A integridade das estruturas portuárias é essencial para promover a atracação de embarcações com segurança e, para isso, o uso de defensas é muito importante e fortemente recomendado. Acompanhe no texto a seguir a análise realizada pela Kot Engenharia para minimizar danos e aumentar a proteção dos portos nas operações de atracação.

1. The problem of mooring vessels

A utilização de navios de elevada capacidade para transporte de commodities é cada vez mais comum. Por exemplo, temos os grandes navios graneleiros, sendo que os da classe VALEMAX projetados em 2010 são os maiores, com uma capacidade de 400 mil toneladas. Como comparação, podemos citar um navio Capesize da década de 70 que carrega 170 mil toneladas, ou seja, os navios da classe VALEMAX são cerca de 2,3 vezes maiores. A Figura 1 mostra uma comparação relativa do tamanho dos navios: 

Figure 1 - Schematic relative size of the various ship classes [Source: Kot Engenharia]

 

Such a capacity and size of vessels results in a challenge for the port infrastructure, which needs to accommodate these ships safely in its berths.

O processo de manobra do navio no porto é executado por um profissional especializado, o Prático, que possui o conhecimento necessário das condições do porto como correntes, comportamento das marés, vento, canal de acesso, obstruções submersas, dentre outros riscos à navegação. Dessa forma, toda a operação é executada sob os comandos do Prático, desde a manobra do ponto de fundeio até a entrada no porto e a atracação final no berço.

 

2. Mooring charges

As cargas atuantes durante a atracação são muito elevadas, e proporcionais ao deslocamento do navio. Logo, com o notório aumento no deslocamento das embarcações nos últimos anos, os aparelhos de defensas devem ser capazes de suportar a energia envolvida no processo de atracação, preservando a infraestrutura dos berços, a maior parte executada em concreto armado.

Existem diversos parâmetros envolvidos no cálculo das cargas atuantes durante a atracação, sendo que o principal é o componente normal ao berço da velocidade de atracação, Vb. A velocidade de atracação depende do tamanho do navio, das condições de ondas e correntes, vento e possíveis restrições de aproximação no berço. Já a velocidade de aproximação pode ser estimada conforme norma BS6349, PIANC, dentre outras normas, que adotam as curvas de Brolsma. Tal curva é baseada em diferentes condições de aproximação e proteção do porto em questão, que variam de 1 a 5, ou de ‘a’ até ‘e’ em algumas literaturas especializadas.

(a) Easy mooring, protected area;

(b) Difficult mooring, protected area;

(c) Easy mooring, unprotected area;

(d) Good mooring, unprotected area;

(e) Difficult mooring, unprotected area.

All the mooring energy must be borne by the fender located at the point of impact. Figure 2 shows some of the parameters involved in calculating mooring loads:

 

Figure 2 - Some of the parameters involved in calculating mooring loads. [Source: Kot Engenharia]

 

An interesting aspect of the mooring process is the watercushion that forms between the side of the vessel and the wall(water cushion effect). During mooring, the movement of water in this area can cushion the loads acting on the process. In some cases, the breakwater is hollowed out due to the pier's piling structures, which allows water to flow between the submerged foundation structures, reducing the water cushion effect.

Fender structures are typically made up of steel structures with elastomers attached to the slabs of the mooring pier.

The fender's main function is to absorb the ship's kinetic energy during the approach, converting it into the elastomer's deformation energy. Some of the energy absorbed in the process is dissipated through hysteresis and heat. Figure 3 shows a graph of the relationship between deflection, reaction and energy absorption in an impact absorption system:

 

Figure 3 - Relationship between reaction force, deflection and absorbed energy for a typical elastomeric fender [Source: Trelleborg]

 

3. Examples of fender structures

Fenders are usually made of elastomers, cylindrical or conical in shape. The elastomeric devices are attached to metal structures coated with polymers to reduce friction with the side of the boat. See Figures 4, 5, 6 and 7 for some examples of fenders.

Figure 4 - Example of a pier SUPERCONE fender fixed directly to the metal pile, note the chains to prevent excessive displacement which could damage the fender.

Figure 5 - Example of a pier fender in reinforced concrete and an elastomer fender with a metal panel.

Figure 6 - Example of a pier damaged by an unsuccessful mooring, strong sea currents resulted in an excessive approach speed of the ship that exceeded the capacity of the fender.

Figure 7 - Metal structure fender fitted with a cylindrical elastomer absorber; friction marks between the fender and the ship's side can be seen.

 

4. Final considerations

Elastomeric devices are fundamental parts of a port's infrastructure, preserving structures during ship berthing. An unsuccessful approach, for example, can result in damage to port infrastructure such as: piers, piles, walls, slabs, and can also cause damage to the ship's side structures. If the ship is damaged, the shipowner will charge for the necessary repairs, causing disputes that can be avoided.

The selection of the ideal fender must take into account the aspects of local port operations, because in less sheltered ports with obstructions and risks to navigation, the approach speed must be considered appropriately, directly influencing the energy capacity required for the fenders. The use of fenders with an energy capacity lower than that calculated is a risk, as berthing operations, even if carried out within the design conditions, could cause damage to port structures.

As the increase in ship capacity is a current reality, the port authority must be vigilant so that the fenders installed in its ports are suitable for the energy capacity required by these heavier vessels.

 

Security is with Kot Engenharia

If you, like our more than 150 clients, are looking for specialized solutions in structural analysis or the prevention of failures such as deformations, vibrations and corrosion, please contact our team and count on Kot Engenharia.

Since 1993, we have been offering engineering consultancy services through technical studies using non-destructive testing, field instrumentation and computer simulations (FEM, DEM and CFD) for highly complex diagnoses of concrete and metal structures and industrial equipment.

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