Ship design and the computer

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SHIP DESIGN AND THE COMPUTER

Horst Nowacki

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December 1969

Delft

THE UNIVERSITY OF MICHIGAN COLLEGE OF ENGINEERING

SHIP DESIGN AND THE COMPUTER

BY

HORST NOWACKI

-DEPARTMENT OF NAVAL ARCHITECTURE AND

MARINE ENGINEERING THE UNIVERSITY OF MICHIGAN

ANN ARBOR, MICHIGAN

EASTERN CANADIAN SECTION SOCIETY OF NAVAL ARCHITECTS

AND MARINE ENGINEERS MONTREAL DECEMBER 2, 1969 " I 1 Mdeling Sch ibliotheek van de nische Hogeschoo eepvaartkunde DAT UMI DOCUMENTATIE

42

ABSTRACT

The paper discusses current trends in computer-aided ship design, and the effects computer use may have on the scope and style of ship design.

INTRODUCTION

Ship design is without doubt receiving strong, fresh impulses from the rapid advances of computer technology. We are witnessing a process of mutual adaptation: Computer systems and new computer methods are providing a growing

variety of the capabilities essential to ship design. In

ship design, on the other hand, a new, somewhat more rational and systematic style is developing where the computer is

used as a tool.

This paper will attempt to evaluate the present stage of developments, and to discuss where current trends seem to be leading. The purpose is not making an exact predic-tion, but rather stimulating a few thoughts and a clarifying discussion before this professional audience of what lies ahead for ship design.

DESIGN OBJECTIVE

The goal. of z design process may be defined in terms of input and output as follows:

Given a functional requirement (say, a certain amount of cargo shall be transported from A to B); given further a number of constraints of technical, physical, or legal nature (stability, strength, ship safety, classification rules; etc.).

Sought an optimal technical solution judged on the basis of a concrete measure of merit.

This comprehensive definition of design, which results from applying the thinking of systems analysis to design, goes probably beyond the notion of ship design many of us have today. Up to now the designer has frequently been satisfied demonstrating the feasibility rather than the

optimality of a technical solution. Moreover, merit compar-isons in the course of a design have not always been based on a single, clearly expressed measure of merit.

Computer-aided design, too, is far from the ideal of completely rational, optimal solutions. But the following will show that the use of computers creates the tendency to more ambitious, comprehensive design objectives.

The economic success of a technical venture is generally measured by the ratio of benefit and effort. To the owner of a merchant ship, profit is the desired benefit, and the

size of the investment measures the effort. Their ratio, the profitability, is to be maximized.

We need not discuss in detail how to express profita-bility in ship design. There are many, more or less equi-valent possibilities (yield, required freight rate, etc.), but there is no reason to be dogmatic since, within

reason-able variation of the criterion, the technical solutions obtained are fairly equivalent, as Benford has frequently

stated, References (1) and (2).

To illustrate how a suitable measure of merit provides a common perspective to all design considerations, let us look at the Ship Merit Factor, SMF, recently proposed by Cheng, Reference (3). One of the versions he gives for SMF is V Wp SMF = k 1 1 WP 1 RV = 8760 _{fs fw fv}

r-T-r-

_{W R P} where

SMF = ship merit factor

k = 8760 fs f f /(1 + f) = service constant w v

W = payload

V = design speed in knots C = AAC = average annual cost

8760 = 24 365 hours/year RATIONAL DECISION CRITERIA

fs

= utilization factor, percent of annual service hours

fw

= load factor, percent of designed payload

fv

= operating speed factor, percent of design

speed

f = port time factor, port time/sea time

C' = C/PB = specific operating cost in dollars

per horsepower-year

PB = power delivered by prime mover

W /W = payload-displacement ratio

R/W = drag-displacement ratio

RV/PB = no

nH ript

s =

propulsive efficiency

,n

,nR,nS = open water, hull, relative rotative, and

shaft transmission efficiencies

The Ship Merit Factor is defined as the inverse of the well-known Required Freight Rate (RFR). The essential point is that equation (1) allows judging the contributions of

various design measures

in different areas to the success of

the whole, and selecting the most promising changes. It is

evident, for example, that good cargo handling and a fast

turnaround are as significant as design improvements in the more traditional domains of resistance, propulsion, and

strength. Of course, each gain must be related to the corresponding cost, that is the effect on C'.

CLOSED LOOP OPTIMIZATION VERSUS OPEN LOOP DESIGN

The computer enables us to generate and compare a

much greater number of design variants than possible before. The question arises naturally how to go about a systematic evaluation of this variety and how to pick a best solution. The effort of evaluating all conceivable variations of the design exhaustively is generally prohibitive even when com-puters are used.

This is why several relatively efficient optimization 1

methods have been applied to design problems in recent years, 11

which tend to converge to the optimum automatically, and 1

fairly fast and selectively, on the basis of a given measure 1

of merit function. (Linear, and nonlinear programming,

random search, direct search, and other search methods). 1

Applications of optimization methods to early stage ship !

design (selection of principal dimensions), and ship struc-tural optimization (midship section) are probably well-known, References (4), (5), and (6).

At the University of Michigan, we have recently

success-fully adapted the direct search technique, Reference (7), and

the SUMT method by Kowalik, Moe, and Lund (Sequential Uncon-strained minimization Technique, References (8), (9)) to the problem of preliminary ship design. Meyer-Detring (10) has 1

presented a tanker design application, and a variety of other 1

studies are underway.

The use of optimization methods in closed form requires

that the nature and scope of the intended design variations 1

be known when the program is written, that is before the 1

design has begun. In the typical operation of a batch 1

computer, the user has no control over the computation during the execution of a run. Consequently, where batch computers are I

used in design optimization the designer is forced to formulate

-explicitly, and ahead of time, his design objectives, the measure of merit, and the range of design variations.

This may have some beneficial effects upon the ratio-nality of our design decisions. But it is also clear that in this kind of computer use we have to break with the traditional style of design,. This may be illustrated by the flow chart of Figure 1 for the example of early stage ship design.

In this figure, the arrows and incomplete loops shall indicate the "open" stiucture of conventional design

pro-cedures. The sequence of steps is actually very flexible

and hardly predictable. It will be decided by the designer on the basis of intermediate results in such a manner that one obtains the most suitable solution by gradual trial and

error improvement, and with the least possible effort. The

designer always reserves the right to learn from the inter-mediate results and to enter new thoughts into the design as

it proceeds.

The "open" logical structure and freedom of traditional manual design thus is not ip harmony with the "closed" format of optimization by a single computer run. Consequently,

optimization methods have been successfully applied only to certain subproblems of ship design, for example the deter-mination of principal dimensions.

A new development is coming about now with the intro-duction of time-sharing computer systems which permit

continuous access to the computer, even during execution of the program, and hence facilitate a free dialog between computer and user. Although this technical development is still in its early stages it can already be concluded that time-sharing and dialog are very suitable media for design

tasks.

This suggests a compromise in the future computer use in design such that the designer controls the gradual step

GIVEN: ROUTE, PAYLOAD, (SPEED)

FIND: SIZE, PROPORTIONS, (SPEED)

V

ESTIMATE A, SPEED, AND

PRINCIPAL DIMENSIONS

POWER ESTIMATE: SHP

-WEIGHT CHECK 1

LINES AND ARRANGEMENTS

FREEBOARD

CHECK OF _CAPACITIES

STABILITY, TRIM, AND MOTIONS

STRENGTH

MEASURE OF MERIT

I OPTIMUM _I

time using optimization subroutines for any desired partial aspect of his design studies.

THE SYSTEMS APPROACH IN SHIP DESIGN

In the present context let us define a system as a number of objects or activities, serving a common purpose and being judged by a common measure of merit. Where ship design decisions are subjected to rational criteria, such

as profitability, it follows naturally that the ship and

its parts are viewed as a system. Moreover, many ship types cannot be designed effectively today without

devot-ing much attention to the interaction between ship and shore based handling and distribution facilities, and to several other details of ship operations. In this connection the

ship is viewed as part of a more comprehensive system. For these reasons the organization of ship design studies follows the systems approach tore and more

fre-quently. Such systems studies tend to be far more elaborate than used to be the case in conventional design, which makes computer use all the more imperative. Figure 2 illustrates the possible scope of systems design studies, and the variables and parameters that may be involved. The figure shows that beside the technical ship design variables many other details of the system have to be determined in mutual harmony where

they are under the system designer's control (routing,

sched-uling, cargo handling, details of ship operations). Moreover,

there are many external influences on the system that are of

uncertain nature and best represented statistically (cargo availability, weather and seaway, other causes for delays). Dealing with such influences properly, necessitates stochastic decision models.

The systems approach not only causes these additional complexities, but fortunately also provides the means for problem solution. Generally the problem is first decomposed into its simplest elements following a formal pattern. Never-theless the analyst will benefit from a thorough technical

-8

CARGO PARAMETERS:

STOWAGE FACTORS,

AVAILABILITY,

SPECIAL REQUIREMENTS

TECHNICAL DESIGN

VARIABLES OF THE SHIP

DISPLACEMENT, SPEED,

LENGTH, BEAM, DRAFT, DEPTH,

C, C,

, ETC.

PORT PARAMETERS:

TERMINAL FACILITIES,

CARGOHANDLING METHODS,

PHYSICAL PORT LIMITATIONS,

DELAYS, AND OTHER

UNCERTAINTIES Is- II

MATHEMATICAL

MODEL OF SYSTEM

MEASURE OF MERIT

IOPTIMIZED SYSTEM

VOYAGE PARAMETERS:

TRADE ROUTE,

UNCERTAINTIES, AND DELAYS

EN ROUTE, ETC.

COST PARAMETERS:

SHIP BUILDING AND

OPERATING COST,

COST FOR TERMINALS,

AND OTHER PARTS OF

THE SYSTEM

understanding of the system. It remains an essential engineering talent to discover how to reduce a complex problem to many simpler, more tractable ones.

In the following step the system is optimized by means of standardized, flexible, and powerful

optimiza-tion methods, often provided as subroutines by computer

systems. The crucial role Of the computer in this kind

of work should be obvious.

-DATA STRUCTURES

The growing scope of design studies puts no small demands on computer storage and cost economy. It is

therefore important to organize the handling of data in the computer efficiently. Modern computer systems offer many features facilitating efficient data structures. The

following are of particular importance.

Random access:

Disk, data cell, and similar secondary storage devices allow direct storage and retrieval of each data element as opposed to the time-consuming scan required in se-quential storage devices (tapes).

Dynamic storage allocation:

In classical FORTRAN style computer software the alloca-tion of storage is done by the programmer rigidly before program execution. In certain modern software environ-ments the system itself is responsible for the allocation of storage and dynamic updating during execution. This

dynamic allocation feature avoids the waste of idle storage

space.

List structures:

Contrary to the usual array structure of logically con-nected data elements in successive storage locations in the computer memory, list structures allow placing the data elements at arbitrary locations in storage, providing the logical connections by pointers associated with each data element. The logical and the physical sequence of

data in storage are thus independent of each other. This

feature is essential to update data scopes conveniently without relocating any data elements.

Time sharing:

User control during program execution may lead to essen-tial cost savings in debugging and whenever intermediate checks are advisable to keep large programs from taking off tangentially.

The advances in computer technology and systems programming discussed in the foregoing have been sufficient to meet the growing demands of the design-oriented user.

COMPUTER GRAPHICS

The picture is an essential ingredient of engineering design. Pictures convey ideas spontaneously, much more immediately than numbers. A design idea is often born as a ,visual concept, and must be displayed graphically to be communicated easily and uniquely.

Many computer systems today are strictly analytically oriented. The user must encode digitally all geometric

input data, and decode the digital output for geometric meaning. At this level there exists a conspicuous mis-match between computer and user.

This deficiency is being removed by computer graphics providing graphic input and output devices. Digital plotters have been playing a useful role for some time (passive

graphics, just output), whereas active graphics by cathode ray tube display and light pen control is beginning to emerge from its experimental stages.

The processing of geometric data is extremely inten-sive in the shipbuilding industry, and it is obvious that computer graphics are going to play an eminent part in all phases of shipbuilding from early stage design through production, Figure 3.

Graphic output on plotters is already an essential feature

in lines fairing applications and as a check medium for nu- i

merically controlled flame-cutting equipment. The Norwegian AUTOKON system, widely usedthroughout the world, uses

graphics intensively in both applications, and demonstrates the key role of graphics in any all-round design-production

system. The CASDOS ship detailing system, under development

for the U. S. Navy by Arthur D. Little, Inc., also depends

heavily on its original solutions in graphics. More details about the two systems below.

CHECK, OPTIMIZE

1

WORKING DRAWING

PRODUCTION PROGRAM

The use of computer graphics in the shipbuilding industry has been directed to ship production primarily; but other industries have already made significant

progress in design applications of graphics. This is certainly true for the developments of the U.S. aircraft and automotive industries, References (11) and (12).

The foregoing shall not give the impression that all

problems of computer graphics have now been resolved. The

cost of many installations is not yet too encouraging

either. But in the course of further technological progress

computer graphics should gradually advance to a standard tool in the ship design office.

-DESIGN AND PRODUCTION

Design and production form a continuum in data pro-cessing. The output from the design phase is input to

the production process, and it is expedient to simplify the transition by identical data formats.

Consequently it has proven successful to integrate design- and production-oriented computer software wherever

large program packages had to be developed for automation purposes. In other words, design-oriented computer

ap-plications have been thriving best in an environment of computer systems thinking motivated by production

tech-nology.

The major efforts of the automobile and aircraft industries spanning design and production have already been mentioned. In our field the best known examples are:

AUTOKON, References (13) and (14):

This software system encompasses hull form definition (fairing), part definition for a great number of struc-tural members of the ship, and preparation of numerical control tapes for flame-cutting equipment.

CASDOS, References (15)1 (16), and (17):

The objective of this system is to generate from contract plans and specifications, with due regard to the peculiar-ities of a given shipyard, all the necessary information for building a ship: Working drawings, bills of materials, and NC tapes. The system is based on man-computer dialog, but the role of the man is largely creative and critically selective, while a great amount of the routine work involved in design detailing is automated. CASDOS is in trial oper-ation at the Puget Sound Naval Shipyard at the present time.

-PROBLEM-ORIENTED LANGUAGES

Widespread computer use in design offices is often hampered by the fact the engineering staff is not

suff-iciently familiar with the whole potential of the computer and the details of programming. Learning a programming language like FORTRAN or ALGOL is perhaps not too difficult but requires a conscious effort in intellectual skills not

too germane to design so that we must

not

expect every

member of a design team to become conversant with such programming languages.

Whenever one is interested in making the computer a

useful, everyday tool for all engineers in a large

organi-zation, rather than relying on the services of a few

computer specialists, introduction of more problem-oriented

languages will be the expedient solution. They will enable

the user to talk to the computer in his own technical

term-inology. CASDOS and AUTOKON point in this direction. In

the AUTOKON code, for example, a single, compact

program-ming statement is sufficient to defihe, practically in

shipbuilding terminology, all the details necessary for the

automatic flamecutting of a floor plate.

The organizational effort required for such languages is considerable. But they nonetheless represent a natural

solution whenever a major programming system is to be made

available to a large engineering organization.

Problem-oriented languages are conducive to efficient sharing of

the work between the systems programmer and the user.

-OUTLOOK

In the preceding, several significant tendencies

have been pointed out with regard to the effects the rapid advances of computer technology may have upon the scope and style of ship design. It depends on the initiative and foresight of our profession to what extent and how soon we shall exploit the new potential. There is no doubt that the successful innovations of other fields will gradually be adopted in ours. But there are encouraging signs that a little more will happen, namely a thorough reevaluation of the methods of ship design, and perhaps the development of a special style of computer use tailored to a modernized

interpretation of the old art of ship design.

-ACKNOWLEDGMENTS

The thoughts presented in this paper summarize in a very condensed way the material I collected last winter

for a new graduate course in "Computer-Aided Ship Design" at The University of Michigan, Reference (18). A grant the Department of Naval Architecture and Marine Engineering had received from the Bethlehem Steel Corporation helped

me during the preparation of my notes. I further want to

acknowledge gratefully the inspiration and advice from my colleagues, especially Harry Benford whose influence this

paper cannot deny, and the constant encouragement I derived

REFERENCES

Benford, H., "Principles of Engineering Economy in Ship Design," Transactions SNAME, 1963

Benford, H., "Fundamentals of Ship Design Economics," Lecture Notes, The University of Michigan, Department of Naval Architecture and Marine Engineering, Ann Arbor, Michigan, 1968

Cheng, H. M., "Performance Comparisons for Marine Vehicles, SNAME New York Metropolitan Section, September 1968

Murphy, R. D., Sabat, D. J., Taylor, R. J., "Least Cost Ship Characteristics by Computer Techniques," Marine Technology, April 1968

Mandel, P., Leopold, R., "Optimization Methods Applied to Ship Design," Transactions SNAME, 1966

Moe, J., Lund, S., "Cost and Weight Minimization of Structures with Special Emphasis on Longitudinal Strength Members of Tankers," De Ingenieur, nos. 47 and 49, 1967, The Hague, Holland

Hooke, R., Jeeves, T. A., "Direct Search Solution of Numerical and Statistical Problems," Journal of the Association for Computing Machines, Vol. 8, April 1962

Kowalik, J., "Nonlinear Programming Procedures and Design Optimization," Acta Polytechnica Scandinavica, no. Ma 13, Troudheim, 1966

Kavlie, D., Kowalik, J., Lund, S., Moe, J., "Design Optimization Using a General Nonlinear Programming Method," European Shipbuilding, no. 4, 1966

Meyer-Detring, D., "Tanker Preliminary Design Economics," SNAME Southeast Section, Miami, September 1969

Chasen, S. H., "Experience in the Application of Interactive Computer Graphics," Section 11, Lecture Notes, Computer-Aided Ship Design, Intensive Short Course, The University of Michigan, Department of Naval Architecture and Marine Engineering, Ann Arbor, Michigan, May 1968

Herzog, B., "Computer Graphics: An Introduction," Section 7, Lecture Notes, Computer-Aided Ship Design, Intensive Short Course, The University of Michigan,

Department of Naval Architecture and Marine Engineering, Ann Arbor, Michigan, May 1968

-Hysing, T., "From Basic Design to Flamecutting," note issued by Central Institute for Industrial Research, Oslo, January 1968

Sorensen, P., "Autokon II, A Preliminary Outline Description," Shipping Research Services, Oslo, July 1969

Nachtsheim, J. J., Romberg, B. W., O'Brien, J. B., "Computer Aided Structural Detailing of Ships," Transactions SNAME, 1967

Cohen, J. B., Gardner, G. 0., Romberg, B. W., "Design Automation in Ship Detailing," Proceedings A.C.M., National Meeting, 1967

Romberg, B. W., "A Computer System for Structural Detailing of Naval Ships," SNAME Chesapeake and Hampton Roads Sections, September 1968

Nowacki H., "Computer-Aided Ship Design," Lecture

Notes, The University of Michigan, Department of

Naval Architecture and Marine Engineering, Ann Arbor, April 1969