Journal of Ship Production, Vol. 3, No. 3, Aug. 1987, pp. 155-164

Journal of Ship Production Shipyard Facilities--New and Old Closures for Dry Docks Jay J. Hassani 1 Among the many old and new shipyard facilities, this paper depicts three types of closures for dry docks, one old and the other two new, and discusses them from the standpoint of operation, capital investment, life cycle costs, and adaptability to specific sites. The first is a sliding caisson, used in the 40's, the second an innovative double keel caisson, and the third is a seldom-used cantilever-type flap gate, which appears to have great potential. In addition, a recently installed intermediate dock gate is presented.

Introduction IN AN EFFORT to cope with today's depression in the shipbuilding and ship repair industry, shipyards are introducing a variety of cost-saving devices such as dock arms, to replace the old portable stages; sophisticated shipguide systems to take the place of manual hauling of ships into dry docks; quick opening and closing dock gates; and intermediate gates to allow repair of smaller ships in larger docks. This paper focuses on old and new entrance closures for dry docks. Among these, the floating caisson is by far the most popular type used in U.S. dry docks, because it is relatively low in initial cost, is reversible, can he used in two or more seats, is mobile, is relatively easy to handle, and can readily be replaced by a spare gate of identical shape and dimensions. It is, however, slow in closing and opening, uses considerable manpower and depends on shore utilities for its operation. The U•S. Navy has adopted some types of floating caissons as a standard and has included them in their design manuals to serve as a design guide for architects/engineers. Two specific types of caissons, one old and the other new, are discussed from the standpoint of operation, capital investment, life-cycle cost and adaptability to a specific site. The first type is a sliding caisson which has been used in some foreign docks, built during World War II, and the other is an innovative type of caisson which never needs to be drydocked for maintenance, painting and small repairs, thereby greatly minimizing the downtime of the dry dock. A third type of entrance closure, a cantilever-type flap gate, which has some desirable features, is a type of gate that has a potential for use in future docks. Table 1 lists important characteristics of dock closures. Quick operation, minimum manpower requirement and low life cycle cost are the most important characteristics which a repair yard owner wants for his dry dock. This has encouraged the development of flap gates, which are described later. In earlier docks, however, the main question was how to design a closure device for docks which were growing in ~Vice president, Century Engineering, Inc., Towson, Maryland. Presented at the January 29, 1986 meeting of the Chesapeake Section of THE SOCIETY OF NAVAL ARCHITECTSAND MARINE ENGINEERS.

AUGUST 1987

width and were getting too large for the old conventional miter gates. During World War II susceptibility to war damage and security became important selection factors, and gradually all seven factors given in Table 1 became major considerations in the selection of the right closure for building and repair docks. F r e q u e n c y of m a n e u v e r i n g a c a i s s o n For each full drydocking and undocking cycle of a ship the caisson will normally be maneuvered four times. If special circumstances permit the admission of the next ship immediately after floatout of the preceding ship, the caisson will be maneuvered only three times. The sequence of operation is as follows: 1. Dock dry, caisson in place, blocks placed and secured (starting condition). 2. Flood dock. 3. Remove caisson• 4. Admit ship into wet dock. 5. Place caisson. 6. Dewater dock chamber, set ship on blocks, wash down floor• 7. Perform required work on drydocked ship.

Table 1

Important characteristics of dock closures

Important for:

. Rapid operation (opening/closing) • Minimum requiredmanpower and tug assistance 3. Ease of maintenance 4. Low capital investment 5. Low life-cycle cost 6. Minimum disruption of dock operation during repair and maintenance of caisson 7. Capability of carrying vehicular traffic

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• • • • •

One dockmaster Three capstan operators Three line handlers One electrician One pump operator

Each maneuver of the caisson (removal or installation) takes approximately 30 minutes of active work, but one docking or undocking evolution of a ship, which includes removal of the caisson, admission or floatout of a ship and placement of the caisson, normally takes four to six hours. Of course

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156

JOURNAL OF SHIP PRODUCTtON

the men working on the caisson will also work on the docking/undocking of the ship. Figure 1 shows diagrammatically the arrangement of the lines, capstans and deck fittings during the removal of a caisson and movement to a nearby pier. The caisson is shown in its closed, intermediate, and stored position with arrows indicating the pull in the lines. It should be noted that some dry docks remove and install the floating caisson with the assistance of the same tugs that are used for the admission or floatout of the ship, and move it to and from a remote berth, Figure 2 shows diagrammatically the installation procedure of a caisson and movement from the berthing position to the closed position.

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where V = volume of water displaced, ft 3 I = moment of inertia of waterplane about longitudinal axis, ft 4 W = weight of water displaced, lb O = angle of inclination from vertical (not exceeding 10 to 15 deg) G B = pendulum, ft In smooth water the pendulum G B may be from 15 to 24 in. and the metacentric height may be large, but in rough waters the metacenter should be kept low in order to ensure ease of movement (see Fig. 3). The metacenter should be about 2 ft above the center of gravity, the pendulum not less than 18 in., and G should be 20 to 24 in. below B. The floating light draft of a caisson is usually fixed by its drydocking requirements. For example, if the caisson needs to be drydocked in a floating dock of limited draft, the minimum draft of the caisson will be limited accordingly. In the design of floating caissons, tedious calculations must be performed to establish the center of gravity of the structure, the center of buoyancy, the weight of the water ballast, and fixed concrete ballast, so that characteristic curves can be drawn showing graphically the conditions of the caisson at various drafts. The important characteristics are: • Minimum freeboard of caisson (maximum draft), when caisson is afloat. Minimum draft (no water ballast) when caisson is afloat. • Maximum height of water in main ballast tank to seat caisson when outside water level is at high water. The Navy requires that the minimum draft of the caisson be such that it can be raised or seated at MLW (mean low water). Therefore, the fixed ballast must be calculated to satisfy this condition. The Navy also requires that the water ballast must be such that the caisson can be seated at MHW (mean high water). Dip pipes (vertical pipe whose lower end is "dipped" in bal-

last water and upper end vented to atmosphere) are required to prevent overfilling the ballast tank.

Sliding caisson The general plan of a dry dock (Fig. 4) shows a sliding

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caisson, a chamber--in the shape of a minidock--to house the sliding caisson when it is in the open position, an intermediate floating caisson to divide the dock into two compartments, and an outer seat---identical to the seat of the intermediate floating caisson seat. This dock, with its sliding and floating gates, built in the 40's is still in operation at the Dorbyl Shipyard in Capetown, South Africa (see Fig. 5). A similar dock with a sliding gate and floating caisson is also in operation at the Dorbyl Shipyard in Durban, South Africa. Photographs taken at the Durban Yard, Figs. 6 through 9, show the caisson being pulled into its chamber. The wooden deck of this sliding caisson, designed for vehicular traffic, folds down as it is pulled into the caisson chamber. This is effected by the hauling chains and mechanism which pull counterweighted vertical supports of the deck around series of hinges, as illustrated diagrammatically in Fig. 5. Grooves and slots could be fitted along the bottom and sides of the entrance to the chamber, for placement of stop logs, so that the chamber could be dewatered for repairs of the sliding caisson in place. In this particular dock, however, a groove is provided just outboard of the sliding caisson, for the placement of the floating caisson, which can serve as the main dock closure in case the sliding caisson is in need of repair. The cross section in Fig. 5 shows diagrammatically the arrangement of the counterweights, and the longitudinal section shows the arrangement of the vertical members and the hinges which facilitate the falling action of the deck. This deck is held in position by a locking device when in the upright position. The main advantage of this type of caisson is its speed of operation. It can be opened or closed in about 7 min, for a dock width of 125 ft. The hauling mechanism provides a crawling speed of 2.5 fpm and a traveling speed of 25 fpm. The sliding caisson has two watertight decks, B and C, which form the flotation chamber. The compartment below the flotation chamber has no bottom, thereby allowing water to rise in this compartment. Two open trunks are provided through the air chamber and allow water to rise to the tidal chamber above the flotation tank. From the standpoint of site adaptability this type of sliding caisson can generally be used at docks which are recessed behind the shoreline, rather than built offshore between two moles, such as the Bethlehem Steel Dock at Sparrows Point (see aerial photograph in the Appendix). AUGUST 1987

Fig. 7 Caisson being pulled into chamber. Note falling deck and reclining handrails

Fig. 8 Top deck of sliding caisson fully reclined. Note difference between yard level and top of caisson

Fig. 9 Closer view of wall recess and watertight seal of caisson 159

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Double-keel caisson When a floating caisson itself is drydocked for maintenance or repair, the dock it serves is out of commission. This means loss of revenue for the shipyard. To overcome this deficiency, in the mid- and late-70's the naval architects at Sunderland Shipyards in England developed a prototype of a caisson that never needs to be drydocked. This caisson, designed for their fully covered dock at Pallion, has a double keel and is designed for ballasting and deballasting through rising and falling tides of the River Weir, is adaptable to only one seat location, and has other features such as tracks on its weather deck for sliding doors. We have studied this type of double-keel caisson for one of our drydock projects to see if we could thereby save the cost of a spare gate. We developed a concept design, considered its operational and functional features, and estimated its life cycle costs to determine if it could be adapted to meet our specific design criteria, which were: 1. Eliminate the need for a spare caisson. 2. All parts of the caisson, inside and outside, shall be accessible for painting, maintenance and minor repairs, while the caisson is in service. 3. Caisson shall be suitable for installation in one of two adjacent seats, so that the inner seat can be repaired while the caisson is in its outer seat. 4. Caisson shall be designed for conditions when dock is superflooded. 5. Removal and installation characteristics shall be comparable to those of a standard caisson. 6. Caisson shall be reversible. Figure 10 shows a plan, a longitudinal section and a typical cross section of the specific caisson we developed. The main advantage of this type of caisson over the conventional type is that the stems and the bottom of the gate are accessible for maintenance and minor repair while the caisson is in service. This is possible because the configuration of the 160

gate abutment provides for recesses for access by maintenance personnel. Fifty percent of the caisson is accessible from the inside of the dock, when the caisson is in place, and the other 50 percent is accessible during the next placement period, when the caisson is reversed. When the caisson is in place, one of the two keels is resting on pedestals located in a long pit, from which all parts of the caisson are accessible for maintenance, painting and minor repair. The cross section and the cutaway plan and longitudinal section of Fig. 10 show the pedestals on which the interior keel rests. The plan view shows the accessibility of the two stems of the caisson via two sets of stairs, and the longitudinal section shows the pedestals which provide bearing for the inner keel. For maintenance of the inside of the ballast tanks, the individual tanks can be emptied one at a time without affecting stability, while the caisson is in place and the dock is dry. Our investigation showed that the interior volume of the ballast tanks on the double-keel caisson is approximately 20 percent greater, and therefore the pump sizes must be increased to compensate and maintain the same deballasting time. The flood valves for the ballast tanks must be increased in size for the same reason. The site adaptability of this type of caisson is the same as that for any other floating caisson.

Cantilever.type flap gates Flap gates are known to be less expensive to build and easier to operate than caissons. Among the different types of bottom-hinged flap gates, we shall focus on the one which is cantilevered from the sill. There are basically four types of flap gates used in dry docks: JOURNAL OF SHIP PRODUCTION

1. Bottom-hinged caissons. These are captive caissons and therefore have a smaller beam than the equivalent freefloating caissons. 2. Bottom-hinged, single stiffened-skin flap gates, strutted on the inside, with auxiliary flotation tanks. 3. Bottom-hinged, single stiffened-skin gates with large flotation tanks operated by compressed air. 4. Bottom-hinged, double-skin gates, cantilevered from the sill, with flotation tanks operated by compressed air. A typical section of the fourth type is shown in Fig. 11. The main features of this type of gate are its simple construction, speed of operation, and the fact that it requires no ropes or winches. Although the cantilever-type flap gate is seldom used in docks (the author knows of only one, at the Sembawang Yard in Singapore), it has potential, provided the site-specific problems are effectively dealt with. These site-specific problems are the accumulation of silt and mud in the below-sill recess, and occasionally the lodgment of an unseen obstacle in the recess, which prevents the flap gate from completely falling into its recess. To alert the dockmaster to such an occurrence some yards use small floating buoys attached to the top of the flap gate by strings of fixed length. If the buoys show up above water when the gate is in its down position, it is an indication that the gate is not fully seated and that a diver's services are required to remove the obstacle. In general, however, the disadvantages are outweighed by the advantages of rapid operation, lower life cycle cost, and the absence of a berthing space requirement, as in the case of a caisson. The design of the cantilever-type flap gate is made for a vertical cantilever section of unit width, resisting the triangular water loading. The design of one unit width can then be applied to any width of dock entrance. This type of flap gate is adaptable to any site where a dry dock can be built. Life c y c l e c o s t s o f c l o s u r e s

The life cycle cost of a complete dock closure installation consists of the following items: 1. Initial cost of the closure itself. 2. Cost of operation. 3. Maintenance and repair costs. 4. Cost of the abutments and their maintenance. 5. Cost of a spare gate, if uninterrupted use of the dry dock is mandatory. Due to the numerous variables which affect any one of the above cost items, and therefore the resulting life cycle cost, a meaningful life cycle cost analysis with realistically estimated figures can be made only when the size of the dock closure, the frequency of its use, labor rates, geographical location of the facility, the specific geotechnical site conditions, and special criteria (roadway over gate, etc.) are known. Here we attempt only to provide some notes and guidelines for computing each one of the aforementioned cost items. First let us consider a realistic period for life cycle analysis. Since the present market conditions are susceptible to change within the next 25 years, it is suggested that life cycle costs for shorter periods, say 10, 15, or 20 years, be computed and used in the decision-making process. Initial cost of dock closure Considering only steel gates or caissons it is best to perform preliminary strength calculations and arrive at steel weights. Although actual weights of previously designed dock closures could be taken as a guide, they are not recommended because of the great number of variants between the gate under investigation and the previously designed gates. AUGUST 1987

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In the case of caissons, the preliminary design may be based on the horizontal trapezoidal water loads to be resisted by the horizontal girders that span the space between the jambs of the dock entrance structure. The horizontal girders can be assumed to be composed of the horizontal deck structures and portions of the skin plating. Such an analysis will result in bending stresses from which the assumed sizes of the girders and skin plate can be checked. Of course, preliminary stability calculations must also be performed, once a satisfactory preliminary structural design has been obtained. In the case of a double-keel caisson a set of similar preliminary calculations may be performed, except that experience has shown that the weight of steel for this type of caisson is approximately 10 percent greater than for the standard-type caisson. Preliminary structural designs for bottom-hinged, doubleskin cantilever-type gates can be performed by analyzing a section of unit width and applying it to the entire width of the dock. The unit width shall consist of a vertical cantilevered section consisting of one frame and the stiffened skin plating. The effect of the watertight decks can be considered in the final design of the cantilevered gate. Of course all other costs, such as the cost of machinery (in caissons) and installation, must be considered. Cost of operations

If a suitable mooring location is available near the dock entrance, so that the caisson can be transferred to its mooring location by ropes and winches, without tug assistance, one docking or undocking evolution will require nine men for approximately four hours. The number of annual dockings and the labor rates will determine the annual cost of manning the gate, and adding the cost of fuel and power will result in an estimate of the annual operating costs. Maintenance and repair costs

Two items of routine maintenance are required for steel caissons and gates: one is painting, outside and inside, once every five years, and the other is the replacement of rubber seals, usually once every ten years. This normally requires 161

Fig. 12 Old dock at Burmeister & Wain, Copenhagen, closed by a ship-type caisson

Fig. 13 Cantilever-type flap gate at Sembawang Drydock, Singapore

drydocking of the caisson unless the caisson is of the doublekeel type, which can be painted and have its seals replaced in place during two consecutive dockings. As for repair costs of the mechanical/electrical equipment of the gate, a reasonable estimate would be 3 percent of the machinery cost, per year.

Intermediate gates

Cost of the entrance structure and its maintenance

The entrance structure is normally a U-shaped concrete structure, with grooves and recesses, forming the sill and the jambs of the gate. It is the outboard end of the dock body and as such its cost is usually considered part of the dock body. For cost study purposes, however, it is best to limit the cost of the dock body to the usable length of the dock, and to consider the part beyond that the entrance structure. Experience has shown that caisson loads on the sill and jambs, when the dock is dry, are easily resisted by the dock floor and walls, whereas loads due to a flap gate of the strutted or cantilever type require special sill designs, which makes the entrance structure of a flap gate more costly than that of a caisson. Maintenance costs of entrance structures are usually minimal. The important elements, the so-called meeting faces of the gate stems and keel with the jambs and sill of the entrance structure, are made of first-class material to alleviate the need for maintenance. These meeting faces used to be made of carefully cut and fitted granite, but today they are constructed of high-strength concrete faced with stainless steel. Docks built in the 20's with granite meeting faces are still in use today. Cost of a spare gate

If uninterrupted use of a dedicated dry dock is mandatory, such as may be the case with some Navy docks, a spare gate of lower quality and lower cost is provided. For this purpose either a concrete caisson or a sectionalized steel gate may be adequate. The concrete caisson, although less expensive (perhaps 60 percent) than a steel caisson, is somewhat more cumbersome to maneuver and the sectionalized steel gate requires more time to install and dismantle. Moreover, cranage is required to handle a sectionalized steel gate. Maintenance cost of the spare gate must be considered, although due to its rare use only minimum maintenance may be required. 162

During the past 20 years intermediate gates were used in very large Far Eastern dry docks for the purpose of building ships in tandem. Today intermediate gates are installed in large docks which are too long and too uneconomical to use for the present shipbuilding and ship repair market. Bethlehem Steel Corporation recently has installed an intermediate gate in their 1200 × 200 × 37.5 ft dry dock at Sparrows Point. The Appendix includes a typical section through the gate and illustrates the docking and undocking procedure of a ship in the forward compartment of the partitioned dock. The three 80-ton sections are placed in position with the aid of the 200-ton dockside revolving cranes. It takes about 30 min to install each of the three sections and make the necessary fine adjustments to seat the gate. Photographs of contrasting-type dock closures are shown in Figs. 12 and 13. Bibliography Design Manual, Naval Facilities Engineering Command, NAVFAC DM 29.1, Graving Docks, May 1982, pp. 29.1-70-29.1-78. Dock and Harbour Engineering, Vol. 1, The Design of Docks, 2nd ed., Dock and Harbour Engineering Magazine (London), 1968. Paterson, D. E., "The Sturrock Graving Dock, Cape Town" in Proceedings, Institution of Civil Engineers, London, Oct. 1947. Sewell and Searle, "A New Design of Flap Gate for No. 4 Dock, Malta" in Proceedings, Institution of Civil Engineers, London, Vol. 33, Feb. 1966, pp. 161-181. Metric Conversion Factors 1 f t = 0.3048 m 1 in. = 25.4 mm

1 ft ~ = 0.0283 m 3 llb = 0.45kg 1 long ton = 1.016047 metric tons

Appendix 2 The intermediate-gate solution During the prosperous shipbuilding years of the 60's and early 70's, large ships were built in large graving docks. Now ten to twenty years 2This Appendix is t a k e n from a n article by the a u t h o r t h a t appeared in the Baltimore Engineer, Dec. 1985 issue. JOURNAL OF SHIP PRODUCTION

later, some shipbuilders are the proud owners of large graving docks but with few or no ships to fill them. To compound matters, a major oil company announced recently that they intend to dispose of their entire fleet of large crude carriers. Other oil companies may follow suit. Yards with large docks have to decide whether to sell or remodel their facilities to suit the current market of ship conversion, and the building and repairing of smaller vessels. One answer to the problem is to partition the dock with an intermediate gate, and use one part for building smaller vessels and the other for repair. Bethlehem Steel Shipyard at Sparrows Point is one of those yards which decided to fabricate a removable intermediate gate.

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It is a removable steel wall placed in a predetermined location in the dock. The wall panels may be vertical or sloping. They are usually strutted to the dock floor, as shown in Fig. 14. Rubber seals on three sides of the panels provide the necessary watertightness. The intermediate gate usually comes in three or more panels, depending on the lifting capacity of the dockside cranes. When the intermediate gate is part of the original design of the dock, its panels are usually vertical and tied down to the dock floor to prevent the uplift caused by the overturning movement of the horizontal water pressure. When the intermediate gate is installed in an existing dock, its panels are usually on a slope, creating no uplift on the floor and causing the water to push down on the rubber seals for watertightness (see Fig. 14).

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163

Fig. 17 The building basin to be equipped with the intermediate gate is shown in the lower portion of this photograph with a 680-ft cargo ship drydocked within. The building ways are in the middle of the photo, with the outfitting piers jutting out in the upper left

Fig. 18 The scheduling of work is important when an intermediate gate is used since tasks on a ship on the outer side of the gate must be completed before work is finished on the ship on the inner side

It is customary to build only one removable intermediate gate, b u t provide two or more locations in the dock where the gate can be placed.

and out of position with the help of one or two of the dockside cranes operating in unison. When not in use, the gate sections are stored in a suitable location in the yard. Each section, weighing approximately 80 tons, can easily be transported by the Yard's heavy transporter. Sloping concrete pilasters on the i n n e r face of t h e dock walls provide a b u t m e n t s for the two end panels of the intermediate gate, and a shallow trench across the floor and s t r u t foundations provide the required support of the gate sections. The floor trenches are covered with steel cover plates when the intermediate gate is not in place. A 4-ft-wide walkway on top of the gate provides personnel access from one side of the dock to the other.

Bethlehem's intermediate gate Bethlehem's dock at Sparrows Point is 1200 ft long by 200 ft wide by 37.5 ft deep and belongs to the class of very large graving docks. It was designed for t a n d e m construction of ships up to 300000 dwt. Its 1200-ft length can accommodate a full length 300 000-dwt crude carrier plus a stern section of a second 300 000-dwt ship. Work on the full-length vessel and the stern section would be in progress simultaneously until the fulllength vessel was ready to be launched. T h e n the dock would be flooded, the full length vessel launched, and the stern section floated to a forward location for assembly of the r e m a i n d e r of the hull. Of course coordinated scheduling is the key to an efficient tandem-type shipbuilding program. The removable intermediate gate at Sparrows Point can be placed in two positions along the length of the dock, one approximately 300 ft a n d the other some 700 ft from the inside face of the entrance gate. The inboard compartment m a y remain dry, and shipbuilding work m a y progress uninterrupted while the outboard compartment is being flooded for docking and undocking of vessels to be serviced. The caisson gate at the entrance of the dock will t h e n perform its function for t h e outboard compartment only. Figures 15 and 16 show schematically the docking and undocking procedures. (See also Figs. 17 and 18.) The intermediate gate consists of three 67-ft sections, to be lifted into

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Benefits to b e d e r i v e d f r o m t h e i n t e r m e d i a t e

gate

Two or more ships can be serviced or worked on simultaneously. The vessel which requires longer time in dock can be accommodated in the inboard and the ship with quicker t u r n a r o u n d time in t h e outboard compartment. With careful planning and some luck, the inboard c o m p a r t m e n t could accommodate two ships of smaller size. Flooding of the outboard compartment, with the intermediate gate placed at some 300 ft from the entrance gate, can be achieved in approximately one hour and dewatered in three hours. Increased utilization of the dock would result in increased profit for the shipyard.

JOURNAL OF SHIP PRODUCTION