Marine structures

C. S. Smith, Design of Marine Structures in Composite Materials, Elsevier AppHed Science Pubhshers, London, 1990. 

There are numerous publications [9,10,16,19-24] and test specifications [8,25] on the formation of cathodic blisters. They are particularly relevant to ships, marine structures and the internal protection of storage tanks. Blister attack increases with rising cathodic polarization. Figures 5-6 and 5-7 show the potential dependence of blister density and the NaOH concentration of blister fluid, where it is assumed that c(Na ) and c(NaOH) are equal due to the low value of c(Cr) [23]. 

Protection with impressed current, with galvanic anodes, and a combination of both processes is used for marine structures and offshore pipelines. Their properties, as well as their advantages and disadvantages, are given in Table 16-1. The protective measures must be optimized for every structure. In the impressed current protection of offshore platforms, for example, the difficulties of maintenance and repair will be of major importance, whereas in harbor installations these problems can be... 

Table 16-1 Comparison of cathodic protection systems for marine structures...

Table 16-2 Protection potential regions for plain carbon and low-alloy steels (YP 800 N mm ) for marine structures...

Marine structure Reference electrode Cu-CuS04 (V) Ag-AgCl in seawater (V) Uzn (V)... 

In the corrosion protection of marine structures, it is often found that the corrosion rate decreases strongly with increasing depth of water, and protection at these depths can be ignored. Investigations in the Pacific Ocean are often the source of these assumptions [7], However, they do not apply in the North Sea and other sea areas with oil and gas platforms. Figure 16-1 is an example of measurements in the North Sea. It can be seen that flow velocity and with it, oxygen access, is responsible for the level of protection current density. Increased flow velocity raises the transport of oxygen to the uncoated steel surface and therefore determines the... 

Marine Structures and Offshore Pipelines 387 16.7.2 Harbor Structures... 

Songa, T., International Conference on Steel in Marine Structures, Paris, 1981, ECSC, Luxembourg (1981)... 

Sacrificial anodes Small land based schemes and for avoidance of interaction problems. Marine structures, e.g. offshore platforms High soil/water resistivities and small driving e.m.f. may require a large number of anodes Reasonably uniform Cannot be applied in high-resistivity environments... 

Disadvantages Possible interaction effects on other buried structures (Section 10.6) subject to the availability of a suitable a.c. supply source or other source of d.c. regular electrical maintenance checks and inspection required running costs for electrical supply (usually not very high except in the case of bare marine structures and in power stations where structures are often bare and include bimetallic couples) subject to power shutdowns and failures. 

Impressed-current systems for power stations are somewhat more sophisticated than those required for pipelines or marine structures inasmuch that a large number of items of plant, with a wide range of current requirements, are protected by one transformer-rectifier. Each section of every water box in order to provide even current distribution requires one or more anodes. In the case of a large circulating water pump as many as 30 anodes may be required to provide the current distribution necessary. Three types of system should be considered as follows ... 

Fig. 4-12(b) Typical load deflection that is characteristic of a marine structure. 

Boat. By far the most important application of RP in marine structures, particularly with respect to volume consumed, has been in boat construction. This has occurred in both civilian and military markets. Growth continues where it already dominates the small boats with the larger boat market growing. 

Washout resistance Silica fiime shotcrete is very resistant to washout by water. It is thus well suited to operations where freshly applied shotcrete will be exposed to running water or tidal flushing, e.g. rehabilitation of marine structures in the intertidal zone, construction of dikes, and creek channelization. 

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