INTRODUCTION
Ship is a complex vehicle. Its production involves the involvement of a huge range of engineering disciplines. Ship design is not an exact science but embraces a mixture of theoretical analysis and empirical data accumulated from previous succesfull designs. Due to the complex interrelationships between features of the technical design, and the construction of the ship and its operation, the final ship design will often represent a compromise between conflicting ship requirements.
The development of the overall ship design and its production can not normally be treated in technical isolation as operational requirements have to be considered. For example, the ship will often form part of a through transport system;In this case specific container systems witth dedicated mega ship operating between specified ports(Eyres, 2007).
Since the late 1950's the ocean cargo container has revolutionized the seaborne and inland transport of general cargo throughout the world. The specialized container ship is a key element in an intermodal system developed to efficiently transport standard sized ocean freight containers(Lamb, 2004).The route and its environment, type of cargo, quantity to be moved, value of the cargo and port facilities are typical features which will be considered when evolving the size, speed and specification of a suitable ship.
In this paper , we will try to figure out the technical requirements for designing a 18000 TEU container ship. Since We will try to reach the approximately usuable data , design of the previous ultra-large container ships should be taken under serious consideration
Technical Design
Shipowners operate ships to make a satisfactory profit on their invesment.The evolution of a technical design can therefore be considered as a component part of an overall economic model.In evolving a ship design it is therefore necessary to assess the operating requirements and the environment in which the vessel is to operate, to evolve the feasible technical design and to economically justifiy the viability of the proposal(Molland, 2008).
In an overall final design process the desing objectives have to be clearly identified and constraints in the process incorporated.Such as ;
Design for Functionability : This is a pre-requisitewithout which the ship does not fulfil its role(Molland, 2008) . (Consider to build a large container ship)
Design for Efficiency and Economy : this is normally also a pre-requisite and might take several forms including designing to minimize running costs, maintenance costs, cranage/turnround time for container ship, all with a view to improving the overall efficiency of the operation. (Molland, 2008)
Design for Production : In this case producibility is important, and savings in construction costs may be assessed.In this case , the analysis maybe trading increases in steel mass ( and hence decrease in deadweight) against decreases in production costs(Molland, 2008).
Design for Maintenance : this will often amount to increase in space and improved access for maintenance of Hull&Machinary.This might entail accepting surplus volume and an increase in ship's first cos(Molland, 2008) t.
Each objective is important in its own right. Whilst achievement of all the objectives is desirable, but unlikely, some weighting as to the relative importance of the various objectives will normally be necessary.
Ship Design Process
The ship design process may be separetad broadly into two stages:
Conceptual and preliminary design
Detailed or tender or contract design
The principal ship dimensions and power to meet the intended service will be evolved at stage (1). If the results of stage (1) are technicall and economically viable then stage (2) will flow. What is concerned by the designer is the evolution of the preliminary ship design and its evolution(Eyres, 2007).
The preliminary design process will normally take the form of a techno-economic appraisal, using a fundamental engineering economy approach.
The increase in effort to improve efficiency has led to an increasing use of economic investigation. Whilst a primary and traditional function of the ship designer or naval architectis to derive a feasible technical design, it is unlikely that this will be achieved in technical isolation without taking account of economic considerations, either directly or indrectly(Molland, 2008).
Many of the techno-economic evaluations amount to an investigation of the trade-off between first and operating costs; it is important to note that the "best" desing need not necessarily be of lowest first cost, but that which shows the most profitable combination of first and operating costs over the life cycle.
Technical Ship Design
The principal requirements of a technical ship desing could be summarized as follows:
1
Is adequate in size and arrangement for intended service
Implies ability to carry a specified volume of cargo and have adequate space for machinery, fuel and crew etc.
2
Floats at correct draught
Implies sum of weights of lightship and deadweight equals force due to buoyancy (function of ship form)
3
Floats upright
Implies adequate stability
4
Achieves correct speed
Implies satisfactory estimates of resistance and propulsive power(plus margins) and installation of suitable engine(s)
5
Is structurally safe/sound
Implies structural design with the ability to withstand forces in the marine environment; typically built to the requirements of a classification society
6
Meets requirements for manoeuvring, coursekeeping and seakeeping
Implies choice of suitable hull form
7
Meets international standards of safety and reliability
Meets requirements of IMO
The derivation of a fesible technical design will take form of an 'iterative process of analysisand synthesis'; i.e. is a repetitive process whereby the design is resolved into simple elements and relevant calculations made, after which the elementsare combined into the total ship design(Molland, 2008).
Vessel Beam and Hatch Opening Width
In order to maximize cargo stowage, the breadth of a container ship is normally a multiple of the container spacing on deck. Transverse spacing between adjacent deck stacks is often 25 mm and therefore the ship breadth is typically a multiple of 2463 mm eith some additional margin.Greater container capacity was first achieved by increasing ship length and by enhancing above deck securing systems to permit containers to stowed in stacks four and five high.Eventually, the panamax size ship faced limitations in stability and longitudinal strength and some ships were designed with high density fixed ballast in the midship double bottom tanks(Lamb, 2004).
Further advances in capacity came about in two ways. One significant change was the increase in beam. The first post-panamax beam ship, the C-10 designated class for American President Lines, were delivered in 1988. With a beam of 39.4 meters, these ships permitted containers to be stowed in 16 stacks across on deck and 12 across below deck.
The second significant design change was to increase the number of below deck stacks by widening the hatch opening, by decreasing the spacing between containers and by either eliminating the longitudinal girders that support the hatch covers or reducing their width(Lamb, 2004). Increasing below deck capacity lowers the center of gravity of the cargo and enhances the overall efficiency by providinf additional useable capacity.
Increasing the hatch width tends to lower the neutral axis of the longitudinal hull girder and leads to a design with increased depth or to the utilization of high strength steels in order to meet longitudinal strength requirements. Large hatch openings also reduce the torsional rigidity of the ship and increase torsional deflection and warping stresses(Lamb, 2004).
Vessel Depth
A typical container ship has a length to depth ratio of about 12, with lengthened vessels having ratios ranging upwards to 15.Reducing the length to depth ratio by inreasing the vessel depth increases hull grider section modulus and may allow a reduction in the thickness of the main deck and bottom plates, as the top and bottom flanges of the hull girder become more effective.However, other design issues must be considered in determining the design depth(Lamb, 2004).An increase in depth increases the gross and net tonnages thereby increasing vessel port fees.Depth affects the height of the center of gravity of the lightship, and an increase may adversely affect stability. A deeper ship may have a greater hull steel weight if the reduction in main deck and bottom plating does not offset the increase in transverse structure(Eyres, 2007).
Machinery Arrangements
Th main criteria for engine selection, is economy rather than high transit speed. The engine for the vessel types, which form the vast majority of the world's fleet, is therefore generally slow speed two-stroke engine. This is directly connected to the propeller shaft. Forward thrust of the fixed pitch propeller is channelled to the frames of the ship by the thrust block(Lamb, 2004). Reverse is obtained by running the engine in reverse. These engines are designed with a variable cam, which allows this to happen. This set up is simple, efficient and "easy" to operate and maintain(Douglas-Westwood, 2006).
The conventional arrangement for mid and large size container ships is to place the engine room at a location roughly three quarters of the length aft of the forward perpendicular.In this arrangement the engine room protrudes into the cargo space.In the three quarter aft arrangement the engine room and shaft alley are under the holds aft of the house and in the fully aft arragement the engine room protrudes into the hold forward of the engine room(Lamb, 2004).
The low speed diesel is almost universal as the main engine for the modern conventional container ship. As the container ships tend to have high speeds and high power requirements for their size, the low specific fuel consumption of the low speed diesel make it well suited for this type of vessel.In addition, container ships generally have the full depth of the hull avaialble for the engine room, so the required overhead clearance for a long stroke, low speed diesel is available(Molland, 2008).
The container ship generally has a conventional engine room layout. Ballast, bilge and fire pumps, sewater and fresh water circulating pumps and purifiers are generally in the lower engine room or on the lower platform deck.The control room, compressors, fresh water generators and workshops are generally on the second deck and upper platform decks or in the casing(Lamb, 2004). The waste heat boiler is normally in the engine casing above.
Electrical power is generally provided by diesel generators, supplemented on some ships by shaft generators. Since modern diesel generator engines burn heavy fuel and have specific fuel consumptions close to that of low speed diesels, there is generally not an economic basis for fitting a shaft generator(Lamb, 2004). Diesel generators are generally located on a platform deck, either adjacent to the main engine or aft of it under the hold. Because of the large electrical power requirements for reefer containers, container ships have larger electrical plants than many other types of ships.Most container ships have an automated engine room with bridge control and remote operation of ballast and fuel transfer. An automated heel control system is often provided to enhance port productivity.
Another method of generatig electrical power for auxiliary use is via the use of a shaft alternator. Shaft alternators take advantage of the propeller shaft rotation, saving diesel fuel and lubricating oil(Lamb, 2004). The shaft generator either reduces the need for independently driven generators or reduces the wear and tear, and hence maintanance requirements, of diesel-gensets. The power generation has to function properly at changing speeds of the propulsion shaft, when the ship travels at different speed ranges or in case of very fast speed changes caused by heavy seas(Molland, 2008).
Stability and Trim Considerations
Container capacity on deck for Mega container ship is a significant percentage of the total, ranging from %55 to as high as %65.Utilization of upper tier slots for cargo stowage may be limited by stability and must often be offset by ballasting the double bottom tanks.Maximizing container capacity below deck and keeping the base height of each individual stack as low as possible helps to lower the center of gravity of the cargo, reducing required ballast and thereby increasing cargo capacity and operating efficiency(Lamb, 2004).
Stowage below deck is generally designed for a mix of standard height(2.59 m) and high cube (2.90 m) containers. For a conventional container ship, this assumption determines the underdeck clear hegiht and the height of the hatch coaming. In turn, this decision also establishes the base height for the majority of the container stacks on deck(Lamb, 2004). It affects the stability of the ship in that each additional high cube container raises the center of the gravity of the stacks above the hatch cover by 0.3 m. Not providing enough high cube container slots can mean that a slot will be lost for each additional high cube container stowed in a stack. The design mix of standard height and high cube containers should reflect the current inventory in the operator's fleet, current trends and the anticipated market. High cube boxes are more desirable for speific trades and markets(Molland, 2008).
Double bottom tanks are generally utilized for salt water ballast for stability reasons. Wing tanks and deep tanks are used for either ballast or fuel oil storage. Fuel oil storage tanks may be located throughout the ship, but locating them forward of the engine room will permit Sea water ballast tanks to be used to compensate for changes in trim and stability as HFO is consumed.
Propulsion for Container Ships
The demand for transport capacity increases by around 10% annualy, and both the number and the size and containerships has increased over the past 15 years to meet this demand.Ultra-Large containerships are the new trend in shipping industry and in this case our 18.000 TEU Giant fits this picture perfectly.This sort of vessel would call for propulsive power up to 100MW(Douglas-Westwood, 2006). These ultra large ships are expected to have speeds of up to 26 knots. Higher sppeds would pay a penalty in a higher fuel consumption(Douglas-Westwood, 2006)..
From an economic point of view, propulsion systems are regarded as a limiting factor for growth in the size of container ship. Until recently, the largest 12 cylinder engines available developed a maximum power of 69MW sufficient to provide adequate propulsion for a post-Panamax ship of some 10,000 TEU with a speed up to 25 knots. Engine manufacturers, however, now are able to offer even larger engines which provide adequate power(Douglas-Westwood, 2006)..
Selecting the most suitable configuration depends on the hull design, and there are many conflicting arguments on the subject.Some reports say that altough a twin screw vessel would be more expensive to construct, it has been estimated that it would need some 3% less power than a single screw ship which would give savings in operating costs(Douglas-Westwood, 2006).. The lower power requirement for a twin-skeg ship is the net result of a combination of factors, namely larger wetted surface area, lower hull efficiency and increased propeller efficiency.Redundancy in the propulsion system would be another advantage. The twin skeg ship would in able to use small propellers with fewer blades but with a larger total disc area compared with the singel screw(Douglas-Westwood, 2006)..
Other reports say that while the total hull surface area is 5% larger in a twin skeg because the extra, rudder and modified aftbody, calculations show that the propeller efficiency was offset by the increased hull ressitance. A twin skeg ship would require the same propulsion power as a single screw ship(Douglas-Westwood, 2006).
In order to have twin propellers it would be necessary to use an appropriate hull design.The argument goes on to say that building larger containerships based on a simple single screw hull is the cheapest solution in terms of operating costs and invesment.
With very large engines, the propeller becomes the limiting factor fr ship size with optimum efficiency and tolerable cavitation being the decisive parameters. At present vessels with a capacity ranging up to 12000 TEU and more would still be able to sail with a single propulsion plant. Possible propulsion developments for the 12000+TEU and in this case our 18000 TEU ultra large container ship relating to the power plant and propulsion systems are twin-screw installation and the use of CRPs either with a podded drive or by the use of a double shaft(Tozer and Penfold, N/A).The CRP pod concept could achieve a substantial increase in installed propulsion power. For example, a 35MW pod could be combined with a forward propeller driven by a 14-cylinder engine to provide a total output of 115 MW which would be more than sufficient for a 16000 TEU ship(Tozer and Penfold, N/A).
Present foundries can single cast six bladed fixed pitch propellers up t o125 tons. It would be necessary to raise this to meet future demand. MAN B&W calculated that a six-bladed single screw ship with a 12-cylinder K108 ME-C engine at a nominal 83.4 MW at 94 rpm, might need a 10m propeller diameter with the weight of around 130 tons. The company calculated that a 103 Mwengine coupled with a six bladed 10m diameter propeller could power an 18000 TEU single screw ship at 25.5 knots. The largest motor currently avaiable - MAN B&W's 14-cylinder k108 ME-C model is rated at 97.3 MW altough this power could be operated by increasing the ball size (Man B&W Diesel A/S , N/A).
