35 Special cases of optimization

Optimization of repeat ships

Conditions for series shipbuilding are different from those for single-ship designs. Some of the advantages of series shipbuilding can also be used in repeat ships. For a ship to be built varying only slightly in size and output from a basis ship, the question arises: 'Should an existing design be modified or a new design developed?' The size can be changed by varying the parallel middlebody. The speed can be changed by changing the propulsion unit. The economic efficiency (e.g. yield) or the initial costs have to be examined for an optimum new design and for modification of an existing design.

The advantages of a repeat design (and even of modified designs where the length of the parallel middlebody is changed) are:

1. Reduced design and detailed construction work can save considerable time, a potentially crucial bargaining point when delivery schedules are tight.

2. Reduced need for jigs for processing complicated components constructed from plates and sections.

3. Greater reliability in estimating speed, deadweight and hold size from a basis ship, allowing smaller margins.

4. Greater accuracy in calculating the initial costs using a 'cost difference' method.

Where no smaller basis ship exists to fit the size of the new design, the objective can still be reached by shortening a larger basis ship. This reduces CB. It may be necessary to re-define the midship area if more than the length of the parallel middlebody is removed. Deriving a new design from a basis ship of the same speed by varying the parallel middlebody is often preferable to developing a new design. In contrast, transforming a basis ship into a faster ship merely by increasing the propulsion power is economical only within very narrow limits.

104 Ship Design for Efficiency and Economy Simplified construction of steel hull

Efforts to reduce production costs by simplifying the construction process have given birth to several types of development. The normal procedure employed in cargo shipbuilding is to keep CB far higher than optimum for resistance. This increases the portion of the most easily manufactured parallel middlebody.

Blohm and Voss adopted a different method of simplifying ship forms. In 1967 they developed and built the Pioneer form which, apart from bow and stern bulbs, consisted entirely of flat surfaces. Despite 3-10% lower building costs, increased power requirement and problems with fatigue strength in the structural elements at the knuckles proved this approach to be a dead end.

Another simple construction method commonly used in inland vessels is to build them primarily or entirely with straight frames. With the exception of the parallel middlebody, the outer shell is usually curved only in one direction. This also increases the power requirement considerably.

Ships with low CB can be simplified in construction—with only little increase in power requirement—by transforming the normally slightly curved surfaces of the outer shell into a series of curved and flat surfaces. The curved surfaces should be made as developable as possible. The flat surfaces can be welded fairly cheaply on panel lines. Also, there is less bending work involved. The difference between this and the Pioneer form is that the knuckles are avoided. CB is lower than in the Pioneer class and conventional ships. Optimization calculations for simple forms are more difficult than for normal forms since often little is known about the hydrodynamic characteristics and building costs of simplified ship forms.

There are no special methods to determine the resistance of simplified ships, but CFD methods may bring considerable progress within the next decade. Far more serious is the lack of methods to predict the building costs by consideration of details of construction (Kaeding, 1997).

Optimizing the dimensions of containerships

The width

The effective hold width of containerships corresponds to the hatch width. The area on either side of the hatch which cannot be used for cargo is often used as a wing tank. Naturally, the container stowage coefficient of the hold, i.e. the ratio of the total underdeck container volume to the hold volume, is kept as high as possible. The ratio of container volume to gross hold volume (including wing tanks) is usually 0.50-0.70. These coefficients do not take into account any partial increase in height of the double bottom. The larger ratio value applies to full ships with small side strip width and the smaller to fine vessels and greater side strip widths.

For constant CB, a high container stowage coefficient can best be attained by keeping the side strip of deck abreast of the hatches as narrow as possible. Typical values for the width of this side strip on containerships are:

The calculated width of the deck strip adjacent to the hatches decreases relative to the ship's width with increasing ship size. The variation in the figure also decreases with size.

If the ship's width were to be varied only in steps as a multiple of the container width, the statistics of the containership's width would indicate a stepped or discontinuous relationship. However, the widths are statistically distributed fairly evenly. The widths can be different for a certain container number stowed across the ship width, and ships of roughly the same width may even have a different container number stowed across the ship. The reason is that besides container stowage other design considerations (e.g. stability, carrying capacity, favourable proportions) influence the width of container-ships. The difference between the continuous variation of width B and that indicated by the number and size of containers is indicated by the statistically determined variation in the wing tank width, typically around half a container width. The practical compromise between strength and construction considerations on the one hand and the requirement for good utilization on the other hand is apparently within this variation.

The length

The length of containerships depends on the hold lengths. The hold length is a 'stepped' function. However, the length of a containership depends not only on the hold lengths. The length of the fore peak may be varied to achieve the desired ship length. Whether the fore end of the hold is made longer or shorter is of little consequence to the container capacity, since the fore end of the hatch has, usually, smaller width than midships, and the hold width decreases rapidly downwards.

The depth

Similarly the depth of the ship is not closely correlated to the container height, since differences can be made up by the hatchway coaming height. The double bottom height is minimized because wing tanks, often installed to improve torsional rigidity, ensure enough tank space for all purposes.

Optimization of the main dimensions

The procedure is the same as for other ships. Container stowage (and thus hold space not occupied by containers) are included at a late stage of refining the optimization model. This subsequent variation is subject to, for example, stability constraints.

The basis variant is usually selected such that the stowage coefficient is optimized, i.e. the deck strips alongside the hatches are kept as narrow as possible. If the main dimensions of the ship are now varied, given constant underdeck capacity and hold size, the number of containers to be stowed below deck will no longer be constant. So the main dimensions must be corrected. This correction is usually only marginal.

Since in slender ships the maximum hold width can only be fully utilized for a short portion of the length, a reduction in the number of containers to be stowed across the width of the midship section would only slightly decrease the number of containers. So the ratio of container volume to hold volume will change less when the main dimensions are varied on slender containerships than on fuller ships.

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