Proceedings of the 7th International Symposium on Practical Design of Ships and Mobile Units, The Hague, The Netherlands, PRADS’98, September 1998

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Practical Design of Ships and Mobile Units

Plenary Session Lectures of the Seventh International Symposium on Practical Design of Ships and Mobile Units, The Hague, The Netherlands, September 1998

Edited by:

M.W.C. Oosterveld and S.G. Tan

MARIN - Maritime Research Institute Netherlands, Wageningen, The Netherlands

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The 7th International Symposium on Practical Design of Ships and Mobile Units. PRADS '98, is held from 20-25 September 1998 in The Netherlands Congress Centre in The Hague. Earlier PRADS

Conferences have been held in Tokyo, Seoul, Trondheim, Varna and Newcastle.

The interest in the PRADS Conferences is growing and this time over 200 proposals for papers were received. The final programme includes 126 papers of high quality which are published in the Proceedings of the Conference by Elsevier in the series "Developments in Marine Technology". 250 participants from 30 different countries will attend the Conference. The largest delegations come from The Netherlands, Japan, Germany, Korea, Italy and UK.

Aspect of the Practical Design of Ships and Mobile Units considered in the selection of the papers were Design Synthesis, Production, Hydromechanics, Structures and Materials. and Offshore Engineering.

The organization of PRADS was supported by MARIN, Delft University of Technology, the

Netherlands Organization for Applied Research, the Royal Institute of Engineers in The Netherlands, the Royal Netherlands Association of Maritime Engineers, and the Royal Netherlands Navy.

During the OPENING & PLENARY SESSION of the Conference: Jr J.J.C.M. van Dooremalen, President Board of Directors, IHC Holland

will welcome the participants on behalf of the Netherlands Maritime Industry and the supporting

Organizations. Mr Van Dooremalen is a member of the Controlling Board of MARIN and a member of the Association of Shipyards in The Netherlands.

Representatives of the supporting organizations formed together the LOCAL ORGANIZING

COMMITTEE. They played a major role in the choice of the Technical Themes and the Paper selection. In the Plenary Session the members of the LOCAL ORGANIZATION COMMITTEE will give their vision about the main themes of the Conference. These Plenary lectures are included in this booklet.

PRADS Local Organizing Committee: Dr M.W.C. Oosterveld. Chairman Jr S.G. Tan, Secretary

Prof. Jr A. Aalbers Ir G.T.M. Janssen Jr P.J. Keuning Prof. Dr J.A. Pinkster Mr J. Veltman Prof. Dr J.H. Vugts

M.C.W. Oosterveld and S.G. Tan Editors

MARIN - MARIN

- Delft University of Technology

Netherlands Organization for Applied Research Royal Netherlands Navy

Delft University of Technology Royal Institute of Engineers

- Royal Netherlands Association of Maritime Engineers - Royal Institute of Engineers

CONTENTS

Page

Safety and Ship design 1

A. Aalbers

Production of Complex Ships 10

A. Aalbers

Aspects of model tests and computations for ships and structures in waves 18

J.A. Pinkster and S.G. Tan

Some considerations on design of ship structures and materials 31

G.T.M. Janssen

Offshore technology in perspective 37

A. Aalbers

Delft University of Technology, Marine Technology, Ship Design Mekelweg 2, 2628 CD Delft, the Netherlands

INTRODUCTION

Two topics, which are always in the mind of a

designer, are Economy and Safety. If a design

is not economical it will never be built and if a

design turns out to be unsafe the designer is

responsible. The practising designer expects

from

the scientists

and

researchers,

as

collected here at the PRADS symposium, that

they

will

contribute

to

the

further

development of tools and methods to judge

and improve economy and safety in an early

stage of the design process.

The concerns from the point of view of a

practising designer are given with respect to

safety, which is always under pressure of

economy.

SAFETY

General

The attitude in

society towards safety is

changing

from

prescriptive

rules

to

prescriptive safety levels. It is not sufficient

for the designer to refer to the prescriptive

rules, and it is not possible to refer to rules for

new ship types, because these are not covered

by the rules.

We will first discuss the hazards for a ship.

Hazards

What is the situation with respect to safety of

ship and crew? A sailor in 1890 still had a

chance of almost 4% to lose his ship, currently

that chance is less than 0.5%. In fig. 1 the

causes of accidents since 1939 are given. In

recent years the division over the various

causes is as follows [1]:

Foundering 40%

In 80% of these accidents the primary cause

was, however, a human error. For instance,

,

t.

V

,

-Figure 1 Development of causes of accidents.

bad seamanship, insufficient knowledge of

equipment,

faults

in

procedures

communication errors and bad ergonomic

design.

Compared with

other

transport

modes

shipping is safe in terms of absolute numbers

of fatalities per year. In Europe there are 300

times more fatalities in road transport and 9

times more in rail transport. Aviation

is

comparable with

shipping.

However, per

passenger kilometre aviation is 25 times more

safe than shipping. [2]. In that respect much

can still be improved.

The large

scale of shipping, for instance

cruise ships with more than 4000 persons on

board, requires a much higher safety level

than that of a motorcar with 4 persons on

board. The reluctance of the aircraft industry

to go beyond 500 passengers per aeroplane is

significant in this respect.

Rules and Regulations

Three independent instruments mainly cover

the safety of ships: the Load Line Convention,

the SOLAS Convention and the Rules of the

Classification Societies.

Load Line Convention. The assignment of a

load

line

is executed by

the

National

Authorities.

SOLAS Convention. This convention deals

with intact stability and damaged stability,

lifesaving equipment and fire protection. The

Grounding 32%

Fire 18%

National Authorities

execute

the SOLAS

convention.

Classification Rules. The classification rules

cover the reliability of ship structures and

propulsion systems. The National authorities

and the Insurance companies require the

fulfilment to the classification rules.

We will now critically discuss some aspects of

the current safety rules.

3. FREEBOARD

History

The assignment of a load line to a ship is the

oldest safety measure in shipping, probably

going back to the Middle Ages when Italian

shipbuilders already applied freeboard/depth

ratio's of 0.25 to their ships. Being the oldest

instrument (SOLAS came later

after the

Titanic disaster) all kind of safety aspects

were covered, implicitly or explicitly, by the

Load Line Convention.

Scope of Load Line Conference

During the last Load Line Conference of 1966

a farewell was given to the tradition that the

Load Line Conference covered virtually all

safety

aspects

of a

ship.

Stability and

subdivision (damaged

stability)

were not

taken

into account explicitly,

but

the

following scope of application was valid [3]:

prevention of entry of water into the hull

adequate reserve buoyancy

protection of the crew

adequate structural strength

limitation of wetness on deck

Unscientific approach

Looking back it

is amazing to read the

unscientific considerations. The results of sea

keeping model tests were just becoming more

generally known, but not properly applied.

For instance, the influence of the block

coefficient and the bow flare on the required

bow height was considered in the wrong way.

Our current freeboards are based on those

invalid considerations.

Some other decisions and discussions, which

have their repercussions in our days, will now

be mentioned.

Bulk carrier losses

Based on the favourable statistics of ships

built in accordance with the previous, 1930

Convention it was decided in 1966 to reduce

the freeboard of ships and to delete the

requirement

of a forecastle. The more

reduction for the larger ships, as they were

considered as being above the weather. This,

however, was not accompanied with extra

requirements for hatch covers. The shipping

world was very

satisfied

with

this

contribution to the economy of ships. This

relaxation of the Load Line has contributed to

the large number of bulk carriers lost in the

last 30 years due to damaged hatch covers,

with the Derbyshire as the most pregnant

example.

Freeboard of small ships

Small ships were traditionally assigned a

relative small freeboard. This is based on the

consideration that they will not meet such

severe weather as larger ships on their large

trips. This resulted in ships under the 1930

Convention with very low dynamic stability,

since no explicit stability requirements were

available. Efforts

at

the

Conference to

increase the freeboard of smaller ships to

improve their stability were not successful

because

stability

was not seen as a responsibility of the Load Line Conference. As

a compromise the Conference decided to

encourage the fitting of superstructures to

small ships, with a length of 35% of the ship

length, in order to increase the dynamic

stability of small ships.

Stability of small ships

Still our small ships are sailing with small

freeboards

and

inherent

small

stability

ranges. To comply with the intact stability

requirements this

is

in

the mean time

counteracted

by large

GM-values. These

cause higher accelerations and unpleasant

rolling behaviour in rough weather.

The sailing area of small ships (say below

100m) is extended considerably to worldwide

operations. As a consequence these ships

meet much more

severe

weather

than

anticipated by the Load Line Conference.

This may lead

to more damage

than

anticipated

to

hatch

covers,

ventilation

trunks and other watertight openings. A

captain of what we used to call a coaster, but

now sailing world wide, told that he is

anxious during every storm on the North

Atlantic

that

his

ventilation

trunks

are

smashed in by the waves, figure 2. His ship

fully complies with the Rules but is in fact

only suitable for sailing in restricted waters.

Feedback of seamen

Seamen have not the attitude to complain

about their ships, but they should do this

more to make the ship designer more aware of

the circumstances at sea. The profession can

not accept that a captain of a large bulk

carrier said: 'the more I study the problem of

bulk carrier failure, the less I trust the naval

architects' [4].

A critical re-assessment of the safety of small

ships with respect to watertight integrity is

required, as is the case for bulk carriers.

Scientific approach?

Currently discussions in IMO are going on

regarding new freeboard rules. There seems

to be more room now for a scientific approach,

see for instance [5]. On the other hand in

these circles discussions take place whether a

rule should be given as a formula or in tables,

as if we are still in the times of the slide rule.

Apparently, no discussion is held on which

...tins at SEaWAv-R V -0 knots Japanese mol Chinese method CONTAINIER SHIP LID-IS Cob 0 35).0 65 5% 0% 20

There is no differentiation for sailing area,

but in stead a mean North Atlantic condition

is taken. We still accept 2.35 m green water

on board with a probability of 30% in a North

Atlantic storm on our ships (figure 3).

The consequences can be seen in figure 4

showing a ship in a North Atlantic storm,

designed with a bow height according to the

Load Line Rules and an alternative one with

a calculated bow height based on a rational

requirement for the rough sailing area of the

ship.

Figure 4 - Minimum and rational bowheight

We should encourage direct calculations of

the freeboard and bow height of ships based

on their actual sailing area. When going

outside their design area the draught of the

ship should be adapted for that specific trip.

4. STABILITY

Intact stability

History

The current intact stability requirements of

IMO are directly based on the work of Rahola

in 1939

[6].

Explicit requirements to the

stability of ships were implemented only very

recently (IMCO 1968 conference). Especially

when compared

with

the date

of

the

investigations of Reed into the capsizing of

the 'Captain' in 1870.

The approach of Rahola and subsequent of

IMO is based on a judgement of the curve of

arm of stability for a large number of

non-capsized ships, apparently safe, and non-capsized

ships, apparently unsafe. To remind this

important work

of

Rahola

one of his

statistical graphs is given in figure 5. The

current IMO requirements misses two of

Rahola's criteria, viz, the maximum capsize

angle and the influence of the dynamic angle

of repose of the cargo.

100 20C 300 400 Snip length Lpp (01)

Figure 3 ILLC probability of green water.

,5 10 2

/

IC LL' 1 50 5. 4.0 %

.°P

Old hull forms and proportions.

The stability requirements are based on old

ship types, old hull forms and old proportions,

not necessarily representing current practice.

The introduction of containerships, with the

possibility of cargo on deck, resulted in the

development of stability oriented designs.

These are characterised by high ratios of B/D,

BIT and Cb/C,p. A large number of model tests

at HSVA revealed that these characteristics

have a deteriorating effect on the capsize

resistance in following waves. [7].

Additional requirements

Based on

these

model

tests,

additional

requirements to the curve of stability are

recommended in IMO resolution A749 (18),

applicable for containerships with a length

greater than 100m.

It

took

15 years to

transfer these HSVA findings into Rules. This

is quite an improvement compared with the

findings of Reed, which took 90 years to

become Rule.

It is interesting to note that the old Rahola

criterion on total area of the curve of stability

is re-introduced. This recommendation can

cause a substantial required increase of the

GZ Iml 0,80 0,50 0,40 0,20 0° 20' Fig. 10 Vessel 111 d .5.00 rn form lector . 0.0627

Fig 5 Extra stability required for high B/D [8]

area of the curve of stability. See for example

figure 5 for a comparison of the existing curve

of stability and the new required curve for a

ship with a B/D of 2.45. [8]

Disproportionate dimensions.

There was a comparable trend with B/D, but

in the opposite direction around 1870. For

reasons of probably tonnage rules and fitting

of shelter decks, the B/D ratio in those years

was around 1.3, leading to bad stability and

high losses of ships. Following the explication

of the losses in a paper by Martell concerning

'disproportionate dimensions of steamers' the

B/D ratio rose sharp to a value of about 1.7 in

1900. For non-shelter deckers this

value

remained up to 1960. [9]. From then on there

was a clear further increase up to values

above 2.0 up to 2.5, for stability reasons,

leading to the results as earlier mentioned.

High GM-values

Compliance

with

these

latest

IMO

recommendations, which are requirements in

Germany, can only be achieved for a design

with given ratios, by an increase of the

GM-value. As was the case with the low freeboard

small ships.

Damaged stability

History

The current probabilistic damaged stability

requirements are of recent date, based on

sound scientific principles, but so complicated

that most designers still handle it as a black

box.

Old statistics

Also here the method is based on statistics

which uses the characteristics of old ships. In

the mean time most ships are of double hull

construction, having a much higher calision

resistance

than

the

single

hull

designs

prevailing at the time of the formulation of

the requirements.

High GM-values.

The damaged

stability

rules

require

a

survival index, which is a weighted value of

the index on partial drafts and on the loaded

draft. One of the possibilities to achieve the

required index is to give the ship a high

GM-value

in

the partly

loaded

drafts while

4 . ... ..wet On eg2t .... ... ..ON = MM._ oh. yam 0300..

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i - ----300.. 0,00.

oo-t , _I

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0.000m 0 .. Figure 5 Jr or Nee, ongle0

minimising the GM (containerships with deck

loads) at the loaded draft.

The unforeseen consequence of this rule is

therefore that the safety of the ship in the

partly loaded conditions decreases due to high

accelerations, while the

safety

at

loaded

GM, partial draft b a ---b

GM{

subdivision draft _draft

Figure 6 Damaged stability index [10]

conditions is not improved at all.

Net safety decreased.

The net safety of the

ship is

therefore

decreased due to the damaged stability rules.

The more when one realises that collisions

forms only 10% of the accidents while the ship

has to sail with the unpleasant GM-values its

whole life. Also

here

the

designer

is

stimulated by the rules and under economic

pressure,

to design

ships

with

large

metacentre heights.

5. CONSEQUENCES

Rules lead to high GM-values.

We saw that low freeboard ships leads to high

GM-values, that the extra requirements for

containerships leads to high GM-values and

that

the damaged

stability requirements

leads to high GM-values.

Seaworthy and seakind

These tendencies leading to higher GM-values

have a detrimental effect on the seakindliness

and accelerations of the ship. This creates

unsafe situations for the crew and large forces

on the cargo. We here see the contradiction

between the vision of the Authorities on a

seaworthy ship and the requirements for a

seakind ship from the point of view of the

crew and the cargo.

Proportions.

The best compromise between seaworthiness

and seakindliness (if there is a contradiction

at all) has to be found. This will quite sure be

found with other proportions than the market

shows now. On longer term this insight

should lead to designs with lower B/D or B/T

values and more cargo below deck.

Punishment for safety

This preferred tendency

is,

however, not

encouraged and even punished by the fact

that ship owners have to pay more

harbour-and canal tolls for more safety harbour-and comfort.

Because these are based on the total volume

of the ship (Gross Tonnage). On the one hand

we see Port Authorities encouraging safety

with Green Awards, while on the other hand

they punish (probably ignorant) by higher

costs for ships with more spare buoyancy

(lower B/D) than strictly required. This has

been emphasised on many occasions by the

very experienced designer Ir. E. Vossnack.

For further examples see [11], [12] and [13].

6. CONSTRUCTION

The Rules for the construction of ships have a

sound scientific base, at least for the response

of the construction on a defined load. Finite

element

methods

are

matured and

are

extensively used in the preliminary and final

design phases by yards and classification

societies. The problem still is not the response

of the construction but the loads imposed by

the sea. The previous experience- based rules

did implicitly

take

into account

the

complicated and difficult to predict sea and

operational

conditions. Large

construction

failures are rare in our times. This is true

with the exemption of the bulk carriers with

their vulnerable hatch covers,

ventilation

trunks and bulkheads.

Mismatch between rules.

The hatch cover is a typical example of

a

mismatch between two sectors of the safety

requirements:

the Load Line

Convention

promises a design load of 1.75 m of water

on

the hatch cover when obeying their rules.

This value is taken over by the classification

societies as a design load. In practice this

1A

lit

E

0 GM,

value can be much more. In the case of the

170000 tdwt bulk carrier Derbyshire it

is

assumed that a water head of more than 11 m

has been on the hatch cover, leading to a

Figure 7 Collapsed hatchcover Derbyshire [14].

collapse with fatal consequences [14]

7. LIFESAVING

Safety barriers

Experience learns that disasters occur after a

sequence of

technical

events

or human

mistakes. But it also learns that the saving of

people on board is prevented by a sequence of

mostly technical obstacles. As example the

unbelievable chain of events leading to the

disaster

and

the

unbelievable chain

preventing the saving of people is given here

on hand of the 'Estonia' disaster [15]. If there

are lifebelts on board then we apparently

accept that some time they have to be used.

Design to enable people

to

be saved

is

therefore important.

Events leading to accident:

Inadequate methods to calculate the wave

loads on the bow visor

Inadequate methods

to

dimension the

construction of the bow visor

Misunderstanding between Class and Flag

Authority on approval procedures leading

to a non-approved visor locking device

No action undertaken after problems on

sister ship with bow visor

Position sensors for signal lamps on wrong

position giving false indication

Seamanship error by persisting on too

high speed after suspect noises were heard

Miscommunication between crew when

reporting the apparent failure of the bow

door

Design

fault:

Second door serving

as

second barrier connected to the bow visor

which also opened when bow visor failed Events preventing people to be saved:

Very fast achievement of large list

Breaking

of windows

near waterline

causing progressive flooding

Black out of main engines caused by list of

30°, no possibility any more to manoeuvre

the ship in better position

Black out of auxiliary engines at angle of

40°, no more lighting and communication

Evacuation very difficult: under large list

transverse stairs became almost vertical

passageways with width of 1 m became 1m

high

Break down of handrails in alleyways and

staircases under the weight of hanging

people

Loose carpets, floors and equipment falling

on people

Impossible to launch boats and rafts under

a large list

Capsizing of life rafts under influence of

wind and waves

Some people survived all these horrors but

died when at last their life raft ruptured

when hoisting on board saving ship

As known IMO has reacted very fast on this

large accident and has given new rules in the

mean time.

8. INTEGRATED SAFETY

Balanced safety

The current Rules and Regulations are not

sufficient based on state-of-the-art scientific

principles and are not related to each other.

This leads to unbalance in the approach of the

various safety aspects of a ship and even to

unsafe

ships. Also

the

introduction

of completely new ship

types

like

fast

catamarans and the introduction of very large

cruise ships makes in necessary to critically

judge the current approach of safety

in shipping.

Two things are important in my opinion to

improve the current set of safety measures,

one concerning the contents of the safety

measures, one concerning the methodology:

Simulation of ships in operational and

extreme conditions using state of the art

knowledge and computer codes

Application of a structured and balanced

methodology known as Formal

Safety

Assessment

Simulation

A very

interesting

and

promising

development is the introduction of simulation

tools for the assessment of the behaviour of

ships in

extreme

conditions,

including

manoeuvring in

waves. At MARIN the

program Fredyn has been developed for some

Navies for the evaluation of new hull forms.

[16] With this simulation tool so far difficult

to

predict

dynamic

phenomena

like broaching,

stability

in following waves,

broaching and parametric rolling can now be

handled.

It was even possible

to

predict

unexpected parametric

rolling

of a large

cruise ship under moderate condition [17].

With a large number of simulations in various

sea states the experience which Rahola had to

gather from accidents in the past can now be

gathered before accidents take place.

The responsible designer of commercial ships

knows these phenomena but so far no tools

were available to predict them. While it is

known that the current regulations do not

take these phenomena into account this is a

big worry for the designer, and perhaps also

for the Authorities. We have to do the utmost

to get these methods available in day-to-day

design work.

But

in

any case

for

the

judgement of new or unusual ship types and

operational conditions.

jnhlr,

13

Total Dynamic Stability Im-rad]

Figure 8 Capsize index as function of dynamic

stability [16].

Formal Safety Assessment

The Formal Safety Assessment (FSA) is due

to its complexity not suitable for day-day to

day design work [18]. The FSA methodology

consists of five steps viz,

identification of

hazards, risk assessment, risk control options,

cost benefit assessment and decision making.

A team of experts executes the identification

of hazards and the risk assessments. The

quality of the judgement can therefore very

much depend on the level and experience of

these experts. The main application of the

FSA-approach will be in the development of

consistent rules.

For designs strongly deviating from current

practice this approach, in combination with

scientific methods, is the only way to judge

the safety level of a design.

New is

the

explicit use of cost/benefit

evaluations. Apart from the selection of the

best alternative, also in economical respect,

there is sense in an example referenced by

Krappinger [19], that the use of too much

steel in a ship, leads to more risk for human

life as steel has to be mined and produced in

dangerous situations.

9. CONCLUSION

Shipping

is a safe

industry. But things

change fast. Faster than in aviation where

types are produced for more than 20 years.

In shipping we see the advent of fast ships,

very large passenger ships and ships with

new hull forms operating worldwide. We have

to be careful

and aware

of these new

developments to maintain the high required

safety level.

We have seen that the various rules intend to

provide safe ships, but that they currently

=1

RISK CONTRIBUTION TR',

lead to sub-optima per safety instrument like

load line,

intact

stability

and damaged

stability, and that even the safety on other

aspects, seakindliness, can deteriorate.

This is especially

the

case

with

the

deteriorating effects of damaged and intact

stability requirements leading to higher

GM-values on the seakindliness.

A plea is made to use advanced methods to

simulate the behaviour of ships in operational

conditions. The reliability of these programs

is sufficient and in any case better than the

approaches based on statistics based on old

ships or rules of thumb (load lines) which are

currently used.

A further plea is made to integrate all safety

aspects of a ship using the FSA-methodology

for the calibration of the rules and for the

assessment of unusual ships.

It

is advised

that

designers

use

these

advanced methods on a voluntary base as

long as these have no statutory base.

The influence of punishment on extra safety

due to

the Gross Tonnage measurement

system should be deleted. A more rational tax

method should be invented.

It might be good to re-introduce the old

approach of the Load Line Conference, which

tried to cover all aspects of ship safety. But

now on

a scientific

base and including

seakindliness and economy.

REFERENCES

I. Clingan, 'Safety at Sea-Its Risk

Management', Bulletin de l'AISM/IALA, 1986/1.

'Priority Measures for Maritime Accident

Reduction', European Transport Safety

Council, August 1997.

D.R. Murray Smith, 'The 1966

International Conference on Load Lines',

Quarterly Transactions, RINA, Vol.111, No.

1, January 1969.

M. Grey, 'Creating a storm over how to

handle ship Lloyd's List, June 8, 1994.

H. Meier, C. Ostergaard, Zur direkten

Berechnung des Freibordes far den

Schiffsentwurf, Jahrbuch STG, 1996.

J. Rahola, 'The judging of the stability of

ships and the determination of the minimum

amount of stability', Helsinki, 1939.

P. Blume, 'On the influence of the

variation of righting levers in waves on

stability requirements'.

H. Hormann, D. Wagner, 'Stability

Criteria for present day ships design', 3rd

International Conference on Stability of Ships

and Ocean Vehicles, Gdansk, 1986.

J.M. Murray, 'Merchant Ships, 1860-1960',

RINA, 1960.

H. Stevelt, J. Dalgaard, 'Design and

optimisation of containervessels'

E. Eisenhardt, Termessungs-formeln

und Vermessungs-vorschriften, ihr Einfluss

auf den Entwurf von Schiffen'. Schiffstechnik

3, 1955/56.

A. Aalbers, 'Ontwerpen van Schepen',

Inaugural Lecture, Delft, 1996.

E. Vossnack, 'Developments in Ship Design', Delft 1997.

R.A. Williams and R. Torchio, `M.V. Derbyshire Surveys. UK/EC Assessors' Report. A summary. 1998.

The Joint Accident Investigation

Commission of Estonia, Finland and Sweden,

Final report on the capsizing of the Ro-Ro Passenger Vessel MV Estonia'

J.O. de Kat, R. Brouwer, K.A.

McTaggart, W.L. Thomas, 'Intact ship

survivability in extreme waves', 5th

International Conference on Stability of Ships

and Ocean Vehicles, 1994.

R.P. Dallinga, J.J. Blok, H.R. Luth,

'Excessive rolling of cruise ships in head and

following waves', RINA International

conference on ship motions and maneuvring,

1998.

'Interim Guidelines for the Application of

FSA to the IMO Rule-making Process'. MSC/Circ.829, 1997.

0.

Krappinger,

Zusammenhange

zwischen

der

Wirtschaftlichkeit

und

Sicherheit von Schiffen, Institut fUr Schiffbau

der Universitat Hamburg, 1967.

Production of Complex Ships

A. Aalbers

Delft University of Technology, Marine Technology Mekelweg 2, 2628 CD Delft, the Netherlands

INTRODUCTION

It

is

a human joy to

create things, and

especially moving things. Perhaps therefore

shipbuilding is a fascinating activity. The

creative process from the first design sketch

through

engineering,

cutting,

welding,

launching and seatrials is completed in a

relative short time and the people involved

can closely follow

the creation

process.

Parents working in shipbuilding have no

problem to explain their children what they

are doing. This is a reason, be it not an

objective one, to strive for the preservation of

shipyards in our expensive countries. It will,

however, be shown

that

despite severe

competition there are objective reasons for

shipbuilding in 'expensive' countries.

We will try to find this based on the cost

structure

of complex ships.

It

will be indicated which

technical

and

other

developments are necessary.

SHIPBUILDING

The following analysis

is

based on the

situation of shipyards in the Netherlands [11

The building of complex ships is a

one-of-a-kind industry with short lead times. This in

contrast

to

the

automobile or

aircraft

industry which are massproduction industries

with long productlifecycles. This means that

the approach of a shipbuilder is difficult to

compare with the approach in other sectors of

industry.

Activities

The following

activities

are

required

to produce a ship:

Marketing: to identify which people are

interested in which type or size of ship

(shipyard approach) or which shipper is

interested

in

transporting

goods (shipowner approach)

Acquisition

and

design: to come on

speaking

terms

with

the

identified

shipowner or shipper and to translate his

requirements in a design proposal with a

price and an operational performance

Engineering

Purchasing and subcontracting

Production of steel (aluminium, GRP etc)

Outfitting

Commissioning

Shipbuilding is still strongly associated with

the production of

the

steel

hull. The

shipbuilder's first interest sometimes seem to

be to have his yard producing steel. But, the

raison d'etre of a shipbuilder is to produce

ships for the market.

The first interest for a shipbuilder should

therefore be to deliver attractive ships for the

market, off course at an attractive price.

Production is therefore only one part, be it an

important and expensive part, in the chain of

necessary activities to realize a ship.

Whether the various mentioned activities are

combined in one company, the shipyard or

divided over various companies, depends on

country and markets. In some countries it is

usual

that

design

and

engineering

are

performed by design agencies on behalf of the

shipowners. The yard has to produce the steel

as a 'jobber' with no input in the design.

This has resulted sometimes in expensive

designs due to insufficient knowledge of the

factors which determine costs. A part of the

'design for production' hype can be attributed

to this insufficient knowledge resulting in

unneccessary expensive designs for instance

with for welders unaccessible constructions or

with too small engine rooms.

Design for production is therefore a derivative

goal and should not be a main goal. The

shipowner is not interested how his ship is

produced as long as the ship is of good quality

and as long

it

is cheap. For

instance

production-friendly bulbs are not acceptable

any more to shipowners when they have a

lower hydrodynamic performance.

When all mentioned activities are combined

in one company we have a complete shipyard.

The advantage is that all elements which can

lead to a successful ship are in one hand and

no sub-optimization takes place. But, more

and more there is a tendency to separate the

activities,

and to

outsource

as much as

possible for cost or strategic reasons.

What then remains as core at the end are

marketing,

design and purchasing.

Even

engineering and outfitting are outsourced.

Whether this

is

to be desired or this

is

possible depends on the type of ship and the

environment

in

which the

yard has

to

operate. Let us have a look at the

West-European and, more

specific,

the Dutch

situation.

Environment

Labourcosts. Like

in

all

West-European

countries the cost of labour is high in the

Netherlands. The economical threat of the

emerging shipbuilding countries is clearly felt

in

the Netherlands.

Therefore

gradually

everybody is realizes that it is important to

work hard, to be inventive and

to use the

advantage

of experience and

technical

infrastructure

to keep

shipbuilding

and

shipping a healthy and important part of our

industry.

Productivity. Also in connection with the high

labour costs,

the

mechanisation

and

productivity in the Netherlands are among

the highest in the world. The social structure

is sound, with only very few labour strikes.

Education. The level of education is generally

high. It is not easy to attract talented young

people for shipbuilding or shipping, which

still has an image of low-valued labour and of

uncertainty of jobs due to past experiences

with

closing

yards.

This factor is also

encountered in Japan, where the shipbuilding

industry has the image of what they call the

three K's, of dangerous, dirty and difficult.

Even in Korea the managers of shipyards are

complaining that the brightest young men are

not choosing any more for a job in

shipbuilding, but in other parts of industry

like electronics. This situation will also come

in other countries climbing in their state of

industrialisation.

Tradition. It is an advantage for countries

when shipbuilding is

not a first

step in

industrialisation, but a long existing and

embedded part

of industry, with

strong

homemarkets for e.g. fishery, inland shipping

and transport of goods and persons.

Infrastucture.

There is

an

excellent

infrastructure

in

the

Netherlands

for

shipbuilding with education and research

institutes and local shipowners with modern

and specialized tonnage.

Size of shipbuilding. The size of the maritime

industry is internationally seen small, and

close to

a critical mass, below which no

sufficient research and developments in the

field of production technology can be

performed. Each individual yard certainly can

not perform sufficient research on its own.

How to be competitive?

Years ago the question was raised how to be

competitive in the sketched environment. The

succesfull yards in the Netherlands adopted

one or more of the following measures:

concentration on complicated one-off 's

high productivity through organisation,

involvement of all people in the company

high productivity through mechanisation

Complex one-offs

The Dutch

shipbuilding

industry

has

succeeded to concentrate on complicated

one-off 's. This is one-off course supported by a strong

home-market in the Netherlands for complex

ships. This follows from for instance the

AWES statistics of delivered ships expressed

in gross tons and in compensated gross tons.

The

relation

between gross

tons

and

compensated gross tons is an indication for

the size and complexity of a ship. Where this

factor for

the total

world

shipbuilding

production amounts to 0.65 to 0.75, this factor

is 1.8 to

2.4 for the Dutch shipbuilding

industry.

With complicated one-off 's it is normally so

that the direct labour costs constitutes a

small part of the total price of the ship, while

on the other hand the design and engineering

with high qualified people forms a substantial

It is the quality of the involved manhours

which is decisive for the competitiveness. Also

with ships with a larger part of the cost price

consisting out of manhours, it is possible to be

competitive,

if

the

required

design or production know-how is available. For

example, if the know how to weld a duplex

stainless steel is available, it takes still more

time to weld stainless steel than normal steel,

but if the know hoe is not available, it takes

very much more time, with still insufficient

results.

Examples of complex one-offs are deepfreeze

trawlers,

suppliers,

cablelayers, hopper

dredgers, passengerferries.

3. PRODUCTION COSTS

Costcomponents

We will now investigate the cost structure of a

complex ship and try to identify the relative

importance

of the

various systems

and

activities. An overall view is given in fig 1 at

the end of this paper.

The first significant division can be made

between labour costs and material costs.

Labour- and materialcosts

About 70 percent of the price of the ship

consists of materialcosts, but this can also be

80 percent or more. This depends on the costs

of the equipment, the extent of subcontracting

and the state of mechanisation/robotization at

the yard.

The material costs

in

this example also

includes the material- and labourcosts of

sub-suppliers for conservation, airconditioning,

electrical installation, insulation. This can

differ appreciably per country or per yard.

The influence of

the

material-

and

subcontracting part on the total price is high.

Influence of subcontractors. An important

consequence is that a shipyard can influence

to a large extent the manhour costs of his

sub-contractors. A good organisation of the yard

allows sub-contractors to offer lower prices. It

is preferred more and more to talk about

co-makers instead of suppliers.

Influence of specification.

The correct specification of a ship is very

important

for

the

costprice. An overspecification

of 5%,

which is easily

achieved, leads to a costprice increase of 3.5

%. A lot of efforts should therefore be put in a

correct specification. Fortunately it turns out

that not more than say 20 single items (main

engine, propeller, winches, cargo equipment

etc) are responsible for 80 % of the material

costs. For

materials

and

equipment

in

principle worldmarket prices have to be paid.

This means that complex ships with their

large equipment content, are less sensible for

manhour rates

than

ships

with

a low

equipment content.

Manhours per system

We will further elaborate the manhours. The

total manhours are split over the various

functional elements out of which a ship is

built up like hull, equipment, accommodation

and machinery and activities like design,

engineering and transport (sometimes called

overhead).

This turns out

for a

typical

Steel. A shipyard is often identified with steel

production, and the impression exists that

steel

production is

the

most important

activity of a shipyard. This is clearly not the

case for complicated one-off's, as follows from

above table. The variation of the importance

of manhours steel for various shiptypes is

shown in the following graph [2]. The more

complex the ship the less important is the

influence

of the

steelproduction

on

the

costprice of the ship.

Influence of Design

Remarkable are the low design costs for a

ship, which is less than 1%. The influence of

the design on the costs is on the other hand

very high, as we already saw when discussing

example as follows:

Steel hull

50

Equipment

10

Accommodation 5

Machinery & Systems 15

Design <1

Engineering

10

General 10

the specification. Some other examples are

given here taken from [3]. The following

examples give cost items which are equivalent

Figure 2 Relative part of steel.

to one manyear designer

one extra ballasttank

100 kW of propulsionpower

20 t of extra steel

deadweight penalty of 5 t

50 m2 of extra accommodation

It pays off therefore for a yard to have a good

design team (which is not the same as a big

design team).

Steel Hull

The steel hull of a complex ship still has a

large contribution to the total weight of the

ship, but attributes for less than 50 % of

manhours.

Fig.3 Hull as collection of mainly flat panels

It is not often realized that by far the largest

part of the steel consists of flat surfaces, and

only about 10 to 20 % of curved surfaces. This

is shown in figure 4. In this graph with a

large variety of shipforms ranging from

Cb=0.58 to 0.78 the division of the various

surfaces is given.

100%

division of plate areas

Figure 4 Division of plate areas.

E Shell curved Shell nat !U Bulkheads

0 Girders In db Decks

Comparison between a complicated and a

simple

ship. A comparison between the

involved costs for a complicated ship, for

instance a deepfreeze factory trawler and a

simple ship, for instance a tanker, as given in

the following graph. This reveals that the

costprice of the trawler consists for only 15 %

out of manhours steel, while for the tanker

this part is about 40 %. (This is under equal

circumstances. From [4] it is known that at

Odense this

figure is

far lower through

massive robotization).

Figure 5 Comparison trawler and tanker

Consequence for investments

This implies that an improvement of 10 % in

steel production for a trawler would give a

total cost reduction of 1.5 %, while the same

improvement for a tanker would result in 4 %

costprice reduction..

The approach of sectionbuilding

and

robotization has to be seen in this perspective:

steel production forms a relative small part of

the costprice of complex ships.

This means that the price at which it

is

economical to invest in for instance robots, is

lower for these yards than for bigger yards

producing much more tons of steel per year,

for ships with a much larger steel part. Apart

from considerations of sizes

of robots

in

relation to the size of sections of

medium-sized ships.

Steel production

We will further zoom into the steel production

activities, and give the relative importance of

the various activities. For that purpose ships

of various types and sizes

are analysed,

ranging

from

reefers

to

stainless

steel

chemical tankers.

Sectionbuilding and assembly

The division between the building of

3D-sections in the steel factory and the assembly

of the 3D-sections on the slipway or in the

buildingdock is an indication of the size of

plates and the weight of sections which a yard

can handle.

sectionbuilding

80%

assembly

20%

total steel

100 %

Allthough the weldingmeters in the assembly

phase are much less than those in the

sectionbuilding phase, still 20 % of the steel

hours are spent in the assembly phase, with

manual fitting of sections to each other and

welding them together

in often confined spaces.

Sectionbuilding

The sectionbuilding consists of

the

preparations of the steelplates and profiles

like cutting and bending, the composition of

subassemblies like stiffened platefields and

webframes, the composition of the 3D section

out of the subassemblies (the fitting and

tackwelding) and the ens welding of the 3D

section.

preparations

15 %

subassemblies

10% webs & frames 10% fitting of sections

35%

welding of sections

20%

total

100 %

Much attention has been paid in the past to

the development of numerical cutting,

one-side-welding

techniques,

panelstreets

for

large and small panels and the fabrication of

subassemblies like webs.

But, still 65 % of the sectionbuilding consists

of

manual

work:

the

fitting

of

the

subassemblies like bulkheads and decks and

the end welding of the 3D- sections, which

takes place in mostly closed and confined

spaces. This is a large amount of handwork

which could potentially be handled by robots.

This should be considered not only for cost

reasons,

but

also for

strategic reasons,

because in the future it will become more and

more difficult to find people willing to enter

confined and enclosed spaces.

Preparations

A further look into the preparation phase

learns that the largest part of the preparation

time is

spent to cutting plates, including

handling and transport.

Bending of plates and profiles shares for 25 %

of the preparations.

The relative part of the preparation activities

in the total costprice is small. For instance

the bending of plates and profiles as part of

the total costprice amounts to (see fig 1 going

from the bottom up)

(0.15 + 0.10)*0.15*0.80*0.30=0.009, : abt 1 %

It is, given the very small influence of the

curves plates on the total costs, not wise to

design a productionfriendly hullform if this

leads to a higher fuel consumption.

It must however be emphasized that large

overall cost savings are achieved by putting

much efforts in these relative small activities

like forming of plates and stiffeners.

The 'bottom-up' influence is much higher than

their part in the costprice would suggest.

High accuracy of

this

elements

greatly

simplifies further fitting and welding. It is

therefore

very costeffective to

invest

in

14

cutting plates

50 % cutting profiles

25%

bending plates

15% bending profiles 10%

total

100 %

improvement of accuracy of shellplating and

frames. Accuracy is further a prerequisite for

eventual introduction of robots or automats.

Real working hours

We have seen in the preceding paragraphs

the division of the hours over the various

aspects or processes.

A last division which can be made, is between

the hours spent for the actual job and all the

hours a welder or fitter has to spent before he

can start is actual job.

In the following figure (from [5]) a division is

given of the activities of fitters and welders on

a Japanese shipyard, divided over the aspects

preparatory, adjustments and main. It turns

out

that

'even' in

the

highly organized

ems.stsrs as is.

So

Figure 6 Division of fitting and welding.

S

Japanese shipyards only one third of the time

is used for the actual job.

This fact of the low usage of machines is not

new. Already in 1921 Wilfrid Ayre [6], was

complaining that his punchingmachine could

not be used for more than 25 % of time, for

reasons

of

insufficient

materialflow

etc.

Apparently not much has improved therefore

in almost 80 years.

Motivation and organisation

Technicians have the attitude to look at

measurable,

mechanical

quantities

like

weldingmeters per hour or tons per day, but

perhaps the largest savings can be made at

the lowest costs in the improvement of the

division of activities of a fitter or a welder, by

preventing that he looses time with looking,

adjusting, checking. This should be improved

by providing high accuracy of components,

motivation and (self)-organisation.

An interesting view on the last mentioned

aspects was given at the ship production

symposium in Delft in 1992, by the Dutch

shipyard IHC [7].

DEVELOPMENTS What has been done?

So far much attention has been paid to the

first stages of the production process, namely

the

computer

generated

production

information, the preparations, the production

of

subassemblies

and

the

panellines.

Although

there

is

still

room for

improvements, especially in the intermediate

transport and storage, quite a lot has been

achieved in the preparatory phases.

Focus from preparations to assemblies

It is now time to focus on the second phase of

the buildingprocess,

the

fitting

of

the

subassemblies and the endwelding, which

amounts to 60 to 70 % of the sectionbuilding

hours. This

is

still

mainly handwork in

confined spaces.

Current

robots have

insufficient room to enter the confined spaces

of medium sized complex ships.

Focus on increase

of

real working hours

There is much room in the increase of real

working hours by motivation, organisation

and accuracy.

CONCLUSIONS Influence

of

steel

manhours steel counts for only 15 % of

total costs of complex ships

influence of curved surfaces on costprice is

small

influence of production accuracy of curved

surfaces on costprice is high

Influence

of

design

cost of design less than 1%

influence of design on cost very high Design

of

construction for production:

good accessibility for endwelders

reducing the number of parts

reducing difficult connections Accurate production:

controlled

heat

input

for controlled deformations

high accuracy of

frames

and

plates

required

reduce non

accurate

elements

like bulbprofiles

Flexible automation:

broadly based robotization not yet feasible

for economic reasons for small to medium

sized yards

handy portable tractor semi-automats for

use in confined spaces

further robotization of production of

sub-assemblies when the price of the robots

allows such

REFERENCES:

H.A. Boer, A. Aalbers, 'Approach of Dutch

Shipyards

to Steelproduction', VVEMT

Conference, Madrid, 1993

H. Kerlen, `Methoden und Verfahren der

Schiffbaufertig,ung Ziele und Aufgaben fill'. die

90er Jahre', Jahrbuch STG 79 (1985).

J. von Haartman, I. Kuutti, C. Schauman,

'Improving design productivity with a Product

model for initial ship design'

'The Yard View', 100A1, 3, 1993.

M. Yuzaki, et al, 'An approach to a new

ship production system based on advanced

accuracy control', Journal of Ship Production,

Vol. 9, No. 2, May 1993

W. Ayre,

'Organization

for Ship

Production', Trans. NECIES, Vol. 27,

1920-1921.

J. van Sliedregt, 'Improving Productivity

in

Shipyards',

First

joint

conference on

marine safety and environment and ship

production, DUP, 1992.

Section Building

'reparation and Subassembly

Sections and Assembly on Slipway

assembly

20%

fitting and welding webs

10%

fitting and welding frames 5% welding plates 5% bending plates 10% cutting plates 30%

Figure 1 - Division of production costs

Materials and Labour

fitting sections 25 %

preparation and subassembly 35%

cutting and bending frames

25 % _{production webs} 20% small panels 10% hull 50% welding sections 25 % materials 70% man hours 30% accommodation

Division of Man Hours

5 %

machinery and piping

15 % equipment

design 10 %

1% general and engineering

19%

section building 80%

Aspects of model tests and computations for ships and other structures in waves

J.A. Pinkstera and S.G. Tan'

a Delft University of Technology, Ship Hydrodynamics Laboratory

Mekelweg 2, 2628 CD Delft, .The Netherlands

Maritime Research Institute Netherlands, Research and Development Department P.O. Box 28, 6700 AA Wageningen, The Netherlands

1. INTRODUCTION

Knowledge on the loads on, and the behaviour of ships and other floating equipment in waves may be required for a variety of reasons related to the

structural design or the operations of such

structures. See Figure 1.

SHIP CHARACTERISTICS SEAKEEPING TOOLS SEAKEEPING OPERATIONAL CHARACTERISTICS CRITERIA DESIGN ASSESSMENT OPERATOR GUIDANCE SEA CONDITIONS

Figure 1. Seakeepina performance analysis

Regarding structural loads, we are concerned with extreme loads which a vessel may have to withstand during its operational life time on the one hand and on the other hand the fatigue loads leading

to the accumulation of fatigue damage.

The expected extreme loads are the prime input

to the structural design of a vessel, besides of

course the input based on the operational demands placed on the vessel. Extreme loads on ships are associated with high sea-states and often with large motion amplitudes. As a consequence non-linear

effects, i.e. that loads and motions are not a linear

function of the wave amplitudes and

that the

frequencies present in the wave loads contain

super-and subharmonics of the wave frequencies, can

become important.

Expected fatigue loads tend to have an impact

on details of the structural design and will not

dictate the overall structural design to the same degree as the extreme loads. Increasingly, however, information on fatigue loads is required as the

structures tend to become lighter and less material is used. This requires that statistical data on the

frequency distribution of load oscillations be

developed for short and long term. Short term in this sense being the statistics related to a particular sea-condition and long term being the statistics of

the loads as related to the lifetime of the vessel.

2. EXTREME BEHAVIOUR OF SHIPS Besides information on the loads due to the continuous action of waves on the structures of ships, information is required on the loads due to extreme events such as slamming and green water. The loads due to slamming are highly non-linear when related to the wave amplitudes and the

frequencies associated with the load oscillation after

an initial wave impact are related more to the

frequencies of the vibratory modes than to the wave

frequencies.

Higher sea-states and larger ship motions may lead to the occurrence of green water on deck.

While this is also

one of those

undesirable

occurrences which, as slamming, is avoided as much as possible, the effects of green water on deck

are potentially so much more dangerous for the crew, the ship and its cargo. Therefore not only is

the probability of occurrence a focal point but

increasingly attention is being paid to the actual behaviour of water on deck and to the effects in

terms of water heights, velocities on deck and

impact pressures on superstructures. Research in this field is aimed at, among others, more rational

design of wave breaking barriers on deck.

For ships at sea the influence of waves on the resistance and propulsion characteristics

are of

importance from the point of view of economy and

the time taken to reach the destination. The

resistance characteristics of a ship are traditionally based on the stil water resistance with corrections for the in-service condition of the hull taking into account in an approximative way, the resistance

increase in waves. Nowadays, more detailed

information on such effects are required in the design stage in order to be able to asses influence of changes in hull form, loading condition, course and speed etc. on the speed loss in waves. See Figure 2,

Blok (1993). 1.0 0.8 0.6 0.4 0.2 0 0.45 (Frigates) 040 cs/Tm<0.25 A c .0 65 (Ferries) o.a. 0.28.<6S /T M<0.6.0 '--006.0.85 (Tankers) 0.50<cr /T s m A a

Figure 2. Thrust increase in waves. Blok (1993)

Motions of ships in waves can be influenced significantly by non-linear effects. A well known aspect in this sense is the rolling of ships in waves

coming from off-bow, off-stern or beam directions.

Another effect which has become more important in recent times is the occurrence of parametric rolling of ships which can occur in head or stern seas. See

Dallinga et al. (1998). This phenomenon is

especially of importance for cruise vessels. See

Figure 3.

Broaching effects of ships in stern or stern-quartering seas have always been of concern to designers and operators. The behaviour of a ship under such conditions is a complex combination of the effects of wave-induced motions and forces on the one hand and on the capacity of the vessels steering system to counteract these forces and the lateral stability of the vessel on the other hand. The onset of broaching and the subsequent behaviour in which large yaw accelerations and course changes

along with large roll motions can occur are elements

of highly complex and non-linear flow phenomena.

See, for

instance, De Kat

(1994). Broaching

becomes of greater importance as the speed of ships

increases. As such, the arrival of large, fast

passenger ferries in sea-areas with significant wave

action are

a cause

for extra concern. Recent

experiences with such vessels has shown that beside broaching, a relatively new phenomenon,

nose-diving, can occur when fast vessels of the catamaran

type, which have relatively small waterplane areas, travel at high speeds in stern or stern-quartering

seas.

With respect to high-speed craft, planing or semi-planing craft form a group on their own. The behaviour in waves of these vessels is heavily

influenced by non-linear effects. At high speeds, high acceleration levels may be reached which can lead to structural damage or personal injury of crew

and passengers.

0.2 0.4 0.6

WAVE 3 PITCH deg ROLL deg FIN ANGLE deg RUO ANGLE deg 15. 15. .II

I1

1/.1, I !I I 11111 t

"

' .1 ' I I -

,./vvv\C

Figure 3. Parametric rolling in stern waves. Dallinga et al. (1998)

3. FLOATING OFFSHORE EQUIPMENT The exploration for oil and gas deposits and the subsequent field developments which took place demanded new floating equipment for the purpose of carrying out new and novel tasks. Many of the aspects of working at sea made use of knowledge and experience gained from the operations of ships and other existing floating equipment. However, the scale and diversity of the activities also demanded equipment and methods to be developed for which no precedents existed. This led to new procedures for the design of floating equipment which rely much more on the application of knowledge of fundamentals regarding hydrodynamics, strength of materials and structures and assessments of the fatigue life of structures than had previously been the case with ships. Much of the knowledge gained over the last thirty years is now being incorporated

in design guide lines and rules.

Floating offshore equipment is required to

work in a variety of conditions and for a wide range

of applications.

Early on in the life-cycle of an offshore oil or gas field, exploration drilling followed by early

production may be carried out using a

semi-submersible drilling rig which is moored on

location by means of anchor lines, or, in case of deeper water, by means of dynamic positioning. The wave-induced motions will be of interest from the point of view of the drilling operations. At the

design stage special attention is paid to

minimisation of the wave-induced heave, roll and

pitch motions

of the

platform in operational

conditions and to the 'air-gap' in survival

conditions. The mooring forces are a result of the

wave, wind and current forces

acting

on the

structure. Wind and current forces contain both lift and drag effects which, beside containing constant parts also contain sub- and superharmonic force fluctuations associated with flow instabilities. Wave forces consist of wave frequency force fluctuations leading to the well known wave frequency motions

and also contain mean and low frequency

components which contribute to the mooring loads. The latter force components contain non-linear