A Closer Look at Prevailing Civil Engineering Practice - European ...

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A Closer Look at Prevailing Civil Engineering Practice, What, Why and How Vincent T. H. CHU Chapter 4. Marine Works 1. Why is larger concrete cover (e.g. 75mm) normally used in marine concrete In marine environment, the cover to reinforced concrete in exposure zones is specified to be much larger than normal concrete (e.g. 75mm). Based on past experimental studies, the concrete cover is identified to be directly related to the corrosion failure of marine structures. In Eurocode 1 under Ultimate Limit State the probability of failure is less than 10 -4 while under Serviceability Limit State the probability of failure is less than 10 -2 . For corrosion aspects, it is normally classified under SLS state. For OPC concrete with 50mm cover, the corrosion probability is found to be more than 10 -2 . However, with the use of 75mm cover, the corrosion probability is about 10 -3 , which fulfills the Eurocode requirement. 2. In mooring of vessels, wire ropes or fibre ropes are commonly used for tying the vessels to mooring system. It is not recommended to use them together in mooring. Why Mooring lines are provided by vessel while the shore provides the mooring points. Wire ropes provide a more rigid mooring system than fibre ropes. When a high degree elastically is required, fibre ropes would be a better choice. The mixed usage of wire ropes and fibre ropes is not recommended because of the uneven tensioning of ropes. Owing to different material properties associated with wire ropes and fibre ropes, it is almost impossible to allocate uniform tension on both types of wires. As a result, there is possible occurrence of overloading in some ropes. 3. Which configuration is better, finger jetty or T-shaped jetty Finger jetty is a more efficient pier structure because it could accommodate vessels at both sides of the jetty. However, there should be sufficient water depth as the berths at finger jetty is relatively close to shoreline when compared with T-shaped jetty so that it is anticipated that vessels are required to berth at shallower water. Moreover, there should be no cross current to enhance berthing at both sides of finger jetty. Also, as mooring points are often located on the jetty, leads are not ideal for larger ships. 30
A Closer Look at Prevailing Civil Engineering Practice, What, Why and How Vincent T. H. CHU T-shaped jetty allows higher water depth for vessels to berth. Moreover, with the installation of breasting dolphins and mooring dolphins, it allows the berthing of larger vessels. Fig. 4.1 Finger jetty and T-shaped jetty 4. What is the difference in design philosophy between flexible dolphin and rigid dolphin (A) Rigid dolphin The impact energy of vessel is absorbed mainly by fender. As such, the dolphin itself is designed as a rigid structure with a group of piles. The piles serve to transfer the reaction force from fender system to the foundation soils. The design of rigid fender is similar to other structures and the strength and stiffness of rigid dolphin should be sufficient to withstand berthing forces without causing excessive deformations. (B) Flexible dolphin The impact energy of vessel is absorbed by lateral deflection of piles. The dolphin itself performs both the functions of fender and berthing structure. Flexible dolphin is particular suitable in deep water region because the energy absorption capacity is a function of pile length. The pile stiffness could not be designed to be too low because large deflection of pile may occur so that the pile may touch the jetty or the vessel may touch the pile. On the other hand, pile stiffness could not be designed to be too high because of potential yielding of piles or vessel’s hull. 31
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