MULTIPLE VESSEL DRY-DOCKING AND OTHER ISSUES RELATED WITH THE OPERATION OF A FLOATING DOCK G. Aguirrezabala, Tsakos Industrias Navales, Uruguay SUMMARY When the main tool of a ship repair shipyard is a floating dock and the market asks for extreme flexibility —like un-docking a bulk-carrier one day and dry-docking six fishing vessels next— the technical procedures must support the commercial efforts to make such operations possible. In this paper, we will analyse the way to dry-dock multiple vessels simultaneously leaning them only on a row of keel blocks and the corresponding side wall of the floating dock. We also consider problems relating to the location of side blocks, in the case of tankers or bulk-carriers, due to the difficulties arising from attempts to match the strong points of the dock and the vessel’s structure. Finally, the paper will discuss how dredging under floating docks is feasible without disturbing their operation. 1. INTRODUCTION Owned and operated by the Tsakos Group, and strategi-cally situated in a natural bay in the Port of Montevideo, Tsakos Industrias Navales is one of the largest shipyards in the region and provides ship repair services to mer-chant vessels plying South Atlantic trading routes and to the South Atlantic operating fishing fleets. A floating dock of 200x40x16m with a 32m internal breadth and 20,000 ton lifting capacity can accommodate vessels up to 75,000 dwt for dry docking, repair, mainte-nance and conversion works. A smaller dock of 60x18x10m with a 14m internal breadth allows the yard to attend dry docking and repairs of small vessels.

Taking into consideration the diversity of the above men-tioned market segments and the dry-docking means at our disposal, a flexible work schedule is mandatory to compete at the top of shipyard’s capacity, keeping the windows open for new projects. In our particular case, added to that tension between commercial and technical possibilities, there is the neces-sity to continuously fight against the mud deposited un-der our floating docks by the harbour currents, threaten-ing the maximum available draft over the keel-blocks and thus our selling capability.

According to Schor [1]: “Shipbuilding involves a long period of construction and many repetitive operations which permit use of detailed production planning and control procedures, similar to those employed in manu-facture. The ship repair yard, on the other hand, does not enjoy sufficiently repetitive work or constancy of volume of work to permit the adoption of rigid production plan-ning and control procedures. However, a flexible system with procedures varied to suit the type of job, can be applied”. The purpose of this paper is to share our experience re-garding how certain operational procedures can help to face the natural menaces and inherent conflicts of the activity. But, before that, we will give a glance to the cauldron where those procedures are boiled. 2. AT THE EDGE OF CHAOS Consider the following typical situation: • After the staying in the floating dry-dock of a bulk-

carrier, a tanker is expected to come. That means that only small changes in the distribution of side blocks are necessary in order to match the strong points of the dock to the new vessel’s structure.

• A sudden delay of the tanker opens a window that can be covered simultaneously dry-docking several fishing vessels whose requests were kept warm in the commercial oven. That is possible only if they can be dry-docked immediately after undocking the bulk-carrier, using the same floating dock but with a totally different blocking layout.

• Once performed that manoeuvre, the shadow of the delayed tanker, like a ghost, urges us to finish the dock works on the fishing vessels before her new es-timated time of arrival. The feasibility of start shap-ing the new cradle for the tanker, using spare blocks, should be analysed to avoid further delays.

• In the middle of the planned dry-docking period, the works in one of the fishing vessels are ready and the owner put pressure on her undocking. Concurrently, the propeller of a harbour tug was damaged, de-manding an emergency dry-docking.

Figure 1: Tsakos Industrias Navales premises.

• Next morning, in the same water, the finished fish-ing vessel is undocked and the tug-boat docked, making happy two owners and frown the others. It was necessary to work during night preparing the new blocking and assuring the water-tightness of the remaining vessels.

• Finally, the expected tanker is canceled, but other ships will come from beyond the horizon… if we adapt ourselves to their requirements.

Such circumstances can properly be considered to occur “at the edge of chaos”. This is not merely a metaphorical expression, it fits our picture into the theoretical frame of complexity and complex adaptive systems (CAS). Between chaos (where prediction is unthinkable) and order (where change is unworkable), there is a land where all is possible: complexity. A complex situation arises when the details cannot be understood but the whole can be appreciated by the ability to make patterns. Like surfing, creativity is only practicable at the complex crest of the wave, nor in the orderly calm sea neither in the chaotic breaking wave. Thus, organizations dealing continuously with complex situations are in the best position to solve them. The ship-repairing industry is a CAS because “is a sys-tem of individual agents, who have the freedom to act in ways that are not always totally predictable, and whose actions are interconnected such that one agent’s actions changes the context for other agents”[2]. In a paper ed-ited almost forty years before, Schor confirms that defini-tion: “The yard organization is not autonomous and does not alone control yard operations. There is a companion external organization of ship owners, naval architects, subcontractors and regulatory agencies which also con-trol operations”[1]. As an individual agent in this world of change, a ship-repair company must learn from experience and foster an environment favourable to the emergence of adaptive patterns of behavior (i.e., operational procedures tailored to the actual marketing requirements and natural restric-tions). Like fractals, CASs are self-similar: equal patterns can be found at different levels. For example, the same complex characteristics of the ship-repair activity as a whole, are reproduced, in a smaller scale, in a typical dry-docking operation. Out there, when the wind changes suddenly and a wire-rope parts, “the edge of chaos” really cuts. In such limit situations, an experienced crew will rely more on self-organized patterns of behavior rather than on a centralized control of details. 3. MULTIPLE VESSELS, ONE DOCK Multiple vessel dry-docking is one of such operational

procedures that allows our shipyard to adapt itself to commercial momentum and physical handicaps. In figure 2 we can see a typical cradle, first step in that procedure. In our floating dock, up to 6 cradles can be shaped to receive the same quantity of fishing vessels (3 per side, supposing a LOA=55m) leaning them only on a row of keel blocks (K) and two side wall pillars (P). The dis-tance between the center line of the keelblocks and the internal face of the pillars is half-breadth plus 25 cm to permit a “leaning list” of approximately two degrees. Empty spaces (E) for echo-sounders and reinforcements (R) where the load concentration is greater (i.e. under engine room) are also considered in the design. Keel-blocks are installed over the longitudinal bulkheads of the floating dock.

In figure 3, we can visualize the next steps. After the dock is flooded, one by one the ships are hauled in until they reach her own reference mark in the corresponding side wall. Once in position, the ship must be listed prop-erly (see fig. 3A) using weights (W). When all the ves-sels are ready, the dock starts rising trying to copy the average trim of the ships in order to reduce the stress in their knuckles (see fig. 4). This is the most difficult step due to the characteristic instability of fishing vessels in docking condition and also due to the their loss of stabil-ity at the first touch between keel and blocks. During this period is necessary to check constantly that the “leaning list” is the correct one in each vessel. Continuing with this process, all the ships finally will rest completely over the keelblocks leaning on the pillars, in isostatic equilib-rium, like shelved books. A set of wire ropes (R) secure the vessels against any unexpected change in the dock’s list (see fig. 3B) that, otherwise, must be kept adrift. Immediately after the pontoon deck is over the water level, our riggers install the bilge blocks (S) to complete the cradle (see fig. 3C). If necessary, the centering of the vessels can be checked with the help of divers, in the stage corresponding with fig. 3B. The advantages of this system are: • In a floating dock designed to dry-dock one or two

ships at the center line, six or more vessels can be

Figure 2: Typical cradle

simultaneously dry-docked using the side walls. • The absence of large bilge blocks allows the en-

trance of vessels with higher drafts due to the lack of interference (see fig. 3A).

• Due to the simplicity of the cradle, only a general arrangement drawing is necessary to design it. Trustworthy docking plans and line drawings not always are available on board fishing vessels (regret-tably, some fishermen and fishing companies think that ships are a necessary evil).

• Pillars help to center the vessel and permit to handle the ship from one side of the dock only. Thus, two vessels can be entering to the dock at the same time.

But it has also some drawbacks: • The list of the ship must be less than three degrees

(in order not to over-stress the pillars) and more than one degree (to avoid the risk of tumbling the ves-sel). To check it continuously during the “touching” stage, in six vessels at the same time, no instrument is better than the naked eye.

• Complete blocking (including bilge blocks) to assure vessel’s stability during dry-docking is possible only after the pontoon deck is over the water level. Movement of weights on board, before the bilge blocks are installed, are absolutely forbidden.

• Welding of pillars to the pontoon deck and to the side wall is a very time consuming task.

Anyway, this procedure proved its reliability during more than thirty years of use —with only two minor incidents in his record, which fostered additional safety measurements—, conjugating the idea of Rear Admiral Dr. Grace Murray Hopper: “A ship in port is safe, but that’s not what ships are built for. Sail out to sea and do new things”. 4. ONE VESSEL, MULTIPLE PROBLEMS Another set of docking problems arises when one big vessel is dry-docked in a floating dry-dock. The structure of a steel floating dock is usually considered weaker than the structure of the vessels to be dry-docked in it. Thus, the primary concern of a Dock Master is to protect the dock and the vessel against local and general over-stresses. For instance, if the structure of the dock is transversal, then the position of the side blocks is defined by the intersection of the longitudinal members of the vessel’s bottom and the transversal bulkheads of the floating dock. Other matching is possible only by coincidence or tailoring. But, in some cases, the fact that vessels are stronger than the floating docks and that they can be considered like rigid bodies, is only a myth. Certain ships (for example, those with thin high tensile steel hull plating and propor-tionally large beams) can hide a transversal hogging that

(C)

(B)

(A)

P

R

W

W

W

S

S

S

S

K

K

P

R

S

S

K

P

Figure 3: Front views of three docking steps.

not always is considered when calculating the height of the side blocks used to support a “flat” bottom. In order to avoid buckling the plates associated to the above men-tioned longitudinal members, such height could be up to 25mm less than the height of the corresponding keel blocks. On the contrary, overhanging is more spectacular but not necessarily harmful. According to Díaz [3], is possible to dry-dock vessels in a floating dock with cantilevers up to 15% of their length without affecting the structural strength of the “dock-ship beam”. 5. DREDGING UNDER Due to the currents present in the Montevideo Harbour, the quantity of mud deposited under our floating docks (the deepest points inside the bay) is very high. To fight against this permanent diminution of the available dry-docking draft we have two alternatives: batch dredging or continuous dredging. The first method implies the movement of the dock in order to free the water surface over the pit for a normal dredging operation. That option not only is very expensive but it is also associated with huge earning losses. The second method, with the help of an “elephant trunk” attached to our suction dredge IHC Beaver 1500 (see fig. 5), permits a permanent dredging operation. The dock itself can assist to this process press-ing, when sinking, the mud into the trenches dredged laterally. Thanks to this solution we can keep good drafts for our customers, in a more efficient way. 6. CONCLUSIONS Moving heavy weights is always an eye-catching task. Most of time slow and clumsy, never faster than a man walking, such movements have a visual inertia, an expec-tation that something extraordinary could happen, that make each of them unique, even tough they are repeated time and again. If you add to that recipe the salty flavour

of the sea, the result is exhilarating. At least for us, shar-ing our experiences dry-docking, lifting and launching vessels in such different ways that make us really rich in possibilities and understanding. A last word about that words (docking, lifting and launching) that guided us to this conference: despite the inevitable shift of our inter-est, technical and economical, from the sea crafts to the space crafts, they will stand in the future. 7. REFERENCES 1. SCHOR, R., ‘A Method for Increasing the Efficiency of Ship Repair Yard Operation’, paper presented at the Spring Meeting, Philadelphia, Pa., of The Society of Naval Architects and Marine Engineers, May 19 and 20, 1955. SNAME Transactions, 1955. 2. PLSEK, P., et al, ‘Some Emerging Principles for Man-aging in Complex Adaptive Systems’, <http://www. plexusinstitute.com/edgeware/archive/think/main_filingl. html> Version: November 25, 1997.

Figure 4: A longitudinal view showing the different trims of the vessels and of the dock

Figure 5: Dredge with attached “elephant trunk”

3. DIAZ, N., ‘Puesta en Seco de Buques en Diques Flo-tantes cuando su Eslora es Apreciablemente Mayor que la Eslora de Picaderos’, Tandanor, 1973. 8. AUTHORS BIOGRAPHY Germán Aguirrezabala holds the current position of Technical Manager at Tsakos Industrias Navales S.A. He is responsible for the technical support activities of the yard, like purchasing, estimating, drawing, special projects, dredging and dry-docking operations since 1996. He is a diplomed Naval Engineer.