupings or if there are other sets of characteristics which could assist in codifying manoeuvring needs? Of all ship performance requirements, the man-machine interface is most crucial in manoeuvring performance and the most difficult to model. Two computer based approaches are becoming available which can deal with this interface. Firstly, there are compact, simple to operate, manoeuvring simulators, which can turn 'numbers' into 2D and 3D displays as in the BMT REMBRANT simulator. Secondly, there is a growing body of 'expert' technology to draw out the real response of an operator in a critical situation and so provide an input to numerate criteria. Have the authors considered these approaches?
AUTHORS' REPLY The authors feel particularly grateful for the interest shown in this paper and would like to thank all the contributors to the discussion for their useful and helpful comments, and for the interesting questions they have raised. The authors will attempt to answer the latter by addressing the contribution of each discusser in turn.
Reply to Dr N.E. Mikelis Dr Mikelis's comments are gratefully received. We would agree that the existing rule on rudder area needs to be checked, and that our developed procedure could help in this direction. We believe, however, that a more suitable method for assessing rudder area can be established by attempting to identify the rudder area necessary for maintaining course in a representative beam wind environment, see (30). This aspect is currently being pursued, and results will soon be
The first point raised by Mr Dorey provides us with the opportunity to offer additional clarification on a key issue of our proposed approach. The main aim in proposing an overall manoeuvrability index is to ensure that inter-relations between different manoeuvring qualities are duly taken into consideration in judging ship manoeuvring capability. Furthermore, such an index will facilitate comparisons between ships, which it is difficult, or even impossible, to do at present on the basis of different and conflicting characteristics. An overall index could be useful in the design process, for insurance and classification purposes and so on. On the specific issue of setting standards, however, we would favour the adoption of minimum levels of performance with respect to individual manoeuvring qualities considered in parallel with the overall performance index, see (15). Mr Dorey also refers to the phenomenon of directional instability, which he generally considers as an unacceptable feature of a ship. The problem is, however, that the quantification of the risks to which a ship is exposed because of directional instability has not been achieved so far. This has led in practice to a large number of ships spending a considerable proportion of their operational lives in a condition favouring the occurrence of directional instability. Furthermore, our own studies have revealed that directional instability might be a feature of ships that is unavoidable in practical terms in realistic environmental conditions, see for example (30). We think, therefore, that before proceedingto the development of a requirement disallowing directional instability, a proper justification should be provided, showing why, for example, one-degree loopwidth directional instability represents adangerous situation, particularly when this might lead to improved turning characteristics.
Reply to Mr M.N. Parker The point raised by Mr Parker has been addressed in our
based on human experience could be obtained, and on this matter we think that a properly conducted investigation on the perceived ranking of specific manoeuvring characteristics would be of considerable assistance using experts as the recipients. It is encouraging to note recent efforts in this direction, see (32).
reply to Mr Dorey. We should like to add, however, that the preferential independence mentioned in his example is exactly the stepping stone needed in the development of the proposed hierarchical procedure. To elaborate a little further, suppose that designs A and B both satisfy the requirements for turning and directional stability but A is superior to B in terms of turning but inferior in terms of directional stability. How would a designer arrive at an optimum solution? We feel that integration is an essential feature of design and this is, in fact, what we advocate here.
To trace the boundary separating 'satisfactory' from 'unacceptable' manoeuvrability is a major difficulty in the process for developing standards because there is no clear safety consideration on the basis of which the specification of this boundary could be produced. Whilst this decision should not be the outcome of a purely technical investigation, we would envisage the following process:
Reply to Mr U.K. Gerry In response to Mr Gerry's comments we should like to point out that not only does our profession insist on manoeuvring ships by means of rudders but such devices are not sized by generally acceptable rational procedures.
(a)
Assessment .procedures pertaining to a specific ship type are carried out, according to which a quantitative correlation between main ship parameter ratios and manoeuvring performance is obtained.
(b)
A parallel study is conducted aimed at identifying the distribution of existing fleets in terms of the same ratios.
(c)
By comparing the two pictures it will be possible to assess the implications of setting the boundary at alternative levels. It is suggested that a limit in the neighbourhood of 0.5 could constitute a good starting point since reference can be made to a number of real-life applications where such a limit is employed. Initial steps in this direction can be found in (30).
We agree with him that the lack of objective quantifiable manoeuvring standards has hindered progress in this direction, and indeed - one could argue - progress in the evolution of vessels with enhanced controllability.
Reply to Professor R.O. Goss In answer to Professor Goss's question we could, of course, say that manoeuvring is as old as the sailing of ships itself, meaning that a fund of experience has been built up which we must try to draw on whenever we apply ourselves to the study of specific problems. However, to answer the question directly, we did not ourselves carry out a specific survey prior to undertaking this research but Panel H-10 of SNAME did exactly that in the early eighties (10). That survey addressed 120 individuals and organisations involving harbour and river pilots from most US coastal ports, and was aimed at deducing the perceived need for inherent ship manoeuvrability by the practitioner.
Reply t o Professor M. Fujino The authors would like to thank Professor Fujino for his kind remarks, and provide the following answers to his specific questions: 1.
The association of casualty-avoidance concerns with the weights' derivation procedure does indeed reflect our initial motivation for adopting this approach. It is disappointing, however, that at present very limited information exists which could help in this direction. This fact indicates the existence of a need for initiating independent studies, probably involving a combination of theoretical considerations with casualty statistics analyses, which would be aimed at revealing the link between a number of critical parameters and the risk of casualty occurrence. It has to be admitted, however, that this presents a difficult problem. In the meantime - and until a satisfactory solution is achieved -we believe that a useful input
2.
We concur with Professor Fujino's idea of associating different degrees of uncertainty with the predicted values regarding each performance measure. We would add that uncertainty will exist not only in the numerical values of the above but also in the weighting factors, although the uncertainty in the latter case seems more difficult to quantify. There is no doubt that the introduction of probabilisticconsiderationswill give the assessment a more pragmatic character.
Reply to Professor J.W. Doerffer Professor Doerffer refers to the activities of IMO and he is, of course, a most appropriate reviewer because of his long association with that Organisation. In the main, we agree with his suggestions and believe that our approach encompasses both the considerations referred to. We should like to elaborate on one point of detail, namely the usefulness of the pull-out manoeuvre in assessing directional stability, which essentially refers to the loopheight of the spiral curve. Our studies have clearly shown that the loopwidth is a more useful and practical measure as it relates to the control space.
Reply to Mr G.H. Fuller Mr Fuller wanders, firstly, how the five proposed manoeuvring groups were derived. This was done on the basis of: The ships' geometrical characteristics; The manoeuvrability requirements for different ship types; 'The number of ships available per type; The significance of the implications of a potential casualty. It has to be admitted, however, that the above aspects were taken into account only in a qualitative sense. Earlier work on this issue has also been taken into consideration. Mr Fuller mentions further that within each group a variety of operating conditions could occur. On this aspect we think that with regard to each manoeuvring quality the condition giving rise to the severest implications for safety should be examined, e.g.. maximum allowed trim by the bow if directional stability is under investigation, ballast condition at slow speed if the effect of wind is being investigated, etc.
Finally, Mr Fuller refers to the role of the operator in the ship control process. His importance is, of course, undeniable. We believe, however, that this matter can be adequately investigated only after a clear picture has been obtained of the inherent manoeuvring characteristics.
REFERENCES
30.
SPYROU, K: 'A New Approach for Assessing Ship Manoeuvrability Based on Dynamical Systems' Theory'. Ph.D Thesis. University of Strathclyde, December 1990.
31.
DELLA LOGIA, B, BRIA, M and COLOMBO, A: 'Manoeuvrability of Full-Scale Ships', Third PolishItalian Seminar on Ship Research, CETENA Publications, Quaderno, No.29.
32.
YOSHIMURA, Y and KOSE, I: 'Studies of Manoeuvrability Standards'. Contribution to 19th IlTC, Spain, 1990.
I 1 I
