Technological and Economic Implications of Mega-Container Carriers

Hans G. Payer, Ph. D. (M)Germanischer Lloyd, Hamburg

Figure 1, Analysis model of 5500 TEU Post-Panamax Carrier

ABSTRACT

Economy of scale has driven the development of container shipping right fromits beginning. The trend towards larger ships has accelerated in recent years andcan be observed with the increasing size of long haul as well as feeder vessels.

The largest Post-Panamax container ships today have a carrying capacity ofover 7.000 TEU and ships with 10.000 TEU are on the drawing board. Thistendency towards larger ships is supported by a continuing healthy growth incontainer volume practically on all major trade routes of the world. Looking intothe future the limits for container ships from a technological point of view are seenat a ship size carrying 15,000, or 18,000 TEU with the “Malacca-Max” containercarrier.

Post-Panamax ships present special challenges to designers as well as to safetyregulators. After a brief overview of the success story of the container ship and alook at the possible future developments and limitations, design and safety aspectsas well as economic implications of the new generation of containerships areaddressed.

1 GENERAL

Container transportation has become the mostsuccessful branch in the history of shipping. Introducedin the mid 1960ies, a new breed of purpose builtcontainer ships was introduced and has since taken oversuccessively a good portion of the general cargo tradeworldwide. Container ships have shown the most

pronounced growth of any ship type in the past decades.The first container carriers of the early sixties were

general cargo ships converted to carry containers ondeck and in the holds. Very soon the pure containership with large deck openings for vertical loading andunloading of containers in the holds was introduced.Since then the container ships became bigger and moreefficient. The idea of shipping cargo in locked

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containers has been widely accepted, resulting in anuninterrupted growth in container shipping, continuinginto the beginning of this century. It was totallyappropriate for Lloyd’s List at the end of the year 1999to appoint Mr. Malcolm McLean one of the ThreeMen of the 20th Century. With the trucking companySea-Land at the time, he changed the face of dry goodsshipping most fundamentally. With his notion of astandardised container for ship, rail and truck transporthe totally transformed transportation, therebyaccelerating world trade.

The container success story continues. Todaycontainers reach the most remote corners of the world.With the East - West container traffic already highlydeveloped and continuing to grow, the North - Southtraffic is now getting more attention from containershipping companies.

The world container fleet has shown a remarkablegrowth. Today the fleet of 2600 fully cellular containerships with a total of 55 million gross tonnage accountsfor about 11% of the world merchant fleet tonnage.Only six years ago this share was at around 5%.Roughly 70% of general cargo is already containerised.It is expected that by 2010 this will grow to more than90%.

The efficiency, safety, cleanliness and economy ofcontainer transportation is so convincing that almostany cargo will stay with the container concept, onceshippers have seen and become used to the benefits.This, together with the growth of world trade has beenthe basis for the phenomenal success in containertransportation.

2 TECHNICAL IMPLICATIONS

The development of container ships has beencharacterised from the very beginning by effortstowards the optimisation of the ship design with respectto the possible number of containers to be carried, theefficiency of loading and unloading of containers and bya steady increase in the size of the ships. Because of thespecial strength and stiffness problems of containerships all developments are being done in close co-operation of the designers with the classificationsocieties.

While the so called container ships of the firstgeneration constructed more than thirty years ago in thelate 1960ies were still confined to the carriage ofapproximately 700 TEU (Twenty Feet Equivalent Unit),the first container ships of the third generation carryingup to 3000 TEU were delivered already in 1972, [1].These ships had Panamax dimensions (length 285 m,breadth 32.2 m) and are only beginning to be phased outtoday.

Today Panamax ships can carry up to approximately4,800 TEU, an increase in transport volume of 50 % asagainst that of the third generation container shipdesigns of the mid seventies, practically with the sameoverall dimensions. The number of crew members at thesame time has been reduced by about 40%.

Economy of scale effects in container shipping haveled to a rapid increase in ship size for all types ofvessels, from feeders to the large intercontinentalcarriers. Attempts are made today to increase thetransport capacity of container ships within Panamaxdimensions to 5,000 TEU. 12-abreast stowage underdeck is being considered as one alternative forachieving this objective. From a classification society’spoint of view this solution needs very carefulconsiderations. It may in fact not be practicallypossible, particularly in connection with the lack ofstiffness of this design. Other alternatives are beingexamined, where additional stowage space is createdunder deck by a reduction of the double bottom heightand by modifications of the transverse bulkheadconfiguration.

A side benefit for both alternatives is the reductionof ballast water quantities required for stabilitypurposes. This will result in further improvements in theeconomic efficiency. Present 4,500 TEU Panmaxvessels have to carry 10,000 t of ballast water and morefor ensuring adequate stability in the fully loadedcondition. They are consequently at a disadvantage inthis respect compared for instance to Post-Panamaxcarriers.

Strength and StiffnessA ship hull, looked at as a box girder, is very strong

and stiff in bending and torsion when it has a largelyclosed cross section. This is true for tankers but appliesalso to conventional general cargo ships or bulkers,where hatch openings are only a fraction of the totaldeck area. For centuries these ships were analysed forhull girder bending and local stresses only.

When the deck openings become significant,however, the girder will be more flexible and highlystressed, particularly with respect to torsion. Theprincipal feature of a containership is the open topdesign which has to be accounted for in the strength andstiffness considerations. The hatch openings are as longand wide as the cargo space in the holds below. Specialstrength considerations are necessary here whichinclude the simultaneous action of bending and torsionloading, both static from the cargo and dynamic fromthe sea way and sea motions. This makes the analysis ofcontainer ships much more complex than that of anyother vessel.

Bending stresses depend only on the magnitude of

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the maximum expected bending loads at any one crosssection in relation to the section modulus at thatsection. Torsion stresses on the other hand depend onthe distribution of torsion loading over the length of thevessel and on the distribution of the stiffnessparameters of the ship’s structure as well. With thelarge hatch openings it suddenly became necessary toextend the strength analysis of the ships from a more orless one dimensional form to two and three dimensions.As the phase relation between vertical and horizontalbending loads on the one hand and torsion loading onthe other also has a significant influence, extra effortsin sea load analysis were called for as well.

Beam and torsion bar theory were used in the earlyyears for the analysis of container ships. The containerships of the so called second and third generation weredesigned on the basis of elaborate approximate methodswhich were developed both in the US and in Europe,based on bending and St. Venant and warping torsiontheories. As the ships became larger and more complexit was however increasingly difficult to take account ofall the influencing parameters within these approximatemethods. The introduction of detailed computermethods for both, the seaway, ship motions and sealoads analysis as well as the structural analysis just atthat time brought the tools necessary for furtherdevelopments of the container ship design. Sea loadsinclude the wave action on the hull as well asacceleration forces from the ship and container massesdue to the ship accelerations. The analysis includes arealistic distribution of the masses in the ship fordifferent loading conditions. The computer modelincludes a description of the containers stacked ondeck, which make up a good part of cargo on theseships, see Fig. 1.

The finite element method for strength and stiffnessanalysis was combined with sea loads analysisaccording to the strip theory and probabilisticdescriptions of the ocean wave climate and expectedcumulative wave load cycles over the ship’s lifetime. The programs became increasingly more sophisticated,eventually also allowing the study of the interactionbetween ship hull and hatch covers. This was aprerequisite for the development of modern containerships, up to Panamax size and beyond.

The computer model for a 9,200 TEU container shipproject is shown in Figure 2. With present daycomputing facilities there are no more storage andcomputing time limitations. The finite element modelcan be generated sufficiently subdivided as necessaryfor the overall analysis including all major details. Themodel realistically describes for instance thelongitudinal and transverse structural components aswell as the transition of the highly loaded longitudinal

bulkheads and coaming from the mid body to the bowand stern structure.

Fig.2 Finite element model of a 9,2000 TEU Post-Panamax Carrier

Detailed sea loads and dynamic strength analyseswere performed for every ship in the earlier days of thecontainer ship history. Today more experience and feedback from ships in operation are available. Therefore aseaway analysis is usually performed only for newdesigns and when going into new ship sizes, in order todetermine the maximum dynamic loads to be expectedfor the vessel. Envelope design loads are derived fromthe results which are then applied to the over-all fe-model as quasi-static loads in a dynamic equilibriumcondition. Bending and torsion loads are superimposedwith differing phase angles in a way to again get anenvelop of maximum stresses and deformations. As anexample deformations are shown for bending andtorsion loads of the 9,200 TEU vessel in Fig. 3.

Fig. 3 Deformations of a 9,200 TEU vessel in bending and torsion

The structure of container ships is highly stressedthroughout. Any design mistake with stressconcentrations going beyond the endurance limits willresult in cracks, frequently already after a short time ofoperation. Critical details such as hatch corners or cellguide supports in hatch corners therefore areinvestigated separately in detail, Fig. 4.

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Fig. 4 Detail of hatch corner and cell guide support

The life time fatigue analysis is finally performed indetail for all hot spot areas of the design, orapproximate methods for checking the fatigue life canbe employed, (Payer and Fricke 1994). A life span of20 years continuous operation in the North Atlantic isusually taken as a basis.

Hull–Hatch cover interactionHatch covers are an integral part of a container ship.

They contribute to the strength and stiffness of the hull.Deformations of the hull under static and dynamicloading cause the hatch covers to slide or to distort,depending on the design of hatch cover supports andstoppers. This is a non-linear so-called contactproblem. The analysis of it is quite complex. Thedistribution of the change in length of the hatchdiagonals as calculated in the FE analysis is shown inFig. 5. This gives an overview of the flexibility and ofthe ships hull and of the magnitude of deformations thehatch cover and the lashing system has to cope with. Itcan be seen from the example that the deformations ofthe ship hull under design loads can result in a changein the length of the diagonal in the order of 100 mm or4 inches.

Fig. 5 Change in length of hatch diagonals in mm

VibrationsGlobal and local ship vibrations are being

considered in detail during the design phase, makinguse of the overall finite element analysis modelavailable from the strength and stiffness calculations.With the large power input and the large main enginesused any resonance between excitation and global orlocal vibration modes must be detected and avoided asfar as possible. If this is not fully possible, an analysisof the forced vibrations has to be performed, (Payerand Asmussen 1985).

The excitation forces from the propeller areestimated usually for the blade and twice the bladefrequency. The probability and extent of cavitation isalso estimated either by calculations or based onhydrodynamic model tests.

The large slow running two stroke Diesel engineshave different types and frequencies of excitation,depending on the engine type and the number ofcylinders. Additionally, with the elastic deformationsof the engine on the foundation, there is the possibilityof internal energy being transmitted into the shipstructure .

Fig 6 shows a calculation model used for theanalysis of forced vibrations, where apart from the shipstructure the engine is also modelled in some detail.The internal engine forces are calculated by a separatecomputer program so that the operation of the enginecan be simulated.

Fig. 6 FE model for the analysis of forced vibrations including adescription of the engine.

With this technique it is possible to makepredictions of the vibration amplitudes at differentfrequencies, in order to determine whether vibrationlimits as stipulated in the contract specifications oraccording to ISO 6954 are exceeded or not

Post-Panamax Carriers – Where are the limits?The development of post-Panamax container vessels

was started in the mid eighties with the C10 classvessels for American President Lines, APL. TheGerman shipyard HDW pioneered the construction ofthese vessels together with APL. During the designphase both American Bureau of Shipping andGermanischer Lloyd supported the shipyard withdetailed strength and fatigue strength investigations.

The new idea to give up the 32.2 m beam constraint

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for the Panama Canal was accepted by the industry.After a time of successful operation in the Pacific theC10 and C11 class APL vessels found many followers.Today more than 270 Post Panamax container vesselsare in operation or on order world wide.

The development of container ships is progressingsteadily, always close to the limits of the technicallyfeasible, (Wittenberg 1998). An 8,000 TEU containership study made by a German industry consortium underthe leadership of the HDW shipyard was completed bythe end of 1997, (Kraus 1997). A classification societyhas taken an active part in this development byperforming intensive calculations regarding the loads aswell as the design and dimensioning of the structures.The results showed that it is indeed possible to copewith the structural problems of a ship of that size oreven beyond.

The so-called S-Class built by Odense shipyard forMaersk Lines became the largest container ship in1998. Although officially put at 6,600 TEU, the ships ofthis class are coming close to the volume of 8,000TEU. One of these ships is shown in dry dock in Fig. 7.

Like many other containerships this ship isconstructed with two intermediate longitudinal hatchcoamings. This design has some advantages, particularlyregarding support for hatch covers. One disadvantage isthat the ship has three times the number of hatchcorners than a design without intermediate girders.Hatch corners are one of the most critical and highlystressed structural elements in container ships. Additionally a ship without these structural elementshas more space and can carry more containers. Severalships of that size and even beyond are underdevelopment or have been contracted at the present.Where are the limits?

Fig. 7. S-Class vessel in dry-dock

Limitations for the development of ULCC’s – UltraLarge Container Carriers, say up to 12,000 TEU - andfor 'Mega Container Ships' beyond that come from thefollowing aspects, Payer (1999) and (2001):- the maximum available size of the propulsion plant- the water depth of the port entrance and in port, and

finally- the cargo handling facilities in container terminals and

logistics and infrastructure requirements in ports.Propulsion Aspects

Because of its reliability and very good economicperformance, single propeller powering units with onelarge slow speed two-stroke Diesel engine have becomethe norm for large container ships.

The largest slow-speed Diesel engines available atpresent, with 12 cylinders of 960 or 980 mm bore and amaximum output of about 68,000 kW (95,000 BHP)provide adequate propulsive power for a Post-Panamaxship of 8,000 TEU for a speed of about 25 knots.

Beyond this ship size, either the ships have to goslower, even larger engines have to be built, or twinpropulsion plants with two propellers would have to beinstalled. It is generally expected that large container

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ships would operate at sea with a speed of 25 to 27knots. With the considerable increase in fuel oil pricesin the past couple of years, more attention is beinggiven today to fuel consumption and efficiency. Severalprojects are being discussed of ships with a carryingcapacity beyond 8,000 TEU however powered by thesame 12 cylinder engines as their smaller predecessors.This means that owner and operator may accept aslightly slower ship with improved economicparameters.

Just-in-time delivery of containers is gettingincreasingly more attention. Speed consequentlycontinues to be an important design parameter,particularly for large ships. Contrary to someconsiderations of slower steaming in light of the higherfuel costs, there are indications that the design speedfor these large vessels may actually go up, to 26 and 27knots. The market appears to be prepared to pay apremium for faster ships. Consequently there will be aneed for even larger engines, if we stay with singlescrew propulsion.

Both of the two designers and licensors for largeslow speed Diesel engines have recently come up withengines of more than 12 cylinders. Sulzer has a 14cylinder, 960 mm bore engine in their new cataloguewith a power output of 80,000 kW. Also MAN-B&Whave announced going beyond 12 cylinders all the wayto 14 or even 16 cylinder versions of their 980 mmbore engine. With about 92,000 kW and 31.5 m lengththe 16 cylinder engine will have a mass of more than2600 tons, Fig. 8.

Fig. 7.. 12, 14 and 16 cylinder engines

It is clear today that shipowners do not want todeviate yet from the well proven one engine / onepropeller concept. Single screw containerships of morethan 10,000 TEU carrying capacity will be possible withthese engines, although we are reaching some newlimitations which will make a twin propulsion system aviable alternative.

Looking back in the history of container ships therewas a time when the container ships of the thirdgeneration were going at significantly higher speedsthan we see today. They were twin screw vessels anddriven by steam and gas turbines their top speed was 30

knots and more. Some of them are still operating today,but at a more moderate speed. Following the energycrisis of the 70ies most of them have been converted tosingle screw Diesel engine propulsion.

One of the practical limitations for the largestengines may be the overall length of the engine. Thismay cause problems with engine rigidity as well asregarding possible interaction with the hull - an aspectrequiring careful examination, particularly in view of itseffects on the engine. The ship hull structure has to bestrong enough to give active support to the engine.Longitudinal side bulkheads in way of the engine roomfor instance improve the stiffness considerably,particularly as the large ships, with the draft limited to14 or 14.5 meters, will have a beam of 42 or 46 metersor more and the engine room therefore would be verywide.

The possibility of a two-stroke V-Engines has beenstudied for some time. Engines of this design wouldhave more rigidity and their overall length would beconsiderably less than the comparable in-line engine.This would be a large step into new territory and no shipowner is yet prepared to venture into that direction.

A chronic problem for the large container ships withone large engine arises in estuary trading. Engine speedsin some cases can not be reduced sufficiently to ensurecompliance with the speed limitations in pilotagewaters. Problems with soot in cylinders and exhaustsystem may arise in the event of long periods of low-load operation. This is a problem which will needattention in the future, particularly when and if ULCC’scome to ports like Hamburg or Tilbury, a considerabledistance from the open sea.

Twin propulsion systems have several advantages,including redundancy in an emergency, more flexibilityregarding partial loads and higher propulsive efficiency.On the other hand, such a system drives up thenewbuilding cost significantly and requires moremaintenance efforts in operation, Fig. 9.

Once owners decide for a twin screw design, theoverall efficiency could again be maximised with two ofthe largest engines available today. For a design speedof 25 knots with two of the presently largest twelvecylinder engines this leads to a Mega Container Carrierof about 18,000 TEU with a total of 103,000 kW. Thismeans the 18,000 TEU Malacca-Max Carrier of Prof.Wijnolst (1999) would also become feasible.

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Fig. 9. Twin screw versus single screw design

Other alternatives to improve the performance ofsingle screw vessels regarding speed in pilotage waterscould include an additional diesel-electric driveconnected to the main shaft by elastic couplings orconnected directly to a counter-rotating propeller(CRP). Apart from the additional output available,further benefits come from the improved propellerefficiency, which may be significant.

Fig 10 shows a possible arrangement where anAzipod drive is added behind the main propeller of asingle screw design and operates in a counter rotatingdirection. Such an arrangement is expensive but hasseveral advantages such as- improved hydrodynamic efficiency when operated

together with the main propeller by 10 to 15%- improved manoeuvrability- propulsion redundancy for an emergency as ‘take-

me-home unit’- the Azipod as power drive for slow speed sailing- using the existing auxiliary power on board for

powering the Azipod

Fig. 10. Counter-rotating azipod to augment single screw propulsion

Propeller limits at present are seen at about 9.5meters diameter. The manufacturers appear to haveproblems regarding diameter and weight of even larger

propellers. We see larger propellers for the very largetankers, but they are built for only a fraction of thepower desired for the large containerships.

Propeller design problems are connected amongstothers to the wake of these large vessels and tocavitation. This is already a problem with the 8 and9,000 TEU ships being designed today. For even largerships of 10,000 to 11,000TEU at the same maximum14.5 m draft, the wake field will become even morecritical. The propellers have to be carefully designedregarding efficiency and propeller excitation. It is cleartoday that pressure pulses of 6 to 9 k Pascal will haveto be accepted, whereas 4 k Pascal has been generallyconsidered the comfortable limit until now. Thepropellers are designed specifically to have thesepulses purely at blade frequency, so that the structuralengineers can avoid resonance. This calls for a detailedanalysis of the forced vibrations, as for instancedescribed by (Payer and Asmussen 1985).

Cargo handling facilities and water depth of portsWhen expanding their loading and unloading

facilities, the main container ports and ports oftranshipment are currently gearing up already for shipswith a beam of 50 m and more, at drafts of up to 14.5 m.This will enable them to handle ships of 8,000 TEU andbeyond. Different concepts for terminal designs arebeing considered and built to cope efficiently with thelarge container ships.

Apart from the extension of container bridgecapacities the increasing size of container shipsrequires an improvement of the ground handling anddistribution systems. An increase of the efficiency ofthose systems is mandatory to achieve furtherreduction of terminal operating costs on the one handand to ensure sufficiently short lay times for the shipsin port on the other. All the possibilities offered bylarge and ultra large containerships and the moderncontainer bridges are wasted if the port side facilitiescan not accommodate the rapid movement of such largevolumes of containers.

Research and development are focusing on theaspects of efficient container stacking, terminaltransportation and control technology. Automatedground transportation systems within the containerterminal have been introduced successfully in severalof the more progressive ports, such as Singapore andRotterdam.

For container ships of around 8,000 TEU a port hasto be capable of an effective handling speed of about330 container moves per hour. The performance of anaverage terminal presently is 120 to 150 moves perhour. New ideas and concepts are needed to keep pacewith the developments of the large container vessels.

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Unloading simultaneously with more than onegantry crane is common practice already today. Thepractical limit for a ship of around 300m in length isabout 5 to 6 gantry cranes simultaneously.

A further increase in handling capability requiresincreased speed for the crane movements, double ortriple trolley cranes and possibly servicing the biggestships from both sides in a berth, (Mascini 1997),(O’Mahony 1998).

A proposal for the layout of a double trolley craneis shown in Fig 11, where the space for the movementsof the two trolleys with the aim to avoid interference isindicated. Triple trolley systems have so far beenconsidered in futuristic development studies only butmay become reality if combined with capableautomation systems.

Fig. 11 – Double trolley cranes for Post-PanMax Ships

Important aspects when further increasing theefficiency of large container cranes are their structuralstrength and stiffness on the one hand and sophisticatedcontrol systems on the other. The control systems willdecrease cycle time by actively minimising sway, skewand pending or swinging of the containers. A highdegree of automation for the handling cycle of thespreader carrying the containers assures the requiredaccuracy of the movements.

Servicing from both sides with the ship moved in aberth, Fig 12 brings further improvements ofproductivity. With six double trolley cranes per side,each dual hoist (trolley) crane producing 55 moves perhour, the total possible productivity could be 660moves per hour. This may be necessary to make full useof the pronounced economic advantages of a megacontainer ship of more than 10,000 TEU. Proposals forservicing from both sides include either gantry cranesarranged on both sides of the berth or with bridgecranes spanning across. Details of the operation stillhave to be worked out.

Fig. 12 – Vessel in a slip with double trolley cranes from both sides

Inadequate water depth is a problem for many portsand their access routes. Increasing the depth of pilotagewaters is being examined carefully in view of theenormous costs involved and the possible ecologicalconsequences from dredging. It may be better first toreduce the draft of the arriving or departing vessels byreducing the ballast water quantities within thesesheltered regions. Investigations show that de-ballastingfor estuary trading in the case of reduced stabilityrequirements may result in a draft reduction ofapproximately half a meter on fully loaded Panamaxvessels. Hydrodynamic shallow water effects, such asdynamic trim, i.e. squat on the other hand increases thedraft again in shallow pilotage waters, particularly athigher speeds.

Presently the Laura Maersk, the first of a series ofsix ships, is getting attention as the smallest Post-Panamax carrier with a stated carrying capacity of only3,700TEU. The ships of this class have a beam of 37 m and alength of 266 m. The draft of the vessels with a dead-weight of 63,000 dwt is given with 14 m. So far two4,200 TEU vessels from P&O Nedlloyd were thought tobe the smallest Post-Panamaxes with a beam of 37.7mand 56,000 dwt.

One advantage of such a design is the possibility forthe owner / operator to use port terminals with bridgeshaving an outreach sufficient to unload the largest Post-Panamax vessels, efficiently also for smaller ships. Therequired quay-length is shorter and the loading andunloading process can be faster than for a comparablePanamax design.

Reefer containersWith the continuation of containerisation of cargoes

of all types, new specialised containers are being builtand sometimes special ships are developed for their

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transportation. Introduced in the sixties for thetransportation of such goods as beer, which had atendency to disappear on route or in port when shippedin open pallets, the container has found its way to mostfinished products. Today this trend is continuingtowards semi-products as well as agricultural productsand even some commodities. Coffee, Tea andrefrigerated as well as frozen cargoes are transported incontainers. Paper products and logs as well aschemicals and liquid products are being containerised.Open top containers for bulky cargoes such as scrapmetals or open side containers for live stock transportare successfully in use today. The advantages oftransporting cargo from origin to destination in lockedand standardised containers is most persuasive.

Reefer containers have an important role in thedesign of modern container ships. Many of the modernPost-Panamax container ships are equipped to carry25% or more of their capacity as reefers. The auxiliarypower required to maintain the required temperatures enroute reaches dimensions otherwise only seen on largepassenger vessels. Medium voltage systems have beenintroduced to improve safety and handling.

Ventilation and cooling of holds have to be designedto cope with the heat generated by the cooling systems.The transverse bulkheads have to be designed toaccommodate air ducts without impairing the structuralstrength. A hatch cover-less design has the advantage ofnatural air exchange. A detailed heat transfer study isneeded whether the holds are open or closed.

An example for a specialised container ship is thefully containerised reefer vessel or, vice versa, thecontainer ship equipped for 100% reefer containers,Fig. 13. A series of these largest reefer container shipsin the world to date has been completed recently. Amajor share of the reefer containers is equipped forcontrolled atmosphere.

Fig. 13 2,000 TEU reefer containership for Dole with hatch-cover-less

holds

Medium Voltage Systems Aboard ShipsThe growing demand for electric power on board

has resulted in an increased installation of mediumvoltage systems. Medium voltage systems have beenselected not only for cruise liners for the economicalpower distribution to major consumers on board suchas bow and stern thrusters, auxiliary machinery, hotelloads and, of course, the main propulsion units. Thetrend to high reefer capacities is leading to the use ofmedium voltage systems also on containerships.

Nominal voltages for 50 or 60 Hz medium voltagesystems range from 3 to 12 kV with 6.6 kV being themost widely applied voltage for marine systems. Thesystem design and protection concepts differconsiderably from low voltage systems. Acontainership with a capacity of approximately 600reefer containers requires auxiliary power installationof about 10,000 kVA depending upon route and cargoprofile. Medium voltage systems are preferable forelectrical loads of this magnitude or more.

Although the components for medium voltagesystems cost more than equivalent low voltage gear,there often are economical advantages due to reducedmachinery size, less cabling and lower installationcosts. The cross sectional area for the cabling of a 6.6kV bow thruster, for example, is only about 7% of thecross section needed for a 440 V system. The cost forsuch long cable runs for large consumers can beconsiderably reduced. As the voltage is 15 timeshigher the current for the same total power areproportionally lower and with it the losses.

For the distribution of electric power to the reefercontainers several configurations can be used. A ringdistribution system is often preferred with step downtransformers located in the passage ways or transversebox girders adjacent to the outlet distribution panels.Such an arrangement minimises installation cost, itdoes however result in higher component costs than acomparable direct distribution system.

Open Top versus Closed Container ShipsThe containership has been traditionally designed

with watertight hatch covers in accordance with SOLASrequirements. It is possible to build ships also withouthatch covers by increasing the freeboard and provingthat the cargo hold pumps have sufficient capacity tocope with the amounts of green water which may enterthe holds from waves coming over the side, or fromrain. These quantities may be calculated but also have tobe demonstrated in model experiments.

Several ships have been built successfully as ‘Open

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Top’ design without hatch covers for some or all holds,Fig. 14.

Fig. 14 Open top container ship with cell guides extending above thedeck

The following advantages are seen for the open topdesign:- Safe transport of boxes in container guides- No sensitive components such as hatch covers with

stoppers and cover supports- No items or parts exposed to tear and wear and

which have to be replaced, such as cover seals andsliding pads

- No lashing bridges – no hatch cover interactions- Less ventilation problems with reefer boxes in the

holds

Disadvantages of the hatch-cover-less designinclude:- Reduced space for carrying containers- Less flexibility for mixed stowage of containers of

differing size- Intermediate supports required for more than 10

tiers.

The conventional design with hatch covers on theother hand also has advantages:- Large slot capacity which may be used to also

carry empty containers- Faster loading and unloading of containers from

the holds

- Larger capacity for containers with dangerousgoods, to be carried only on deck

- More flexibility for mixed stowage of 20’ and 40’boxes

The disadvantages of the design with hatch coverson the other hand include:- No heavy hatch covers- Proper design and analysis for lashing bridges

required- Errors with lashing may have disastrous effects- Higher investment cost

The main problem of hatch-cover-less containerships is their increased tonnage measurement inconnection with their added freeboard as compared to aship with hatch covers. As pilot and harbour dues aswell as canal passage taxes are based on tonnage, theseships are being penalised.

In many ways they are safer and the containers arebetter protected. It is an anachronism that owners arediscouraged to decide for the improved safety aspectsof these vessels by higher operating costs. This costpenalty in operation discourages most owners fromdeciding for a hatch-cover-less design.

Container Feeder ships and Fast Feeder ShipsThe container feeder ship concept has become very

successful in many regions of the world, particularly inAsia, where the inland infrastructure is not yet as highlydeveloped as for instance in Europe. The subdivision oftasks between intercontinental carriers calling only atvery few hub ports and the more local ship traffic hasproved to be very successful. With a continuingincrease in container volume on all trade routes also thetransport capacity of feeder systems has to increase.Today even ships with a capacity of more than 2,000TEU are employed as feeder ships.

The cost of transhipment still is a severe restrictionfor the hub and feeder system. Transhipment of acontainer in a European port today costs about $300. Itis more economical for even large ships call at themajor ports of final destination themselves than tounload at a central hub-port with the containerscontinuing their voyage to their final destination aboardfeeder ships.

As the different steps of transhipment are beingimproved and automated, it may be expected that thiscost will go down. This will give the feedertransportation chain a big lift.

There may even be multilevel feeder systems,ultimately as shown in Fig.15 (de Monie, 1997)

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Fig 15 Multilevel feeder system

Weak points and risks in container shippingWhere do we expect the main problems with the

future technological developments? Regarding thestructural design of very large containerships themaximum plate thickness for the deck and coamingstructure has to be addressed. Today a plate thickness of70 or 80 millimetres, perhaps 100mm is seen as thelimit. This material has to be cut, prepared and weldedto reliably withstand high dynamic stresses. We can notexclude that further progress will be made both with thematerial and the welding techniques, but today this is alimiting factor. The strength requirements of very largecontainerships can for instance be met by designing thelongitudinal hatch coaming as box girders on either sideof the holds. Thus it is possible to structurallydimension 12,000 TEU vessels and even larger designsadequately, Fig 16.

Fig. 16 Double coaming with 75 mm plating

We have already addressed some aspects of thepropulsion of the very large ships. Problems are notonly connected with the large engines, theirdeformations and their limited capabilities for partialload operation. As the power continues to grow we arealso faced with

design problems regarding the propeller. With the draftof the container vessels limited to about 14 or 14.5meters, the propeller diameter is reaching practicallimitations already with the present designs of 8,000 or9,000 TEU vessels.

Cavitation has to be accepted with these highlyloaded powerful propellers, and we are seeing cavitationproblems with the rudder. There have been cases ofsevere erosion of the forward structure of the rudderand rudder horn. On some ships this has been detectedafter not more than the ship yard trials.

In order to accommodate larger diameter propellers,the owners and designers will have to go for a largerdraft for these ships. Ship owners will just have toaccept that some of the ships of the near future will belimited to operate only between ports which do not havethe 14 m draft limitation. We can compare this stepwith the transition from Panamax to Post-Panamaxvessels. A barrier was broken down by one progressiveoperator with the courage to give up the possibility forthis class of ship to pass through the Panama canal.

There are many problems related to ports. They willnot be addressed here. It is a fact that shippingcompanies interested in breaking barriers and goingbeyond the presently established limits will have to co-operate with ship designers and with the other membersin the transportation chain to further optimise thetransportation system.

Container ships so far have a very good safetyrecord compared for instance to bulk carriers orgeneral cargo ships. Apart from collisions or groundingcaused by operational errors, very few container shipshave suffered serious damage or were lost at sea.Particularly for the big vessels very few cases of majordamage have been observed so far.

MS Carla in fact was the first container vessel tobreak in two in the winter storms of 1997/98. Thisaccident in fact demonstrated that container ships withhigh freeboard and closely spaced water tighttransverse bulkheads are intrinsically very safe. Bothparts of the ship remained afloat and were salvagedafter the storm.

As the container ships get larger and closer packedwith containers their strength and stiffness need verydetailed attention. New designs are being analyzed bymodern computer methods for strength, deformationsand fatigue life in the expected operating conditions.As the structures of container ships are highly stressedin service, the design of structural details is of utmostimportance.

Feed back from ships in service shows that due tothe high stresses the structure of container ships isparticularly unforgiving regarding design or fabricationerrors. Any hot spot, any stress concentration may

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result in a crack sooner or later.There are however other risks for container ships

such as excessive wave loads, loss of containers at sea,a lack of dynamic stability and the danger of fire incontainers. Some of these risks increase with size,others decrease.

Excessive wave loadsContainer ships are often built with considerable

bow-flare to accommodate as many containers on deckas possible. Some of the smaller and medium sizedcontainer ships have a bow form comparable to that of asleek cruise liner. To compensate for extra resistancein heavy seas and remain within the generally tightschedule, these ships are usually equipped withsignificant reserve power, Fig. 17.

Fig. 17- Waveloads on bow of fast containership

There have been many cases where the bow or thestern structure of modern containerships wereoverloaded and could not withstand the heavy impactloads from the waves. The structure sufferedconsiderable damage, Fig. 18. For the officers it isoften difficult to recognize that the ship is beingoverloaded, as the view to the bow is blocked bycontainers on deck and no green water comes on board.The damage usually is not dangerous to the overallsafety of the ship but this certainly needs attention inthe container transportation at sea. Germanischer Lloydhas significantly increased the design loads for shipswith large bow flare and has made ship owners andoperators aware of this danger. They are advised toinstruct the officers to reduce speed and avoid heavybow flare impacts in operation when heavy weather is

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encountered.

Fig. 18 - Damage to stern structure of a containership

Safety of Containers on DeckThe safety of large container ships has recently

been called into question in connection with the largevolume of containers carried on deck. Freeboardregulations, developed in the early days of ship designdeserve revisions to accurately reflect the safetyrequirements of modern container ships.

Present-day large container ships are neverthelessbasically safe. Each ship is carefully checked for itsstability, and limitations of weight and number ofcontainers loaded on deck are clearly defined. Theaccident of the APL China, a Post Panamax containership which survived a 24 hour hurricane, for one houreven with a total black out on board, is often cited as anindication that we are going too far with the design ofthese ships, Fig. 19. When analyzed unemotionally thisevent is actually a proof of the basic safety of theseships under an extreme load case going beyond theordinary design conditions. Some other ships may nothave survived.

Fig. 19 - APL-China after sea damage

Some believe that the APL China and some otherPost-Panamax carriers experienced a condition ofresonance between the ships rolling frequency and thewave frequency of encounter. It is well known thatcontainer ships, with their usually fine lines, canexperience heavy rolling in following seas, dependingon the wave conditions in relation to the dimensions ofthe ship. This phenomenon, referred to as parametricrolling, may in rare cases even lead to the capsizing ofthe vessel, if no appropriate action is taken by the crew.Up to now the usual remedy was for the master to turnthe ship into the seas.

Theoretical as well as experimental investigationsindicate that, as the container ships get larger and theirstability increases, parametric rolling may in factdevelop when the ship is heading into the seas. The shipis very stiff with a short rolling period, the resonancefrequency of encounter with large waves is found inhead seas rather than following seas. This means thatthe crew will need more specific instructions for theirvessel on how to respond in heavy weather, dependingon the loading condition of the vessel and sea and waveconditions. More research is necessary, and studies areon-going both in Europe as well as in the USA.

Container stowage on deck has come into focusworld wide with respect to damage to the containers inheavy weather. An increasing number of containers arelost. Apart from the associated material loss, floatingcontainers are posing a risk to small and medium sizedvessels.

In port containers are stacked on deck of the ship ontop of each-other, up to 7 high, connected by twist-locks of differing design. Securing and releasingparticularly the upper layers requires dock workers toclimb up to about 15 m above deck. As loading andunloading takes place around the clock in efficientports this so-called coning and de-coning has to bedone under all circumstances.

Tens of millions of containers annually, (90 millionhas been estimated for the year 2000), are being loadedand unloaded in ports world wide. Dock workers areexposed to the risk of falling from great height whileconing or de-coning millions of times every year.Although the number of such accidents appears to berelatively small, the consequences of a fall of a dockworker can be very serious.

No reliable statistics on accidents connected withconing and de-coning of containers in port are availableto date. There is no doubt that the safety of dockworkers is a very important issue. It has to berecognized additionally that where-ever there is anelevated risk of an accident occurring the speed of theprocess will be decreased.

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A number of measures have been proposed orintroduced to reduce, or even eliminate the risk to dockworkers associated with securing of containers ondeck. This includes: a lifeline connecting the dockworkers to the spreader; the use of safety flats, whereinthe dock workers are seated and the safety flat movedover the container stacks by the gantry crane; semi-automatic or automatic twist-locks; or the design ofhatch-cover-less container ships.

The use of lifelines or safety flats seriously hamperthe loading / unloading process and decreaseproductivity of the gantry cranes. Hatch-cover-lesscontainer vessels equipped with overhead cell guides onthe other hand do not require any lashing or securingdevices at all. As mentioned earlier such ships arepunished severely by the presently applicable load lineconventions. This is very unfortunate as, apart fromextra freeboard being connected with extra ship safety,this design would practically eliminate the problems ofconing and de-coning just addressed.

3. ECONOMIC IMPLICATIONS

The increase in the volume of cargo carried incontainers is continuing from the last decades into thebeginning of this new century. Container transport hasobtained such a central role in world trade, that itsgrowth continues even through economic downturns, aswe have seen with the Asian crisis towards the end ofthe nineties. It can be expected that also the presentslow-down in the economies of the major tradepartners of the world will only have a limited effect onthe growth of container-transport. Most predictionsregard this slow-down only as temporary. After agrowth of the world gross domestic product, GDP, ofonly slightly more than 3% in 2001, a return to ahealthier 3.5 to 4% is predicted already for 2002.World trade and world transport with it can be expectedto generally grow about twice that rate.The container volume has had such a pronounced growthduring the past twenty years that on most major routeswe are experiencing a doubling of volume in less thanten years. Experience shows that with falling transportrates more cargo turns up in container shipping. Thusalso the shipping crisis of 1998/99, when containerrates dropped to less than 50% of what they were onlytwo years earlier on most routes, did have the beneficialeffect of bringing more cargoes and new kinds of cargoto be shipped in containers.

In order to demonstrate the economic advantage oflarge ships economic analyses were conducted withinthe feasibility and design studies for the 8000 TEUconcept, (Kraus et. al., 1997) as well as for theMalacca-Max study, (Wijnholst, 1999).

It is shown for instance in the 8,000 TEU study that

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the fuel consumption per TEU-mile decreases with anincrease in size of the ship as well as with a reduction inspeed.

The cost for sea and land transport were analysed fora series of 360 different designs and sizes of containerships, between 5,000 and 8,000 TEU. The round-tripvoyage was considered. The results clearly show abeneficial effect of size which is particularlypronounced for higher speeds around 25 knots andabove.

The highest yearly return was found for an 8,000TEU vessel steaming at 24 knots, followed by the shipof the same size sailing at 26 knots.

In the study by Wijnolst et al. (1999) slot costs perTEU and day were calculated and compared forrepresentative container ships between 3,000 and18,000 TEU, Fig. 20. It is shown that larger ships canachieve significant economies of scale on total slotcost. This effect is less pronounced when the timecharter equivalent rate is used as a benchmark. In thisstudy it is found that the major savings are achievedfrom the reduced fuel consumption per TEU on thelarger ships. Construction cost per TEU were found tobe less influenced by ship size.

Fig.20 Slot costs for large container ships (Wijnolst)

The cost for the deep-sea transport leg betweenSingapore and Rotterdam is shown as a function of shipsize in Fig. 21.

Fig.21 TEU-Transport cost between Singapore and Rotterdam asfunction of ship size (Wijnolst)

4. CONCLUSION

We are living in a time of dynamic growth anddevelopment world-wide. No branch is so closelyconnected with the liberalisation and globalisation ofworld trade as the shipping industry, particularlycontainer shipping. The container shipping industry hashad a significant effect on world trade. The contributionof the container ship to the changes in the globaleconomy is sometimes compared to that of themicrochip. With the growth in ship size and TEUcapacity shipping has become more efficient andreliable in big steps.

It may be assumed that further progress will be madein the development of container ships in the future, wellbeyond 8,000 TEU. Naval architects and marineengineers will no doubt be in the position to meet therequirements of the market for even larger units.Container ships have always been developed close to thelimits of what was considered to be technically feasible.

As it continues to grow rapidly, the container shipmarket will be reaching a degree of maturity, even if

we are far from the end of the development yet. The bigships will not replace the smaller ones, they will co-exist.

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There will probably not be one or a few standard shiptypes but a whole diversity of vessels. Different designswill be operated side by side and compete with each-other.

In the longer run there may well be very large ships,some within the presently considered draft limits,others going beyond the 14.5 meters. Some owners maybe prepared to limit the operation of their ships to onlya few hub ports without draft restrictions. There will besingle propeller vessels with and without additionaltake-me-home propulsors like Azipods and otherdesigns, and there will be twin screw vessels.

There may well be quite a different development ifthe Panama canal is widened to allow ships to pass withsay a 50 meter beam. This will define a new Panamaxdimension, which probably will be observed by manyowners interested in more flexibility for their ships topass between the Pacific and the Atlantic. This will beof particular interest for the trade between the Far Eastand the US East coast or the Atlantic and the US Westcoast.

Containerships have to be reliable, economical andsafe, with their important role for the world economyand the well being of the people. This has been achallenge for naval architects and marine engineers inthe past and continue to be so in the future.

References

DE MONIE, G. The Future is Mega Hubs, Cargo Systems, August 1997

KRAUS et al. Container-Transportation System of the Future, Final Report, HDW, Kiel (1997)

O’MAHONY, H. (editor) Opportunities for Container Port; Cargo Systems, IIR-Publications Ltd. London,1998

MASCINI, H. The Terminal of the Future: Famas; Terminal Operations Conference (TOC) Barcelona, June,1997.

PAYER, H. G., ASMUSSEN, I. Vibration Response on Propulsion-Efficient Container Vessels; Trans. SNAME,Vol.93, 1985.

PAYER, H.G., FRICKE, W; Rational Dimensioning and Analysis of Complex Ship Structures; Trans. SNAME,Vol.102, 1994

PAYER, H. G. Larger and Faster Containerships – Where is the Limit?; Liner Shipping Conference, London,March 1999

PAYER, H. G. Is the Desire for Bigger and Faster Container Ships compromising Cargo Safety? – TheRegulator’s View; London, 4th Annual International Containerisation Conference March 2001

WIJNOLST, N., SCHLOTENS, M., WAALS, F. Malacca-Max – The Ultimate Container Carrier, DelftUniversity Press, 1999

WITTENBERG, L. Large Container Ships - Present Position and Future Prospects, Germanischer Lloyd,Hamburg, 1998

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