Some Aspects Of Large Floating Docks

The Authors Tell Why They Feel That Floating Drydocks Are Best Suited For Docking VLCCs — They Can Closely Align To The Ship's Bottom.

When planning a new repair yard or increasing the resources of an existing one, the choice of dock type comes under discussion at a very early stage. It usually turns out that the geographical location of the yard is an overriding factor; that is, the depths of water and the subsurface conditions at the site are largely decisive.

Where a large dock is required, a graving dock can often turn out to be the most economical solution, especially if it can be built as a naturally drained dock and can be engineered without excessive cofferdam arrangements and, nonetheless, in fairly easily excavated soil. But if the local ground conditions mean that the graving dock must be of the gravity type with bottom anchoring, then the floating dock will often be a cheaper solution.

It must be remembered, however, that initial costs alone cannot determine the choice of dock type; the type of work to be done must be allowed to exercise an influence — perhaps an overriding influence — on the choice.

The advantages of a floating dock include the following: It is more easily oriented relative to winds and current so as to bring the most prevalent effect of these in the line of entry. This will, of course, facilitate every docking and undocking and will reduce the risk of damage.

In a floating dock, the time required to lift and reimmerse the dock is directly proportional to the size of the ship, whereas the opposite is true in a graving dock yard or increasing the resources of an existing one, the choice of dock type comes under discussion at a very early stage. It usually turns out that the geographical location of the yard is an overriding factor; that is, the depths of water and the subsurface conditions at the site are largely decisive.

Where a large dock is required, a graving dock can often turn out to be the most economical solution, especially if it can be built as a naturally drained dock and can be engineered without excessive cofferdam arrangements and, nonetheless, in fairly easily excavated soil. But if the local ground conditions mean that the graving dock must be of the gravity type with bottom anchoring, then the floating dock will often be a cheaper solution.

It must be remembered, however, that initial costs alone cannot determine the choice of dock type; the type of work to be done must be allowed to exercise an influence — perhaps an overriding influence — on the choice.

The advantages of a floating dock include the following: It is more easily oriented relative to winds and current so as to bring the most prevalent effect of these in the line of entry. This will, of course, facilitate every docking and undocking and will reduce the risk of damage.

In a floating dock, the time required to lift and reimmerse the dock is directly proportional to the size of the ship, whereas the opposite is true in a graving dock —that is, the bigger the ship, the quicker pumping-out and refilling can be achieved.

*Mr. Andersson, executive vice president, Gotaverken AB, Goteborg, Sweden; Mr. Buczkowski, naval architect, Prorem, Gdansk, Poland, and Mr.

Doerffer, professor, Technical University, Gdansk, Poland, presented a paper entitled "Some Aspects of the Design and Building of Large Floating Docks" at the recent Annual Meeting of The Society of Naval Architects and Marine Engineers. The article appearing here is an abstract of that paper. The full paper may be obtained from the Publications Department, The Society of Naval Architects and Marine Engineers, 74 Trinity Place, New York, N.Y. 10006.

A floating dock can always be re-sited.

The very character of a floating dock, similar to a ship's, although with much longer life expectancy, means that it can be registered and mortgaged with far greater ease than a graving dock—which creditors would find very difficult to take away with them in case of a default.

Where tidal variation is minimal, access for vehicles to the pontoon deck is easily arranged.

The open-ended nature of the dock leads to more favorable paint-drying conditions.

Vessels with excessive trim can often be docked by a floating dock, where a graving dock cannot be used.

There are several disadvantages, which could be overcome as follows: Bottom loading has always been the major problem in floating docks, but these difficulties can be virtually eliminated by suitable compensatory arrangements at given parts of the floating dock's bottom.

Another and perhaps often decisive difficulty has been found in the provision of adequate cranage for a floating dock. The width of the sidewalls is a major limiting factor. But, by using a gantry crane, the entire area of the dock can be reached. At Gotaverken AB, we used a gantry crane with a capacity of 150 tons, which ought to be enough in a repair dock. The crane is provided with a side extension of 52V-> feet, enabling it to pick up a load from barges moored alongside the dock.

There remains the difficulty of finding sufficient storage space in convenient proximity to the dock.

To reduce this disadvantage, the Gotaverken dock has been provided with a ramp for loads up to 150 tons that can be moved by hydraulic transporters, making it easy to pick up and readily position sections, machinery, and such in shops or in storage yards.

A floating dock usually costs more to maintain, but with modern anticorrosive preparations it is possible to extend its life considerably.

The steady advance of techniques for underwater inspection and treatment makes it unnecessary to render the dock "self-docking." Divers and underwater cameras may be relied upon.

As a result of the use of ramps, securing of the dock to dolphins has been preferred to the use of anchors and chain cable.

Large floating docks are better suited for docking VLCCs than graving docks.

Floating docks of 50,000-tonsand- over lifting capacity in operation according to Lloyd's Register of Shipping Appendix 1974-75 were: Bethlehem Steel Corporation's San Francisco, Calif, yard (65,000 tons) ; Howaldswerke- Deutsche Werft, Hamburg, West Germany (53,000 tons) ; Overseas Drydock Co., Tsing Yi, Hong Kong (50,000 tons) ; Italcantieri, Castellamare di Stabia, Italy (52,500 tons) ; Rotterdam Dockyard Company, Rotterdam, the Netherlands (54,000 tons), and Gotaverken, Goteborg, Sweden (55,000 tons). There is very little information published about these large docks.

Dock Dimensions The principal dimensions of large docks are based upon the dimensions and weight of ships that are intended for docking.

The breadth of the dock depends mainly on the breadth of the biggest ship to be docked. On each side of the docked ship a certain space must be left free for staging, as well as for working devices for mechanized cleaning and painting of the ship's sides and to give sufficient air, light, and access to the ship's bottom. The width of that space on large docks should not be less than 10 feet. The minimum width of inner wall shoring stage — if fitted — should be 2V2 feet.

The width of sidewalls is governed by dock stability requirements, by space requirements for workshops, accommodations for crew, etc., and by dock cranage arrangements. Generally, it is considered that a 161/2-foot width should be sufficient.

The height of the sidewalls depends on the maximum draft of the ship in docking condition, the height of the keelblocks (approximately 6V-> feet), the clearance between the keelblocks and the ship bottom (about IV2 feet), the freeboard (usually 3Vt feet), and the height between the freeboard and upper decks (usually about 10 feet).

The height of the pontoon depends on the buoyancy requirements, the amount of residual water and ballast water, and the freeboard to the pontoon deck at centerline when supporting a ship whose displacement equals the lifting capacity of the dock (not less than 1 foot). The height of residual water is generally about 1 foot. The usual amount of water ballast equals 30 percent of the lifting capacity of the dock.

The length of the pontoon is the length of the bottom caisson exclusive of the portable end platforms. It is governed by the length of the biggest ship that is meant to be docked. For big ships with full ends, the length of the pontoon equals 0.9 times the length of the ship.

Supporting The Ship The ship in a dock should be supported in such a manner that her weight is most evenly distributed over all the supporting blocks without causing any concentration of loads upon individual blocks or upon certain areas.

Such concentration may cause damage not only to the blocks, but to the ship's bottom structure.

Thus far, the support of large, all-welded ships with flat bottoms of large extent has been investigated very little. Occasional damage to keelblocks or to ships' bottoms has been attributed to lack of skill in docking.

Gotaverken AB has recognized the importance of this problem and has made a thorough investigation in order to establish the exact distribution of the loads on the blocks supporting the ship and the longitudinal bending of the ship-dock system.

It was thought that the best method of measuring the load distribution was to measure the compression caused by the docked ship on individual wooden supporting blocks. Thus, it was necessary to establish with a high degree of accuracy the characteristics of wooden supports under loading. For this purpose, three sets of keelblocks were used, one from the after end of the dock, one from midships, and one from the forward end. The keelblocks were compressed in a 375-ton hydraulic press.

The compression tests proved that of the two kinds of timber used, oak and pine (old and new), the old and water-soaked timber behaves more uniformly than new timber. The flow limit, which is well marked in new pine, disappears in old pine that has been compressed many times and contains a lot of water. There was no marked crushing of timber under the test loads, with the exception of the new pine, in which several cracks appeared along the grain.

The deformation of pine under a similar load was about five times greater than that of oak.

Having established the characteristic deformation of timbers and keelblocks, it was possible to measure the distribution of the load over all supporting structure.

Special measuring sticks were fitted on each keelblock and sideblock. These sticks were displaced while the blocks were compressed under the weight of the ship, and the amount of displacement was the measure of load taken by each block.

Conclusions The following conclusions were drawn from this investigation: Floating docks are better suited for docking large vessels than any other type of docks, because they can closely align themselves to the bottom line of the ship and thus prevent excessive bottom pressure that could cause damage to the ship.

Floating docks can be trimmed during docking operations, thus decreasing the heel pressure on the few after keelblocks.

In order to avoid excessive bottom pressure and excessive longitudinal bending of the ship-dock system, the displacement of the dock should correspond with the weight curve of the docked ship.

Thus the water ballast in the dock must be distributed in tanks in accordance with this requirement.

All the supporting blocks should be of the same design, have the same combination of timber, and be well leveled in order to arrive at the most even distribution of the load.

The layout of supporting blocks should correspond with the internal structure of the ship to avoid excessive stresses in the ship's bottom structure. As all large ships are very wide and very seldom have a centerline bulkhead, supporting them on keelblocks only is not acceptable. Widely spaced sideblocks are not sufficient to support ships with two longitudinal side bulkheads. Thus, two or three rows of supporting blocks in way of the flat part of the bottom should be arranged.

Side rows should be arranged each time to correspond with the bulkhead spacing of individual ships.

Moving of closely spaced sideblocks is labor demanding and impedes access to the center part of the flat bottom. The investigations showed, as regards the loading on the dock, that no great variations would be encountered if about half of the ship's length was supported only on sideblocks.

It should be possible, therefore, to avoid the complications of rise of floor and evolve a block system which would not require individual adjustment for each ship.

Docks should be fitted with instruments to measure stresses in the dock bottom and sidewalls, as well as loads on supporting blocks.

When the relatively even and controlled distribution of pressure on the blocks can be ensured in this way, it is possible to reduce their number considerably. Thus, larger areas of the bottom are exposed for both painting and repairs, and it also becomes easier to mechanize such routine jobs.